Summer 2011 Intern Project- Jonathan Waltman
EQUIVALENT CIRCUIT AND CHARGE TRANSPORT MODELING OF GaN/InGaN PHOTOVOLTAIC DEVICES
UC Santa Barbara
Mentor: Matt Laurent
Faculty Advisor: Umesh Mishra
Department: Electrical and Computer Engineering
High efficiency solar cells require multiple junctions optimized for different wavelengths, and indium gallium nitride (InGaN) has the potential to further improve the efficiency of existing multi-junction devices. Since there is currently no commercial material that efficiently converts high-energy photons into electricity, InGaN can be used as the uppermost absorbing layer in a multi-junction cell to accomplish this task. However, InGaN solar cells still require much research and development before it is commercially feasible to implement them in such a manner. In order to aid in the optimization of InGaN solar cells, I have developed an equivalent circuit model that simulates current flow in the devices. Solar cells are simply diodes that are designed to deliver current to a load under illumination. To assess solar cell operation, we illuminate the device with a lamp that mimics the solar spectrum and measure current as a function of applied bias voltage. This measurement provides key solar cell parameters such as the short-circuit current, open-circuit voltage, and maximum power output. However, no detailed information about the p-n junction is gained. To provide this missing information, the model fits to the experimental data and extracts values – such as the series and shunt resistances, reverse saturation current density, and ideality factor – that are useful in understanding device performance and areas of inefficiency. This information provides supplementary insight into the physical behavior of the solar cell and thus aids in the refinement of crystal growth processes.