Radiative Recombination in Bulk‐Heterojunction Solar Cells

We have studied the time-dependent charge transfer emission in a photovoltaic device made of blend 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, system showing low band-gap suitable as absorber for bulk-heterojunction solar cells. find short-lived component (∼1 ns) no evidence long-lived (μs) expected if photo-generated free carriers would recombine radiatively via state. Our results explain, why open circuit voltage loss investigated cell is much larger compared to GaAs-based or perovskite cells highlight one key processes limiting performance