Band Gap Tuning via Lattice Contraction and Octahedral Tilting in Perovskite Materials for Photovoltaics.

Tin and lead iodide perovskite semiconductors of the composition AMX3, where M is a metal and X is a halide, are leading candidates for high efficiency low cost tandem photovoltaics, in part because they have band gaps that can be tuned over a wide range by compositional substitution. We experimentally identify two competing mechanisms through which the A-site cation influences the band gap of 3D metal halide perovskites. Using a smaller A-site cation can distort the perovskite lattice in two distinct ways: by tilting the MX6 octahedra or by simply contracting the lattice isotropically. The former effect tends to raise the band gap, while the latter tends to decrease it. Lead iodide perovskites show an increase in band gap upon partial substitution of the larger formamidinium with the smaller cesium, due to octahedral tilting. Perovskites based on tin, which is slightly smaller than lead, show the opposite trend: they show no octahedral tilting upon Cs-substitution but only a contraction of the lattice, leading to progressive reduction of the band gap. We outline a strategy to systematically tune the band gap and valence and conduction band positions of metal halide perovskites through control of the cation composition. Using this strategy, we demonstrate solar cells that harvest light in the infrared up to 1040 nm, reaching a stabilized power conversion efficiency of 17.8%, showing promise for improvements of the bottom cell of all-perovskite tandem solar cells. The mechanisms of cation-based band gap tuning we describe are broadly applicable to 3D metal halide perovskites and will be useful in further development of perovskite semiconductors for optoelectronic applications.

Organic phototransistors using poly(3-hexylthiophene) nanofibres.

Here we report the fabrication of nanofibre-based organic phototransistors (OPTs) using preformed poly(3-hexylthiophene) (P3HT) nanofibres. OPT performance is analysed based on two important parameters: photoresponsivity R and photosensitivity P. Before testing the devices as OPTs, the normal organic field-effect transistor (OFET) operation is characterized, revealing a surface-coverage-dependent performance. With R reaching 250 A W(-1) in the on-state (V(GS) = -40 V) and P reaching 6.8 × 10(3) in the off-state (V(GS) = 10 V) under white light illumination (I(inc) = 0.91 mW cm(-2)), the best nanofibre-based OPTs outperform the OPTs fabricated from a solution of P3HT in chlorobenzene, in which no preformed fibres are present. The better performance is attributed to an increase in active layer crystallinity, a better layer connectivity and an improved edge-on orientation of the thiophene rings along the polymer backbone, resulting in a longer exciton diffusion length and enhanced charge carrier mobility, linked to a decreased interchain coupling energy. In addition, the increased order in the active layer crystallinity induces a better spectral overlap between the white light emission spectrum and the active layer absorption spectrum, and the absorption of incident light is maximised by the favourable parallel orientation of the polymer chains with respect to the OPT substrate. Combining both leads to an increase in the overall light absorption. In comparison with previously reported solution-processed organic OPTs, it is shown here that no special dielectric surface treatment or post-deposition treatment of the active device layer is needed to obtain high OPT performance. Finally, it is also shown that, inherent to an intrinsic gate-tuneable gain mechanism, changing the gate potential results in a variation of R over at least five orders of magnitude. As such, it is shown that R can be adjusted according to the incident light intensity.

The Influence of UV-Ozone, O2 Plasma, and CF4 Plasma Treatment on the Droplet-Based Deposition of Diamond Nanoparticles.

Surface treatment is critical for homogeneous coating over a large area and high-resolution patterning of nanodiamond (ND) particles. To optimize the interaction between the surface of a substrate and the colloid of ND particles, it is essential to remove hydrocarbon contamination by surface treatment and to increase the surface energy of the substrate, hence improving the diamond film homogeneity upon its deposition. However, the impact of substrate surface treatment on the properties of coatings and patterns is not fully understood. This study explores the impact of UV-ozone, O2 plasma, and CF4 plasma treatments on the wetting properties of the fused silica glass substrate surface. We identify the optimal time interval between the treatment and subsequent ND coating/patterning processes, which were conducted using inkjet printing and ultrasonic spray coating techniques. Our results showed that UV-ozone and O2 plasma resulted in hydrophilic surfaces, while CF4 plasma treatment resulted in hydrophobic surfaces. We demonstrate the use of CF4 plasma treatment before inkjet printing to generate high-resolution patterns with dots as small as 30 µm in diameter. Ultrasonic spray coating showed homogeneous coatings after using UV-ozone and O2 plasma treatment. The findings of this study provide valuable insights into the hydrocarbon airborne contamination on cleaned surfaces over time even in clean-room environments and have a notable impact on the performance of liquid coatings and patterns. We highlight the importance of timing between the surface treatment and printing in achieving high resolution or homogeneity.

A molecular toolkit for the functionalization of titanium-based biomaterials that selectively control integrin-mediated cell adhesion.

We present a click chemistry-based molecular toolkit for the biofunctionalization of materials to selectively control integrin-mediated cell adhesion. To this end, α5ß1-selective RGD peptidomimetics were covalently immobilized on Ti-based materials, and the capacity to promote the selective binding of α5ß1 was evaluated using a solid-phase integrin binding assay. This functionalization strategy yielded surfaces with a nine-fold increased affinity for α5ß1, in comparison to control samples, and total selectivity against the binding of the closely related integrin αvß3. Moreover, our methodology allowed the screening of several phosphonic acid containing anchoring units to find the best spacer-anchor moiety required for establishing an efficient binding to titanium and to promote selective integrin binding. The integrin subtype specificity of these biofunctionalized surfaces was further examined in vitro by inducing selective adhesion of genetically modified fibroblasts, which express exclusively the α5ß1 integrin. The versatility of our molecular toolkit was proven by shifting the cellular specificity of the materials from α5ß1- to αvß3-expressing fibroblasts by using an αvß3-selective peptidomimetic as coating molecule. The results shown here represent the first functionalization of Ti-based materials with α5ß1- or αvß3-selective peptidomimetics that allow an unprecedented control to discriminate between α5ß1- and αvß3-mediated adhesions. The role of these two integrins in different biological events is still a matter of debate and is frequently discussed in literature. Thus, such bioactive titanium surfaces will be of great relevance for the study of integrin-mediated cell adhesion and the development of new biomaterials targeting specific cell types.

Selective protein immobilization onto gold nanoparticles deposited under vacuum on a protein-repellent self-assembled monolayer.

The immobilization of proteins on flat substrates plays an important role for a wide spectrum of applications in the fields of biology, medicine, and biochemistry, among others. An essential prerequisite for the use of proteins (e.g., in biosensors) is the conservation of their biological activity. Losses in activity upon protein immobilization can largely be attributed to a random attachment of the proteins to the surface. In this study, we present an approach for the immobilization of proteins onto a chemically heterogeneous surface, namely a surface consisting of protein-permissive and protein-repellent areas, which allows for significant reduction of random protein attachment. As protein-permissive, i.e., as protein-binding sites, ultra pure metallic nanoparticles are deposited under vacuum onto a protein-repellent PEG-silane polymer layer. Using complementary surface characterization techniques (atomic force microscopy, quartz crystal microbalance, and X-ray photoelectron spectroscopy) we demonstrate that the Au nanoparticles remain accessible for protein attachment without compromising the protein-repellency of the PEG-silane background. Moreover, we show that the amount of immobilized protein can be controlled by tuning the Au nanoparticle coverage. This method shows potential for applications requiring the control of protein immobilization down to the single molecule level.

Evidence for phase separation of ethanol-water mixtures at the hydrogen terminated nanocrystalline diamond surface.

Interactions between ethanol-water mixtures and a hydrophobic hydrogen terminated nanocrystalline diamond surface, are investigated by sessile drop contact angle measurements. The surface free energy of the hydrophobic surface, obtained with pure liquids, differs strongly from values obtained by ethanol-water mixtures. Here, a model which explains this difference is presented. The model suggests that, due to a higher affinity of ethanol for the hydrophobic surface, when compared to water, a phase separation occurs when a mixture of both liquids is in contact with the H-terminated diamond surface. These results are supported by a computational study giving insight in the affinity and related interaction at the liquid-solid interface.

Electron-donating amine-interlayer induced n-type doping of polymer:nonfullerene blends for efficient narrowband near-infrared photo-detection.

Inherently narrowband near-infrared organic photodetectors are highly desired for many applications, including biological imaging and surveillance. However, they suffer from a low photon-to-charge conversion efficiencies and utilize spectral narrowing techniques which strongly rely on the used material or on a nano-photonic device architecture. Here, we demonstrate a general and facile approach towards wavelength-selective near-infrared phtotodetection through intentionally n-doping 500-600 nm-thick nonfullerene blends. We show that an electron-donating amine-interlayer can induce n-doping, resulting in a localized electric field near the anode and selective collection of photo-generated carriers in this region. As only weakly absorbed photons reach this region, the devices have a narrowband response at wavelengths close to the absorption onset of the blends with a high spectral rejection ratio. These spectrally selective photodetectors exhibit zero-bias external quantum efficiencies of ~20-30% at wavelengths of 900-1100 nm, with a full-width-at-half-maximum of ≤50 nm, as well as detectivities of >1012 Jones.

Metallization of ultra-thin, non-thiol SAMs with flat-lying molecular units: Pd on 1, 4-dicyanobenzene.

Self-assembled monolayers of 1,4-dicyanobenzene on Au(111) electrodes are studied by cyclic voltammetry, in-situ STM and ex-situ XPS. High-resolution STM images reveal a long-range order of propeller-like assemblies each of which consists of three molecules, all lying flat on the gold substrate with the cyano groups oriented parallel to the metal surface. It is demonstrated that both functional groups can act as complexation sites for metal ions from solution. Surprisingly, such arrangements still allow the metal to be deposited on top of the molecules by electrochemical reduction despite the close vicinity to the Au surface. The latter is demonstrated by angle-resolved XPS which unequivocally shows that the metal indeed resides on top of the organic layer rather than underneath, despite the flat arrangement of the molecules.

Metallization of organic surfaces: Pd on thiazole.

Results from electrochemical studies, in situ STM, and ex situ angle-resolved XPS on self-assembled monolayers (SAMs) of thiazole on Au(111) are reported for the first time. Although STM seems to indicate a low density of molecules organized in small ordered domains, a quantitative chemical analysis of the sample surface by XPS clearly points toward the formation of a densely packed molecular layer. The stability of the thiazole SAM against reductive desorption is found to be very comparable with that for thiol-SAMs on gold. This results from the formation of Au-S bonds between the molecules and their support as evidenced by XPS, thereby rebuting speculations that the ring nitrogen is responsible for the attachment of such molecules to gold surfaces. Consequently, the N-atoms terminating the molecular layer are available as active sites for the complexation with Pd ions thereby allowing the deposition of Pd islands with monatomic height on top of the thiazole SAM. The importance of such studies for metal-molecule interconnections is briefly addressed.

Improved Field Electron Emission Properties of Phosphorus and Nitrogen Co-Doped Nanocrystalline Diamond Films.

Nanocrystalline diamond (NCD) field emitters have attracted significant interest for vacuum microelectronics applications. This work presents an approach to enhance the field electron emission (FEE) properties of NCD films by co-doping phosphorus (P) and nitrogen (N) using microwave plasma-enhanced chemical vapor deposition. While the methane (CH4) and P concentrations are kept constant, the N2 concentration is varied from 0.2% to 2% and supplemented by H2. The composition of the gas mixture is tracked in situ by optical emission spectroscopy. Scanning electron microscopy, atomic force microscopy (AFM), transmission electron microscopy, and Raman spectroscopy are used to provide evidence of the changes in crystal morphology, surface roughness, microstructure, and crystalline quality of the different NCD samples. The FEE results display that the 2% N2 concentration sample had the best FEE properties, viz. the lowest turn-on field value of 14.3 V/µm and the highest current value of 2.7 µA at an applied field of 73.0 V/µm. Conductive AFM studies reveal that the 2% N2 concentration NCD sample showed more emission sites, both from the diamond grains and the grain boundaries surrounding them. While phosphorus doping increased the electrical conductivity of the diamond grains, the incorporation of N2 during growth facilitated the formation of nano-graphitic grain boundary phases that provide conducting pathways for the electrons, thereby improving the FEE properties for the 2% N2 concentrated NCD films.

Inkjet Printing of PEDOT:PSS Based Conductive Patterns for 3D Forming Applications.

This paper presents the formulation, inkjet printing, and vacuum forming of a conductive and stretchable polymer, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), ink on a stretchable and transparent thermoplastic polyurethane (TPU) substrate. The formulation of the conductive and stretchable ink is achieved by combining PEDOT:PSS with additional solvents, to achieve the right inkjet properties for drop-on-demand (DoD) inkjet printing. A conductive pattern can be printed from the 21 µm orifice on a flexible and stretchable TPU substrate, with a linewidth down to 44 µm. The properties of the printed pattern, in terms of sheet resistance, morphology, transparency, impact of weather conditions, and stretching are investigated and show sheet resistances up to 45 Ohm/sq and transparencies as high as 95%, which is comparable to indium tin oxide (ITO). Moreover, in contrast to ITO, one-time stretching up to 40% can be achieved, increasing the sheet resistance up to 214 Ohm/sq only, showing the great potential of this ink for one-time stretching. Finally, as a proof of this one-time stretching, the printed samples are vacuum formed around a 3D object, still showing sufficient conductivity to be applied as a capacitive touch sensor.

Laser-Patternable Graphene Field Emitters for Plasma Displays.

This paper presents a plasma display device (PDD) based on laser-induced graphene nanoribbons (LIGNs), which were directly fabricated on polyimide sheets. Superior field electron emission (FEE) characteristics, viz. a low turn-on field of 0.44 V/µm and a large field enhancement factor of 4578, were achieved for the LIGNs. Utilizing LIGNs as a cathode in a PDD showed excellent plasma illumination characteristics with a prolonged plasma lifetime stability. Moreover, the LIGN cathodes were directly laser-patternable. Such superior plasma illumination performance of LIGN-based PDDs has the potential to make a significant impact on display technology.

Structure-Property Relations of Methylamine Vapor Treated Hybrid Perovskite CH3NH3PbI3 Films and Solar Cells.

The power conversion efficiency of halide perovskite solar cells is heavily dependent on the perovskite layer being sufficiently smooth and pinhole-free. It has been shown that these features can be obtained even when starting out from rough and discontinuous perovskite film by briefly exposing the film to methylamine (MA) vapor. The exact underlying physical mechanisms of this phenomenon are, however, still unclear. By investigating smooth, MA treated films based on very rough and discontinuous reference films of methylammonium triiode (MAPbI3) and considering their morphology, crystalline features, local conductive properties, and charge carrier lifetime, we unraveled the relation between their characteristic physical qualities and their performance in corresponding solar cells. We discovered that the extensive improvement in photovoltaic performance upon MA treatment is a consequence of the induced morphological enhancement of the perovskite layer together with improved electron injection into TiO2, which in fact compensates for an otherwise compromised bulk electronic quality simultaneously caused by the MA treatment.

Reversible restructuring of supported Au nanoparticles during butadiene hydrogenation revealed by operando GISAXS/GIWAXS.

Periodically arranged, monodisperse gold nanoparticles supported on flat silicon substrates were studied for the hydrogenation of 1,3-butadiene under operando conditions using Grazing Incidence Small- and Wide-Angle X-ray Scattering (GISAXS/GIWAXS). It was found that the composition and shape of the nanoparticles depends very much on the chemical environment; the particles are shown to be dynamic, undergoing reversible size and shape change particularly during catalytic reaction, highlighting a dynamism often not observed in traditional studies. Specifically, the size of the Au nanoparticles increases during butadiene hydrogenation and this is attributed to the partial removal of a Au2O3 at the metal-oxide interface and consequential shape change of the nanoparticle from a more hemispherical particle to a particle with a larger height to width ratio.

Steering the Properties of MoOx Hole Transporting Layers in OPVs and OLEDs: Interface Morphology vs. Electronic Structure.

The identification, fine-tuning, and process optimization of appropriate hole transporting layers (HTLs) for organic solar cells is indispensable for the production of efficient and sustainable functional devices. In this study, the optimization of a solution-processed molybdenum oxide (MoOx) layer fabricated from a combustion precursor is carried out via the introduction of zirconium and tin additives. The evaluation of the output characteristics of both organic photovoltaic (OPV) and organic light emitting diode (OLED) devices demonstrates the beneficial influence upon the addition of the Zr and Sn ions compared to the generic MoOx precursor. A dopant effect in which the heteroatoms and the molybdenum oxide form a chemical identity with fundamentally different structural properties could not be observed, as the additives do not affect the molybdenum oxide composition or electronic band structure. An improved surface roughness due to a reduced crystallinity was found to be a key parameter leading to the superior performance of the devices employing modified HTLs.

Assessing the toxicity of Pb- and Sn-based perovskite solar cells in model organism Danio rerio.

Intensive development of organometal halide perovskite solar cells has lead to a dramatic surge in power conversion efficiency up to 20%. Unfortunately, the most efficient perovskite solar cells all contain lead (Pb), which is an unsettling flaw that leads to severe environmental concerns and is therefore a stumbling block envisioning their large-scale application. Aiming for the retention of favorable electro-optical properties, tin (Sn) has been considered the most likely substitute. Preliminary studies have however shown that Sn-based perovskites are highly unstable and, moreover, Sn is also enlisted as a harmful chemical, with similar concerns regarding environment and health. To bring more clarity into the appropriateness of both metals in perovskite solar cells, we provide a case study with systematic comparison regarding the environmental impact of Pb- and Sn-based perovskites, using zebrafish (Danio Rerio) as model organism. Uncovering an unexpected route of intoxication in the form of acidification, it is shown that Sn based perovskite may not be the ideal Pb surrogate.

A Universal Deposition Protocol for Planar Heterojunction Solar Cells with High Efficiency Based on Hybrid Lead Halide Perovskite Families.

A robust and expedient gas quenching method is developed for the solution deposition of hybrid perovskite thin films. The method offers a reliable standard practice for the fabrication of a non-exhaustive variety of perovskites exhibiting excellent film morphology and commensurate high performance in both regular and inverted structured solar cell architectures.

Perovskite-perovskite tandem photovoltaics with optimized band gaps.

We demonstrate four- and two-terminal perovskite-perovskite tandem solar cells with ideally matched band gaps. We develop an infrared-absorbing 1.2-electron volt band-gap perovskite, FA0.75Cs0.25Sn0.5Pb0.5I3, that can deliver 14.8% efficiency. By combining this material with a wider-band gap FA0.83Cs0.17Pb(I0.5Br0.5)3 material, we achieve monolithic two-terminal tandem efficiencies of 17.0% with >1.65-volt open-circuit voltage. We also make mechanically stacked four-terminal tandem cells and obtain 20.3% efficiency. Notably, we find that our infrared-absorbing perovskite cells exhibit excellent thermal and atmospheric stability, not previously achieved for Sn-based perovskites. This device architecture and materials set will enable "all-perovskite" thin-film solar cells to reach the highest efficiencies in the long term at the lowest costs.