Lead acetate (PbAc2), as a lead (Pb2+) source, has extraordinary advantages in large-area solution processing of uniform and pinhole-free MAPbX3 perovskite films at low temperature for photovoltaic applications. However, PbAc2-derived MAPbI3 thin films usually exhibit small crystallites (200 nm) and high density of grain boundaries and their associated trap states, hindering charge carrier transport, resulting in lower photovoltaic performance compared with the state-of-the-art lead Iodide (PbI2) derived thin films. In this study, we show that the use of acetate charge additives in PbAc2-based precursor solutions can overcome these problems and significantly increase the carrier mobility in thin films, leading to high fill factors (FF). It was also found that some chloride ions remained in the final film at the grain boundaries, passivating the trap states generated by the defects. Therefore, the FF of planar inverted perovskite solar cells (PSCs) based on the optimized Cl-doped MAPbI3 layer with PEDOT:PSS and PCBM as charge-transport layers is improved by up to 82%, which is one of the best FF values for PSCs, while Compared with devices using pristine PbAc2-based MAPbI3 films with an average FF of 74.5%, the PCE exceeds 16% and the environmental stability is improved. This study provides insight into factors affecting FF in PSCs and suggests strategies to further improve this parameter to reach its radiative limit.
Although several approaches have been proposed, such as using additives during film fabrication or post-film deposition treatments and passivation (Lee et al., 2017, Zhu et al., 2017) to produce high-quality perovskite films, Few studies have investigated the effect of charge transport properties on the photovoltaic performance of PSCs, especially FF. Although the conductivity and thickness of the charge transport layer in PSCs may affect FF, the maximum achievable parameter is determined by the perovskite layer. In addition to being affected by non-radiative recombination, FF, like voltage open circuit (VOC), also depends on charge carrier transport within the perovskite layer (Qiu et al., 2020). In this regard, several reports have shown that controlling the grain size and orientation in thin films is the key to achieve enhanced charge conduction and thus high FF. Jiang and Wu developed a method to control the growth of MAPbI3 thin films with reduced grain boundaries and large grain size (up to 1.5 μm) exhibiting favorable crystallographic orientations, thereby achieving a remarkable fill factor of 86% ( Chiang and Wu, 2018). In another study, Bolink and colleagues showed that doping strontium (Sr3+) at the Pb2+ sites of the MAPbI3 lattice can extend the carrier lifetime to about 40 μs and enhance the material mobility, thereby increasing the cell photocurrent and FF