Ultraviolet–visible bipolar photodetector for hardware–software collaborative encrypted information transmission

Secure optical communication increasingly demands encryption strategies that go beyond software-based algorithms and penetrate the physical layer. Here, we demonstrate an ultraviolet–visible bipolar photodetector that enables hardware–software collaborative encrypted information transmission by introducing signal polarity as an intrinsic physical key. The device is constructed using a vertical back-to-back N–I–P–I–N (N, I, and P denote n-type semiconductor, intrinsic layer, and p-type semiconductor, respectively) architecture based on a perovskite/organic hybrid heterojunction, where a thin p-type interlayer reconstructs the built-in electric field distribution and enables wavelength-dependent polarity reversal. As a result, the photodetector exhibits balanced bipolar responsivity with peak values of −0.16 and +0.13 A·W⁻¹ in the ultraviolet and visible regions, respectively, microsecond-level response time, and a high specific detectivity exceeding 10¹² Jones. By combining polarity-encoded optical signals with software-based principal component analysis and spatial permutation, a multi-key image transmission system is realized. Image reconstruction requires simultaneous access to both the material-defined polarity key and the algorithmic key. The image reconstructed after decryption at the receiving end has high fidelity (the peak signal-to-noise ratio (PSNR) > 30 dB). This work demonstrates how rational device architecture design can elevate photodetectors from passive receivers to active security primitives, opening a new avenue for physical-layer encryption in high-speed and covert optical communication.