Photocurrent enhancement in GaAs solar cells using whispering gallery modes of dielectric nanospheres
Light trapping is a critical requirement in thin film photovoltaics, and dielectric texturing is a viable method to induce light trapping, but thin film device quality often suffers upon direct texturing of the semiconductor active material. Thus it is desirable to develop a design method in which textured dielectric layers provide for light trapping on smooth planar thin film cells. We propose here an approach for coupling light into smooth untextured thin film solar cells of uniform thickness using periodic arrangements of resonant dielectric nanospheres deposited as a continuous film on top of a thin cell. It is shown that guided whispering gallery modes in the spheres can be coupled into particular modes of the solar cell and significantly enhance its efficiency by increasing the fraction of incident light absorbed. We numerically demonstrate this enhancement using full field finite difference time domain (FDTD) simulations of a SiO_2 nanosphere array above a 1 μm thick Gallium arsenide (GaAs) solar cell structure featuring back reflector and double anti-reflection coating. The incoupling element in this design has advantages over other schemes as it is a lossless dielectric material and its spherical symmetry naturally accepts a wide angle of incidence range. Moreover, analytical models show that for SiO_2 nanospheres of a given dielectric material, a large number of resonant modes can be supported which can give rise to a 2.5% absorption enhancement in the GaAs absorber layer at several wavelengths between 300 nm and 870 nm and reach a current density of J=28.23 mA/cm^2. Also, the SiO_2 nanosphere array can be fabricated using simple, well developed self assembly methods and is easily scalable.