Atomic scale structure and bond stretching force constants in stoichiometric and off-stoichiometric kesterites

The deviation from stoichiometry and the understanding of its consequences are key factors for the application of kesterites as solar cell absorbers. Therefore, this study investigates the local atomic structure of off-stoichiometric Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 (CZTSe) and Cu2ZnGeSe4 (CZGSe) by means of Extended X-ray Absorption Fine Structure Spectroscopy. Temperature dependent measurements yield the bond stretching force constants of all cation-anion bonds in stoichiometric CZTS and CZTSe and nearly stoichiometric CZGSe. Low temperature measurements allow high precision analysis of the influence of off-stoichiometry on the element specific average bond lengths and their variances. The overall comparison between the materials is in excellent agreement with measures like ionic/atomic radii and bond ionicities. Furthermore, the small uncertainties allow the identification of systematic trends in the Cu-Se and Zn-Se bond lengths of CZTSe and CZGSe. These trends are discussed in context of the types and concentrations of certain point defects, which gives insight into the possible local configurations and their influence on the average structural parameters. The findings complement the understanding of the effect of off-stoichiometry on the local structure of kesterites, which affects their electronic properties and thus their application for solar cells.

Favorable Bonding and Band Structures of Cu2ZnSnS4 and CdS Films and Their Photovoltaic Interfaces

Thin-film photovoltaic cells using Cu₂ZnSnS₄ (CZTS, p-type) have many advantages, such as high photoconversion, low cost, and great tunability with earth-abundant, nontoxic elements, all of which are necessary to be long-term contributors to next-generation solar energy harvesting. Accurate measurements of bonding and band structures of both the thin-film materials and their interfaces are paramount to designing the solar devices layer-by-layer. Here, finely tuned 1 μm thick CZTS films, 50 nm thick CdS layers (n-type), and their 1 μm/2 nm p-n junction were fabricated inexpensively using our previously studied methods and investigated extensively for maximizing the key interface in the CZTS solar devices. Synthesized bulk CZTS and CdS were analyzed for structural deviations and crystal defects using synchrotron-based (SR) X-ray absorption fine structure (XAFS) along with simulated XAFS patterns. The structural properties of the two materials were designed to favor photovoltaic activity. Interface valence band structures of the CZTS/CdS p-n junction were measured through SR X-ray photoelectron spectroscopy (SR-XPS) and compared with the ones simulated using density functional theory. A full band diagram was constructed from XPS of the bulk films and SR-XPS of the interface, providing guidelines in optimizing charge-carrier extraction from the CZTS absorber to CdS buffer layer. It turns out that a small spike-like interface in the conduction band overlap was formed, maintaining a strong internal bias, while favoring a small energy barrier to prevent large-scale recombination from occurring. A large open-circuit voltage was obtained in the preliminary solar cell devices built on the small spike-like interface.

Element-specific investigations of ultrafast dynamics in photoexcited Cu2ZnSnS4 nanoparticles in solution

Ultrafast, light-induced dynamics in copper-zinc-tin-sulfide (CZTS) photovoltaic nanoparticles are investigated through a combination of optical and x-ray transient absorption spectroscopy. Laser-pump, x-ray-probe spectroscopy on a colloidal CZTS nanoparticle ink yields element-specificity, which reveals a rapid photo-induced shift of electron density away from Cu-sites, affecting the molecular orbital occupation and structure of CZTS. We observe the formation of a stable charge-separated and thermally excited structure, which persists for nanoseconds and involves an increased charge density at the Zn sites. Combined with density functional theory calculations, the results provide new insight into the structural and electronic dynamics of CZTS absorbers for solar cells.