All-Solid-State Self-Powered Bipolar Photodetector Based on an FTO/Sb2S3/Bi2Te3 Back-to-Back Heterojunction for Secure Optical Communication

Bipolar photodetectors with wavelength-dependent polarity switching show broad prospects in secure optical communication, but existing devices are limited by liquid electrolyte reliance, temperature susceptibility, material stability, and sluggish response. Herein, an all-solid-state self-powered bipolar photodetector with FTO/Sb2S3/Bi2Te3 back-to-back heterostructure is developed. Driven by two oppositely oriented built-in electric fields at the heterointerfaces, the device achieves photovoltaic-effect-driven carrier separation and transport without external bias, exhibiting precise wavelength-selective bipolar response across the 405–940 nm broad spectrum. It outputs negative photocurrent under short-wavelength illumination (<685 nm) and positive photocurrent under long-wavelength illumination (>685 nm), with microsecond-scale response speed and a –3 dB bandwidth up to 75 kHz. A hardware-encrypted optical communication system is further demonstrated, where valid signals are effectively masked via polarity cancellation, while conventional unipolar detectors cannot decode the encrypted information. This work provides a facile strategy for high-performance bipolar photodetectors for secure optical communication applications.