Optimized Designs and Materials for Nanostructure-Based Solar Cells
Qinghui Shao
PhD Defense
When: Tuesday, November 3, 2009
Time: 1:30 PM
Location: BOURNS A277
Committee: Dr. Alexander Balandin (Chair), Dr. Mihri Ozkan, Dr. Sakhrat Khizroev
Abstract:
Nanostructure-based solar cells are attracting significant attention as possible candidates for drastic improvement in photovoltaic (PV) energy conversion efficiency. Although such solar cells are expected to be more expensive there is growing need for the efficient and light-weight solar cells in aero-space and related industries. In this dissertation I present results of the theoretical, computational and experimental investigation of novel designs for quantum dot superlattice (QDS) based photovoltaic elements and advanced materials for transparent solar cells. In the first part of the dissertation I describe possible implementation of the intermediate-band (IB) solar cells with QDS. The IB cells were predicted to have PV efficiency exceeding the Shockley-Queisser limit for a single junction cell. The parameters of QDS structure have to be carefully tuned in order to achieve the desired charge carrier dispersion required for the IB operation. The first-principles theoretical models were used to calculate the electrical properties and light absorption in QDS. This approach allowed me to determine the dimensions of the quantum dots and inter-dot spacing for inducing the carrier mini-band in the band-gap region where the mini-band can play the role of the IB. Using the detailed balance theory it was determined that the upper-bound theoretical PV efficiency of such IB solar cells can be as high as ~51%. The required QDS dimensions for the IB implementation on the basis of InAsN/GaAsSb are technologically challenging but feasible: ~2-6 nm. Using the developed simulation tools I proposed several possible designs of QDS solar cells including one, which combined the benefits of the IB concept and the advanced tandem cell design. It is implemented on the basis of QDS and an original layered structure. The second part of the dissertation presents a feasibility study of applications of graphene layers as optically transparent electrically conductive electrodes for the top surface of the PV cells. The graphene layers were mechanically exfoliated from bulk graphite and characterized with micro-Raman spectroscopy. It was found that graphene electrodes have good electrical conductivity, which reveals unusual temperature dependence beneficial for the proposed application. The decrease in resistance with temperature was explained by the thermal generation of the electron-hole pairs in the conditions when the carrier mobility is limited by the defect scattering. The final part of the dissertation presents results of simulation of electrical current transport in graphene ribbons, which can be used as transparent electrodes or interconnects.