Insights on local chemical structure of chalcogenide photovoltaic materials using solid state NMR spectroscopy

Quaternary chalcogenides with the formula I2-II-IV-VI4 have emerged as promising candidates for photovoltaic materials, but many details about their structures remain poorly understood. This thesis describes the synthesis, structure determination, and optical properties of the sulfides Cu2MTtS4 (M = Zn, Cd, Hg; Tt = Ge, Sn), prepared as bulk samples by reaction of the elements. Their long- and short-range structures were elucidated by a combination of powder X-ray diffraction and multinuclear solid-state magnetic resonance spectroscopy (63/65Cu, 67Zn, 73Ge,113Cd, 119Sn, 199Hg). Within the Cu2Zn1−xCdxSnS4 series, 113Cd NMR spectroscopy confirmed a phase transition at x = 0.4 and revealed the coexistence of phases with subtly different structures (space groups I4 ̅ and I4 ̅2m) in the intermediate region (x = 0.3–0.4), which were previously not apparent from XRD data alone. The 119Sn NMR spectra indicated the Sn atoms are surrounded by a random distribution of Zn and Cd atoms within the second coordination sphere; these distinct local environments were resolved by their isotropic chemical shifts and quantified by their peak areas. A longstanding problem about whether disorder of Cu and Zn atoms occurs in Cu2ZnSnS4 was resolved by analyzing high-field (21.1 T) 63/65Cu, 67Zn solid-state NMR spectra, in conjunction with DFT calculations. Similar problems plague the related compounds Cu2HgSnS4 and Cu2GeSnS4. Evidence from 119Sn and 199Hg NMR spectra suggests that ordering occurs in Cu2HgSnS4 after annealing, a phenomenon that was not previously detected from XRD data. Within the Cu2ZnSn1−xGexS4 series, the influence of Ge substitution for Sn on the local structure was monitored by 67Zn solid-state MAS NMR spectroscopy, which showed excellent sensitivity. For all these compounds, optical band gaps were determined experimentally from UV-visible diffuse reflectance spectra. Electronic structure calculations were performed to bear further insight into the bonding and charge distribution in these compounds.