Simple hole-transporting materials containing twin-carbazole moiety and unconjugated flexible linker for efficient and stable perovskite solar cells

Three twin-carbazole-based hole-transporting materials (HTMs) bearing an unconjugated flexible linker and four peripheral diphenylamine groups with different methoxy positions (pp-, pm-, and po-) are first synthesized (GJ-pp, GJ-pm, and GJ-po) and successfully employed in perovskite solar cells (PSCs). The HTMs were obtained from simple synthetic steps and facile purification techniques. The thermal stability, optical and electrochemical properties, density functional theory calculations, hole-transporting properties, X-ray diffraction, hole transfer dynamics, hydrophobic properties, surface morphology, and photovoltaic performances were measured and discussed. Device performance depends on the methoxy group positions. GJ-pp shows the best power conversion efficiency (17.23%) among the three twin-carbazole-based HTMs, and slightly lower than that of the spiro-OMeTAD-based devices (17.96%). However, the GJ-pp-based devices show outstanding thermal stability compared with the spiro-OMeTAD-based ones. Laboratory synthetic cost of GJ-pp is merely 29.0% of the purchasable spiro-OMeTAD price, and the concentration of GJ-pp is lower than that of spiro-OMeTAD (50.0 vs. 72.3 mg mL(-1)). The results demonstrate that the twin-carbazole moiety can endow enhanced charge transport properties due to its potential intermolecular interactions, and the conjugated core is unnecessary for promising HTMs.