SOLUTION PROCESSING OF SILVER-BASED KESTERITE: FROM NANOPARTICLES TO THIN FILM SOLAR CELLS

2019-08-13T20:05:45Z (GMT) by Xianyi Hu
Because of the limited reserve of fossil fuels and issues brought up by their combustion, the demand on renewable energy is considerably increasing. Solar energy is one of the most promising renewable energy sources considering the large amount of solar irradiation received by Earth and solar cell is such a device that allows us to directly convert sunlight directly into electricity. In this thesis, kesterite (I2-II-IV-IV4) system is the main focus as the light absorber material in thin film solar cells.

Cu2ZnSn(S,Se)4 (CZTSSe) has been first studied intensively. However, due to the band tailing resulting from Cu-Zn anti-site defects, further improvement on power conversion efficiency of this material has been hindered. Substituting Cu with Ag is expected to solve this problem by decreasing this defect density as a result of the high formation energy of Ag-Zn antisite defects. Herein, different concentrations of Ag are used to substitute Cu in the kesterite system through a nanoparticle-ink route for the fabrication of light absorber thin films. For Ag-alloying concentration less than 50%, it suggests that the Ag can induce inhomogeneity as well as secondary phase formation during nanoparticle formation. Moreover, Ag alloying is shown to enlarge the grain size and reduce film roughness after selenization, which are beneficial for the optoelectronic properties and device performance.

Additionally, the synthesis process for kesterite Ag2ZnSnS4 nanoparticles is explored. AZTS nanoparticles are achieved by solvent-thermal reaction. The reaction pathway during reaction is investigated by different material characterization methods to shed light on the Ag-based nanoparticle synthesis. The final nanoparticles obtained have high crystallinity and homogeneous composition, demonstrating great potential as light absorber materials. Also, the sulfide nanoparticles are converted into selenide thin films in Se vapor at elevated temperature (selenization). The selenization conditions, including temperature, heating ramp and selenization time, are optimized for the pure phase kesterite AZTSe thin films with large and dense grains. The optoelectronic properties are explored on these films and an initial research already demonstrates a 0.35% power conversion efficiency as the first solution processed AZTSe device.

In summary, multiple material characterization techniques are utilized to understand the microstructure evolution, phase transformation, and composition change for solution-processed nanoparticles and their resulting thin films. The material characteristics, process methods and film optoelectronic properties are associated for the future analysis and development of kesterite thin films for photovoltaic applications.