Valence-State Controllable Fabrication of Cu_2–<i>x</i>O/Si Type-II Heterojunction for High-Performance Photodetectors

Cuprite, nominally cuprous oxide (Cu₂O) but more correctly Cu_2–<i>x</i>O, is widely used in optoelectronic applications because of its natural p-type, nontoxicity, and abundant availability. However, the photoresponsivity of Cu₂O/Si photodetectors (PDs) has been limited by the lack of high-quality Cu_2–<i>x</i>O films. Herein, we report a facile room-temperature solution method to prepare high-quality Cu_2–<i>x</i>O films with controllable <i>x</i> value which were used as hole selective transport layers in crystalline n-type silicon-based heterojunction PDs. The detection performance of Cu_2–<i>x</i>O/Si PDs exhibits a remarkable improvement via reducing the <i>x</i> value, resulting in the optimized PDs with high responsivity of 417 mA W^–1 and fast response speed of 1.3 μs. Furthermore, the performance of the heterojunction PDs can be further improved by designing the pyramidal silicon structure, with enhanced responsivity of 600 mA W^–1 and response speed of 600 ns. The superior photodetecting performance of Cu_2–<i>x</i>O/n-Si heterojunctions is attributed to (i) the matched energy level band alignment, (ii) the low trap states in high-quality Cu₂O thin films, and (iii) the excellent light trapping. We expect that the low-cost, highly efficient solution process would be of great convenience for large-scale fabrication of the Cu_2–<i>x</i>O thin films and broaden the applications of Cu_2–<i>x</i>O-based optoelectronic devices.