Self-Powered Broadband Ta₂NiSe₅/PdSe₂ Heterojunction Photodetector with Mid-Infrared Response

Broadband photodetectors capable of extending into the mid-infrared region are of great interest for applications requiring access to molecular vibrational and thermal radiation information, yet their realization under low-power or self-powered conditions remains challenging. Here, we report a self-powered van der Waals heterojunction photodetector composed of narrow-band gap Ta₂NiSe₅ and semimetal PdSe₂. First-principles calculations reveal band alignment giving rise to a strong built-in electric field that enables efficient carrier separation under zero bias. Benefiting from this interfacial band engineering, the device delivers a responsivity of 212 mA/W and a detectivity of 9 × 10⁹ Jones at 638 nm, while maintaining a measurable of 0.05 mA/W at 4000 nm. At a low modulation frequency of 7 Hz, a noise-equivalent power as low as 1.06 × 10^-10 W Hz^-1/2 is obtained, allowing sensitive low-frequency weak-light detection. Stable and relatively fast photoresponse is achieved, with rise and decay times of 407 and 457 μs, respectively. Moreover, the heterojunction exhibits a broadband spectral response into the mid-infrared regime around 4 μm. These results highlight Ta₂NiSe₅/PdSe₂ heterojunctions as a promising platform for low-power, broadband optoelectronic applications.