Geometry-Engineered Bipolar Photodetectors for Multivalued Logic-Gate Encrypted Optical Communication

Bipolar photodetectors, with positive-negative photocurrent responses and coupling-enabled multifunctionality, show great promise for logic-gate-encrypted optical communication, where precise photocurrent balancing is essential to minimize polarity-induced logic errors and ensure transmission accuracy. Here, a parallel-structured, self-powered photoelectrochemical photodetector (PEC-PD) based on fully amorphous p-NiO_X and n-GaO_X thin films is developed, exhibiting dual-band (254 nm/365 nm) bipolar photoresponses. To address the intrinsic photocurrent asymmetry, we first employ thermal annealing to mitigate the imbalance. More significantly, leveraging the unique parallel architecture, we propose a universal geometry-engineered strategy to accurately balance positive and negative photocurrents by simply adjusting the area ratio of the two photoactive materials. This approach enables robust photocurrent symmetry across diverse wavelength combinations, ensuring broad adaptability to complex optical environments. Based on this tunability, reconfigurable binary and ternary exclusive OR (XOR and TXOR) logic gates are demonstrated as encryption frameworks in optical communication systems. The ternary encryption scheme enhances information throughput by ∼1.585 × and expands the key space by 89% compared to binary systems. This work presents a universal strategy for implementing balanced bipolar photodetectors, facilitating their application in secure, high-throughput underwater optical communication.