NiOx nanoparticles as p-dopants for instant oxidation of Spiro-OMeTAD enabling high-performance and stable perovskite solar cells

Abstract

Perovskite solar cells (PSCs) have demonstrated remarkable advancements, achieving power conversion efficiencies (PCEs) exceeding 26 %. A critical component in these devices is 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (Spiro-OMeTAD), which serves as the conventional hole transport material (HTM) in “n-i-p” structural PSCs. However, the optimal functionality of Spiro-OMeTAD necessitates doping with lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) and 4-tert-butylpyridine (tBP), both of which introduce stability challenges. Li-TFSI is highly hygroscopic, leading to moisture-induced degradation, while tBP exhibits high volatility, compromising the long-term integrity of the hole transport layers (HTLs). Additionally, the preparation of Spiro-OMeTAD typically requires a prolonged oxidation process (8–72 h) to achieve a sufficiently oxidized state with enhanced charge transport properties. In this study, stable oleyl amine (OAm) ligand nickel oxide nanoparticles (noted as the oil-NiO_x NPs) were employed as p-type dopants and instantaneous oxidization agents to accelerate the oxidation of Spiro-OMeTAD. This substitution enhances the dispersion of Li-TFSI in chlorobenzene (CB) and promotes the formation of a uniform HTL film, while simultaneously introducing an immediate oxidation pathway that markedly accelerates the oxidation of Spiro-OMeTAD. In this process, Li⁺ interacts with the NiO_x lattice to generate Ni³⁺ species, which act as transient oxidizing agents and drive the rapid oxidation of Spiro-OMeTAD. Consequently, power conversion efficiency (PCE) of 24.20 % was achieved, along with enhanced stability under moisture (55 % RH), thermal (85°C), and light exposure conditions, maintaining 94 %, 61 %, and 80 % of their initial efficiency after more than 800 h of continuous operation.