Ligang Yuan

By means of first‐principles simulations, the electronic structure and optical properties of intimately mixed and phase‐segregated lead halide perovskites are contrasted. While the bandgap can be tuned over a broad range by changing the halide ratio in solid‐state solutions, phase segregation, even down to the nm scale, results in increased effective masses and energetic disorder, both detrimental to charge transport.

Abstract

Halide mixing is a key strategy to tune the emission wavelength in lead halide perovskites but is also effective in improving the performance of halide perovskite solar cells. Yet, a clear and global picture of how halide alloying and related spatial in/homogeneous distribution influence the electronic and optical properties of halide perovskites is currently lacking. Considering the preeminent mixed iodine/bromine perovskite as a case study, state‐of‐the‐art hybrid density functional theory calculations are performed, exploring the full space of chemical composition and accounting for phase segregation effects. It is shown that, at low doping regime, halide impurities do not act as trap states and that a quasi‐linear opening of the bandgap occurs with increasing the bromine content. Phase segregation at the nanoscale, in turn, drastically affects the electronic structure of mixed halide systems, with namely bromine rich domains acting as barrier to hole diffusion. The energetic disorder probed by optical absorption in these mixed systems is surprisingly insensitive to configurational disorder, but mostly dominated by the coexistence of phases with various degrees of halide segregation.

Nature Communications, Published online: 13 January 2021; doi:10.1038/s41467-020-20582-6

Achieving bright and efficient blue emission in metal halide perovskite light-emitting diodes has proven to be challenging. Here, the authors demonstrate high EQE and spectrally stable blue light-emitting diodes based on mixed halide perovskites, with emission from 490 to 451 nm by using a vapour-assisted crystallization technique.

Nature Energy, Published online: 11 January 2021; doi:10.1038/s41560-020-00756-8

Bifacial solar cells can outperform monofacial cells by exploiting sunlight reflected off the ground surface. De Bastiani et al. show that bifacial perovskite/silicon tandem with an optimized bandgap can deliver a power density of 26 mW cm–2 and compare its performance to monofacial cells under outdoor conditions.

Self‐assembled P3HT‐COOH is an excellent hole extraction layer to fabricate robust, high‐performance, and extremely reproducible perovskite solar cells. The well‐aligned self‐assembled P3HT‐COOH generates a dipole layer between indium tin oxide and perovskite, substantially retarding interface charge recombination and producing highly sensitive devices to dim light. The enhanced crystallinity and preferred out‐of‐plane orientation play a key role to suppress the device degradation process.

Abstract

Crystallinity and crystal orientation have a predominant impact on a materials’ semiconducting properties, thus it is essential to manipulate the microstructure arrangements for desired semiconducting device performance. Here, ultra‐uniform hole‐transporting material (HTM) by self‐assembling COOH‐functionalized P3HT (P3HT‐COOH) is fabricated, on which near single crystal quality perovskite thin film can be grown. In particular, the self‐assembly approach facilitates the P3HT‐COOH molecules to form an ordered and homogeneous monolayer on top of the indium tin oxide (ITO) electrode facilitate the perovskite crystalline film growth with high quality and preferred orientations. After detailed spectroscopy and device characterizations, it is found that the carboxylic acid anchoring groups can down‐shift the work function and passivate the ITO surface, retarding the interface carrier recombination. As a result, the device made with the self‐assembled HTM show high open‐circuit voltage over 1.10 V and extend the lifetime over 4,300 h when storing at 30% relative humidity. Moreover, the cell works efficiently under much reduced light power, making it useful as power source under dim‐light conditions. The demonstration suggests a new facile way of fabricating monolayer HTM for high efficiency perovskite devices, as well as the interconnecting layer needed for tandem cell.

The coupling of quasi‐2D perovskite layers is demonstrated by Kai Wang, Rui Chen, Wallace C. H. Choy, and co‐workers in article number 2005570. Their results show that a bifunctional ligand can simultaneously diminish the weak van der Waals gap and passivate perovskite defects for efficient energy transfer and radiative recombination. They fabricate blue PeLEDs that show high performance with an EQE of 10.11% and a long stability of 81.3 min.

Publication date: 17 February 2021

Source: Joule, Volume 5, Issue 2

Author(s): Ziming Chen, Zhenchao Li, Zhen Chen, Ruoxi Xia, Guangruixing Zou, Linghao Chu, Shi-Jian Su, Junbiao Peng, Hin-Lap Yip, Yong Cao

Publication date: 17 February 2021

Source: Joule, Volume 5, Issue 2

Author(s): Jin Hyuck Heo, Fei Zhang, Chuanxiao Xiao, Su Jeong Heo, Jin Kyoung Park, Joseph J. Berry, Kai Zhu, Sang Hyuk Im

Passive attack: The α‐FAPbI₃ perovskite layer in a solar cell is stabilized without deteriorating the spectral features by passivating with 2,3,4,5,6‐pentafluorobenzyl phosphonic acid (PFBPA). High‐quality perovskite solar cells with an improved efficiency of 22.25 % was achieved with excellent moisture stability maintaining >90 % of its initial efficiency at high humidity levels.

Abstract

Perovskite solar cells (PSCs) have shown great promise for photovoltaic applications, owing to their low‐cost assembly, exceptional performance, and low‐temperature solution processing. However, the advancement of PSCs towards commercialization requires improvements in efficiency and long‐term stability. The surface and grain boundaries of perovskite layer, as well as interfaces, are critical factors in determining the performance of the assembled cells. Defects, which are mainly located at perovskite surfaces, can trigger hysteresis, carrier recombination, and degradation, which diminish the power conversion efficiencies (PCEs) of the resultant cells. This study concerns the stabilization of the α‐FAPbI₃ perovskite phase without negatively affecting the spectral features by using 2,3,4,5,6‐pentafluorobenzyl phosphonic acid (PFBPA) as a passivation agent. Accordingly, high‐quality PSCs are attained with an improved PCE of 22.25 % and respectable cell parameters compared to the pristine cells without the passivation layer. The thin PFBPA passivation layer effectively protects the perovskite layer from moisture, resulting in better long‐term stability for unsealed PSCs, which maintain >90 % of the original efficiency under different humidity levels (40–75 %) after 600 h. PFBPA passivation is found to have a considerable impact in obtaining high‐quality and stable FAPbI₃ films to benefit both the efficiency and the stability of PSCs.

Publication date: 1 April 2021

Source: Chemical Engineering Journal, Volume 409

Author(s): Kyungeun Jung, Weon-Sik Chae, Yun Chang Park, Nam-Gyu Park, Man-Jong Lee

Publication date: 1 April 2021

Source: Chemical Engineering Journal, Volume 409

Author(s): Kai-Chi Hsiao, Bo-Ting Lee, Meng-Huan Jao, Ting-Han Lin, Cheng-Hung Hou, Jing-Jong Shyue, Ming-Chung Wu, Wei-Fang Su

The time‐resolved grazing‐incidence wide‐angle X‐ray scattering technique provides real‐time insights on the phase‐transition during the organic cation coating and perovskite quantum wells (PQWs)/3D architecture formation mechanism. With fluorinated poly(triarylamine) (PTAA) as a dopant‐free hole‐transport layer, this PQWs/3D architecture leads to stable perovskite photovoltaics with power conversion efficiency of >22%.

Abstract

The combination of a bulk 3D perovskite layer and a reduced dimensional perovskite layer (perovskite quantum wells (PQWs)) is demonstrated to enhance the performance of perovskite solar cells (PSCs) significantly in terms of stability and efficiency. This perovskite hierarchy has attracted intensive research interest; however, the in‐depth formation mechanism of perovskite quantum wells on top of a 3D perovskite layer is not clearly understood and is therefore the focus of this study. Along with ex situ morphology and photophysical characterization, the time‐resolved grazing‐incidence wide‐angle X‐ray scattering (TS‐GIWAXS) technique performed in this study provides real‐time insights on the phase‐transition during the organic cation (HTAB ligand molecule) coating and PQWs/3D architecture formation process. A strikingly strong ionic reaction between the 3D perovskite and the long‐chain organic cation leads to the quick formation of an ordered intermediate phase within only a few seconds. The optimal PQWs/3D architecture is achieved by controlling the HTAB casting, which is assisted by time‐of‐flight SIMS characterization. By controlling the second ionic reaction during the long‐chain cation coating process, along with the fluorinated poly(triarylamine) (PTAA) as a hole‐transport layer, the perovskite solar cells demonstrate efficiencies exceeding 22% along with drastically improved device stability.

Nonfullerene acceptors dominate organic solar cell research due to their promising high device efficiencies. However, key challenges for achieving high stability in commercially viable devices still remain. In this review, recent progress and challenges toward stable organic solar cells are discussed correlating molecular design and device engineering to device stability.

Abstract

Organic solar cells (OSCs) based on nonfullerene acceptors (NFAs) have made significant breakthrough in their device performance, now achieving a power conversion efficiency of ≈18% for single junction devices, driven by the rapid development in their molecular design and device engineering in recent years. However, achieving long‐term stability remains a major challenge to overcome for their commercialization, due in large part to the current lack of understanding of their degradation mechanisms as well as the design rules for enhancing their stability. In this review, the recent progress in understanding the degradation mechanisms and enhancing the stability of high performance NFA‐based OSCs is a specific focus. First, an overview of the recent advances in the molecular design and device engineering of several classes of high performance NFA‐based OSCs for various targeted applications is provided, before presenting a critical review of the different degradation mechanisms identified through photochemical‐, photo‐, and morphological degradation pathways. Potential strategies to address these degradation mechanisms for further stability enhancement, from molecular design, interfacial engineering, and morphology control perspectives, are also discussed. Finally, an outlook is given highlighting the remaining key challenges toward achieving the long‐term stability of NFA‐OSCs.

Field‐induced formation of dopant‐free radial junctions at the Al₂O₃/n‐c‐Si (crystalline silicon) interface is demonstrated. Atomic layer deposition of Al₂O₃ conformally coats tapered c‐Si microwire arrays to form the radial junctions. A dopant‐free radial junction solar cell is fabricated based on this technique. At 20.1%, the device obtains the highest efficiency compared with that of previously reported radial junction solar cells.

Abstract

Radial junctions on crystalline silicon (c‐Si) microwire structures considerably reduce the diffusion length of photoinduced minority carriers required for energy generation by decoupling light absorption and carrier separation in orthogonal spatial directions. Hence, radial junctions mitigate the need for high‐purity materials, and thus reduce the fabrication cost of c‐Si solar cells. In this study, the formation of dopant‐free radial junctions from atomic layer deposition (ALD) of Al₂O₃ on an n‐c‐Si microwire surface is reported. ALD‐Al₂O₃ generates a p⁺ inversion layer, which eventually forms the radial junction on the n‐c‐Si surface. The width of depletion region induced by the p⁺ inversion layer is calculated from PC1D simulation as 900 nm. The fabricated dopant‐free radial junction c‐Si solar cells exhibit a power conversion efficiency of 20.1%, which is higher than those of previously reported microwire‐based radial junction solar cells. Notably, internal quantum efficiencies of over 90% are obtained in the 300–980 nm wavelength region, thereby verifying the successful formation of radial junctions.

Replace Pb²⁺ cations in the lead halide perovskite with other suitable environmentally friendly metal cations can address the toxicity of lead and the structure‐induced intrinsic instability of lead halide perovskites. Moreover, it can maintain the perovskite crystal structure and excellent photoelectric properties. Herein, a systematic review summarizes recent progress of the lead‐free halide perovskite photodetectors.

Abstract

Lead halide perovskite (LHP) has been widely researched in the photovoltaic field due to its highly attractive optoelectronic properties. Among the LHP‐based devices, the detectivity of the photodetector is as high as 10¹⁵ Jones. However, their practical application is limited by the toxicity of lead in perovskite and the inherent instability induced by the perovskite structure against moisture, heat, and light. To address these issues, tremendous efforts have been made to replace Pb²⁺ with other environmentally friendly metal cations such as Sn²⁺, Bi³⁺, Cu²⁺, Sb³⁺, and Ge²⁺. Thus, considerable breakthroughs in device performance and stability using lead‐free metal halide perovskite (LFMHP) have been made in recent years. In this review, the synthesis methods and strategies are focused for enhancing the material quality and photoelectric properties of LFMHPs and the recent research progress of LFMHP‐based photodetectors is summarized. This research provides some promising perspectives for high‐performance LFMHP photodetectors to achieve a broader range of practical applications in the future.

In article number 2005445, Ki‐Yeon Kim, Garam Park, In‐Hwan Oh, and co‐workers synthesize few‐layered organic‐inorganic halide perovskite ultrathin films with their own intrinsic magnetic order by chemically exfoliating bulk compounds using a binary co‐solvent and spin coating technique. The atomically thin two‐dimensional organic‐inorganic halide perovskite magnets for spintronic applications are just around the corner.

Two sequentially deposited SnO₂ layers doped with a low and a high amount of ammonium chloride, respectively, boost the open‐circuit voltage and fill factor of perovskite solar cells. The main effect of the novel electron transport layer is a change in the energy level alignment with the perovskite interface leading to decreased carrier recombination.

Abstract

Tin oxide (SnO₂) is an emerging electron transport layer (ETL) material in halide perovskite solar cells (PSCs). Among current limitations, open‐circuit voltage (V _OC) loss is one of the major factors to be addressed for further improvement. Here a bilayer ETL consisting of two SnO₂ nanoparticle layers doped with different amounts of ammonium chloride is proposed. As demonstrated by photoelectron spectroscopy and photophysical studies, the main effect of the novel ETL is to modify the energy level alignment at the SnO₂/perovskite interface, which leads to decreased carrier recombination, enhanced electron transfer, and reduced voltage loss. Moreover, X‐ray diffraction reveals reduced strain in perovskite layers grown on bilayer ETLs with respect to single‐layer ETLs, further contributing to a decrease of carrier recombination processes. Finally, the bilayer approach enables the more reproducible preparation of smooth and pinhole‐free ETLs as compared to single‐step deposition ETLs. PSCs with the doped bilayer SnO₂ ETL demonstrate strongly increased V _OC values of up to 1.21 V with a power conversion efficiency of 21.75% while showing negligible hysteresis and enhanced stability. Moreover, the SnO₂ bilayer can be processed at low temperature (70 °C), and has therefore a high potential for use in tandem devices or flexible PSCs.

Highly efficient CsPbI_1.5Br_1.5 perovskite solar cells (PSCs) are achieved via introducing fluorescein isothiocyanate (FITC) organic dye as passivator. FITC not only reduces the metal ion related trap states but also improves film crystallinity, resulting in an enhancement of device efficiency from 12.3% to 14.05%. In addition, it is demonstrated that CsPbI_1.5Br_1.5 perovskite shows the optimal halide composition for inorganic PSCs.

Abstract

All‐inorganic perovskite solar cells (PSCs) have recently received growing attention as a promising template to solve the thermal instability of organic–inorganic PSCs. However, the thermodynamic phase instability and relatively low device efficiency pose challenges. Herein, highly efficient and stable CsPbI_1.5Br_1.5 compositional perovskite‐based inorganic PSCs are fabricated using an organic dye, fluorescein isothiocyanate (FITC), as a passivator. The carboxyl and thiocyanate groups of FITC not only minimize the trap states by forming interactions with the under‐coordinated Pb²⁺ ions but also significantly increase the grain size and improve the crystallinity of the perovskite films during annealing. Consequently, perovskite films with superior optoelectronic properties, prolonged carrier lifetime, reduced trap density, and improved stability are obtained. The resulting device yields a champion efficiency of 14.05% with negligible hysteresis, which presents the highest reported efficiency for inorganic CsPbI_1.5Br_1.5 solar cells reported thus far. In addition, FITC can be generally adopted as attractive passivator to improve the performance of CsPbI₂Br‐ and CsPbIBr₂‐based PSCs. Furthermore, with a comprehensive comparison of mixed‐halide inorganic perovskites, it is demonstrated that CsPbI_1.5Br_1.5 compositional perovskite is a promising candidate with the optimal halide composition for high‐performance inorganic PSCs.