Species selective charge transfer dynamics in a P3HT/MoS2 van der Waals heterojunction: fluorescence lifetime microscopy and core hole clock spectroscopy approaches

Hybrid van der Waals heterojunctions based on organic polymers and 2D materials have emerged as a promising solution for developing more efficient optoelectronic devices. Herein, we investigated the charge transfer (CT) dynamics at the interface of the poly[3-hexylthiophene-2,5-diyl] (P3HT) organic polymer and a MoS2 monolayer. A global picture of the charge transfer dynamics of a P3HT/MoS2/SiO2 heterojunction was elucidated from photoluminescence (PL) spectroscopy and the fluorescence lifetime decay profile. Rapid interfacial charge transfer between P3HT and MoS2 was indicated by strong PL quenching and a reduction in the average fluorescence lifetime (tau(av)) of the P3HT/MoS2/SiO2 heterojunction. The role of specific electronic states in the interfacial CT process was investigated by applying the core hole clock approach. CT times (tau(CT)) on femtosecond and sub-femtosecond timescales were estimated using the S1s core-hole lifetime as the internal clock. Sub-femtosecond CT was observed for electrons excited to S3p(z) (0.34 fs) electronic states of MoS2 and to pi* (C-C) (0.45 fs) electronic states of P3HT in the P3HT/MoS2/SiO2 heterojunction. These fast bidirectional CT processes result from strong coupling between these two electronic states in the P3HT/MoS2/SiO2 heterostructure. However, the reduction of the tau(CT) values in the heterojunction compared with those of the isolated films shows that interfacial CT from the P3HT species to MoS2 is more efficient. Interfacial CT was not observed for electrons excited to electronic states S3p(x,y) (MoS2) and sigma* (S-C) (P3HT). We conclude that the pi* (C-C) electronic state of the P3HT species is the main pathway for interfacial ultrafast CT in a P3HT/MoS2/SiO2 heterojunction.