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 the solar cell, we carried out the study of mixing both the electron acceptor and the electron donor. The ternary organic heterojunction formed results in panchromatic absorption spectra. The overall efficiency of the organic solar cell based on solvent vapor treated ternary active layer showed power conversion efficiency of 8.94% due to the efficient light harvesting, improved energy levels alignment, and better charge balance of electronic holes and electrons.