New designs for graded refractive index antireflection coatings A. Mahdjoub a, * , L. Zighed b a De ´partement de physique, Centre universitaire L.Benmhidi, BP358 OEB 04000 O.E.Bouaghi, Algeria b De ´partement d’Electronique, Universite ´ de Skikda, Algeria Received 25 March 2004; accepted in revised form 11 November 2004 Available online 10 December 2004 Abstract The incessant progress in thin layers technology, especially the graded index inhomogeneous dielectrics, allows the realization of antireflection coatings (ARCs). Graded refractive index silicon oxynitrides are deposited by electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR-PECVD) controlled in situ by monochromatic ellipsometry. While avoiding the complexity of the classical multilayer ARCs, graded coatings permit to obtain the same performances, or furthermore to improve solar cells efficiency. A theoretical model, validated by confrontation to ellipsometric spectra, and reflectance measurements were used to optimize different suggested profiles. Calculation predicts an enhancement of photogenerated current exceeding 45% and a weighted reflectance (between 300 and 1100 nm) around 5.6%. On texturized surfaces, these ARCs should enhance short-circuit current by 52.79%. D 2004 Elsevier B.V. All rights reserved. PACS: 42.79; 84.60-Jt; 78.40-q; 81.15-z; 81.15-Gh Keywords: ARCs; Oxinitrides; Ellipsometry; Graded refractive index 1. Introduction Materials used for the manufacture of solar cells (Si, GaAs, InP, CdTe, etc.) present high refractive indices; thus, more than 35% of incident sunlight is lost by reflection without antireflection coating (ARC) [1]. The quality of ARC is therefore an essential parameter to obtain high- efficiency solar cells [2–4]. The simplest way to realize ARC, usually used in photovoltaic industry, consists in depositing a quarter wavelength dielectric layer with high refractive index (TiO 2 , Si 3 N 4 , SiO, etc.). For example, for silicon solar cells, Si 3 N 4 ARC centered on the maximum of AM0 solar spectrum, reduces reflections to 12% on average in the 400–1100 nm wavelength range. Short-circuit current is increased by 45% [1]. Multilayer systems combined with surface texturization can reduce reflection losses to few percents over all the useful range of solar spectrum [2,5–7]. However, the more technically complicated is the produc- tion, the higher is the cost. Another practical manner to reduce reflection consists in depositing an inhomogeneous dielectric with a gradual decrease of refractive index from substrate to the ambient. It is, therefore, necessary to optimize the refractive index profile to get minimum reflectance. Neuman [7] reported that antireflection performance of graded index coatings is less sensitive to thickness changes compared to classical ARCs. The steady progress in thin layers technology, espe- cially inhomogeneous dielectrics with variable refractive index, arouses researchers’ interest who suggested new deposition techniques [8–10], modelization and character- ization methods [11–13]. Therefore, our studies were guided toward graded refractive index ARC for solar cells applications. The most important advantage of the manufacture of these layers is the possibility to make a full system having a complex profile at one deposition stage without interruption of plasma process; hence, in so doing, the problem of interfaces will be avoided. Among the studied inhomogeneous dielectrics, oxynitrides seem particularly interesting [14,15]. Since they are intermediate compounds between silica and nitrides, oxinitrides encom- 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.11.119 * Corresponding author. Tel.: +213 32 42 39 70; fax: +213 32 42 10 36. E-mail address: abdelmah@yahoo.com (A. Mahdjoub). Thin Solid Films 478 (2005) 299 – 304 www.elsevier.com/locate/tsf