Photovoltaic Characterization under Artificial Low Irradiance Conditions Using Reference Solar Cells

Due to the rapidly growing interest in energy harvesting from indoor ambient lighting for the powering of internet-of-things devices, accurate methods for measurements of the current vs voltage characteristics of light-harvesting solar photovoltaic devices must be established and disseminated. A key requirement when conducting such characterizations is to create and measure the irradiance from the test light, whose spectral output approximates the profile of some agreed-upon standard reference. The current methods for measuring the irradiance from indoor ambient lighting (e.g., illuminance meters) can yield unacceptable discrepancies in measurements from one lab to another. Here, we take the first steps in establishing a more accurate alternative: using a calibrated reference solar cell to measure the total irradiance of the test light when establishing the test light level, and then, once set, while collecting the characterization data for the test specimen. The method involves establishing multiple reference indoor lighting spectra that meet desired illuminance requirements, while also offering precise spectral irradiance profiles. Regardless of whether these proposed spectra are formally adopted, the test method is available and useful. The proposed approach facilitates inter-lab measurements, allows for a way to calculate an accurate power conversion efficiency, and establishes a dialogue between National Metrology Institutes to begin the process of drafting standards for solar cell testing under conditions that are significantly different than the well-established standard reporting condition used for rating solar modules that are deployed outdoors.