New insight into light utilization efficiency: an evaluation of semitransparent solar cells for building-integrated photovoltaic windows

Semitransparent solar cells hold significant potential for application in energy-efficient buildings. The light utilization efficiency (LUE), defined as the product of power conversion efficiency (PCE) and average visible transmittance (AVT), is widely adopted as the figure of merit for semitransparent solar cells. However, their performance evaluation is complicated by the inherent trade-off between PCE and AVT. In this study, we first demonstrate that LUE is not a standalone criterion for assessing the performance in terms of energy usage for semitransparent solar cells, as its validity depends on the defined transmittance boundaries. Secondly, we clarify the priority between PCE and AVT under a specific LUE and reveal that this priority is region-dependent. The results indicate that in Nanjing (N32°49', Humid Subtropical climate) and San Diego (N32°43', Warm Mediterranean), solar cells with higher PCE (lower AVT) perform better, whereas in Guangzhou (N23°10', Humid Subtropical climate), solar cells with higher AVT show greater advantages. Furthermore, we analyze the impact of AVT on indoor visual comfort by calculating useful daylight illuminance and daylight glare probability. The transmittance thresholds suitable for different regions are identified: below 50% for Nanjing and San Diego, and up to 55% for Guangzhou for unshaded conditions.