Angle-dependent reflectance measurements on photovoltaic materials and solar cells

Results of angle-dependent reflectance measurements on photovoltaic materials and solar cells are presented and discussed. The optical apparatus employs an integrating sphere of 40 cm diameter and it is suitable for working at variable wavelengths and incidence angles of the light beam. Directional/hemispherical reflectance measurements can be performed and both the diffuse and the total components of the reflected light can be obtained. Samples of any size can be measured by this apparatus, including small prototype solar cells (less than or equal to 2 cm), medium size materials and solar cells (10-12 cm), as well as glass sheets and entire photovoltaic modules. Results of reflectance on silicon materials, commercial and record efficient silicon solar cells are here presented as functions of the incidence angle and wavelength of the light beam. The measurements evidence the superior light trapping performances of the inverted pyramids (PERL) and of the new 'honeycomb' structures with respect to the traditional upright random pyramids texturization. A simple analytical model describing the effect of the grid is derived and discussed. The model was used to calculate the reflectance of the exposed semiconductor region of the cell only. It was validated by comparing the calculated reflectance of the semiconductor region with that measured on 'gridless' cells. The spectral reflectance measurements at different angles matched the light collection performances of the cells, and are very useful for their optimization in view of their outdoor application. (C) 1999 Published by Elsevier Science B.V, All rights reserved.