The introduction of the strong donor DTP leads to increased HOMO energies and thus decreased open-circuit voltage (Voc) (compared to previously published semi-random polymers) as well as an amorphous character of P3HTT-DTP, P3HTT-BTD-DTP and P3HTT-TP-BTD resulting in low hole mobilities and moderate solar cell efficiencies (0.18% to 0.36%). All polymers (except for the all donor polymer P3HTT-DTP) show considerably broadened absorption compared to P3HT due to the donor-acceptor effect and their multichromophoric nature.
Novel semi-random poly(3-hexylthiophene) (P3HT) based polymers P3HTT-DTP, P3HTT-BTD-DTP, P3HTT-TP-DTP and P3HTT-DPP-DTP containing the strong donor dithienopyrrole (DTP) as well as different acceptors (benzothiadiazole (BTD), thienopyrazine (TP) or diketopyrrolopyrrole (DPP)) were synthesized by Stille copolymerization and their optical, electrochemical, charge transport, and photovoltaic properties were investigated. Our results therefore provide clear evidence of the role of nanomorphology in determining the nature of recombination dynamics in such donor/acceptor blends. However, this transition does correlate with nanomorphology data that indicate that both pyrazolinofullerenes yield a much finer phase segregation with correspondingly smaller domain sizes than observed with PCBM. We show that this transition cannot be explained by a difference in interfacial energetics. Transient absorption data indicate that replacement of PCBM with either of the pyrazolinofullerene derivatives results in a transition from nongeminate to monomolecular (geminate) recombination dynamics.
Furthermore, we compare the charge recombination dynamics observed in films comprising P3HT blended with three fullerene derivatives: PCBM and two alternative pyrazolinofullerenes. This correlation is discussed in the context of studies of charge photogeneration for other organic donor/acceptor blend films, including other polymers, blend compositions, and the widely used electron phenyl-C61-butyric acid methyl ester(PCBM). For the PDI film series, we observe a close correlation between the PDI electron affinity and the efficiency of charge separation. In particular, we undertake transient optical studies of films comprising regioregular poly(3-hexylthiophene) (P3HT) blended with a series of perylene-3,4:9,10-tetracarboxydiimide (PDI) fullerene electron acceptors.
We focus upon the role of interfacial energetics and morphology in influencing the separation of CT states into dissociated charge carriers.
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