Summer 2010 Intern Project- Ali Al-Heji
EFFECT OF SURFACE ROUGHNESS ON THE EXTERNAL QUANTUM EFFICIENCY OF INGAN-BASED SOLAR CELLS
Ali A. Al-Heji
UC Santa Barbara
Mentor: Robert M. Farrell
Faculty Advisor: James S. Speck
Indium gallium nitride (InGaN) solar cells show promise for absorbing high-energy photons with wavelengths shorter than 500 nm. This region of the solar spectrum is converted inefficiently by conventional narrow bandgap solar cells, as the excess energy of these photons is dissipated as heat rather than electrical energy. Unfortunately, the total thickness of the active region in InGaN-based solar cells is limited by the relatively large lattice mismatch between alloys of InxGa1-xN and GaN, resulting in incomplete light absorption in conventional InGaN-based solar cell structures. Nevertheless, it is possible to improve the absorption in the active region of InGaN-based solar cells by increasing the path length of light through the device. This can be achieved by intentionally roughening the surface of the solar cell, scattering the incident light at an angle that is oblique to the surface of the device, thus increasing the path length of light through the active region. By optimizing the growth conditions for our solar cells, we have found a way to intentionally roughen the surface of our devices. The goal of this study is to quantify the effects of surface roughness on the efficiency of our solar cells. Initial results show an increase in external quantum efficiency of greater than 30% for solar cells with intentionally roughened surfaces compared to devices with smooth surfaces.