Analysis of quantum efficiency and optical enhancement in amorphous Si p-i-n solar cells

The effect of i-layer thickness, tin oxide texture, and back reflector (BR) on optical enhancement has been systematically studied in a series of 20 a-Si p-i-n solar cells. The internal quantum efficiency has been analyzed by a simple model based on the work of Schade and Smith. The enhancement of optical absorption is characterized by m, a wavelength-dependent fitting parameter representing the increase in optical pathlength relative to the i-layer thickness d. Solar cells with an Al BR have negligible optical enhancement, with m < 1.5, consistent with large parasitic absorption at the Al/Si interface as reported by others. Solar cells on highly textured SnO2 with ZnO/Al or ZnO/Ag BR have peak values of m similar to 3-4, with ZnO/Ag having slightly larger values than ZnO/Al. It was found that m has a strong dependence on the product alpha d, and that maximum values of m increase with reflectivity of the BR. It is shown that a major source of parasitic absorption loss at long wavelengths is light trapping in the textured SnO2 front contact. Copyright (C) 2002 John Wiley & Sons, Ltd.