Near-infrared organic photodetectors are highly attractive for flexible wearable systems, with organic phototransistors offering particular promise due to their intrinsic photogating gain that enables amplification of weak optical signals. However,the performance of such devices is often constrained by the limited driving force for exciton dissociation in narrow-bandgap donors and the short exciton diffusion length typical of organic semiconductors, both of which hinder efficient photocarrier generation and separation and subsequently suppress the photogating effect. To overcome this sensitivity bottleneck, the introduction of a well-designed excitondissociation interface is critical. In this work, we fabricate a PDPPBTT/PC₆₁BM layer heterojunction via self-assembly transfer and integrate it into a bottom-gate, top-contact transistor architecture. We systematically investigate the role of this heterojunction interface in facilitating exciton dissociation, charge trapping, and modulation of channel conductance. The p-type polymer PDPPBTT serves a dual function as both the near-infrared absorber and the hole-transporting channel, while PC₆₁BM acts as the electron-accepting layer. The resulting donor-acceptor interface provides a substantial driving force for exciton dissociation and enables efficient spatial separation of electrons and holes. Optical and electrical characterization reveal that photogenerated electrons are transferred to the PC₆₁BM layer and interfacial trap states, forming longlived negative charges that are effectively separated from holes. These trapped charges induce a pronounced photogating effect, which continuously modulates the hole density in the PDPPBTT channel and significantly boosts the photocurrent. Under near-infrared illumination, the device exhibits a maximum responsivity of 1.26×10⁶ A·W^-1 and a specific detectivity of 6.96×10¹⁵ Jones, estimated under the dark-current shot-noise limit, with rise and fall times of 0.26 s and 0.21 s, respectively. Furthermore, the device is capable of recording transmissive photoplethysmographic (PPG) signals, allowingfor a preliminary estimation of blood oxygen saturation through dual-wavelength PPG measurements. Collectively, these findings demonstrate that engineering donor- acceptor layer heterojunctions offers an effective route to overcoming the performance limitations of near-infrared organic phototransistors and provides a solid experimental foundation for their application in detecting weak physiological optical signals.