In a common research project of Flabeg GmbH & Co. KG, Merck KGaA and the Fraunhofer Institutes ISC and ISE, a production process for AR solar glass was developed, allowing the coating of panels up to 1.5 x 2.5 square meters. After edge processing and surface cleaning, the porous SiO2 AR layers are obtained by sol-gel dipping of the glass followed by a thermal curing at 630°C taking place simultaneously with the glass toughening, and therefore minimising the production costs. The critical steps in the preparation of the glazing are the glass cleaning and a good control of the SiO2 sol particle size, in the range of 30 nm. A precise control of this size is necessary to achieve films with good adhesion properties and the desirable refractive index. Typical AR layers are about 130-140 nm thick, with a refractive index of 1.27, and are deposited on both sides of the panel. The AR coating process can be applicable to any kind of glass (float glass or rolled glass with texture). Fig. 1 shows the light transmittance through low iron patterned glasses with and without the AR layer. The broad transmission maximum at 99 % around 600-650 nm indicates that the % X condition for reflection minimum is fulfilled in this range. Note that the reflection measured through a glass is roughly the sum of the first air-glass reflection and of the second glass air reflection (which adds 4% to the primary air-glass reflection).

An important issue is the stability of the AR layer, because modules are expected to stay several tens of years in the field. To clarify this point, two main test procedures were performed: accelerated durability tests under severe conditions in accordance to IEC 1215 and real life outdoor exposure tests.

The following laboratory tests were passed successfully:

• Condensed water climate test at 40°C and 100% relative humidity (acc. DIN 50017): no significant degeneration or changes

• Mechanical resistance by Crockmeter-test (acc. DIN EN 1096-2): after 1000 cycles no significant visible changes, change in solar transmittance < 1%

• Humidity-heat-test of AR-PV laminate modules at 85°C and 85% relative humidity for 1000 hours (acc. IEC 1215): no visual degeneration, change in energy output < 5%

• Temperature-cycle-test of AR-PV laminate modules (-40°C to +85°C for 200 cycles, acc. IEC 1215): no visual degeneration, change in energy output < 5%

Besides, climate test in SO2 atmosphere (cycling: 40°C, 100% rel. humidity, 8 hours to 18- 28°C, 75% rel. humidity, 16 hours, 5 ppm SO2, 23 cycles), freezing test (-20°C, 48 days) and boiling test (100°C, 10 minutes) did not modify the optical properties of the layer. Laboratory tests even under severe conditions can only give an indication of the durability behaviour of new materials or systems. Therefore outdoor weathering tests have also to be performed to check the suitability of the layers in real conditions.

Test samples have been exposed at different locations spread over Germany and monitored for 3 years at the date of writing this paper. The AR glasses have been evaluated by visual inspection and solar reflectance measurement. During this test period no effects and changes could be observed. Fig. 1b shows a summary of the test data.

100 80 60 40 20 0

Fig.1. a): Optical transmission as a function of the wavelength for a low iron glass with and without the porous SiO2 AR layer (AR layer on both sides). b) Solar Reflectance (reflectance weighted over the solar spectrum) of AR-coated and uncoated glass as a function of outdoor exposure time.

In addition, the outdoor results showed no negative effect of contamination or soiling of the glasses due to the AR coating. A possible increased adherence of dust or particles in or on the porous AR structure is not observed. The porosity of the AR layer is in the range of a few nanometers and therefore much lower compared to the typical size of dust particles in the range of several microns. Because of these different dimensions, a particular interaction has indeed not to be expected.

As a conclusion, the mechanical resistivity, as well as the chemical inactivity and the nanostructure of the AR layer seem to combine to confer porous SiO2 AR glass the necessary stability for long term outdoor applications.