CHARACTERIZATION AND OPTIMIZATION OF ABSORBING PLASMA-ENHANCED CHEMICAL-VAPOR-DEPOSITED ANTIREFLECTION COATINGS FOR SILICON PHOTOVOLTAICS

We have optimized plasma-enhanced chemical vapor deposition (PECVD) of SM-based antireflection (AR) coatings with special consideration for the short-wavelength (<600 nm) parasitic absorption in SiN. Spectrosco pic ellipsometry was used to measure the dispersion relation for both the refractive index n and the extinction coefficient k, allowing a pr ecise analysis of the trade-off between reflection and absorption in S iN-based AR coatings. Although we focus on photovoltaic applications, this study may be useful for photodetectors, IR optics, and any device for which it is essential to maximize the transmission of light into silicon. We designed and optimized various AR coatings for minimal ave rage (spectrally) weighted reflectance ([R-w]) and average weighted ab sorptance ([A(w)]), using the air mass 1.5 global solar spectrum. In m ost situations [R-w] decreased with higher n, but [A(w)] increased bec ause k increased with n. For the practical case of a single-layer AR c oating for silicon under glass, an optimum refractive index of similar to 2.23 (at 632.8 nm) was determined. Further simulations revealed th at a double-layer SiN stack with an n = 2.42 film underneath an n = 2. 03 film gives the minimum total photocurrent loss, Similar optimizatio n of double-layer SiN/SiO2 coatings for silicon in air revealed an opt imum of n = 2.28 for SiN. To determine the allowable tolerance in inde x and film thickness, we generated isotransmittance plots, which revea led more leeway for n values below the optimum than above because abso rption begins to reduce photocurrent for high n values. (C) 1997 Optic al Society of America.