Over 19.1% efficiency for sequentially spin-coated polymer solar cells by employing ternary strategy

Series of sequentially spin-coated polymer solar cells (SS-PSCs) are fabricated with donor layers containing PM1, D18 or PM1:D18 and sequentially spin-coated L8-BO acceptor layer. Photon harvesting of donor layers could be enhanced by optimizing the weight ratios of PM1 to D18 due to their distinct optical band-gap. The optimized ternary SS-PSCs exhibit a power conversion efficiency of 19.13%, resulting from the simultaneously enhanced short-circuit-current density (J_SC) of 27.20 mA/cm², open-circuit voltage (V_OC) of 0.90 V and fill factor of 78.53% in comparison to those of PM1/L8-BO based SS-PSCs. The increased J_SCs of ternary SS-PSCs is due to the enhanced photon harvesting and efficient exciton utilization assisted by an additional channel from D18 to PM1 through energy transfer. The increased V_OC of optimal ternary SS-PSCs benefits from the little larger V_OC of binary SS-PSCs with D18 as donor layer. The donor/acceptor interfacial energy is slightly increased by incorporating D18 in donor layer, which is conducive to form the optimized vertical phase separation to decrease charge recombination loss. This work indicates that ternary strategy is also a versatile and effective method in improving the performance of SS-PSCs.