Panchromatic Triple Organic Semiconductor Heterojunctions for Efficient Solar Cells

Two new molecules having D-A-D configuration, symbolized as LCS01 and EP02, have been synthesized, characterized, and used for both fullerene- and non-fullerene-based organic solar cells (OSC). Both molecules have been employed as p-type materials (donors) in combination with the fullerene derivative PC71BM and a nonfullerene molecule, namely, MPU3, as electron acceptors for the fabrication of solar cells. Careful control of the photoactive layer, the LCS01:PC71BM-, EP02:PC71BM-, LCS01:MPU3-, and EP02:MPU3-based organic solar cells, leads to light-to-electrical energy conversions of 6.35, 5.59, 8.22, and 8.92%, respectively. Moreover, the optimized ternary heterojunction active layer EP02:PC71BM:MPU3 (1:0.5:1)-based OSC showed a remarkable power conversion efficiency (PCE) of 9.62% with an energy loss of 0.58 eV, which is superior to that for the LCS01:PC71BM:MPU3 counterpart (9.16% with an energy loss of 0.69 eV). The improvement in the PCE for the ternary OSCs compared to the binaries is attributed to the formation of a correct energy-level cascade alignment in the ternary active layer that guarantees efficient charge transfer and energy transfer, suppression of both bimolecular and trap-assisted recombination mechanisms. Therefore, the use of a ternary active layer consisting of a wide-bandgap small-molecule donor, PC71BM, and a low-bandgap nonfullerene molecule affords an effective approach for developing efficient single-junction organic solar cells.