Control Over Vertical Composition in Low Complexity Polymer Solar Cells

Inverted organic solar cells are promising due to their better stability compared to conventional structures. Donors with low synthetic complexity are desirable to lower costs. However, inverted devices are rarely used in low-complexity systems. To investigate the reasons, the low-complexity PTQ10:BTP-eC9 binary system is benchmarked against the high-complexity PM6:BTP-eC9 system. In PTQ10:BTP-eC9, where the efficiency of inverted devices lags the conventional structure significantly, distinct wetting layers are observed in conventional and inverted device structures. Conversely, the vertical distribution of PM6:BTP-eC9 remains unaffected by changes in interlayer materials. The surface is always enriched in BTP-eC9, but less for PM6. Importantly, the addition of PC₇₁ BM reduces the nonuniform vertical composition gradients. As the PC₇₁ BM concentration increases, the efficiency of the inverted PTQ10 devices approach that of the conventional devices and PTQ10:BTP-eC9:PC₇₁ BM (1:1.2:0.4) exhibits negligible efficiency differences between inverted (14.01%) and conventional (14.49%) architectures. The concentration-gradients aredriven by the interfacial energy between the active layer and interlayer materials and the casting kinetics in the case of the surface. Understanding the thermodynamic and kinetic aspects provides valuable insights for optimizing the performance of inverted organic solar cells, bringing them closer to practical applications.