Anti-reflection coatings and optical interference in photovoltaics

Light reflection from the glass surface of a photovoltaic (PV) module is a significant source of energy loss for all types of PV devices. The reflection at the glass and air interface accounts for ~4% of the total energy. Single layer anti-reflection coatings with sufficiently low refractive index have been used, such as those using magnesium fluoride or porous silica, but these are only effective over a narrow range of wavelengths. Multilayer-antireflection coatings reduce the weighted average reflection over the wavelength range used by solar technologies more effectively by utilising interference effects. Multilayer stacks consisting of silica and zirconia layers deposited using reactive magnetron sputtering and single layer porous silica coatings were compared in terms of effectiveness and durability. Details of the stack design, sputter deposition process parameters, and the optical and micro-structural properties of the layers of the multilayer coating are provided and similar properties where applicable for the single layer coatings. Anti-reflection coatings on glass exposed to the outdoors must not degrade over the lifetime of the module. A comprehensive set of accelerated environmental durability tests has been carried out in accordance with IEC 61646 PV qualification tests. The durability tests confirmed no damage to the coatings or performance drop as a result of thermal cycling or damp heat. All attempts to perform pull tests on either coating resulted in either adhesive or substrate failure, with no damage to the coating itself. Scratch resistance, abrasion resistance, and adhesion tests have also been conducted. The optical performance of the coatings was monitored during these tests and the coatings were visually inspected for any sign of mechanical failure. These tests provide confidence that broadband anti-reflection coatings are highly durable and will maintain their performance over the lifetime of the solar module. Additionally heat treatment experiments demonstrated both coatings can withstand up to 600°C temperatures and can thereby withstand CdTe manufacturing processes allowing for pre-coated glass. Additionally experiments demonstrated that multi-layer coatings are resistant to acid attack. Thin film photovoltaic devices are multilayer opto-electrical structures in which light interference occurs. Light reflection at the interfaces and absorption within the window layers reduces transmission and, ultimately, the conversion efficiency of photovoltaic devices. Optical reflection losses can be reduced by adjusting the layer thicknesses to achieve destructive interference within the structure of the cell. The light transmission to the CdTe absorber of a CdS/CdTe cell on a fluorine doped tin oxide transparent conductor has been modelled using the transfer matrix method. The interference effect in the CdS layer and high resistance transparent buffer layers (SnO2 and ZnO) has been investigated. The modelling shows that due to relatively high absorption within the SnO2 layer, there are modest benefits to engineering anti-reflection interference in the stack. However, a ZnO buffer layer has limited absorption and interference can be exploited to provide useful anti-reflection effects. Additionally the light transmission to the perovskite absorber of a thin film solar cell using fluorine doped tin oxide (FTO) transparent conductor has been modelled. Alternative transparent conductor materials have also been investigated including aluminium doped zinc oxide (AZO) and indium tin oxide (ITO) and shown to be beneficial to transmission.