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 Ech=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.
The influence of reaction order and trap-assisted recombination on continuous-wave photoinduced absorption measurements is clarified through analytical calculations and numerical simulations. The results reveal the characteristic influence of different trap distributions and enable distinguishing between shallow exponential and Gaussian distributions as well as systems dominated by direct recombination by analyzing the temperature dependence of the in-phase and quadrature signals. The identifying features are the intensity dependence of the in-phase at high intensity, PA I ∝ I γ HI , and the frequency dependence of the quadrature at low frequency, PA Q ∝ ω γ LF . For direct recombination γ HI and γ LF are temperature independent, for an exponential distribution they depend on the characteristic energy E ch as γ HI = 1/(1 + E ch /kT ) and γ LF = kT /E ch while a Gaussian distribution shows γ HI and γ LF as functions of I and ω, respectively. a) nwilson@abo.fi
The generation and recombination of long-lived photoexcitations is clarified in TQ1 films and TQ1:PC 71 BM 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 (γ TPA ) constant to be 1.12 × 10 −14 and 3.10 × 10 −13 cm 3 s −1 for the TQ1:PC 71 BM 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:PC 71 BM blends. Using frequency measurements we show that the TQ1:PC 71 BM 1:1 blend shows nondispersive kinetics (ζ = 1), while the 1:3 blend exhibits dispersive kinetics (ζ = 0.87). Using intensity-dependent measurements, we show that trap-assisted recombination is the dominating recombination mechanism for polarons in TQ1:PC 71 BM 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.
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