Wavelength dependent bidirectional photoconductivity in carbon quantum dot embedded in indigo molecular layer with enhanced detectivity†

We report a wavelength-dependent bidirectional photoconductivity along with reversible switching behavior in a room-temperature, air-stable, two-terminal optoelectronic device based on a composite of carbon quantum dots (CQDs) dispersed in an indigo molecular matrix. The distinct dual-mode photoresponse arises from the complementary absorption characteristics of CQDs and indigo molecules. Specifically, negative photoconductivity (NPC) is observed under ultraviolet (UV) excitation, while exposure to visible light induces positive photoconductivity (PPC). The transition from NPC to PPC is governed by wavelength-dependent mechanisms involving charge carrier recombination, competitive trapping of photo-excited electrons in defect states associated with CQDs, and the efficient generation of electron–hole pairs within the indigo molecular framework. Notably, the switching between PPC and NPC is fully controlled by the excitation wavelength without any change in device configuration or bias polarity. Under UV illumination at 275 nm, the device exhibits high responsivity (R = 1947.66 mA W⁻¹), an ON/OFF ratio of 5.82, and enhanced detectivity (D* = 3.94 × 10¹⁴ Jones). Furthermore, both PPC and NPC modes demonstrate significant improvement in detectivity compared to previously reported organic and many inorganic photodetectors. This dual-mode photoconductive behavior in a single device architecture not only enables spectral selectivity but also paves the way for multifunctional optoelectronic applications. The CQD–indigo composite is thus a highly promising active material for broadband photodetection, optoelectronic memory, and neuromorphic device platforms.