Dopants and Traps in Nanocrystal-Based Semiconductor Thin Films: Origins and Measurement of Electronic Midgap States

Different models are proposed to explain the origin of electronic midgap states that limit charge transport in nanocrystal thin films. Here, we investigate midgap states using optoelectronic and electrochemical techniques, supported by theoretical calculations. We find that charge trapping in PbS NC thin films can be explained by (1) nanocrystal dimerization and (2) charging of doped nanocrystals. In these models, there are no states within the band gap of individual nanocrystals; the nanocrystals themselves act as trap states. These findings allow us to formulate strategies to improve transport by increasing free carrier densities while reducing the number density of trap states.