Radiative Recombination in Bulk-Heterojunction Solar Cells

Abstract

We have studied the time-dependent charge transfer emission in a photovoltaic device made of a blend of poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclo-penta[2,1-b;3,4-b’]-dithiophene)-alt-4,7-(2,1,3 benzothiadiazole)] and [6,6]-phenyl C61-butyric acid methyl ester, a system showing a low band-gap suitable as absorber for bulk-heterojunction solar cells. We find a short-lived component (∼1 ns) and no evidence of a long-lived emission (μs) expected if photo-generated free charge carriers would recombine radiatively via the charge transfer state. Our results explain, why the open circuit voltage loss in the investigated solar cell is much larger compared to GaAs-based or perovskite solar cells and highlight one of the key processes limiting the performance of bulk-heterojunction solar cells.