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

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 (H$_2$Pc), enhances the photoresponse of ML MoS$_2$ dramatically and at the same time also significantly extends 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 MoS$_2$/H$_2$Pc interface with a large energy offset of 1 eV, which is sufficient to drive the excited state charge transfer. Time-resolved PL measureme...