Exploration of external light trapping for photovoltaic modules

Abstract

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. Electron. Dev. 33, 234-239 (1986). 9. A. Gabor, "Cell-to-module gains and losses in crystalline silicon pv, gabor photovoltaics consulting," July 10, 2013Intersolar NA. 10. G. Peharz, W. Nemitz, V. Schmidt, S. Schweitzer, W. Mühleisen, and C. Hirschl, "Investigations on the photonrecycling properties of different back-sheets," in Proceedings of EUPVSEC, (2014) (111) and (100) silicon/silicon oxide structure," in Proceedings of EUPVSEC, (2008), pp. 244-247. 16. K. Xiong, S. Lu, D. Jiang, J. Dong, and H. Yang, "Effective recombination velocity of textured surfaces," Appl.Phys. Lett. 96, 193107 (2010). 17. M. Green, "Limits on the open-circuit voltage and efficiency of silicon solar cells imposed by intrinsic auger processes," IEEE Trans. Electron. Dev. 31, 671-678 (1984).

Concepts for external light trapping and its utilization in colored and image displaying photovoltaic modules

“…(3) [43,44,48,49]. The potential impact of non-uniform illumination on the cell performance as well as several solutions to re-homogenize the light has been discussed previously [9,50]. For the designs presented here, the intensity variations are mild, and therefore, no severe impact on cell performance is expected [51].…”

“…The space between the reflector and the white back sheet forms a cavity (or cage) in which the light is trapped. The spacer thickness determines the divergence (i.e., width) of the beam in the cavity and the uniformity of the cell illumination [9]. Its thickness needs to be such that (most of) the reflected light from the cell is redirected backwards by the reflector, instead of escaping through an opening after the first reflection.…”

“…Moreover, they confine light within the active cell area but do not recover light reflecting from the bus bars or the optically dead area between the solar cells. In contrast, light confining cavities have a broadband enhancement and are a more comprehensive solution as they can trap the light irrespective of its origin [9]. In recent work, we demonstrated optical confinement using a universal external light trap on nc-Si:H [10], organic [11], and c-Si solar cells [9].…”

“…In contrast, light confining cavities have a broadband enhancement and are a more comprehensive solution as they can trap the light irrespective of its origin [9]. In recent work, we demonstrated optical confinement using a universal external light trap on nc-Si:H [10], organic [11], and c-Si solar cells [9]. The concept of optical confinement by an external light trap is illustrated in Figure 1.…”

“…In the successful initial proof-of-concept experiments [9][10][11], we employed prototype optical elements that are not well suited for module integration. It is not straightforward to directly sandwich the metalized parabolic mirrors (or concentrators) into a conventional module.…”