Ligang Yuan

A newly designed amorphous‐TiO₂‐encapsulated CsPbBr₃ nanocrystal is prepared for photocatalytic CO₂ reduction reaction, leading to a maximum 6.5‐fold increment on electron consumption by quenching the radiative recombination and increasing CO₂ feedstock adsorption. This study emphasizes the pivotal issues in designing halide perovskite photocatalyst and its solution by composite material concept.

Abstract

Artificially photocatalytic reduction of CO₂ into valuable chemicals, responding to the call of carbon neutral economy, has aroused considerable interests so far. Among those photocatalysts screened, an emerging and promising alternative of inorganic CsPbBr₃ perovskite has recently been reported. Here, to attain preferable photocatalytic performance, an amorphous‐TiO₂‐encapsulated CsPbBr₃ nanocrystal (CsPbBr₃ NC/a‐TiO₂) hybrid is demonstrated through a solution processing strategy. After optimizing the a‐TiO₂ matrix amount by tuning the tetrabutyl titanate precursor volume, the CsPbBr₃ NC/a‐TiO₂ composite exhibits a marvelous 6.5‐fold improvement on the consumption of photoelectrons in photocatalytic CO₂ reduction reaction when compared with the individual CsPbBr₃ NC. Such significant enhancement is ascribed to the accelerated electron–hole separation and the multiplied CO₂ adsorption. Thus, as an available prototype, this work offers a rational encapsulation design for efficient halide perovskite photocatalyst.

With the scaling trends in photovoltaics moving toward thinner active materials, two‐dimensional (2D) materials with their exciting optical and electronic properties are an obvious choice for integration with next‐generation solar cells. The role of 2Dmaterials in solar photovoltaics is presented so that they can be employed for formulating a future roadmap of various photovoltaic technologies.

Abstract

2D materials have attracted considerable attention due to their exciting optical and electronic properties, and demonstrate immense potential for next‐generation solar cells and other optoelectronic devices. With the scaling trends in photovoltaics moving toward thinner active materials, the atomically thin bodies and high flexibility of 2D materials make them the obvious choice for integration with future‐generation photovoltaic technology. Not only can graphene, with its high transparency and conductivity, be used as the electrodes in solar cells, but also its ambipolar electrical transport enables it to serve as both the anode and the cathode. 2D materials beyond graphene, such as transition‐metal dichalcogenides, are direct‐bandgap semiconductors at the monolayer level, and they can be used as the active layer in ultrathin flexible solar cells. However, since no 2D material has been featured in the roadmap of standard photovoltaic technologies, a proper synergy is still lacking between the recently growing 2D community and the conventional solar community. A comprehensive review on the current state‐of‐the‐art of 2D‐materials‐based solar photovoltaics is presented here so that the recent advances of 2D materials for solar cells can be employed for formulating the future roadmap of various photovoltaic technologies.

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

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.

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.

The percentage of all photogenerated charge carriers that undergo recombination during a simulated experiment based on the time‐delayed collection field (TDCF) method is discussed. A generation rate independent of the electric field is used and only bimolecular recombination is considered. The recombination increases with decreasing field thus producing an apparent field‐dependence of the generation when analyzed as performed in literature.

Charge carrier generation in organic solar cells is often reported to depend on the electric field. This is, however, not measured directly but derived from transient charge carrier extraction experiments based on the time delayed collection field (TDCF) method. In this work, numerical simulations of TDCF experiments are presented which – when analyzed in the same way as reported in literature – result in a field‐dependence of charge carrier generation despite the fact that a field‐independent generation is used. This discrepancy is shown to be caused by recombination of photogenerated charge carriers occurring in the time range prior to and during extraction. This apparent field‐dependence becomes more pronounced for larger recombination coefficients and decreasing charge carrier mobilities, very much in accordance with experimental TDCF data from literature. Even an apparent voltage‐ and time‐dependence of the bimolecular recombination coefficient is reproduced in the simulations although a constant, voltage‐independent one is used. These findings strongly question whether TDCF is an appropriate method to detect a potential field‐dependence of the photocurrent generation and the recombination coefficient. Our study shows that all experimental results can consistently be explained without the assumption of a field‐dependence of the charge carrier generation and the bimolecular recombination coefficient.