Nanocrystal synthesis and thin film formation for earth abundant photovoltaics
Providing access to on-demand energy at the global scale is a grand challenge of our time. The fabrication of solar cells from nanocrystal inks comprising earth abundant elements represents a scalable and sustainable photovoltaic technology with the potential to meet the global demand for electricity. Solar cells with Cu2ZnSn(S,Se)4 (CZTSSe) absorber layers are of particular interest due to the high absorption coefficient of CZTSSe, its band gap in the ideal range for efficient photovoltaic power conversion, and the relative abundance of its constituent elements in the earth’s crust. Despite the promise of this material system, CZTSSe solar cell efficiencies reported throughout literature have failed to exceed 12.6%, principally due to the low open-circuit voltage (VOC) achieved in these devices compared to the absorber band gap. The work presented herein primarily aims to address the low VOC problem. First, the fundamental cause for such low VOC’s is investigated. Interparticle compositional inhomogeneities identified in the synthesized CZTS nanocrystals and their effect on the absorber layer formation and device performance are characterized. Real-time energy-dispersive x-ray diffraction (EDXRD) elucidates the role of these inhomogeneities in the mechanism by which a film of CZTS nanocrystals converts into a dense absorber layer comprising micron-sized CZTSSe grains upon annealing in a selenium atmosphere (selenization). Additionally, a direct correlation between the nanocrystal inhomogeneities and the VOC in completed devices is observed. Detailed characterization of CZTSSe solar cells identifies electrical potential fluctuations in the CZTSSe absorber – due to spatial composition variations not unlike those observed in the nanocrystals – as a primary V OC inhibitor. Additional causes for low VOC’s in CZTSSe solar cells proposed in the literature involve recombination at the interface between the CZTSSe absorber and: (1) the n-type, CdS buffer layer, or (2) the Mo back contact, and work is presented to address these issues. A chemical-mechanical polishing procedure is developed to afford modification of the CZTSSe absorber surface, and in turn the CZTSSe/CdS interface. However, such treatment results in a failure of the resultant solar cells to produce any photogenerated current without annealing/selenizing the absorber again prior to the deposition of CdS. For nanocrystal-based CZTSSe solar cells, the possibility of V OC limitations due to back surface recombination may be related to the formation of the C- and Se-rich layer of fine grains between the CZTSSe layer and the Mo contact, which is largely attributed to the long-chain organic solvent and ligand typically utilized in formulating the nanocrystal ink. CZTS nanocrystals are synthesized using a lighter but chemically similar organic solvent, and while films selenized from these particles appear to contain qualitatively less carbon, devices with these absorbers fail to produce noteworthy efficiencies. Finally, Cu2ZnSn(S,Se)4 (CMTSSe) is investigated as a proof-of-concept PV material due to its structural similarity to CZTSSe and the magnetic properties of Mn, which may prove advantageous in spintronic photovoltaic hybrid devices. While initial results demonstrate diode behavior and photoresponse from a CMTSSe/CdS junction, further processing optimization is necessary to realize meaningful device efficiencies.
Agrawal, Purdue University.
Chemical engineering|Electrical engineering|Materials science
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