Boosted Efficiency Over 18.1% of Polymer Solar Cells by Employing Large Extinction Coefficients Material as the Third Component

A series of binary and ternary polymer solar cells (PSCs) is successfully fabricated. The optimal ternary PSCs achieve a power conversion efficiency (PCE) of 18.14%, benefiting from the increased short circuit current density (J_SC) of 26.53 mA cm⁻² and fill factor (FF) of 78.51% in comparison with the J_SCs (25.05 mA cm⁻² and 25.65 mA cm⁻²) and the FFs (77.13% and 76.55%) of the corresponding binary PSCs. The photon harvesting ability of ternary active layers can be enhanced, which can be confirmed from the EQE spectral difference of the optimized ternary and binary PSCs, especially in the wavelength range from 680 nm to 800 nm. The refractive index and extinction coefficients of binary and ternary blend films are measured, which can well support the enhanced photon harvesting ability in different wavelength ranges. Photogenerated exciton distribution in active layers is simulated by the transmission matrix method based on the Beer–Lambert law. The photogenerated exciton density can be enhanced in the middle of the active layers by incorporating a third component in acceptors, which is conducive to charge collection by individual electrodes, resulting in the simultaneously enhanced J_SC and FF of the optimal ternary PSCs.