Bridging the Micro-to-Nano Gap: A Thermo-Coupled Broadband Photodetector for Photon–Phonon Dual Harvesting

Self-powered broadband photodetectors face severe thermal degradation limits at high excitation powers due to the parasitic heat generated by upconversion nanoparticles (UPC). To overcome this thermal bottleneck, we propose a monolithic UPC/FTO/TiO2/Bi2Te3 heterostructure that simultaneously harvests both photonic and thermal energies at the integrated multilayer scale. Unlike conventional architectures that suffer from efficiency saturation, our design utilizes the narrow-bandgap Bi2Te3 layer as a deliberate thermal confinement bottleneck. This configuration converts the localized multi-phonon waste heat into a dynamic thermoelectric potential (VTE ≈ 304 µV) via the Seebeck effect, which synergistically modulates the built-in potential of the Type-II staggered junction. To ensure high physical fidelity, our model explicitly incorporates non-ideal interface physics, quantifying the voltage drops induced by lattice mismatch, deep-level trap states, and Shockley–Read–Hall (SRH) recombination centers at the heterojunction. Reduced-order multiphysics drift-diffusion and heat transfer boundary value problem (BVP) simulations demonstrate that this thermo-coupled mechanism provides a robust stabilizing effect against thermal degradation. Within our strictly bounded non-ideal model, the device achieves a realistic short-circuit current density of 11.45 mA/cm2, an open-circuit voltage of 0.342 V, and a Fill Factor of 85.4% under zero-bias operation. Furthermore, to bridge the massive lithographic gap between the bulk-like UPC layer (114.5 µm) and the thin-film TiO2 (4 µm), we propose a hybrid thick-film integration strategy to prevent thermo-mechanical delamination and structural cracking. Our findings provide a new design paradigm for achieving highly stable detectivity in self-powered optoelectronic devices by transforming thermal losses from a parasitic vulnerability into an operational compensating force.