Summer 2010 Intern Project- Charles Buhler
DESIGNING A MIDDLE ANTIREFLECTIVE COATING TO ALLOW LIGHT TO THE LOWER BANDGAP CELLS AND REFLECT HIGHER ENERGY PHOTONS BACK TO THE TOP CELLS TO INCREASE PERFORMANCE FOR A FOUR-JUNCTION SOLAR CELL
UC Santa Barbara
Mentor: Chieh-Ting Lin
Faculty Advisor: John Bowers
Department: Electrical and Computer Engineering
As photovoltaic cells are made to absorb a larger part of the solar spectrum, antireflective (AR) coatings need to be better designed to accommodate for this wide spectrum. One method for solar cells to absorb more of the solar spectrum is to have multiple cells, or junctions, stacked on top of each other, each tuned to absorb a different wavelength range. The UCSB Optoelectonics Group is creating a four-junction cell that can absorb radiation from ~300-1500 nanometers. The four junction cell is fabricated by bonding, which allows us to insert an AR coating to increase the amount of light to enter the lower bandgap cell and reflect higher energy photons back to the higher bandgap cells to increase optical absorption of the top cells, thus lead to better performance. To design this coating, material absorption data is loaded into thin-film optimization software to determine the best material type and thickness for given layers of the coating. The specifications are then inserted into a computer simulation of the proposed cell design where solar spectrum data imitates physical conditions. This gives an idea of the effectiveness of the coating design when in the actual cell. Once a suitable antireflective coating is designed and simulated, the coating can be fabricated in the clean room and tested once more to assure that predicted performance matched actual performance.