Acetamidinium based 2D alternating cation perovskite for efficient solar cells

Quasi-two-dimensional (Q-2D) layered perovskite with alternating cations in the interlayer space (ACI) is a potential candidate for highly efficient and environmental stable solar cells. Here, we report the fabrication of photovoltaic devices using a novel layered Q-2D perovskite (AcA)MA(n)Pb(n)I3(n+1) (n = 4) containing acetamidinium (AcA(+)) and methylammonium (MA(+)) cations as spacer. With introducing appropriate amount of methylammonium chloride (MACl) as an additive in precursor solution, (AcA)MA(4)Pb(4)I(13) film presents micrometer sized crystalline grain and high crystalline quality, resulting in more than 40% enhancement of power conversion efficiency (PCE) compared to that without additive. Specially, an optimized (AcA)MA(4)Pb(4)I(13) device achieves the best PCE of 15.04% and retains more than 85% of the original PCE after 30 days under ambient condition (similar to 40% relative humidity, 25 degrees C) without encapsulation, while there is no sign of degradation for accompanying (AcA)MA(4)Pb(4)I(13) film. Using ultrafast transient spectroscopy, we find that MACl additive in precursor solution results in slower charge transfer from small n phase to 3D-like phases in (AcA)MA(4)Pb(4)I(13) film, while there is no photoluminescence observed from small n materials; thus we may deduce that electrons and holes are separated efficiently in small n phases as well. We believe our results not only demonstrate that AcA(+) can act as an effective spacer cation fabricating perovskite for efficient solar cells, but also help to understand the unique working mechanism of ACI perovskites in-depth.