Internal and External Light Trapping for Solar Cells and Modules

Abstract

Renewable energy resources are essential to realize a sustainable society and a clean environment. In virtually all energy scenarios, solar power will supply a significant share of the world energy demand within a few decades. This energy transition can be significantly supported and accelerated when the power conversion efficiency of solar cells improves. This will bring down the cost per delivered unit of energy and thereby solar cells become even more financially competitive with burning fossil fuels. The efficiency of solar cells is to a large extend determined by their light absorptance. Conventional solar cells do not absorb all light; instead they reflect some light to space. If this reflected light can be recycled, then a higher light to electricity conversion efficiency is realized. This thesis focuses on two optical solutions for improved absorptance of light in solar panels: internal and external light trapping. For internal light trapping the solar cell is internally modified to trap the light in the solar cell; for external light trapping optical elements are placed in front of the solar panel, forcing the light to pass the solar cell several times, thereby increasing its absorption. Internal light trapping is demonstrated in a nano-crystaline silicon solar cell. An array of glass nanocylinders overcoated with silver is applied at the backside of the planar nano-crystaline silicon layer. Due to the geometry and the contrast of the dielectric index of the glass, silicon, and silver the light scatters when it interacts with this back reflector. By total internal reflection in the silicon, most of the scattered light will travel a much longer distance through the cell compared to a flat cell. This leads to a significant increase in the absorptance and thereby an increase in cell efficiency. A special feature of this structure is its ability to improve the absorptance without inducing damage to the solar cell. External light trapping is accomplished by trapping the light that reflects from the solar cell in a cage between the solar cell and a mirror above the solar cell. Thereby, the reflected light from the solar cell is redirected back to the solar cell by the mirror, see figure. {Use figure 1 of the following publication for an image of an external light trap: http://onlinelibrary.wiley.com/doi/10.1002/pip.2702/full; I can email this figure in high resolution.} A lens is used to guide the light into this cage through a small aperture in the mirror. The light trapping results in higher absorptance and improves the power conversion efficiency. We successfully demonstrate a 3D-printed external light trap on top of a nano-crystalline silicon solar cell. Furthermore, the opportunities for external light trapping on a large area were explored by making a matrix of lenses which is tested on an organic solar cell. Subsequently, a series of external light traps was fabricated by an industrial milling process which resulted in significantly enhanced the performance of a crystalline silicon solar cell. Finally, we explored several technical options for low-cost integration of external light trapping in solar panels. These results pave the way towards a new class of solar panels with a higher light to electricity conversion efficiency and novel esthetic features. For example, one could make colored solar panels, integrate LEDs in the solar panel for lighting at night, and solar panels that display an image (e.g. art, photo or advertisement). This is expected to be of high interest for many applications such as building integrated photovoltaics.