Control over morphology and crystallinity of metal halide perovskite films is of key importance to enable high-performance optoelectronics. However, this remains particularly challenging for solution-printed devices due to the complex crystallization kinetics of semiconductor materials within dynamic flow of inks. Here we report a simple yet effective meniscus-assisted solution printing (MASP) strategy to yield large-grained dense perovskite film with good crystallization and preferred orientation. Intriguingly, the outward convective flow triggered by fast solvent evaporation at the edge of the meniscus ink imparts the transport of perovskite solutes, thus facilitating the growth of micrometre-scale perovskite grains. The growth kinetics of perovskite crystals is scrutinized by in situ optical microscopy tracking to understand the crystallization mechanism. The perovskite films produced by MASP exhibit excellent optoelectronic properties with efficiencies approaching 20% in planar perovskite solar cells. This robust MASP strategy may in principle be easily extended to craft other solution-printed perovskite-based optoelectronics.

In this study we investigated, theoretically and experimentally, the unique photoactive behavior of pristine and defected indium oxide surfaces providing fundamental insights into their excited state properties as well as an explanation for the experimentally observed enhanced activity of defected indium oxide surfaces for the gas-phase reverse water gas shift reaction, CO2 + H2 + hν→ CO + H2O in the light compared to the dark. To this end, a detailed excited-state study of pristine and defected forms of indium oxide (In2O3, In2O3-x, In2O3(OH)y and In2O3-x(OH)y) surfaces was performed using time dependent density functional theory (TDDFT) calculations, the results of which were supported experimentally by transient absorption spectroscopy and photoconductivity measurements. It was found that the surface frustrated Lewis pairs (FLPs) created by a Lewis acidic coordinately unsaturated surface indium site proximal to an oxygen vacancy and a Lewis basic surface hydroxide site in In2O3-x(OH)y become more acidic and basic and hence more active in the ES compared to the GS. This provides a theoretical mechanism responsible for the enhanced activity and reduced activation energy of the photochemical reverse water gas shift reaction observed experimentally for In2O3-x(OH)y compared to the thermochemical reaction. This fundamental insight into the role of photoexcited surface FLPs for catalytic CO2 reduction could lead to improved photocatalysts for solar fuel production.

Ultrafast two-photon photoemission has been used to study electron solvation at two-dimensional metal/polar-adsorbate interfaces. The molecular motion that causes the excess electron solvation is manifested as a dynamic shift in the electronic energy. Although the initially excited electron is delocalized in the plane of the interface, interactions with the adsorbate can lead to its localization. A method for determining the spatial extent of the localized electron in the plane of the interface has been developed. This spatial extent was measured to be on the order of a single adsorbate molecule.

No abstract

The development of strategies for increasing the lifetime of photoexcited charge carriers in nanostructured metal oxide semiconductors is important for enhancing their photocatalytic activity. Intensive efforts have been made in tailoring the properties of the nanostructured photocatalysts through different ways, mainly including band-structure engineering, doping, catalyst-support interaction, and loading cocatalysts. In liquid-phase photocatalytic dye degradation and water splitting, it was recently found that nanocrystal superstructure based semiconductors exhibited improved spatial separation of photoexcited charge carriers and enhanced photocatalytic performance. Nevertheless, it remains unknown whether this strategy is applicable in gas-phase photocatalysis. Using porous indium oxide nanorods in catalyzing the reverse water-gas shift reaction as a model system, we demonstrate here that assembling semiconductor nanocrystals into superstructures can also promote gas-phase photocatalytic processes. Transient absorption studies prove that the improved activity is a result of prolonged photoexcited charge carrier lifetimes due to the charge transfer within the nanocrystal network comprising the nanorods. Our study reveals that the spatial charge separation within the nanocrystal networks could also benefit gas-phase photocatalysis and sheds light on the design principles of efficient nanocrystal superstructure based photocatalysts.

Conventional squaraine dyes exhibit an intense absorption band in the red region of the solar spectrum and with appropriate design can also have high energy absorption as well, making them interesting building blocks toward achieving panchromatic dyes for dye sensitized solar cell (DSSC) applications. In this report, eight squaraine dyes with thiophene, 4-hexyl-4H-dithieno­[3,2-b:2′,3′-d]­pyrrole, dithieno­[3,2-b:2′,3′-d]­thiophene, and 4,4-bis­(2-ethylhexyl)-4H-silolo­[3,2-b:4,5-b′]­dithiophene (DTS) π-bridges with cyanoacetic acid (CA) and cyanophosphonic acid (PA) acceptor/anchoring groups are synthesized to extend the squaraine absorption into the 450–550 nm region and to provide different spatial arrangements of solubilizing groups. Squaraines with CA anchoring groups have higher power conversion efficiencies compared to their PA analogs, with the highest being 8.9% for the DTS-based dye, which is among the highest reported in the literature for squaraine dyes. This is due to high short circuit currents (J SC) and increased open circuit voltages (V OC). Dyes with PA anchoring groups exhibited lower J SC resulting from decreased charge injection efficiency, as determined by femtosecond transient absorption spectroscopy. This study suggests that out-of-plane bulky substituents may increase DSSC performance not only by increasing J SC through decreased aggregation but also by increasing V OC through decreased TiO2/electrolyte recombination.

In 2 O 3-x (OH) y nanoparticles have been shown to function as an effective gas-phase photocatalyst for the reduction of CO 2 to CO via the reverse water-gas shift reaction. Their photocatalytic activity is strongly correlated to the number of oxygen vacancy and hydroxide defects present in the system. To better understand how such defects interact with photogenerated electrons and holes in these materials, we have studied the relaxation dynamics of In 2 O 3-x (OH) y nanoparticles with varying concentration of defects using two different excitation energies corresponding to above-band-gap (318-nm) and near-band-gap (405-nm) excitations. Our results demonstrate that defects play a significant role in the excited-state, charge relaxation pathways. Higher defect concentrations result in longer excited-state lifetimes, which are attributed to improved charge separation. This correlates well with the observed trends in the photocatalytic activity. These results are further supported by density-functional theory calculations, which confirm the positions of oxygen vacancy and hydroxide defect states within the optical band gap of indium oxide. This enhanced understanding of the role these defects play in determining the optoelectronic properties and charge carrier dynamics can provide valuable insight toward the rational development of more efficient photocatalytic materials for CO 2 reduction.indium oxide | solar fuels | CO 2 hydrogenation | transient absorption | surface defects C oncerns over climate change and the projected rise in global energy demand have motivated researchers to develop alternative, more sustainable ways to generate energy from naturally abundant and renewable sources (1-3). An important challenge associated with using renewable energy sources, such as solar, is their inherent intermittent nature (4-6). The emerging field of solar fuels seeks to address this issue by storing radiant solar energy in chemical bonds, which can then be released on demand and act as a drop-in replacement for traditional fossil fuels (7-11). By using the greenhouse gas, CO 2 , currently regarded as a waste product, as a feedstock and converting it into valuable products such as solar fuels or platform chemicals, we could simultaneously address concerns over climate change and energy security while creating significant economic benefits (12)(13)(14). However, despite recent advances in the development of active materials that can drive the photocatalytic reduction of CO 2 into useful chemical species, much remains unknown about the fundamental physical properties that control the activity of a photocatalyst. To facilitate the rational development and improvement of photocatalytic materials, a detailed understanding of the complex interplay between chemical, optical, and electronic processes is needed.Indium oxide has many favorable optical, electronic, and surface properties that make it a compelling choice as a photocatalyst for CO 2 reduction. It has a relatively high conduction band, and common defects such as oxyge...

We have studied ultrafast dynamics in thin films of Eu-doped zinc oxide (ZnO), prepared by radio-frequency sputtering onto sapphire substrates. Following UV excitation of ZnO, a red emission is observed. Postdeposition annealing in an oxygen atmosphere improves the crystallinity and emission intensity of the films, which are highly sensitive to the dopant concentration. Transient-absorption spectroscopy shows that the excited semiconductor host transfers energy to rare-earth ions on a time scale of only a few picoseconds. The dynamics as a function of the probe wavelength change dramatically after annealing, with annealed films showing the fastest dynamics at much lower wavelengths. Our results show that annealing greatly affects the defect energy levels of the films and the dynamics of the trapped carriers. Unannealed films show dynamics consistent with energy transfer from O vacancies to the dopant, while energy transfer in annealed samples involves acceptor-type defects such as Zn vacancies as intermediates.