Utilising Chromium-Based p-Type Transparent Conducting Oxides in Photovoltaic Devices

Abstract

This thesis deals with the growth of modern, highly performing p-type transparent conducting oxides and their utilisation in two types of photovoltaic devices. X-ray spectroscopic techniques are used to understand the electronic structure of these materials and the various interfaces they form when used in these devices.

The initial chapters provide an overview of the history and current state of the art of transparent conducting oxides in general with specific reference being paid to to the materials used in this work: Cr2O3, Cr2O3:Mg, and CuCrO2.

Cr2O3 and Cr2O3:Mg were used as an anode buffer layer in simple organic solar cells, while CuCrO2 and Cr2O3:Mg were used as a transparent p-type layer of a novel silicon heterojunction solar cell. Both Cr2O3 and Cr2O3:Mg anode buffer layers were found to improve the efficiency of organic solar cells by a factor of ~3 compared to cells produced with no buffer layer. All cells produced exhibited an "S''-shaped current-voltage curve. To understand why this occurred, band alignment measurements were performed using in-situ high resolution x-ray and ultraviolet photoelectron spectroscopy. An ultra-high vacuum transfer system necessary for the band alignment measurements was built to enable the spectroscopic analysis of samples after growth, prior to being exposed to ambient conditions.

Simple n-Si/CuCrO2 solar cells were produced by spray pyrolysis while n-Si/Cr2O3:Mg solar cells were produced by molecular beam epitaxy. Solar cells produced on low resistivity silicon did not demonstrate any power generation, while cells produced on standard resistivity silicon demonstrated lower performance than expected. Band offset measurements were performed on the n-Si/Cr2O3:Mg cells using photoelectron spectroscopy, revealing that an oxide layer present on the silicon surface is the primary factor responsible for their poor solar cell characteristics.