Characterization of the dominating bulk recombination in bulk-heterojunction blends using photoinduced absorption

In this work we show how to clarify the dominating bulk recombination in organic solar cells by using photoinduced absorption. We show how to use the intensity and frequency dependence of the in-phase and quadrature signals to obtain the effective reaction order. For trap-assisted recombination, we can show using a multiple trapping and retrapping model with an exponential tail-state distribution that a temperature dependent reaction order is obtained which allows for determination of the characteristic energy of the exponential distribution of trap-states. In the model system pBTTT:PC60BM, we show that trap-assisted recombination is the dominating bulk recombination in 1:1 blends with a characteristic energy of the exponential trap distribution E-ch = 44 +/- 5 meV. The 1:4 blend, on the other hand, shows temperature independent behavior in good agreement with a dominating 2D Langevin bulk recombination.