Interfacial reactions between atomic layer deposited NiOx hole transport layers and metal halide perovskites in n-i-p perovskite solar cells

Abstract

The hole transport layer (HTL) plays a critical role for the stability and efficiency of perovskite solar cells (PSCs) as it forms a direct interface with the metal halide perovskite (MHP) and contact electrode. In the widely researched n-i-p architecture PSCs, organic HTLs like p-doped spiro-OMeTAD (2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene) and PTAA (poly-[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]) are commonly used to fabricate high-efficiency devices. Despite the high efficiency, the inherent instability and hygroscopic nature of these doped organic HTLs are of major concern for the stability of PSCs. In this work, we incorporated atomic layer deposition (ALD) based nickel oxide NiO_x HTLs into n-i-p PSCs, which serve the function of effective charge selective transport and pre-encapsulation layers and investigated its interface formation with the MHP using synchrotron-based hard X-ray photoelectron spectroscopy. Our study reveals that the ALD-NiO_x film grown on the MHP contained a high concentration of hydroxide and oxy-hydroxide species. Additionally, defect species, including nitrogen and lead-based compounds, formed in the perovskite layer at the interface, which adversely affected PSC performance. To mitigate undesirable chemical reactions that occur during the ALD process, we introduced a 20 nm PTAA interlayer as a buffer between the MHP and ALD-NiO_x layers. This resulted in improved device efficiency and enhanced operational stability.