Enhancing molecular aggregation and decreasing the optical gap by a dual-additive to reduce the energy loss of all-polymer organic solar cells

Abstract

All-polymer solar cells (all-PSCs) have attracted significant attention due to their unique advantages, such as intrinsic stretchability and high thermal stability. However, in the PM6:PYIT system, controlling the phase-separated aggregation size of the acceptor PYIT is particularly crucial because of the difficulties in choosing suitable solvents. High-boiling-point solvents such as chlorobenzene may lead to excessive aggregation of the acceptor due to the slow solvent evaporation, and low-boiling-point solvents like chloroform (CF) may result in small aggregation sizes of the acceptor due to its fast evaporation. To optimize the aggregation size of the acceptor PYIT, we employ a strategy of using two high-boiling-point solvent additives (diphenyl ether: DPE and chloronaphthalene: CN) to prolong the aggregation and film formation time of the acceptor PYIT. This result shows that the aggregation state of PYIT is optimized effectively after the evaporation of CF, with the slow evaporation process of DPE and CN acting synergistically. Moreover, DPE with lower surface energy tends to make PM6 aggregate more in the bottom anode region, while CN with higher surface energy drives PYIT to aggregate upwards, forming a vertical phase separation distribution structure, which is conducive to efficient exciton dissociation and charge transport. Further analysis indicates that the introduction of the dual-additive leads to a corresponding increase in the external quantum efficiency (EQE) spectrum and an effective reduction in the absorption bandgap (E_g) of the solar cells. Consequently, the PCE of the optimized device increases notably from 14.58% to 16.67% and a reduction in E_loss from 0.500 eV to 0.476 eV by adding 0.6 vol% of 1-CN and 0.4 vol% of DPE simultaneously is also observed.