Effect of Charge Recombination on the Fill Factor of Small Molecule Bulk Heterojunction Solar Cells

Solution-processed organic BHJ solar cells based on 3,6-bis[5-(benzofuran-2-yl)thiophen-2-yl]-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(TBFu)(2)) or poly(3-hexylthiophene) blended with [6,6]-phenyl-C-60(70)-butyric acid methyl ester (PC60(70) BM) behave differently under various irradiation intensities. Small molecule-based DPP(TBFu)(2):PC60 BM solar cells show up to 5.2% power conversion efficiency and a high fill factor at low light intensity. At 100 mW cm(-2) illumination, the efficiency and fill factor decrease, resulting in stronger power losses. Impedance spectroscopy at various light intensities reveals that high charge recombination is the cause of the low fill factor in DPP(TBFu)(2):PC60 BM.