Efficient Non-polymeric Heterojunctions in Ternary Organic Solar Cells

We describe the use of a near IR non-fullerene low molecular weight organic semiconductor molecule, as electron acceptor, in combination with a wide band gap organic semiconductor electron donor, in efficient bulk heterojunction organic solar cells. The electron acceptor is a 2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione-based molecule while the electron donor is a carbazole derivative. These two small molecules have complementary absorption spectra, and, thus, the heterojunction absorbs from 450 to 950 nm. The optimized photoactive layer shows power conversion efficiency of 7.44% under standard measurement conditions with an estimated energy loss as low as 0.55 eV. Identical solar cells, but using fullerene derivative, PC71BM, result in lower efficiency values (PCE = 5.07%) and greater energy losses (0.86 eV). The better performance of the non-fullerene-based small molecule organic solar cells is due to the higher LUMO energy level for the electron acceptor. Moreover, to improve further the efficiency of th...