Synthesis, Processing, and Fundamental Phase Formation Study of CZTS Films for Solar Cell Applications

Authors

Osama Awadallah, Florida International UniversityFollow

Document Type

Dissertation

Degree

Doctor of Philosophy (PhD)

Major/Program

Materials Science and Engineering

First Advisor's Name

Zhe Cheng

First Advisor's Committee Title

Committee chair

Second Advisor's Name

Surendra Saxena

Second Advisor's Committee Title

Co-committee chair

Third Advisor's Name

Chunlei Wang

Third Advisor's Committee Title

committee member

Fourth Advisor's Name

Nezih Pala

Fourth Advisor's Committee Title

committee member

Fifth Advisor's Name

Hebin Li

Fifth Advisor's Committee Title

committee member

Keywords

Solar cells, Kesterite, CZTS, Thin films, Raman scattering

Date of Defense

4-2-2018

Abstract

Copper zinc tin sulfide (Cu₂ZnSnS₄ or CZTS) kesterite compound has attracted much attention in the last years as a new abundant, low cost, and environmentally benign material with desirable optoelectronic properties for Photovoltaic (PV) thin film solar cell applications. Among various synthesis routes for CZTS thin films, sol-gel processing is one of the most attractive routes to obtain CZTS films with superior quality and low cost.

In this study, sol-gel sulfurization process parameters for CZTS thin films were systematically investigated to identify the proper process window. In addition, temperature dependent Raman spectroscopy was employed to monitor the CZTS sulfurization process in real time and gain fundamental information about the phase formation and degradation mechanisms of CZTS under the relevant processing conditions. It was found that CZTS thin films with different Cu stoichiometry can be prepared using parts-per-million (ppm) level of hydrogen sulfide (H₂S) gas as opposed to high percentage level of H₂S (e.g., ≥ 5%) in all previous studies. Samples sulfurized at lower temperatures of ~350°C and 125°C revealed the formation of CZTS phase as confirmed by XRD, Raman micro-spectroscopy, and sheet resistance measurement.

Local EDS analysis indicates that CZTS films prepared at those low temperatures have a near-stoichiometric composition and are sometimes accompanied by the formation of Cu_2-xS phase(s). Also, stoichiometric and Cu-rich precursor solutions tend to yield CZTS samples with better crystallinity and superior optical properties compared with the Cu-deficient solution.

Moreover, in situ Raman monitoring of phase formation of CZTS material was carried out from room temperature up to 350°C in a 100 ppm H₂S+4%H₂+N₂ gas mixture. The results showed that CZTS phase formed in about 30 min via a direct reaction between the metal oxide precursor film and the H₂S-H₂ gas mixture at an intermediate temperature of 350°C and remained stable upon extended exposure. In comparison, at a lower temperature (170°C), the oxide precursor film had to be reduced first (e.g., in 4% H₂/N₂ forming gas) and then the CZTS phase emerged. However, continued sulfurization at a lower temperature (e.g., 170°C) led to the disintegration of CZTS and the formation of CuS impurity, which remains stable upon cooling the sample down to room temperature. Furthermore, results of in situ Raman monitoring of CZTS films in an oxygen-rich atmosphere at elevated temperatures up to 600°C suggested that CZTS oxidizes first at ~400°C to form tin oxide (SnO₂) and binary sulfides of mainly copper sulfide (Cu_2-xS) and zinc sulfide (ZnS). Then, at temperatures higher than 400°C, the remaining sulfides oxidize to form zinc oxide (ZnO).

The outcomes of the current study set the directions for optimizing the CZTS film structure and stoichiometry toward developing low cost and high-performance CZTS solar cells in future.

Identifier

FIDC006910

Recommended Citation

Awadallah, Osama, "Synthesis, Processing, and Fundamental Phase Formation Study of CZTS Films for Solar Cell Applications" (2018). FIU Electronic Theses and Dissertations. 3725.
https://digitalcommons.fiu.edu/etd/3725