Optoelectronic properties and depth profile of charge transport in nanocrystal films

We investigate the charge transport in nanocrystal (NC) films using field effect transistors (FETs) of silicon NCs. By studying films with various thicknesses in the dark and under illumination with photons with different penetration depths (UV and red light), we are able to predictably change the spatial distribution of charge carriers across the films' profile. The experimental data are compared with photoinduced charge carrier generation rates computed using finite-difference time-domain (FDTD) simulations complemented with optical measurements. This enables us to understand the optoelectronic properties of NC films and the depth profile dependence of the charge transport properties. From electrical measurements, we extract the total (bulk) photoinduced charge carrier densities (${n}_{\text{photo}}$) and the photoinduced charge carrier densities in the FETs channel (${n}_{\text{photo}}^*$). We observe that the values of ${n}_{\text{photo}}$ and their dependence on film thickness are similar for UV and red light illumination, whereas a significant difference is observed for the values of ${n}_{\text{photo}}^*$. The dependencies of ${n}_{\text{photo}}$ and ${n}_{\text{photo}}^*$ on film thickness and illumination wavelength are compared with data from FDTD simulations. Combining experimental data and simulation results, we find that charge carriers in the top rough surface of the films cannot contribute to the macroscopic charge transport. Moreover, we conclude that below the top rough surface of NC films, the efficiency of charge transport, including the charge carrier mobility, is homogeneous across the film thickness. Our work shows that the use of NC films as photoactive layers in applications requiring harvesting of strongly absorbed photons such as photodetectors and photovoltaics demands a very rigorous control over the films' roughness.