Reduced Energy Offsets and Low Energy Losses Lead to Efficient (∼10% at 1 sun) Ternary Organic Solar Cells

Two organic semiconductor molecules, CS01 and MPU3, an electron donor and electron acceptor, respectively, are described for use in organic bulk heterojunction solar cells. The highest occupied molecular orbital (HOMO) energy offset (ΔΕ_HOMO) between these two molecules is as low as 0.29 eV. Moreover, the lowest unoccupied molecular orbital (LUMO) energy offset (ΔΕ_LUMO) is as low as 0.14 eV. Nonetheless, the interfacial charge-transfer process upon light excitation is extremely efficient, and solar cells with efficiencies, under normal conditions, as high as 7.81% have been fabricated. Furthermore, the incorporation of PC₇₁BM (another electron acceptor molecule) to fabricate a triple organic heterojunction led to efficiencies close to 10% at 1 sun irradiation conditions. The improvement is due to a better panchromatic harvesting of the sunlight and a better charge balance, which allows the rapid extraction of carriers before they can recombine.