As the front runner among emerging photovoltaic technologies, perovskite solar cells (PSCs) with certified power conversion efficiencies (PCEs) over 26% show great promise for scale-up and future commercialization due to relatively simple and low-cost solution processes. However, the disordered stoichiometric compositions at surfaces generate abundant defects in the solution-processed perovskite films, particularly at surfaces and grain boundaries. Such defects shorten the carrier lifetime and limit photovoltaic performance. Moreover, these defects are responsible for accelerated ion migration and the initial invasion of moisture or oxygen, ultimately causing device instability. The defects also hinder the scale-up of PSCs, thus restricting commercialization. Efficient and stable PSCs with a simple active layer are desirable for manufacturing. Organic halide salt passivation is considered to be an essential strategy to reduce defects in state-of-the-art PSCs. This strategy, however, suffers from the inevitable formation of in-plane favored two-dimensional (2D) perovskite layers with impaired charge transport, especially under thermal conditions, impeding photovoltaic performance and device scale-up.
Firstly, the inclusion of larger ammonium salts is demonstrated leading to a trade-off between improved stability and efficiency, which is attributed to the perovskite films containing a 2D component.[1] The addition of 0.3 mole percent of fluorinated lead salt into the three-dimensional methylammonium lead iodide perovskite enables low-temperature fabrication of simple inverted solar cells with a maximum PCE of 21.1%. The perovskite layer has no detectable 2D component at salt concentrations of up to 5 mole percent. The high concentration of fluorinated material found at the film-air interface provides greater hydrophobicity, increased size and orientation of the surface perovskite crystals, and unencapsulated devices with increased stability to high humidity.
Secondly, the energy barrier of 2D perovskite formation from ortho-, meta- and para-isomers of (phenylene)di(ethylammonium) iodide (PDEAI₂) that were designed for tailored defect passivation was studied.[2] Treatment with the most sterically hindered ortho-isomer not only prevents the formation of surficial 2D perovskite film, even at elevated temperatures but also maximizes the passivation effect on both shallow- and deep-level defects. The ensuing PSCs achieve an efficiency of 23.9% with long-term operational stability (over 1000 hours). Importantly, a record efficiency of 21.4% for the perovskite module with an active area of 26 cm² was achieved.