High-efficiency fullerene free ternary organic solar cells based with two small molecules as donor

Herein, we have designed a ternary system comprising of two small molecules (B2 and B3), as donor and a narrow bandgap non-fullerene small molecule acceptor Y6. The chemical structures of B2 and B3 are close to each other but their absorption spectra are complementary with different energy levels. Using these smallmolecules, a ternary organic solar cell was fabricated. The presence of B2 in the B3:Y6 blend increases the photon harvesting as well as also forms cascade energy level arrangement which benefits assisting the balancing between the dissociation of excitons into free charge carriers and their subsequent charge transfer between the two donors (B2 and B3) and the acceptor (Y6). Moreover, the photoluminescence spectroscopy analysis has revealed efficient energy transfer from B2 to B3, which resulted in the effective excitons utilization in ternary device. This way, we have been able to achieve over efficiency of about 12.88% with energy loss of 0.54 eV for the ternary device, greater than that for binary devices i.e., 8.73% with energy loss of 0.51 eV and 9.26% with energy loss of 0.61 eV for B2:Y6 and B3:Y6, respectively. This low energy loss is one of the lowest in the fullerene free ternary photovoltaic devices based on all small molecules (donor and acceptors).