Trap-induced photoresponse of solution-synthesized MoS2

We investigated, for the first time, the photoresponse characteristics of solution-synthesized MoS2 phototransistors. The photoresponse of the solution-synthesized MoS2 phototransistor was solely determined by the interactions of the photogenerated charge carriers with the surface adsorbates and the interface trap sites. Instead of contributing to the photocurrent, the illumination-generated electron-hole pairs were captured in the trap sites (surface and interface sites) due to the low carrier mobility of the solution-synthesized MoS2. The photogenerated holes discharged ions (oxygen and/or water) adsorbed onto the MoS2 surface and were released as neutral molecules. At the same time, the photogenerated electrons filled the traps present at the interface with the underlying substrate during their transport to the drain electrode. The filled trap sites significantly relieved the band bending near the surface region, which resulted in both a negative shift in the turn-on voltage and an increase in the photocurrent. The time-dependent dynamics of the solution-synthesized MoS2 phototransistors revealed persistent photoconductance due to the trapped electrons at the interface. The photoconductance was recovered by applying a short positive gate pulse. The instantaneous discharge of the trapped electrons dramatically reduced the relaxation time to less than 20 ms. This study provides an important clue to understanding the photoresponses of various optoelectronic devices prepared using solution-synthesized two-dimensional nanomaterials.