Si Microwire-Array Solar Cells

Citation

Putnam, Morgan Charles (2010) Si Microwire-Array Solar Cells. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/JSVM-J029. https://resolver.caltech.edu/CaltechTHESIS:06072010-170215356

Abstract

By allowing for the fabrication of flexible crystalline-Si (c-Si) solar cells that employ ~1/100^th) the Si of a traditional wafer-based c-Si solar cell, while maintaining high photovoltaic efficiencies, vertically aligned arrays of c-Si microwires provide a novel photovoltaic geometry that has the potential to dramatically reduce the cost of solar electricity. In this thesis we report on 1) the growth of Si microwire arrays, 2) the chemical and electrical characterization of Si microwire arrays, and 3) the fabrication of Si microwire-array solar cells.

Using the vapor-liquid-solid (VLS) growth mechanism in combination with photolithographic patterning, vertically aligned arrays of Si microwires, with nominally identical heights and diameters, were fabricated over areas > 1 cm². Chemical characterization of the Si wires was then performed using secondary ion mass spectrometry to measure the incorporation of the Au VLS-catalyst into the Si wire. The incorporation of the VLS-catalyst into the Si wires at its thermodynamic equilibrium concentration suggested that the use of Cu as a VLS-catalyst was less likely to limit the photovoltaic performance of Si microwire-array solar cells. Switching to the Cu-catalyzed growth of Si wires, in-situ doping with BCl₃ was used to demonstrate control of the electrically active dopant concentration from 8 x 10¹⁵ to 4 x 10¹⁹ dopants cm^-3. Scanning photocurrent measurements were then made to measure the minority-carrier diffusion length. The observation of 10 μm minority-carrier diffusion lengths indicated that solar cells with efficiencies of 17.5% should be possible. With the knowledge that highly efficient solar cells were possible, methods for the fabrication of a p-n junction and a transparent top contact in a solid-state solar cell were developed. This culminated in the demonstration of Si microwire-array solar cells with Air Mass 1.5 Global photovoltaic conversion efficiencies of up to η = 7.9%. Through improved device processing and the use of an amorphous Si passivation layer at the top contact, ~15% efficient solar cells should be possible.

Thesis Files