Aggregation Kinetics of Metal Chalcogenide Nanocrystals: Generation of Transparent CdSe (ZnS) Core (Shell) Gels

Transparent CdSe (ZnS) core (shell) sol–gel materials have potential uses in optoelectronic applications such as light-emitting diodes (LEDs) due to their strong luminescence properties and the potential for charge transport through the prewired nanocrystal (NC) network of the gel. However, typical syntheses of metal chalcogenide gels yield materials with poor transparency. In this work, the mechanism and kinetics of aggregation of two sizes of CdSe (ZnS) core (shell) NCs, initiated by removal of surface thiolate ligands using tetranitromethane (TNM) as an oxidant, were studied by means of time-resolved dynamic light scattering (TRDLS); the characteristics of the resultant gels were probed by optical absorption, transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS). At low concentrations of NCs (ca. 4 × 10^–7 M), the smaller, green-emitting NCs aggregate faster than the larger, orange-emitting NCs, for a specific oxidant concentration. The kinetics of aggregation have a significant impact on the macroscopic properties (i.e., transparency) of the resultant gels, with the transparency of the gels decreasing with the increase of oxidant concentration due the formation of larger clusters at the gel point and a shift away from a reaction-limited cluster-aggregation (RLCA) mechanism. This is further confirmed by analyses of the gel structures by SAXS and TEM. Likewise, the larger orange-emitting particles also produce larger aggregates at the gel point, leading to lower transparency. The ability to control the transparency of chalcogenide gels will enable their properties to be tuned in order to address application-specific needs in optoelectronics

Enhanced Conductivity in CZTS/Cu_2–<i>x</i>Se Nanocrystal Thin Films: Growth of a Conductive Shell

Poor charge transport in Cu₂ZnSnS₄ (CZTS) nanocrystal (NC) thin films presents a great challenge in the fabrication of solar cells without postannealing treatments. We introduce a novel approach to facilitate the charge carrier hopping between CZTS NCs by growing a stoichiometric Cu₂Se shell that can be oxidized to form a conductive Cu_2–<i>x</i>Se phase when exposed to air. The CZTS/Cu₂Se core/shell NCs with varying numbers of shell monolayers were synthesized by the successive ionic layer adsorption and reaction (SILAR) method, and the variation in structural and optical properties of the CZTS NCs with varying shell thicknesses was investigated. Solid-phase sulfide ligand exchange was employed to fabricate NC thin films by layer-by-layer dip coating and a 2 orders of magnitude rise in dark conductivity (∼10^–3 S cm^–1 at 0 monolayer and ∼10^–1 S cm^–1 at 1.5 monolayers) was observed with an increase in the number of shell monolayers. The approach described herein is the first key step in achieving a significant increase in the photoconductivity of as-deposited CZTS NC thin films

Synthetic Control of Quinary Nanocrystals of a Photovoltaic Material: The Clear Role of Chalcogen Ratio on Light Absorption and Charge Transport for Cu_2–<i>x</i>Zn_1+<i>x</i>Sn(S_1–<i>y</i>Se_<i>y</i>)₄

Photovoltaic (PV) devices based on bulk polycrystalline Cu₂ZnSn­(S_1–<i>x</i>Se_<i>x</i>)₄ (CZTSSe) as the absorber material have historically shown the best efficiency with high Se compositions. The selenization process, which is employed in the formation of absorber layer, has been shown to result in maximum device efficiency at a lower than predicted optimal band gap (<i>E</i>_g= ∼1.1 eV as compared to the 1.34 eV predicted by the Shockley–Queisser detailed balance model). It is still not clear if this deviation is due to changes in the chalcogen composition, grain growth in the film, or increased order in the lattice. In contrast, CZTSSe nanocrystals (NCs) offer a unique opportunity to evaluate the effect of chalcogen ratio on light absorption, charge transport, and photovoltaic performance excluding the impact of the uncertain effects of the conventional selenization step and, importantly, offer a potential path to a dramatic reduction in PV manufacturing cost. Despite an abundance of literature reports on this compound, there is to date <i>no systematic study of the effects of controlled composition of the chalcogen on photocarrier generation and extraction at an optimal and constant cation ratio in a single system.</i> This is required to determine the interplay between light absorbance and transport without compositional convolution and, in turn, to identify the best chalcogen ratio for the unannealed NC PV devices. Here we show that the entire family of Cu_2–<i>z</i>Zn_1+<i>z</i>Sn­(S_1–<i>y</i>Se_<i>y</i>)₄ NCs can be made by a simple one-pot synthetic method with exquisite control over cation content and particle size across the entire range of chalcogen compositions. These NCs are then used to make solution-processed and electrically conductive CZTSSe NC films in the full range of S/(S + Se) ratios via ligand exchange without postdeposition annealing. The transport properties assessed by Hall-effect measurements revealed an intrinsic increase in film conductivity with selenium incorporation. These measurements are then correlated with the PV performance at the full range of band gaps (<i>E</i>_g = 1.0–1.5 eV), leading to an observed maximum in power conversion efficiency centered around <i>E</i>_g = 1.30 eV, which is much closer to the predicted Shockley–Queisser optimal band gap, an outcome predominantly dictated by the compromise between electrical conductivity and band gap

Uniform Thin Films of CdSe and CdSe(ZnS) Core(Shell) Quantum Dots by Sol–Gel Assembly: Enabling Photoelectrochemical Characterization and Electronic Applications

Optoelectronic properties of quantum dot (QD) films are limited by (1) poor interfacial chemistry and (2) nonradiative recombination due to surface traps. To address these performance issues, sol–gel methods are applied to fabricate thin films of CdSe and core(shell) CdSe(ZnS) QDs. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging with chemical analysis confirms that the surface of the QDs in the sol–gel thin films are chalcogen-rich, consistent with an oxidative-induced gelation mechanism in which connectivity is achieved by formation of dichalcogenide covalent linkages between particles. The ligand removal and assembly process is probed by thermogravimetric, spectroscopic, and microscopic studies. Further enhancement of interparticle coupling <i>via</i> mild thermal annealing, which removes residual ligands and reinforces QD connectivity, results in QD sol–gel thin films with superior charge transport properties, as shown by a dramatic enhancement of electrochemical photocurrent under white light illumination relative to thin films composed of ligand-capped QDs. A more than 2-fold enhancement in photocurrent, and a further increase in photovoltage can be achieved by passivation of surface defects <i>via</i> overcoating with a thin ZnS shell. The ability to tune interfacial and surface characteristics for the optimization of photophysical properties suggests that the sol–gel approach may enable formation of QD thin films suitable for a range of optoelectronic applications