Ligang Yuan

Perovskite solar cells still have huge room for improvement in photoelectric conversion efficiency. One of the constraints is the defects at the interface between the perovskite and the transport layer. Passivation is considered a key measure to limit defects. This paper systematically categorizes the effective passivation strategies for perovskites in recent years and gives a future outlook.

Abstract

The disorderly distribution of defects in the perovskite or at the grain boundaries, surfaces, and interfaces, which seriously affect carrier transport through the formation of nonradiative recombination centers, hinders the further improvement on the power conversion efficiency (PCE) of perovskite solar cells (PSCs). Several defect passivation strategies have been confirmed as an efficient approach for promoting the performance of PSCs. Herein, recent progress in the defect passivation toward efficient perovskite solar cells are summarized, and a classification of common passivation strategies that elaborate the mechanism according to the location of the defects and the type of passivation agent is presented. Finally, this review offers likely prospects for future trends in the development of passivation strategies.

In this Review, the important role of the interface in tin‐based perovskites and their PSCs device is demonstrated. The up‐to‐date studies on interface engineering of tin‐based PSCs are summarized. At last, a future perspective and remaining challenges in this field are given to provide some new thoughts on interface engineering for efficient tin‐based PSCs device.

Abstract

As a rising star of lead‐free perovskite solar cells (PCSs), tin‐based PSCs have drawn much attention and made promising progress during the past few years. Notably, interfaces in the tin‐based PSCs device have great impacts on performance enhancements. In this Review, the authors first demonstrate why the interface is especially crucial for tin‐based PSCs device. It is proposed that the engineering of i) interface between perovskite grains in the film and ii) interface within the PSCs device are of great significance on the improvement of device functionality and stability. Then, the up‐to‐date studies on interface engineering of tin‐based PSCs are reviewed, including the following strategies: i) passivation of trap states; ii) modification of interfacial layers; iii) construction of 2D/3D structure. At last, a future perspective and remaining challenges in this field are given, aiming to provide a comprehensive understanding of interfaces in tin‐based PSCs and give some new thoughts on interface engineering for efficient PSCs device.

Perovskite solar cells have reached certified efficiencies of 25.2% within just ten years due to their excellent optoelectronic properties. Nonradiative recombination at the interface between the perovskite absorber and charge‐transporting layers is identified as the major source of open‐circuit‐voltage losses in state‐of‐the‐art devices, requiring advanced strategies to study and to control efficiency‐limiting interfacial processes.

Abstract

Perovskite solar cells combine high carrier mobilities with long carrier lifetimes and high radiative efficiencies. Despite this, full devices suffer from significant nonradiative recombination losses, limiting their V _OC to values well below the Shockley–Queisser limit. Here, recent advances in understanding nonradiative recombination in perovskite solar cells from picoseconds to steady state are presented, with an emphasis on the interfaces between the perovskite absorber and the charge transport layers. Quantification of the quasi‐Fermi level splitting in perovskite films with and without attached transport layers allows to identify the origin of nonradiative recombination, and to explain the V _OC of operational devices. These measurements prove that in state‐of‐the‐art solar cells, nonradiative recombination at the interfaces between the perovskite and the transport layers is more important than processes in the bulk or at grain boundaries. Optical pump‐probe techniques give complementary access to the interfacial recombination pathways and provide quantitative information on transfer rates and recombination velocities. Promising optimization strategies are also highlighted, in particular in view of the role of energy level alignment and the importance of surface passivation. Recent record perovskite solar cells with low nonradiative losses are presented where interfacial recombination is effectively overcome—paving the way to the thermodynamic efficiency limit.

Mixed A₁ cations are employed in layered two‐dimensional perovskites to investigate the interplay between alkylamine cations and unsaturated alkylamine cations with π‐electrons. It is revealed that alkylamine spacer cations are able to facilitate precursor assembly, which results in the orientated growth of perovskite crystals. Unsaturated alkylamine cations lead to reduced exciton binding energy, which improves the carrier pathway.

Abstract

Organic spacer cations in layered 2D (A₁)₂(A₂) _{n −1}B _n X_{3 n +1} (where A₁ is an organic cation acting as a spacer between the perovskite layers, A₂ is a monovalent cation, e.g., Cs⁺,CH₃NH₃ ⁺, CH(NH₂)₂ ⁺) perovskite materials improve the long‐term stability of the resulting solar cells, but hamper their power conversion efficiency due to poor carrier generation/transportation. Rational guidelines are thus required to enable the design of organic spacer cations. Herein, mixed A₁ cations are employed in layered 2D perovskites to investigate the interplay between alkylamine cations and unsaturated alkylamine cations. It is revealed that alkylamine spacer cations are able to facilitate precursor assembly, which results in the orientated growth of perovskite crystals. Unsaturated alkylamine cations further lead to reduced exciton binding energy, which improves carrier pathway in the 2D perovskites. By mixing both cations, substantially improved open circuit voltage is observed in the resultant photovoltaic cells with the efficiency of 15.46%, one of the highest one based on (A₁)₂(A₂)₃Pb₄I₁₃ layered 2D perovskites. The generality of the design principle is further extended to other cation combinations.

Selectivity is a figure of merit for the usefulness of a contact in a photovoltaic device. Here, previous quantitative definitions of selectivity are compared and combined. The influence of the built‐in potential on the selectivity is investigated and it is illustrated how this quantity can be measured.

Abstract

The electronic properties of the contacts to a photovoltaic absorber material are important for the final efficiency of any type of solar cell. For highly efficient solar cells based on high quality absorber materials like single‐crystalline silicon, polycrystalline Cu(In,Ga)Se₂, CdTe, or metal‐halide perovskites, contact formation is even the decisive processing step determining the final efficiency. The present paper combines recently developed quantitative concepts for the description of contacts to solar cells in terms of their selectivity toward a more general description that is valid for all types of solar cells and all types of contacts. It is shown that the built‐in voltage is an important parameter to influence the selectivity of contacts to photovoltaic absorber materials. It is also shown that the contact selectivity is mathematically related to the collection efficiency which can be measured by luminescence based techniques.

Herein, acetate anion (Ac⁻) is used to partially replace I⁻ in the CsPbI₂Br framework. Ac⁻ doping changes the morphology, electronic properties, and band structure of the host CsPbI₂Br film. The obtained CsPbI_{2− x} Br(Ac) _x perovskite solar cells exhibit a power conversion efficiency of 15.56%, an open circuit voltage of 1.30 V, and great air stability.

Abstract

The Cs‐based inorganic perovskite solar cells (PSCs), such as CsPbI₂Br, have made a striking breakthrough with power conversion efficiency (PCE) over 16% and potential to be used as top cells for tandem devices. Herein, I⁻ is partially replaced with the acetate anion (Ac⁻) in the CsPbI₂Br framework, producing multiple benefits. The Ac⁻ doping can change the morphology, electronic properties, and band structure of the host CsPbI₂Br film. The obtained CsPbI_{2− x} Br(Ac) _x perovskite films present lower trap densities, longer carrier lifetimes, and fast charge transportation compared to the host CsPbI₂Br films. Interestingly, the CsPbI_{2− x} Br(Ac) _x PSCs exhibit a maximum PCE of 15.56% and an ultrahigh open circuit voltage (V _oc) of 1.30 V without sacrificing photocurrent. Notably, such a remarkable V _oc is among the highest values of the previously reported CsPbI₂Br PSCs, while the PCE far exceeds all of them. In addition, the obtained CsPbI_{2− x} Br(Ac) _x PSCs exhibit high reproducibility and good stability. The stable CsPbI_{2− x} Br(Ac) _x PSCs with high V _oc and PCE are desirable for tandem solar cell applications.

The various additives adopted for perovskite solar cells (PSCs) are reviewed and their functioning mechanisms and influence on device performance are described. The main roles of additives modulating morphology of perovskite films are summarized; stabilizing phase of formamidinium and cesium‐based perovskites, adjusting energy level alignment in PSCs, suppressing nonradiative recombination in perovskite, eliminating hysteresis, and enhancing operational stability of PSCs.

Abstract

Additives are widely adopted for efficient, stable, and hysteresis‐free perovskite solar cells and play an important role in various breakthroughs of perovskite solar cells (PSCs). Herein the various additives adopted for PSCs are reviewed and their functioning mechanism and influence on device performance is described. The main roles of additives, modulating morphology of perovskite films, stabilizing phase of formamidinium (FA) and cesium (Cs)‐based perovskites, adjusting energy level alignment in PSCs, suppressing nonradiative recombination in perovskites, eliminating hysteresis, enhancing operational stability of PSCs, are summarized.

Lead-based organic-inorganic hybrid perovskite (OIHP) solar cells can attain efficiencies over 20%. However, the impact of ion mobility and/or organic depletion, structural changes, and segregation under operating conditions urge for decisive and more accurate investigations. Hence, the development of analytical tools for accessing the grain-to-grain OIHP chemistry is of great relevance. Here, we used synchrotron infrared nanospectroscopy (nano-FTIR) to map individual nanograins in OIHP films. Our results reveal a spatial heterogeneity of the vibrational activity associated to the nanoscale chemical diversity of isolated grains. It was possible to map the chemistry of individual grains in CsFAMA [Cs_0.05FA_0.79MA_0.16Pb(I_0.83Br_0.17)₃] and FAMA [FA_0.83MA_0.17Pb(I_0.83Br_0.17)₃] films, with information on their local composition. Nanograins with stronger nano-FTIR activity in CsFAMA and FAMA films can be assigned to PbI₂ and hexagonal polytype phases, respectively. The analysis herein can be extended to any OIHP films where organic cation depletion/accumulation can be used as a chemical label to study composition.

Herein, a facile method is provided to fabricate the CsPbIBr₂ inorganic perovskite solar cells under low temperatures. The ZnO electron transport layer modification and band‐alignment engineering contribute to the outstanding power conversion efficiency of 10.16%, representing the highest efficiency for CsPbIBr₂ when the fabrication temperature is lower than 160 °C.

Recently, the thermally stable and facilely fabricated inorganic CsPbIBr₂ perovskite solar cells (PSCs) have attracted tremendous attention where the electron transport layer (ETL) is vital. However, the typical sintering temperature for the widely used electron transport material, that is, TiO₂, is more than 400 °C, elevating the cost and hindering the application. Due to high electron mobility and low fabrication temperature, ZnO becomes a desirable alternative for TiO₂, as the ETL in CsPbIBr₂ PSCs, albeit with low open‐circuit voltage (V _oc). Herein, this work introduces a trace of NH₄Cl to the sol–gel‐derived ZnO precursor to decrease the work function of the ZnO film, tune the surface morphology of the perovskite film, and thus suppress the density of trap states in the CsPbIBr₂ films. Consequently, full‐coverage and pure‐phase CsPbIBr₂ films consisting of micron‐size and high‐crystallinity grains are obtained. More importantly, for the optimal NH₄Cl‐modified ZnO, a shining improvement in V _oc from 1.08 to 1.27 V boosts the champion CsPbIBr₂ PSCs to obtain a power conversion efficiency of 10.16%, which is the highest value reported among pure‐CsPbIBr₂ PSCs under a low fabrication temperature of 160 °C. In addition, the NH₄Cl‐modified ZnO ETL reduces the severe hysteresis and increases the device stability significantly.

Chlorine‐Substituted Graphdiyne

In article number 1900241, Tonggang Jiu and co‐workers introduce chlorine‐substituted graphdiyne into MAPbI₃‐based perovskite solar cells to produce a peak efficiency of 20.34% with suppressed J‐V hysteresis, which results from the interaction of derivated graphdiyne and PCBM, to be exact, four types of non‐covalent bonds. These findings suggest that derivated graphdiyne could have potential applications in solar cells and other photoelectric devices.

Interface Modification Engineering

In article number 19000162, Qiuhong Cui, Yanbing Hou, and co‐workers construct perovskite solar cells with discrete SnO₂ nanoparticle‐modified layers by spin coating the SnO₂ dispersions on PEDOT:PSS. The discrete SnO₂ nanoparticle film lets holes pass and block electrons to diffuse toward PEDOT:PSS, which enhances the extraction efficiency, leading to an increase in power conversion efficiency.

The power conversion efficiency of inorganic perovskite solar cells (PSCs) is still low compared with hybrid PSCs. The use of lithium fluoride on SnO₂ and PbCl₂ additive in perovskite is reported for reducing the charge recombination; 18.64% efficiency of CsPbI_3–x Br _x solar cells is demonstrated; and the devices show over than 1000 h light soaking stability.

Abstract

Cesium‐based inorganic perovskite solar cells (PSCs) are promising due to their potential for improving device stability. However, the power conversion efficiency of the inorganic PSCs is still low compared with the hybrid PSCs due to the large open‐circuit voltage (V _OC) loss possibly caused by charge recombination. The use of an insulated shunt‐blocking layer lithium fluoride on electron transport layer SnO₂ for better energy level alignment with the conduction band minimum of the CsPbI_{3‐ x} Br _x and also for interface defect passivation is reported. In addition, by incorporating lead chloride in CsPbI_{3‐ x} Br _x precursor, the perovskite film crystallinity is significantly enhanced and the charge recombination in perovksite is suppressed. As a result, optimized CsPbI_{3‐ x} Br _x PSCs with a band gap of 1.77 eV exhibit excellent performance with the best V _OC as high as 1.25 V and an efficiency of 18.64%. Meanwhile, a high photostability with a less than 6% efficiency drop is achieved for CsPbI_{3‐ x} Br _x PSCs under continuous 1 sun equivalent illumination over 1000 h.

A method of combined electron transporting bilayer is reported to reduce energy loss and inhibit defects in the perovskite solar cells (PSCs) by combining the commercially accessible SnO₂ and home‐made TiO₂ nanoparticles. Consequently, the PSCs devices acquire a high efficiency of 20.50%, which is superior to that based on SnO₂ layers with a efficiency of 18.09%.

Herein, commercially accessible SnO₂ and home‐made TiO₂ nanoparticles as a combined electron transporting bilayer (ETBL) are applied to achieve highly efficient planar perovskite solar cells (PSCs). With the formed cascade‐aligned energy levels from the proper stacking of SnO₂ and TiO₂ layers and the excellent defect‐passivation ability of TiO₂, SnO₂/TiO₂ ETBLs effectively reduce energy loss and inhibit defects formation both at the electron transporting layers (ETL)/perovskite interfaces and within the bulk of perovskite layer as revealed by a comprehensive analysis of photoelectric characteristic analysis, including ultraviolet photoelectron spectroscopy, photoluminescence, and electrochemical impedance spectroscopy. Consequently, the PSC devices acquired a power conversion efficiency (PCE) of 20.50% with a V _oc of 1.10 V, a J _sc of 24.2 mA cm⁻² and an fill factor of 77%, which are superior to the values of the control device based on single SnO₂ layer with a PCE of 18.09% (a 13.3% boosting on PCE). Moreover, there was no degradation after 49 days, indicating the great stability after adding TiO₂ layer. Herein, it is demonstrated that the cascaded alignment of energy levels between the electrode and perovskite layer by ETBLs could be an effective approach to improve the photovoltaic performance of the PSCs with excellent long‐term stability.

A hybrid electron transport layer (ETL) of SnO₂ and carbon nanotubes (CNTs) is designed by simple thermal decomposition of a mixed solution of SnCl₄·5H₂O and pretreated CNTs. Based on the hybrid ETL, a high efficiency of 20.33% is achieved in the hysteresis‐free perovskite solar cell, which shows 13.58% enhancement compared with the conventional device (power conversion efficiency = 17.90%).

Tin oxide (SnO₂) has recently received increasing attention as an electron transport layer (ETL) in planar perovskite solar cells (PSCs) and is considered a possible alternative to titanium oxide (TiO₂). However, planar devices based on pure solution‐processed SnO₂ ETL still have hysteresis, which greatly limits the application of SnO₂ in high‐efficiency solar cells. Herein, to address this issue, a hybrid ETL of SnO₂ and carbon nanotubes (CNTs) is fabricated by a simple thermal decomposing of a mixed solution of SnCl₄·5H₂O and pretreated CNTs (termed SnO₂–CNT). The addition of CNTs can significantly improve the conductivity of SnO₂ films and reduce the trap‐state density of SnO₂ films, which benefit carrier transfer from the perovskite layer to the cathode. As a result, a high efficiency of 20.33% is achieved in the hysteresis‐free PSCs based on SnO₂–CNT ETL, which shows 13.58% enhancement compared with the conventional device (power conversion efficiency = 17.90%).

A method of combined electron transporting bilayer is reported to reduce energy loss and inhibit defects in the perovskite solar cells (PSCs) by combining the commercially accessible SnO₂ and home‐made TiO₂ nanoparticles. Consequently, the PSCs devices acquire a high efficiency of 20.50%, which is superior to that based on SnO₂ layers with a efficiency of 18.09%.

Herein, commercially accessible SnO₂ and home‐made TiO₂ nanoparticles as a combined electron transporting bilayer (ETBL) are applied to achieve highly efficient planar perovskite solar cells (PSCs). With the formed cascade‐aligned energy levels from the proper stacking of SnO₂ and TiO₂ layers and the excellent defect‐passivation ability of TiO₂, SnO₂/TiO₂ ETBLs effectively reduce energy loss and inhibit defects formation both at the electron transporting layers (ETL)/perovskite interfaces and within the bulk of perovskite layer as revealed by a comprehensive analysis of photoelectric characteristic analysis, including ultraviolet photoelectron spectroscopy, photoluminescence, and electrochemical impedance spectroscopy. Consequently, the PSC devices acquired a power conversion efficiency (PCE) of 20.50% with a V _oc of 1.10 V, a J _sc of 24.2 mA cm⁻² and an fill factor of 77%, which are superior to the values of the control device based on single SnO₂ layer with a PCE of 18.09% (a 13.3% boosting on PCE). Moreover, there was no degradation after 49 days, indicating the great stability after adding TiO₂ layer. Herein, it is demonstrated that the cascaded alignment of energy levels between the electrode and perovskite layer by ETBLs could be an effective approach to improve the photovoltaic performance of the PSCs with excellent long‐term stability.

Herein, a facile method is provided to fabricate the CsPbIBr₂ inorganic perovskite solar cells under low temperatures. The ZnO electron transport layer modification and band‐alignment engineering contribute to the outstanding power conversion efficiency of 10.16%, representing the highest efficiency for CsPbIBr₂ when the fabrication temperature is lower than 160 °C.

Recently, the thermally stable and facilely fabricated inorganic CsPbIBr₂ perovskite solar cells (PSCs) have attracted tremendous attention where the electron transport layer (ETL) is vital. However, the typical sintering temperature for the widely used electron transport material, that is, TiO₂, is more than 400 °C, elevating the cost and hindering the application. Due to high electron mobility and low fabrication temperature, ZnO becomes a desirable alternative for TiO₂, as the ETL in CsPbIBr₂ PSCs, albeit with low open‐circuit voltage (V _oc). Herein, this work introduces a trace of NH₄Cl to the sol–gel‐derived ZnO precursor to decrease the work function of the ZnO film, tune the surface morphology of the perovskite film, and thus suppress the density of trap states in the CsPbIBr₂ films. Consequently, full‐coverage and pure‐phase CsPbIBr₂ films consisting of micron‐size and high‐crystallinity grains are obtained. More importantly, for the optimal NH₄Cl‐modified ZnO, a shining improvement in V _oc from 1.08 to 1.27 V boosts the champion CsPbIBr₂ PSCs to obtain a power conversion efficiency of 10.16%, which is the highest value reported among pure‐CsPbIBr₂ PSCs under a low fabrication temperature of 160 °C. In addition, the NH₄Cl‐modified ZnO ETL reduces the severe hysteresis and increases the device stability significantly.

A general approach for lab‐to‐manufacturing translation is developed to achieve high‐performance flexible organic solar modules without obvious efficiency loss. The shear impulse during the coating/printing process is applied to control the morphology evolution of the bulk heterojunction layer for both fullerene and nonfullerene acceptor systems. A quantitative transformation factor of shear impulse between slot‐die printing and spin‐coating is detected.

Abstract

The blossoming of organic solar cells (OSCs) has triggered enormous commercial applications, due to their high‐efficiency, light weight, and flexibility. However, the lab‐to‐manufacturing translation of the praisable performance from lab‐scale devices to industrial‐scale modules is still the Achilles' heel of OSCs. In fact, it is urgent to explore the mechanism of morphological evolution in the bulk heterojunction (BHJ) with different coating/printing methods. Here, a general approach to upscale flexible organic photovoltaics to module scale without obvious efficiency loss is demonstrated. The shear impulse during the coating/printing process is first applied to control the morphology evolution of the BHJ layer for both fullerene and nonfullerene acceptor systems. A quantitative transformation factor of shear impulse between slot‐die printing and spin‐coating is detected. Compelling results of morphological evolution, molecular stacking, and coarse‐grained molecular simulation verify the validity of the impulse translation. Accordingly, the efficiency of flexible devices via slot‐die printing achieves 9.10% for PTB7‐Th:PC₇₁BM and 9.77% for PBDB‐T:ITIC based on 1.04 cm² . Furthermore, 15 cm² flexible modules with effective efficiency up to 7.58% (PTB7‐Th:PC₇₁BM) and 8.90% (PBDB‐T:ITIC) are demonstrated with satisfying mechanical flexibility and operating stability. More importantly, this work outlines the shear impulse translation for organic printing electronics.

Ruddlesden–Popper 2D perovskites with a formula of (A′)₂(A) _{n −1}B _n X_{3 n +1} are excellent materials for the next generation of optoelectronic devices. Their properties can be tuned by selecting different organic amines, metal halides, and number of layers for light‐emitting diodes and solar cells.

Abstract

Ruddlesden–Popper perovskites with a formula of (A′)₂(A) _{n −1}B _n X_{3 n +1} have recently gained widespread interest as candidates for the next generation of optoelectronic devices. The variations of organic cation, metal halide, and the number of layers in the structure lead to the change of crystal structures and properties for different optoelectronic applications. Herein, the different synthetic methods for 2D perovskite crystals and thin films are summarized and compared. The optoelectronic properties and the charge transfer process in the devices are also delved, in particular, for light‐emitting diodes and solar cells.