Cooperating with additives: low-cost hole-transporting materials for improved stability of perovskite solar cells

The widespread adoption of perovskite-based solar technologies is strictly related to the cost reduction of the hole-transporting component in the device, while maintaining compatibility with its absorbing active layer. To date, several organic systems have been developed to compete with the pioneering 2,2 ',7,7 '-tetrakis(N,N-di-4-methoxyphenylamino)-9,9 '-spirobifluorene (Spiro-OMeTAD) used as the benchmarking hole-transporting material (HTM). However, an easily accessible platform to construct economically competitive HTM scaffolds as alternatives to Spiro-OMeTAD is still lacking. In this study, we propose a straightforward route (excluding organometallic cross-coupling reactions) to prepare nonconventional HTMs (BTF and BTC) based on a bithiophene core decorated with unsymmetrical triarylamine groups. The two HTMs are implemented in dopant-free n-i-p perovskite solar cells (PSCs) to evaluate their performance and long-term behaviour. Despite enhancing hole extraction and transport at the perovskite/HTM interface compared to the Spiro-OMeTAD benchmark, BTC does not perform exceptionally as an undoped HTM in PSCs (PCE = 14.0% vs. 16.5% of the doped Spiro-OMeTAD reference). Moreover, the efficiencies of unencapsulated devices rapidly degraded over time (T80: similar to 57 days) due to weak HTM adhesion at the perovskite interface. Conversely, using tert-butylpyridine as the sole additive slightly increases performance (PCE = 14.8%) and remarkably improves device resilience to ambient exposure (PCE = 15.4% after 401 days), representing one of the longest shelf-stability experiments ever reported. Other dopant/additive formulations are unproductive in terms of both efficiencies and device resistance. These results indicate that focusing on the molecular design of low-cost HTMs and investigating the appropriate HTM/additive systems can be a promising strategy for developing efficient and stable PSCs.