First Conventional Solution Sol–Gel-Prepared Nanoporous Materials of Nickel Oxide for Efficiency Enhancing and Stability Extending MAPbI₃ Inverted Perovskite Solar Cells

With a judicious selection of the nickel complex (precursor) and stabilizer, that is, nickel formate and enthylenediamine, respectively, a conventional sol–gel method followed by a sintering process at 450 °C, nanoporous NiOx (np-NiOx) thin films have been successfully prepared. However, the alcohol solvent used in the sol–gel process is critical that only low-viscosity ones, such as methanol, ethanol, 2-methoxyethanol, and iso-propanol, afford np-NiOx instead of plate/flake-like compact NiOx (cp-NiOx). Some stabilizers, such as 1,3-diaminopropane, diethylenetriamine, and N,N-dimethylethylenediamine, provide nothing but seemly amorphous NiOx fragmentary thin films, which are unsuitable for the hole-transporting material (HTM) in the inverted perovskite solar cells (PVSCs). SEM or AFM study has a clear differentiation between np-NiOx and cp-NiOx. Moreover, we have characterized that the np-NiOx thin films show a better conductivity, a higher hole mobility, a stronger perovskite photoluminescence quenching, and a superior transparency than those of cp-NiOx thin films. Using MAPbI3, inverted PVSCs were fabricated with either np-NiOx or cp-NiOx as HTM. A high power conversion efficiency approaching 20% was achieved by np-NiOx-based PVSCs, which was better than our best PVSC (17.95%) based on cp-NiOx. Electrochemical impedance spectroscopy (EIS) has observed a lower charge recombination resistance but a higher charge transport resistance for the cp-NiOx device comparing with those of each np-NiOx devices. Moreover, np-NiOx MAPbI3 PVSCs exhibit a less hysteresis effect and an extended lifetime in terms of shelf stability compared with those of cp-NiOx MAPbI3 PVSCs. The SEM and XRD have demonstrated the larger grain size and less gain boundary of MAPbI3 thin films on np-NiOx HTM, where MAPbI3 is in fact more hydrophobic, evident by the water contact angle.