Volume 10, Issue 8 - Special Issue: Photonics for Enhanced Perovskite Optoelectronics; Guest Editors: Hairen Tan, Li Na Quan and Michael Saliba

Perovskite/silicon tandem solar cells are regarded as a promising candidate to surpass current efficiency limits in terrestrial photovoltaics. Tandem solar cell efficiencies meanwhile reach more than 29%. However, present high-end perovskite/silicon tandem solar cells still suffer from optical losses. We review recent numerical and experimental perovskite/silicon tandem solar cell studies and analyse the applied measures for light management. Literature indicates that highest experimental efficiencies are obtained using fully planar perovskite top cells, being in contradiction to the outcome of optical simulations calling for textured interfaces. The reason is that the preferred perovskite top cell solution-processing is often incompatible with usual micropyramidal textures of silicon bottom cells. Based on the literature survey, we propose a certain gentle nanotexture as an example to reduce optical losses in perovskite/silicon tandem solar cells. Optical simulations using the finite-element method reveal that an intermediate texture between top and bottom cell does not yield an optical benefit when compared with optimized planar designs. A double-side textured top-cell design is found to be necessary to reduce reflectance losses by the current density equivalent of 1 mA/cm 2 . The presented results illustrate a way to push perovskite/silicon tandem solar cell efficiencies beyond 30% by improved light management.

Perovskite semiconductor nanocrystals have emerged as a promising family of materials for optoelectronic applications including light-emitting diodes, lasers, light-to-electricity convertors and quantum light emitters. The performances of these devices are fundamentally dependent on different aspects of the excited-state dynamics in nanocrystals. Herein, we summarize the recent progress on the photoinduced carrier dynamics studied by a variety of time-resolved spectroscopic methods in perovskite nanocrystals. We review the dynamics of carrier generation, recombination and transport under different excitation densities and photon energies to show the pathways that underpin the photophysics for light-emitting diodes and solar cells. Then, we highlight the up-to-date spin dynamics and coherent exciton dynamics being manifested with the exciton fine levels in perovskite semiconductor nanocrystals which are essential for potential applications in quantum information technology. We also discuss the controversial results and the possible origins yet to be resolved. In-depth study toward a comprehensive picture of the excited-state dynamics in perovskite nanocrystals may provide the key knowledge of the device operation mechanism, enlighten the direction for device optimization and stimulate the adventure of new conceptual devices.

Due to stable and high power conversion efficiency (PCE), it is expected that silicon heterojunction (SHJ) solar cells will dominate the photovoltaic market. So far, the highest PCE of the SHJ-interdigitated back contact (IBC) solar cells has reached 26.7%, approximately approaching the theoretical Shockley–Queisser (SQ) limitation of 29.4%. To break through this limit, multijunction devices consisting of two or three stacked subcells have been developed, which can fully utilize the sunlight by absorbing different parts of the solar spectrum. This article provides a comprehensive overview of current research on SHJ-based tandem solar cells (SHJ-TSCs), including perovskite/SHJ TSCs and III–V/SHJ TSCs. Firstly, we give a brief introduction to the structures of SHJ-TSCs, followed by a discussion of fabrication processes. Afterwards, we focus on various materials and processes that have been explored to optimize the electrical and optical performance. Finally, we highlight the opportunities and challenges of SHJ-TSCs, as well as personal perspectives on the future development directions in this field.

Metal halide perovskites have emerged in recent years as promising photovoltaic materials due to their excellent optical and electrical properties, enabling perovskite solar cells (PSCs) with certified power conversion efficiencies (PCEs) greater than 25%. Provided radiative recombination is the dominant recombination mechanism, photon recycling – the process of reabsorption (and re-emission) of photons that result from radiative recombination – can be utilized to further enhance the PCE toward the Shockley–Queisser (S-Q) theoretical limit. Geometrical optics can be exploited for the intentional trapping of such re-emitted photons within the device, to enhance the PCE. However, this scheme reaches its fundamental diffraction limits at the submicron scale. Therefore, introducing photonic nanostructures offer attractive solutions to manipulate and trap light at the nanoscale via light coupling into guided modes, as well as localized surface plasmon and surface plasmon polariton modes. This review focuses on light-trapping schemes for efficient photon recycling in PSCs. First, we summarize the working principles of photon recycling, which is followed by a review of essential requirements to make this process efficient. We then survey photon recycling in state-of-the-art PSCs and propose design strategies to invoke light-trapping to effectively exploit photon recycling in PSCs. Finally, we formulate a future outlook and discuss new research directions in the context of photon recycling.

Low-dimensional metal halide perovskites have emerged as promising alternatives to the traditional three-dimensional (3D) components, due to their greater structural tunability and environmental stability. Dion-Jacobson (DJ) phase two-dimensional (2D) perovskites, which are formed by incorporating bulky organic diammonium cations into inorganic frameworks that comprises a symmetrically layered array, have recently attracted increasing research interest. The structure-property characteristics of DJ phase perovskites endow them with a unique combination of photovoltaic efficiency and stability, which has led to their impressive employment in perovskite solar cells (PSCs). Here, we review the achievements that have been made to date in the exploitation of DJ phase perovskites in photovoltaic applications. We summarize the various ligand designs, optimization strategies and applications of DJ phase PSCs, and examine the current understanding of the mechanisms underlying their functional behavior. Finally, we discuss the remaining bottlenecks and future outlook for these promising materials, and possible development directions of further commercial processes.

Perovskite semiconductors have experienced meteoric rise in a variety of optoelectronic applications. With a strong foothold on photovoltaics, much focus now lies on their light emission applications. Rapid progress in materials engineering have led to the demonstration of external quantum efficiencies that surpass the previously established theoretical limits. However, there remains much scope to further optimize the light propagation inside the device stack through careful tailoring of the optical processes that take place at the bulk and interface levels. Photon recycling in the emitter material followed by efficient outcoupling can result in boosting external efficiencies up to 100%. In addition, the poor ambient and operational stability of these materials and devices restrict further commercialization efforts. With best operational lifetimes of only a few hours reported, there is a long way to go before perovskite LEDs can be perceived as reliable alternatives to more established technologies like organic or quantum dot-based LED devices. This review article starts with the discussions of the mechanism of luminescence in these perovskite materials and factors impacting it. It then looks at the possible routes to achieve efficient outcoupling through nanostructuring of the emitter and the substrate. Next, we analyse the instability issues of perovskite-based LEDs from a photophysical standpoint, taking into consideration the underlying phenomena pertaining to defects, and summarize recent advances in mitigating the same. Finally, we provide an outlook on the possible routes forward for the field and propose new avenues to maximally exploit the excellent light-emitting capabilities of this family of semiconductors.

Quasi two-dimensional (2D) lead halide perovskite materials have shown outstanding performance in various photoelectric devices, including perovskite light-emitting diodes (LEDs) and perovskite optical pumping lasers. Due to the structure diversity of bulky organic cation, the photoelectric property for quasi-2D perovskite materials is flexible to be tuned. The spontaneously formed quantum-well structures allow rapid and efficient energy funneling from low- n domains to high- n domains, contributing to high exciton utilization for perovskite LEDs and low threshold for amplified spontaneous emission (ASE) and optical pumping perovskite lasers. Moreover, the hydrophobic bulky organic cations benefit to improve the environmental and operating stability owning to the better moisture tolerance and defects passivation ability. In this review, we will primarily introduce the quasi-2D lead halide perovskite materials from the structure to their optical and electrical properties. Then, we will focus on the advances of optical pumping lasers based on quasi-2D lead halide perovskite materials as gain mediums. Especially, more attention will be paid to perovskite lasers using distributed feedback (DFB) and distributed Bragg reflector (DBR) cavities. Furthermore, the key issues to realize quasi-2D perovskite-based electrical pumping lasers will be discussed.

Lead (Pb) halide perovskites have witnessed highly promising achievements for high-efficiency solar cells, light-emitting diodes (LEDs), and photo/radiation detectors due to their exceptional optoelectronic properties. However, compound stability and Pb toxicity are still two main obstacles towards the commercialization of halide perovskite-based devices. Therefore, it is of substantial interest to search for non-toxic candidates with comparable photophysical characteristics. Metal-halide double perovskites (MHDPs), A 2 BBʹX 6 , are recently booming as promising alternatives for Pb-based halide-perovskites for their non-toxicity and significantly enhanced chemical and thermodynamic stability. Moreover, this family exhibits rich combinatorial chemistry with tuneable optoelectronic properties and thus a great potential for a broad range of optoelectronic/electronic applications. Herein, we present a comprehensive review of the MHDPs synthesized so far, and classified by their optical and electronic properties. We systematically generalize their electronic structure by both theoretical and experimental efforts to prospect the relevant optoelectronic properties required by different applications. The progress of the materials in various applications is explicated in view of the material structure-function relationship. Finally, a perspective outlook to improve the physical and optoelectronic properties of the materials is proposed aiming at fostering their future development and applications.

Photodetectors based on semiconducting materials are vital building blocks for modern systems containing optoelectronic modules. Although commercial semiconductors have established good performances, they are plagued by complex processing procedures and stalled performances. Recently, lead halide perovskites with superior semiconducting attributes have achieved stunning progress in optoelectronics including photodetectors. However, the toxicity of lead and the ill stability significantly handicap their practical use. Great efforts thus have been devoted to developing lead-free alternatives with improved stability and uncompromised traits. In this review, we thoroughly summarize recent progress in photodetectors based on lead-free halide perovskite variants. The substitution of lead with new elements usually induces a change in structure and ensuingly optoelectronic particularities, which afford unique suitability for a collection of functionality-specified photodetectors. Especially, the family of lead-free variants witnesses a range of bandgaps that construct a broadband photon detection spanning from near-infrared (NIR) to visible regimes. Besides, stress is laid on the X-ray detection capability based on especially bismuth-type lead-free perovskites, of which the strong X-ray absorption, large bulk resistance, suppressed ion migration, and efficient charge collection enable superior X-ray sensitivities and ultralow detection limits. Finally, the challenges and visions are discussed.

In most perovskite nanocrystal (PeNC)-based optoelectronic and photonic applications, surface ligands inevitably lead to a donor–bridge–acceptor charge transfer configuration. In this article, we demonstrate successful modulation of electron transfer (ET) rates from all-inorganic CsPbBr 3 PeNCs to mesoporous titanium dioxide films, by using different surface ligands including single alkyl chain oleic acid and oleylamine, cross-linked insulating (3-aminopropyl)triethoxysilane and aromatic naphthoic acid molecules as the ligand-bridge. We systematically investigated the ET process through time-resolved photoluminescence spectroscopy. Calculations verified the ligand-bridge barrier effect of the three species upon the ET process. Transient absorption measurements excluded carrier-delocalization effect of the naphthoic acid ligands and confirmed the bridge-barrier effect. Our work provides a perspective for composable and appropriate ligands design for diverse practical purposes.

The unique optoelectronic properties of metal halide perovskite quantum dots (QDs) make them promising candidates for applications in light-emitting diodes (LEDs), scintillators, and other photonic devices. The automated micropipetting synthesis platform equipped with an optical reader enables the opportunity for high throughput synthesis and photoluminescent (PL) characterization of metal halide perovskite QDs for the first time. Here, we explore the compositional dependence of the PL behavior and stability of the combinatorial library of cesium lead halide (CsPbX 3 ) perovskites QDs via the automated platform. To study the stability of synthesized QDs in the binary and ternary configurations, we study the time-dependent PL properties using previously developed machine learning analysis. To systematically explore the PL behavior in the ternary CsPbX 3 QDs system, we introduce the Bayesian inference framework that allows the probabilistic fit of multiple models to the PL data and establishes both optimal model and model parameter robustly. Furthermore, these behaviors can be used as a control parameter for the navigation of the multidimensional compositional spaces in automated synthesis. This analysis shows the nonuniformity of the PL peak behavior in the ternary CsPbX 3 QDs system. Further, the analysis confirms narrow size distribution and good quality of CsPbBr 3 QDs alloyed with low concentrations of iodide and chloride. We note that Bayesian Inference fit parameters can be further used as a control signal for navigation of the chemical spaces in automated synthesis.

Since single junction c-Si solar cells are reaching their practical efficiency limit. Perovskite/c-Si tandem solar cells hold the promise of achieving greater than 30% efficiencies. In this regard, optical simulations can deliver guidelines for reducing the parasitic absorption losses and increasing the photocurrent density of the tandem solar cells. In this work, an optical study of 2, 3 and 4 terminal perovskite/c-Si tandem solar cells with c-Si solar bottom cells passivated by high thermal-budget poly-Si, poly-SiO x and poly-SiC x is performed to evaluate their optical performance with respect to the conventional tandem solar cells employing silicon heterojunction bottom cells. The parasitic absorption in these carrier selective passivating contacts has been quantified. It is shown that they enable greater than 20 mA/cm 2 matched implied photocurrent density in un-encapsulated 2T tandem architecture along with being compatible with high temperature production processes. For studying the performance of such tandem devices in real-world irradiance conditions and for different locations of the world, the effect of solar spectrum and angle of incidence on their optical performance is studied. Passing from mono-facial to bi-facial tandem solar cells, the photocurrent density in the bottom cell can be increased, requiring again optical optimization. Here, we analyse the effect of albedo, perovskite thickness and band gap as well as geographical location on the optical performance of these bi-facial perovskite/c-Si tandem solar cells. Our optical study shows that bi-facial 2T tandems, that also convert light incident from the rear, require radically thicker perovskite layers to match the additional current from the c-Si bottom cell. For typical perovskite bandgap and albedo values, even doubling the perovskite thickness is not sufficient. In this respect, lower bandgap perovskites are very interesting for application not only in bi-facial 2T tandems but also in related 3T and 4T tandems.

Wide-bandgap (wide- E g , ∼1.7 eV or higher) perovskite solar cells (PSCs) have attracted extensive attention due to the great potential of fabricating high-performance perovskite-based tandem solar cells via combining with low-bandgap absorbers, which is considered promising to exceed the Shockley–Queisser efficiency limit. However, inverted wide- E g PSCs with a minimized open-circuit voltage ( V oc ) loss, which are more suitable to prepare all-perovskite tandem devices, are still lacking study. Here, we report a strategy of adding 1,3,5-tris (bromomethyl) benzene (TBB) into wide- E g perovskite absorber to passivate the perovskite film, leading to an enhanced average V oc . Incorporation of TBB prolongs carrier lifetimes in wide- E g perovskite due to reduction of defects in perovskites and makes a better energy level matching between perovskite absorber and electron transport layer. As a result, we achieve the power conversion efficiency of 17.12% for our inverted TBB-doped PSC with an enhanced V oc of 1.19 V, compared with that (16.14%) for the control one (1.14 V).

Lead halide perovskites have attracted tremendous attention in photovoltaics due to their impressive optoelectronic properties. However, the poor stability of perovskite-based devices remains a bottleneck for further commercial development. Two-dimensional perovskites have great potential in optoelectronic devices, as they are much more stable than their three-dimensional counterparts and rapidly catching up in performance. Herein, we demonstrate high-quality two-dimensional novel perovskite thin films with alternating cations in the interlayer space. This innovative perovskite provides highly stable semiconductor thin films for efficient near-infrared light-emitting diodes (LEDs). Highly efficient LEDs with tunable emission wavelengths from 680 to 770 nm along with excellent operational stability are demonstrated by varying the thickness of the interlayer spacer cation. Furthermore, the best-performing device exhibits an external quantum efficiency of 3.4% at a high current density (J) of 249 mA/cm 2 and remains above 2.5% for a J up to 720 mA cm −2 , leading to a high radiance of 77.5 W/Sr m 2 when driven at 6 V. The same device also shows impressive operational stability, retaining almost 80% of its initial performance after operating at 20 mA/cm 2 for 350 min. This work provides fundamental evidence that this novel alternating interlayer cation 2D perovskite can be a promising and stable photonic emitter.

All-inorganic CsPbBr 3 perovskite is an attractive emission material for high-stability perovskite light-emitting diodes (PeLEDs), due to the high thermal and chemical stability. However, the external quantum efficiencies (EQEs) of CsPbBr 3 based PeLEDs are still far behind their organic–inorganic congeners. Massive defect states on the surface of CsPbBr 3 perovskite grains should be the main reason. Lewis base additives have been widely used to passivate surface defects. However, systematic investigations which relate to improving the passivation effect via rational molecule design are still lacking. Here, we demonstrate that the CsPbBr 3 film’s optical and electrical properties can be significantly boosted by tailoring the hardness–softness of the Lewis base additives. Three carboxylate Lewis bases with different tail groups are selected to in-situ passivate CsPbBr 3 perovskite films. Our research indicates that 4-(trifluoromethyl) benzoate acid anion (TBA − ) with the powerful electron-withdrawing group trifluoromethyl and benzene ring possesses the softest COO − bonding head. TBA − thus acts as a soft Lewis base and possesses a robust combination with unsaturated lead atoms caused by halogen vacancies. Based on this, the all-inorganic CsPbBr 3 PeLEDs with a maximum EQE up to 16.75% and a half-lifetime over 129 h at an initial brightness of 100 cd m −2 is thus delivered.

Metal halide perovskites have recently been reported as excellent scintillators for X-ray detection. However, perovskite based scintillators are susceptible to moisture and oxygen atmosphere, such as the water solubility of CsPbBr 3 , and oxidation vulnerability of Sn 2+ , Cu + . The traditional metal halide scintillators (NaI: Tl, LaBr 3 , etc.) are also severely restricted by their high hygroscopicity. Here we report a new kind of lead free perovskite with excellent water and radiation stability, Rb 2 Sn 1- x Te x Cl 6 . The equivalent doping of Te could break the in-phase bonding interaction between neighboring octahedra in Rb 2 SnCl 6 , and thus decrease the electron and hole dimensionality. The optimized Te content of 5% resulted in high photoluminescence quantum yield of 92.4%, and low X-ray detection limit of 0.7 µGy air s −1 . The photoluminescence and radioluminescence could be maintained without any loss when immersing in water or after 480,000 Gy radiations, outperforming previous perovskite and traditional metal halides scintillators.

All-inorganic cesium lead halide perovskite quantum dots have recently received much attention as promising optoelectronic materials with great luminescent properties and bright application prospect in lighting, lasing, and photodetection. Although notable progress has been achieved in lighting applications based on such media, the performance could still be improved. Here, we demonstrate that the light emission from the perovskite QDs that possess high intrinsic luminous efficiency can be greatly enhanced by using metallic thin films, a technique that was usually considered only useful for improving the emission of materials with low intrinsic quantum efficiency. Eleven-fold maximal PL enhancement is observed with respect to the emission of perovskite QDs on the bare dielectric substrate. We explore this remarkable enhancement of the light emission originating from the joint effects of enhancing the incident photonic absorption of QDs at the excitation wavelength by means of the zero-order optical asymmetric Fabry–Perot-like thin film interference and increasing the radiative rate and quantum efficiency at the emission wavelength mediated by surface plasmon polaritons. We believe that our approach is also potentially valuable for the enhancement of light emission of other fluorescent media with high intrinsic quantum efficiency.