Generation of Photoexcitations and Trap-Assisted Recombination in TQ1:PC71BM Blends

The generation and recombination of long-lived photoexcitations is clarified in TQ1 films and TQ1:PC71BM 1:1 and 1:3, by weight, blends using photoinduced absorption measurements. At 80 K triplets are formed in TQ1 films, while both triplets and polarons are formed in the 1:1 and 1:3 blends. We suggest that the triplet state acts as a loss mechanism for generation of free charges in these blends and suggest an energy diagram for the photoexcitations in the blends. We estimate the triplet polaron annihilation (gamma(TPA)) constant to be 1.12 x 10(-14) and 3.10 x 10(-13) cm(3) s(-1) for the TQ1:PC71BM 1:1 and 1:3 blends, respectively. At 300 K triplets are mainly formed in the TQ1 films, while only polarons are present in the TQ1:PC71BM blends. Using frequency measurements we show that the TQ1:PC71BM 1:1 blend shows nondispersive kinetics (zeta = 1), while the 1:3 blend exhibits dispersive kinetics (zeta = 0.87). Using intensity-dependent measurements, we show that trap-assisted recombination is the dominating recombination mechanism for polarons in TQ1:PC71BM blends. Assuming an exponential trap-density we show that the characteristic energy E-ch, the mean trap-depth, is E-ch = 38.7 +/- 2 meV in 1:1 blends and slightly deeper in 1:3 blends, E-ch = 48.4 +/- 2 meV. The trap density is shown to be higher in the 1:1 compared to the 1:3 blends.