Energy production advantage of independent subcell connection for multijunction photovoltaics

Increasing the number of subcells in a multijunction or “spectrum splitting” photovoltaic improves efficiency under the standard AM1.5D design spectrum, but it can lower efficiency under spectra that differ from the standard if the subcells are connected electrically in series. Using atmospheric data and the SMARTS multiple scattering and absorption model, we simulated sunny day spectra over 1 year for five locations in the United States and determined the annual energy production of spectrum splitting ensembles with 2–20 subcells connected electrically in series or independently. While electrically independent subcells have a small efficiency advantage over series‐connected ensembles under the AM1.5D design spectrum, they have a pronounced energy production advantage under realistic spectra over 1 year. Simulated energy production increased with subcell number for the electrically independent ensembles, but it peaked at 8–10 subcells for those connected in series. Electrically independent ensembles with 20 subcells produce up to 27% more energy annually than the series‐connected 20‐subcell ensemble. This energy production advantage persists when clouds are accounted for. As the solar spectrum varies over the course of a year, multijunction cells with series‐connected subcells suffer efficiency losses as various subcells are over or under illuminated. Consequently, series‐connected multijunction cells will produce fewer kW‐hr of energy per year than equivalent ensembles with electrically independent subcells. We find that the energy production advantage of the electrically independent subcells increases with the number of junctions, from 4.9% for two junctions, 13.3% for six junctions, and up to 27.1% for 20 junctions.