Photo-induced carrier generation and recombination dynamics probed by combining time-resolved microwave conductivity and transient absorption spectroscopy

We examined photoinduced charge-generation dynamics of the poly(3-hexylthiophene) (P3HT)/titanyl phthalocyanine (TiOPc) bilayer and the P3HT/TiOPc/C₆₀ trilayer using the combination of flash-photolysis time-resolved microwave conductivity experiments (fp-TRMC) and classic pump-probe transient absorption (TA) spectroscopy following dominant excitation of the P3HT layer. The superlinear increase of φΣμ for the P3HT/TiOPc bilayer, compared to the φΣμ sum of each P3HT and TiOPc layer suggest photoinduced carrier-generation. Furthermore, the superlinear increase of φΣμ of the P3HT/TiOPc/C₆₀ trilayer with respect to the each P3HT/TiOPc and TiOPc/C₆₀ bilayers evinces charge migration from one interface to the other interface. In addition, with selective photoexcitation on the P3HT layer, both amorphous and H-aggregated P3HT domains participate in electron transfer ([P3HT∗/TiOPc]→[P3HT^•+/TiOPc^•-]), contrasting to the previous observation where with selective excitation of the TiOPc layer, only the H-aggregated P3HT domain involves in hole transfer ([P3HT/TiOPc→[P3HT^•+/TiOPc^•-]) to produce P3HT^•+/TiOPc^•- in J. Phys. Chem. B 119(24), 7729 - 7739 (2015). These results under different excitation conditions are consistent with calculated energetic driving force (ΔE_CS) for charge generation which is -0.58 eV and -0.73 eV for amorphous and H-aggregated P3HT domains under the P3HT layer excitation, while 0.04 eV and -0.11 eV for amorphous and H-aggregated P3HT domains under the TiOPc layer excitation.