Student: Aditi Krishnapriyan
Mentor: Dennis Nordlund
First-Principles Computational Study on Developing a Better Photovoltaic Solar Cell
The copper indium gallium selenide (CIGS) direct band gap semi-conductor is best known as a versatile and useful photovoltaic solar cell material. A portion of this solar cell consists of a CIGS absorber layer, which is underneath a CdS transparent emitter layer. However, the poor transparency of the CdS layer is a limitation to the efficiency of the overall solar cell due to the band gap leading to absorption in the blue region, thereby decreasing the incident solar radiation on the CIGS absorber. The electronic structure of the CdS transparent emitter layer is studied by density functional theory using semi-local functionals and x-ray spectroscopy to find a suitable and more efficient transparent emitter layer, while also mitigating defects, minimizing the conduction band offset with the CIGS absorber and the ZnO window later above the CdS layer, and having a transparent but not insulating band gap. The CdS transparent emitter layer is perturbed and the effect of various defects, such as the removal of different cadmium and sulfur ions, and replacements, such as replacing cadmium and sulfur with copper and oxygen, are studied. The x-ray spectroscopy and transition energies of sulfur to the first excited state are evaluated, as well as the ground state electronic properties and the density of states of the different cell defects and replacements. These different results are compared with each other, as well as with other compiled experimental data obtained at the National Renewable Energy Laboratory (NREL) and the Stanford Synchrotron Radiation Lightsource (SSRL) through spectroscopic and structural synchrotron radiation methods. Overall, the replacement of sulfur with oxygen may show some promise in further increasing the efficiency of the CdS transparent emitter layer and the projected improvement of the CIGS cell performance is about 10% or a 2% overall efficiency gain above the current efficiency record.