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 (τ_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 (τ_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 π* (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 τ_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 σ* (S-C) (P3HT). We conclude that the π* (C-C) electronic state of the P3HT species is the main pathway for interfacial ultrafast CT in a P3HT/MoS₂/SiO₂ heterojunction.