On the Use of Photocurrent Imaging To Determine Carrier Diffusion Lengths in Nanostructured Thin-Film Field-Effect Transistors

Scanning photocurrent microscopy (SPCM) has been widely used as a powerful experimental technique to investigate charge transport and recombination in nanostructured field-effect transistors (FETs). Photocurrent mapping modulated by transverse electric field provides critical insights into local electronic band bending and carrier transport. However, the analysis of experimental results is often based on unjustified assumptions. In particular, the inhomogeneous carrier concentration induced by gate bias and local photoexcitation may significantly influence the photocurrent distribution, but these effects have not been considered in previous work. Furthermore, carrier lifetime is a function of carrier concentration and can have a large spatial variation in a gated channel, which may lead to further complication. Here, we perform rigorous two-dimensional cross-sectional modeling of thin-film FETs under local photoexcitation. Our simulation results validate that accurate minority carrier diffusion lengths can be extracted from SPCM measurements regardless of the substantial nonuniformity in carrier density and potential that is characteristic of these devices. However, at high excitation intensity, the photocurrent decay lengths deviate from the minority carrier diffusion lengths as a result of light-induced carrier drift. We also verify that thermoelectric effects due to laser heating can be ignored. By accounting for a carrier concentration-dependent recombination lifetime, we find that the photocurrent decay length corresponds to an average diffusion length of carriers distributed throughout the channel. Although we focus here on colloidal quantum dot thin films, our conclusions can be extended to SPCM measurements of any thin-film or nanowire FET