Towards intermediate-band photovoltaic absorbers: theoretical insights on the incorporation of Ti and Nb in In 2 S 3

Creation of a partially filled intermediate band in a photovoltaic absorber material is an appealing concept for increasing the quantum efficiency of solar cells. Recently, we showed that formation of a partially filled intermediate band through doping a host semiconductor with a transition metal dopant is hindered by the strongly correlated nature of d-electrons and the antecedent Jahn–Teller distortion, as we have previously reported. In present work, we take a step forward and study the delocalization of a filled (valence-like) intermediate band throughout the lattice: a case study of Ti- and Nb-doped In2S3. By means of hybrid density functional calculations, we present extensive analysis on structural properties and interactions leading to electronic characteristics of Ti- and Nb-doped In2S3. We find that Nb creates an occupied doublet, which can become delocalized onto the crystal at high but feasible concentrations (around 2.5 at% and above). As a consequence, doping In2S3 with adequately high concentrations of Nb allows the subgap intermediate band to conduction band absorption, which leads to higher photocurrent densities compared to pure In2S3. Ti on the other hand forms an occupied singlet intermediate band, which remains strongly localized even at high concentration of 5 at%.