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Zn-VI/Cu2O Heterojunctions for Earth-Abundant Photovoltaics

Citation

Wilson, Samantha Stricklin (2015) Zn-VI/Cu2O Heterojunctions for Earth-Abundant Photovoltaics. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9N58J9H . http://resolver.caltech.edu/CaltechTHESIS:05212015-091546304

Abstract

The need for sustainable energy production motivates the study of photovoltaic materials, which convert energy from sunlight directly into electricity. This work has focused on the development of Cu2O as an earth-abundant solar absorber due to the abundance of its constituent elements in the earth's crust, its suitable band gap, and its potential for low cost processing. Crystalline wafers of Cu2O with minority carrier diffusion lengths on the order of microns can be manufactured in a uniquely simple fashion — directly from copper foils by thermal oxidation. Furthermore, Cu2O has an optical band gap of 1.9 eV, which gives it a detailed balance energy conversion efficiency of 24.7% and the possibility for an independently connected Si/Cu2O dual junction with a detailed balance efficiency of 44.3%.

However, the highest energy conversion efficiency achieved in a photovoltaic device with a Cu2O absorber layer is currently only 5.38% despite the favorable optical and electronic properties listed above. There are several challenges to making a Cu2O photovoltaic device, including an inability to dope the material, its relatively low chemical stability compared to other oxides, and a lack of suitable heterojunction partners due to an unusually small electron affinity. We have addressed the low chemical stability, namely the fact that Cu2O is an especially reactive oxide due to its low enthalpy of formation (ΔHf0 = -168.7 kJ/mol), by developing a novel surface preparation technique. We have addressed the lack of suitable heterojunction partners by investigating the heterojunction band alignment of several Zn-VI materials with Cu2O. Finally, We have addressed the typically high series resistance of Cu2O wafers by developing methods to make very thin, bulk Cu2O, including devices on Cu2O wafers as thin as 20 microns. Using these methods we have been able to achieve photovoltages over 1 V, and have demonstrated the potential of a new heterojunction material, Zn(O,S).

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Photovoltaics, Solar, Energy, Earth-abundant, semiconductor, Cu2O, ZnO, ZnS
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Materials Science
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Atwater, Harry Albert
Thesis Committee:
  • Atwater, Harry Albert (chair)
  • Greer, Julia R.
  • Johnson, William Lewis
  • Minnich, Austin J.
Defense Date:10 December 2014
Record Number:CaltechTHESIS:05212015-091546304
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:05212015-091546304
DOI:10.7907/Z9N58J9H
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:8874
Collection:CaltechTHESIS
Deposited By: Samantha Wilson
Deposited On:04 Jan 2016 19:51
Last Modified:26 Oct 2016 20:47

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