Crystallinity Effects on Near-Infrared Photoresponse in Y6-Based Organic Phototransistors

Organic phototransistors (OPTs) are appealing devices for the fabrication of low-cost, tuneable and highly sensitive light detectors. This study investigates the potential of single-component OPTs based on the non-fullerene acceptor Y6 to detect near-infrared (NIR) light. Y6 films are deposited via blade coating and subjected to thermal annealing between 200°C and 250°C. Morphological and structural characterization reveals a transition from amorphous to a polycrystalline phase above 200°C. Electrical measurements demonstrate enhanced electron mobility and reduced hysteresis in annealed devices. Under 840 nm illumination, amorphous films exhibit photoconductive behavior, while polycrystalline films show photogating/photogain effects, resulting in a significantly higher photoresponsivity and faster transient response times. The highest photoresponsivity of 127 A·W^-1 and shot-noise-limited specific detectivity of 6·10¹² Jones (6·10¹⁰ Jones under full noise analysis) are achieved in polycrystalline devices under low NIR light intensity (0.32 µW·cm^-2) at V_DS = V_GS = 10 V, associated with enhanced charge carrier mobility rather than enhanced free charge generation. The devices also exhibit high cycling stability under continuous measurements and long-term shelf-stability when encapsulated. These findings establish thermally annealed Y6 films as a viable single-component material for efficient NIR photodetection.