Outstanding Thermal Stability of Perovskite Solar Cells Based on Zn(TFSI)₂-Doped Spiro-MeOTAD

Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is the most general dopant for 2,2′,7,7′-tetrakis[ N, N -di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-MeOTAD), which has been dominantly used as a hole transport material to achieve record efficiencies of perovskite solar cells (PSCs) in spite of resultant poor thermal stability. Recently, zinc(II) bis(trifluoromethanesulfonyl)imide (Zn(TFSI) 2 ) was proposed as an excellent candidate to replace LiTFSI. In this study, the use of Zn(TFSI) 2 as a dopant for spiro-MeOTAD is explored to investigate the thermal stability of PSCs. A complexation of Zn(TFSI) 2 along with tert -butylpyridine during the doping process of spiro-MeOTAD is favorable for the device operation at 85 °C, while PSCs with LiTFSI demonstrate a rapid performance degradation. Differential scanning calorimetry, X-ray diffraction, and morphology confirm that LiTFSI-doped spiro-MeOTAD is prone to involving higher crystallinity at the elevated temperature, compared to the one with Zn(TFSI) 2, directly affecting the photovoltaic performance by aggravating the charge recombination at the interface.