Tunable Bipolar Perovskite Photodiodes Enabled by Electric Field Flipping for Secure Optical Communication

The wavelength-dependent positive and negative responses make bipolar photodiodes essential for applications in sensing, secure communication, and imaging systems. However, existing bipolar photodiodes typically rely on tandem light-absorbing layers or the synergy of multiple physical effects, leading to complex architectures and limited adaptability. Here, we present an isotype transport layer-structured bipolar single-layer perovskite photodiode (n-i-n type) with an adjustable bipolar response range through an electric field flipping mechanism. The device features two electric fields in opposite directions. Through the in situ modulation engineering of the energy band of nanomaterials, the electric field at the perovskite/SnS₂ (electron transport layer) interface is precisely modulated, resulting in flipping of the overall device electric field, driving carriers generated at different depths toward opposite electrodes, and producing a wavelength-dependent bipolar response. In particular, tuning the internal electric field expands the positive response range from 300-480 nm to 300-700 nm. A secure optical communication system constructed by this bipolar photodiode can transmit positive and negative signals, enabling the secure transmission of Fourier-transformed color images. The optical communication system demonstrated excellent security. When intercepted by unipolar photodiodes, the information leakage rate is only 10.85%, and the difference rate of transmitted images reaches a perfect 100%. This work utilized an interfacial potential engineering strategy to fabricate tunable bipolar perovskite photodiodes, offering a promising route toward compact, high-speed, and intrinsically secured optoelectronic communication systems.