The investigation of inorganic–organic hybrid perovskite solar cells is of major interest since its massive increase in power conversion efficiency (PCE) from 3.8% [
1] in 2009 to 22.1% in 2016 [
2]. The highest PCEs so far were achieved in n-i-p device architectures, using mesoporous TiO
2 as electron transport layer (ETL) and spiro-OMeTAD as hole transport layer (HTL), respectively [
3]. Recently, Saliba et al. introduced a triple cation perovskite, using a combination of methylammonium, formamidinium and cesium ions (MA/FA/Cs) and a mixture of bromide and iodide reaching PCE values of more than 20% together with enhanced device stability. Maximum PCEs of 21.1% and 20.96% were reached with a Cs
x(MA
0.17FA
0.83)
(100−x)Pb(I
0.83Br
0.17)
3 perovskite absorber layer where x = 5 and 10, respectively. This A-site cation combination results in the suppression of the photovoltaic non-active “yellow phase” and enhances perovskite crystallinity. However, so far this route is mainly investigated in n-i-p based perovskite solar cells [
4‐
6]. In these structures, TiO
2 is used as ETL in most cases in combination with HTLs such as spiro-OMeTAD or other organic HTLs [
7]. Despite reaching high efficiencies, commonly high temperature processing is required for the preparation of the compact and mesoporous TiO
2 films limiting low cost and energy efficient fabrication. Compared to the n-i-p device structures containing mesoporous TiO
2, p-i-n planar devices offer low temperature fabrication, which is favourable for roll-to-roll fabrication [
8,
9]. In p-i-n perovskite devices, usually PCBM is used as ETL and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as HTL. However, due to its acidic nature and hygroscopic character, devices with PEDOT:PSS layers might degrade faster than devices with e.g. NiO
x hole transport layers, which have a similar valence band energy as PEDOT:PSS films [
10‐
14]. Some research groups also reported better crystallinity of the perovskite film on a NiO
x layer than on PEDOT:PSS [
15,
16]. NiO
x is an inorganic metal oxide semiconductor, which exhibits high optical transmittance, high stability, easy processability, and a wide band gap that allows high hole mobility and good electron blocking ability [
17,
18]. There exist several methods for NiO
x deposition, like atomic layer deposition, flame spray synthesis, pulsed laser deposition, sputtering and electrodeposition [
16,
19‐
22]. However, for fast and large scale roll-to-roll fabrication, coating or printing processes for the application of the NiO
x films are of interest. In this regard, several methods using spin coating followed by an additional annealing step are reported [
23‐
25]. Interestingly, higher temperature treatment of the NiO
x films significantly affects the device performance. Jiang et al. reported that annealing NiO
x above 150 °C reduces the device performance due to the formation of Ni
2O
3 species [
26]. A reduction of the device performance was also seen by Hou et al. when heating the NiO
x film above 140 °C [
16]. Moreover, it was shown that p-i-n devices with MAPbI
3 as absorber layer reveal low to almost negligible hysteresis, especially if NiO
x is used as HTL [
9,
27]. Recently, a PCE of 18.4% for perovskite-based solar cells with a NiO
x HTL was reported by Bai et al. who investigated the perovskite crystal growth on NiO
x films and the formation of intermediate layers (MAPbI
3-DMSO) with increasing DMSO concentration [
25].
In this work, we investigate and compare different hole transport layers (HTLs), namely solution based NiOx, sputtered NiOx, and PEDOT:PSS, in solar cells using the triple cation lead halide perovskite Cs0.08(MA0.17FA0.83)0.92Pb(I0.83Br0.17)3 as absorber layer in terms of solar cell performance and hysteresis properties of the devices.