Synthesis of spirodithienogermole with triphenylamine units as a dopant-free hole-transporting material for perovskite solar cells

Spiro-condensed dithienogermoles with electron-donating aromatic substituents were prepared by Pd-catalyzed Stille coupling reactions of spirobi(dibromodithienogermole) and the corresponding trimethylstannylarene derivatives. Their optical and electrochemical properties were examined in comparison with those of related model compounds without the germole ring systems and the spiro-condensed unit, indicating that the spiro-condensed dithiengermole systems play roles in the enhancement of the conjugation. Perovskite solar cells (PSCs) were fabricated using triphenylamine-containing spirobi(dithienogermole) (sDTG-tpa) as the hole-transporting material (HTM). The PSCs showed the maximal photo-current conversion efficiency of 14.67%, which was higher than that of a similar device with spiro-OMeTAD as the HTM (9.53%), indicating the superior performance of sDTG-tpa as HTM compared with spiro-OMeTAD, a typical HTM for PSCs.