Solvent Engineering of a Dopant-Free Spiro-OMeTAD Hole-Transport Layer for cm-Scale Perovskite Solar Cells having High Efficiency and Thermal Stability.

High efficiency and environmental stability are mandatory performance requirements for commercialization of perovskite solar cells (PSCs). Herein, efficient cm-scale PSCs with improved stability were achieved by incorporating an additive-free 2,2',7,7'-tetrakis[N,N-di(p-methoxyphenyl)amino]-9,9'-spirobifluorene (spiro-OMeTAD) hole transporting material (HTM) through simply substituting the usual chlorobenzene solvent with pentachloroethane (PC). A stabilized power conversion efficiency of 16.1% under simulated AM 1.5G 1-sun illumination with an aperture of 1.00 cm2 was achieved for PSCs using an additive-free spiro-OMeTAD layer cast from PC. X-ray analysis suggests chlorine radicals from pentachloroethane transfer partially to spiro-OMeTAD and retain in the HTM film, resulting of conductivity improvement. Moreover, unencapsulated PSCs having cm-scale active area cast from PC retained >70% of their initial PCE after aging at 80 °C for 500 h, in contrast with less than 20% retention for control devices. Morphological and X-ray analysis of the aged cells revealed that the perovskite and HTM layers remain almost unchanged in the cells with spiro-OMeTAD layer cast from PC whereas serious degradation occurred in the control cells. This study not only reveals the decomposition mechanism of PSCs in the presence of HTM-additives, but also opens up a broad range of organic semiconductors for radical doping.