Ultrafast Photocurrent and Absorption Microscopy of Few-Layer Transition Metal Dichalcogenide Devices That Isolate Rate-Limiting Dynamics Driving Fast and Efficient Photoresponse

Despite inherently poor interlayer conductivity, photodetectors made from few-layer stacked 2D transition metal dichalcogenides (TMDs) such as WSe2 and MoS2 often yield a desirable fast (less than or similar to similar to 90 ps) and efficient (epsilon > similar to 40%) photocurrent response. To unambiguously separate the competing electronic escape and recombination rates, we combine ultrafast photocurrent (U-PC) and transient absorption (TA) microscopy methods. U-PC and TA kinetics obtained on WSe2 photodetectors yield matching interlayer electronic escape times that accelerated from similar to 1.6 ns to 86 ps with the applied E-field. These ultrafast rates predict the actual device PC efficiencies realized of 40-45%. The roughly linearly increasing electronic escape rates with applied voltage in TA and U-PC decay kinetics both give out-of-plane electron and hole mobilities of 0.129 and 0.031 cm(2)/(V s), respectively, in WSe2. Above similar to 10(12) photons/cm(2) incident flux, defect-assisted Auger scattering greatly lowers the efficiency by trapping carriers at vacancy defects. Both TA and PC spectra identify a metal vacancy subgap peak with 5.6 ns lifetime as one primary trap capturing carriers as they drift between layers. TA and U-PC microscopy independently provide the kinetics of electronic escape and recombination that determine PC device efficiency. For few-layer TMD devices, this simple rate law further predicts the observed nonlinear in PC dependence over a 10(5) range of incident power.