Wavelength‐Selective Bipolar Photodetector Based on FTO/Sb ₂ S ₃ /Sb ₂ Se ₃ Heterostructure for Secure Optical Information Processing

Abstract The growing demand for advanced optical information processing necessitates photodetectors capable of wavelength‐selective responses. This work reports a bipolar photodetector based on an FTO/Sb ₂ S ₃ /Sb ₂ Se ₃ back‐to‐back heterostructure. At zero bias, the device exhibits intrinsic wavelength‐dependent polarity switching: negative photocurrent for λ < 775 nm and positive photocurrent for λ > 775 nm. External bias allows dynamic modulation of spectral selectivity; a forward bias (+113 mV) enhances the response at 940 nm while suppressing that at 660 nm, whereas a reverse bias (−80 mV) yields the opposite behavior. Key performance metrics include microsecond‐scale response speeds (e.g., rise/decay times of 68/178 µs at 940 nm), operational stability over 1800 s, and a peak responsivity of 58.4 nA mW ⁻¹ at 700 nm. This allows the device to decrypt a valid optical signal (a 460‐nm encoded “panda” image) masked by interference (a 940‐nm encoded “cat” image)—a task that conventional unipolar detectors cannot accomplish due to signal superposition and their inability to discern current direction. Combining self‐powered operation, active spectral tuning, broadband response, and straightforward fabrication, this platform shows promise for advanced optoelectronic applications.