A Dual Layer of NiOx 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 NiO_x, a nanoporous NiO_x (np-NiO_x) on a compact NiO_x (cp-NiO_x), outperforms a single layer of cp-NiO_x. With a dual layer of cp/np-NiO_x HTM, we have achieved a PCE as high as 20.7%, which is one of the highest PCEs among inverted PVSCs using NiO_x as HTM without doping NiO_x or an interlayer between NiO_x 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-NiO_x than with cp-NiO_x. Direct current conductivity measurements showed a higher conductivity of cp/np-NiO_x compared to cp-NiO_x. A deeper HOMO energy level was found for cp/np-NiO_x compared to cp-NiO_x. The space-charge limited current (SCLC) method estimated a higher trap-state density in cp-NiO_x than in cp/np-NiO_x. The studies of light intensity-dependent open-circuit voltage (V_OC) and short-circuit current density (J_SC) revealed a less charge recombination in cp/np-NiO_x than in cp-NiO_x PVSCs. We have also found and verified that using a larger volume of the ethylenediamine stabilizer in the preparation of np-NiO_x results in a higher-performance cp/np-NiO_x HTM.