Exploration of external light trapping for photovoltaic modules.

Abstract

The reflection of incident sunlight by photovoltaic modules prevents them from reaching their theoretical energy conversion limit. We explore the effectiveness of a universal external light trap that can tackle this reflection loss. A unique feature of external light traps is their capability to simultaneously recycle various broadband sources of reflection on the module level, such as the reflection from the metal front grid, the front interfaces, the reflective backside of the cell, and the white back sheet. The reflected light is recycled in the space between the solar cell and a mirror above the solar cell. A concentrator funnels the light into this cage through a small aperture in the mirror. As a proof-of-principle experiment, a significant reflectance reduction of a bare crystalline silicon (c-Si) photodiode is demonstrated. In contrast to conventional light trapping methods, external light trapping does not induce any damage to the active solar cell material. Moreover, this is a universally applicable technology that enables the use of thin and planar solar cells of superior electrical quality that were so far hindered by limited optical absorption. We considered several trap designs and identified fabrication issues. A series of prototype millimeter-scale external metal light traps were milled and applied on an untextured c-Si photodiode, which is used as a model for future thin solar cells. We determined the concentrator transmittance and analyzed the effect of both the concentration factor and cage height on the absorptance and spatial intensity distribution on the surface of the solar cell. This relatively simple and comprehensive light management solution can be a promising candidate for highly efficient solar modules using thin c-Si solar cells.