Christophe Ballif, Swiss Federal Laboratories for Materials Testing and Research, Feuerwerkerstrasse 39, CH-3602 Thun, E-mail: christophe. ballif@empa. ch Jochen Dicker, Fraunhofer ISE, Heidenhofstr. 2, D-79110 Freiburg,

Dietmar Borchert, Fraunhofer ISE, Auf der Reihe 2, D-45884 Gelsenkirchen,, Thomas Hofmann, FLABEG GmbH & Co. KG, SiemensstraRe 3, D-90766 Furth

The deposition of a thin porous SiO2 antireflection (AR) layer on glass leads to an enhanced light transmittance by reducing the reflection at the air-glass interface. Such a layer can be used to increase the performance of glasses for photovoltaic modules or for solar collectors.

This contribution describes first an industrial sol-gel process established at Flabeg for the preparation of large area AR panels and addresses the issue of the layer stability. All laboratory tests, as well as outdoor tests are successfully passed and no degradation of the layer is observed even after prolonged outdoor exposure.

The base results of our study are obtained on sets of commercial multicrystalline silicon solar cells encapsulated with low-iron glasses, with or without an AR coating. The measurements performed under standard test conditions (STC) show a current gain of 2.65% with the AR glass, when compared to glass without aR layer. An additional current gain is obtained at higher light incidence angles. Based on the results obtained on mini-modules and on the outdoor monitoring of test modules to evaluate temperature effects, simulations are performed to asses the yearly photovoltaic energy yield gain at different geographical locations. An energy yield increase of 3.4%-3.7 % is expected depending on the location.

In the last part, we briefly summarise the market situation for AR glasses. Based on the current production capacity increase and on the significant improvement given by the AR layer, we think that AR glasses are likely to play an important role for a more efficient photovoltaic energy production.

1. Introduction

Because of the refractive index mismatch between air and glass (n ~ 1.5), around 4% of the light hitting a glass surface is reflected by the outer face of a glass panel. As several authors have already shown [1-3], porous SiO2 thin films can be prepared with a refractive index varying between 1.1 and 1.5. This allows hence the deposition of single or multilayer antireflection (AR) coatings even on materials with a low refractive index such as glass. These layers can, therefore, be used to increase the performance of glasses for photovoltaic modules (referred to as PV glasses in the text) [3, 4] or for thermal collectors. Various techniques for the proportion of porous SiO2 layers have been reported in the past, including sol-gel deposition [1, 5], acid etching of the glass [6], or plasma enhanced chemical vapor deposition (PECVD) [7]. For mass production and for long-term outdoor application however, low production costs of the order of a few Euros per square meter as well as long-term stability of the layers are decisive factors. The development of processes filling these two criteria have only been achieved recently and two companies (Flabeg in Germany and Sunarc in Denmark) propose now commercial AR coating of glass for solar applications (thermal collectors or photovoltaic modules).

The purpose of the present contribution is threefold: first we want to present the industrial process developed at Flabeg and address the important issue of layer stability. Second we will show in detail which effects such an AR coating can be expected to have when used to
encapsulate standard multicrystalline silicon solar cells. To achieve this goal, we have prepared 22 mini-modules composed of a single solar cell, and for each cell the efficiency, the spectral and the angular response have been determined before and after embedding. Besides, we tested modules in outdoor conditions to evaluate the effect of the AR layer on the module temperature: in particular we will show that besides providing an efficiency increase of around 2.65% in standard test conditions (STC), the improved angular response leads to an additional yearly energy increase of around 3.5% when compared to a glass without the AR layer. Third we will briefly consider the market situation for AR glasses, which are already implemented by a number a PV module manufacturers, and describe future market perspectives.