Growth and characterization of III-V semiconductor solar cells.

Abstract

A new Molecular Beam Epitaxy system that can be pumped down to 10-11torr is set up. The ultra high vacuum environment was achieved through multiple rounds of baking before and after material loading into the source cells. Growth calibration was done for doped and undoped GaAs films, where Si and Be were introduced as the donor and acceptor respectively. Hall measurements were conducted to determine the p and n doping of the GaAs at different Be and Si cell temperature. The MBE system was used to grow single junction III-V solar cells. The top electrode of the solar cell was designed to be a grid pattern to enhance the collection of photocurrent, at the same time reduces the shadowing effect by the electrodes. Photolithography was carried out to fabricate the fine grid pattern on top of the device. AuGe:Ni alloy was used as the electrodes on the GaAs devices due to its ability to form ohmic contact on GaAs surface. Conversion efficiency of the solar cell was determined by measuring the I-V curve produced by the devices using a solar light simulator of AM1.5 spectrum. The conversion efficiency of the unoptimized GaAs single junction solar cell was 5.6% while the fill factor was 39.6%.The growth of quantum dot structures was explored by varying the growth parameters, including In beam flux and deposition thickness of InAs. Its growth rate was estimated by observing RHEED pattern evolution, and the structures were characterized using AFM analysis. Analysis showed that quantum dot structures with higher In beam flux resulted in higher dots density. Silvaco Atlas was used in simulating III-V solar cell with different layer structures and materials. Efficiencies of single junction solar cells were optimized by adding window layer and back surface field layer. InGaP/GaAs dual junction solar cells were built by incorporating tunneling junction in between the sub-cells. Highest efficiency of the dual-junction solar cell reported was 33.7%.