Preparation of single-phase SnSe thin-films and modification of electrical properties via stoichiometry control for photovoltaic application
Single-phase SnSe thin films were prepared via thermal co-evaporation using a Se thermal cracker. By carefully tuning the stoichiometry of the SnSe, we found that the composition range of single phase SnSe is very narrow, a Se/Sn ratio of 0.95–0.99; outside of this range secondary phases (metallic Sn or SnSe2) formed. Electrical properties were found to be very sensitive to even small changes in the stoichiometry. Three orders of magnitude difference in the carrier concentration was observed within the stoichiometry range for single-phase SnSe, which can be explained by changes in the shallow level defect density. To further control carrier concentration, we introduced In and Sb as counter-dopants into the SnSe thin films and found that they were deep level donors with the ionization fraction of ∼10−4. Finally, we demonstrate the potential of SnSe thin films as an absorber layer in photovoltaic applications. Our study demonstrates the importance of fine-tuning stoichiometry of SnSe to achieve desired electrical properties.