Energy-level modulation of coumarin-based molecular donors for efficient all small molecule fullerene-free organic solar cells

In fullerene-free single-junction organic solar cells despite a small interfacial energy offset between the donor and acceptor materials efficient hole transfer occurs, resulting in power conversion efficiencies (PCE) >16%. Here, we demonstrate efficient organic solar cells by tuning of the electronic properties through simple structural modulation of coumarin-based electron donors. The OSCs with C2:BThIND-Cl:Y6 ternary blends exhibit a PCE of 13.54%, with an increased open circuit voltage (VOC) compared to the C1:BThIND-Cl:Y6-based devices (11.27%). Interestingly, an efficient charge transfer is revealed despite a reduced HOMO orbital offset of 0.02-0.04 eV between the C2 donor and BThIND-Cl and Y6 acceptors, resulting in a lower energy loss of 0.45 eV in ternary devices while still maintaining a maximum EQE of similar to 80%@820 nm. The work demonstrates the importance of simple coumarin-based donors and their role in performance improvement for fullerene-free ternary OSCs by facilitating charge carrier transport with suppressed recombination.

Automated summary

Efficient organic solar cells with power conversion efficiencies (PCE) >16% are achieved by tuning the electronic properties through structural modulation of coumarin-based electron donors. The ternary blend devices exhibit increased open circuit voltage (VOC) compared to the binary devices, with efficient charge transfer despite reduced HOMO orbital offset, resulting in lower energy loss. The study highlights the importance of simple coumarin-based donors in performance improvement for fullerene-free ternary organic solar cells.