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The photovoltaic performance of organic-inorganic hybrid perovskite solar cells has reached a bottleneck after rapid development in last few years. Further breakthrough in this field requires deeper understanding of the underlying mechanism of the photoelectric conversion process in the device, especially the dynamics of charge-carrier recombination. Originating from dye-sensitized solar cells (DSSCs), mesoporous-structured perovskite solar cells (MPSCs) have shown many similarities to DSSCs with respect to their photoelectric dynamics. Herein, by applying the multiple-trapping model of the charge-recombination dynamic process for DSSCs in MPSCs, with rational modification, a novel physical model is proposed to describe the dynamics of charge recombination in MPSCs that exhibits good agreement with experimental data. Accordingly, the perovskite- and TiO2-dominating charge-recombination processes are assigned and their relationships with the trap-state distribution are also discussed. An optimal balance between these two dynamic processes is required to improve the performance of mesoporous-structured perovskite devices. [ABSTRACT FROM AUTHOR]

Organic-inorganic halide perovskite solar cells have rapidly come to prominence in the photovoltaic field. In this context, CH3NH3PbI3, as the most widely adopted active layer, has been attracting great attention. Generally, in a CH3NH3PbI3 layer, unreacted PbI2 inevitably coexists with the perovskite crystals, especially following a two-step fabrication process. There appears to be a consensus that an appropriate amount of unreacted PbI2 is beneficial to the overall photovoltaic performance of a device, the only disadvantageous aspect of excess residual PbI2 being viewed as its insulating nature. However, the further development of such perovskite-based devices requires a deeper understanding of the role of residual PbI2. In this work, PbI2-enriched and PbI2-controlled perovskite films, as two extreme cases, have been prepared by modulating the crystallinity of a pre-deposited PbI2 film. The effects of excess residual PbI2 have been elucidated on the basis of spectroscopic and optoelectronic studies. The initial charge separation, the trap-state density, and the trap-state distribution have all been found to be adversely affected in PbI2-enriched devices, to the detriment of photovoltaic performance. This leads to a biphasic recombination process and accelerates the charge carrier recombination dynamics. [ABSTRACT FROM AUTHOR]

Behind the outstanding photovoltaic performance of perovskite solar cells (PSCs), the correlation between the structural configuration of the PSCs and the mechanism of photoelectric conversion is still not fully understood. In this paper, two types of PSCs with different structural configurations, that is, mesoporous (meso) and planar, were prepared, and systematic optoelectronic transient studies were performed on these devices. Although the photovoltaic performances of the tested meso- and planar-PSCs were comparable, the different structural configurations resulted in distinct charge dynamics and photoelectric conversion mechanisms. By taking the dynamic results into account, the influence of a meso-TiO2 layer on charge accumulation, transport, and recombination was elucidated. More importantly, an excessively slow temporal response component in the optoelectronic transients was observed, which was proven to be linked to specific hysteresis phenomena in the PSCs. The experimental results collectively indicated that the hysteresis, which showed strong dependence on device structure and charge dynamics, was irrelevant to charge trapping or ferroelectricity but probably resulted from interfacial charge accumulation and/or ion migration. This work provides deep insight into the structure-function relationship in PSCs and highlights the effect of hysteresis on charge dynamics. [ABSTRACT FROM AUTHOR]

Organolead trihalide perovskite films with a large grain size and excellent surface morphology are favored to good‐performance solar cells. However, interstitial and antisite defects related trap‐states are originated unavoidably on the surfaces of the perovskite films prepared by the solution deposition procedures. The development of post‐growth treatment of defective films is an attractive method to reduce the defects to form good‐quality perovskite layers. Herein, a post‐treatment tactic is developed to optimize the perovskite crystallization by treating the surface of the one‐step deposited CH3NH3PbI3 (MAPbI3) using formamidinium iodide (FAI). Charge carrier kinetics investigated via time‐resolved photoluminescent, open‐circuit photovoltage decay, and time‐resolved charge extraction indicate that FAI post‐treatment will boost the perovskite crystalline quality, and further result in the reduction of the defects or trap‐states in the perovskite films. The photovoltaic devices by FAI treatment show much improved performance in comparison to the controlled solar cell. As a result, a champion solar cell with the best power conversion efficiency of 20.25% is obtained due to a noticeable improvement in fill factor. This finding exhibits a simple procedure to passivate the perovskite layer via regulating the crystallization and decreasing defect density. Formamidinium iodide (FAI) post‐treatment is used on MAPbI3 surfaces to obtain high‐quality perovskite films, which leads to traps‐states or defects reduction, thus reducing hysteresis. The FAI post‐treated solar cells show improved device performance, with the average efficiency increasing from 16.86% to 18.40%, and the power conversion efficiency reaching 20.25%. [ABSTRACT FROM AUTHOR]

The photovoltaic performance of planar perovskite solar cell is significantly influenced by the morphology of perovskite film. In this work, five kinds of devices with different perovskite film morphologies were prepared by varying the concentration of CH 3 NH 3 Cl in precursor solutions. We found that best morphology of perovskite film results in the excellent photovoltaic performance with an average efficiency of 15.52% and a champion efficiency of 16.38%. Transient photovoltage and photocurrent measurements are performed to elucidate the mechanism of photoelectric conversion processes, which shows that the charge recombination is effectively suppressed and the charge transport is obviously promoted by optimized morphology. [ABSTRACT FROM AUTHOR]

[Display omitted] • The migration time scales for different ions in operational perovskite devices are identified. • The cations play a decisive role in hysteresis effect rather than anions. • Steric hindrance effects play a crucial role in determine the ion migration dynamics. The migration time scales of different ions in methylammonium lead iodide perovskite devices are identified by a facile photovoltage-buildup technique. The cation-related slow migration is found to be the primary factor that determines the hysteresis, given a great match between the migration dynamics and the scanning rate of photoelectric measurements. The steric hindrance effects can significantly tune the ion migration dynamics. Introducing a large cation (formamidinium) or a little amount of small anion (bromide) will slow down or accelerate the cation migration, respectively. The proposed modification method should be an ideal strategy for suppressing the hysteresis phenomenon in perovskite solar cells. [ABSTRACT FROM AUTHOR]

Fill factor is a critical parameter for characterizing the performance of perovskite solar cells and is speculated to be closely related to the trap states, while the underlying correlation remains unclear. In this work, the effect of the energetic distribution of trap states in perovskite on the fill factor of the device is systemically investigated. Perovskite films with distinct trap states are prepared with/without the dimethyl sulfoxide additive, based on which the corresponding devices exhibit different power conversion efficiency and fill factors, but similar open-circuit voltages and short-circuit currents. Subsequently, the recombination dynamics process, trapping/detrapping efficiency and transport property of the photo-generated charge carrier are fully studied. Moreover, it is found that the difference in the energetic distribution of trap states leads to the variation of charge recombination/transport dynamics. The energetic distribution of trap states also determines the activation energy of thermal-assisted carrier detrapping and charge collection efficiency, which further affects the FF behavior. The passivation of trap state, especially deep trap states, could reduce the charge recombination loss, make the charge detrapping process more efficient, modify the charge transport property, improve the charge collection efficiency and eventually lead to the enhancement of FF. Image 1 • Effect of trap states on the charge collection efficiency and fill factor is studied. • The energetic distribution of trap states determines charge collection efficiency. • Fill factor is the manifestation of photo-generated charge collection efficiency. • Trap state passivation shallows its energetic distribution and improve fill factor. [ABSTRACT FROM AUTHOR]