Operando EFISHG Visualization of Trapped-Charge Photogating in DPP–TTT/g-C ₃ N ₅ Organic Phototransistors

Organic phototransistors (OPTs) enable lightweight, low-temperature processability, and mechanically flexible photodetection, yet high sensitivity often compromises charge transport due to trap-assisted field screening and altered internal electrostatics. Here, we incorporate graphitic carbon nitride (g-C3N5) nanosheets into a high-mobility donor–acceptor (D–A) polymer (DPP–TTT) and examine their coupled effects on charge transport and internal electrostatic modulation. An optimum incorporation of 3 wt % g-C3N5 results in the highest saturation mobility, a 2.1-fold enhancement over pristine devices. Out-of-plane X-ray diffraction indicates improved structural organization of polymer matrix, with a slight lamellar contraction and an increased coherence length suggesting enhanced intermolecular ordering and more efficient charge transport pathways. Under 730 nm illumination, g-C3N5-added devices show enhanced photosensitivity and a larger positive threshold-voltage shift; the photoinduced threshold shift persists only when the device is illuminated under off-state reverse bias, consistent with field-assisted charge separation and trapping. Operando EFISHG directly visualizes the internal electric field near the source; the observed gate-voltage shift in the field–response curve correlates with trapped negative charge, while transient EFISHG measurements reveal accelerated field screening in g-C3N5 blends. These results identify g-C3N5 as an additive that simultaneously improves packing-driven transport and amplifies photogating via enhanced trapped-charge formation.