Thin films built up by 3D assemblies of bismuth(III) sulfide and indium(III) sulfide quantum dots: Charge carrier transport mechanism

Abstract

The transport of charge carriers through 3D assemblies composed by In2S3 and Bi2S3 quantum dots deposited as thin films was studied. Combining temperature-dependent resistivity measurements with structural and optical absorption studies, thorough insights into the mechanism governing charge transfer throughout the QD solids were derived. In cubic In2S3 QD solids, charge transport was found to be governed by thermionic emission (TIE) involving trap states situated below the Fermi level, with significant contribution of the variable range hopping (VRH). Tunneling through intercrystalline barriers was found to be of marginal importance. The conductivity activation energy for this system is 0.85eV and it was found to be equal to the barrier height at inter-nanocrystalline boundaries. Such effective complex interplay between the two charge carrier transport mechanisms at first sight appears to be in a contradiction with the Debye's length of this system calculated using relative permittivity value of the bulk solid, which is almost 50% larger than QD size. However, accounting for the possibility of several-fold decrease of the last quantity upon entrance of strong size-quantization regime, clarifies the apparent predominance of the TIE mechanism. In nanostructured films built up by Bi2S3 QDs in weak size-quantization regime, Debye's length was found to be smaller than QD size. VRH mechanism was here found to govern the overall charge transport at lower temperatures, with significant contribution of TIE, involving trap states below the Fermi level.