Perovskite Tandem Solar Cells

The meteoric rise of perovskite single‐junction solar cells has been accompanied by similar stunning developments in perovskite tandem solar cells. Debuting with efficiencies less than 14% in 2014, silicon–perovskite solar cells are now above 25% and will soon surpass record silicon single‐junction efficiencies. Unconstrained by the Shockley–Quiesser single‐junction limit, perovskite tandems suggest a real possibility of true third‐generation thin‐film photovoltaics; monolithic all‐perovskite tandems have reached 18% efficiency and will likely pass perovskite single‐junction efficiencies within the next 5 years. Inorganic–organic metal–halide perovskites are ideal candidates for inclusion in tandem solar cells due to their high radiative recombination efficiencies, excellent absorption, long‐range charge‐transport, and broad ability to tune the bandgap. In this progress report, the development of perovskite tandem cells is reviewed, with presentation of their key motivations and challenges. In detail, it presents an overview of recombination layer materials, bandgap‐tuneability, transparent contact architectures, and perovskite compounds for use in tandems. Theoretical estimates of efficiency for future tandem and triple‐junction perovskite cells are presented, outlining roadmaps for future focused research.     

Imaging Spatial Variations of Optical Bandgaps in Perovskite Solar Cells

A fast, nondestructive, camera‐based method to capture optical bandgap images of perovskite solar cells (PSCs) with micrometer‐scale spatial resolution is developed. This imaging technique utilizes well‐defined and relatively symmetrical band‐to‐band luminescence spectra emitted from perovskite materials, whose spectral peak locations coincide with absorption thresholds and thus represent their optical bandgaps. The technique is employed to capture relative variations in optical bandgaps across various PSCs, and to resolve optical bandgap inhomogeneity within the same device due to material degradation and impurities. Degradation and impurities are found to both cause optical bandgap shifts inside the materials. The results are confirmed with micro‐photoluminescence spectroscopy scans. The excellent agreement between the two techniques opens opportunities for this imaging concept to become a quantified, high spatial resolution, large‐area characterization tool of PSCs. This development continues to strengthen the high value of luminescence imaging for the research and development of this photovoltaic technology.     

Trap States,Electric Fields,and Phase Segregation in Mixed‐Halide Perovskite Photovoltaic Devices

Mixed‐halide perovskites are essential for use in all‐perovskite or perovskite–silicon tandem solar cells due to their tunable bandgap. However, trap states and halide segregation currently present the two main challenges for efficient mixed‐halide perovskite technologies. Here photoluminescence techniques are used to study trap states and halide segregation in full mixed‐halide perovskite photovoltaic devices. This work identifies three distinct defect species in the perovskite material: a charged, mobile defect that traps charge‐carriers in the perovskite, a charge‐neutral defect that induces halide segregation, and a charged, mobile defect that screens the perovskite from external electric fields. These three defects are proposed to be MA⁺ interstitials, crystal distortions, and halide vacancies and/or interstitials, respectively. Finally, external quantum efficiency measurements show that photoexcited charge‐carriers can be extracted from the iodide‐rich low‐bandgap regions of the phase‐segregated perovskite formed under illumination, suggesting the existence of charge‐carrier percolation pathways through grain boundaries where phase‐segregation may occur.     

Benzylamine‐Treated Wide‐Bandgap Perovskite with High Thermal‐Photostability and Photovoltaic Performance

Mixed iodide‐bromide organolead perovskites with a bandgap of 1.70–1.80 eV have great potential to boost the efficiency of current silicon solar cells by forming a perovskite‐silicon tandem structure. Yet, the stability of the perovskites under various application conditions, and in particular combined light and heat stress, is not well studied. Here, FA_0.15Cs_0.85Pb(I_0.73Br_0.27)₃, with an optical bandgap of ≈1.72 eV, is used as a model system to investigate the thermal‐photostability of wide‐bandgap mixed halide perovskites. It is found that the concerted effect of heat and light can induce both phase segregation and decomposition in a pristine perovskite film. On the other hand, through a postdeposition film treatment with benzylamine (BA) molecules, the highly defective regions (e.g., film surface and grain boundaries) of the film can be well passivated, thus preventing the progression of decomposition or phase segregation in the film. Besides the stability improvement, the BA‐modified perovskite solar cells also exhibit excellent photovoltaic performance, with the champion device reaching a power conversion efficiency of 18.1%, a stabilized power output efficiency of 17.1% and an open‐circuit voltage (V _oc) of 1.24 V.     

Monolithic Perovskite Tandem Solar Cells: A Review of the Present Status and Advanced Characterization Methods Toward 30% Efficiency

Tandem solar cells are the next step in the photovoltaic (PV) evolution due to their higher power conversion efficiency (PCE) potential than currently dominating, but inherently limited, single‐junction solar cells. With the emergence of metal halide perovskite absorber materials, the fabrication of highly efficient tandem solar cells, at a reasonable cost, can significantly impact the future PV landscape. The perovskite‐based tandem solar cells have already shown that they can convert light more efficiently than their standalone sub‐cells. However, to reach PCEs over 30%, several challenges have to be overcome and the understanding of this fascinating technology has to be broadened. In this review, the main scientific and engineering challenges in the field are presented, alongside a discussion of the current status of three main perovskite tandem technologies: perovskite/silicon, perovskite/CIGS, and perovskite/perovskite tandem solar cells. A summary of the advanced structural, electrical, optical, radiative, and electronic characterization methods as well as simulations being utilized for perovskite‐based tandem solar cells is presented. The main findings are summarized and the strength of the techniques to overcome the challenges and gain deeper knowledge for further performance improvement is assessed. Finally, the PCE potential in different experimental and theoretical limits is compared with an aim to shed light on the path towards overcoming the 30% efficiency threshold for all of the three herein reviewed tandem technologies.     

Fabrication of Efficient and Stable CsPbI3 Perovskite Solar Cells through Cation Exchange Process

Inorganic lead halide perovskites have attracted attention due to their tolerance to higher processing temperature and higher bandgap suitable for tandem solar cell application. Not only do they improve cell stability and efficiency, they also reveal many interesting and un‐anticipated material qualities. This work reports a simple cation exchange growth (CEG) method for fabricating inorganic high‐quality cesium lead iodide (CsPbI₃) by adding methylammonium iodide (MAI) additive in the precursor. X‐ray diffraction results reveal a multi‐stage film formation process whereby i) MAPbI₃ perovskite first formed that acts as a perovskite template for ii) subsequent ion exchange whereby the MA⁺ ions in the MAPbI₃ are replaced by Cs⁺ (as temperature ramps up) and iii) form g‐phase perovskite CsPbI₃. Optical microscopy, photoluminescence, and electrical characterizations reveal that the CEG process produces high‐quality film with better absorption, uniform and dense film with better interface, lower defects, and better stability. Using the CEG approach, the power conversion efficiency of the best CsPbI₃ solar cell is significantly increased up to 14.1% for the device fabricated using 1.0 m MAI additive. The outcome is beneficial for further improvement of inorganic perovskite solar cells and their application in perovskite‐silicon tandem devices.     

Improved Optics in Monolithic Perovskite/Silicon Tandem Solar Cells with a Nanocrystalline Silicon Recombination Junction

Perovskite/silicon tandem solar cells are increasingly recognized as promi­sing candidates for next‐generation photovoltaics with performance beyond the single‐junction limit at potentially low production costs. Current designs for monolithic tandems rely on transparent conductive oxides as an intermediate recombination layer, which lead to optical losses and reduced shunt resistance. An improved recombination junction based on nanocrystalline silicon layers to mitigate these losses is demonstrated. When employed in monolithic perovskite/silicon heterojunction tandem cells with a planar front side, this junction is found to increase the bottom cell photocurrent by more than 1 mA cm^?2. In combination with a cesium‐based perovskite top cell, this leads to tandem cell power‐conversion efficiencies of up to 22.7% obtained from J–V measurements and steady‐state efficiencies of up to 22.0% during maximum power point tracking. Thanks to its low lateral conductivity, the nanocrystalline silicon recombination junction enables upscaling of monolithic perovskite/silicon heterojunction tandem cells, resulting in a 12.96 cm² monolithic tandem cell with a steady‐state efficiency of 18%.     

Reducing Saturation‐Current Density to Realize High‐Efficiency Low‐Bandgap Mixed Tin–Lead Halide Perovskite Solar Cells

The unsatisfactory performance of low‐bandgap mixed tin (Sn)–lead (Pb) halide perovskite subcells has been one of the major obstacles hindering the progress of the power conversion efficiencies (PCEs) of all‐perovskite tandem solar cells. By analyzing dark‐current density and distribution, it is identified that charge recombination at grain boundaries is a key factor limiting the performance of low‐bandgap mixed Sn–Pb halide perovskite subcells. It is further found that bromine (Br) incorporation can effectively passivate grain boundaries and lower the dark current density by two–three orders of magnitude. By optimizing the Br concentration, low‐bandgap (1.272 eV) mixed Sn–Pb halide perovskite solar cells are fabricated with open‐circuit voltage deficits as low as 0.384 V and fill factors as high as 75%. The best‐performing device demonstrates a PCE of >19%. The results suggest an important direction for improving the performance of low‐bandgap mixed Sn–Pb halide perovskite solar cells.     

Vacuum‐Assisted Growth of Low‐Bandgap Thin Films (FA0.8MA0.2Sn0.5Pb0.5I3) for All‐Perovskite Tandem Solar Cells

All‐perovskite multijunction photovoltaics, combining a wide‐bandgap (WBG) perovskite top solar cell (E_G ≈1.6–1.8 eV) with a low‐bandgap (LBG) perovskite bottom solar cell (E_G < 1.3 eV), promise power conversion efficiencies (PCEs) >33%. While the research on WBG perovskite solar cells has advanced rapidly over the past decade, LBG perovskite solar cells lack PCE as well as stability. In this work, vacuum‐assisted growth control (VAGC) of solution‐processed LBG perovskite thin films based on mixed Sn–Pb perovskite compositions is reported. The reported perovskite thin films processed by VAGC exhibit large columnar crystals. Compared to the well‐established processing of LBG perovskites via antisolvent deposition, the VAGC approach results in a significantly enhanced charge‐carrier lifetime. The improved optoelectronic characteristics enable high‐performance LBG perovskite solar cells (1.27 eV) with PCEs up to 18.2% as well as very efficient four‐terminal all‐perovskite tandem solar cells with PCEs up to 23%. Moreover, VAGC leads to promising reproducibility and potential in the fabrication of larger active‐area solar cells up to 1 cm².     

Record Open‐Circuit Voltage Wide‐Bandgap Perovskite Solar Cells Utilizing 2D/3D Perovskite Heterostructure

In this work, the authors realize stable and highly efficient wide‐bandgap perovskite solar cells that promise high power conversion efficiencies (PCE) and are likely to play a key role in next generation multi‐junction photovoltaics (PV). This work reports on wide‐bandgap (≈1.72 eV) perovskite solar cells exhibiting stable PCEs of up to 19.4% and a remarkably high open‐circuit voltage (V_OC) of 1.31 V. The V_OC‐to‐bandgap ratio is the highest reported for wide‐bandgap organic?inorganic hybrid perovskite solar cells and the V_OC also exceeds 90% of the theoretical maximum, defined by the Shockley–Queisser limit. This advance is based on creating a hybrid 2D/3D perovskite heterostructure. By spin coating n‐butylammonium bromide on the double‐cation perovskite absorber layer, a thin 2D Ruddlesden–Popper perovskite layer of intermediate phases is formed, which mitigates nonradiative recombination in the perovskite absorber layer. As a result, V_OC is enhanced by 80 mV.     

Understanding the Film Formation Kinetics of Sequential Deposited Narrow‐Bandgap Pb–Sn Hybrid Perovskite Films

Developing efficient narrow bandgap Pb–Sn hybrid perovskite solar cells with high Sn‐content is crucial for perovskite‐based tandem devices. Film properties such as crystallinity, morphology, surface roughness, and homogeneity dictate photovoltaic performance. However, compared to Pb‐based analogs, controlling the formation of Sn‐containing perovskite films is much more challenging. A deeper understanding of the growth mechanisms in Pb–Sn hybrid perovskites is needed to improve power conversion efficiencies. Here, in situ optical spectroscopy is performed during sequential deposition of Pb–Sn hybrid perovskite films and combined with ex situ characterization techniques to reveal the temporal evolution of crystallization in Pb–Sn hybrid perovskite films. Using a two‐step deposition method, homogeneous crystallization of mixed Pb–Sn perovskites can be achieved. Solar cells based on the narrow bandgap (1.23 eV) FA_0.66MA_0.34Pb_0.5Sn_0.5I₃ perovskite absorber exhibit the highest efficiency among mixed Pb–Sn perovskites and feature a relatively low dark carrier density compared to Sn‐rich devices. By passivating defect sites on the perovskite surface, the device achieves a power conversion efficiency of 16.1%, which is the highest efficiency reported for sequential solution‐processed narrow bandgap perovskite solar cells with 50% Sn‐content.     

Graded Bandgap Perovskite with Intrinsic n–p Homojunction Expands Photon Harvesting Range and Enables All Transport Layer‐Free Perovskite Solar Cells

Organic–inorganic halide perovskites are promising materials for next‐generation photovoltaic device due to their attractive photoelectrical properties such as strong light absorption, high carrier mobility, and tunable bandgap. Generally, perovskite solar cells require carrier transport layers (CTL) to provide a built‐in electric field and reduce the recombination rate. However, the construction of suitable electron‐ and hole‐transport layers is not cost effective, impairing the commercial application of the devices. An n–p perovskite homojunction absorber with a graded bandgap is developed by introducing a three‐step dynamic spin‐coating strategy and variable valence Sn elements. The bandgap of the perovskite absorber is gradually manipulated from 1.53 eV (the bottom) to 1.27 eV (the top). The electronic behavior is also transformed from n‐type (excess PbI₂, the bottom) to p‐type (Sn vacancy, the top) in a very short distance (50 nm). This designed perovskite homojunction electronic structure not only expands the light harvesting range from 800 to 970 nm which provides potential to break the PCE limits, but also promotes oriented carrier transportation and weakens the dependence on CTL. The demonstrated asymmetrical active layer shows a brand‐new approach to simplify the device structure and boost the performance of CTL‐free perovskite solar cells.     

Wide‐Bandgap Perovskite/Gallium Arsenide Tandem Solar Cells

Gallium arsenide (GaAs) photovoltaic (PV) cells have been widely investigated due to their merits such as thin‐film feasibility, flexibility, and high efficiency. To further increase their performance, a wider bandgap PV structure such as indium gallium phosphide (InGaP) has been integrated in two‐terminal (2T) tandem configuration. However, it increases the overall fabrication cost, complicated tunnel‐junction diode connecting subcells are inevitable, and materials are limited by lattice matching. Here, high‐efficiency and stable wide‐bandgap perovskite PVs having comparable bandgap to InGaP (1.8–1.9 eV) are developed, which can be stable low‐cost add‐on layers to further enhance the performance of GaAs PVs as tandem configurations by showing an efficiency improvement from 21.68% to 24.27% (2T configuration) and 25.19% (4T configuration). This approach is also feasible for thin‐film GaAs PV, essential to reduce its fabrication cost for commercialization, with performance increasing from 21.85% to 24.32% and superior flexibility (1000 times bending) in a tandem configuration. Additionally, potential routes to over 30% stable perovskite/GaAs tandems, comparable to InGaP/GaAs with lower cost, are considered. This work can be an initial step to reach the objective of improving the usability of GaAs PV technology with enhanced performance for applications for which lightness and flexibility are crucial, without a significant additional cost increase.     

Infrared Light Management Using a Nanocrystalline Silicon Oxide Interlayer in Monolithic Perovskite/Silicon Heterojunction Tandem Solar Cells with Efficiency above 25%

Perovskite/silicon tandem solar cells are attractive for their potential for boosting cell efficiency beyond the crystalline silicon (Si) single‐junction limit. However, the relatively large optical refractive index of Si, in comparison to that of transparent conducting oxides and perovskite absorber layers, results in significant reflection losses at the internal junction between the cells in monolithic (two‐terminal) devices. Therefore, light management is crucial to improve photocurrent absorption in the Si bottom cell. Here it is shown that the infrared reflection losses in tandem cells processed on a flat silicon substrate can be significantly reduced by using an optical interlayer consisting of nanocrystalline silicon oxide. It is demonstrated that 110 nm thick interlayers with a refractive index of 2.6 (at 800 nm) result in 1.4 mA cm^?² current gain in the silicon bottom cell. Under AM1.5G irradiation, the champion 1 cm² perovskite/silicon monolithic tandem cell exhibits a top cell + bottom cell total current density of 38.7 mA cm^?2 and a certified stabilized power conversion efficiency of 25.2%.     

Targeting Ideal Dual‐Absorber Tandem Water Splitting Using Perovskite Photovoltaics and CuInxGa1‐xSe2 Photocathodes

Efficient sunlight‐driven water splitting devices can be achieved by pairing two absorbers of different optimized bandgaps in an optical tandem design. With tunable absorption ranges and cell voltages, organic–inorganic metal halide perovskite solar cells provide new opportunities for tailoring top absorbers for such devices. In this work, semitransparent perovskite solar cells are developed for use as the top cell in tandem with a smaller bandgap photocathode to enable panchromatic harvesting of the solar spectrum. A new CuIn_xGa_1‐xSe₂ multilayer photocathode is designed, exhibiting excellent performance for photoelectrochemical water reduction and representing a near‐ideal bottom absorber. When pairing it below a semitransparent CH₃NH₃PbBr₃‐based solar cell, a solar‐to‐hydrogen efficiency exceeding 6% is achieved, the highest value yet reported for a photovoltaic–photoelectrochemical device utilizing a single‐junction solar cell as the bias source under one sun illumination. The analysis shows that the efficiency can reach more than 20% through further optimization of the perovskite top absorber.     

High Efficiency Perovskite‐Silicon Tandem Solar Cells: Effect of Surface Coating versus Bulk Incorporation of 2D Perovskite

Mixed‐dimensional perovskite solar cells combining 3D and 2D perovskites have recently attracted wide interest owing to improved device efficiency and stability. Yet, it remains unclear which method of combining 3D and 2D perovskites works best to obtain a mixed‐dimensional system with the advantages of both types. To address this, different strategies of combining 2D perovskites with a 3D perovskite are investigated, namely surface coating and bulk incorporation. It is found that through surface coating with different aliphatic alkylammonium bulky cations, a Ruddlesden–Popper “quasi‐2D” perovskite phase is formed on the surface of the 3D perovskite that passivates the surface defects and significantly improves the device performance. In contrast, incorporating those bulky cations into the bulk induces the formation of the pure 2D perovskite phase throughout the bulk of the 3D perovskite, which negatively affects the crystallinity and electronic structure of the 3D perovskite framework and reduces the device performance. Using the surface‐coating strategy with n‐butylammonium bromide to fabricate semitransparent perovskite cells and combining with silicon cells in four‐terminal tandem configuration, 27.7% tandem efficiency with interdigitated back contact silicon bottom cells (size‐unmatched) and 26.2% with passivated emitter with rear locally diffused silicon bottom cells is achieved in a 1 cm² size‐matched tandem.     

Hybrid Perovskite‐Organic Flexible Tandem Solar Cell Enabling Highly Efficient Electrocatalysis Overall Water Splitting

Perovskite‐organic tandem solar cells are attracting more attention due to their potential for highly efficient and flexible photovoltaic device. In this work, efficient perovskite‐organic monolithic tandem solar cells integrating the wide bandgap perovskite (1.74 eV) and low bandgap organic active PBDB‐T:SN6IC‐4F (1.30 eV) layer, which serve as the top and bottom subcell, respectively, are developed. The resulting perovskite‐organic tandem solar cells with passivated wide‐bandgap perovskite show a remarkable power conversion efficiency (PCE) of 15.13%, with an open‐circuit voltage (V_oc) of 1.85 V, a short‐circuit photocurrent (J_sc) of 11.52 mA cm^?2, and a fill factor (FF) of 70.98%. Thanks to the advantages of low temperature fabrication processes and the flexibility properties of the device, a flexible tandem solar cell which obtain a PCE of 13.61%, with V_oc of 1.80 V, J_sc of 11.07 mA cm^?2, and FF of 68.31% is fabricated. Moreover, to demonstrate the achieved high V_oc in the tandem solar cells for potential applications, a photovoltaic (PV)‐driven electrolysis system combing the tandem solar cell and water splitting electrocatalysis is assembled. The integrated device demonstrates a solar‐to‐hydrogen efficiency of 12.30% and 11.21% for rigid, and flexible perovskite‐organic tandem solar cell based PV‐driven electrolysis systems, respectively.     

Progress in Tandem Solar Cells Based on Hybrid Organic–Inorganic Perovskites

Owing to their high efficiency, low‐cost solution‐processability, and tunable bandgap, perovskite solar cells (PSCs) made of hybrid organic‐inorganic perovskite (HOIP) thin films are promising top‐cell candidates for integration with bottom‐cells based on Si or other low‐bandgap solar‐cell materials to boost the power conversion efficiency (PCE) beyond the Shockley‐Quiesser (S‐Q) limit. In this review, recent progress in such tandem solar cells based on the emerging PSCs is summarized and reviewed critically. Notable achievements for different tandem solar cell configurations including mechanically‐stacked, optical coupling, and monolithically‐integrated with PSCs as top‐cells are described in detail. Highly‐efficient semitransparent PSC top‐cells with high transmittance in near‐infrared (NIR) region are critical for tandem solar cells. Different types of transparent electrodes with high transmittance and low sheet‐resistance for PSCs are reviewed, which presents a grand challenge for PSCs. The strategies to obtain wide‐bandgap PSCs with good photo‐stability are discussed. The PCE reduction due to reflection loss, parasitic absorption, electrical loss, and current mismatch are analyzed to provide better understanding of the performance of PSC‐based tandem solar cells.     

How to Report Record Open‐Circuit Voltages in Lead‐Halide Perovskite Solar Cells

Open‐circuit voltages of lead‐halide perovskite solar cells are improving rapidly and are approaching the thermodynamic limit. Since many different perovskite compositions with different bandgap energies are actively being investigated, it is not straightforward to compare the open‐circuit voltages between these devices as long as a consistent method of referencing is missing. For the purpose of comparing open‐circuit voltages and identifying outstanding values, it is imperative to use a unique, generally accepted way of calculating the thermodynamic limit, which is currently not the case. Here a meta‐analysis of methods to determine the bandgap and a radiative limit for open‐circuit voltage is presented. The differences between the methods are analyzed and an easily applicable approach based on the solar cell quantum efficiency as a general reference is proposed.     

Long‐Lasting Nanophosphors Applied to UV‐Resistant and Energy Storage Perovskite Solar Cells

Recently, considerable progress is achieved in lab prototype perovskite solar cells (PSCs); however, the stability of outdoor applications of PSCs remains a challenge due to the high sensitivity of perovskite material under moist and ultraviolet (UV) light conditions. In this work, the UV photostability of PSC devices is improved by incorporating a photon downshifting layer—SrAl₂O₄: Eu²⁺, Dy³⁺ (SAED)—prepared using the pulsed laser deposition approach. Light‐induced deep trap states in the photoactive layer are depressed, and UV light‐induced device degradation is inhibited after the SAED modification. Optimized power conversion efficiency (PCE) of 17.8% is obtained through the enhanced light harvesting and reduced carrier recombination provided by SAED. More importantly, a solar energy storage effect due to the long‐persistent luminescence of SAED is obtained after light illumination is turned off. The introduction of downconverting material with long‐persistent luminescence in PSCs not only represents a new strategy to improve PCE and light stability by photoconversion from UV to visible light but also provides a new paradigm for solar energy storage.