Thickness-dependent enhanced optoelectronic performance of surface charge transfer-doped ReS₂ photodetectors

Surface charge transfer doping has been widely utilized to tune the electronic and optical properties of semiconductor photodetectors based on low-dimensional materials. Although many studies have been conducted on the performance (response time, responsivity, etc.) of doped photodetectors and their mechanisms, they merely examined a specific thickness and did not systematically explore the dependence of doping effects on the number of layers. This work performs a series of investigations on ReS₂ photodetectors with different numbers of layers and demonstrates that the p-dopant tetrafluorotetracyanoquinodimethane (F₄-TCNQ) converts the deep trap states into recombination centers for few-layer ReS₂ and induces a vertical p-n junction for thicker ReS₂. A response time of 200 ms is observed in the decorated 2-layer ReS₂ photodetector, more than two orders of magnitude faster than the response of the pristine photodetector, due to the disappearance of deep trap states. A current rectification ratio of 30 in the F₄-TCNQ-decorated sandwiched ReS₂ device demonstrates the formation of a vertical p-n junction in a thicker ReS₂ device. The responsivity is as high as 2,000 A/W owing to the strong carrier separation of the p-n junction. Different thicknesses of ReS₂ enable switching of the prominent operating mechanism between transforming deep trap states into recombination centers and forming a vertical p-n junction. The thickness-dependent doping effect of a two-dimensional material serves as a new mechanism and provides a scheme toward improving the performance of other semiconductor devices, especially optical and electronic devices based on low-dimensional materials.