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As a two-dimensional (2D) material, palladium diselenide (PdSe 2 ) has attracted extensive research attention due to its unique asymmetric crystal structure and extraordinary optoelectronic properties, showing great potential in electronic, optoelectronic, and other application fields. Thinner PdSe 2 exhibits semiconductor properties, while the photoresponse of the photodetectors based on this film is weaker. Although increasing the thickness of the PdSe 2 film can improve the photoresponse, thicker PdSe 2 exhibits metallic-like properties, which is not conducive to the formation of the heterojunction. In this work, a PdSe 2 2D material with a quantum island structure is prepared by a simple thermal-assisted conversion method. A new type of photodetector with a PdSe 2 /n - -Si/n + -Si vertical PIN-like structure is innovatively proposed. Broad spectral absorption from 532 to 2200 nm and a high rectification ratio (10 6 ) of the device are achieved. The introduced n - -Si layer concentrates the electric field in the depletion region, thereby shortening the transit time and accelerating the separation and collection of the carriers, resulting in the enhancement of the responsivity and 3 dB frequency compared to the traditional device with a PN structure. A recorded highest 3 dB frequency of ∼25 kHz is achieved for the PdSe 2 2D-3D PIN-like device.