Effect of Fluorine Substitution in a Hole Dopant on the Photovoltaic Performance of Perovskite Solar Cells

Most of the high-efficiency perovskite solar cells (PSCs) are based on the doped spiro-MeOTAD as a hole-transporting layer. Lithium sulfonyl imides with the general chemical formula of LiN(SO2CnF2n+1)2 (n = 0, 1, and 2 for LiFSI, LiTFSI, and LiPFSI, respectively) are candidates for dopants. Although LiTFSI is generally used, it is argued that the power conversion efficiency (PCE) is better for LiPFSI than for LiTFSI due to higher fluorine substitution. In this report, we investigate the effect of the amount of fluorine substitution on photovoltaic performance. Four different researchers fabricated independently PSCs, which reveals that the PCE is highest in the following order, LiTFSI ≈ LiFSI > LiPFSI. The relatively lower performance for LiPFSI is attributed to the interfacial problem and aggregate formation leading to a nonuniform film, which is related to the hydrophobicity and lipophobicity increased by perfluorination. LiFSI however has poor moisture stability due to being less hydrophobic. We draw the conclusion that LiTFSI is a suitable dopant among the studied candidates in terms of both efficiency and stability due to moderate hydrophobicity.