Type-II Charge-Transfer Photogating in a DDCI-4/MoS2 Heterojunction Phototransistor

Abstract

Molecular sensitization of two-dimensional semiconductors offers a route to enhanced photodetection, yet the interfacial charge-transfer mechanisms that govern gain in these hybrid devices are often inferred rather than directly established. Here, we report a multilayer MoS2 field-effect phototransistor interfaced with vacuum-sublimated films of the cationic cyanine dye 1,1′-diethyl-4,4′-dicarbocyanine iodide (DDCI-4), in which density-functional theory establishes a clean type-II band alignment: the DDCI-4 LUMO sits 0.52 eV above the MoS2 conduction-band minimum, providing a thermodynamic driving force for photoexcited electron injection into the channel while the complementary hole remains localized on the dye. This charge-transfer photogating mechanism is corroborated by a systematic negative shift of the field-effect threshold voltage with increasing DDCI-4 coverage, consistent with ground-state n-type doping, and by response times that grow monotonically with coverage (τdecay ≈ 230–405 ms), consistent with a hole-trapping-limited photoconductive gain. Channel-geometry analysis (L = 1.04 µm, W = 3.46 µm) establishes field-effect mobilities of 29–42 cm2 V−1 s−1 across all coverages, confirming that the enhanced photoresponse originates from interfacial charge transfer rather than degraded channel transport. At the most strongly depleted negative gate bias studied (VG = −16 V, VDS = 50 mV), the 5.0 Å DDCI-4 device delivers a responsivity of R = 116 A W−1, an external quantum efficiency of EQE = 26,657%, and a shot-noise-limit estimate of specific detectivity of D* = 4.25 × 1011 Jones; all photoresponse measurements reported here were acquired in this negative-VG (electron-depletion) regime. The photocurrent spectra additionally display two narrow, coverage-independent features at 608 nm and 658 nm separated by ∼1250 cm−1, a spacing consistent with a vibronically resolved molecular aggregate transition; three-component Voigt deconvolution isolates this dye-derived contribution from the intrinsic MoS2 background with R2 = 0.983–0.997 across all devices and gate voltages, and quantifies its share of the total photocurrent at 62.6–80.8% depending on coverage and bias. Together, these results identify type-II charge-transfer photogating, rather than photoinjection or intrinsic MoS2 photoconductivity, as the dominant mechanism of photoresponse enhancement, and establish molecularly thin cyanine layers as an effective, minimal-complexity sensitization strategy for 2D optoelectronic devices.