Improving thin-film crystalline silicon solar cell efficiency with back surface field layer and blaze diffractive grating

Both 2D electromagnetic and electrical semiconductor simulations are performed sequentially in this study in order to better understand the structural principles of thin-film crystalline solar cells with back surface field and blaze diffractive grating. In the absence of adequate approximations for blazed gratings, we simulate silicon solar cells electromagnetically and electrically in order to deal with the geometrical complexity produced by the blazed grating with a BSF on top of it. Thin-film crystalline silicon solar cells (TF-c-Si SCs) typically exhibit poor quantum efficiency both at shorter wavelengths and longer wavelengths with sharp drops in spectral response. Longer wavelength spectral response (from 0.6 μm to 1.2 μm) is addressed here first by considering the influence of blaze gratings on the enhancement of effective optical absorption in thin-film crystalline silicon (TF-c-Si) solar cells. The effect of the back surface field layer (BSF) in terms of improving minority carrier collection is also taken into account. In the 2D electromagnetic simulation, polarization dependent two-dimensional (2D) numerical simulations based on rigorous coupled wave analysis (RCWA) and finite element method (FEM) are implemented for the optimization of optical absorption of the solar cell structure. A rather large tolerance in design parameters of the optimized blaze grating structure was found. The optimized blaze grating structures help in improving the cell efficiency, especially for weak absorption thin cell structures. The enhancement of equivalent optical path length reveals the efficient light trapping effect caused by the diffractions of the blaze grating structures, especially in the longer wavelength range. In the electrical semiconductor simulation, the BSF, which arises from the heavy acceptor doping that creates the concentration gradient, is set atop the blaze grating in order to provide an extra small drift field for the collection of minority electrons. Incorporating the optimized antireflection coating along with a BSF layer and a blaze-grating in the 2 μm cell doubles cell efficiency. The use of blazed gratings in thin-film solar cells, which can be performed upon silicon by means of lithography and ion-beam etching, is promising for low cost and high-efficient solar cell applications.

► How to improve long wavelength spectral response of TF-c-Si solar cells is addressed.
► Blaze gratings for the enhancement of effective optical absorption in TF-c-Si SCs.
► Back surface field on the improvement of minority carrier collection is simulated.
► Polarization dependent 2D numerical simulations for blaze gratings.
► Incorporating a blaze-grating along with a BSF in the 2 μm Si doubles cell efficiency.