Flexible quintuple cation perovskite solar cells with high efficiency

Flexible perovskite solar cells (PSCs) present the most promising photovoltaic technologies due to their flexibility, light weight, and low temperature processing. However, their power conversion efficiency (PCE) is still far behind those of rigid PSCs. The enhancement of the PCE of flexible devices remains challenging. Here, two strategies are developed to improve the PCE of flexible PSCs. One strategy is to use quintile monovalent cations to obtain high-quality perovskite films. Rubidium (Rb) and potassium (K) cations are added to a basic perovskite film containing cesium, methylammonium, and formamidinium cations to improve film quality for efficient PSCs. It is revealed that the coexistence of Rb + and K + cations can effectively reduce the recombination within PSCs and thus improve the photovoltaic performance. The second strategy is to insert an ultrathin HfO 2 layer between indium tin oxide (ITO) and the tin oxide layer by atomic layer deposition at low temperature. The recombination between ITO and perovskite films is found to be effectively suppressed after the introduction of an optimized HfO 2 layer. As a result, a record PCE for the PSC devices based on polyethylene naphthalate (PEN)/ITO substrates is obtained as high as 19.11%, which is the highest reported PCE for flexible PSCs to date.