Wavelength-Selective Photocurrent Polarity Switching in Back-to-Back MoO _x /Sb ₂ S ₃ /Sb ₂ Te ₃ Heterojunctions for Secure Optical Encryption

Bipolar photodetectors (BPDs) featuring switchable photocurrent polarity represent a transformative advancement in optoelectronic systems, unlocking functionalities beyond conventional intensity-based detection. In this work, we present an MoO x /Sb 2 S 3 /Sb 2 Te 3 heterojunction device specifically engineered to achieve wavelength-dependent reversal of photocurrent polarity. Through strategic band alignment mediated by a MoO x interlayer, the device exhibits distinct photocurrent polarities: negative responses for wavelengths ≤ 700 nm and positive responses for wavelengths ≥ 775 nm. The optimized heterostructure delivers impressive performance metrics, with responsivities reaching −48.7 nA/mW at 685 nm and +49.0 nA/mW at 775 nm, coupled with a −3 dB bandwidth of 46.4 kHz. Critically, we leverage the device’s capability for polarity-based signal discrimination to validate its practical utility in encrypted optical communication, successfully reconstructing both ASCII codes and images from superimposed interference signals. By combining self-powered operation, structural simplicity, straightforward fabrication, spectral programmability, and functional versatility, this work not only advances the fundamental understanding of bipolar photoresponse mechanisms but also paves the way for low-cost, scalable BPDs in secure optoelectronic systems.