Textured AZO on silicon oxy-nitride barrier films for enhanced light trapping in micromorph tandem junction solar cells

Aluminum doped Zinc Oxide (AZO) films have been reported as the transparent front contact layer in single and tandem junction thin film silicon solar cells. AZO films form highly transparent and conductive layers on glass surfaces with the possibility of further enhancement of solar cell performance by light trapping through wet-chemical texturing of the AZO surface. However, most reports have demonstrated such AZO films either on alumino-borosilicate glasses which are prohibitively expensive for low cost solar modules, or on commercial glass with barrier films deposited by other deposition methods. This paper demonstrates that well-textured AZO films can be fabricated on a manufacturable barrier layer of silicon oxy-nitride deposited on commercial low-iron glass, thus enabling the production of high-performance micromorph solar cells. Silicon oxy-nitride is chosen as the barrier layer for two reasons: (1) it forms an effective diffusion-barrier to contaminant migration from the commercial glass substrate to the active layers of the cell, and (2) its refractive index can be tailored to allow maximum optical coupling of incoming light into the solar cell. The disadvantage is that AZO film growth on the silicon oxy-nitride surface is more difficult to control, and thereby the surface texture obtained after wet-etching. Using a multi-step deposition process, we show that uniformly textured AZO films can be deposited on silicon oxy-nitride barrier films on commercial grade low-iron glass substrates. Measurements of the optical, electrical, microstructure and surface texture characteristics of the pulsed DC magnetron sputtered AZO films from a 1%wt aluminum doped zinc oxide target are shown to meet or exceed design criteria. Performance measurements on tandem junction solar cells made with an AZO front contact processed in this manner are presented.