Highly efficient GaAs solar cells by limiting light emission angle

Abstract

In a conventional flat plate solar cell under direct sunlight, light is received from the solar disk, but is re-emitted isotropically. This isotropic emission corresponds to a significant entropy increase in the solar cell, with a corresponding drop in efficiency. Here, using a detailed balance model, we show that limiting the emission angle of a high-quality GaAs solar cell is a feasible route to achieving power conversion efficiencies above 38% with a single junction. The highest efficiencies are predicted for a thin, light trapping cell with an ideal back reflector, though the scheme is robust to a non-ideal back reflector. Comparison with a conventional planar cell geometry illustrates that limiting emission angle in a light trapping geometry not only allows for much thinner cells, but also for significantly higher overall efficiencies with an excellent rear reflector. Finally, we present ray-tracing and detailed balance analysis of two angular coupler designs, show that significant efficiency improvements are possible with these couplers, and demonstrate initial fabrication of one coupler design.

Experimental demonstration of enhanced photon recycling in angle-restricted GaAs solar cells

“…Despite this theoretical prediction, until recently even the highest efficiency solar cells were not close enough to the radiative limit for such an effect to be observed. [5][6][7] However, with the introduction of cells lied off the growth substrate, GaAs cells have shown signicant gains in efficiency due to V oc increases, indicating an increase in the external radiative efficiency (ERE) of the cell. [2][3][4] In these lied-off GaAs cells radiatively emitted photons are reected from a metallized back surface instead of being absorbed in the substrate, resulting in a large increase in ERE and V oc .…”

Enhanced absorption of thin-film photovoltaic cells using an optical cavity

“…Modeling shows that the optical cavity used in conjunction with patterned PV cells can even exceed the Yablonovitch limit for absorption, especially for ultrathin (<2µm) solar cells. The effect of the cavity can be even stronger for thin film PV cells with higher radiative recombination emission rates (such as GaAs), as the cavity-imposed emission angular selectivity also enables re-cycling of emitted photons [16,21].…”

“…Most strategies for enhancing thin-film cell absorption pursued up to this point have used cell surface texturing to scatter photons into a wider range of angles into the cell with the aim of approaching the Yablonovitch limit [10][11][12][13][14]. However, the PV cell efficiency can also be increased by limiting the angular range through which photons (both trapped and emitted in the process of radiative recombination) can escape the cell [15][16][17][18]. For angle limited PV cells, the maximum absorption enhancement factor is increased from to where is the half-angle of the absorption cone of the PV cell [19,20].…”

Maximal power output by solar cells with angular confinement

“…The modal structure of light in thin cells is considered according to [20]. The Auger recombination is, in the case of intrinsic or very lightly doped material, given by [2]:…”

“…With such a system, it is possible to reach the highest efficiencies [19]. A high quality broadband angularly selective filter is described in [20], which employs micro-concentrators at the front side to act as angle confining elements. A good light-trapping structure is a Lambertian Diffuser, which is considered in the following.…”