Influence of Defect States on Charge Transport in CuInSe$_{\mathrm{2-x}}$S$_{\mathrm{x}}$ Quantum Dot Films

CuInSe$_{\mathrm{2-x}}$S$_{\mathrm{x}}$ quantum dots (QDs) are environmental-friendly alternatives to Cd- or Pb-based QDs for solar energy applications. The key to using QD thin films in opto-electronic devices like solar cells is understanding their charge-transport properties, which are known to be influenced by defects that can serve as carrier traps. Here, we combine field effect transistor (FET) and ultrafast transient photocurrent (u-TPC) measurements to obtain a more complete picture of the nature and role of trap sates in CuInSe$_{\mathrm{2-x}}$S$_{\mathrm{x}}$ QD thin films. FET devices employing indium contacts exhibit $n$-type transport with electron mobility of 5.34 \texttimes 10$^{\mathrm{-4}}$ cm$^{\mathrm{2}}$/Vs, but they also indicate high concentrations of electrons in the films. Early-time dynamical signatures revealed in u-TPC suggest that this high carrier density arises from the presence of trap states in CuInSe$_{\mathrm{2-x}}$S$_{\mathrm{x}}$ QDs. In order to reduce the density of trap states, atomic layer deposition was used to infill the CuInSe$_{\mathrm{2-x}}$S$_{\mathrm{x}}$-based devices with amorphous alumina, which results in both higher FET mobilities, and a reduction in trap-related decay signatures in u-TPC measurements.