Newfangled progressions in the charge transport layers impacting the stability and efficiency of perovskite solar cells

Solar cell generations and types

There have been different compositional, morphological and technical variations in the emerging phase of SCs leading to different generations of SCs (Ahmad et al., 2020a). The three generations of SCs have been devised in accordance with the developmental progression with passage of time. Crystalline silicon based PV technology forms the first generation of SCs. Such SCs were composed of either single crystal (c-Si) structure or a combination of small sized crystals. The highest price ranges associated with silicon based PV technologies, even surpassing the fossil fuels are not in conformity with the principles of green chemistry. This gave rise to the inception of second generation SC composed of thin films. Present day cadmium telluride (CdTe), copper indium gallium selenide in addition to the amorphous silicon are classic examples of second generation of SC. The SCs are primarily composed of the thin films with thickness spanning over a range of very few nanometers to microns. The thickness of second generation SCs was considerably reduced than the previous generation SCs with employment of as thin as 200 nm flakes. Despite cheaper costs of second generations SCs, the developed SCs could not excel the first generation c-Si SC in terms power conversion efficiency (PCE). However, the scientific communities have been eager to find a solution for boosting PCEs of the comparatively cheaper second generation solar cells. The third generation SCs are being researched heavily and are in experiential phase (Ahmad et al., 2020b). They include different types of SCs i.e. polymeric, organic, dye-sensitized, nano-crystalline, quantum dots, micro-morphs and perovskite SC. Among these types of third generation SCs, perovskites have been marked by an eminent and expeditious progress (Park, 2015).

Perovskite solar cells

The demand for energy for human survival has been estimated to grow at a double rate by 2040 (Iqbal et al., 2020; Tahir et al., 2020). Such demand can be sustainably met if renewable energy resources particularly solar energy has been used as an alternative to the conventional fossil fuel driven energy modes. Halide based organic inorganic perovskite solar cells (PSCs) are characterized by exceedingly high dielectric properties and the active layer upon incidence of direct solar irradiance produces freely moving electrons and holes instead of the attached excitons (Green and Ho-Baillie., 2017; Ijaz and Zafar, 2020; Ijaz et al., 2020). PSCs were introduced in 2009 (Kojima et al., 2009) and in less than a decade of their inception, PSCs have remarkably overtaken c-Si SC in terms of competitive PCEs (Yang et al., 2017; Hörantner et al., 2017). The astonishing executional progress made by PSCs in PV technology has been further strengthened by the achievement of an augmented efficiency and brought PSCs at the core of PV based researches carried in the world (Rizwan et al., 2019). The researches at the present stage are majorly focused on the development of advanced version PSCs offering an enhanced device surficial area (Galagan et al., 2016) in addition to the efforts being made for augmentation of the chemical stability of PSCs in the moist conditions (Yang et al., 2017). PSCs are the future high efficiency SC technology possessing high level compatibility with lower costs, easier processing at an alleviated temperature, pliant substrates and fabrication methods for larger areas. PSCs in a completely solidified form were materialized soon after the utilization of the methylammonium lead halide perovskite material in form of dyes in SCs developed by Miyasaka in 2009 having liquid electrolyte previously sensitized by dyes (Kojima et al., 2009). Starting with the certified PCEs of 3.8% (Kojima et al., 2009), the present days PCSs have succeeded in attaining PCEs of 25.2%.

Chemical structure of PSCs is predominantly composed of a perovskite material, which is in general a halide substance attached with the organic-inorganic lead or tin. This perovskite material has a dual functionality of harnessing the solar energy and carries out the electron and hole conduction by acting as a charge carrier conductor. The typical chemical formula used to represent perovskites class is ABX₃ (Figure 1). In ABX₃, A and B represent the cations and X refers to the anion possessing opposite charge and dimensionality. Furthermore, A is a mono-cation having size greater than the B di-cation. Figure 1 represents the three-dimensional (3D) crystalline structure of the frequently used, methyl-ammonium lead halides perovskite, CH₃NH₃PbX₃ (X = I, Br or Cl). In this structure, CH₃NH₃ ⁺ cation is fenced by octahedra of PbX₆.The X anion in the perovskite structure is a mobile specie marked by its meandering behavior throughout the crystalline structure. X anion possess an activation energy of 0.6 eV and the migration of this anion is induced by the development of the vacancy (Eames et al., 2015).

Figure 1:

Three dimensional (3D) structure of perovskite materials with general formula of ABX₃ expressing the larger monocation as A, smaller dication as B and oppositely charged anion as X. Reprinted with permission from Park, 2015. Copyright 2015, Elsevier Ltd.

Halide perovskite materials having three dimensional crystal lattice are often semiconductors having direct band gaps and are marked by competing characteristics i.e. considerably higher absorption coefficients of light (Pazos-Outón et al., 2016), spectral ranges of absorption that can be easily adjusted (De Wolf et al., 2014), enhanced mobility of charge carriers, longer diffusion lengths of charges, stronger photoluminescence and the reduced rates of the charge combination in case of non-radiation. For all these characteristics, perovskite materials have assumed a conspicuous place in the PV market for acting as a photoactive substrate for different devices i.e. SCs, light-emitting diodes (LEDs), photodetectors (PDs) and lasers.

Configurational and compositional transformation

Perovskite solar cells are developed in different architectures (Figure 2) with a continuous progression in structure. These device architectures are primarily inclusive of classic mesoporous or planner structure. The mesoporous structures are usually consists of an infiltrated n–i–p junction while the planner structure is devoid of any mesoporosity. Planner architectures of PSCs employing SnO₂ processed at lower temperatures possess parallel PCEs in comparison to the mesoporous analogues processed at higher temperature ranges (Anaraki et al., 2016; Baena et al., 2015). In terms of practical benefit to be driven from PSCs by means of commercializing them, considerable effort needs to be done for enhancement in the efficiency and stability for longer time periods. Furthermore, the accurate optimization of the energy levels between the perovskite absorber layer and charge transporting layer can also be beneficial in achievement of an improved functionality (Jeon et al., 2018). The potential of organic–inorganic PSCs for development into future generation PV cells seems to be quite pronounced by the virtue of quicker advancements in the PCS, nevertheless, there is an urgent need to comprehend the functional mechanisms regarding photo and bias-induced impacts inside PSCs in the nano-range. Scanning probe microscopic techniques has been utilized for the thorough comprehension of the morphology based transformations occurring in perovskite absorber layer (PAL) as a result of photo incidence (Figure 3). There is a considerable evidence regarding the morphological evolution of the domains contributing towards the providence pf the transferal pathways for holes to a HTL expressing positive bias (Kim et al., 2019).

Figure 2:

Perovskite solar cells inception and morphological evolution: (a) perovskite sensitized surface adsorptive interactions by utilization of the nanodot perovskite, (b) non-injecting scaffolding layer forming meso-super structural configuration, (c) nanoscale oxide acting as a construction block based pillared structural configuration, and (d) pin heterojunction expressing planar configuration. TiO₂ is shown in spheres in (a) and (c) and Al₂O₃ in (b). FTO, Fluorine doped tin oxide; BL, blocking layer; HTM, hole transport material; Au, gold; Ag, silver. Reprinted with permission from Park, 2015. Copyright 2015, Elsevier Ltd.

Figure 3:

Strip domains and augmentation in the piezo-response as a result of solar incidence in the active perovskite absorber layer: (a) scanning probe micrograph of the absorber layer placed on the photo-anode with structural configuration of FTO/mesoporous-TiO₂/compact-TiO₂/PAL, and (b) Amplitude map of the piezo-response in 3D morphology exhibited in the absence of illumination. Reprinted with permission from Kim et al., 2019. Copyright 2019, Nature Publishing Group.

The remarkable optoelectronic characteristics associated with PSCs have contributed to their stardom. The evolution and maturation phase of PSCs have been particularly very rapid as expressed from the number of publications exceeding 2000. There has also been a demystification of the hysteresis and ferroelectricity which have remained the most debated topics of perovskite materials so far. In practical aspect, present day PSCs are usually solution processable thin films based and thus, causing a remarkable alleviation in the device fabrication costs due to employment of the physical vapor deposition techniques. In this domain, the effort has also been done for abatement of toxicity by synthesizing lead free perovskites has achieved a PCE of up to 9% giving rise to a myriad of researches in progress in solution processable forms (Sani et al., 2018).

The wide ranging applications of organic-inorganic lead halide perovskites in thin films based PV has been pronounced (Green et al., 2014). Inside PSC, the electronic flow is attracted by the photo-anode i.e. electron transport layer (ETL) and the migration of holes is towards the cathodic region i.e. hole transport layer (HTL).

With passage of time, a wide range of materials have been utilized for obtaining maximum efficiencies (Table 1). A perfect alignment and balancing is mandatory for the energetics and mobility aspects between the layers for the minimization of agglomeration of the charges in addition to the prevention of charge carriers i.e. electrons or holes recombination. Thus, the presence of HTL in a PSC ensures the provisioning of the pathway for the conduction of the holes and also blocks the electrons, consequently giving rise to an alleviated charge recombination and an elevated PCE and fill factor (FF) (Galatopoulos et al., 2017). Current review is aimed at summarizing the profound role of the ETL and HTL in influencing the functionality of PSCs. The review particularly covers the characteristics of PSCs, latest advancements done for modifying the structure and role of ETL and HTL in the expeditiously progressing field of PV technologies with basic focus on the PSCs.

Hysteresis free electron transporters

The adoption of TiO₂ as an ETL in PSCs is due to its transparent nature, easier tunability of electronic characteristics, perfect matching of energy levels with the perovskite layer, economic viability and is fabricated by utilization of solution containing Ti-precursor. Planar PSCs with tri-layers were analyzed by formulation of the model considering the mobility of ion vacancy and transport of charge. Obtained results were employed for the effective substitution of the conventionally used ETL (TiO₂) and HTL (spiro-OMeTAD) with advanced materials marked by alleviated permittivity. Furthermore, the doping of other materials was found to cause a shifting behavior of the scan rates for hysterical phenomenon. Through such investigations, the researchers succeeded in comprehending the role of organic ETLs in yielding superficially “hysteresis-free” PSC devices, however at lower temperatures, the phenomenon of hysteresis is quite pronounced (Courtier et al., 2019). Though, the significance of the compatibility of TiO₂ for improved PSC device has been established. In the PSCs with planer n-i-p architecture, the determination of transferal characteristics of charge has been done by TiO₂ ETL and exerts stronger influence on the morphological and crystal aspects of the perovskite absorber layer. In addition to TiO₂, organic ETLs have also been employed as an under-layer for perovskite absorber films for beneficially influencing the interfacial matching and electronic stabilization. Predominant PSC device functionalities have been obtained by utilization of the ETLs with bilayer compositions containing metallic oxides and the organic components e.g. PEIE (polyethyleneimine ethoxylated)/Y-TiO₂ (Zhou et al., 2014), TiO₂/PCBM ([6,6]- phenyl-C₆₁-butyric acid methyl ester) and graphene/TiO₂ (Wang et al., 2013) etc.

Suppression of hysteresis via functionalization

Efforts have been made for the suppression of the hysteresis associated with the current–voltage response by using functionalization mechanism employing amino groups, consequently enhancing the stability of the PSC device. Comparatively simpler, single step, low temperature and non-hydrolytic method has been adopted for the synthesis of in situ TiO₂ nanoparticles functionalized with the amino groups (NH₂–TiO₂ NPs). NH₂–TiO₂ NPs were formed by establishing a chemical bond between (–NH₂) and TiO₂ by means of Ti N bonds on the exterior region of the TiO₂ NPs followed by incorporation of these NH₂–TiO₂ NPs as an effective ETL in PSC with n–i–p planar heterojunction producing a PCE of >21%. The comparison of PCE between the immaculate ETL (only TiO₂) and the amino functionalized TiO₂ expressed the boost in PCE from 19.82% for pristine TiO₂ ETL to 21.33% for functionalized ETL (Hu et al., 2019). Improved PSC device functioning with negligible hysterical behavior has been fabricated by bi-functionalization of the ETL interface linker in form of self-assembled monolayer (SAM). 4-picolinic acid SAM was utilized as an interface linker between the ETL and perovskite absorber layer. The bi-functionalized interface linker highly influenced the crystallinity of the perovskite and the transport mechanism of the electrons between the ETL and absorber layer. Furthermore, SAM interface linker adjusted PSC yielded a PCE of 18.90% and a picayune hysterical index of 0.03 in comparison to the unadjusted pristine device having a hysteresis index of 14.65% under direct solar irradiance of AM 1.5G (100 mW cm⁻²). Results of the SAM modified PSC device were indicative of the prospects of the grappling of the bifunctional SAM with the TiO₂ causing an increment in the granular size of the MAPbI₃ absorber layer and causing an impressive defect passivation between the TiO₂ ETL and the absorber layer interface. The modified assemblage consequently gave rise to the perfect balancing of the electronic and holes transferal and enhancing the PSC device functioning (Han et al., 2019).

Procedural engineering of neoteric electron transporters

TiO₂, the frequently used ETL, though contributing towards the PCE of perovskite materials has been debated for its dilapidation whenever exposed to the heat and UV-light leading to thermal and photocatalytic deterioration. For that reason, the challenges of degradation needs to be solved before adopting PSCs as an alternative to the c-Si based cells. Researches regarding thermal stability of PSCs at 60 °C have been scanty since the performance of PSCs at such temperature has always been found poor. Adoption of novel ETLs exhibiting favorable features e.g. inertness towards photosensitivity, remarkable thermal stability and electronic mobilization is a crucial factor. In this regard, there is an utmost need for effectively controlling the morphological orientation of the ETLs during fabrication which is mostly not comprehended well due to film deposition giving rise to lower PCEs. Blade-coating has been developed recently for ETL deposition at considerably lower temperatures, completely solution processable and scalable method for perovskite material. However, it has not been examined for the ETLs deposition yet (Cao et al., 2019; Zhong et al., 2018). In order to resolve the issues of photo-stability, other inorganic materials e.g. ZnO and SnO₂ have been adopted as ETLs in PSCs for their capability of inertness towards the photocatalytic degradation (Cao et al., 2019; Tan et al., 2017). Furthermore, different experimentations have been done on PSCs at the ambient temperatures for comprehending the challenges regarding stability (Lee et al., 2017).

Atomic layer deposition has also been employed for the thin films of TiO₂ thin films for planar architecture PSCs. Thin films of the perovskite absorber layer (MAPbI_{3− x} Cl _x ) was manufactured on ETL via progressive vapor processing. This technique produced the thin films of perovskite with commendable uniformity and pinhole-free morphology yielding a granular size of ∼370 nm. Furthermore, the crystal aspects of the material was favorable and obtained bandgap was ∼1.58 eV. The PSC device gave produced PCE of ∼11.6% in comparison to the traditionally used TiO₂ (Na et al., 2019). TiO₂ has been employed frequently as an ETL in PSCs but its adoptability on the practical scale is cramped by the requirements for higher temperature for fabrication and inherently alleviated mobility of the carriers. Thus, myriad of researches have been done by substitution of different materials as ETLs in the PSCs e.g. PCBM (Lee et al., 2017), Nb₂O₅ (Feng et al., 2017), Zn₂SnO₄ (Wu et al., 2018), N,N′-bis(3-(dimethylamino)propyl)-5,11-dioctylcoronene-2,3,8,9-tetracarboxdiimide (CDIN)) (Zhu et al., 2016), ZnO (Mahmood et al., 2017), WO _x (Wang et al., 2015), CdS (Zhao et al., 2015), Fe₂O₃ (Hu et al., 2017), SnO₂ (Halvani Anaraki et al., 2018) etc.

Electron transporters doping with carbonaceous materials

Carbonaceous materials e.g. graphene oxide (GO), graphene, single walled carbon nanotubes (SWNTs) etc. have also been utilized for the modification of ETL of PSC devices as effective dopants for possessing exceptional conductivity aspects. PSC device instability is caused usually due to interface erosion up on exposure to the moisture and thus hindering the commercialization of PSCs in ambient conditions. PSC devices stable for extended durations with higher PCEs can be developed by melioration of the organic ETL with solvent resistant characteristics associated with the effective tackling the challenges of charge extraction, crystallization of perovskite absorber layer and stability parameters. Fullerene [6,6]-phenyl-C₆₁-butyric styryl dendron ester (PCBSD) was modified by utilization of large π-conjugated graphdiyne (GD) for improvement of the orientation of the film (Li et al., 2018).

PSCs fabricated at low temperature have been with modified ETL in planar architecture have been stability enhanced by doping of (4-(1,3-dimethyl-2,3- dihydro-1H-benzoimidazol-2-yl) phenyl)dimethylamine into the fullerene derivative. In this design, ETL was a side chain of triethylene glycol having hydrophilic nature. Doping of ETL enhanced the stability and produced PCE of 18.5% without any hysterical response in comparison to the unmodified ETL having PCE of 16.2% (Liu et al., 2019a). Fullerene derivatives are one of the remarkable the n-type semiconductors used in the optoelectronic devices. Effective electronic mobility and resistance towards thermal or moisture exposure is exhibited by them (Jeng et al., 2013; Lin et al., 2015; Malinkiewicz et al., 2014).

The adoption of organic ETLs in comparison to the pristine TiO₂ ETLs has been augmented at a rapid rate due to the possibility of organic ETL as n-type materials at lower temperatures in comparison to the TiO₂ requiring temperatures > 450 °C for processing. PSCs in planar configuration containing C₆₀ as an electron selective layer (ESL) have been fabricated by utilization of the soluble C₆₀–9-methylanthracene mono-adduct (C₆₀(9MA)) functioning as a thermal precursor to C₆₀ (Figure 7). Highest PCE with such PSCs obtained was 15.0% in the simulated solar irradiance of AM 1.5G one sun illumination, excelling the archetypical TiO₂ ESL (12.9%). The modified device expressed a superior FF of 0.723 and surpassed the pristine TiO₂ based ESL for having FF of 0.671. The enhanced FF was obtained due to comparatively picayune charge transfer resistance at the interface between C₆₀ ESL and active perovskite absorber layer (Umeyama et al., 2017).

Figure 7:

Organic electron transport material engineering by incorporation of fullerenes: (a) schematic illustration of thermally assisted transformation of C₆₀(9MA) to C₆₀ in the thin film state and the PSC device configuration, (b) scanning electron micrograph of the modified ETL, and (c) differences in the energy levels of the materials employed for PSC with FTO/C₆₀/perovskite absorber/hole transport layer/Au. Reprinted with permission from Umeyama et al., 2017. Copyright 2017, ECS Ltd.

Phenyl-C₆₁-butyric acid methyl ester (PCBM), is a fullerene derivative, which has been employed for the stability enhancement and hysteresis prevention in PSCs with planar p-i-n architecture. However, the PCEs of PCBM containing ETL devices have not yet surpassed the TiO₂ based ETLs. Niggardly existing electrical conductivity and remarkable interfacial reincorporation rate associated with the PCBM. Effective conductivity enhancement of PCBM can be done by adopting doping mechanisms. Especially, this strategy can be used for alleviation in the charge recombination at the interface between PCBM and electrode region. The interlayer of polyethyleneimine (PEI) between electrode region and PCBM can significantly cause an improvement in the device stability. Fullerene derivatives have also been substituted with different hydrophilic substances. Such PSCs expressed better PCEs in comparison to PSCs containing PCBM ETLs. Planar PSCs with hydrophilic triethylene glycol type side chain (PTEG-1) as an ETL has expressed a PCE of 15.71% (Shao et al., 2016). Currently developed PSC with semiconductor ETLs are faced with the challenges of efficient electronic mobility and energy level matching with the active perovskite absorber layer. Recently, a fledging allotropic form of carbon known as T-carbon has been employed as an ETL for addressing these challenges. By utilization of calculations of first principles and deformation potential theory, it was revealed that T-carbon possess 2.273 eV direct band gap and is a semiconductor. T-carbon exhibited the superior electronic injection force and there was a difference of energy levels between the conduction band (CB) and the active perovskite, the energy level of CB being lower by 0.5 eV (Figure 8). T-carbon as an ETL also expressed superior level electronic mobility of up to 2.36 × 10³ cm² s⁻¹ V⁻¹, excelling the archetypical ETLs e.g. TiO₂, ZnO and SnO₂ and providing a better facilitation of the effective electronic separation in addition to causing higher instantaneous diffusion at a considerable distance from their locus of production inside the active perovskite absorber layer. Results were indicative of the highest suitability of the T-carbon for development into an effective ETL in PSCs (Sun et al., 2019).

Figure 8:

Suitability of T-carbon as an electron transport material in PSCS: (a) comparative energy levels diagram for T-carbon with other materials employed as photo anodes in PSCs. Corresponding band gaps of the materials have been shown by blue color, and (b) Cluster model of 200 atoms containing T-carbon expressing the highest occupied molecular orbital (HOMO) and LUMO calculated at TD-B3LYP/6-31G level. Excitation energies are denoted by E and λ refers to the highest absorption wavelength. Reprinted with permission from Sun et al., 2019. Copyright 2019, Elsevier Ltd.

Inorganic substitutes of TiO₂ photo-anode

Meritorious photo-physicality aspects associated with the halide perovskites have given rise to an augmentation in the PCEs. Further enhancement of PCEs can be achieved by the thorough comprehension of the dynamics of the charge carriers between the perovskite absorber layer and the linked interfacial components. Currently, PSCs have been anticipated to achieve PCE of 25% by optimization and modification of the device architecture. ZnO has also been employed as an ETL in PSCs for possessing favorable aspects e.g. processability at the lower temperature and higher mobility of electrons. Comparatively stable PSC devices were fabricated with PCEs up to 20.62% in mesostructured architecture exhibiting zero hysteresis. ETL was further modified by development into the bilayer at low temperature processing of the barium hydroxide hybridized boron-doped ZnO (B:ZnO). Modified ETLs with relatively alleviated trap states was obtained yielding higher PCEs [92]. ZnO has been employed as an ETL recently by blade coating ZnO nanorods in PSCs signifying cost effective, scalable and facile fabrication mode with commercialization aspects (Mahmood et al., 2019).

Remarkably efficient PV technologies have been using semiconductor oxides (Hagfeldt et al., 2010; Pérez-Tomás et al., 2018) for different device fabrications e.g. PSCs. These oxides are usually employed in form of thin films of higher density or as mesoporous oxides. These SOs work as the ETLs or HTLs in PSCs architectures either in normal or inverted configuration (Tian et al., 2018). Recently, focus has been shifted to the utilization of less explored complex oxides possessing singular characteristics e.g. oxides with characteristics of ferroic, ferroelectricity and magnetism. Recently, triple cation halide PSCs with aided with the complex ferroelectric oxide Pb(Zr _x Ti_1-x )O₃ (x¼ 0.6) (PZT) functioning as an ETL. Results were supportive of the better PV characteristics of the PSC device upon polling the PZT modified SC in the air and UV-light causing an improvement in the transport properties. The improved response of the PSC was attributed to the PZT thickness with ideal response obtained at 10 nm giving a PCE of 11% upon UV-light and air exposure with utilization of any encapsulation (Pérez-Tomas et al., 2019).

Recently, carbon based PSCs comprising of CsPbBr₃ films spin coated on the compact TiO₂ (c-TiO₂) ETL were fabricated. Passivation in c-TiO2 ETL was done by introduction of thin layer of SnO₂. The utilization of SnO₂ for ETL modification is associated with the improvement in the surficial morphology of the ETL in addition to the profound reduction in pathways for current shunting in the ETL. Furthermore, the fabricated bilayer ETL containing TiO₂/SnO₂ exhibited an extraordinary capacity for electronic extraction, charge transportation and elimination of the charge recombination mediated by interfacial traps. The modified champion device gave a PCE of 8.79% with 0.817 of fill factor. The PCE obtained is highest reported in case of CsPbBr₃ based PSCs. Furthermore, the fabricated PSC also expressed considerable stability towards heat and humidity at the ambient conditions (25 °C) for 30 days (Liu et al., 2019c).

In addition to different ETLs used in PSCs, arrays of In₂S₃ nanoflakes have also been found effective for enhancing the functioning of the PSCs in terms of augmented efficiency and prevention of the hysteresis. Thin films of In₂S₃ have excelled TiO₂ ETLs expressing commendable mobility of the carriers i.e. 17.6 cm² V⁻¹ s⁻¹ vs. 10⁻⁴ cm² V⁻¹ s⁻¹. Nevertheless, the depositional process for thin films of In₂S₃ is quite a labyrinthine process which takes place by chemical bath deposition requiring numerous hours marked by time consumptiveness and energy intensiveness. In terms of commercialization fullerene derivative compounds, important factor is an extended thermal stability for longer operational time (>1000 h) at 60 °C. In this regard, In₂S-based ETLs are known for expressing an outstanding functioning by producing higher PCEs and infinitesimal hysterical response. Recently, organic-inorganic PSC PV devices have been developed by utilization of thin films of In₂S₃ as an ETL fabricated at optimized conditions. In₂S₃ ETL excelled the archetypical TiO₂ ETL in PCE and electronic extraction efficiency (Yu et al., 2019).

PSCs with WS₂ have been designed and simulated as an effective ETL. SCAPS-1D numerical simulator has been utilized for the quantitative analysis of the interface between ETL/perovskite/HTL of the PSC having an active perovskite absorber i.e. CH₃NH₃PbI_3−x X _x in addition to the influence of numerous amphoteric defect states. Results in this regard were supportive of the commendable influence on the absorber layer defect states in comparison to the interface defects layer (IDL) (Figures 9 and 10). The quantifiable tolerable scale of the CH₃NH₃PbI_3−x X _x absorber layer and IDL was revealed to be <1015 cm⁻³ and 1016 cm⁻³, respectively. The highest performance for WS₂ ETL containing PSC has been found to span over a range of 10–40 °C with consequential dilapidation at the increased temperatures. Furthermore, the device produced a PCE of 25.70% (V _oc = 1.056 V, J _sc = 25.483 mA/cm², and FF = 88.54%) (Sobayel et al., 2019).

Figure 9:

Energy level diagram exhibiting band alignment of WS₂ based electron transport layer of MAPI₃ PSCs Reprinted with permission from Sobayel et al., 2019. Copyright 2019, Elsevier Ltd.

Figure 10:

Influence of the change on WS₂ based electron transport layer: (a) efficacy with the density of the defect sites, (b) V _oc in accordance with the position of the defect sites vs. density of defects, (c) fill factor, and (d) J _sc with the defect sites in the absorber layer. Reprinted with permission from Sobayel et al., 2019. Copyright 2019, Elsevier Ltd.

Phylogenesis of hole transporters

HTLs are not only significant for their holes transporting characteristics but they are also known to block the electrons and thus, causing a retardation in the charge recombination. Additionally, the dual functionality of HTLs is also expressed by its role as an encapsulation layer ensuring the protection of the underlying perovskite absorber layer from possible dilapidation caused upon exposure to atmospheric oxygen or moisture. Encumbrances are associated with the all the archetypical HTLs e.g. remarkably increased synthetic costs, instability towards thermal or chemical exposure etc. Such challenges can be solved by the utilization of advanced material e.g. Phthalocyanines comprising of 18π electrons can be an effective candidate to be used as an ETL possessing relatively lower costs and germane redox potential (Sfyri et al., 2016). The hydrophobicity aspect is not true for PEDOT:PSS which is a hydrophilic substance (Sun et al., 2014). P-type semiconductor, Cuprous oxide (Cu₂O) having 2.17 eV band gap has been gaining considerable attention for HTM utilization in PSCs (Chatterjee and Pal, 2016). Cu₂O HTL has produced an efficiency of 11% expressing the auspiciousness of inorganic HTMs in PSCs (Tang et al., 2016).

Myriad of PSCs with different HTLs having organic or inorganic nature have been fabricated in different configurations. However, the functioning of the majority of the HTLs is improved by addition of the minute quantity of dopants for enhancement of the charge conveyance characteristics. The frequently employed state of the art HTL, organic in nature i.e. spiro-OMeTAD, poly(3-hexylthiophene) (P3HT), and poly(triarylamine) (PTAA), are often doped with the bis(trifluoromethane)sulfonamide lithium salt (LiTFSI) for yielding improved FF and PCE (Hawash et al., 2018; Ko et al., 2018; Jeon et al., 2018) (Figure 11). HTLs of organic nature, especially PTAA, P3HT and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) are often employed in the PSCs for exhibiting remarkable inherent mobility and conductivities (Bush et al., 2016). There is a rapid progression in the polymeric HTLs for PSCs due to their favorable solution-processability, profound thermal stability and hydrophobic nature (Choi et al., 2018; Dongxue and Liu 2017; Pitchaiya et al., 2018).

Figure 11:

Chemical structures of (a) Spiro-OMeTAD, (b) Li-TFSI, (c) P3HT, and (d) PTAA.

Transpositioning of Spiro-OMeTAD

2,2′,7,7′-tetrakis(N,N-di-pmethoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) is the most frequently used material for HTL. Spiro-OMeTAD has been primitively synthesized for dye-sensitized solar cells in solid-state. However, Spiro-OMeTAD HTLs containing PSCs are beneficial for cardinal lab based researches but there are challenges in the commercialization of due to the higher synthetic disbursement, impoverished thermal stability, lower inherent conductivity and transport. Thus, currently the scientific community is researching for efficient substitutes with lower costs. Sufficiently higher oxidation potentials and higher V _oc are observed for HTLs in case of the satisfactory hole injection. Nevertheless, a multitude of factors plays a significant role in exerting arduous impacts regarding HTM’s ionization potentials (IPs). Such factors involve the inherent mobility of the charge, higher conductivity acquisition by means of doping strategy and the specialized cooperation with the active perovskite absorber surface. This fact signifies the rare reporting of the newly developed HTLs, in spite of the prodigiously produced HTLs (Kim et al., 2018). Recently, HTMs comprising benzene compounds substituted at hexa positions namingly, HFB-OMeDPA and HPBOMeDPA have expressed better solubility of the compounds, higher hole transport and appropriate energy levels. PCEs associated with the substituted compounds HFB-OMeDPA and HPBOMeDPA were 15.9 and 16.7%, respectively having an equivalency with the conventionally used spiro-OMeTAD (Liu et al., 2019b).

PSCs with dopant free HTMs comprising of poly[2-methoxy-5-(2-ethylhexyloxy)- 1,4-phenylenevinylene] (MEH-PPV) and poly(3-hexylthiophene-2,5-diyl) (P3HT) fabricated via two solution procedures exhibited suitability of non-doped MEH-PPV in a cost effective manner. The enhanced PCEs obtained by such polymeric HTMs can be sttributed to the perfect band alignment (Figure 12) of the materials aimed at fabrication of a complete PSC device (Chen et al., 2016). The development of radical cations of the spiro-OMeTAD in case of prototypical spiro-OMeTAD, Li-TFSI is the major contributing factor in the presence of oxygen. Consequently, such radical cations formation gives rise to the promotion of the charge hopping and causes aggrandizement in the mobility pattern of the holes and the conductivity aspects of the HTL. Nevertheless, there is a profound inconsistency associated with the radical cation formation since it is not in relevance with the quantitative measure of the dopant but is affected by the exposure to the atmospheric conditions.

Figure 12:

Energy diagram expressing the energy bands of different hole transport materials. Reprinted with permission from Chen et al., 2016. Copyright 2016, Nature Publishing Group.

The thermal stability of PSCs has been enhanced by the formulation of the triarylamine-based organic hole transport materials (HTMs) by using salts of HTL in oxidized and stabilized form. PSC devices comprising of an organic HTL having triarylamine doped with EH44/EH44-ox (which is also its oxidized salt analogue) without any encapsulation have expressed higher stability in the atmospheric conditions. Generally, it is mandatory for the dopants comprising triarylamine to contain a minimum of two electrons at para position donating groups. Such design is signified by the stabilization of the radical cations for prevention of any impurity development leading to the poor functioning of the PSC device. EH44/EH44-ox and Li⁺-doped spiro-OMeTAD HTLs expressed a commendable PSCs device stability without any encapsulation layer upon exposure to the incessant load and solar illumination at 50 °C. Additionally, highly economical and stable PSCs can be fabricated by means of the mixing and matching of different varieties of the smaller molecules based HTLs having non identical nature (Schloemer et al., 2019).

Despite the rapid and rigorous research done in the field of perovskite materials and the thorough comprehension of the smaller molecules of organic nature for HTLs, LiTFSI and tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III) tri[bis(trifluoromethane)sulfonimide] (FK209) remains the most applied dopant material for HTL for enhanced charge conveyance characteristics i.e. augmented mobility and conduction of the holes. However, these dopants are also known for adversely impacting the PSCs device stability and lifespan (Schloemer et al., 2019a). Recently, different PSC devices in series and fully evaporated forms, have been fabricated by using HTLs of variable ionization energies. Results were indicative of the determination of the open circuit voltage by the bulk and surficial alliance of charges in the active absorber layer in comparison to the off set of energetics between the perovskite absorber valence band and the organic HTL’s HOMO (Dänekamp et al., 2018).

Phthalocyanines, suitable material for the HTLs of PSCs contains the core metal and marginal groups of the Pc ring is associated with playing an influential role in enhancement of the PSC functioning by diversifying the energies of HOMO and LUMO of the phthalocyanines (Torabi et al., 2017). The crystal lattice changes in the phthalocyanine leading to the alterations in the mobility of charge in the phthalocyanine materials is actually due to diversified core metals and marginal groups causing π-π stacking in different orientations. Investigations in exploring the PSC’s marginal groups and core metals are scarce in addition to the tunability of the hole transport yielding higher functioning and stability in solar irradiance at elevated temperatures. Phthalocyanine based HTL without any dopant were developed. The material design comprised of the phthalocyanine core consisting of (4-methyl formate) phenoxy or (4-butyl formate) phenoxy in form of marginal groups, while copper and zinc were used as core metals. Synthesized complexes revealed statuesque thermal stability, suitable energy levels and appropriate mobility of holes. Phthalocyanine based HTL PSC device exhibited 15.74% PCE under 100 mA cm⁻² standard AM 1.5G solar irradiance. Therefore, the selection of the suitable marginal groups and core metals is a crucial factor for brining improvements in the PSCs by modifying HTLs (Guo et al., 2019).

Organic HTL, spiro-OMeTAD is also modified by providing spirobifluorene framework in spiro-OMeTAD with one carbonyl group producing an unprecedented phenanthrenone-based HTL known as spiro-PT-OMeTAD. Results revealed the significant changes caused due to insertion of only one carbonyl group altering the light absorptivity, mobility of holes and energy levels of marginal molecular orbital. PSCs containing spiro-PT-OMeTAD modified HTLs, without any dopant introduction expressed remarkable functioning with J _sc 22.36 mA/cm², V _oc 0.99 V, FF 0.62% beneath 1 sun irradiance giving rise to PCE of 13.83% excelling the pristine spiro-OMeTAD based HTLs with PCE of 10.5% (Zou et al., 2019). Scarcely any newly developed HTMs have surpassed the frequently used spiro-OMeTAD HTM in producing superior V _oc and PCE. Comparatively stable and efficient PSCs have been developed by synthesizing fluorene-terminated HTL material with the finely tuned energy level. The modified HTL also exhibited an exhilarated glass transition temperature. Fluorene-terminated HTL based PSC PV devices have exhibited a PCE of 23.2% efficiency (under reverse scanning) and 22.85% inalterable efficiency for the diminutive area of ∼0.094 cm² and an efficiency of 21.7% (under reverse scanning) was obtained for the voluminous area of ∼1 cm² (Jeon et al., 2018).

PSCs in inverted planar configuration and organic solar cells (OSCs) have been employing PEDOT:PSS layer as an HTL (Figure 13). PEDOT:PSS in form of aqueous solution has been employed for the fabrication of the polymeric layer with good conduction characteristics by solution synthetic processes e.g. spin-coating etc. Solution-processable thin films of PEDOT:PSS with an augmented conductivities are fabricated by utilization of dissimilar organic solvents e.g. sorbitol, dimethylsulfoxide (DMSO) and N,N-dimethylformamide (DMF). Organic solvents processed PEDOT:PSS thin film expressed a considerable improvement with 2–3 orders of magnitude in conductivity aspect in comparison to the archetypical PEDOT:PSS HTL film having similar thickness. Additionally, the resistance to moisture was augmented while the contact barrier was lowered between the HTL and active perovskite material interface by means of surficial transformation of the PEDOT:PSS HTL. This was achieved by utilization of GO dissolved in organic solvent which led to the effective removal of the fragment of hygroscopic PSS substance (Choi et al., 2015).

Figure 13:

PSCs comprising PEDOT:PSS and CPE-K containing hole transporters: (a, b) scanning electron micrograph of active perovskite absorber thin film spin coated in PEDOT:PSS and CPE-K HTLs, respectively, photo-luminescent behavior of active absorber layer on variable substrates: (c) PL spectra in steady state, and (d) PL decay transient in time resolved manner. Reprinted with permission from Choi et al., 2015. Copyright 2015, Nature Publishing Group.

Nonetheless, the utilization of DMF and DMSO for transubstantiation of PEDOT:PSS is not as much felicitations for consequent production due to their noxiousness, higher costs and harmful towards environment. The performance of PSCs comprising PEDOT:PSS HTL has also been improved by using sodium citrate in terms of maturation of active persovskite absorber layer and charge derivation aspects. The comparative analysis of the sodium citrate modified PEDOT:PSS HTL expresses an augmentation of >20% in PCE i.e. modified HTL produced an efficiency of >11.50% in comparison to the archetypical PEDOT:PSS HTL (9.13%). PEDOT:PSS HTL engineering with the sodium citrate enhanced the functioning of PSC by assisting crystal maturation for enhancement of the domain size of perovskite in addition to the enhanced favorability of charge aggregation in PSCs (Syed et al., 2019). HTM consisting of di(1-benzothieno)[3,2-b:2ʹ,3ʹ-d]pyrrole framework having a small molecular orientation has been found effective yielding. 18.09% (Azmi et al., 2018). Furthermore, the utilization of a dopant free HTM composed of A polythiophene-based copolymer (PDVT-10) revealed an extraordinary hole transport characteristics and produced a PCE of 13.4% under 100 mW cm⁻² illumination (Liu et al., 2018a).

Perplexities associated with HTL doping

Li-TFSI is associated with the creation of the 2,2′,7,7′-tetrakis(N,N-di-p-methoxy phenyl amine)-9,9′-spiro bifluorenedi[bis-(trifluoromethanesulfonyl)imide] (Spiro-(TFSI)₂) or Spiro-OMeTAD-(TFSI), which in turn causes an enhancement in the HTL conductivity. HTLs doped with the Spiro-(TFSI)₂ have expressed fairly larger contact angles with water and picayune density of the pin holes created in comparison to the Li-TFSI doped HTL. The stronger resistance of the doped HTL towards moisture expresses the suiatability of Spiro-(TFSI)₂ doped Spiro-OMeTAD HTL as a favorable candidate for production of the stable PSCs. Subsequently, the modified PSCs expresses better prospects of commercialization.

Though the utilization of doping strategy might prove as a facilitator of the charge conveyance and avoiding the recombination, subsequently enhancing PCE but the presence of dopants can seriously cause a hindrance in the way of PSCs to adopt overall effective and stabilized modes upon exposure to the atmospheric conditions containing oxygen. Supplementarily, the hygroscopic nature of Li⁺ cations have been revealed for their migration in the complete device assemblage causing an enhancement in the moisture ingress (Christians et al., 2015). The use doping strategy for conductivity enhancement of HTL is been investigated rigorously but it is associated with the profound anfractuous device development methods leading to the dissimilitudinous energy levels. Furthermore, the use of Li-TFSI can highly contribute to the dilapidation of PSC device functionality deliquescence in behavior. For this reason, PSC device architectures recently have been designed with dopant free HTL composition while equally considering the need for better charge mobility aspects (Rakstys et al., 2017).

There is a considerable evidence about the diffusion of CH₃NH₃ ⁺ (MA⁺) and I⁻ ions from the active absorber layer into the Spiro-OMeTAD HTL at the room temperature up to 80 °C and above. Such diffusion triggers the inhibition of Spiro-OMeTAD oxidation and alleviation of electrical characteristics leading to the grave dilapidation of the efficiency. There is a need for development of HTMs having effectual nature with profound hydrophobicity and thermal stability to be used as an alternative to Spiro-OMeTAD. Previously, the photophysicality and photochemistry of porphyrins in addition to their derivatives have been utilized as HTLs in dye-sensitized solar cells (DSSCs) (Yella et al., 2011) and organic solar cells owing to the cogent intermolecular π-π stacking. But the current stage investigation of HTMs based on porphyrins have rarely been reported. Porphyrin derivatives containing acylhydrazone expressed commendable charge mobility characteristics (Wu et al., 2017; Zhang et al., 2016). PSC with the HTL having porphyrin derivatives-acylhydrazone expressed a PCE of 17.8% by the virtue of efficacious passivation of electronic defects of Pb atoms that are un-coordinated in the active absorber layer (Lv et al., 2019).