Electricity produced by cadmium telluride (CdTe) photovoltaic modules has one of the lowest-costs in the solar industry, and is now much cheaper than fossil fuel-based sources in many regions of the world. Recent substantial efficiency gains have been achieved by alloying selenium into the CdTe absorber, improving efficiency from 19.5% to the current record value of 22.1%. This paper in Nature Energy reports directly correlated cathodoluminescence and NanoSIMS results showing that selenium passivates critical defects in the bulk of the absorber layer, and this understanding provides promising directions for further efficiency improvements.