Light harvesting and carrier separation in N-doped graphene clusters/Si heterojunctions for photodetection and spatial image recognition

Two-dimensional (2D) graphene photodetectors have limited light absorption, short carrier lifetimes, and a dependence on external bias, which hinder sensitivity and power efficiency. This study presents a three-dimensional (3D) heterojunction of nitrogen (N)-doped graphene clusters, featuring 82% porosity, constructed on a silicon (Si) substrate. The 3D structure enhances light–matter interaction and trapping through localized optical field confinement in concave areas. Scanning kelvin probe microscopy and conductive atomic force microscopy results reveal that N doping enhances band alignment, boosts the built-in electric field, and improves carrier transport. The device enables broadband self-powered detection from 440 to 1550 nm without bias. At 1550 nm, it has a responsivity of 24.5 A/W and specific detectivity of 1.2 × 1012 Jones (under −3 V bias and 40 mW/cm2), and a low noise power spectral density of 3 × 10−26 A2/Hz. It features a −3 dB bandwidth of 1.5 kHz and rise/fall times of 173/189 μs, maintaining stable performance for 4 months. By combining spatial photocurrent mapping with a deep learning model, it achieves over 97% recognition accuracy for various images. This work presents a strategy for graphene optoelectronic devices featuring broadband, high-sensitivity, self-powered, and intelligent sensing.