Charge Density Dependent Nongeminate Recombination in Organic Bulk Heterojunction Solar Cells

Abstract

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 donor–acceptor 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 current–voltage characteristics. It is shown that the charge carrier density n dependence of the mobility μ and the recombination prefactor are different for P3HT:PC₆₁BM at temperatures below 300 K and PTB7:PC₇₁BM at room temperature. Therefore, in addition to μ(n), a detrapping limited recombination in systems with at least partial donor–acceptor phase separation is required to explain the high recombination orders.