Date of Award

Spring 1-1-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical, Computer & Energy Engineering

First Advisor

Bart Van Zeghbroeck

Second Advisor

John Perkins

Third Advisor

Philip Parilla

Fourth Advisor

Garret Moddel

Fifth Advisor

Won Park

Abstract

This thesis focuses on developing earth abundant materials and compounds for solar energy applications. Thin films were grown and studied in search of improved p-type transparent contact layers in photovoltaic devices. Combinatorial co-sputtering was used to make thin films with compositional and temperature gradients across glass substrates. Materials were characterized using x-ray diffraction, x-ray fluorescence, Rutherford backscattering spectrometry, Kelvin probe measurements, four point probe measurements, Seebeck measurements, and optical transmission & reflection measurements.

The pseudo-ternary oxide system of Zn-Ni-Co-O was deposited on ambient temperature glass substrates to serve as a p-type selective contact. The properties of the ambient temperature-deposited Zn-Ni-Co-O thin films were compared to films grown at higher substrate temperatures (350 °C) with the same compositions. It was found that the optical absorption and work function of the materials were similar in both cases. The optical absorption at 1.8 eV was large (~ 105 cm-1) and the work functions were in the range of 5.1 to 5.8 eV. However, the conductivity and crystallinity of the ambient-temperature-grown samples was poorer than the high temperature samples. The maximum conductivity was found to be 35 S/cm and corresponded to the region with the most crystalline disorder. A minimum grain size for the spinel films and disordered films were both estimated to be near 5 nm based on grain size analysis and neglecting strain effects. Overall, a factor of three decrease in the conductivity was the main difference observed when Zn-Ni-Co-O films were deposited on ambient temperature substrates when compared to 350 °C depositions.

A growth technique for sulfide and oxide-sulfide thin films is introduced. An RF solids single atom source (cracker) is used in conjunction with co-sputtering to deposit both sulfide and oxide sulfide compounds. Reactive sputtering from metallic and ceramic targets is demonstrated for binary sulfide growth. A range of oxygen to sulfur ratios is obtained by growing BiOxSy thin films to demonstrate the tunability of sulfur content using this technique. Lastly, independent tuning of two cation species and two anion species is demonstrated by the growth of the quaternary compound BiCuOS.

The properties of BiCuOS thin films are measured and compared to reported values for bulk crystals. The electrical conductivity and Seebeck coefficient are measured to show the electrical properties of the material and majority carrier type. The electrical conductivity was determined to be 0.059 +/- 0.005 S/cm and the Seebeck coefficient was positive indicating a p-type material. The optical transmission and reflection is used to determine the absorption and band gap. Tauc plots showed an indirect band gap at 1.03 +/- 0.01 eV with a first direct transition at 1.34 +/- 0.02 eV. The properties of the BiCuOS thin films are very similar to the bulk crystals further indicating successful growth of this material. Bi-Cu-O-S films were grown using combinatorial sputtering and the properties were measured as the composition was varied. The films remained in the BiCuOS crystal structure as both the copper and bismuth contents were each individually increased in the films. The electrical conductivity increased as excess copper was added into the films and the conductivity decreased as excess bismuth was added. Seebeck measurements indicated that the materials remained p-type as the composition was varied. Tauc plots indicated that the energy transitions remained relatively constant as both excess bismuth or excess copper were added into the material. These results provided a basis for a combinatorial study of the Bi-Cu-O-S material system that can be expanded with future experiments. These results also provided the foundation for combinatorial studies of oxide-sulfide compounds in search of p-type transparent contacts. The study of LaCuOS and La2O2SnS3 can now be attempted using these techniques and the insight gained from this study.

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