A highly transparent NiO layer was prepared by a solution processing method with nickel(II) 2‐ethylhexanoate in non‐polar solvent and utilized as HTM in perovskite solar cells. Excellent optical transmittance and the matched energy level lead to the enhanced power conversion efficiency (PCE, 18.15 %) than that of conventional sol–gel‐processed NiO‐based device (12.98 %).

Methylammonium lead halide perovskites (CH3NH3PbI3) have been extensively studied as a photoactive layer for next‐generation efficient solution‐processed photovoltaic devices.1-5, 6 It was first reported by Miyasaka and co‐workers in 2009.7 Organic–inorganic hybrid perovskite solar cells (PSCs) have reached PCE values of 22.1 %,8 which is excellent compared to other thin film photovoltaics.9 The PSCs can be divided into two classes, n‐i‐p and p‐i‐n device. Nevertheless, because of major hysteresis in conventional structured devices, the truth of the photocurrent density–voltage (J–V) characteristics is difficult to judge.10-13 Therefore, p‐i‐n‐type devices are beneficial for suppressing hysteresis because of the compensation for the relatively shorter hole‐diffusion length in the CH3NH3PbI3 layer than the electron diffusion length.13

At present, poly(3,4‐ethylenedioxythiophene) poly(styrene‐sulfonate) (PEDOT:PSS) is a widely used hole transport material (HTM) in p‐i‐n‐type PSCs. However, its hygroscopicity and high acidity significantly impede the devices long‐term operation.14 In this regard, various inorganic materials have been used to replace PEDOT:PSS. In particular, NiOx is the most popular inorganic material due to its excellent properties, such as high optical transmittance, suitable work function and low‐lying valence band.15 Jeng et al. first used NiOx films as HTM and achieved a PCE of 7.8 % in the p‐i‐n‐type devices.16 Subsequently, they built a double HTM, or changed the approach of preparing NiOx films. The open circuit voltage (VOC) was improved, but the short circuit current density (JSC) and the fill factor (FF) were still low.17 To improve the performance, Jen et al. utilized Cu‐doped NiOx films and achieved a PCE of 15.4 %, and the cells with pure NiOx only provided a PCE of 8.73 %.18 Later, they have modified film‐forming process using a combustion method and achieved a PCE of 17.74 %.14 Recently, Yang et al. used a molecular monolayer to modify NiOx layer surfaces and exhibited a higher PCE than the reference device.19 Park et al. prepared NiO films via pulsed laser deposition, and reached a PCE of 17.3 %.13 To date, the devices based vapor‐deposited NiO films could achieve good performance, however, vapor deposition needs energy for high vacuum. As a comparison, the method of liquid‐deposition has a greater advantage for commercial development. The efficiency of p‐i‐n‐type PSCs on based liquid‐deposited NiOx HTM has been largely enhanced, unfortunately, thus‐prepared NiOx layers need to be doped with metal ions, or modified by a molecular monolayer. The PCE of the devices based pure NiO layers is still low, which is mainly limited by JSC and FF. The basic reason could be related to the low quality of NiO film and the poor contact between NiO and conducting substrate. We assume that the poor NiO film is due to the reaction system. As reported precursor solution is polar system, during the sol–gel process, small pores usually occur accompanied with the evaporation of water or other by‐products. Additionally, the precursor solution of perovskite has a poor wettability on such NiO films, which would affect the charge transfer and injection, leading to the low PCE. Replacement of the sol–gel system by using the organometal precursor system could be a simple and effective way to overcome the shortcoming.

Here we report a solution processed NiO film prepared from nickel(II) 2‐ethylhexanoate in non‐polar solvent, as the HTM in inverted planar PSCs and exhibits the highest PCE of 18.15 %. Which is much better than that of the cell based on sol–gel‐processed NiO15 film prepared from a polar system, with a PCE of 12.98 %. We attribute the high performance to the match of function of solution‐processed NiO with the occupied molecular orbital (HOMO) of the CH3NH3PbI3 perovskite and fluorine‐doped tin oxide (FTO), which minimize the energy loss for hole extraction and transport and optimize the magnitude of FF. The higher of JSC results from the better optical transmittance of solution‐processed NiO film and the increased light absorption of the perovskite layer. Our results indicate that a non‐polar solution system enhances the PCE.

Solution‐processed NiO films were prepared from nickel(II) 2‐ethylhexanoate in n‐hexane, more details are shown in the experimental section. The morphological properties of the resulting films were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). SEM highlights the full coverage of NiO on the FTO substrate (Figure 1 A). The film is interrupted by cracks along the surface. AFM provides the surface roughness parameter (RMS, root mean square roughness). The RMS value of the bare surface of FTO (Figure S1) is 18.499 nm, the value for as prepared NiO film (Figure 1 B) shows smoother surface, with a RMS value of 14.673 nm. The SEM and AFM results indicate that the as‐prepared NiO film on FTO has good coverage and a smooth surface.