Significantly Enhancing Response Speed of Self-Powered Cu₂ZnSn(S,Se)₄ Thin Film Photodetectors by Atomic Layer Deposition of Simultaneous Electron Blocking and Electrode Protective Al₂O₃ Layers

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) thin film is a promising material for optoelectronic devices. In this work, we fabricate Mo/CZTSSe/CdS/ZnO/ITO (ITO, indium tin oxide) heterojunction photodetectors with favorable self-powered characteristics. The photodetector exhibits exceptional high-frequency photoresponse performance whose −3 dB bandwidth and rise/decay time have reached 1 MHz and 240/340 ns, respectively. For further improvement, ultrathin Al2O3 layer prepared via atomic layer deposition (ALD) process is introduced at the Mo/CZTSSe interface. The influence of ALD-Al2O3 layer thickness and its role on the photoresponse performance are investigated in detail. The interfacial layer proved to serve as a protective layer preventing selenization of Mo electrode, resulting in the reduction of MoSe2 transition layer and the decrease of series resistance of the device. Accordingly, the −3 dB bandwidth is remarkably extended to 3.5 MHz while the rise/decay time is dramatically improved to 60/77 ns with 16 cycles of ALD-Al2O3 layer, which is 4–5 orders of magnitude faster than the other reported CZTSSe photodetectors. Simultaneously, it is revealed that the ALD-Al2O3 interfacial layer acts as an electron blocking layer which leads to the effective suppression of carrier recombination at the rear surface. Consequently, the responsivity and detectivity are enhanced in the entire range while the maximum values are up to 0.39 AW–1 and 2.04 × 1011 Jones with 8 cycles of ALD-Al2O3, respectively. Finally, the CZTSSe photodetector is successfully integrated into a visible light communication system and obtains a satisfying transfer rate of 2 Mbps. These results indicate the satisfying performance of CZTSSe-based thin film photodetectors with great potential applications for communication.