Facile synthesis of spiro-core-based hole-transporting material for high-performance and stable perovskite solar cells

Two new spirocore-based hole-transporting materials (HTMs), spiro-1 and spiro-2, with 5H-spiro(benzo[1,2-b:6,5-b']dithiophene-4,4′-cyclopenta[2,1-b:3,4-b′]dithiophen)-5-one moiety connected with multiple triphenylamine donor substituents are synthesized via two-step synthesis with low cost and simple procedures and used as HTM in perovskite solar cells (PSCs). The correlation between different HTM structures and the performance of the PSCs was investigated systematically. Experiments results on the thermal stability, photophysical, electrochemical, charge-transporting, as well as film-forming properties show that spiro-1 and spiro-2 are suitable candidate as HTMs for PSCs. The fabricated PSCs with spiro-1 and spiro-2 as HTMs achieved an excellent power conversion efficiency (PCE) of 21.67 % and 19.65 % respectively, both with negligible hysteresis. The obtained PCE of 21.67 % based on spiro-1 is higher than that of the reference solar cell using traditional HTM of spiroOMeTAD (20.59 %) under the same conditions. It may be ascribed to the higher hole mobility, the more efficient hole extraction at the HTM/perovskite interface, the better film morphology. Moreover, the devices based on the new HTM showed good long-term stability and maintained over 80 % of their initial efficiency under continuous stressing at 85 °C and light illumination at 50 °C for 500 h in air without any encapsulation, which were superior to that of spiroOMeTAD. This work demonstrates that the newly developed spirofused molecules are promising HTMs for highly efficient and stable PSCs for the future commercialization of PSCs.