Bipolar Photodetector with MoO_x-Tuned Sb₂S₃/SnTe Heterojunction for Optical Encryption Communications

The escalating demand for secure optical communication requires integrated encryption strategies that balance security with detection performance. Bipolar photodetectors (BPDs) offer wavelength-tunable photocurrent polarity, enabling simultaneous spectral discrimination and signal encryption. Here, we report a self-powered BPD based on an FTO/MoO_x/Sb₂S₃/SnTe/Au back-to-back heterostructure. The device generates negative/positive photocurrents under short/long wavelengths (620/735 nm) with responsivities of -40.7 and 76.1 μA/W and fast response times (88.8/59.3 μs rise/decay at 620 nm). The MoO_x interlayer critically tunes heterojunction band alignment, shifting the polarity-switching threshold from 405 to 660 nm. Nonadditive photocurrents under dual-wavelength illumination create unique signal fingerprints, enabling an optical encryption system. Using 620 nm (valid signal) and 735 nm (encryption key) LEDs, binary data streams are encrypted such that hybrid outputs are indecipherable by unipolar detectors. This work demonstrates hardware-level security for next-generation optical networks.