Organolead halide perovskites: Synthetic routes, structural features, and their potential in the development of photovoltaic

Peter A. Ajibade; Adewale O. Adeloye; Abimbola E. Oluwalana; Mamothibe A. Thamae

doi:10.1515/ntrev-2022-0547

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Organolead halide perovskites: Synthetic routes, structural features, and their potential in the development of photovoltaic

Peter A. Ajibade , Adewale O. Adeloye , Abimbola E. Oluwalana und Mamothibe A. Thamae

Veröffentlicht/Copyright: 1. August 2023

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Aus der Zeitschrift Nanotechnology Reviews Band 12 Heft 1

Abstract

About 13 TW of energy is needed to sustain the lifestyle of people worldwide but an additional 10 TW clean energy will be required by 2050. The increase in the world population and the demand for energy that rely on fossil fuels has resulted in global warming that necessitates the need for alternative energy such as solar. Solar energy is abundant and readily available, and its use will contribute to sustainable development. Metal halide perovskites are promising materials for the development of next-generation solar cells. The power conversion efficiency (PCE) of 25.8% obtained for organolead halide perovskite is close to the polycrystalline solar cell’s efficiency at 26.3% and these materials offer great prospects for future photovoltaic development. To approach the theoretical efficiency limit, it is very important to study the development of perovskite solar cells in terms of material composition, fabrication techniques, and device architectures with emphasis on charge transport layers and electrodes. Limitations to PCE and stability of perovskites, optoelectronic properties, lifetime and stability, wide-scale applications, components of the perovskites solar cell, the standard for testing conditions for good stability and its evolution into the lower layered perovskite solar cells were examined in the current review.

Keywords: perovskite synthetic route; stability; power conversion efficiency

1 Introduction

The increase in the number of installations of photovoltaic (PV) modules worldwide makes the development of viable PV cells one of the most viable power generation technology alternatives to fossil fuels that we relied on at present. The common single-junction solar PV cells in the market are inherently limited in power conversion efficiency (PCE). Among the PV modules being used at present, perovskite solar cells (PSCs) have received great attention due to noticeable improvements in the PCE that exceed 25% [1,2].

Organometal halide perovskites are compounds with the formula ABX₃, where A is an organic cation with a monovalent oxidation state, B is a metal ion with a divalent oxidation state and X is a halide ion with (−1) oxidation state (A is usually larger than B, for example, A = CH₃NH₃ ⁺(MA⁺), CH(NH₂)₂ ⁺(FA⁺); B = Pb²⁺, Sn²⁺; X = Cl⁻, Br⁻, I⁻) [3]. The compound typically forms an MX₆ ⁴⁻ octahedral complex. In this three-dimensional structure, the A-cation is often found within the eight adjacent octahedra of the interstitial space as depicted in Figure 1 [4]. In addition, A being a monovalent organic cation can also exist as an inorganic cation with coordination number 12 [5]. Perovskites are classified as mixed-cation, organic-inorganic halide, and mixed halide types [5,6]. Their constituent ions determine the physical properties of perovskites as they dictate their structures to be either pseudo-cubic or distorted cubic. Therefore, a wide range of band gap values, high absorption coefficient, and excellent carrier transportation [7,8] may be obtained by changing the chemical identity of the constituent ions.

Figure 1

The crystal structure of perovskites.

The development of perovskites in PVs started with the substitution of a pigment of dye-sensitized solar cells (DSSCs) with CH₃NH₃PbBr₃ and CH₃NH₃PbI₃ leading to PCEs of 3.13 and 3.81%, respectively [9]. The introduction of 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OmeTAD) and mesoporous titanate that serve as a p–n junction resulted in 9.7% efficiency [10], while the replacement of the n-type TiO₂ with Al₂O₃ (an insulator) resulted in meso-super structured PSCs. The mixing of PSCs with halogenated perovskite (MAPbI_3−xCl_x) as a photoactive layer enhanced the PCE up to 10.9%, which was ascribed to the Al₂O₃ materials serving as an insulating scaffold capable of avoiding a drop in the voltage as related to the TiO₂ bond tails that generate 1.3 V [11]. The use of various synthetic routes leads to an increase in PCE up to 12.8% [12,13,14] under solar irradiation with ambient stability above 1,000 h using a one-step deposition method [15,16] with PCE in the range of 19.3 to 20.1%.

This considerable increase in the PCE of perovskite solar cells from 3 to 22% over a decade [17,18] astonished the PV community and generates great interest. PSCs show great promise for commercialization as a result of their promising PCE, cost-effectiveness in terms of the process of fabrication, band-gap tunability due to broad spectral absorption [19], good coefficient of absorption [8], electron–hole diffusion lengths that are longer than the absorption depth [20], solution process ability at a reduced temperature [21], and the charge selective layer (especially the n-type semiconductors) promoting the transportation of light absorbers from the photoactive layer to the front electrode, which is critical in the fabrication of perovskites with high performance [22,23,24,25]. Over the years, several efforts have been made to understand the physical and chemical functionality [26], architectural design [27,28], crystallization modulation [29,30], and interfacial engineering [31] of perovskite. These efforts have led to an increase in the PCE while the stability issues are being addressed, especially in the presence of moisture, because perovskites are ionic crystals [32]. Hence, improvements in the stability of PSCs are essential and some of the synthetic routes are explored in this review. At present, the PCE of PSCs has reached a certified 25.5% [33,34,35].

2 Synthetic routes of PSCs

Perovskite may be synthesized through the solution-processed methods or the vapour-deposition method depending on the need and functional requirements.

2.1 Solution-processed methods

This technique is used to prepare pure and crystalline perovskites through a layered solution or the top-seeded solution growth (TSSG) method. In the layered solution approach, (C₆H₅C₂H₄NH₃)₂PbCl₄ perovskites are synthesized at room temperature by the addition of a methanolic solution of C₆H₅C₂NH₂ and concentrated hydrochloric acid containing lead chloride [36]. TSSG methods may be used to prepare large- and small-sized methylammonium lead iodide [37]. Solution synthetic processes may be completed through thermal annealing to remove the solvent residue and promote phase transformation since perovskites are known to precipitate out of solution either by heating or spin-coating the precursor metal halide and organic halide [38,39]. Moreover, laser illumination may be used to achieve homogenous perovskite structures with enhanced optical properties [40] to impede incomplete intermediate phase transformation, which could lead to poor optical properties, heterogeneity, and low PCE in solar cells. Perovskite could also be obtained easily at room temperature through the coating of the methylammonium iodide (MAI) and lead iodide mixture in dimethylformamide (DMF) on the polycrystalline TiO₂ film [9].

The solution process method is divided into one- and two-step deposition methods. The main disadvantage of the one-step deposition is the uncontrollability of large morphological varieties due to the mixture of the precursors (metal and ammonium halides) in a solution. Inverted PSCs of ITO/HTM/MAPbI₃/BCP/Ag fabrication using the spin-coating method gave an efficiency of 17.6%. An increase in PCE to 20.3% was obtained with the MAPbI₃ PSC using the chlorobenzene (CBZ)/perylene mixed antisolvent [41]. The use of the spin-coating anti-solvent technique increases the efficiency to 23.4% with high stability [42]. The advantages derived from a single-step deposition technique include additives that create heterogeneous nucleation sites that support uniform crystallization with full coverage [43], reduced charge carrier recombination, low defect density, and enhanced photocurrent conversion efficiency [44,45,46]. A typical two-step spin coating method for the preparation of the perovskite layer is illustrated in Figure 2, wherein the solutions of solvent mixtures of DMF and DMSO in appropriate volume ratios were prepared as the precursor solvents [47].

Figure 2

The two-step spin coating method for the preparation of the perovskite layer.

Studies of anti-solvent dripping conditions on inverted planar CH₃NH₃PbI₃ solar cells under environmental conditions [48] revealed that the right combination of relative humidity and anti-solvent dripping time affects the (110) orientation of crystal growth, which results in pinhole-free films with light harvesting and charge carrier reduction. The two-step process involves the preparation of the metal halide by spin coating followed by exposure to ammonium halide through spin or dip coating [49,50]. This process gives perovskites better morphology, versatility [51], and reproducibility [52]. Single crystals can also be grown by the vapour-assisted crystallization approach, which dissolves MAPbX₃ in a solvent in which low solubility is attained as temperature increases. This method is called solvent engineering technology that produces a full solution-processed PSC with 18.53% PCE certification under standard conditions. For example, MAPbI₃ perovskites were prepared by solvent engineering technology [53]. The composition of the perovskite led to high stability in the ambient atmosphere. Deposition of perovskite by mixing Lewis’ base urea as an additive controls the crystallization dynamics to form large-grain crystals [53]. The PCE is better than the 16.2% PCE initially reported [54] with the use of γ-butyrolactone, DMSO, and toluene as solvents.

The morphology of perovskites influences their transportation properties and trap density [55]. The use of the spray deposition technique results in uncontrollable morphology with pinholes that makes reproductivity difficult [56] even though perovskites prepared in this manner are highly efficient. Therefore, as revealed in Figure 3, solvent engineering of perovskite solar cell film is important as the use of antisolvent results in shiny, pinhole-free, and full coverage of the perovskite film [47,57].

Figure 3

Scanning electron microscopy (SEM) images of the perovskite surface prepared on the hydrophilized microscope slides with different solvents: (a) pure DMF, (b) mixture of DMF/DMSO, 4:1, (c) mixture of DMF/DMSO, 1:4, (d) pure DMSO, and (e) cross-sectional SEM image of a complete cell fabricated with the mixture of DMF/DMSO, 4:1 as the precursor solvent. Adapted with permission from Hosseinmardi et al. [47]. Copyright: Journal of Materials Science: Materials in Electronics.

2.2 Thermal evaporation method

The thermal evaporation method gives perovskites better uniformity and crystallinity. Thermal evaporation techniques could be through co-evaporation or single-source evaporation. Co-evaporation makes it possible to prepare precisely controlled layered perovskite thin films with great flexibility [58], while single-source evaporation requires a complicated device for fabrication [59]. The co-evaporation process is complicated due to the balance needed between the perovskite organic and inorganic components. CH₃NH₃PbI_3−xCl_x were synthesized with a vapour pressure at 65°C, and they exhibited lower absorption, full film coverage, large grain size, and long-range and uniform thin-film morphology [60]. An improvement was achieved by using vapour deposition to produce the fabricated perovskite that was homogenous and dense with a low-density effect [61]. The MAI vapour pressure can be controlled by adjusting the MAI source temperature with a constant PbI₂ evaporation rate, with the working total pressure of 4.5 ± 0.3 × 10⁻³ Pa at 175°C [62]. The MAI source temperature results in highly dense and uniform morphology that is pin-hole defective [62].

Using a triple-source co-evaporation system as shown in Figure 4, enhanced annealing properties and the PV performance of PSCs were established. In comparison to a normal thermal evaporation method used in the preparation of MA-free formamidinium lead triiodide (FAPbI₃) perovskite layers which only afford PCE of 14.2%. The product is accompanied by a pure FAPbI₃ layer, which leads to an inactive yellow PV δ-phase at room temperature and prevents the introduction of Cs⁺ into the perovskite structure. With the introduction of a triple-source co-evaporation system, the optical and structural properties and the PV performance of FAI-poor and stochiometric devices are improved when compared to FAI-rich devices with a negative impact (Figure 5) [63]. Studies have also revealed that the formamidinium (FA)-based perovskite has advantages over MAPbI₃ in terms of a wider absorption wavelength range of up to 850 nm and higher stability that further increases the J _sc and efficiency [64,65,66,67].

Figure 4

Schematic diagram of the (a) triple-source co-evaporation system and (b) PV device structure.

Figure 5

SEM images of (a) and (d) FAI-poor, (b) and (e) stoichiometric, (c) and (f) FAI-rich perovskite samples prepared on glass substrates before and after annealing at 100°C, respectively. Scale bar: 1 µm. Adapted with permission from [63]. Copyright: Journal of Materials Chemistry C.

Recently, PCE of over 20% was attained for methylammonium lead iodide perovskite in the p–i–n configuration [68] but the fabrication process was not reproducible. Hence, it is important to understand the effect of different components of PSCs on the PCE to be able to develop reproducible PSCs. The effects of structural components on the efficiency of PSCs were further examined in the review.

2.3 Electrodeposition (ED) method

Apart from the well-developed solution and thermal methods of preparation of PSCs, recent development and special interest have been given to the use of the ED method that serves as a hybrid approach with superior advantages of the chemical and physical thin-film deposition techniques. ED is an electrochemical technique used in the deposition of thin films onto a conducive substrate to form redox reaction products. It is a method that allows direct control of the nucleation and formation of target films [69]. General features of ED that showed the electrochemical setup for PV purposes and electrodeposited perovskites solar cells are shown in Figure 6. It is a three-electrode cell containing the working electrode (WE), counter electrode (CE), and reference electrode (RE) (Figure 6a), with the corresponding voltammogram (Figure 6b) and chronoamperogram (Figure 6c).

Figure 6

(a) Schematic representation of the ED setup with a three-electrode electrochemical cell: WE, CE, and RE. WE represents the ED substrate while CE assures the passage of the current through the electrolyte and warrants electrical continuity with the external circuit. RE allows the control of the potential of WE with respect to the constant potential value of RE (non-polarizable electrode). (b) Typical cyclic voltammetry with the nucleation loop at high potential (regime of oxidative ED). (c) Representative current transient recorded during the potentiostatic ED process of NiOOH on ITO. Adapted with permission from [69], Copyright: Solar RRL.

Two different pathways were identified for the electrochemical preparation of perovskites that can go either through lead oxide deposition and/or the inclusion of MAI when the lead oxide film is first converted to lead iodide by hydrogen hydride leaching. Nonetheless, a cathodic ED of the lead ion solution made from metallic lead has also been reported [70,71,72,73]. In comparison to the record efficiencies of perovskites PV, which are based on small area devices, ED is suitable for enhancement of the production method [74]. The ED technique is the most suitable process for large-scale area preparation with commercial viability and controllability but still suffers because of the varied intrinsic properties embedded in active perovskite layers. Thus, the use of the ED technique for improving efficiency hinged on selective contacts, solubility of materials in appropriate solvents, and types of substrates/electrodes [75,76].

3 Perovskites’ structures and efficiency

Optimization of the several functional components of PSCs is important for a reduction in the degradation process and increases the photoconversion efficiency. In the recent decade, PSCs with different structural components have been fabricated and their PCE investigated [77,78,79]. The most common architectural plan for PSCs’ fabrication is either the glass/transparent conductive oxide (TCO)/p-type hole transport layer (HTL)/perovskite/n-type electron transport layer (ETL)/metal electrode or glass/TCO/n-type ETL/perovskite/p-type HTL/metal electrode. Perovskites are sandwiched between the ETL and HTL though the device can be fabricated as hole-conductor free; hence, the morphology of ETL helps in the smoothness of the perovskite growth and formation, which also helps to improve the opto-photoelectric properties of the device [80]. Perovskites consist of three types of configurations: regular mesoporous supersaturated solar cells [11,81], regular planar solar cells [82], and inverted planar solar cells [79,83].

3.1 Regular mesoporous supersaturated solar cells

PSCs are classified into two groups: mesoscopic and planar types. Unlike the mesoscopic type which consists of a mesoporous layer among other device structures, the planar-type cell does not possess mesoporous layers but has direct contact between the perovskite and semiconducting compact layer. Each device structure possesses unique characteristics that tend to favour the efficiency of the PSC. In mesoporous supersaturated solar cells, Al₂O₃ is added as an insulator to the mesoporous TiO₂ layer to form a heterogeneous TiO₂/Al₂O₃ composite as a mesoporous scaffold, as shown in Figure 7. This composite formation leads to improved mesoporous TiO₂ perovskite’s surface wettability property and efficiency upon the addition of Al₂O₃, yielding thick perovskite capping layers and better surface coverage. The overall improvement observed in PV cells depends on the optimum Al₂O₃–TiO₂ composite ratios [84]. Solar cells, also known as porous architecture devices, can act as both conductors and insulators. They are classified as mesoporous metal oxide perovskite for mesoporous conductors and mesoporous scaffold solar cells for mesoporous insulators [85].

Figure 7

Electron transport in PSCs based on mesoporous films with varying TiO₂/Al₂O₃ composition ratios.

An insulator is spin-coated onto the compact metal oxide as an additional layer on top of the pre-cleaned fluorine-doped tin oxide (FTO) conductive substrates. Mesoporous n-type TiO₂ (mp-TiO₂) is the most common, and it not only extracts photo-excited electrons generated in the absorbing layer but also increases the perovskite crystal transformation. Besides the mostly used TiO₂ nanocrystals, others such as nanosheets, nanorods, nanotubes, and nanofibres/nanowires have been employed in the fabrication of mesoscopic TiO₂ structures [86] as their physicochemical properties affect the intrinsic electronic structure and morphology. Variation of the size of mp-TiO₂ from 10 to 15 nm layer gave increased efficiency from 11.4 to 12.8% because it is much easier to fill the perovskite with a bigger pore size [87]. Yang et al. reported the size effect of TiO₂ nanoparticles on printable (5-AVA)_x(MA)_1−xPbI₃ mesoscopic PSCs. The TiO₂ nanoparticle size was varied from 15 to 30 nm using the ZrO₂ layer and a carbon CE in a hole-free conductor, and the best efficiency of 13.41% was obtained from TiO₂ nanoparticles of 25 nm [88]. Accordingly, the transmission electron microscopy (TEM) patterns of the different-sized TiO₂ nanoparticles, as observed in Figure 8, show that the sizes of TiO₂ particles have effects on the precursor and the contact between perovskite crystals and TiO₂, which at the same time affects the interface of the perovskite/TiO₂ charge transfer kinetics. Thus, the shape of the TiO₂ nanoparticles influences the pore structure and connectivity.

Figure 8

TEM patterns of different-sized TiO₂ nanoparticles. Adapted with permission from [88]. Copyright: Journal of Materials Chemistry A.

The irregularities in the TiO₂ mesoporous layer are found to have a significant influence on the formation of the perovskite film and performance. An efficient solution-based technique was therefore used to grow thin films of homogeneous TiO₂ nanoparticles with tuneable crystal size, porosity, and roughness on a transparent conducting oxide (TCO) that affects the optimization of the perovskite morphology. The ability to control the morphology of the individual components within PSCs and the optimization of interfacial interactions between the layers is a predominant factor that could be used to enhance the solar cells’ efficiency. It was also observed that DMF and CBZ played an important synergistic role in facilitating crystal growth, increasing the grain size, densifying the film, and improving crystallinity and surface smoothness. Surface smoothness and surface coverage could be used to improve the performance of the device and reproducibility [89].

TiO₂ films are widely used in perovskite fabrication because of their chemical stability, electron transfer ability, and optical and electrical characteristics. Various techniques have been used to deposit on TiO₂ layers. These include magnetron-sputtering, sol-gel, e-beam evaporation, atom layer deposition, spray pyrolysis, thermal oxidation, spin-coating, and electrochemical deposition [90,91,92,93,94,95]. Among these techniques, spray pyrolysis, spin-coating, and atomic layer deposition (ALD) are the mostly used techniques. ALD has been used in a layering style to give a compact TiO₂ layer. Unfortunately, post-treatment via thermal oxidation leaves electronic pinholes. In addition, spray pyrolysis required a high temperature that led to high energy consumption. Sol-gel requires low-temperature, although it is difficult to prevent pinholes on the blocking surface layer [91]. Magnetron sputtering is an inspiration for obtaining low-cost and high-performance blocking layers, although thermal oxidation leaves titanium particles behind, thus increasing the recombination charges [80,96,97].

Several synthetic methods have been used in the preparation of PSC, e.g. Anuratha et al. [85] reported the galvanostatic anodic deposition method in the fabrication of a bifunctional nanoporous TiO₂ thin film. The fabricated PSC module made from the bifunctional TiO₂ film gave 17.06% efficiency, which was higher when compared to that reported for the commercially made TiO₂ nanoparticle. A schematic representation of TiO₂ porous layers by different methods is summarized in Figure 9. The results show that incorporation of soft template materials often leads to a reduction in electron recombination and appreciable enhancement of the PV performance and sunlight utilization [63].

Figure 9

Schematic representations of the fabrication of TiO₂-porous layers using different methodologies.

Lee et al. reported a new route for the fabrication of perovskite layers. In their work, a conformal methylammonium lead iodide (MAPbI₃) film was prepared on different substrates of varied morphologies using the ED method that was followed by the vapour reaction [98]. This method provides high-quality perovskite films for planar or textured substrates. A similar approach was reported by Li et al. [99], where PbI₂ was coated on TiO₂ films and subsequently exposed to CH₃NH₃I to form a MAPbI₃ layer. The SEM image of the PbI₂ is shown in Figure 10.

Figure 10

SEM images of the PbI₂ deposited electrochemically for (a) 30 min, (b) 40 min, (c) 50 min, and (d) 60 min. Adapted with permission from [99]. Copyright: Solar Energy.

The effect of morphological defects (pinholes) has been reported to increase the recombination on the perovskite surface layer due to improper alignment of the perovskite surface layer and ETL [80], which further causes hysteresis. This was considered by Liu et al. [100] in a synthesis where o-dichlorobenzene was used to suppress the volatilization of the solvent. This results in surface defect passivation, which favours the transportation of photocarriers in the active layer. PCE of 20.72% was attained, and after 2,400 h, 85% of the initial PCE remained. Hence, it is important to optimize synthetic parameters to reduce the pinhole effect.

3.2 Regular planar PSCs

Mesoporous PSCs, which are the original device structures, have achieved relatively higher PCE values. However, these device structures are complicated and require high-temperature processing and various fabrication steps [101,102,103,104]. By comparison, the planar PSCs, a much simpler structure that can easily be fabricated, are receiving a lot of attention in recent times [105,106]. Planar architecture with less effective charge extraction lacks a mesoporous scaffold layer, so the perovskite layer is located between the ETL and HTL. In planar PSC configuration, the thickness of the perovskite film is important because a too-thin or too-thick film affects the absorption of sunlight and the collection of charges at the p–n junction, respectively. Furthermore, the mesoporous oxide film is not applicable, and only a compact layer of TiO₂ is used to serve as an electron-selective contact layer. In either case, the planar/mesoporous ETL is important to attain high open-circuit voltage (V _oc) and fill factors (FFs) to obtain high PCE [106]. A combination of TiO₂ nanoparticles and TiO₂ nanorods as an ETL [107] for the fabrication of Cs_0.05(MA_0.17FA_0.83)_0.015Pb(I_0.83Br_0.17)₃ PSCs resulted in a PCE of 17.2%, which is higher than that of PSCs fabricated with an ETL of TiO₂ nanoparticles by 20%.

To improve the electrical stability of the TiO₂ ETL due to its low conductivity property, ethanol-soluble C₆₀RT₆ (fullerene derivative) was prepared by Wang et al. [108] as an additive for reduced TiO₂. They observed that better conductivity, negligible hysteresis, higher surface hydrophilicity, long-term stability, and high-PCE were obtained by using the additive compared to using pure TiO₂. TiO₂ is widely utilized as an ETL in regular planar PSCs. SnO₂ was used by Xu et al. [105] due to its low fabrication temperature, high electron mobility, and suitable energy alignment attributes. To increase the wettability properties of SnO₂, ethanol was used as a solvent for dispersion, as this reduced the aggregation of SnO₂ and the quality of PSCs with a PCE of 18.84% attained. This indicates that other metal oxides can be considered as ETL for regular planar PSCs to improve environmental, electrical, and thermal stability development.

3.3 Inverted planar PSCs

An inverted planar heterojunction with p-type-intrinsic-n-type (p–i–n) structures has attracted attention due to low hysteresis in comparison with n-type-intrinsic-p-type (n–i–p) structural planar heterojunction devices [109,110,111,112]. Furthermore, an inverted planar heterojunction with p–i–n structures can also be fabricated through a low-temperature route using poly(3,4-ethylenedioxythiophene) polystyrene sulphonate (PEDOT:PSS) films yielding flexible solar cells [113,114] offering a PCE of 21%. Unfortunately, PEDOT:PSS has stability issues due to high levels of acidity and hygroscopic nature demonstrated in PVs and light-emitting diodes (LEDs) [115]. To surpass stability concerns, inorganic hole transport materials were employed to replace PEDOT:PSS layers such as CuSCN films and NiO_x [116,117,118]. While the research on NiO_x-based PSCs is exceptional, film deposition uses an expensive pulse layer deposition method, which renders it unsuitable for large-scale fabrication. For example, a solution derived from NiO_x-based inverted planar heterojunction PSCs by employing PEDOT:PSS films, NiO_x nanoparticles, and NiO_x films by depositing the perovskite films (CH₃NH₃PbI₃) via one-step coating was reported [119]. Cerdàn-Pasaràn et al. [120] reported on the post-synthesis treatment of inverted planar ITO/PEDOT:PSS/MAPbI₃/[6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM)/Ag at different temperatures (100, 120, and 150°C), as shown in Figure 11. Increased efficiency of 8.2–14.2% was obtained at an optimum temperature of 100°C and was attributed to boosted morphology that tweaked the charge–carrier interaction. However, it was observed that below 100°C, an incomplete conversion and non-homogenous film-forming islands were obtained as well as the disintegration of the perovskite layer at higher temperatures above 100°C.

Figure 11

SEM images of (a) the MAPbI₃ film deposited on PEDOT:PSS and (b–d) perovskite film after the bromide solution post-synthesis treatment annealed at 100, 120, and 150°C. Adapted with permission from [120]. Copyright: Solar Energy.

Zhang et al. [121] reported the use of conjugated polyelectrolytes (DTB-Na) that have ionic pair with similar wettability properties as PEDOT:PSS and give a PCE of 19.92%. The high efficiency obtained was attributed to defect passivation on the PSL surface and fewer defects in the deep trap state for DTB-Na. This means reduced trap state increases the transport layer recombination, which improves the PSCs’ performance.

Masi et al. [122] used a mixed cation perovskite layer with tropolone as a chelating agent for lead. This led to the development of a somewhat ideal perovskite surface morphology with a PCE of 20% and an increase in the short-circuit current. This kind of surface morphology modification is another area of PSCs that can be considered for the development of PSL morphology. A summary of the device structures and PCE (%) as reported by Bai et al. [123] is listed in Table 1.

Table 1

A summary of comparison of devices containing mesoporous structure, planar n–i–p structure, and planar p–i–n structure with published PSCs

Device structure	Light source	Ageing condition	Degradation factor	Initial	Estimated T ₈₀ (h)
Device structure	Light source	Ageing condition	Degradation factor	PCE (%)	Estimated T ₈₀ (h)
Mesoporous structure
FTO/c-TiO₂/Li-doped meso-TiO₂	White LED	Nitrogen, MPP,	Light (without UV),	∼17	∼1,700
+ perovskite/poly(triarylamine) (PTAA)/Au		500 h	85°C		(burn-in)
FTO/c-TiO₂/Li-doped meso TiO₂	White LED	Nitrogen, MPP,	Light (without UV),	∼20	∼5,700
+ perovskite/CuSCN/r-GO/Au		1,000 h,	60°C
FTO/meso-TiO₂/meso-ZrO₂/	—	Ambient air,	Light (–), air	∼11	—
Perovskite/carbon		1,008 h
Planar n–i–p structure
FTO/BaSnO₃:La/perovskite/	Metal–halide	Ambient air, sealed,	Light (with UV)	∼14	—
NiO/FTO	lamp	Open-circuit, 1,000 h
FTO/SnO₂/PCBM/perovskite/	Xenon lamp	Ambient air, sealed,	Light (with UV)	∼17	∼3,900
Spiro-OmeTAD/Au		Open-circuit, 2,400 h	50–60°C		(Burn-in)
ITO/TiO₂-Cl/perovskite/	Xenon lamp*	Nitrogen, MPP, 500 h	Light (without UV)	∼20	∼1,600
Spiro-OMeTAD/Au
ITO/C₆₀-SAM/SnO_x/PCBM/
Perovskite/polymer/Ta-WO_x/Au	White LED	Nitrogen,	Light (without UV)	∼20	∼1,600
		Open-circuit, 1,000 h			(Light-soaking)
		Ambient air, MPP,	Light (with UV), air,	∼12	∼2,500
FTO/SnO₂/perovskite/EH44/	Sulphur	1,000 h	∼ 30°C		(Light-soaking)
MoO_x/Al	plasma lamp	Nitrogen, MPP,	Light (with UV),	∼16	—
		1,500 h	∼30°C
Planar p–i–n structure
FTO/LiMgNiO/perovskite/PCBM	Xenon lamp*	Ambient air, sealed,	Light (without UV),	∼16	∼2,200
/Nb-TiO₂/Ag		MPP, 1,000 h	45–50°C
ITO/NiO/perovskite/PCBM/	Sulphur	Ambient air, MPP	Light (with UV),	∼13	∼2,000
SnO₂/ZTO/ITO/LiF/Ag	plasma lamp	1,000 h	∼35°C		(Light-soaking)
ITO/PEDOT:PSS/2D	Xenon lamp	Ambient air, sealed,	Light (with UV)	—	—
perovskite/PCB/Al		Open-circuit, 2,250 h

Adapted with permission from [123]. Copyright: Nature.

4 Hybrid PSCs

Hybrid perovskite is known to exhibit ambipolar properties and can serve as an electron and hole transporter simultaneously, thereby allowing PSC to be structurally flexible, though two architectural devices are associated with PSCs, which are mesostructured (nanostructured) and planar structures [77,124]. A good optical absorption coefficient, low trap density, and efficient charge-transport properties are important for the excellent efficiency of PSCs. Materials with electrons as major carriers (n-type) such as electron-accepting fullerene and mesoporous metal oxide scaffold serve as good-electron transporters in PSCs; hence, the photoexcited electrons are extracted efficiently and collected by TCOs [20,125].

4.1 Binary cation

Mixed organic cationic perovskites have unique properties, such as tunable band gap, high defect tolerance, high absorption coefficient, and long carrier diffusion length. FA_xMA_1−xPbI₃ films [126] were prepared by varying the concentration of FA in methyl ammonium (MA), and it was observed that the concentration of FA⁺ affected the optical and photoactivity of the film. The morphology and crystallinity can be improved with the addition of ethyl acetate which serves as an anti-solvent. To obtain solar cells with negligible hysteresis and PCE of 15.8% [127], a sequential vapour-processing route can be used to fabricate n–i–p (FA, MA)PbI₃ solar cells using TiO₂ and Spiro-OMeTAD as transport layers. When phenylethylammonium iodide (PEAI) is used as a post-treatment for FA_1−xMA_xPbI₃ to reduce perovskite film surface defects rather than 2D perovskite. PEAI filled the grain boundary thereby passivating the surface, and a certified PCE of 23.32% was attained [128]. To improve the hole extraction in FA_0.85MA_0.15Pb(I_0.85Br_0.15)₃ PSCs [129], lithium bis(trifluoromethanesulphonyl) imide (Li-TFSI) was replaced with molybdenum tri(1-(trifluoroethanol)-2-(trifluoromethyl) ethane-1,2-dithiolene) as a dopant for spiro-OMeTAD, which gave an improved performance at 21.2% and long-term thermal stability. Recently, Manteen et al. [130] produced mixed perovskites by the incorporation of MAI-PbI₂ as an intermediate layer on FABr solution. High-quality mixed perovskite films with improved grain size, morphology, and reduce defect density were obtained with a conversion efficiency of 20.08%. The methodical approach used was simple and can be scaled up industrially in the future. When FA_0.6MA_0.4PbI₃ was used as an absorber layer with PTAA as an HTL, an efficiency of 22.8% [131] was obtained. PTAA hydrophilicity enhanced the diffusion of perovskite solvent complexes that led to favourable growth of crystal films.

4.2 Mixed halide PSCs

Reyna et al. [132] reported mixed halide perovskites in real outdoor conditions that are stable for more than 1,000 h. A high open-circuit voltage and high FF were observed at very low light irradiation, a response attributed to the ionic–electronic properties that characterize halide perovskites. A “double electronic–ionic transport model” was made to reproduce the real outdoor operation conditions of the devices and showed that the ionic component dominates at low intensities but is annihilated at high irradiances where the electronic component is dominant. This response is possibly affected by the “pre-treatment” of PSCs in the dark, where charge accumulation triggers high V _oc values once the sample is irradiated at low light intensities, demonstrating the importance of the device pre-treatment and the history of the device. The extraordinary effect of ion migration observed at low irradiation can contribute towards understanding the working mechanisms of the cells under actual conditions and may encourage the PSCs’ community to fabricate more.

Lead mixed-halide perovskite CH₃NH₃PbI_3−2xBr_2x films were prepared by a two-step sequential method [133]. Ethanol treatment was introduced into the process to influence the size, shape, and microstructure of the perovskite. Ethanol fastens the crystallization of Pb(I_1−xBr_x)₂, which exhibited a porous structure; hence, the complete conversion of PbI₂ was facilitated at the same time resulting in a highly pure and crystallized CH₃NH₃Pb_3−2xBr_x with 15.33% efficiency. After exposure to air for 14 days, it retained 66% of its initial PCE (x = 0.2). A two-step sequential deposition method provides a reproducible way of obtaining perovskite films with controllable morphologies with uniform and dense films [134,135]. Perovskites prepared by two-step fabrication lead to relatively small particle size, which means a large amount of grain boundary exists in perovskite films, and it may serve as trapping sites for carriers and affect the FFs negatively. Chen et al. [136] used tin(iv) oxide/PCMB (SnO₂/PCBM) as a double interfacial modifier to increase the performance of mixed-cation mixed-halide perovskites using a low-temperature ALD method. The PCE of 19.45% was obtained with better light stability, increased charge transport, and reduced charge recombination. The effect of different solvent ratios (DMF/DMSO) on the MAPbI_3−xCl_x performance was evaluated. High performances were obtained in cells made from precursors that are soluble in DMF due to their interaction material [137]. Unfortunately, up-scaling the mixed perovskite still proves challenging; to this effect, Guo et al. [138] used the spray deposition technique to fabricate MAPbI_3−xBr_x PSCs with a PCE of 15.60%.

A new trend in the use of natural products and their derivatives was seen in the work of Cheng et al. [139]. They used a chlorophyll derivative to prepare sodium copper chlorophyllin (NaCu–Chl) that was employed in different ways to suppress the defect-induced nonradiative recombination expected to improve film morphology and create efficient hole extraction capable of transferring from perovskite to Spiro-OMeTAD. A PCE of 20.27% was subsequently reported from NaCu–Chl-treated MAPdI₃ combinations as depicted in Figure 12.

Figure 12

Summary of PV performances of PSCs assisted with natural chlorophyll derivatives. Adapted with permission from [139]. Copyright: ACS Appl. Energy Mater Interfaces.

5 Transport layer

It is believed that a device's performance is determined by how efficiently charge carriers are transported and extracted across the interface by both the ETL and HTL. In an inverted structure PSC, the ETL is typically a metal oxide (e.g. zinc oxide, titanium oxide, etc.), and the HTL is usually composed of organic molecules, such as Spiro-OMeTAD, benzodithiophene polymers (e.g. PTB7), or polythiophenes (e.g. P3HT) [140,141,142]. Some of these organic molecules have shown promising performances with the best PCE for these molecules in the range of 10–20%. It is relatively easy to analyse the structures of the ETL and perovskite photoactive layers due to their high crystallinity.

A typical planar heterojunction cell has a photoactive layer inserted in the middle of the ETL and the HTL [103,104,143]. By absorbing the incoming photons, holes and electrons are created inside the photoactive layer and are extracted by the ETL and HTL, respectively. Moreover, grain boundaries were not only passivated but also heterojunctions were formed, thus improving the electron extraction rate, and the device had enhanced charge transport and suppressed charge recombination. The process proved to be undemanding and a practical way for printable PSCs [144].

5.1 Hole-transport layer materials (HTMs)

HTMs play a significant role in the PCE, and hence there is a need to understand the principle behind these materials. Numerous HTMs exist, as shown in Figure 13, but Spiro-OMeTAD is the most common [145]. Other HTMs have been explored with a PCE of over 20%. The chemical properties of Spiro-OMeTAD depend on their doping content, so the doping content of Spiro-OMeTAD can be reduced to lower polaronic absorption [146]. There is still a need for further research on inexpensive HTMs with good stability and high efficiency with reproducible results. The limitation of polymer HTMs-based PSCs is their degradation on exposure to ultraviolet light more than humidity or oxygen, which reduces their efficiency.

Figure 13

Chemical structures of HTMs.

The energy level diagram of the components of a p-type organic semiconductor employed in PSCs and their corresponding reported PCEs, especially for the current best performing HTMs are shown in Figure 14. It is common to find most HTMs with methoxy group (–OCH₃) as a substituent. It has been found that the –OCH₃ group is capable of exhibiting both electron-withdrawing and electron-donating properties but this depends on whether the group is on the para- or meta-substitution position. Their presence has been ascribed to be responsible for the adjustment of the HOMO level of the material. The methoxy group also serves as anchoring agent on the perovskite layer, a role like the carboxylic acid group on the TiO₂ semiconductor in DSSCs [4].

Figure 14

Schematic energy-level diagram of the components used in PSCs including the most efficient perovskite light absorbers MAPbI₃, MAPbI_3−xCl_x, (FAPbI₃ )_0.85 (MAPbBr₃)_0.15, HTM, and TiO₂. Power-conversion efficiencies are shown for the best-performing HTMs along with their HOMO levels [4]. Copyright: Angew. Chem. Int. Ed.

Recently, Elnaggar et al. [164] reported the preparation of a new series of PTAAs as HTMs for p–i–n PSCs. The positional influence of substituent groups on aryl backbones, as depicted in Figure 15a, led to the formation of compact films with better charge-transport characteristics and superior solar cell characteristics and efficiency greater than 20%. Another work involving a new series of linear HTMs based on a fluorenyl core was carried out by Sun et al. [165] where it was discovered that substituting the middle carbon atom with different alkyl chain lengths affects both the photophysical and electronic properties, as well as the hole mobility, transmittance, thermal stability, melting point, intermolecular stacking, and the optimal energy level alignment of perovskite (MAPbI_3−xCl_x) types leading to an efficiency greater than 22% for the performance of p–i–n PSCs. As shown in Figure 15b, the efficiency increases with N increase in THE alkyl chain length of the linear compounds. A similar report by Wang et al [166] showed the influence of increasing the number and varying THE position of fluorine substitutions on pyrrolo[3,2-b]pyrrole derivatives prepared as dopant-free HTMs for PSCs. An increased number of fluorine atoms enhance the photochemical and PV performances to afford PCE of 20.1%, as shown in Figure 15c. To overcome the intrinsic surface defects, poor stability, and retard charge recombination in PSCs, Fu et al. [167] prepared two solution-processable two-dimensional (2D) conjugated polymers with efficiencies between 23 and 24%, as shown in Figure 15d

Figure 15

Various HTMs for (MAPbI_3-xCl_x) PSCs with improved PCE efficiencies. Adapted with permission from (a) [164] (b) [165] (c) [166] and (d) [167]. Copyright: ACS Appl. Energy Mater.

5.2 ETL materials

The ETL is an excellent collector and carrier of electrons where electrons are injected from the absorbing layer and then transported through the electron transporting material and collected by the electrode. It is important that ETM satisfies band alignment with the perovskite layer and high transmittance in the UV-Vis region to make the passage of photons easily and readily absorbed by perovskites. The proper selection of ETL is important as it plays a significant role in the ambient, thermal stability, and efficiency of PSCs [168].

The ETL mostly exists as porous nanostructures such as nanowires, nanoparticles, nanorods, nanosheets, and nanoplatelets and is coated directly on TCO substrates for the improvement of optical and photoelectrical properties. There is an increase in the amount of incident light passage from ETL to the photoactive layer, which could assist with the infiltration properties and enhances the efficiency of the PV device [169]. Nanostructured ETLs have been found to give efficient electron injection, short carrier path for electrons, enhanced transport properties, and better charge collection efficiency through improved surface morphology and roughness [170]. The structure, uniformity, shape, and size of ETM in perovskite are key to PCE improvement [171,172].

TiO₂, ZnO₂, SnO₂, Zn₂SnO₄, V₂O₅ WO₃, SrTiO_3, and BaSnO₃, have been used as alternative ETLs due to their unique attributes such as ferroelectricity, superconductivity, excellent conduction band edge, higher electron mobility at room temperature, thermoelectricity, and transparency [146,173,174,175,176,177]. TiO₂ is preferred because of its superior electronic properties such as external electroluminescence, intramolecular exchange, chemical stability, and high conducting ability [10]. For better surface coverage of TiO₂, spray pyrolysis is the best technique compared with spin coating, which results in a layer with high pinhole density [178]. Due to thermal stress in the formation of the layer, high temperatures after compacting the TiO₂ layer lead to breakage or lower pinhole density. For this reason, the low-temperature method for the ultimate lowest cost and versatility for perovskite processing in solution is sort.

The replacement of the mesoporous electron selective layer is better for commercialization. Planar TiO₂ has shown unstabilized PCE [80]. Barrier-free energetic configuration was obtained with an almost hysteresis-free PCE of 18.4% and a voltage of 1.19 V, performed via ALD [179]. The combined spin coating–chemical bath deposition method shows promising long-term stability under continuous visible light irradiation compared to other deposition methods [180]. Deposition of mesoporous TiO₂ with high-efficient PSCs was done via the sintering method to improve crystallinity [181], electronic conductivity, and inhibition of interface charge recombination, which also applies to the SnO₂ electron selective layer.

SnO₂ is less hygroscopic, with high thermal and photostability compared to TiO₂ and ZnO, making it a better alternative for fabrication that can take place at low temperatures with flexible substrates that reduce the cost. An n–i–p planar flexible substrate was fabricated by Kam et al. [182] using SnO₂ as an ETL; an efficiency of 12.8% was achieved with excellent thermal and moisture stability. The effect of annealing temperature on SnO₂ electron selective layers (ESLs) was studied. Low-temperature SnO₂ produces better film coverage, wider band gap, and lower electron density than higher-temperature SnO₂ [183]. Further development was made using spin-coating and post-treatment chemical bath deposition of the SnO₂ layer to improve the conformity and blocking capabilities, which are essential for shunt-free PSCs [184]. PSCs yielded an efficiency close to 21% with gained in fill-factor when compared to the conformal blocking layers deposited by ALD. The combination of all these resulted in more effective electron transportation and hole blocking resulting in lower interface recombination.

ZnO is another alternative for ESL apart from SnO₂ and TiO₂ with ZnO preparation being the best among them in low-temperature processing. ZnO can be readily printed at low temperatures with the potential for large-scale roll-to-roll manufacturing of flexible PVs at low cost. However, rapid degradation of perovskite was observed when deposited on ZnO due to the inability of CH₃NH₃PbI₃ to withstand high temperatures on ZnO-based devices, thus affecting stability [185].

Fluorine and aluminium co-doped ZnO films (FAZO) were tested against commercial ITO and FTO films. Their efficiencies were 16.24, 15.92 and 12.45%, respectively. The FAZO film was developed using the radio frequency (RF) magnetron-sputtering method, and the use of F and Al as co-dopants made extra provision for electrons in the conduction bands of A and O sites [186]. Modification of TiO₂ with NiO nanocrystals for the ETL formed p–n island-like morphology with high transparency [187]. It was found that NiO nanocrystals decreased the frequency capacitance drastically resulting in the elimination of J–V hysteresis, hence achieving the high PV performance of 19.42%. Using NiO nanocrystals on TiO₂ serves as crystal nucleation centres that impact the morphology and crystallinity of the perovskite layer.

The morphology of the n-type ETL is either compact or mesoscopic and it determines the formation, grain size, and uniformity of the perovskite crystals, which in turn affects the efficiency [188]. The design of the ETL is of paramount interest in the manipulation of perovskite crystallization and reproducibility. ETLs that selectively and efficiently extract the photogenerated electrons from the perovskite light absorber are of significant interest [89,189]. Recently, Eliwi et al. [190], for instance, studied the optimization of different types of nanostructured single- or bilayer SnO₂ ETLs for PSCs. A reduction in the degree of ion migration was among other factors found to be responsible for the increased PCE at 18.5% as compared to previous PCE efficiencies, which are found to be as low as 12.5% for lithium (Li)-doped compact SnO₂ (c(Li)-SnO₂).

5.2.1 n-type organic ETMs

ETLs for inverted PSCs use fullerene (C₆₀), PCBM indene-C₆₀ bis-adduct, poly(3,4-ethylenedioxy thiophene)poly(styrene-sulphonate) (PEDOT:PSS) n-type organic materials (Figure 16) with good electron collection properties due to low-temperature processing, high extinction coefficient, easy film deposition technique, higher LUMO level than the perovskites, increase in the magnitude of V _oc, and the overall performance of the solar cells due to improved light harvesting [191]. Buffer layers or interlayers are introduced between the organic ETL and electrode to reduce the charge electron transfer at the interface for better band energy alignment [192]. PCBM acceptor molecules can be used to improve the film quality by filling the vacancies and grain boundary of the perovskite film. It has high conductivity than perovskite, which signifies better charge transport. It also makes the mobility of electrons and holes balanced with long carrier diffusion lengths, which benefits charge collection efficiencies [193]. Huang’s group demonstrated that the PCE of the CH₃NH₃PbI₃ planar heterojunction PV cell could increase when the energy disorder in the fullerene ETL is reduced by the solvent annealing technique; the PCE of CH₃NH₃PbI₃ PSCs increase from 17.1 to 19.4% and the V _oc from 1.04 to 1.13 V without giving up FF and circuit current density (J _sc) [194]. The demerits of PCBM as ETL are ambient instability and photochemical transformation from mono to polymers on exposure to light, which is a cause of concern.

Figure 16

n-type electron transport layer organic materials.

The flexible solar cell n–i–p configuration was fabricated at a lower temperature (>150°C) using SnO₂ as the ETL and CPTA as the interface modifier; the PCE of 18.36% was obtained with good ambient stability and mechanical durability [195]. Interfacial modification enhances the carrier selectivity of electrons in the film. The use of ITIC as a SnO₂ surface modifier improved the conduction band edge significantly, reduced the charge resistance, and effective surface passivation of SnO₂ was achieved [196]. Most of the fullerene ETL are not stable to moisture, which hinders the efficiency of PSCs. Xu et al. considered the use of hydrophilic fullerene derivative [6,6]-phenyl-C61-butyric acid-(3,4,5-tris(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-phenyl) methanol ester (PCBB-OEG) [197] as an additive in MAPbI₃ perovskite film fabrication. A high-quality perovskite film with high crystallinity, fewer trap-states, and dense-grained uniform morphology that could grow on both hydrophilic (poly (3,4-ethylenedioxy thiophene)/poly(styrene sulphonic acid)) and hydrophobic (PTAA) HTLs was obtained.

Apart from fullerene-based ETL, amino-functionalized copolymer with a conjugated backbone with fluorene, naphthalene diimide, and thiophene as its component, discotic coronene, and perylene diimide are designed and applied as ETLs; the π-conjugated plane along the short axis enables efficient face-on π–π stacking in the solid state that provides good charge carrier mobility [198]. Non-fullerene small molecule-based ETMs successfully function as efficient ETMs with negligible hysteresis and ambient stability with efficiency greater than that of PCBM ETMs. A combination of these properties shows that they are good n-type polymers and alternative candidates to PCBM as electron acceptors in PSCs [199]. In addition, the amino-substitute perylene diimide derivative was used as an alternative for TiO₂ in n–i–p PSCs to serve as an effective ETM with high-performance PSCs that draw molecular design guidelines for electron-selective contacts [200]. The low-temperature solution-processed method makes the construction of flexible PSCs easy with high efficiency [198]. The P═O of PFPDI interacts with Pb in perovskite films to form forming nanostructure that helps the phosphate ether groups to modify the interface coupled with their solubility in CBZ and enhance grain size crystallinity. PFPDI [201] has a lower HOMO level of −5.66 eV compared to perovskite (−5.4 eV); this enables it to block holes effectively making it more stable when compared to PCBM. Thiophene-flanked diketopyrrolopyrrole (TDPP) as an ETL in comparison to PEDOT: PSS was found to have greater electron density, which indicates better electron extraction and charge transport behaviour, air stability, and excellent electron mobility [202]. Fullerene-based ETL has poor photochemical and thermal stability and has low open-circuit voltage due to interfacial defects. Hence, there is a need to develop new types of ETL polymers. Chen et al. [203] developed a thienylthiazole imide-based ETL polymer (PDTzTI) with 20.86% efficiency which is better than PCBM. The properties of PDTzTI such as hydrophilicity and mobile ion blockage result in higher efficiency and long-term device stability. These results confirm that the choice of ETL influences the device's stability and performance.

6 Fundamental properties of organometal halide perovskites

The length of diffusion charge carriers is essential in the PCE of organometal halide perovskites. It has been observed that the CH₃NH₃PbI₃ perovskite absorber has a lower diffusion length for electrons in comparison to that of holes which limits the thickness to hundreds of nanometres for active layers. Therefore, the mesoporous structure is usually used for an absorber composite, halide-based perovskites, that could improve the electron diffusion length using a planar structure. The structural, electronic, and optical properties of organometal halide perovskites are of paramount importance that determine the efficiency of perovskite solar cells. These properties are further examined in this review.

6.1 Optical properties

The optical properties of perovskites such as the refractive index, extinction coefficient, real and imaginary parts of the dielectric constant, and direct and indirect band gaps are of interest because these properties affect the structural phase transition, strength, carrier diffusion length, and delocalization of electrons within the perovskites. All these properties make perovskites fascinating optoelectronics devices [34]. They have clear narrow emission exciton and wide absorption bands around 390–600 nm; thus, their absorption in the visible region can be tuned by changing the metal, halogen, or inorganic sheet [204].

The choice of organic cations in the organometal halide perovskite influences the photoluminescence emission [205], which makes them useful in light-emitting devices, lasers, and optical sensors. Zadeh et al. [206] studied the optical properties of CH₃NH₃PbI₃ coated on mp-TiO₂ using a one-step method (OSM) and hot-casting technique (HCT). They observed that CH₃NH₃PbI₃ prepared by the OSM have a better refractive index, lower optical band gap, higher optical conductivity, and higher dielectric imaginary constant compared to those prepared by HCT, which make them better light harvesters with excellent conversion efficiency. The use of acetic acid was reported [207] to increase the absorbance, refractive index, extinction co-efficient, dielectric properties, and optical conductivity of CH₃NH₃PbI₃ perovskites with a decrease in transmittance and optical band gap. The enhanced absorption was attributed to structure ordering by acetic acid. Perovskites with better surface morphology have been reported [208] to have high optical absorption and this is influenced by the deposition method; some examples are given in Table 2.

Table 2

Effect of deposition techniques on CH₃NH₃PbI₃ and CH(NH₂)₂PbI₃ band gaps

Deposition technique	Band gap (eV)	Perovskite	Ref.
One-step spin coating	2.3	CH₃NH₃PbI₃	[209]
One-step spin coating	1.59	CH(NH₂)₂PbI₃	[210]
Two-step spin coating	1.47–1.50	CH₃NH₃PbI₃	[207]
Two-step spin coating	1.48	CH(NH₂)₂PbI₃	[66]
Vapour	1.55	CH₃NH₃PbI₃	[211]
Vapour	1.46	CH(NH₂)₂PbI₃	[212]
Solution	1.58	CH₃NH₃PbI₃	[213]
Solution	1.50	CH(NH)₂PbI₃	[214]
Solution crystallization	1.50	CH₃NH₃PbI₃	[215]
Solution crystallization	1.54	CH(NH₂)₂PbI₃	[216]
Single-source thermal evaporation	1.58	CH₃NH₃PbI₃	[217]
Dip coating	1.50–1.60	CH₃NH₃PbI₃	[48]

Hartono et al. [218] studied the influence of the processing method of preparation of perovskites on structure morphology and optoelectronic properties of PSCs. It was observed that different absorption properties are obtained for the same materials prepared in solution and/or by an evaporation method, of which the latter gave a lower absorption band gap in a 2D structure, whereas the former presented as a dimer with larger band gaps. The use of organic solvents containing sulphuryl groups during processing leads to a potential change in the compound morphology due to the chemical combination with an A-site cation. In addition, optoelectronic properties also depend on the type of metal used as an A-site cation. As shown in Figure 17, a wide range of colours was observed when cations are changed, for example, between caesium, rubidium, and potassium [219]. The solar performance in multidimensional lead-halide perovskites showed a decrease in mobility by a factor of as much as 20 and a low efficiency of 2 orders of magnitude was observed when more low-dimensional perovskites based on MA-based 3D perovskites containing larger A-site cations were mixed with dimethylammonium, iso-propylammonium, and tert-butylammonium lead iodide perovskites where the results were attributed to poor recombination dynamics. Recent reports on the basic crystal structures of hybrid halide perovskite materials in relation to their optoelectronic properties have revealed other very important intrinsic properties that contribute including the ambipolar transport features in a long-range, low exciton binding energies, high dielectric constants, and ferroelectric polarizations; however, all these properties are tunable to increase the PCE efficiency [220].

Figure 17

Optical properties of A₃Sb₂I₉ perovskites: (a) film absorptance calculated using 100−T−R, where T and R are transmittance and reflectance, respectively, with 2 nm wavelength resolution. Photographs of films deposited by spin-coating solutions of A₃Sb₂I₉, where the (a) site is (b) Cs, (c) Rb, and (d) K. Adapted with permission from [219]. Copyright: Chem. Mater.

6.2 Electrical properties

The synthetic method influences the mobility, concentration, and carrier type of a perovskite [221]. Stoumpos et al. [222] prepared a series of organic–inorganic hybrid metal iodide perovskites using different synthetic approaches, which affects their physical and chemical properties, thus making them behave as either p- or n-type semiconductors. The samples with the lowest carrier concentration obtained from those prepared from solutions are n-type, whereas the p-type samples are obtained through solid-state reactions. A facile tendency towards oxidation was noted in the case of Sn compounds and, hence, tin compounds were doped with Sn⁴⁺ and consequently behaved as p-type semiconductors, thereby displaying a metal-like conductivity. It is known that CH₃NH₃Sn_1-xBr_x and CH₃NH₃Sn_1-xI_x have high electrical conductivity on doping with Sn⁴⁺, whereas the pure compound showed high resistivity properties due to reduced conductivity. Conductivity was affected by the dopants and temperature [223]. The material composition of perovskite influences its electrical properties, energy-saving techniques, method of fabrication, and low hysteresis and gives perovskites an edge over traditional semiconductors [224]. The ionic migration that occurs in halide perovskite materials does have significant effects on the electrical properties as this is attributed to the native point defect density such as vacancies [225]; defects in the density can be tuned by voltage regulation [226]. Ren et al. have shown that metal nanoparticles that have highly confined electric fields and large scattering cross-section show high optoelectronic performance through vacancy blockage [227]. The use of doped metal oxides as an ETL has also been shown to improve s charge mobility and energy band levels [228].

Moradbeigi and Razaghi [229] recently reported the preparation of perovskite tandem solar cells using three-dimensional (3D) optical-electrical simulation of non-Pb and flexible four-terminal (4T), which led to PCE above 24%. Meanwhile, the molecular engineering of the coating layers of the substrate was accomplished by adding antireflection agents and incorporating periodic nano-texture patterns and the elimination of the buffer layer resulted in a power efficiency increase of 30.14% accompanied by long-term moisture resistance. The best tandems that have also been previously reported for perovskite/silicon, perovskite/CIGS, and all-perovskite by the Oxford PV company gave PCEs of 28, 23.3, and 24.8%, respectively [230–232]. However, other significant reports recently showed impressive PCEs of 23 and 25.4% [233,234] using a 4T tandem architecture and varying structure absorbers with excellent electrical resistivities. In another dimension, Ren et al. [2] proposed a method to eliminate nonradiative recombination and carrier transport losses by generalized optoelectronic reciprocity relations as a means of improving PCEs in large-area PSCs.

7 Perovskites solar cell: Stable or unstable?

Perovskite PCE is highly competitive with traditional crystalline solar cells. Thus, its operational stability on prolonged exposure to humidity, heat, light, and oxygen is of paramount interest and this can be solved by knowing the structural stable phase of the perovskite. PSCs must be able to operate outdoors for up to 25 years to be considered for commercialization [235]; hence, more research is now focused on long-term stability, as a result, increased recognition of the instability of the component materials under conditions such as humidity, heat, light, and oxygen [236].

It was discovered that at higher relative humidity, the perovskite tends to decompose at a faster rate while the inverse happens at a lower relative humidity. Though it has been reported that oxygen has little or no effect on the degradation of the perovskite [237], nonetheless, moisture in an ambient environment primarily causes perovskite degradation. Perovskite degradation is also influenced by the rate of formation, dissociation, and recombination processes, which are important to the PCE [18,238]. Some of the ways to protect PSCs against humidity include the following but are not limited to them: (i) surface treatment of perovskite films with hydrophilic material like 2-aminoethanol hydroiodide [239], (ii) a mixture of hydrophilic polymers that contain C═O and O atoms that can have synergistic effect between Pb²⁺ and H atoms in the perovskite layer [240], (iii) capping with ultra-thin 2D layer that have bulk cations as this helps self-healing and are easily recoverable [241], and (iv) fabrication of mixed-cation mixed halide perovskites with Pb(SCN) as additives [242]. The use of Pb(SCN) reduces pinholes and trap density and increases the grain size and charge carrier mobility. All the methods stated can help achieve PSCs with extra stability, which could lead to commercialization of the fabricated solar cells.

Tan et al. [243] reported efficient and stable solution-processed planar PSCs at low temperatures (<150°C) via contact passivation strategy using a chlorine-capped TiO₂ colloidal nanocrystal film, which at low temperatures improves the binding of the interface and reduces interfacial recombination with a small active area possessing higher efficiency and at the larger area has lower efficiency.

Tunnelling junction has been known to restrain charge recombination at contact, making the surface unreactive, thereby increasing the efficiency of the device. This was applied in silicon solar cells, the most effective PV. Based on this, Wang et al. [244] inserted an insulating polymer (tunnelling layer) between the perovskite and the ETL to selectively conduct one type of charge while blocking the other type, which spatially separates photogenerated electrons and holes to reduce the recombination effect between the perovskite and ETL because the charge selection electrodes or charge transport layers have energy matching the electronic states for one type of charges to tunnel into but not for the other.

De Wolf et al. [245] demonstrated that photothermal deflection spectroscopy (PDS) and Fourier transform photocurrent spectroscopy (FTPS) can be applied to perovskite layers for monitoring moisture effects by comparing the results of CH₃NH₃PbI₃ layers immersed in Fluorinert FC-72 liquid and CH₃NH₃PbI₃ layer exposed to ambient air. A comparison of the two showed no change in the PDS spectrum of the former, while the latter had a spectrum showing the reliability of the technique. Also, the PSCs were fabricated using double-layer mesoporous TiO₂ and ZrO₂ as a scaffold without the HTL, with the addition of 5-ammonium valeric acid iodide to methyl ammonium lead triiodide [15]. The fabricated perovskite was stable for more than 1,000 h in ambient air under full sunlight with a PCE of 12.8%.

Cerium oxide as an ETL is used to compact titanium oxide for the fabrication of perovskite using the sol-gel method at low temperatures. PCE of 17.04% with greater stability was obtained for perovskite and that of compact titanate was 14.32% with lower stability [246].

7.1 Structural stability

Structural stability is the ability of a material to exist in a perovskite phase and function as PV within certain temperatures and pressures for optimum PV performance [247]. It is also important for the energy level of PSCs to properly match the functional level to enable improved performance of the device. Optimization of the functional layer is one of the most effective ways to improve the efficiency of perovskites.

The equilibrium position of the “A” cation can be determined by its position and tilting of the MI₆ octahedral for a given set of bond angles. The tilting of the octahedral was interpreted in terms of lattice vibrational modes, and some of the tilt system corresponds to important modes connected to the phase transition in the system such as tetragonal and orthorhombic [248]. Band gap, carrier lifetime, and mobility of perovskites can be tuned if the distortion is relatively minimal in the orthorhombic and tetragonal phases [249]. It was discovered that a small fraction of excess PbI₂ led to the degradation of the perovskite layer under illumination even when it was stored in an inert atmosphere. Moreover, the PSCs with excess PbI₂ degrade faster than PSCs without excess PbI₂ when illuminated in an ambient atmosphere [250].

Hoke et al. [251] reported reversible light-induced transformation in the MAPb(I_1−xBr_x)₃ compound that could be formed but suffer from rapid light-induced halide degradation. They stated that photoexcitation that involves halide migration may cause segregation of halide into iodide-rich minority and bromide-rich majority regions (resulting in smaller band gaps that lower the energy of excitons that could drive halide segregation), which act as recombination centre trap sites that pin the photoluminescence and limit device performances. Halide migration occurs through halogen vacancies. Ion’s mobility becomes more facile at the grain boundary compared to bulk materials. Reduction of defects density and grain boundaries should reduce the rate at which phase segregation occurs. Due to this, the fabrication of MAPb(I_1−xBr_x)₃ or FAPb(I_1−xBr_x)₃ compounds with a high band gap, prolonged stability, and efficiency were not possible [252].

Leijtens et al. [247] expressed concern over methylammonium volatility and its thermal instability, which led to replacement with FA cations. The large cations do not form a black cubic or tetragonal perovskite phase at room temperature but exist as a yellow film with the δ-phase, which limits the performance and stability tending to degrade when exposed to moisture, mechanical stress, and heat. Structural stability plays an important role in the ability of perovskite solar cells to withstand stresses due to PV. Zhao et al. [253] fabricated FAPbI₃ using the seed crystal growth of perovskites to reduce trap density and the efficiency increased to 21%. The obtained device was stable for up to 140 h after one sun illumination. When formadinium was changed to Cs, the PCE of the Cs-device dropped on exposure to sun illumination for 300 s. Combination of FA and Cs on the A site to make FA_1−yCsPb(I_1−xBr_x)₃ resulted in a promising breakthrough towards structural stability in which the smaller ionic size of caesium (Cs) makes up for the larger size of fluorine (F). The size of the cation is vital as it can cause the entire network to enlarge or compress [254]. In addition, it was structurally stable with excellent charge carrier mobility and less disposed to light-induced halide segregation, which indicates that there is a link between the device composition and structural stability [255].

The use of smaller-sized molecules can help passivate surface trap density. Cetyl trimethylammonium bromide (CTAB) [256] was used to dope PEDOT: PSS, which increased the built-in voltage and the PCE from 10.21 to 12.5%. It was also established that the efficiency of inverted PSCs is stable for 30 days under ambient conditions. Tetraphenyl dibenzoperoflanthene (DBP) was used as an interface modification layer for planar inverted PSCs for an ITO/PEDOT:PSS/CH₃NH₃PbI₃/DBP/PCBM/Bphen/Ag device architecture [257]; the same device without DBP was also fabricated for comparison purposes. The PCE obtained was 15.61 and 14.26% for devices with and without DBP, respectively. This was attributed to the ability of DBP to passivate perovskite surface defects and enhance energy level alignment and its hydrophilic nature, which prevents moisture from entering the perovskite layer and makes the device stable.

Structural stability of PSCs can be achieved through surface passivation of the perovskite layer [258], modification of additives for organic–inorganic hybrid PSCs [259], use of lead indicators that can retard the rate of perovskite films crystallization (e.g. dithizone) [260], inhibition of ion migration using Lewis-base (pyridine or thiophene) [261], stacking of perovskite layers with 2D perovskites with defect passivation and self-healing effects (butylammonium iodide) [262], and incorporation of metal sulphide to the ETL for charge recombination reduction and charge transport facilitation [263].

7.2 Thermal stability

Perovskites should be thermally stable outdoor in the temperature range −40 to 85°C. At lower temperatures, the material symmetry is reduced as it approaches the orthorhombic phase. The material would not degrade structurally at 0–100°C. Dualeh et al. [264] carried out thermogravimetric analysis (TGA) on MAPbI₃ and MAPbCl₃ to study the sublimation behaviour of the organic component with respect to temperature. In addition, differential scanning calorimetry (DSC) was used to probe the crystalline phases of perovskites. The TGA curves of MAPbI₃ and MAPbCl₃ show that each lost 100% of their organic components at 234 and 185°C, respectively, through sublimation. About 90 and 95% of MAPbI₃ and MAPbCl₃ inorganic components were lost at 646 and 714°C, respectively. The authors concluded that the thermal decomposition of the organic components seems to be intrinsically thermally stable at relevant operational PV temperatures. The main concern is the sublimation temperature of 250°C, which indicates that the organic component can sublime steadily within the device operation temperature [264], as shown in Scheme 1. The scheme shows that methylammonium lead halide decomposes to the respective lead halide, methyl amine, and hydrogen halide.

Scheme 1

Decomposition of methylammonium lead halide [264].

The main challenge of perovskite development is poor stability at high temperatures, which can be overcome by incorporating caesium into the perovskite structure to make an organic–inorganic hybrid perovskite or solely an inorganic perovskite [265]. Eperon et al. [266] prepared PSCs by introducing a small amount of hydroiodic acid to the precursor solution and forming metastable CsPbI₃ crystals that decompose on exposure to moisture. CsPbBr₃ is structurally more stable than CsPbI₃ because of the smaller ionic radium of bromide in comparison to iodide. The volatility of PSCs is of great concern due to thermal instability. Therefore, attempts were made to replace methylamine (MA) with FA, a larger cation in small and large compositions but the resulting perovskites are still unstable. However, the introduction of Cs drastically increases the perovskite stability without reducing the high efficiency of the perovskites [243].

FA_0.83CsPb(I_1−xBr_x)₃-based perovskites did not degrade above 130°C for over 6 h in an inert atmosphere, and the compound existed in a single phase irrespective of the compositional structure [255]. The thermal stability of PyPbI₃–(CH₄)₄NH₂PbI₃ was tested using XRD at 135°C, which shows that the compound is a stable and viable material for the development of PSCs. The study was carried out at 135°C, which is higher than the transformation phase temperature in MAPbI₃ [267].

Metal-contact-induced degradation and escape of volatile species PSCs cause thermal instability that requires the introduction of a diffusion barrier layer [268]. The presence of lone pair of electrons on S, N, and O atoms on the organic HTL and HTL allows Lewis-base interaction with uncoordinated Pb²⁺ sites, which results in defect passivation [269]. Unfortunately, temperature causes phase transitions and this can be reduced with the addition of TiO₂ aerogel-based suspension, which showed stability between 30 and 80°C [270]. ETL and HTL are important for the control of the charge separation process between the perovskite and electrode interface, which can be improved by encapsulation, modification, and doping to enhance thermal stability [271–273].

7.3 Atmospheric stability

The performance of semiconductors is influenced by many external factors such as light, heat, moisture, and oxygen, but moisture and oxygen affect stability by dissolving the functional layers through oxidation.

7.3.1 Oxygen-induced degradation

The perovskite PV is stable under an inert/ambient atmosphere and maintains the optimal power output with no change in the short circuit photocurrent for perovskite based on metal oxide ETL (HTL), while those based on organic charge transport layer degraded dramatically due to adsorption of oxygen from the organic layer [77,274]. It was observed that oxygen has a degradation effect only when PSCs are exposed to light. Research shows that methylammonium lead iodide undergoes photo-oxidation easily. Pearson et al. [275] reported the mechanism of photo-oxidation and subsequent decomposition based on the combined action of light and oxygen on the photoactive layers using fluorescent molecular probe studies. They showed that O₂ ^‒ species is generated by the reaction of photoexcited electrons in the perovskite and molecular oxygen, which is triggered by methylammonium, the organic component in the perovskite photoactive layer due to its acidic proton, hence it was suggested that the methylammonium component should be replaced with a species having no acidic protons to enhance oxygen stability. The use of TiO₂-based PSCs undergo two-stage degradation processes. The first stage involves the transformation of Ti³⁺-oxygen vacancy (Ti³⁺-V_o) to active trap states (Ti⁴⁺-V_o), which causes photocarrier loss. The second stage involves the conversion of Ti⁴⁺-V_o to Ti³⁺-V_o through oxidation of I^‒, which causes I₃ ^‒ accumulation. This can be controlled by the modification of TiO₂ with materials that can reduce photocarrier loss or complete replacement of TiO₂ [276]. Chen et al. [277] reported that prolonged atmospheric-pressure dielectric barrier discharge (DBD) treatment on CH₃NH₃PbI₃ planar n–i–p perovskite leads to the degradation of cells [278].

The replacement of MA with FA, and caesium gives perovskites that are resistant to oxidation with reduced decomposition. When FA_0.83Cs_0.17Pb(I_0.6Br_0.4)₃ was compared with FAPbI₃ solar cells, FAPbI₃ solar cells decomposed rapidly with a T ₈₀ lifetime of 20 h, while those with FA_0.83Cs_0.17Pb(I_0.6Br_0.4)₃ had superior T ₈₀ lifetime of over 650 h and still operated at 10% PCE [279]. Methanol was used as an additive to enhance MAPbI₃ absorption and improve coverage. The film has improved uniformity and crystallinity with no pinholes. The compact perovskite layer and the charge carrier extraction and collection ability were also enhanced. The fabricated solar cell efficiency was 19.5% with great air stability and reduced hysteresis after 30 days [278]. The use of polyurethane-based resin as a PSC encapsulator showed good stability under ambient light at 1,000 lux, controlled humidity of 65%, temperature in the range of 18–30°C, and maintained 94% of its initial PCE after 4 months [280]. Another method involves the use of oxygen passivation to improve stability and efficiency in the absence of light illumination [281].

7.3.2 Moisture-induced degradation

PSCs have progressed as solution-processable devices in which humidity facilitates the crystallization process. Unfortunately, the CH₃NH₃I₃ semiconductor decomposes quickly in moist air, reducing its industrial potential. It was discovered that exposure to moisture not only changes CH₃NH₃PbI₃ to PbI₂ but it also coordinates with perovskite in the dark to form a hydrate (CH₃NH₃)₄PbI₆·2H₂O, thereby decreasing the absorption across the visible region. The presence of moisture, especially small polar and hydrogen bonding molecules, results in an irreversible degradation in the presence of an electric field [282,283].

Perovskite materials often undergo rapid degradation upon exposure to humidity, which increases upon exposure to light [284,285]; the possible mechanism of decomposition in moisture is shown in Scheme 2.

Scheme 2

The decomposition mechanism of 3D PSCs [286].

The use of tunnelling insulating layer (polymeric insulating layer) enables the capping of perovskite films by super-hydrophobic insulating layers, which dramatically enhances the resistance of the PSCs to water-induced damage without the needed encapsulation. The thickness of the tunnelling layer significantly influences the electron extraction efficiency and the efficiency of the device. Insulating polymers used for the tunnelling layer can inhibit the intrusion of moisture into the perovskite structure while simultaneously inhibiting the evaporation of MAI. Polymers such as polystyrene, Teflon, polyvinylidene-trifluoro ethylene copolymer, and poly(methyl methacrylate) are used as tunnelling layers [244,282,287]. These devices have been tested outdoors and are stable for long periods without reduction in performance and have thus been further developed for potential applications [288]. Bella and colleagues proposed the use of multifunctional photopolymers as a comprehensive promising solution to PSCs’ instability. This showed rapid-light-induced free-radical polymerization at ambient temperatures, which produces multifunctional fluorinated photopolymer coatings that confer luminescence and easy-cleaning features of the device while simultaneously forming a strong hydrophobic barrier towards environmental moisture on the back-contact side. PCE of 19% was obtained under 1.5 AM sun illumination without loss of functional performance for more than 6 months [289].

Seok et al. [290] showed that the introduction of bromide (Br ion) onto the X site affects moisture stability as Br has low sensitivity to moisture. This was associated with its compact and stable structure, as the substitution from a larger atom to a smaller atom (I to Br) leads to a reduction of the lattice constant and a transition to a cubic phase with greater stability. Also, Jiang and colleagues replaced two iodides in CH₃NH₃PbI₃ with two pseudohalide thiocyanate ions, as pseudohalides have similar chemical behaviours and properties as true halides. The lone pair of electrons from S and N atoms in SCN^‒ interact strongly with Pb ions, which stabilize the structure of CH₃NH₃Pb(SCN)₂I. The resulting perovskite was tested for stability, and it had no significant degradation after being exposed to air of 90% with better relative humidity, open circuit voltage (V _oc), and FF values than CH₃NH₃PbI₃ [291]. More studies have been carried out with the introduction of SCN⁻ into the perovskite structure to form CH₃NH₃PbI_3−x(SCN)₂ films. These films are of high quality and can still be prepared even when the relative humidity is above 70%. A PCE of 13.49% was obtained and the device was stable in humid air without encapsulation; a reduced recombination rate observed was due to the pin-hole free morphology, which can prevent carrier recombination at the HTM/TiO₂ interface, and the other is the lower density trap states [292]. Wang et al. [293] fabricated FA-PSCs under ambient air using N-methyl pyrrolidine (NMP) as an additive. A smooth highly crystallized α-FAPbI₃ phase was obtained with a PCE of 17.29%, which is moisture resistant. The use of NMP suppressed non-radiative recombination. The replacement of lithium bis(trifluoro methylsulphonyl) imide (Li-TFSI), an hydroscopic additive, with a hydrophilic 2,2′,7,7′-tetrakis (N,N-di-p-methoxy phenyl amine)-9,9′-spiro bifluorenedi[bis-(trifluoromethane sulphonyl)-imide] (Spiro-(TFSI)₂) increased the moisture stability of PSCs with a PCE of 19.1% [148].

7.4 Electrodes and encapsulation

Electrodes can function as a capping layer and the most common electrodes are gold, silver, and aluminium which give devices good efficiency but they are also susceptible to degradation. The rate of metal halide formation is very fast with silver and aluminium, while that of gold is relatively slow; hence, gold is the metal of choice even though it is very expensive. Interface deterioration by a chemical reaction between the perovskite layer and the metal electrode under an ambient environment has been reported. Hence, other metal electrodes such as copper, nickel, iron, and chromium were investigated. These metal electrodes showed slightly improved performance when compared to gold, silver, and aluminium due to the low reaction of perovskite with these metal electrodes. Chromium can alleviate fast device aging at elevated temperatures [294].

Saliba et al. [295] replaced the commonly used spiro-OMeTAD (hole-transport material) with a thin layer of the PTAA polymer and proposed that gold is not able to diffuse through this polymer layer, as it is highly stable; however, the fabricated PSCs lost 10% of their initial efficiency during maximum power point tracking over 500 h of illumination at 85°C in a nitrogen environment. Metal electrodes in perovskites are susceptible to corrosion by the reaction with halides; thus, the use of a pinhole-free metal oxide layer has been explored to prevent metal–halide interactions. A wide band-gap semiconductor such has aluminium-doped ZnO has been used as an electron-selective buffer layer to prevent sputter damage of the organic layer capped with ITO without using a metal electrode [296]. It was observed that slow degradation occurred due to the lack of an edge seal on the solar cell, but encapsulation of the solar cells reduces the degradation of the fabricated PSCs. The deposition of silver electrode via thermal deposition results in the diffusion of the electrode in inverted ITO/PEDOT:PSS/FA0.5MA0_.5Sn_0.5Pb_0.5I₃/PCBM/Ag that increases the trapping of electron density due to high open-circuit voltage loss. It was discovered that the placement of ZnO between the ETM and the cathode electrode results in the suppression of silver diffusion into the perovskite which reduces high open-circuit voltage [297].

The use of carbon as a top contact electrode has been explored due to its stability and simple fabrication method. Carbon is sort after because it is inert to ion migration, has low work function, high conductivity, is readily available, and is resistant to moisture [298]. Interfacial properties between the carbon electrode and perovskite layer are important parameters necessary for the development of carbon-electrode-based PSCs. The use of ammonium chloride in a carbon electrode [299] resulted in the fabrication of a stable device with approximately 45% humidity and 9.89% PCE, of which 96% is retained after 24 days. Liu et al. [300] fabricated MAPbI₃/MAPbI_xBr_3−x perovskite using the stacking method with 16.2% PCE, of which 85% is retained after 55 days with negligible hysteresis. While MAPbI₃-based PSCs have 10.7% efficiency with poor stability. The method was found to reduce interfacial recombination significantly with better photostability. Yin et al. [301] reported that CH₃NH₃PbI₃ perovskite is stable after 5 years with comparable photoelectrochemical properties. The hydrophobic effect was prevented by the tetrafluoroethylene polymer, which prevents the invasion of external moisture and air. To reduce environmental degradation, mechanical strength was increased against impact and prolonged device lifetime; additional encapsulation layers such as epoxy sealant and carbon nanotube/polymer composites are required to prevent the permeation of moisture and protect the organic layer from oxidation [302–304].

8 Upgrade of bulk PSCs to a lower dimension

2D perovskites with layered structures are derived from the 3D AMX₃ perovskite structure, in which A and X ions within the planes are cut into halves [305,306]. The general formula is (A)₂(MA)_n−1MX_3n+1 (n is an integer), where A is a primary aliphatic or aromatic alkylammonium/monovalent inorganic cation, M is a divalent metal ion, X is a halide anion, and n is a variable that indicates the number of metal cation layers between the two layers of the organic chain. They are comparable to the Ruddleson–Popper (K₂NiF₄) crystal phase. 2D perovskite is an alternative to conventional 3D PSCs in terms of stability although it is reported to have a van der Waals gap in its Ruddleson–Popper phase, thus destabilizing the device. The gap was bridged by the introduction of diammonium cations [307] to produce the Dion–Jacobson phase with perovskite that was stable to humidity, heat, moisture, and light illumination.

If the small MA⁺ or FA⁺ is replaced by a much larger organic primary ammonium cation, the 3D perovskite would change to a 2D layered structure due to steric effects. In recent years, 2D layered perovskites have attracted attention due to their moisture stability, presence of bulky and hydrophobic organic layers, and their controllability in the fabrication of photoelectronic devices [308–310]. Further development led to the use of 2D perovskite compounds and their analogues with better moisture resistance [311,312]. 2D perovskite compounds show both high moisture stability and exceptional optical properties [313], as the organic layers act as insulating barriers and the inorganic layers behave as semiconductors. The interfaces between the inorganic and organic layers are intrinsically flat and it is assumed that excitons are confined and make the device an ideal 2D system [314]. These stable excitons led to exceptional optical properties, such as strong photoluminescence and high optical nonlinearity, so that organic–inorganic layered perovskites have the potential for application in a wide range of optical materials and the inorganic layers of 2D perovskites can act as semiconductors for PCE when the 2D planes are positioned parallel to the substrate [315].

2D materials do not have electronic properties typically associated with good solar cell absorbers. To access more favourable electronic properties of 3D structures, Smith et al. [316] developed an intermediate structure between n = 1 and n = ∞ by synthesizing n = 3-member series of (PEA)₂(MA)_n−1[PbI_3n+1], a bulkier organic cation where PEA is C₆H₅(CH₂)₂NH₃ ⁺ as opposed to the original MA of CH₃NH₃ ⁺ by reacting (PEA)I, (MA)I, and PbI₂ in a mixture of nitromethane/acetone by the non-solvent crystallization method. It was observed that reducing the dimension of the inorganic components from the 3D structure ((MA)[PbI₃]) to (PEA)₂(MA)₂[Pb₃I₁₀] increases the band gap with a low PCE of 4.73% [316,317] due to a low absorption coefficient and poor carrier transportation.

A computational method was used to establish the origin of instability with complementary studies of the physical and optoelectronic properties of quasi-2D perovskites in order to identify optimal conditions for designing and fabricating chemically stable, high-efficiency PV devices. Quan et al. [315] fabricated dimensionally tuned perovskite films using the single-spin cooling method. PEAI was synthesized according to Scheme 3. A series of different dimensional perovskite [(C₆H₅C₂H₄NH₃)₂ (CH₃NH₃)_n−1PbI_3n+1] solutions were prepared by dissolving specific stoichiometric quantities of lead iodide, MAI, and PEAI in DMF/DMSO, which was then fabricated using single-step spin-coating followed by a two-step spin method. Then, it was annealed at 100°C for better crystallization.

Scheme 3

Synthesis of PEAI.

The material stability as a function of dimensionality was investigated by storing the perovskite films in an environment where humidity was controlled for an extended period; the 3D perovskite film absorbance at 500 nm was suppressed significantly, which indicates its decomposition to PbI₂. However, pure 2D perovskite exhibited excellent stability over the same period. This is the result of van der Waals interaction between the capping molecules acting as a key driver of increased material stability [315]. Xing et al. [318] synthesized PEA₂IPA_n−1PbX_3n+1 and used in the fabrication of highly efficient PL thin films, which showed stable blue colour emission with 88% photoluminescence quantum yield (PLQY).

Xiong’s group reported CH₃NH₃PbX₃ nanoplatelets, which were prepared using a two-step method; in the first step, vapour transport chemical deposition system was used followed by the conversion of the as-grown nanoplatelets to perovskite via gas–solid hetero-phase reaction with methyl ammonium halide molecules. The nanoplatelets obtained were about 100 nm and ranged from 5 to 10 µm [66,155], which was used to fabricate near-infrared solid-state lasers. Liao et al. synthesized CH₃NH₃PbBr₃ microdisks using a facile one-step solution self-assembly method. This was done by mixing equal amounts of CH₃NH₃Br and PbBr₂ in DMF; a glass slide was dipped in the stock solution and sealed with a parafilm to control the rate of evaporation. The microdisks had smooth outer surfaces with low defect density [319]. The method of synthesis is close to the anti-solvent vapour-assisted crystallization method [320].

Arai et al. [321] reported two-dimensional perovskite compounds that incorporate ammonium cations and carboxyl groups, which were fabricated and found to form dimer structures based on the hydrogen bonds of the carboxyl moieties. The films of these compounds were randomly oriented with respect to the substrate. In addition, the structural organization allowed for the control of layer orientation to obtain favourable solar cells and increase the thermal stability of the perovskites, while maintaining quantum confinement, which is due to the inhibition of the phase transitions of organic layers upon introduction of carboxyl groups.

The colloidal approach for MAPbBr₃ nanoplatelets, at room temperature for the growth and nucleation of the platelets, was achieved by injecting acetone into a mixture of the precursors. In this way, it was possible to control the growth of the plate thickness from 3 to 5 monolayers of lateral submicron dimensions with single unit cell thickness [322]. Narrow photoluminescence with strong excitonic absorption was observed because of the confinement of the two-dimensional perovskites at small vertical sizes. The exciton dynamics were found not to depend on the extent of the 2D confinement. The mixture of anions or changing anions can lead to various shapes and sizes of nanoplatelets.

Dou et al. [313] synthesized two-dimensional organic–inorganic hybrid perovskites with a thickness of about 1.6–3.4 nm using the solution phase growth. The product was a single crystalline (C₄H₉NH₃)₂PbBr₄ 2D hybrid perovskite with a well-defined square shape and large particle size that is different from other 2D materials. It exhibited unusual structural relaxation, efficient photoluminescence, and colour tuning properties similar to its 3D. Colour tuning can be achieved by varying the sheet thickness and also the composition of the material. Huang et al. [323] fabricated a (DA₂PbI₄)_0.05MAPbI₃ hybrid perovskite film with good uniform morphology and large grain size with 19.05% PCE. The authors observed that the incorporation of the two-dimensional diethyl ammonium iodide (DAI) resulted in stable PSCs. The device retained 80% of initial stability after 60 days, and the large grain size reduced grain boundaries, which reduce hysteresis. Zhao and Zhu used the thermal decomposition process to prepare MAPbI₂Br nanosheets from PbI₂, MABr, and MACl precursors. A solar cell was fabricated using the nanosheets with 10% efficiency and a band gap of 1.8 eV [320].

2D nanoplatelets are interesting materials due to their excitonic nature and low dielectric screening/strong exciton binding energy that led to fast or directional exciton dynamics [324,325]. 2D nanoplatelets with narrow absorption, emission, and small Stokes shift make them an ideal material for studying strong coupling and light–matter interactions. High photoluminescence has been observed in nanoplatelets, nanowires, microdisks, and nanosheets perovskites with high quantum yields [326–328]. 2D layered perovskites that have been fabricated into solar cells are presented in Table 3. It was observed that large cations gave high FFs, short circuit density (J _sc), and open current voltage (V _oc), which results in high PCE. The study shows that all these films were stable to heat and humidity. Hence, the low-dimensional structure can be considered for commercialization. However, the search for easier and cheaper techniques that can facilitate not only moisture-stable but also highly efficient devices is an active area of research at present.

Table 3

2D PSCs using different bulk cations and their electrical properties

Perovskites	J _sc (mA/cm²)	V _oc (mV)	FF (%)	Efficiency (%)	Ref.
SBLC/MAPbI₃	22.36	1.19	75.7	20.14	[329]
IBA₂FAPb₂I₇	25.36	1.09	79.2	22.1	[330]
BA₂PbI₄MAPbI₃	22.54	1.11	78.8	20.26	[331]
(PNA)₂FAPb₂I₇	23.82	1.16	81.87	22.62	[332]
(EDA₂)(CH₃NH₃)Pb_nI_3n+1	21.94	0.90	75.00	15.02	[333]
(Eu-pyP)_0.5MA_n−1Pb_nI_3n+1	22.44	1.08	74.81	18.13	[334]
(PDA)(MA)₃Pb₄I₁₃	19.50	0.98	69	13.3	[307]
DJ:RP PDA: PEA	19.80	1.04	67	13.8	[335]

9 Tandem PSCs

Perovskite-based tandem solar cells have shown significant impact for the future build-up of PV technology with PCE as high as >30% when compared to single-junction solar cells. The performance recorded so far is still based on the emergence of new metal halide perovskite absorber materials and methods of fabrication [1]. As the configuration of PSCs (Figure 18) is a sandwich structure containing both the active layer that absorbs light and housing a p-type HTL as well as an n-type ETL, the timely separation of the generated carriers in the active layers often determines the efficiency of the fabricated device [336–338].

Figure 18

Introduction of tandem PVs and metal halide perovskites. Schematic illustration showing light absorption in single (a) and multijunction (b) PVs. Four-terminal (c) and two-terminal (d) tandem PVs. (e) Crystal structure of metal halide perovskites.

At present, perovskites with high PCEs are typically fabricated via a low-temperature solution method and the three main perovskite tandem new technologies are perovskite/silicon, perovskite/CIGS, and perovskite/perovskite tandem solar cells. Performance is examined based on the arrangement of active layers. We briefly report the status of each type and the possible challenges mitigating against fundamental understanding and integration of these devices that limit potential commercialization.

In conjunction with the different possible bottom cell technologies for perovskite-based tandem solar cells, silicon receives by far the highest attention. In addition, PV efficiency and stability relate to the improvement strategies for most perovskite/silicon two-terminal (2T) tandem solar cells, which rely on several directions in the heterojunction bottom cells fabricated by precursor doping, selection of HTL/ETL, and tandem solar cell structure. All of these affect the charge extraction and charge transportation characteristics [339]. Common perovskite-tandem solar cells are based on 2-terminal (2T) or 4-terminal (4T) configurational structures to form multi-junction tandem solar cells, typically found in perovskite/silicon, perovskite/CIGS, and perovskite/perovskite for rapid improvement in the PCE of monolithic perovskite/silicon tandem solar cells. The observed improvements in the PCE from 25.2 to 29.2% within a short time are products of the changes made to absorption active layers and the propensity to tuning the band gap properties of the metal halide materials [340–342]; a key challenge is to maximize the PCEs to TSCs apart from the additional semi-transparent requirement for the top cell.

The work of Goesten and Hoffmann [343] and Li and Zhang [344] suggested the replacement of the halides or decreasing the band gaps of perovskite for bottom cells for all PTSCs through partial substitution of Pb with Sn; in addition, enhancing the intrinsic and environmental stabilities often result in higher PCE percentage. According to Mazzarella et al. [345], the use of n-type nc-SiO_x:H between the perovskite and silicon absorber and the electron-selective contact layer for the bottom SHJ solar cell provides two important advantages: reflection can be spectrally tuned in relation to the layer thickness at a certain wavelength and adjusting the refractive index between the neighbouring layers. A thickness of about 95 nm was reported both for simulations and experimental results for ideal monolithic perovskite/silicon tandem solar cells incorporating n-type nc-SiO_x:H [346–348].

In their previous report on simulation, Hörantner and Snaith [349] examined the power output for an ideal band gap of a perovskite top cell of 1.65 eV resulting in 32% PCE when combined with a silicon rear cell. It was found that the efficiency of the tandem cell at a standardized air mass of 1.5 is not significantly depleted by real-world spectral variations but the location of installation may contribute to lower efficiency; whereas, for the top cell, a good mechanical stack, optical coupling, and monolithically integrated multi-junction solar cell designs may result in profound improvement in energy yield [348,350].

At present, a world record of 29.8% PCE of the perovskite/silicon tandem solar cell was reported in 2022 with a schematic diagram of the perovskite/silicon structure in Figure 19. The device made of the periodically nanotextured interface between the silicon bottom cell and the solution-processed perovskite top cell exhibits a significant reflection reduction (−0.5 mA/cm² current density equivalent) compared to previous devices with planar perovskite–silicon interface and combined short-circuit current densities in perovskite and silicon of up to 40 mA/cm². Some selected high-power conversion-efficient materials and structure compositions are reported in Table 4. A recent review paper by Li et al. [351] proffers technical ways to further improve the efficiency of perovskite/silicon tandem solar cells, focusing on the light management in the device structure, phase stabilization, and trap reduction commonly for wide-band gap perovskite materials, as well as development of interlayer materials.

Figure 19

Schematic diagram of the perovskite/silicon tandem solar cell structure. Adapted with permission from [351]. Copyright: ACS Appl. Mater. Interfaces.

Table 4

Selected high-energy converting tandem solar cells and their electrical properties

Si or perovskite bottom cell	Perovskite top cell	J _sc (mA/cm²)	V _oc (mV)	FF (%)	Efficiency (%)	Ref.
4T IBC c-Si	FA_0.75MA_0.15Cs_0.10Rb (I_0.66Br_0.33)₃ + 5% RbI (1.63 eV)	19.4, 18.8	1.12, 0.73	0.73, 0.80	26.4	[352]
4T SHJ c-Si	MAPbI₃ (1.55 eV)	20.1, 16.0	1.07, 0.69	0.76, 0.80	25.2	[353]
2T SHJ c-Si	FA_0.83Cs_0.17Pb(I_0.83Br_0.17)₃ (1.63 eV)	18.1	1.65	0.79	23.6	[354]
2T n-type HJ c-Si	Cs_0.07Rb_0.03FA_0.765MA_0.135Pb(I_0.85Br_0.15)₃ (1.62eV)	17.6	1.75	0.73	22.5	[355]
2T SHJ c-Si	Cs_0.19MA_0.81PbI₃ (1.58 eV)	16.8	1.75	0.75	22.0	[356]
2T SHJ c-Si	MAPbI₃ (1.55 eV)	15.9	1.69	0.80	21.2	[357]
4T FA_0.6MA_0.4Sn_0.60Pb_0.40I₃ (1.25 eV)	FA_0.8Cs_0.2Pb(I_0.7Br_0.3)₃ (1.75 eV)	17.5, 12.3	1.20, 0.81	0.74, 0.74	22.9	[358]
4T FA_0.6MA_0.4Sn_0.60Pb_0.40I₃ (1.25 eV)	MAPbI₃ (1.55 eV)	20.1, 4.8	1.14, 0.81	0.80, 0.74	21.0	[359]
4T FA_0.75Cs_0.25Sn_0.50 Pb_0.50I₃ (1.27 eV)	FA_0.83Cs_0.17Pb(I_0.83Br_0.17)₃ (1.63 eV)	20.3, 7.9	0.97, 0.74	0.79, 0.73	20.3	[360]
2T MASn_0.50Pb_0.50I₃ (1.22 eV)	MA_0.9Cs_0.1Pb(I_0.6Br_0.4)₃ (1.82 eV)	12.3	1.98	0.73	18.5	[361]
2T FA_0.75Cs_0.25Sn_0.50 Pb_0.50I₃(1.27 eV)	FA_0.83Cs_0.17Pb(I_0.50Br _0.50)₃ (1.85 eV)	14.5	1.66	0.70	17.0	[360]
2T MAPbI₃(1.55 eV)	FA_0.85Cs_0.15Pb(I_0.30Br _0.70)₃ (2 eV)	9.8	2.29	0.80	18.1	[362]
2T MAPbI₃(1.55 eV)	MAPbI₃ (1.55 eV)	6.6	1.89	0.56	7.0	[363]

10 Potentials and commercialization of PSCs in the PV industry

The PV industry has come into the limelight in the last few decades because of the advancements made in the development of materials that could easily harvest solar radiation and convert it into electrical energy. The advantages of PV solar cell technologies include low cost and environmental friendliness in handling these materials as most do not constitute another form of pollution to the environment. Quite a large number of organic and metallo-organic complexes have been examined as photosensitizers in PV DSSCs, where TiO₂ is used as a semi-conducting agent giving the highest PCE of 14% in the last three decades when the technology was introduced as compared to the silicon-made PV solar cells with over 30% efficiency, although the same Si-PV technology still suffered on the basis of expensive processes involved and in the costs of production [364–368].

As a result of these limitations in dye photosensitizers, the development of hybrid halide PSCs technology has continued to take centre stage in PV society with a surge in interest to find alternative sources for the generation of solar cells with enhanced PCEs [369,370]. The materials so far have achieved 25.8% PCE according to the latest NREL reports [34] compared to the 3.8% initial efficiency recorded in 2009 [9,371–373]. A boost in PV efficiency of perovskite films through control of charge-generating films formation and improved current transfer to the electrodes encourage further advancement in the preparation of new materials and methodologies aimed towards the production of low-cost and efficient PV cells [374] because inorganic/organic lead halide perovskite materials show great promises compared to others for the next generation of solar devices based on their high-PCE. Perovskite materials possess low trap density, high band gaps, high mobility, and high diffusion length together with other intrinsic properties that enhance their optoelectronic properties and applications in PV [101,375–380].

11 Conclusion

The development of PSCs with high PCE has received tremendous attention in recent years but full commercialization is limited by degradation at high temperatures, moisture, oxygen, and the life span. Caesium-based PSCs were able to overcome problems associated with thermal stability. To circumvent the problem associated with low PCE in single-junction solar cells, caesium was mixed with organic perovskite in a moderate proportion to give thermally stable PSCs with high PCE. The presence of cations in the structure of perovskite often affects its size and energy band gap. In the fabrication of PSCs, the use of hydrophobic cations can reduce moisture that causes structural instability while large cations can tune the band gap, carrier lifetime, and mobility of the PSCs. Tuning of the perovskite structure will help the device withstand when exposed to moisture, mechanical stress, and heat. The use of additives can also help combat oxidative stress and reduce the degradation of the solar cell. The use of other cation with similar tolerance factors to methylammonium and FA could lead to an increase in the temperature, moisture, and humidity, and consequently, yield high-PCE.

The use of TiO₂ as an ETL results in thermal and moisture stability as well as low efficiencies due to the loss of photocarrier ions. This can be overcome through doping or encapsulation or the addition of additives, which can slow down the oxidation process, and increase the energy band gap, fermi level, and voltage density. Low-dimensional perovskites have been shown to be relatively stable to moisture, temperature, and oxygen. However, the fabrication of highly stable PSCs from low-dimensional perovskites is extremely difficult and arises from the compositional structure, which is susceptible to decomposition. Fabrication of stable PSCs and/or tandem PSCs with high-PCE depends on balanced structural components with regard to the TCO, hole transport material, photoactive layer, cathode, and electron transporting material. The composition of these components and their ratios affect the overall efficiency of the fabricated PSCs. The viability of PSCs with high-PCE depends on being able to design and fabricate chemically stable, perovskites with good structural features that comply with the Shockley–Queisser limit for high efficiency with any prospect of future commercialization.

Funding information: The authors acknowledge the financial support of the National Research Foundation (Grant Number 129275) and SASOL South Africa.
Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Conflict of interest: The authors state no conflict of interest.

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Received: 2022-05-03

Revised: 2023-01-07

Accepted: 2023-04-11

Published Online: 2023-08-01

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/ntrev-2022-0547

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perovskite synthetic route; stability; power conversion efficiency

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