CdTe thin film solar cells have demonstrated efficiencies over 20%, but CdTe has a somewhat higher band gap than optimal for single junction terrestrial solar-cell power generation. A reduction in the band gap could therefore result in an overall improvement in performance. To reduce the band gap, selenium was alloyed with CdTe using a novel co-sublimation extension of the close-space-sublimation process. Co-sublimated layers of CdSeTe with various selenium concentrations were characterized for optical absorption and atomic concentrations, as well as to track changes in their morphology and crystallinity. The lower band-gap CdSeTe films were then incorporated into the front of CdTe cells. This two-layer band gap structure demonstrated higher current collection and increased quantum efficiency at longer wavelengths. (C) 2016 Elsevier B.V. All rights reserved.