Abstract
Recent reports have shown that nickel oxide (NiO) when adopted as a hole transport layer (HTL) in combination with organic layers, such as PTAA or self-assembled monolayers (SAMs), leads to a higher device yield for both single junction as well as tandem devices. Nevertheless, implementing NiO in devices without PTAA or SAM is seldom reported to lead to high-performance devices. In this work, we assess the effect of key NiO properties deemed relevant in literature, namely- resistivity and surface energy, on the device performance and systematically compare the NiO-based devices with those based on PTAA. To this purpose, (thermal) atomic layer deposited (ALD) NiO (NiO(Bu-MeAMD)), Al-doped NiO (Al:NiO(Bu-MeAMD)), and plasma-assisted ALD NiO (NiO(MeCp)) films, characterized by a wide range of resistivity, are investigated. Although Al:NiO(Bu-MeAMD) (∼400 Ω cm) and NiO(MeCp)(∼80 Ωcm) films have a lower resistivity than NiO(Bu-MeAMD) (∼10 kΩ cm), the Al:NiO(Bu-MeAMD) and NiO(MeCp)-based devices are found to have a modest open circuit voltage (V (OC)) gain of ∼30 mV compared to NiO(Bu-MeAMD)-based devices. Overall, the best-performing NiO-based devices (∼14.8% power conversion efficiency (PCE)) still lag behind the PTAA-based devices (∼17.5%), primarily due to a V (OC) loss of ∼100 mV. Further investigation based on light intensity analysis of the V (OC) and FF and the decrease in V (OC) compared to the quasi-Fermi level splitting (QFLS) indicates that the V (OC) is limited by trap-assisted recombination at the NiO/perovskite interface. Additionally, SCAPS simulations show that the presence of a high interfacial trap density leads to a V (OC) loss in NiO-based devices. Upon passivation of the NiO/perovskite interface with Me-4PACz, the V (OC) increases by 170-200 mV and is similar for NiO(Bu-MeAMD) and Al:NiO(Bu-MeAMD), leading to the conclusion that there is no influence of the NiO resistivity on the V (OC) once interface passivation is realized. Finally, our work highlights the necessity of comparing NiO-based devices with state-of-the-art HTL-based devices to draw conclusion about the influence of specific material properties on device performance.