Development of novel coatings for solar cover glass

The burning of fossil fuels to provide energy for humanity over the last two centuries has released large amounts of greenhouse gases into the atmosphere, resulting in serious environ-mental crises. The move away from fossil fuels towards renewable sources of energy is vital to the future of life on earth. Solar photovoltaics (PV), creating electricity from sunlight, is a key technology in providing clean and renewable energy for future generations.

A significant area of current research in photovoltaics is coatings for the cover glass that protects commercial solar panels, to address important challenges such as reflection of light from the surface and the accumulation of dirt (known as soiling). This report focuses on the development of multilayer coatings designed to address these issues. Durability is also a key consideration for cover glass coatings, especially resistance to abrasion.

Reflection of light from the front surface of solar cover glass accounts for a loss of just over 4%. This loss can be addressed by the use of antireflection (AR) coatings applied to the glass surface. Multilayer, broadband AR coatings for multiple PV technologies are designed and fabricated, with the coatings consisting of alternate layers of SiO2 and ZrO2. Multiple coating designs consisting of different materials and numbers of layers are modelled, before a brief evaluation and selection of the chosen design. The chosen designs are then fabricated on glass substrates using reactive pulsed-DC magnetron sputtering. The optical performance of the coatings are verified using spectrophotometry measurements, and the efficiency of solar cells is measured before and after addition of AR coatings, to determine the increase in power output. The AR coating designed for silicon cells results in a 2.34% increase, and for Cd-SeTe/CdTe cells the improvement is 3.54%. The abrasion resistance of these coatings is also tested, in comparison to the porous SiO2 coatings currently used commercially, and the mul-tilayer coatings show significantly higher resistance to abrasion across multiple test methods.

The accumulation of dirt and dust on the module surface, a process known as soiling, can significantly reduce module power output by blocking light transmission. The problem is significant in dry and dusty areas such as the Middle East. Current mitigation methods cen-tre around manual cleaning with large, tractor-mounted brushes, but this can be expensive depending on factors such as labour costs and water availability. The abrasion from brushes also damages the AR coatings on the module. An alternative solution to cleaning is to use an anti-soiling coating, preferably hydrophobic (water contact angle > 90◦) to reduce dirt accumulation and aid in removal. Current coatings use organic fluoropolymers, which are effective but are vulnerable to degradation under UV exposure. Anti-soiling coatings should also have an anti-reflective (AR) aspect to ensure that gains from reduced soiling are not canceled out by loss of transmission. Work presented here shows the effective combination of these organic hydrophobic coatings with the multilayer AR coating from the previous chapter to increase the water contact angle of the coating from 7◦ to 114◦ with negligible impact on optical performance, combining both properties without comprising either, al-though durability remains an issue. These durability issues show the need for more durable anti-soiling coatings. Inorganic rare earth oxides (REOs) offer a potential solution as they are significantly more stable than polymer-based anti-soiling coatings. This work combines the multilayer AR coating with thin Er2O3 films to provide stable coatings for anti-soiling and anti-reflection. The addition of a 5nm film lowers reflectance even further, and can pro-vide a water contact angle of over 100◦. However, the method of attaining hydrophobicity for REOs is in question, with work to improve hydrophobicity presented alongside coating fabrication.

A major problem with silicon solar cells is that they lose efficiency with increased op-erating temperature, at a rate of about 0.5% per 1◦C increase above 25◦C. This causes a significant reduction in power output, particularly in hot climates where temperatures can reach 65◦C. A potential solution in the form of an optical coating is presented, which reflects infra-red (IR) radiation to limit the module temperature increase. The coating consists of alternate SiO2 and indium tin oxide (ITO) and is made up of 4 layers, building on the design of AR coatings in the previous chapters. Modelling results show that the weighted average reflectance (WAR) is reduced by 2.5% in the wavelength range associated with the band gap of silicon. The optical coating then reflects up to 70% of the sub-bandgap light. Optimisation of oxygen flow and heat treatments for deposition of the individual ITO layers is presented, before fabrication of the full 4-layer stack. Optical performance of the stack is then tested, showing reflection performance in excellent agreement with the theoretical model. Transmission, however, is much lower and suggests unwanted absorption in the films. This unwanted absorption leads to an overall reduction in Jsc compared to uncoated glass. Further work is needed to reduce this absorption, and temperature reduction testing is on-going.