Reconciling LED and monochromator-based measurements of spectral responsivity in solar cells

Modelling photovoltaic soiling losses through optical characterization

The accumulation of soiling on photovoltaic (PV) modules affects PV systems worldwide. Soiling consists of mineral dust, soot particles, aerosols, pollen, fungi and/or other contaminants that deposit on the surface of PV modules. Soiling absorbs, scatters, and reflects a fraction of the incoming sunlight, reducing the intensity that reaches the active part of the solar cell. Here, we report on the comparison of naturally accumulated soiling on coupons of PV glass soiled at seven locations worldwide. The spectral hemispherical transmittance was measured. It was found that natural soiling disproportionately impacts the blue and ultraviolet (UV) portions of the spectrum compared to the visible and infrared (IR). Also, the general shape of the transmittance spectra was similar at all the studied sites and could adequately be described by a modified form of the Ångström turbidity equation. In addition, the distribution of particles sizes was found to follow the IEST-STD-CC 1246E cleanliness standard. The fractional coverage of the glass surface by particles could be determined directly or indirectly and, as expected, has a linear correlation with the transmittance. It thus becomes feasible to estimate the optical consequences of the soiling of PV modules from the particle size distribution and the cleanliness value.

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.

Understanding Photovoltaic Energy Losses under Indoor Lighting Conditions

The external luminescence quantum yield as a function of the solar cell current density when exposed to low indoor light was estimated based on absolute electroluminescence measurements and a self-consistent use of the electro-optical reciprocity relationship. By determining the luminescence yield at current densities corresponding to the cell operation at the maximum power point, we can compute energy losses corresponding to radiative and nonradiative recombination. Combined with other major energy losses, we can obtain a clear picture of the fundamental balance of energy within the cell when exposed to room light with a typical total illuminance of 1000 lx or less.