Direct Determination of Total Hemispherical Emittance of Perovskite and Silicon Solar Cells

Abstract

The performance and lifetime of a solar cell in the field is sensitive to its operating temperature. A knowledge of the total radiative heat transfer is required to predict the operating temperature. This requires an accurate knowledge of the total hemispherical thermal emittance as a function of temperature. Calorimetric methods performed under vacuum are considered to be the “gold standard” method for determining total hemispherical emittance as they are direct and absolute. Here, we report a novel, cost-effective and rapid transient calorimetric method to measure directly the total hemispherical thermal emittance of a solar cell. Examples of standard and high efficiency passivated-emitter-rear-localised contact (PERL) silicon solar cell and perovskite solar cells with and without encapsulation were measured over their operating temperature range for both front and rear surfaces. Results show that the encapsulation material of a cell and the type of rear electrode strongly influence the total hemispherical thermal emittance of the cell. Specifically, using soda-lime glass as a cover layer in encapsulated cells increases the emittance due to the high infrared (IR) absorptance of glass. Metal rear electrodes in a semi-transparent cell decrease the emittance considerably by creating a broad band of high reflectance. These findings demonstrate the importance of front and rear materials selection in the optimization of radiative cooling for efficiency and lifetime. Additionally, total hemispherical emittance of solar cells measured in this work is considerably lower than spectral emittance values previously measured which were used to calculate solar cell operating temperature in the context of radiative cooling. Therefore, radiative heat transfer calculation of photovoltaics will need to be re-examined in the light of this work.