Enhancing Conductivity of Silver Nanowire Networks through Surface Engineering Using Bidentate Rigid Ligands.

Solution processable metallic nanomaterials present a convenient way to fabricate conductive structures, which are necessary in all electronic devices. However, they tend to require post-treatments to remove the bulky ligands around them to achieve high conductivity. In this work, we present a method to formulate a post-treatment free conductive silver nanowire ink by controlling the type of ligands around the silver nanowires. We found that bidentate ligands with a rigid molecular structure were effective in improving the conductivity of the silver nanowire networks as they could maximize the number of linkages between neighboring nanowires. In addition, DFT calculations also revealed that ligands with good LUMO to silver energy alignment were more effective. Because of these reasons, fumaric acid was found to be the most effective ligand and achieved a large reduction in sheet resistance of 70% or higher depending on the nanowire network density. The concepts elucidated from this study would also be applicable to other solution processable nanomaterials systems such as quantum dots for photovoltaics or LEDs which also require good charge transport being neighboring nanoparticles.

Active Metamaterials with Negative Static Electric Susceptibility.

Although well-established textbook arguments suggest that static electric susceptibility χ(0) must be positive in "all bodies," it has been pointed out that materials that are not in thermodynamic equilibrium are not necessarily subject to this restriction. Media with inverted populations of atomic and molecular energy levels have been predicted theoretically to exhibit a χ(0) < 0 state, however the systems envisioned require reduced temperature, reduced pressure, and an external pump laser to maintain the population inversion. Further, the existence of χ(0) < 0 has never been confirmed experimentally. Here, a completely different approach is taken to the question of χ(0) < 0 and a design concept to achieve "true" χ(0) < 0 is proposed based on active metamaterials with internal power sources. Two active metamaterial structures are fabricated that, despite still having their power sources implemented externally for reasons of practical convenience, provide evidence in support of the general concept. Effective values are readily achieved at room temperature and pressure and are tunable throughout the range of stability -1 < χ(0) < 0, resulting in experimentally-determined magnitudes that are over one thousand times greater than those predicted previously. Since χ(0) < 0 is the missing electric analog of diamagnetism, this work opens the door to new technological capabilities such as stable electrostatic levitation.

Influence of Multistep Surface Passivation on the Performance of PbS Colloidal Quantum Dot Solar Cells.

The performance of devices containing colloidal quantum dot (CQD) films is strongly dependent on the surface chemistry of the CQDs they contain. Multistep surface treatments, which combine two or more strategies, are important for creating films with high carrier mobility that are well passivated against trap states and oxidation. Here, we examine the effect of a number of these surface treatments on PbS CQD films, including cation exchange to form PbS/CdS core/shell CQDs, and solid-state ligand-exchange treatments with Cl, Br, I, and 1,2-ethanedithiol (EDT) ligands. Using laboratory-based and synchrotron-radiation-excited X-ray photoelectron spectroscopy (XPS), we examine the compositions of the surface layer before and after treatment, and correlate this with the performance data and stability in air. We find that halide ion treatments may etch the CQD surfaces, with detrimental effects on the air stability and solar cell device performance caused by a reduction in the proportion of passivated surface sites. We show that films made up of PbS/CdS CQDs are particularly prone to this, suggesting Cd is more easily etched from the surface than Pb. However, by choosing a less aggressive ligand treatment, a good coverage of passivators on the surface can be achieved. We show that halide anions bind preferentially to surface Pb (rather than Cd). By isolating the part of XPS signal originating from the topmost surface layer of the CQD, we show that air stability is correlated with the total number of passivating agents (halide + EDT + Cd) at the surface.

Two-Photon Absorption and Two-Photon-Induced Gain in Perovskite Quantum Dots.

Perovskite quantum dots (PQDs) emerged as a promising class of material for applications in lighting devices, including light emitting diodes and lasers. In this work, we explore nonlinear absorption properties of PQDs showing the spectral signatures and the size dependence of their two-photon absorption (2PA) cross-section, which can reach values higher than 106 GM. The large 2PA cross section allows for low threshold two-photon induced amplified spontaneous emission (ASE), which can be as low as 1.6 mJ/cm2. We also show that the ASE properties are strongly dependent on the nanomaterial size, and that the ASE threshold, in terms of the average number of excitons, decreases for smaller PQDs. Investigating the PQDs biexciton binding energy, we observe strong correlation between the increasing on the biexciton binding energy and the decreasing on the ASE threshold, suggesting that ASE in PQDs is a biexciton-assisted process.

The passivating effect of cadmium in PbS/CdS colloidal quantum dots probed by nm-scale depth profiling.

Achieving control of the surface chemistry of colloidal quantum dots (CQDs) is essential to fully exploit their properties in solar cells, but direct measurement of the chemistry and electronic structure in the outermost atomic layers is challenging. Here we probe the surface oxidation and passivation of cation-exchanged PbS/CdS core/shell CQDs with sub nm-scale precision using synchrotron-radiation-excited depth-profiling photoemission. We investigate the surface composition of the topmost 1-2.5 nm of the CQDs as a function of depth, for CQDs of varying CdS shell thickness, and examine how the surface changes after prolonged air exposure. We demonstrate that the Cd is localized at the surface of the CQDs. The surface-localized products of oxidation are identified, and the extent of oxidation quantified. We show that oxidised sulfur species are progressively eliminated as Cd replaces Pb at the surface. A sub-monolayer surface 'decoration' of Cd is found to be effective in passivating the CQDs. We show that the measured energy-level alignments at PbS/CdS colloidal quantum dot surfaces differ from those expected on the basis of bulk band offsets, and are strongly affected by the oxidation products. We develop a model for the passivating action of Cd. The optimum shell thickness (of around 0.1 nm, previously found to give maximised power conversion efficiency in PbS/CdS solar cells) is found to correspond to a trade-off between the rate of oxidation and the introduction of a surface barrier to charge transport.

Narrow Band Gap Lead Sulfide Hole Transport Layers for Quantum Dot Photovoltaics.

The band structure of colloidal quantum dot (CQD) bilayer heterojunction solar cells is optimized using a combination of ligand modification and QD band gap control. Solar cells with power conversion efficiencies of up to 9.33 ± 0.50% are demonstrated by aligning the absorber and hole transport layers (HTL). Key to achieving high efficiencies is optimizing the relative position of both the valence band and Fermi energy at the CQD bilayer interface. By comparing different band gap CQDs with different ligands, we find that a smaller band gap CQD HTL in combination with a more p-type-inducing CQD ligand is found to enhance hole extraction and hence device performance. We postulate that the efficiency improvements observed are largely due to the synergistic effects of narrower band gap QDs, causing an upshift of valence band position due to 1,2-ethanedithiol (EDT) ligands and a lowering of the Fermi level due to oxidation.

High Performance PbS Quantum Dot/Graphene Hybrid Solar Cell with Efficient Charge Extraction.

Hybrid colloidal quantum dot (CQD) solar cells are fabricated from multilayer stacks of lead sulfide (PbS) CQD and single layer graphene (SG). The inclusion of graphene interlayers is shown to increase power conversion efficiency by 9.18%. It is shown that the inclusion of conductive graphene enhances charge extraction in devices. Photoluminescence shows that graphene quenches emission from the quantum dot suggesting spontaneous charge transfer to graphene. CQD photodetectors exhibit increased photoresponse and improved transport properties. We propose that the CQD/SG hybrid structure is a route to make CQD thin films with improved charge extraction, therefore resulting in improved solar cell efficiency.

Poly(3-hexylthiophene-2,5-diyl) as a Hole Transport Layer for Colloidal Quantum Dot Solar Cells.

Lead sulfide colloidal quantum dot (CQD) solar cells demonstrate extremely high short-circuit currents (Jsc) and are making decent progress in power conversion efficiencies. However, the low fill factors (FF) and open-circuit voltages have to be addressed with urgency to prevent the stalling of efficiency improvements. This paper highlights the importance of improving hole extraction, which received much less attention as compared to the electron-accepting component of the device architecture (e.g., TiO2 or ZnO). Here, we show the use of semiconducting polymer poly(3-hexylthiophene-2,5-diyl) to create efficient CQD devices by improving hole transport, removing interfacial barriers, and minimizing shunt pathways, thus resulting in an overall improvement in device performance stemming from better Jsc and FF.

Transfer printed silver nanowire transparent conductors for PbS-ZnO heterojunction quantum dot solar cells.

Transfer-printed silver nanowire transparent conducting electrodes are demonstrated in lead sulfide-zinc oxide quantum dot solar cells. Advantages of using this transparent conductor technology are increased junction surface energy, solution processing, and the potential cost reduction of low temperature processing. Joule heating, device aging, and film thickness effects are investigated to understand shunt pathways created by nanowires protruding perpendicular to the film. A V(oc) of 0.39 ± 0.07 V, J(sc) of 16.2 ± 0.2 mA/cm(2), and power conversion efficiencies of 2.8 ± 0.4% are presented.

Polystyrene templated porous titania wells for quantum dot heterojunction solar cells.

Polystyrene spheres are used to template TiO2 with a single layer of 300 nm wells which are infilled with PbS quantum dots to form a heterojunction solar cell. The porous well device has an efficiency of 5.7% while the simple planar junction is limited to 3.2%. Using a combination of optical absorption and photocurrent transient decay measurement we determined that the performance enhancement comes from a combination of enhanced optical absorption and increased carrier lifetime.

Interactions of Pb and Te atoms with graphene.

PbTe nanocrystals were deposited onto the surface of graphene and used as a reservoir of Pb and Te atoms. Electron beam irradiation at 80 kV caused Pb and Te atoms to mobilize and disperse across the surface of graphene. We studied the dynamics of these atoms in real time using aberration-corrected transmission electron microscopy. The Pb and Te atoms were found to attach to the surface layer of amorphous carbon that resides upon the graphene, as well as its edge. Pb and Te atoms were not found residing on pristine graphene, but were found to bond to the free edge states along graphene hole edges. Small PbTe nanoclusters tended to only form on the surface of the amorphous carbon regions and not on pristine graphene.

Low temperature phase selective synthesis of Cu(2)ZnSnS(4) quantum dots.

The application of indium-free quaternary chalcogenides, such as Cu(2)ZnSnS(4) (CZTS), in photovoltaics has created tremendous interest in recent years. In this paper we develop a method to synthesize high quality CZTS nanoparticles with thermodynamically stable kesterite and wurtzite phases via a simple, one-pot, low-cost solution method.

The transitional heterojunction behavior of PbS/ZnO colloidal quantum dot solar cells.

The nature of charge separation at the heterojunction interface of solution processed lead sulphide-zinc oxide colloidal quantum dot solar cells is investigated using impedance spectroscopy and external quantum efficiency measurements to examine the effect of varying the zinc oxide doping density. Without doping, the device behaves excitonically with no depletion region in the PbS layer such that only charge carriers generated within a diffusion length of the PbS/ZnO interface have a good probability of being harvested. After the ZnO is photodoped such that the doping density is near or greater than that of the PbS, a significant portion of the depletion region is found to lie within the PbS layer increasing charge extraction (p-n operation).

High-performance, single-layer antireflective optical coatings comprising mesoporous silica nanoparticles.

Mesoporous silica nanoparticles are used to fabricate antireflectance coatings on glass substrates. The combination of mesoporous silica nanoparticles in conjunction with a suitable binder material allows mechanically robust single layer coatings with a reflectance <0.1% to be produced by simple wet processing techniques. Further advantages of these films is that their structure results in broadband antireflective properties with a reflection minimum that can tuned between 400 nm and 1900 nm. The ratio of binder material to mesoporous nanoparticles allows control of the refractive index. In this report, we discuss how control of the structural properties of the coatings allows optimization of the optical properties.

Vacuum-deposited planar heterojunction polymer solar cells.

The vacuum thermal evaporation of poly(3-hexylthiophene) (P3HT) for application in photovoltaic cells is demonstrated. Structural changes before and after evaporation are determined using GPC, UV-vis absorption spectroscopy, NMR, and FTIR. GPC showed that the polymer molecular weight is reduced during evaporation, leading to a blue-shift of the absorption spectra. FTIR and NMR were used to examine the change in chemical structure: it was found that conjugation remains mostly intact; however, the conjugation length decreases and side chains dissociate from the backbone. Bilayer heterojunction solar cells were fabricated by sequential deposition of P3HT and C60 and the photovoltaic response measured.

SnS/PbS nanocrystal heterojunction photovoltaics.

We report advances in the growth, characterization and photovoltaic properties of SnS nanocrystals, with controlled < 10 nm size, and their inclusion into a lead chalcogenide solar cell. The SnS/PbS nanocrystalline film heterojunction is shown to display a type II band alignment, in which the direction of flow of the photocurrent depends on the order of the layers and not the relative work functions of the contacts. On placing the SnS layer next to the indium tin oxide (ITO) cathode we observe a dramatic increase in V(oc) to as much as 0.45 V. Our results suggest that SnS nanocrystal films can be used in multi-junction solar cells, that a SnS/PbS heterojunction on its own shows photovoltaic behaviour, and that a SnS layer in an ITO/SnS/PbS/Al device is acting to suppress the flow of an electron injection current.

Colloidal synthesis of lead oxide nanocrystals for photovoltaics.

Lead oxide nanocrystals are synthesised by injecting oxygen gas into an air and moisture free complex of Pb oleylamine and oleic acid in octadecene. Using various characterization methods including fabrication and testing of photovoltaic devices we explore the material properties and photovoltaic application of lead oxide nanocrystal films.

Superstructures of PbS nanocrystals in a conjugated polymer and the aligning role of oxidation.

We present a method to directly align PbS nanocrystals in micron-sized superstructures within a conjugated polymer. First, lead sulfide nanocrystals are directly synthesized in a MEH-PPV suspension via a single pot, surfactant-free method. Post-synthesis precipitation of the composite solution involving mild oxidation of the nanocrystals results in the formation of nanocrystal-polymer and nanocrystal-oxide superstructures. Detailed TEM is used to study the crystallographic nature of these structures and the roles of polymer and lead sulfate. An epitaxial relationship between lead sulfide and lead sulfate at the nanoscale is shown, giving insight into the oxidation rates of the PbS nanocrystals' facets.

Grating of single Lu@C(82) molecules using supramolecular network.

A supramolecular grating of single Lu@C(82) molecules was obtained by depositing Lu@C(82) molecules onto a room temperature PTCDI-melamine network.

Rotating fullerene chains in carbon nanopeapods.

The rotation of fullerene chains in SWNT peapods is studied using low-voltage high resolution transmission electron microscopy. Anisotropic fullerene chain structures (i.e., C300) are formed in situ in carbon nanopeapods via electron beam induced coalescence of individual fullerenes (i.e., C60). A low electron accelerating voltage of 80 kV is used to prevent damage to the SWNT. The large asymmetric C300 fullerene structure exhibits translational motion inside the SWNT and unique corkscrew like rotation motion. Another asymmetric fullerene chain containing mixed fullerene species is prepared by fusing smaller C60 fullerenes to a larger Sc@C82 fullerene, and this also exhibits corkscrew rotational motion. Chains of Sc3C2@C80 in SWNT peapods adopt a zigzag packing structure, and the entire zigzag chain rotates inside the SWNT to induce structural modifications to the SWNT diameter and cross-sectional shape of the SWNT. The expansion and contraction of the diameter of the SWNT is measured as 17%, demonstrating nanoactuation behavior in carbon nanopeapods.