Redefining artificial lighting through spectral engineering of light sources for well-being.

Light-emitting diodes (LEDs) have revolutionized artificial lighting, but also exposed the detrimental health effects that stem from insufficient exposure to natural light. Human-centric artificial lighting requires both visual quality and circadian lighting performance that mimics daylight's evolving spectral power distribution (SPD). Here, we present a color-tunable LED-based light source that achieves SPDs similar to various conditions of daylight and incandescent lighting over the range of visible wavelengths.

Delayed Luminescence in 2-Methyl-5-(penta(9-carbazolyl)phenyl)-1,3,4-oxadiazole Derivatives.

2,5-Diphenyl-1,3,4-oxadiazole has been widely used as an acceptor portion of donor-acceptor fluorophores that exhibit thermally activated delayed fluorescence (TADF), but analogous 2-alkyl-5-phenyl-1,3,4-oxadiazoles have been much less widely investigated. Here the properties of carbazole-substituted 2-methyl-5-phenyl-1,3,4-oxadiazoles are compared to those of their 2,5-diphenyl analogues. The fluorescence of each of the former compounds is blue-shifted by ca.

Skin-like low-noise elastomeric organic photodiodes.

Stretchable optoelectronics made of elastomeric semiconductors could enable the integration of intelligent systems with soft materials, such as those of the biological world. Organic semiconductors and photodiodes have been engineered to be elastomeric; however, for photodetector applications, it remains a challenge to identify an elastomeric bulk heterojunction (e-BHJ) photoactive layer that combines a low Young’s modulus and a high strain at break that yields organic photodiodes with low electronic noise values and high photodetector performance. Here, a blend of an elastomer, a donor-like polymer, and an acceptor-like molecule yields a skin-like e-BHJ with a Young’s modulus of a few megapascals, comparable to values of human tissues, and a high strain at break of 189%.

Increasing Volume in Conjugated Polymers to Facilitate Electrical Doping with Phosphomolybdic Acid.

Molecular p-type electrical dopants have been proven useful to fine-tune the optoelectronic properties of bulk organic semiconductors and their interfaces. Here, the volume in polymer films and its role in solution-based electrical p-type doping using phosphomolybdic acid (PMA) are studied. The polymer film volume was controlled using two approaches.

Large-area low-noise flexible organic photodiodes for detecting faint visible light.

Silicon photodiodes are the foundation of light-detection technology; yet their rigid structure and limited area scaling at low cost hamper their use in several emerging applications. A detailed methodology for the characterization of organic photodiodes based on polymeric bulk heterojunctions reveals the influence that charge-collecting electrodes have on the electronic noise at low frequency. The performance of optimized organic photodiodes is found to rival that of low-noise silicon photodiodes in all metrics within the visible spectral range, except response time, which is still video-rate compatible.

Control of Singlet Emission Energy in a Diphenyloxadiazole Containing Fluorophore Leading To Thermally Activated Delayed Fluorescence.

2-(4-(9,9-Dimethylacridin-10(9)-yl)phenyl)-5-phenyl-1,3,4-oxadiazole has an energy difference between the lowest excited singlet and triplet states (Δ ) of ca. 0.24 eV.

Host-Free Yellow-Green Organic Light-Emitting Diodes with External Quantum Efficiency over 20% Based on a Compound Exhibiting Thermally Activated Delayed Fluorescence.

Thermally activated delayed fluorescent (TADF) materials are advantageous as emitters in organic light-emitting diodes (OLEDs) due to their ability to utilize all excited states formed by charge recombination for light emission, potentially leading to 100% internal quantum efficiency. As in conventional fluorescent or phosphorescent OLEDs, TADF emitters are commonly doped at a relatively low concentration in a host matrix. However, increasing evidence suggests that balanced ambipolar transport properties and small aggregation-induced fluorescence quenching allow TADF emitters to be used alone in so-called host-free OLEDs.

Langmuir-Blodgett Thin Films of Diketopyrrolopyrrole-Based Amphiphiles.

We report on two π-conjugated donor-acceptor-donor (D-A-D) molecules of amphiphilic nature, aiming to promote intermolecular ordering and carrier mobility in organic electronic devices. Diketopyrrolopyrrole was selected as the acceptor moiety that was disubstituted with nonpolar and polar functional groups, thereby providing the amphiphilic structures. This structural design resulted in materials with a strong intermolecular order in the solid state, which was confirmed by differential scanning calorimetry and polarized optical microscopy.

Stable organic thin-film transistors.

Organic thin-film transistors (OTFTs) can be fabricated at moderate temperatures and through cost-effective solution-based processes on a wide range of low-cost flexible and deformable substrates. Although the charge mobility of state-of-the-art OTFTs is superior to that of amorphous silicon and approaches that of amorphous oxide thin-film transistors (TFTs), their operational stability generally remains inferior and a point of concern for their commercial deployment. We report on an exhaustive characterization of OTFTs with an ultrathin bilayer gate dielectric comprising the amorphous fluoropolymer CYTOP and an AlO:HfO nanolaminate.

Efficient Colorful Perovskite Solar Cells Using a Top Polymer Electrode Simultaneously as Spectrally Selective Antireflection Coating.

Organometal halide perovskites have shown excellent optoelectronic properties and have been used to demonstrate a variety of semiconductor devices. Colorful solar cells are desirable for photovoltaic integration in buildings and other aesthetically appealing applications. However, the realization of colorful perovskite solar cells is challenging because of their broad and large absorption coefficient that commonly leads to cells with dark-brown colors.

Solution-based electrical doping of semiconducting polymer films over a limited depth.

Solution-based electrical doping protocols may allow more versatility in the design of organic electronic devices; yet, controlling the diffusion of dopants in organic semiconductors and their stability has proven challenging. Here we present a solution-based approach for electrical p-doping of films of donor conjugated organic semiconductors and their blends with acceptors over a limited depth with a decay constant of 10-20 nm by post-process immersion into a polyoxometalate solution (phosphomolybdic acid, PMA) in nitromethane. PMA-doped films show increased electrical conductivity and work function, reduced solubility in the processing solvent, and improved photo-oxidative stability in air.

Organic Field-Effect Transistors with a Bilayer Gate Dielectric Comprising an Oxide Nanolaminate Grown by Atomic Layer Deposition.

We report on top-gate OFETs with a bilayer gate dielectric comprising an AlO /HfO nanolaminate layer grown by atomic layer deposition and an amorphous fluoro-polymer layer (CYTOP). Top-gate OFETs display average carrier mobility values of 0.9 ± 0.

Semiconductor Polymer/Top Electrode Interface Generated by Two Deposition Methods and Its Influence on Organic Solar Cell Performance.

In this Research Article, the effect of two techniques for top-electrode deposition in organic photovoltaics (OPVs) cells with the configuration ITO/PEDOT:PSS/PTB7-Th:PCBM/PFN/top-electrode is analyzed. One deposition was made by evaporation under high vacuum, meanwhile the other was carried out at normal room atmosphere; for the former, a double layer of Ca and the eutectic alloy Field's metal (FM) was thermally evaporated, while for the latter FM was deposited just by melting and dropping it on top of the delimited active area at temperatures about 90 °C. The average short-circuit photocurrent density, open circuit voltage and fill factor for devices with either Ca/FM (evaporated) or FM (by dripping) cathode, were very similar: around 13.

A Study on Reducing Contact Resistance in Solution-Processed Organic Field-Effect Transistors.

We report on the reduction of contact resistance in solution-processed TIPS-pentacene (6,13-bis(triisopropylsilylethynyl)pentacene) and PTAA (poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]) top-gate bottom-contact organic field-effect transistors (OFETs) by using different contact-modification strategies. The study compares the contact resistance values in devices that comprise Au source/drain electrodes either treated with 2,3,4,5,6-pentafluorothiophenol (PFBT), or modified with an evaporated thin layer of the metal-organic molecular dopant molybdenum tris-[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd)3), or modified with a thin layer of the oxide MoO3. An improved performance is observed in devices modified with Mo(tfd)3 or MoO3 as compared to devices in which Au electrodes are modified with PFBT.

Experimental investigation of defect-assisted and intrinsic water vapor permeation through ultrabarrier films.

In the development of ultrabarrier films for packaging electronics, the effective water vapor transmission rate is a combination of permeation through pinhole defects and the intrinsic permeation through the actual barrier film. While it is possible to measure the effective permeation rate through barriers, it is important to develop a better understanding of the contribution from defects to the overall effective barrier performance. Here, we demonstrate a method to investigate independently defect-assisted permeation and intrinsic permeation rates by observing the degradation of a calcium layer encapsulated with a hybrid barrier film, that is, prepared using atomic layer deposition (ALD) and plasma enhanced deposition (PECVD).

Simultaneous cross-linking and p-doping of a polymeric semiconductor film by immersion into a phosphomolybdic acid solution for use in organic solar cells.

Poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) is shown to be simultaneously cross-linked and p-doped when immersed into a phosphomolybdic acid solution, yielding conductive films with low solubility that can withstand the solution processing of subsequent photoactive layers. Such a modified PCDTBT film serves to improve hole collection and limit carrier recombination in organic solar cells.

Organometallic dimers: application to work-function reduction of conducting oxides.

The dimers of pentamethyliridocene and ruthenium pentamethylcyclopentadienyl mesitylene, (IrCp*Cp)2 and (RuCp*mes)2, respectively, are shown here to be effective solution-processable reagents for lowering the work functions of electrode materials; this approach is compared to the use of solution-deposited films of ethoxylated poly(ethylenimine) (PEIE). The work functions of indium tin oxide (ITO), zinc oxide, and gold electrodes can be reduced to 3.3-3.

Stable low-voltage operation top-gate organic field-effect transistors on cellulose nanocrystal substrates.

We report on the performance and the characterization of top-gate organic field-effect transistors (OFETs), comprising a bilayer gate dielectric of CYTOP/Al2O3 and a solution-processed semiconductor layer made of a blend of TIPS-pentacene:PTAA, fabricated on recyclable cellulose nanocrystal-glycerol (CNC/glycerol) substrates. These OFETs exhibit low operating voltage, low threshold voltage, an average field-effect mobility of 0.11 cm(2)/(V s), and good shelf and operational stability in ambient conditions.

Tetracyano isoindigo small molecules and their use in n-channel organic field-effect transistors.

N,N'-Dihexyl-6,6'-dicyanoisoindigo, N,N'-didecyl-5,5',6,6'-tetracyanoisoindigo, N,N'-dihexyl-5,5',6,6'-tetracyanoisoindigo, and N,N'-dihexyl-5,5',6,6'-tetracyanothienoisoindigo have been synthesised in moderate yields by the reaction of corresponding di and tetrabromo species with CuCN, with microwave heating leading to higher yields and fewer side products for the tetrasubstituted species. Di- and tetracyano substitution anodically shifts the molecular reduction potential relative to the unsubstituted cores by ca. 0.

Organic photovoltaic cells with stable top metal electrodes modified with polyethylenimine.

Efficient organic photovoltaic cells (OPV) often contain highly reactive low-work-function calcium electron-collecting electrodes. In this work, efficient OPV are demonstrated in which calcium electrodes were avoided by depositing a thin layer of the amine-containing nonconjugated polymer, polyethylenimine (PEIE), between the photoactive organic semiconductor layer and stable metal electrodes such as aluminum, silver, or gold. Devices with structure ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/poly(3-hexylthiophene):indene-C60-bis-adduct (P3HT:ICBA)/PEIE/Al demonstrated overall photovoltaic device performance comparable to devices containing calcium electron-collecting electrodes, ITO/PEDOT:PSS/P3HT:ICBA/Ca/Al, with open-circuit voltage of 775±6 mV, short-circuit current density of 9.

Systematic reliability study of top-gate p- and n-channel organic field-effect transistors.

We report on a systematic investigation on the performance and stability of p-channel and n-channel top-gate OFETs, with a CYTOP/Al2O3 bilayer gate dielectric, exposed to controlled dry oxygen and humid atmospheres. Despite the severe conditions of environmental exposure, p-channel and n-channel top-gate OFETs show only minor changes of their performance parameters without undergoing irreversible damage. When correlated with the conditions of environmental exposure, these changes provide new insight into the possible physical mechanisms in the presence of oxygen and water.

Stable organic field-effect transistors for continuous and nondestructive sensing of chemical and biologically relevant molecules in aqueous environment.

The use of organic field-effect transistors (OFETs) as sensors in aqueous media has gained increased attention for environmental monitoring and medical diagnostics. However, stable operation of OFETs in aqueous media is particularly challenging because of electrolytic hydrolysis of water, high ionic conduction through the analyte, and irreversible damage of organic semiconductors when exposed to water. To date, OFET sensors have shown the capability of label-free sensing of various chemical/biological species, but they could only be used once because their operational stability and lifetime while operating in aqueous environments has been poor, and their response times typically slow.

Recyclable organic solar cells on cellulose nanocrystal substrates.

Solar energy is potentially the largest source of renewable energy at our disposal, but significant advances are required to make photovoltaic technologies economically viable and, from a life-cycle perspective, environmentally friendly, and consequently scalable. Cellulose nanomaterials are emerging high-value nanoparticles extracted from plants that are abundant, renewable, and sustainable. Here, we report on the first demonstration of efficient polymer solar cells fabricated on optically transparent cellulose nanocrystal (CNC) substrates.

Ultrafast nonlinear mirrors with broad spectral and angular bandwidths in the visible spectral range.

A novel nonlinear mirror structure is presented. A 23 nm-thick Au thin film separated from a 100 nm-thick Ag film by a dielectric spacer is used to drive the nonlinear optical response of the mirror. The linear and nonlinear optical properties of the mirror can be tuned by optimizing its layer thickness distribution.