Method for measurement of the hemispherical/hemispherical reflectance of photovoltaic devices

Abstract

A novel method and a relative apparatus, which allow the study of the reflectance properties at diffuse light of photovoltaic (PV) devices, are presented. The optical apparatus is provided with a 40 cm diameter integrating sphere (IS) which works like a lambertian source of diffuse light and, at the same time, collects diffuse light reflected by the sample. The reflectance measurements are based on the fact that the illumination intensity of the IS depends, besides on the input light power, also on the average reflectivity of its internal surface and then on the reflectance properties of any sample held against one of its apertures. The hemispherical/hemispherical reflectance, R hh, of the sample under test is obtained by measurements of light irradiance inside the sphere in correspondence with the sample and with a selected number of standards of diffuse reflectance. The described method was used to optically characterise prototype PV modules (to be used in buildings), realised by encapsulating different types of normal and “gridless” monocrystalline-Si (mono-Si) and multicrystalline-Si (multi-Si) cells with different colours. The reflectance of the optically active area of the prototype modules was measured and the optical effect of the grid evaluated. Also the optical loss at diffuse white light was found to be approximately 4–5% for the mono-Si cells and 6–9% for the multi-Si cells. The quantity R hh, when measured for very heterogeneous samples, such as multi-Si or poly-Si solar cells can be taken to represent the optical loss of the PV device operating outdoors, in the absence of sun tracking. This method, and the corresponding apparatus, can be extended to measurements on different plane surface samples.