Charge Transfer and Retention in 2D Passivated Perovskite-C Systems.

2D perovskites and organic ligands are often implemented as passivating interlayers in perovskite solar cells. Herein, five such passivates are evaluated by using time-resolved spectroscopy to study the carrier dynamics at the perovskite-C interface. The impact of passivation on factors such as charge transfer rate, charge retention in the acceptor layers, surface recombination, and uniformity are mapped onto the solar cell performance. The charge transfer was found to take place in tens of nanoseconds, and the charge retention without any passivate lasted a few hundred nanoseconds. The passivate that produced the best solar cells, ethylenediammonium iodide, extended the charge retention time up to one microsecond, which significantly increased the open-circuit voltage. It also had the best uniformity and hence least variance in power conversion efficiency. Curiously, it did not merely adjust surface energy states to enhance charge transfer but also extracted charges by itself without the C, resulting in higher short-circuit current.