Optimization of Top Contact for Cu(In,Ga)Se2 Solar Cells

As world energy demands continue to increase, the need to generate electricity from a broader variety of sources, including renewables, is more critical than ever. With costs still 30% higher than those of natural gas, solar energy is a viable contender, but more progress is needed to level the playing field with other forms of energy generation. The overall energy security can be enhanced by diversifying the energy supply. Among them, Cu(In,Ga)Se2 (CIGS) has gained significant momentum as a possible high efficiency and low cost thin film solar cell material. The capacity to scale up any photovoltaic technology is one of the criteria that will determine its long term viability. In the case of CIGS, many manufacturers are showing the way for GW-scale production capacity. However, as CIGS technology continues to increase its share of the market, the scarcity and high price of indium will potentially affect its ability to compete with other technologies. One way to avoid this bottleneck is to reduce the importance of indium in the fabrication of the cell simply by reducing its thickness without significant efficiency loss. Reducing the thickness of CIGS thin film will not only save the material but will also lower the production time and the power needed to produce the cell. As the properties of the absorber and buffer layers are modified with each enhancement, it is also important to continue developing a better and effective light trapping mechanism. The overall reflection losses can be minimized to a great extent by applying an efficient anti-reflective (AR) coating, thus increasing the power conversion efficiency of the device. We describe a method using in-situ real time spectroscopic ellipsometry and optical modeling allowing for the optimization of the thickness of the anti-reflective (AR) coating for Cu(In1-xGax)Se2 (CIGS) solar cells. The model is based on a transfer matrix theory as well as accurate measurement of the dielectric function and thickness of each layer in the stack by spectroscopic ellipsometry. The AR coating thickness is then optimized in real time to optically enhance the performance of the device for various device configurations by varying the thickness and properties of different layers. In ultra-thin CIGS solar cells, multi-layered anti-reflective coatings are essential since a single layer AR coating is not capable of suppressing the reflectance as it increases. Thus it is very important to obtain an enhanced light trap in the red and near infra-red region. Multi-layer AR coatings are used to obtain at least five passes in the internal reflection from the bottom surface of the cell.