Electron and hole transport layers have critical impacts on the overall performance of perovskite solar cells (PSCs). Herein, for the first time, a solution-processed cobalt (Co)-doped NiO film was fabricated as the hole transport layer in inverted planar PSCs, and the solar cells exhibit 18.6% power conversion efficiency. It has been found that an appropriate Co-doping can significantly adjust the work function and enhance electrical conductivity of the NiO film. Capacitance-voltage ( C- V) spectra and time-resolved photoluminescence spectra indicate clearly that the charge accumulation becomes more pronounced in the Co-doped NiO -based photovoltaic devices; it, as a consequence, prevents the nonradiative recombination at the interface between the Co-doped NiO and the photoactive perovskite layers. Moreover, field-dependent photoluminescence measurements indicate that Co-doped NiO -based devices can also effectively inhibit the radiative recombination process in the perovskite layer and finally facilitate the generation of photocurrent. Our work indicates that Co-doped NiO film is an excellent candidate for high-performance inverted planar PSCs.

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In this article, two different types of spacer cations, 1,4‐butanediamonium (BDA2+) and 2‐phenylethylammonium (PEA+) are co‐used to prepare the perovskite precursor solutions with the formula of (BDA)1‐a(PEA2)aMA4Pb5X16. By simply mixing the two spacer cations, the self‐assembled polycrystalline films of (BDA)0.8(PEA2)0.2MA4Pb5X16 are obtained, and BDA2+ is located in the crystal grains and PEA+ is distributed on the surface. The films display a small exciton binding energy, uniformly distributed quantum wells and improved carrier transport. Besides, utilizing mixed spacer cations also induces better crystallinity and vertical orientation of 2D perovskite (BDA)0.8(PEA2)0.2MA4Pb5X16 films. Thus, a power conversion efficiency (PCE) of 17.21% is achieved in the optimized perovskite solar cells with the device structure of ITO/PEDOT:PSS/Perovskite/PCBM/BCP/Ag. In addition, the complementary humidity and thermal stability are obtained, which are ascribed to the enhanced interlayer interaction by BDA2+ and improved moisture resistance by the hydrophobic group of PEA+. The encapsulated devices are retained over 95% or 75% of the initial efficiency after storing 500 h in ambient air under 40 ± 5% relative humidity or 100 h in nitrogen at 60 °C.