Charge Density Dependent Nongeminate Recombination in Organic Bulk Heterojunction Solar Cells

Apparent recombination orders exceeding the value of two expected for bimolecular recombination have been reported for organic solar cells in various publications. Two prominent explanations are bimolecular losses with a carrier concentration dependent prefactor due to a trapping limited mobility and protection of trapped charge carriers from recombination by a donoracceptor phase separation until re-emission from these deep states. In order to clarify which mechanism is dominant temperature- and illumination-dependent charge extraction measurements are performed under open circuit and short circuit conditions at poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PC61BM) and PTB7:PC71BM (poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]) solar cells in combination with currentvoltage characteristics. It is shown that the charge carrier density n dependence of the mobility mu and the recombination prefactor are different for P3HT:PC61BM at temperatures below 300 K and PTB7:PC71BM at room temperature. Therefore, in addition to mu(n), a detrapping limited recombination in systems with at least partial donoracceptor phase separation is required to explain the high recombination orders.