Andrew D. Dillon

High Chloride Doping Levels Stabilize the Perovskite Phase of Cesium Lead Iodide

Cesium lead iodide possesses an excellent combination of band gap and absorption coefficient for photovoltaic applications in its perovskite phase. However, this is not its equilibrium structure under ambient conditions. In air, at ambient temperature it rapidly transforms to a nonfunctional, so-called yellow phase. Here we show that chloride doping, particularly at levels near the solubility limit for chloride in a cesium lead iodide host, provides a new approach to stabilizing the functional perovskite phase. In order to achieve high doping levels, we first co-deposit colloidal nanocrystals of pure cesium lead chloride and cesium lead iodide, thereby ensuring nanometer-scale mixing even at compositions that potentially exceed the bulk miscibility of the two phases. The resulting nanocrystal solid is subsequently fused into a polycrystalline thin film by chemically induced, room-temperature sintering. Spectroscopy and X-ray diffraction indicate that the chloride is further dispersed during sintering and a polycrystalline mixed phase is formed. Using density functional theory (DFT) methods in conjunction with nudged elastic band techniques, low-energy pathways for interstitial chlorine diffusion into a majority-iodide lattice were identified, consistent with the facile diffusion and fast halide exchange reactions observed. By comparison to DFT-calculated values (with the PBE exchange-correlation functional), the relative change in band gap and the lattice contraction are shown to be consistent with a Cl/I ratio of a few percent in the mixed phase. At these incorporation levels, the half-life of the functional perovskite phase in a humid atmosphere increases by more than an order of magnitude.

Transparent, conductive solution processed spincast 2D Ti2CTx (MXene) films

ABSTRACT Herein, we spincast aqueous colloidal Ti2CTx (MXene) solutions into conductive, transparent films with figures of merit (FOM), that are as good as Ti3C2Tx or un-doped chemically vapor-deposited graphene. When normalized by the number of transition metal atoms, the FOM is the highest ever reported for a MXene film. At about 2.7 × 105 cm–1 the absorbance coefficient of Ti2CTx is quite comparable to that of Ti3C2Tx. Quantitative relationships between film properties—conductance and transparency—and colloidal solution concentration and spin speeds are developed providing a road map for future work. GRAPHICAL ABSTRACT IMPACT STATEMENT In a first, we spincast aqueous colloidal Ti2CTx (MXene) solutions into conductive, transparent films with figures of merit—5—that are as good as Ti3C2Tx or un-doped CVD graphene.

The Influence of Compact, Inorganic Surface Ligands on the Electrophoretic Deposition of Semiconductor Nanocrystals at Low Voltage

For electrophoretic deposition (EPD) to achieve its potential as a method for assembling functional semiconductors, it will be necessary to understand both what governs the threshold voltage for deposition and how to reduce that threshold. Herein we demonstrate that postsynthetic modification of the surface chemistry of all-inorganic copper zinc tin sulfide (CZTS) nanocrystals (NCs) enables EPD at voltages of as low as 4 V, which is a 3-fold or greater reduction over previous examples of nonoxide semiconductors. The chemical exchange of the original surfactant-based NC-surface ligands with selenide ions yields essentially bare, highly surface-charged NCs. Thus, both the electrophoretic mobility and electrochemical reactivity of these particles are increased, favoring deposition. In situ imaging of the reactor during deposition provides a quantitative measure of the electric field in the bulk of the reactor, yielding fundamental insight into the reaction mechanism and mass transport in the low-voltage regime. A crossover from mass-transport-limited to reaction-rate-limited EPD is observed. Under the latter conditions, the influence of gravity can result in boundary-layer instabilities that are severely deleterious to the uniformity of the deposited film, despite the gravitational stability of the colloids in the absence of electric fields. This knowledge is applied to deposit thick, uniform, and crack-free films without sintering, from stable, well-dispersed colloidal starting materials.