Doping porphyrin-based bulk heterojunction solar cells with LITFSI and TFSA

Abstract

Strong electron-withdrawing materials of bis(trifluoromethanesulfonyl)amide (TFSA, [CF3SO2]₂NH) and bis(trifluoromethane)sulfonimide lithium salt (LiTFSI, [CF3SO2]₂NLi) are incorporated into the active layers of a porphyrin small molecule TDPPEZnP and PC₆₁BM for bulk heterojunction solar cells. While the solar cells based on the undoped devices show a power conversion efficiency (PCE) of 6.11% with a V_oc of 0.70 V, a J_sc of 13.92 mA cm⁻² and a FF of 62.71%, doping the active layers with very low loadings of LITFSI and TFSA leads to improved PCEs of 6.85% and 7.01% with J_sc values of 14.97 and 15.39 mA cm⁻² and FF values of 64.73% and 63.92%, accounting for 14% and 16% PCE enhancement, respectively. The improved performance is ascribed to the enhanced charge carrier transport, which is supported by charge mobility, impedance spectroscopy and transient spectroscopy analyses.