HOMO−CBM Alignment Window for Low-Voltage Molecular-Sensitized MoS2 Photodetectors

Abstract

Organic-molecule functionalization enhances the photoresponse of MoS2 field-effect transistor (FET) photodetectors, but predictive design rules linking molecular electronic structure to device performance remain limited. We compare three electronically distinct sensitizers cobalt phthalocyanine (CoPc), metal-free phthalocyanine (H2Pc), and methylene blue (MB)—deposited by single-step ultrahigh-vacuum thermal evaporation on trilayer MoS2 FETs spanning HOMO−CBM offsets of 0.84−1.7 eV. Photoconductive gain peaks within a narrow 0.8−1.0 eV alignment window: the CoPc device achieves responsivity of 127 A W−1, external quantum efficiency of 2.6 × 104%, a specific detectivity of 2.39 × 1012 Jones, and gain of 260 at a drain bias of only 50 mV. Thermal-emission analysis confirms that the operative mechanism is photogating through molecular cation states acting as hole traps. These results establish that molecular design of MoS2 photodetectors is governed not by a monotonic offset−performance relationship but by an optimum HOMO−CBM alignment window near 0.8−1.0 eV, within which trap-mediated photogating is maximized. This finding provides a quantitative, predictive descriptor for molecular selection in manufacturable, low-power 2D photodetectors.