Diffractive gratings for crystalline silicon solar cells - optimum parameters and loss mechanisms

In this paper, we present guidelines for the design of backside gratings for crystalline silicon solar cells. We use a specially developed method based on a combination of rigorous 3D wave optical simulations and detailed semiconductor device modeling. We also present experimental results of fabricated structures. Simulation-based optimizations of grating period and depth d of a binary grating and calculations of the optical and electrical characteristics of solar cells with optimized gratings are shown. The investigated solar cell setup features a thickness of dbulk=40m and a flat front surface. For this setup, we show a maximum increase in short-circuit current density of jSC=1.8mA/cm2 corresponding to an efficiency enhancement of 1% absolute. Furthermore, we investigate different loss mechanisms: (i) an increased rear surface recombination velocity S0,b because of an altered surface caused by the introduction of the grating and (ii) absorption in the aluminum backsi de reflector. We analyze the trade-off point between gain due to improved optical properties and loss due to corrupted electrical properties. We find that, increasing the efficiency by 1% absolute due to improved light trapping, the maximum tolerable recombination velocity is S0,b(max)=5.2×103cm/s. From simulations and measurements, we conclude that structuring of the aluminum backside reflector should be avoided because of parasitic absorption. Adding a dielectric buffer layer between silicon and the structured aluminum, absorption losses can be tuned. We find that for a planar reflector, the thickness of a SiO2 buffer layer should exceed dSiO2=120nm.