Excited-State Charge Transfer Enabling MoS₂/Phthalocyanine Photodetectors with Extended Spectral Sensitivity

Abstract

Monolayer (ML) transition-metal dichalcogenides (TMDCs) are an attracting new class of two-dimensional direct band gap semiconducting materials for optoelectronic device applications. The combination of TMDCs with organic semiconductors holds the promise to further improve device properties with added functionality. Here, we demonstrate that excited-state charge transfer from a thin organic absorber layer, i.e., metal-free phthalocyanine (H2Pc), enhances the photoresponse of ML MoS2 dramatically at the same time also significantly extending it to spectral regions where the TMDC is transparent. The fundamental processes enabling this boost in photodetector performance are unraveled by a combination of photoemission (PES), photoluminescence (PL), and photocurrent action spectroscopy. Direct and inverse PES reveal a type II energy level alignment at the MoS2/H2Pc interface with a large energy offset of 1 eV, which is sufficient to drive the excited-state charge transfer. Time-resolved PL measurements evidence highly efficient dissociation of excitons generated in H2Pc when they are in contact with MoS2. Exciton dissociation results in the formation of a charge-separated state at the hybrid interface with an energy gap of ca. 1.2 eV, in accordance with PES. This state then dissociates into free carriers and markedly contributes to the current in the photodetector, as demonstrated by photocurrent action spectroscopy. This reveals that the photoconductivity within the MoS2 ML is generated by light directly absorbed in the TMDC and, notably, with comparable efficiency by the absorption by H2Pc. The present demonstration of a highly efficient carrier generation in TMDC/organic hybrid structures paves the way for future nanoscale photodetectors with very wide spectral sensitivity.