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 (Cu₂O) substrate, the Cu₂O 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 (Cu₂O) 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 Al₂O₃ 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.