A Dual Layer of NiO_<i>x</i> Hole-Transporting Material Boosting the Efficiency of Inverted Perovskite Solar Cells up to 20.7%

For the hole transport material (HTM) of methylammonium lead(II) triiodide (MAPbI3)-inverted perovskite solar cells (PVSCs), we have demonstrated that a dual layer of NiOx, a nanoporous NiOx (np-NiOx) on a compact NiOx (cp-NiOx), outperforms a single layer of cp-NiOx. With a dual layer of cp/np-NiOx HTM, we have achieved a PCE as high as 20.7%, which is one of the highest PCEs among inverted PVSCs using NiOx as HTM without doping NiOx or an interlayer between NiOx and the perovskite. In this report, scanning electron microscopy (SEM), atomic force microscopy (AFM), and photoluminescence (PL) spectroscopy have demonstrated that the interfacial contact of MAPbI3 is better with np-NiOx than with cp-NiOx. Direct current conductivity measurements showed a higher conductivity of cp/np-NiOx compared to cp-NiOx. A deeper HOMO energy level was found for cp/np-NiOx compared to cp-NiOx. The space-charge limited current (SCLC) method estimated a higher trap-state density in cp-NiOx than in cp/np-NiOx. The studies of light intensity-dependent open-circuit voltage (VOC) and short-circuit current density (JSC) revealed a less charge recombination in cp/np-NiOx than in cp-NiOx PVSCs. We have also found and verified that using a larger volume of the ethylenediamine stabilizer in the preparation of np-NiOx results in a higher-performance cp/np-NiOx HTM.