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. In this talk, several molecular approaches toward passivated defective states leading to stabilized perovskite devices will be presented.

Firstly, 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.[1] 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, an efficiency of 21.4% for the perovskite module with an active area of 26 cm2 was achieved.

Secondly, as a follow-up, passivating salts based on the ortho-methylammonium iodide functional units were molecularly engineered to study the steric hindrance-driven passivation effect further. The incorporation of the fluorine atoms in passivating agents is beneficial not only for maximized defects passivation effect ensuring improved charge transport but also for significantly enhanced hydrophobicity of the perovskite film leading to enhanced device stability. The highest power conversion efficiency (PCE) of over 24% has been achieved on surficial passivated PSCs based on fluorinated cation PFPDMAI₂. Importantly, long-term operational stability over 1500 h is demonstrated showing a great prospect of a simple passivation strategy forming a thin organic halide salt layer instead of a 2D perovskite layer on the surface.