Reaction Dynamics between Formamidinium Lead Iodide and Copper Oxide.

Copper oxide (CuO) has been announced as a very promising hole-transporting layer for perovskite solar cells. However, in our previous work, we have shown that once a formamidinium lead triiodide (FAPI) perovskite is spin-coated on a spray-coated cuprous oxide (CuO) substrate, the CuO diffuses into and reacts with the FAPI film. In order to verify if the degradation products are related to the oxidation state of CuO and/or its preparation method, in this work, we first prepared CuO films by thermal oxidation at temperatures ranging from 120 to 300 °C. While increasing the process temperature, a transformation from copper I (CuO) to copper II (CuO) oxidation states was observed. For both oxidation states of copper, FAPI perovskite degradation was found; however, some alterations in the reaction products were noticed. In contrast to our expectations, the introduction of an ultrathin plasma-enhanced atomic layer deposited AlO layer in between both films only partially blocked the CuO migration into the FAPI film. It can be concluded that regardless of the chemical composition and/or preparation method of CuO, the overlayered FAPI film gets decomposed. In order to use CuO as a hole-transporting layer in solar cells, new strategies must be developed to limit these unwanted chemical reactions.