On the VOC loss in NiO-based inverted metal halide perovskite solar cells†

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.