shuhui

A novel cryogenic process has universal applicability to prepare mixed perovskite films. Excellent film quality and consequently promising device performance result from decoupling of nucleation and crystallization phases during the formation of perovskites. The cryogenic temperature suppresses premature reactions of the precursors and prevents premature coalescence of nuclei into large crystallites, enabling uniform film formation following the blow‐drying and annealing processes.

Abstract

A cryogenic process is introduced to control the crystallization of perovskite layers, eliminating the need for the use of environmentally harmful antisolvents. This process enables decoupling of the nucleation and the crystallization phases by inhibiting chemical reactions in as‐cast precursor films rapidly cooled down by immersion in liquid nitrogen. The cooling is followed by blow‐drying with nitrogen gas, which induces uniform precipitation of precursors due to the supersaturation of precursors in the residual solvents at very low temperature, while at the same time enhancing the evaporation of the residual solvents and preventing the ordered precursors/perovskite from redissolving into the residual solvents. Using the proposed techniques, the crystallization process can be initiated after the formation of a uniform precursor seed layer. The process is generally applicable to improve the performance of solar cells using perovskite films with different compositions, as demonstrated on three different types of mixed halide perovskites. A champion power conversion efficiency (PCE) of 21.4% with open‐circuit voltage (V _OC) = 1.14 V, short‐circuit current density ( J _SC) = 23.5 mA cm⁻², and fill factor (FF) = 0.80 is achieved using the proposed cryogenic process.

Publication date: November 2018

Source: Nano Energy, Volume 53

Author(s): Xixia Liu, Bichen Li, Nengduo Zhang, Zhimeng Yu, Kuan Sun, Baoshan Tang, Diwen Shi, Hongyan Yao, Jianyong Ouyang, Hao Gong

Abstract

Graphical abstract

Publication date: 19 December 2018

Source: Joule, Volume 2, Issue 12

Author(s): Meng Zhang, Benjamin Wilkinson, Yuanxun Liao, Jianghui Zheng, Cho Fai Jonathan Lau, Jincheol Kim, Jueming Bing, Martin A. Green, Shujuan Huang, Anita Wing-Yi Ho-Baillie

Context & Scale

Summary

Graphical Abstract

Machine learning determines the optimal doping content of Mn²⁺ in CZTSSe films effectively and rapidly for the best solar cell efficiency. A CM_0.05Z_0.95TSSe solar cell with an efficiency of 8.93% is achieved in the experiment. The doped Mn²⁺ decreases the Cu_Zn defect and boosts the improvement of CZTSSe solar cells.

Isoelectronic cation substitution is a potential method to decrease the density of Cu‐Zn anti‐site defects in CZTSSe, thus improving the V _OC and performance of CZTSSe solar cells. The proper doping concentration is determined traditionally by the trial and error approach, costing much time, and materials. How to shorten the time to find the proper doping concentration is a big challenge for the development of solar cells. Here, by utilizing the machine learning model, the authors carry out an adaptive design for predicting the optimal doping ratio of Mn²⁺ ions in CZTSSe solar cells for improved solar cell efficiency. With the help of machine learning prediction, the authors rapidly and efficiently find the optimal doping ratio of Mn²⁺ in CZTSSe solar cells to be 0.05, achieving a highest solar cell efficiency of 8.9% in experiment. Further experimental characterizations of Mn‐doped CZTSSe show that the defect in CZTSSe after Mn doping is changed from an anti‐site Cu_Zn defect to V _Cu defect. Our findings suggest that machine learning is a very powerful and efficient approach to aid the development of solar cell materials for its application in the photovoltaic field.

Interface engineering in n‐i‐p metal halide perovskite solar cells is achieved by introducing 2D perovskites, functional molecules, quantum dots, and an insulating layer. Their roles include achieving better energy‐level alignment, passivating traps, resisting moisture and suppressing ion migration, contributing to improved performance, enhanced long‐term stability, and eliminated photocurrent hysteresis.

Recent years have witnessed continuous progress in metal halide perovskite (MHP) solar cells with a certified power conversion efficiency (PCE) exceeding 22%. However, the commercialization of MHP solar cells continues to encounter various challenges including stabilization, scalability and repeatability. Of all problems related to MHP materials, interface recombination is the most prominent, resulting in severe PCE loss within a short time. Fortunately, interface engineering has been identified as an efficient means of achieving better energy‐level alignment, reduced charge recombination, trap passivation, elimination of photocurrent hysteresis, and enhanced long‐term device stability. This review examines the relationship between specific interface modification layers and their roles in interface engineering based on device physics, revealed by several characterization methods. The latest research advances in interface modification layers according to their roles and properties are also summarized.

Publication date: November 2018

Source: Nano Energy, Volume 53

Author(s): Yuanyuan Jiang, Congcong Wu, Liurui Li, Kai Wang, Zui Tao, Fan Gao, Weifeng Cheng, Jiangtao Cheng, Xin-Yan Zhao, Shashank Priya, Weiwei Deng

Abstract

Graphical abstract

Two‐dimensional perovskites typically have a general formula of (RNH₃)₂MA _{n −1}M _n X_{3 n +1}, and can be arbitrarily categorized into strict 2D (n = 1), quasi‐2D (n = 2−5), and quasi‐3D (n > 5). Research progress in each category is summarized, and it is proposed that designing functional organics to manipulate the optoelectronic properties of 2D perovskites can lead to unique device applications.

Abstract

2D perovskites are recently attracting a significant amount of attention, mainly due to their improved stability compared with their 3D counterpart, e.g., the archetypical MAPbI₃. Interestingly, the first studies on 2D perovskites can be dated back to the 1980s. The most popular 2D perovskites have a general formula of (RNH₃)₂MA _{n −1}M _n X_{3 n +1}, where n represents the number of metal halide octahedrons between the insulating organic cation layers. The optoelectronic properties of 2D perovskites, e.g., band gap, are highly dependent on the thickness of the inorganic layers (i.e., the value of n). Herein, 2D perovskites are arbitrarily divided into three classes, strict 2D (n = 1), quasi‐2D (n = 2–5), and quasi‐3D (n > 5), and research progress is summarized following this classification. The majority of existing 2D perovskites only employ very simple organic cations (e.g., butyl ammonium or phenylethyl ammonium), which merely function as the supporting layer/insulating barrier to achieve the 2D structure. Thus, a particularly important research question is: can functional organic cations be designed for these 2D perovskites, where these functional organic cations would play an important role in dictating the optoelectronic properties of these organic–inorganic hybrid materials, leading to unique device performance or applications?

Interface engineering in n‐i‐p metal halide perovskite solar cells is achieved by introducing 2D perovskites, functional molecules, quantum dots, and an insulating layer. Their roles include achieving better energy‐level alignment, passivating traps, resisting moisture and suppressing ion migration, contributing to improved performance, enhanced long‐term stability, and eliminated photocurrent hysteresis.

Recent years have witnessed continuous progress in metal halide perovskite (MHP) solar cells with a certified power conversion efficiency (PCE) exceeding 22%. However, the commercialization of MHP solar cells continues to encounter various challenges including stabilization, scalability and repeatability. Of all problems related to MHP materials, interface recombination is the most prominent, resulting in severe PCE loss within a short time. Fortunately, interface engineering has been identified as an efficient means of achieving better energy‐level alignment, reduced charge recombination, trap passivation, elimination of photocurrent hysteresis, and enhanced long‐term device stability. This review examines the relationship between specific interface modification layers and their roles in interface engineering based on device physics, revealed by several characterization methods. The latest research advances in interface modification layers according to their roles and properties are also summarized.

Intermolecular arrangements of PBDB‐T with ITIC and PC71BM are revealed by atomistic simulations. Owing to smaller side‐chain steric hindrance, both acceptors are prone to approach polymer A units, especially for ITIC considering better matching in size and shape. Importantly, PBDB‐T/ITIC docking occurs mainly via local π–π interaction between electron‐withdrawing end groups of ITIC and A units of the polymer, which is beneficial for efficient exciton dissociation.

Donor/acceptor (D/A) interfaces play a crucial role in photoelectric conversion for organic solar cells. However, it is impossible to experimentally probe D/A interfaces at the atomistic level to date, in particular for organic solar cells based on nonfullerene acceptors due to their anisotropic structures. In this work, we have investigated the interfacial structures of a representative D‐A copolymer donor PBDB‐T with a well‐known A‐D‐A structured nonfullerene acceptor ITIC, in comparison with a fullerene acceptor PC₇₁BM, by means of atomistic simulations. It is found that owing to different side‐chain steric hindrance between the polymer A and D units, both acceptors are more likely to approach the polymer A units, and more apparently for ITIC in consideration of the size and shape matching between the acceptors and the polymer A and D units. Importantly, docking of ITIC with polymer occurs mainly through local π–π interaction between the terminal moieties of ITIC and the A units of the polymer, and such interfacial structures are favorable for efficient exciton dissociation. Our work sheds light on the impact of side‐chain nature and location as well as acceptor structures on D/A interfaces and charge‐transfer dynamics, which will be very helpful for further improving the performance of organic photovoltaics.