Planar perovskite solar cells (PSCs) made entirely via solution processing at low temperatures (<150°C) offer promise for simple manufacturing, compatibility with flexible substrates, and ...perovskite-based tandem devices. However, these PSCs require an electron-selective layer that performs well with similar processing. We report a contact-passivation strategy using chlorine-capped TiO₂ colloidal nanocrystal film that mitigates interfacial recombination and improves interface binding in low-temperature planar solar cells. We fabricated solar cells with certified efficiencies of 20.1 and 19.5% for active areas of 0.049 and 1.1 square centimeters, respectively, achieved via low-temperature solution processing. Solar cells with efficiency greater than 20% retained 90% (97% after dark recovery) of their initial performance after 500 hours of continuous room-temperature operation at their maximum power point under 1-sun illumination (where 1 sun is defined as the standard illumination at AM1.5, or 1 kilowatt/square meter).
Earth-abundant first-row (3d) transition metal–based catalysts have been developed for the oxygen-evolution reaction (OER); however, they operate at overpotentials substantially above thermodynamic ...requirements. Density functional theory suggested that non-3d high-valency metals such as tungsten can modulate 3d metal oxides, providing near-optimal adsorption energies for OER intermediates. We developed a room-temperature synthesis to produce gelled oxyhydroxides materials with an atomically homogeneous metal distribution. These gelled FeCoW oxyhydroxides exhibit the lowest overpotential (191 millivolts) reported at 10 milliamperes per square centimeter in alkaline electrolyte. The catalyst shows no evidence of degradation after more than 500 hours of operation. X-ray absorption and computational studies reveal a synergistic interplay between tungsten, iron, and cobalt in producing a favorable local coordination environment and electronic structure that enhance the energetics for OER.
Bandtail states in disordered semiconductor materials result in losses in open-circuit voltage (V
) and inhibit carrier transport in photovoltaics. For colloidal quantum dot (CQD) films that promise ...low-cost, large-area, air-stable photovoltaics, bandtails are determined by CQD synthetic polydispersity and inhomogeneous aggregation during the ligand-exchange process. Here we introduce a new method for the synthesis of solution-phase ligand-exchanged CQD inks that enable a flat energy landscape and an advantageously high packing density. In the solid state, these materials exhibit a sharper bandtail and reduced energy funnelling compared with the previous best CQD thin films for photovoltaics. Consequently, we demonstrate solar cells with higher V
and more efficient charge injection into the electron acceptor, allowing the use of a closer-to-optimum bandgap to absorb more light. These enable the fabrication of CQD solar cells made via a solution-phase ligand exchange, with a certified power conversion efficiency of 11.28%. The devices are stable when stored in air, unencapsulated, for over 1,000 h.
Advances in colloidal quantum dots
The confinement found in colloidal semiconductor quantum dots enables the design of materials with tunable properties. García de Arquer
et al
. review the recent ...advances in methods for synthesis and surface functionalization of quantum dots that enable fine tuning of their optical, chemical, and electrical properties. These important developments have driven the commercialization of display and lighting applications and provide promising developments in the related fields of lasing and sensing. —MSL
A review highlights advances in the synthesis of colloidal quantum dots that have enabled numerous applications.
BACKGROUND
Semiconductor materials feature optical and electronic properties that can be engineered through their composition and crystal structure. The use of semiconductors such as silicon gallium arsenide sparked technologies from computers and mobile phones to lasers and satellites. Semiconductor quantum dots (QDs) offer an additional lever: Because their size is reduced to the nanometer scale in all three dimensions, the restricted electron motion leads to a discrete atom-like electronic structure and size-dependent energy levels. This enables the design of nanomaterials with widely tunable light absorption, bright emission of pure colors, control over electronic transport, and a wide tuning of chemical and physical functions because of their large surface-to-volume ratio.
ADVANCES
The bright and narrowband light emission of semiconductor QDs, tunable across the visible and near-infrared spectrum, is attractive to realize more efficient displays with purer colors. QDs are engineered compositionally and structurally to manipulate energy states and charge interactions, leading to optical gain and lasing, relevant to light emission across visible and infrared wavelengths and fiberoptic communication. Their tunable surface chemistry allows application as optical labels in bio-imaging, made possible by tethering QDs with proteins and antibodies. The manipulation of QD surfaces with capping molecules that have different chemical and physical functions can be tailored to program their assembly into semiconducting solids, increasing conductivity and enabling the transduction of photonic and chemical stimuli into electrical signals. Optoelectronic devices such as transistors and photodetectors lead to cameras sensitive to visible and infrared light. Highly crystalline QDs can be grown epitaxially on judiciously chosen substrates by using high-temperature and vacuum conditions, and their use has led to commercially viable high-performance lasers. The advent of colloidal QDs, which can be fabricated and processed in solution at mild conditions, enabled large-area manufacturing and widened the scope of QD application to markets such as consumer electronics and photovoltaics.
OUTLOOK
From a chemistry perspective, further advances in QD fabrication are needed to sustain and improve desired chemical and optoelectronic properties and to do so with high reproducibility. This entails the use of inexpensive synthesis methods and precursors that are able to retain laboratory-scale QD properties to market-relevant volumes. A better understanding of the yet-incomplete picture of QD surfaces, atomic arrangement, and metastable character is needed to drive further progress. From a regulatory perspective, added attention is needed to achieve high-quality materials that do not rely on heavy metals such as Cd, Pb, and Hg. The role of nanostructuring in toxicity and life cycle analysis for each application is increasingly important. From a materials and photophysics perspective, exciting opportunities remain in the understanding and harnessing of electrons in highly confined materials, bridging the gap between mature epitaxial QDs and still-up-and-coming colloidal QDs. The yet-imperfect quality of the latter—a price paid today in exchange for their ease of manufacture—remains a central challenge and must be addressed to achieve further-increased performance in devices. From a device perspective, colloidal QD manufacturing must advance to translate from laboratory-scale to large-area applications such as roll-to-roll and inkjet printing. Photocatalysis, in which light is used to drive chemical transformations, is an emerging field in which QDs are of interest. Quantum information technologies, which rely on the transduction of coherent light and electrons, bring new challenges and opportunities to exploit quantum confinement effects. Moving forward, opportunities remain in the design of QD-enabled new device architectures.
Semiconductor quantum dot technologies.
Quantum dots feature widely tunable and distinctive optical, electrical, chemical, and physical properties. They span energy harvesting, illumination, displays, cameras, sensors, communication and information technology, biology, and medicine, among others. These have been exploited to realize efficient lasers, displays, biotags, and solar harvesting devices available in the market and are emerging in photovoltaics, sensing, and quantum information.
In quantum-confined semiconductor nanostructures, electrons exhibit distinctive behavior compared with that in bulk solids. This enables the design of materials with tunable chemical, physical, electrical, and optical properties. Zero-dimensional semiconductor quantum dots (QDs) offer strong light absorption and bright narrowband emission across the visible and infrared wavelengths and have been engineered to exhibit optical gain and lasing. These properties are of interest for imaging, solar energy harvesting, displays, and communications. Here, we offer an overview of advances in the synthesis and understanding of QD nanomaterials, with a focus on colloidal QDs, and discuss their prospects in technologies such as displays and lighting, lasers, sensing, electronics, solar energy conversion, photocatalysis, and quantum information.
Electrochemical reduction of carbon dioxide (CO^sub 2^) to carbon monoxide (CO) is the first step in the synthesis of more complex carbon-based fuels and feedstocks using renewable electricity1-7. ...Unfortunately, the reaction suffers from slow kinetics7,8 owing to the low local concentration of CO^sub 2^ surrounding typical CO^sub 2^ reduction reaction catalysts. Alkali metal cations are known to overcome this limitation through non-covalent interactions with adsorbed reagent species9,10, but the effect is restricted by the solubility of relevant salts. Large applied electrode potentials can also enhance CO^sub 2^ adsorption11, but this comes at the cost of increased hydrogen (H^sub 2^) evolution. Here we report that nanostructured electrodes produce, at low applied overpotentials, local high electric fields that concentrate electrolyte cations, which in turn leads to a high local concentration of CO^sub 2^ close to the active CO^sub 2^ reduction reaction surface. Simulations reveal tenfold higher electric fields associated with metallic nanometre-sized tips compared to quasi-planar electrode regions, and measurements using gold nanoneedles confirm a field-induced reagent concentration that enables the CO^sub 2^ reduction reaction to proceed with a geometric current density for CO of 22 milliamperes per square centimetre at -0.35 volts (overpotential of 0.24 volts). This performance surpasses by an order of magnitude the performance of the best gold nanorods, nanoparticles and oxidederived noble metal catalysts. Similarly designed palladium nanoneedle electrocatalysts produce formate with a Faradaic efficiency of more than 90 per cent and an unprecedented geometric current density for formate of 10 milliamperes per square centimetre at -0.2 volts, demonstrating the wider applicability of the field-induced reagent concentration concept.
Perovskites for Light Emission Quan, Li Na; García de Arquer, F. Pelayo; Sabatini, Randy P. ...
Advanced materials (Weinheim),
11/2018, Letnik:
30, Številka:
45
Journal Article
Recenzirano
Next‐generation displays require efficient light sources that combine high brightness, color purity, stability, compatibility with flexible substrates, and transparency. Metal halide perovskites are ...a promising platform for these applications, especially in light of their excellent charge transport and bandgap tunability. Low‐dimensional perovskites, which possess perovskite domains spatially confined at the nanoscale, have further extended the degree of tunability and functionality of this materials platform. Herein, the advances in perovskite materials for light‐emission applications are reviewed. Connections among materials properties, photophysical and electrooptic spectroscopic properties, and device performance are established. It is discussed how incompletely solved problems in these materials can be tackled, including the need for increased stability, efficient blue emission, and efficient infrared emission. In conclusion, an outlook on the technologies that can be realized using this material platform is presented.
Metal halide perovskites are a promising platform in light of their excellent charge transport and bandgap tunability. Especially low‐dimensional perovskites, spatially confined at the nanoscale, have further extended the degree of tunability and functionalities. Advances in perovskite materials for light emission applications and their materials properties, photophysical and electrooptic spectroscopic properties, and device performance are discussed.
Abstract
Electrochemical reduction of CO
2
(CO
2
R) to formic acid upgrades waste CO
2
; however, up to now, chemical and structural changes to the electrocatalyst have often led to the deterioration ...of performance over time. Here, we find that alloying p-block elements with differing electronegativities modulates the redox potential of active sites and stabilizes them throughout extended CO
2
R operation. Active Sn-Bi/SnO
2
surfaces formed in situ on homogeneously alloyed Bi
0.1
Sn crystals stabilize the CO
2
R-to-formate pathway over 2400 h (100 days) of continuous operation at a current density of 100 mA cm
−2
. This performance is accompanied by a Faradaic efficiency of 95% and an overpotential of ~ −0.65 V. Operating experimental studies as well as computational investigations show that the stabilized active sites offer near-optimal binding energy to the key formate intermediate *OCHO. Using a cation-exchange membrane electrode assembly device, we demonstrate the stable production of concentrated HCOO
–
solution (3.4 molar, 15 wt%) over 100 h.
The stability of solution-processed semiconductors remains an important area for improvement on their path to wider deployment. Inorganic caesium lead halide perovskites have a bandgap well suited to ...tandem solar cells
but suffer from an undesired phase transition near room temperature
. Colloidal quantum dots (CQDs) are structurally robust materials prized for their size-tunable bandgap
; however, they also require further advances in stability because they are prone to aggregation and surface oxidization at high temperatures as a consequence of incomplete surface passivation
. Here we report 'lattice-anchored' hybrid materials that combine caesium lead halide perovskites with lead chalcogenide CQDs, in which lattice matching between the two materials contributes to a stability exceeding that of the constituents. We find that CQDs keep the perovskite in its desired cubic phase, suppressing the transition to the undesired lattice-mismatched phases. The stability of the CQD-anchored perovskite in air is enhanced by an order of magnitude compared with pristine perovskite, and the material remains stable for more than six months at ambient conditions (25 degrees Celsius and about 30 per cent humidity) and more than five hours at 200 degrees Celsius. The perovskite prevents oxidation of the CQD surfaces and reduces the agglomeration of the nanoparticles at 100 degrees Celsius by a factor of five compared with CQD controls. The matrix-protected CQDs show a photoluminescence quantum efficiency of 30 per cent for a CQD solid emitting at infrared wavelengths. The lattice-anchored CQD:perovskite solid exhibits a doubling in charge carrier mobility as a result of a reduced energy barrier for carrier hopping compared with the pure CQD solid. These benefits have potential uses in solution-processed optoelectronic devices.
A very basic pathway from CO2 to ethyleneEthylene is an important commodity chemical for plastics. It is considered a tractable target for synthesizing renewable resources from carbon dioxide (CO2). ...The challenge is that the performance of the copper electrocatalysts used for this conversion under the required basic reaction conditions suffers from the competing reaction of CO2 with the base to form bicarbonate. Dinh et al. designed an electrode that tolerates the base by optimizing CO2 diffusion to the catalytic sites (see the Perspective by Ager and Lapkin). This catalyst design delivers 70% efficiency for 150 hours.Science, this issue p. 783; see also p. 707Carbon dioxide (CO2) electroreduction could provide a useful source of ethylene, but low conversion efficiency, low production rates, and low catalyst stability limit current systems. Here we report that a copper electrocatalyst at an abrupt reaction interface in an alkaline electrolyte reduces CO2 to ethylene with 70% faradaic efficiency at a potential of −0.55 volts versus a reversible hydrogen electrode (RHE). Hydroxide ions on or near the copper surface lower the CO2 reduction and carbon monoxide (CO)–CO coupling activation energy barriers; as a result, onset of ethylene evolution at −0.165 volts versus an RHE in 10 molar potassium hydroxide occurs almost simultaneously with CO production. Operational stability was enhanced via the introduction of a polymer-based gas diffusion layer that sandwiches the reaction interface between separate hydrophobic and conductive supports, providing constant ethylene selectivity for an initial 150 operating hours.
Electrochemical carbon dioxide reduction (CO2) is a promising technology to use renewable electricity to convert CO2 into valuable carbon‐based products. For commercial‐scale applications, however, ...the productivity and selectivity toward multi‐carbon products must be enhanced. A facile surface reconstruction approach that enables tuning of CO2‐reduction selectivity toward C2+ products on a copper‐chloride (CuCl)‐derived catalyst is reported here. Using a novel wet‐oxidation process, both the oxidation state and morphology of Cu surface are controlled, providing uniformity of the electrode morphology and abundant surface active sites. The Cu surface is partially oxidized to form an initial Cu (I) chloride layer which is subsequently converted to a Cu (I) oxide surface. High C2+ selectivity on these catalysts are demonstrated in an H‐cell configuration, in which 73% Faradaic efficiency (FE) for C2+ products is reached with 56% FE for ethylene (C2H4) and overall current density of 17 mA cm‐2. Thereafter, the method into a flow‐cell configuration is translated, which allows operation in a highly alkaline medium for complete suppression of CH4 production. A record C2+ FE of ≈84% and a half‐cell power conversion efficiency of 50% at a partial current density of 336 mA cm‐2 using the reconstructed Cu catalyst are reported.
Electrochemical CO2 reduction is a promising route to facilitate large‐scale storage of intermittent renewable electrons in the form of chemical bonds. A facile wet‐oxidation approach to enhance the CO2‐reduction selectivity toward C2+ products on a copper‐chloride‐derived catalyst is reported. The C2+ Faradaic efficiency of ≈84% at a partial current density of 336 mA cm−2 is demonstrated.
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