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Panchromatic Triple Organic Semiconductor Heterojunctions for Efficient Solar Cells

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journal contribution
posted on 02.12.2020, 21:30 by Cristina Rodríguez-Seco, Lydia Cabau, Maria Privado, Pilar de la Cruz, Fernando Langa, Ganesh D. Sharma, Emilio Palomares
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.