Characterization of UV–Vis–NIR optical constants of encapsulant for accurate determination of absorption and backscattering losses in photovoltaics modules

The determination of the optical properties of encapsulant for photovoltaic modules is essential for cell-to-module (CTM) performance analysis, UV-durability studies and as input parameters of modelling tools. Some innovative polymer encapsulants used in heterojunction module manufacturing, such as polyolefin elastomer or thermoplastic polyolefin, show a non-negligible optical scattering behaviour in addition to refraction and absorption mechanisms, making challenging the extraction of optical properties in general, and optical indices in particular. In this work, a 4-flux optical model was applied on total and diffuse transmittance and reflectance measurements of a Thermoplastic PolyOlefin (TPO) encapsulant layer to extract values of refractive index, absorption, scattering coefficient and anisotropy coefficient of scattered light. This model was found mandatory to obtain a satisfactory fitting and plausible value of the optical parameters, in opposition to simpler models based on collimated light, or only diffuse light, or isotropic 4-flux model. The model was applied on two diffusive TPO: a low and a high UV cut-off. The absorption was found respectively to be 1.7% and 3.8%. Therefore, a new Cell-To-Module source of loss was identified, induced by the backscattered light in the encapsulant bulk, whose value is near 0.8% of the incoming light in both samples.

Julien Eymard and Raphael Clerc and Vincent Duveiller and Benjamin Commault and Mathieu Hebert

Solar Energy Materials and Solar Cells

Cell to module source of loss,Light scattering,Optical indices measurement,PV Module encapsulant

2022

6

10.1016/J.SOLMAT.2022.111717

0927-0248

111717

North-Holland

240