Preparation, Characterization and Reaction in Oxygen of Cu2S Films

The purpose of this paper is to present an extended summary of the oral presentation given at the 24th SPIE Symposium. Details of the rationale for the study, methods, procedures, and results are available.1 The object of this work was to prepare cuprous sulfide films by sulfiding copper in a mixture of hydrogen sulfide and hydrogen, to characterize the films, and to establish that solar cell device grade cuprous sulfide could be prepared by a method that permits characterization of the pure material before, during, and after reaction in degrading environments. Copper films, deposited in vacuum onto a substrate suspended from a quartz ultramicrobalance, were sulfided in mixtures of hydrogen sulfide and hydrogen until the composition cuprous sulfide was reached. With the gravimetric apparatus, the optical transmittance (T) and reflectance (R) were also obtained in situ at intervals during the deposition and sulfiding processes. The sulfided films were characterized by SEM, ISS, ESCA, SIMS, diffraction, profilometry, resistivity, and Hall techniques after removal from the balance. Between 70 and 94°C and with hydrogen/hydrogen sulfide ratios between 11 and 39:1, copper films can be sulfided to cuprous sulfide in 30-36 hours to produce cuprous sulfide films 100 to 400 nm thick. The SEM data show the films are smooth enough to deduce the optical constants from the T and R data. The carrier mobility and optical properties were found to be comparable with those reported for device grade cuprous sulfide. Composition in depth profiles show only copper and sulfur can be detected in the films, except for a trace of surface oxygen. The well-characterized cuprous sulfide films were found to be stable up to 100°C when heated in 13.3 kPa of oxygen, and after a slow reaction up to ca. 225°C, formed cuprous oxide and cupric sulfate at 300°C.