Bias-programmable linear-to-nonlinear photoresponse for filter-free single-pixel VIS-NIR discrimination

Filter-free discrimination of visible (VIS) and near-infrared (NIR) light within a single photodetector pixel remains challenging because conventional photodiodes typically require optical filters or multiple detector elements. This work reports a monolithic nonlinear back-to-back photodiode (NL-BPD) that enables bias-programmable reconfiguration between high-linear-dynamic-range (LDR) photodetection and nonlinear spectral-discrimination modes. By changing the bias from - 0.5 V to + 0.5 V, the device's initially independent linear responses in both VIS and NIR channels (LDRs ≥ 100 dB) evolve into a nonlinear regime in which the VIS response reverses polarity with increasing light intensity while the NIR response remains monotonic. This behavior is enabled by defect-assisted transport pathways formed through partial interfacial reactions between the highly conductive PEDOT:PSS interlayer and MAPbI₃. Together with the optimized device architecture, this interfacial current-path engineering reduces resistive bottlenecks and promotes cooperative carrier extraction from both absorbers. The resulting electrical signatures allow mixed VIS-NIR signals to be decoded from a single electrical output without optical filters or spatially separated pixels. The NL-BPD enables accurate spectral reconstruction (average R² = 0.9996), demonstrates single-point pulse oximetry, and achieves > 96% accuracy in normalized-difference-vegetation-index-based vegetation health classification. These results establish bias-programmable nonlinear photodetection as an effective strategy for single-pixel multispectral sensing.