Prediction of intermediate band in Ti/V doped γ-In₂S₃

Materials with an intermediate energy band (IB) introduced in the forbidden gap are viable alternatives to tandem configurations of solar cells for increasing the photon-conversion efficiency. One of the aspiring designs proposed for the intermediate band concept is hyperdoped (Ti, V):In₂S₃. Being very important in copper indium gallium sulfide (CIGS) solar cells, indium thiospinel (In₂S₃) is known for its three different temperature as well as pressure, polymorphs. The most stable β-In₂S₃ was experimentally shown to have an isolated intermediate band (IB) and exhibits sub-band gap absorption due to the completely filled IB after V-doping. Though experimental observation holds a positive signature, recent DFT studies did not show a metallic intermediate band for the V dopant in the 3+ charge state. In order to clarify this, we have taken incentive from experimental XRD analysis that V-doped β-In₂S₃ shows peaks from disordered In vacancies (either α or γ), in addition to the ordered In vacancies expected. Hence, we have carried out state-of-the-art DFT based computations on pure and Ti, V-doped In₂S₃ in the γ-phase which has not been studied yet. We considered the Ti and V dopants in various charge states. Our theoretical study including hybrid functional, does in fact find the IB in V-doped γ-In₂S₃. However, at equilibrium the IB lies in between the Fermi level (E _F) and conduction band minimum (CBM).