Understanding light trapping by light scattering textured back electrodes in thin film n-i-p-type silicon solar cells

For substrate n-i-p-type cells rough reflecting back contacts are used in order to enhance the short-circuit currents. The roughness at the electrode/silicon interfaces is considered to be the key to efficient light trapping. Root-mean-square (rms) roughness, angular resolved scattering intensity, and haze are normally used to indicate the amount of scattering, but they do not quantitatively correlate with the current enhancement. It is proposed that the lateral dimensions should also be taken into account. Based on fundamental considerations, we have analyzed by atomic force microscopy specific lateral dimensions that are considered to have a high scattering efficiency. Textured back reflectors with widely varying morphologies have been developed by the use of sputtered Ag and Ag:AlOx layers. For these layers we have weighted the rms roughness of the surface with the lateral dimensions of the effective scattering features. A clear correlation is found between the current generation under (infra)red light in microcrystalline (mu c-Si:H) n-i-p solar cells and the weighted rms value of the Ag back contacts. Furthermore, the surface plasmon absorption of the rough Ag back contact has been found to be a significant limiting factor for the current enhancement. Using Ag:AlOx layers on glass, deposited at substrate temperatures below 300 degrees C, a mu c-Si:H n-i-p solar cell is obtained with an efficiency of 8.1%. Using textured Ag layers made at a higher substrate temperature on a stainless steel substrate we have developed a hot-wire chemical vapor deposited mu c-Si:H n-i-p-type solar cell with 8.5% efficiency. (c) 2007 American Institute of Physics.