Thermally Stable Perovskite Solar Cells with Fluoropolymer Coating

Halide perovskites are promising as the light absorbers of solar cells with efficient solar power conversion. However, why the degradation of perovskite solar cells (PSCs), especially at high temperatures, happens has not been completely understood to date. Herein, it is shown that evaporation of 4-tert-butylpyridine (4-tBP) from the hole transport layer (HTL) of 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD) is one of possible degradation mechanisms in PSCs at a high temperature of 85 °C. In fresh PSCs, the chemical doping of the spiro-OMeTAD HTL with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is not so efficient because of the formation of a LiTFSI:4-tBP complex in the HTL. When PSCs are placed at 85 °C, 4-tBP gradually evaporates from the HTL, resulting in the dissociation of the LiTFSI:4-tBP complex. This 4-tBP evaporation enhances the chemical doping of spiro-OMeTAD by LiTFSI and makes the hole transport level of the spiro-OMeTAD HTL deeper, thereby impeding hole extraction at the perovskite/spiro-OMeTAD/Au interfaces. Herein, the 4-tBP evaporation by covering PSCs with a fluoro-polymer CYTOP layer, significantly improving the high-temperature durability of PSCs, is suppressed. The basic understanding obtained in this study would help promote the spread of more thermally durable PSC products in the future.