Thin-film CdS/CdTe solar cells are a promising materials system for large scale deployment of solar photovoltaic modules. Here we report on an extensive side-by-side characterization effort comparing the device properties of a highly optimized cell made on commercially available transparent conductive oxide/glass with a certified efficiency of 15.2% to a non-optimized device with a respectable efficiency of 13.0%. We perform these characterizations with an eye on distinguishing which 3rd level metrics are the drivers for the performance improvement, specifically the open-circuit voltage. Based on the data presented in this paper we found that the biggest difference between the best device and a good device is in the built-in potential of the device, which is likely higher due to better band-alignment at the interfaces. (C) 2012 Elsevier B.V. All rights reserved.