2, Department of Micro and Nanotechnology, Middle East Technical University, Ankara, , Turkey
3, Department of Electrical Engineering, Stanford University, Serra Mall, California, United States
4, Department of Electrical and Electronics Engineering, Middle East Technical University, Ankara, , Turkey
A high efficiency silicon solar cell combines high electrical performance with high optical performance. A common method to improve the optical performance is to utilize subwavelength,high aspect ratio structures, resulting in a surface with a graded refractive index and low reflectance throughout the wavelength spectrum. Although such high aspect ratio structures are relatively harder to passivate than conventional structures, reasonably high surface passivation quality have also been achieved by several research groups. However, surface recombination losses remain to be one of the major loss mechanisms for high efficiency silicon solar cells. One of the widespread methods to achieve high passivation performance on undiffused silicon surfaces is to use passivating dielectric layers with high density of fixed dielectric charges. The combination high aspect ratio structures and dielectric charges is commonly expected to result in an enhanced field effect passivation, resulting in less recombination losses in the textured region than what would be expected from the high surface area enhancement. Yet, despite the comprehensive experimental studies, quantitative analyses of the field effect passivation are mostly based on one-dimensional theory utilizing semi-infinitely thick structures1. A detailed numerical analysis of the field effect passivation in multi-dimensional structures is still missing.
In this work, we provide numerical and analytical analyses of field effect passivation for two-dimensional and undiffused surface textures coated with dielectrics. We show that when the dielectric charge density is very large, an enhancement in field effect passivation is only very minor at surfaces upon texturing. Then, the local surface recombination rate is very similar for textured and planar surfaces with similar interfaces and surface recombination losses increase in proportion with surface area. For moderately-large dielectric charge densities, on the other hand, we show the effect the enhanced field effect is much more significant at the surfaces and surface recombination losses increase sub-linearly with surface area. Thus, through proper material and process choice enabling moderate dielectric charge densities and high interface quality, surface recombination losses are expected to decrease further by taking full advantage the field effect.
1. Von Gastrow, G., Alcubilla, R., Ortega, P., Yli-Koski, M., Conesa-Boj, S., i Morral, A. F., & Savin, H. (2015). Analysis of the atomic layer deposited Al2O3 field-effect passivation in black silicon. Solar Energy Materials and Solar Cells, 142, 29-33.