Origin of Anomalous Transient Photocurrent in Solution-Processed WS₂ Nanosheet-Based Self-Powered Photodetectors

Solution-processed self-powered photodetectors were fabricated by using tungsten disulfide (WS 2 ) nanosheets (NSs) in a vertical structure. The device shows a high on–off ratio of ∼2.4 × 10 5 under illumination from a light-emitting diode of wavelength (λ) ∼ 625 nm and an intensity of 100 mW/cm 2 . The photoresponsivity of the device peaks around λ ∼ 630 and 523 nm, which correspond to the characteristic absorption peaks of WS 2 NSs. The devices exhibit fast photoresponse within a few milliseconds under the pulsed illumination and an anomalous slow decay of photocurrent during the pulse width (pulse-on time ∼50 s). The slow decay of the photocurrent of the photodetector under constant illumination is studied in detail using steady-state and transient photocurrent measurements. The intensity-dependent photocurrent measurements show a sublinear behavior with photocurrent due to the presence of traps and trap-limited conduction in the devices. Similarly, the time-dependent photocurrent measurement under various illumination intensities depicts the accumulation and trapping of charges at the interfaces, which act as recombination centers that lead to charge carrier recombination loss. Additionally, the photoresponse measurements under different vacuum conditions indicate the influence of the adsorbate molecules in the atmosphere. Furthermore, the interfacial effects and performance-limiting factors are analyzed by probing the photocurrent dynamics of these photodetectors as a function of illumination intensity, background illumination, ambient pressure, and temperature. Our investigations on time-dependent photocurrent measurements shed light on how charges were extracted, accumulated, and recombined at the WS 2 -transport layer interfaces.