Detailed Balance Analysis of Photovoltaic Windows

There are several technical and socio-economic factors converging to position photovoltaic (PV) windows as a transformative technology for the energy landscape of the future: 1) Urban areas currently account for 67–76% of global final energy consumption. 2) 70% of the world’s population will live in urban areas by 2050. 3) The overwhelming architectural trend is away from opaque building components and toward all-glass facades. 4) Photovoltaics are becoming extremely affordable, and the most expensive components in a conventional module are the glass and transparent metals—components that are already in highly-insulating glazing. Rational design of PV windows is of paramount importance to realize their impact. In this work, we provide an analysis on the theoretical performance of PV windows using a detailed balance model to understand the complex design space of power conversion efficiency (PCE), visible light transmittance (VLT), solar heat gain coefficient, and color. We find there are two distinct regimes for PV window absorber design. The first low-VLT regime validates the most prevalent approach to semitransparent PV windows in which conventional absorber materials are thinned to allow visible light to pass through—the bandgap for peak PCE is consistent with the classic work of Shockley and Queisser. However, we identify a second, high-VLT regime in which the ideal bandgap for maximum PCE increases monotonically from 2 to 3 eV with increasing VLT. In this tuned-bandgap regime, the solar cell exhibits lower losses and tunable solar heat gain and color.