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.     

Spontaneously Self‐Assembly of a 2D/3D Heterostructure Enhances the Efficiency and Stability in Printed Perovskite Solar Cells

As perovskite solar cells (PSCs) are highly efficient, demonstration of high‐performance printed devices becomes important. 2D/3D heterostructures have recently emerged as an attractive way to relieving the film inhomogeneity and instability in perovskite devices. In this work, a 2D/3D ensemble with 2D perovskites self‐assembled atop 3D methylammonium lead triiodide (MAPbI₃) via a one‐step printing process is shown. A clean and flat interface is observed in the 2D/3D bilayer heterostructure for the first time. The 2D perovskite capping layer significantly suppresses nonradiative charge recombination, resulting in a marked increase in open‐circuit voltage (V_OC) of the devices by up to 100 mV. An ultrahigh V_OC of 1.20 V is achieved for MAPbI₃ PSCs, corresponding to 91% of the Shockley–Queisser limit. Moreover, notable enhancement in light, thermal, and moisture stability is obtained as a result of the protective barrier of the 2D perovskites. These results suggest a viable approach for scalable fabrication of highly efficient perovskite solar cells with enhanced environmental stability.     

Self‐Assembled 2D Perovskite Layers for Efficient Printable Solar Cells

2D organic–inorganic hybrid Ruddlesden–Popper perovskites have emerged recently as candidates for the light‐absorbing layer in solar cell technology due largely to their impressive operational stability compared with their 3D‐perovskite counterparts. The methods reported to date for the preparation of efficient 2D perovksite layers for solar cells involve a nonscalable spin‐coating step. In this work, a facile, spin‐coating‐free, directly scalable drop‐cast method is reported for depositing precursor solutions that self‐assemble into highly oriented, uniform 2D‐perovskite films in air, yielding perovskite solar cells with power conversion efficiencies (PCE) of up to 14.9% (certified PCE of 14.33% ± 0.34 at 0.078 cm²). This is the highest PCE to date for a solar cell with 2D‐perovskite layers fabricated by nonspin‐coating method. The PCEs of the cells display no evidence of degradation after storage in a nitrogen glovebox for more than 5 months. 2D‐perovskite layer deposition using a slot‐die process is also investigated for the first time. Perovskite solar cells fabricated using batch slot‐die coating on a glass substrate or R2R slot‐die coating on a flexible substrate produced PCEs of 12.5% and 8.0%, respectively.     

Energetics and Energy Loss in 2D Ruddlesden–Popper Perovskite Solar Cells

2D Ruddlesden–Popper perovskites (RPPs) are emerging as potential challengers to their 3D counterpart due to superior stability and competitive efficiency. However, the fundamental questions on energetics of the 2D RPPs are not well understood. Here, the energetics at (PEA)₂(MA)_n?1Pb_nI_3n+1/[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) interfaces with varying n values of 1, 3, 5, 40, and ∞ are systematically investigated. It is found that n–n junctions form at the 2D RPP interfaces (n = 3, 5, and 40), instead of p–n junctions in the pure 2D and 3D scenarios (n = 1 and ∞). The potential gradient across phenethylammonium iodide ligands that significantly decreases surface work function, promotes separation of the photogenerated charge carriers with electron transferring from perovskite crystal to ligand at the interface, reducing charge recombination, which contributes to the smallest energy loss and the highest open‐circuit voltage (V_oc) in the perovskite solar cells (PSCs) based on the 2D RPP (n = 5)/PCBM. The mechanism is further verified by inserting a thin 2D RPP capping layer between pure 3D perovskite and PCBM in PSCs, causing the V_oc to evidently increase by 94 mV. Capacitance–voltage measurements with Mott–Schottky analysis demonstrate that such V_oc improvement is attributed to the enhanced potential at the interface.     

New Strategies for Defect Passivation in High‐Efficiency Perovskite Solar Cells

Lead halide perovskite solar cells now show excellent efficiencies and encouraging levels of stability. Further improvements in performance require better control of the trap states which are considered to be associated with vacancies and defects at crystallite surfaces. Herein, a reflection on the ways in which these traps can be mitigated is presented by improving the quality of the perovskite layer and interfaces in fully assembled device configurations. In this review, the most recent design strategies reported in the literature, which have been explored to tune grain orientation, to passivate defects, and to improve charge‐carrier lifetimes, are presented. Specifically, the advances made with single‐cation, mixed‐cation and/or mixed‐halide, and 3D/2D bilayer‐based light absorbers are discussed. The interfacial, compositional, and band alignment engineering along with their consequent effects on the open‐circuit voltage, power conversion efficiency, and stability are a particular focus.     

Reverse Bias Behavior of Halide Perovskite Solar Cells

The future commercialization of halide perovskite solar cells relies on improving their stability. There are several studies focused on understanding degradation under operating conditions in light, but little is known about the stability of these solar cells under reverse bias conditions. Reverse bias stability is important because shaded cells in a module are put into reverse bias by the illuminated cells. In this paper, a phenomenological study is presented of the reverse bias behavior of halide perovskite solar cells and it is shown that reverse bias can lead to a partially recoverable loss in efficiency, primarily caused by a decrease in V _OC. A general mechanism is proposed, supported by drift–diffusion simulations, to explain how these cells breakdown via tunneling caused by accumulated ionic defects and suggests that the reversible loss in efficiency may be due to an electrochemical reaction of these defects. Finally, the implications of these phenomena are discussed and how they can possibly be addressed is also discussed.     

Oxygen Degradation in Mesoporous Al2O3/CH3NH3PbI3‐xClx Perovskite Solar Cells: Kinetics and Mechanisms

The rapid pace of development for hybrid perovskite photovoltaics has recently resulted in promising figures of merit being obtained with regard to device stability. Rather than relying upon expensive barrier materials, realizing market‐competitive lifetimes is likely to require the development of intrinsically stable devices, and to this end accelerated aging tests can help identify degradation mechanisms that arise over the long term. Here, oxygen‐induced degradation of archetypal perovskite solar cells under operation is observed, even in dry conditions. With prolonged aging, this process ultimately drives decomposition of the perovskite. It is deduced that this is related to charge build‐up in the perovskite layer, and it is shown that by efficiently extracting charge this degradation can be mitigated. The results confirm the importance of high charge‐extraction efficiency in maximizing the tolerance of perovskite solar cells to oxygen.     

Mixed 3D–2D Passivation Treatment for Mixed‐Cation Lead Mixed‐Halide Perovskite Solar Cells for Higher Efficiency and Better Stability

Layered low‐dimensional perovskite structures employing bulky organic ammonium cations have shown significant improvement on stability but poorer performance generally compared to their 3D counterparts. Here, a mixed passivation (MP) treatment is reported that uses a mixture of bulky organic ammonium iodide (iso‐butylammonium iodide, iBAI) and formammidinium iodide (FAI), enhancing both power conversion efficiency and stability. Through a combination of inactivation of the interfacial trap sites, characterized by photoluminescence measurement, and formation of an interfacial energetic barrier by which ionic transport is reduced, demonstrated by Kelvin probe force microscopy, MP treatment of the perovskite/hole transport layer interface significantly suppresses photocurrent hysteresis. Using this MP treatment, the champion mixed‐halide perovskite cell achieves a reverse scan and stabilized power conversion efficiency of 21.7%. Without encapsulation, the devices show excellent moisture stability, sustaining over 87% of the original performance after 38 d storage in ambient environment under 75 ± 20% relative humidity. This work shows that FAI/ i BAI, is a new and promising material combination for passivating perovskite/selective‐contact interfaces.     

Reverse‐Graded 2D Ruddlesden–Popper Perovskites for Efficient Air‐Stable Solar Cells

2D Ruddlesden–Popper perovskites (RPPs) have emerged as a promising solar cell material. A group of novel RPPs with cyclohexane methylamine (CMA) as a spacer cation is presented. Unlike previously reported RPPs, the deposited films of (CMA)₂(MA)_n?1Pb_nI_3n+1 (MA is CH₃NH₃⁺, n = 1, 2, 3, …) exhibit multiple phases with reverse‐graded quantum well (QW) distribution; small n (n = 2) RPPs are located at the surface and large n (n ≥ 10) RPPs at the bottom. This has three advantages: (a) The outer, more moisture resistant, small n RPPs create a stable barrier that protects the vulnerable large n RPP lattice from being attacked by water molecules. (b) It forms a type‐II band alignment between different phases, which favors self‐driven charge transport. (c) The natural structure of graded QWs expands the range of photon collection. Attributed to these properties, the best efficiency of 15.05%, with high open‐circuit voltage (V_oc) of 1.10 V for a first‐generation solar cell containing (CMA)₂(MA)₈Pb₉I₂₈, is achieved. A notable enhancement in short wavelength is observed in the Incident photon‐to‐current conversion efficiency spectra. This device shows significantly improved long‐term stability, retaining ≈95% of the initial efficiency after 4600 h exposure in ambient conditions with 40–70% relative humidity.     

Efficient Solution‐Processed Bulk Heterojunction Perovskite Hybrid Solar Cells

Efficient conventional bulk heterojunction (BHJ) perovskite hybrid solar cells (pero‐HSCs) solution‐processed from a composite of CH₃NH₃PbI₃ mixed with PC₆₁BM ([6,6]‐phenyl‐C61‐butyric acid methyl ester), where CH₃NH₃PbI₃ acts as the electron donor and PC₆₁BM acts as the electron acceptor, are reported for the first time. The efficiency of 12.78% is twofold enhancement in comparison with the conventional planar heterojunction pero‐HSCs (6.90%) fabricated by pristine CH₃NH₃PbI₃. The BHJ pero‐HSCs are further optimized by using PC₆₁BM/TiO₂ bi‐electron‐extraction‐layer (EEL), which are both solution‐processed and then followed with low‐temperature thermal annealing. Due to higher electrical conductivity of PC₆₁BM over that of TiO₂, an efficiency of 14.98%, the highest reported efficiency for the pero‐HSCs without incorporating high‐temperature‐processed mesoporous TiO₂ and Al₂O₃ as the EEL and insulating scaffold, is observed from PC₆₁BM modified BHJ pero‐HSCs. Thus, the findings provide a simple way to approach high efficiency low‐cost pero‐HSCs.     

Ruddlesden–Popper 2D Component to Stabilize γ‐CsPbI3 Perovskite Phase for Stable and Efficient Photovoltaics

The highest certified power conversion efficiency (PCE) of black phase based CsPbI₃ perovskite solar cells has exceeded 18%, and become a hotspot in recent progress. However, the black phase of CsPbI₃ rapidly transforms to yellow phase in ambient conditions due to its thermodynamic instability. Here, a Ruddlesden–Popper 2D structure is introduced into γ‐CsPbI₃ film to stabilize the black phase via reducing dimensionality. It is found that a judicious amount of phenylethylammonium iodide can adjust the dimensionality of γ‐CsPbI₃ film from 2D to quasi‐2D and 3D phase. Comprehensive consideration to obtain both the stability and high PCE, quasi‐2D (n = 40) γ‐CsPbI₃ delivers a reproducible PCE of 13.65% with negligible hysteresis. By utilizing femtosecond transient absorption and time‐resolved PL decay, similar carrier kinetics in n = 40 and ∞ samples are observed, meaning an efficient charge extraction. More importantly, when the device is placed at 80 °C in N₂ condition or in air with RH of 25–30%, its PCE keeps ≈88% and ≈89% of its initial PCE after 12 days, respectively. Such results are better than the 3D one (≈69% and ≈16%, respectively).     

Metal Halide Perovskite Materials for Solar Cells with Long‐Term Stability

Metal halide perovskite solar cells (PSCs) have emerged as promising candidates for photovoltaic technology with their power conversion efficiencies over 23%. For prototypical organic–inorganic metal halide perovskites, their intrinsic instability poses significant challenges to the commercialization of PSCs. Recently, the scientific community has done tremendous work in composition engineering to develop more robust light‐absorbing layers, including mixed‐ion hybrid perovskites, low‐dimensional hybrid perovskites, and all‐inorganic perovskites. This review provides an overview of the impact of these perovskites on the efficiency and long‐term stability of PSCs.     

Inorganic CsPb1−xSnxIBr2 for Efficient Wide‐Bandgap Perovskite Solar Cells

Recently, the stability of organic–inorganic perovskite thin films under thermal, photo, and moisture stresses has become a major concern for further commercialization due to the high volatility of the organic cations in the prototype perovskite composition (CH₃NH₃PbI₃). All inorganic cesium (Cs) based perovskite is an alternative to avoid the release or decomposition of organic cations. Moreover, substituting Pb with Sn in the organic–inorganic lead halide perovskites has been demonstrated to narrow the bandgap to 1.2–1.4 eV for high‐performance perovskite solar cells. In this work, a series of CsPb_1?xSn_xIBr₂ perovskite alloys via one‐step antisolvent method is demonstrated. These perovskite films present tunable bandgaps from 2.04 to 1.64 eV. Consequently, the CsPb_0.75Sn_0.25IBr₂ with homogeneous and densely crystallized morphology shows a remarkable power conversion efficiency of 11.53% and a high V_oc of 1.21 V with a much improved phase stability and illumination stability. This work provides a possibility for designing and synthesizing novel inorganic halide perovskites as the next generation of photovoltaic materials.     

2‐Thiopheneformamidinium‐Based 2D Ruddlesden–Popper Perovskite Solar Cells with Efficiency of 16.72% and Negligible Hysteresis

Formamidinium (FA)‐based 3D perovskite solar cells (PSCs) have been widely studied and they show reduced bandgap, enhanced stability, and improved efficiency compared to MAPbI₃‐based devices. Nevertheless, the FA‐based spacers have rarely been studied for 2D Ruddlesden–Popper (RP) perovskites, which have drawn wide attention due to their enormous potential for fabricating efficient and stable photovoltaic devices. Here, for the first time, FA‐based derivative, 2‐thiopheneformamidinium (ThFA), is successfully synthesized and employed as an organic spacer for 2D RP PSCs. A precursor organic salts‐assisted crystal growth technique is further developed to prepare high quality 2D (ThFA)₂(MA)_n?1Pb_nI_3n+1 (nominal n = 3) perovskite films, which shows preferential vertical growth orientations, high charge carrier mobilities, and reduced trap density. As a result, the 2D RP PSCs with an inverted planar p‐i‐n structure exhibit a dramatically improved power conversion efficiency (PCE) from 7.23% to 16.72% with negligible hysteresis, which is among the highest PCE in 2D RP PSCs with low nominal n‐value of 3. Importantly, the optimized 2D PSCs exhibit a dramatically improved stability with less than 1% degradation after storage in N₂ for 3000 h without encapsulation. These findings provide an effective strategy for developing FA‐based organic spacers toward highly efficient and stable 2D PSCs.     

Control over Self‐Doping in High Band Gap Perovskite Films

It is reported how differences in the composition of high bandgap Pb bromide‐based perovskites affect their carrier diffusion length and junction type. Pb‐based, APbX₃, halide perovskite (HaP) films and devices are studied, where A can be a mixture of formamidinium, methylammonium (MA), and Cs, and X a mixture of Br and Cl, using a combination of dark‐ and photoconductivity and steady‐state photocarrier grating. The results show the cation and anion compositions affect both majority and minority carrier diffusion lengths. In particular, using electron beam‐induced current measurements, FTO\dTiO₂mp‐TiO₂HaP\PTAA (poly‐triarylamine)\Au devices are studied. The results enable identifying junction and built‐in voltage formation and track position and size of the space charge region width with changes in the HaP composition. As far as it is known, it is found for the first time that a mixed‐cation HaP forms a junction that has characteristics of a p‐i‐n one, with relatively long and comparable carrier diffusion lengths, while the single cation‐based bromide HaPs form clear p‐n junctions at the interface with the TiO₂ [pure CsPbBr₃ and MAPbBr₃(Cl)] or a buried one (MAPbBr₃) and shorter diffusion lengths. These differences are attributed to lower carrier density in MAPbBr₃, and especially in the mixed cation HaP, which is comparable to iodide‐based HaP films.