I will present recent results on charge generation, recombination, and triplet state formation in various bulk heterojunction photovoltaic blends investigated by femto- to microsecond broadband Vis-NIR transient absorption (TA) pump-probe spectroscopy. Specifically, we looked into the processes following exciton dissociation in low-bandgap polymer and small molecule solar cells using either fullerene or non-fullerene acceptors. As a first example I will show results obtained on the donor-acceptor copolymers PCPDTBT and its silicon-substituted analogue PSBTBT.[1] Here, broadband TA experiments in combination with sophisticated data analyses techniques such as evolving factor analysis (EFA) and multivariate curve resolution with alternating least squares (MCR-ALS) [2] revealed that after exciton dissociation and free charge formation is completed fast sub-nanosecond non-geminate recombination occurs and leads to a substantial population of the polymer’s triplet state. The extent to which triplet states are formed depends on the initial concentration of free charges, which itself is controlled by the microstructure of the blend, especially in case of PCPDTBT:PC60BM. Interestingly, PSBTBT:PC70BM blends showed a higher charge generation efficiency, but less triplet state formation at similar free charge carrier concentrations. This indicates that the solid-state morphology and interfacial structure of PSBTBT:PC70BM blends reduce non-geminate recombination and thus triplet state formation, leading to increased device performance compared to optimized PCPDTBT:PC60BM blends. Based on the aforementioned observations I will extend the discussion to other material systems relevant for organic photovoltaic devices such as PBDTTPD:PCBM,[3] PBDTTT-C:PCBM and DPP-based copolymers.
[1] F. Etzold, I.A. Howard, N. Forler, A. Melnyk, D. Andrienko, M.R. Hansen,
F. Laquai, Energy Environ. Sci. 2015, accepted. [2] I.A. Howard, H. Mangold, F. Etzold, D. Gehrig, F. Laquai, in Ultrafast Dynamics in Molecules, Nanostructures and Interfaces, World Scientific (2013). [3] C. Dyer-Smith, I.A. Howard, C. Cabanetos, A. El Labban, P.M. Beaujuge, F. Laquai, Adv. Energy Mater. 2015, accepted.