Search

In this paper, we report the fabrication and characterization of high-quality distributed Bragg reflectors (DBRs) deposited by low-energetic thermal evaporation. This technique allows deposition of high-quality thin films with an accurate control of thickness at the nanoscale. We investigated, for the first time, the use of tungsten oxide (WO3) and calcium fluoride (CaF2) as high (2.15) and low (1.4) refractive index materials, respectively, for DBR fabrication. They consist of nine pairs of WO3/CaF2 layers, with a central wavelength λc tuned at 640 nm and a maximum reflectance of 99.6%. A passive microcavity consisting of a λ/2 thick spacer of CaF2 sandwiched between 8.5 pairs of WO3/CaF2 has also been fabricated. We investigated the optical behavior of this microresonator after post-deposition thermal treatment at different temperatures, showing a blue shift of the resonant mode and an increase in its intensity. Indeed, heat treatment induced a compaction of the DBR multilayer, leading to more defined and clearer interfaces, as demonstrated by scanning electron microscopy cross section measurements. This behavior results in an improvement of the microcavity quality factor (up to 400) as the temperature increases, which is currently the highest value obtained for microcavities obtained by thermal evaporation processes, so far. These results represent a further step forward toward the development of high-reflectivity mirrors and high-quality microresonators fabricated by the low-energy deposition technique. Such a structure could find extensive application in nano-photonics, optoelectronics, and sensors, even based on soft organic materials.

Highly conductive PEDOT:PSS is one of the most promising materials for indium tin oxide (ITO) substitution in printed electronics. Here, we report the development and optimisation of two PEDOT:PSS ink formulations for the fabrication of inkjet-printed transparent conductive layers. Starting from aqueous commercial solutions, co-solvents and a non-ionic surfactant were employed to modify the surface tension, improve the wetting capability of the ink, and obtain uniform and homogeneous thin films. In particular, the quantities of ethanol and surfactant were systematically adjusted to determine the optimal conditions for inkjet printing. The results demonstrate that a surface tension value between 28 and 40 mN/m and approximately 40 vol.% of a low-boiling-point co-solvent are fundamental to ensure the proper wetting of the glass substrate and a quick-drying process that confers uniformity to the printed thin film. The printed PEDOT:PSS thin films show good morphological, optical, and electrical properties that are similar to those observed for the corresponding spin-coated layers. The organic light-emitting diodes (OLEDs) fabricated with the inkjet-printed PEDOT:PSS electrodes showed a maximum quantum efficiency of 5.5% and maximum current efficiency of 15 cd/A, which is comparable to spin-coated reference devices. These results demonstrate the great potential of polymeric electrodes for the fabrication of high-efficiency printed OLED devices that are compatible with flexible and stretchable substrates.

Top-emitting OLEDs (TOLEDs) represent a promising technology for the development of next-generation flexible and rollable displays, thanks to their improved light outcoupling and their compatibility with opaque substrates. Metal thin films are the most used electrodes for the manufacturing of TOLEDs, but they show poor resistance to mechanical deformation, which compromises the long-term durability of flexible devices. This paper reports the exploitation of a dielectric mirror (DBR) based on seven pairs of TiO2 and SiO2 combined with a polymeric electrode as an alternative to the bottom metal electrode in flexible TOLEDs. The DBR showed a maximum reflectivity of 99.9% at about 550 nm, and a stop-band width of about 200 nm. The reflectivity remained unchanged after bending and treatment with water and solvents. Green TOLED devices were fabricated on top of DBRs, and demonstrated good stability in terms of electro-optical and colorimetric characteristics, according to varying viewing angles. These results demonstrate that the combination of the flexible DBR with the polymeric anode is an interesting strategy for improving the durability of flexible TOLEDs for display applications, implemented on different kinds of free-standing ultra-thin substrates.

Two dimensional (2D) perovskites are promising materials for photonic applications, given their outstanding nonlinear optical properties, ease of fabrication and versatility. In particular, exploiting their high oscillator strength, the crystalline form of 2D perovskites can be used as excitonic medium in optical microcavities, allowing for the study of their optical properties in the strong light-matter coupling regime. While polariton condensation has been observed in different materials at room temperature, here we observe for the first time two distinct threshold processes in a 2D perovskite, a material that has never shown spontaneous phase transition up to now. In particular, we demonstrate lasing from the bi-exciton state which contributes to populate the lower polariton branch and, at higher excitation powers, eventually leads to the formation of a polariton condensate. The emission linewidth narrowing and a spatial coherence over 50 x 50 um2 area are the smoking gun, the formation of a quantum coherent state in 2D hybrid perovskite. Our results not only show the formation of a polariton condensate in 2D perovskites but they are also crucial for the understanding of the physical mechanisms that leads to coherent phase transition in perovskite-based polariton microcavities.

Electrochromic devices (ECDs) represent one of the most promising energy saving and solar control technology for the market of energy-efficient building and optoelectronic devices. A continuous and intense effort is currently devoted to the development of effective solid-state ECDs and their integration in multifunctional systems, such as photoelectrochromics. Here, the fabrication of simplified all-solid-state WO3 based ECDs on single-substrate is reported, demonstrating how the rational design of highly interconnected WO3 columnar nanostructures with Nafion polymer matrix remarkably decreases the charge transport barrier at the hybrid electrolyte/electrochromic interface (EEI), thus determining an impressive improvement of overall device performances. The soft polymer substrate of the electrolyte plays a key role on the formation of WO3 pillar-like structures and on the increase of interfacial contact area by affecting the vacuum-deposition WO3 growth. Apart from providing higher transmittance in bleached state, the resulting device, entirely manufactured at room temperature by bottom-up process, exhibits lower activation voltages (0.5–3 V) and faster switching kinetics (5–10 s) compared with monolithic ECDs based on both bulk and mesoporous WO3 films. Furthermore, the enhanced EEI enables the scale-up on large area and flexible substrate ensuring simultaneously a wide optical contrast (ΔT = 70%), and high coloration efficiency.

Large single-crystal flakes of 2D perovskite are able to sustain strong polariton nonlinearities at room temperature., Polaritonic devices exploit the coherent coupling between excitonic and photonic degrees of freedom to perform highly nonlinear operations with low input powers. Most of the current results exploit excitons in epitaxially grown quantum wells and require low-temperature operation, while viable alternatives have yet to be found at room temperature. We show that large single-crystal flakes of two-dimensional layered perovskite are able to sustain strong polariton nonlinearities at room temperature without the need to be embedded in an optical cavity formed by highly reflecting mirrors. In particular, exciton-exciton interaction energies are shown to be spin dependent, remarkably similar to the ones known for inorganic quantum wells at cryogenic temperatures, and more than one order of magnitude larger than alternative room temperature polariton devices reported so far. Because of their easy fabrication, large dipolar oscillator strengths, and strong nonlinearities, these materials pave the way for realization of polariton devices at room temperature.

We report a double emission layer structure, incorporating a double phosphorescent ldquoguest/host systemrdquo of tris (1-Phenylisoquinoline) iridium (III) (Ir(piq)3) 15wt% doped into two different host molecules, 4,4'-bis[N-(1-napthyl)-N-phenyl-amino] biphenyl (NPB) and bis-(2-methyl-8-quinolinolato)-4-(phenyl-phenolato) aluminum-(III) (BAlq). The proposed configuration reduces the efficiency rolloff, providing high stability at high operating luminance. A maximum current efficiency of 9.5 cd/A, at a driving voltage of 3 V, and a remarkably long device lifetime of more than 2500 h at a starting luminance of 6000 cd/m2 have been measured.

Extremely low voltage white organic light-emitting devices (WOLEDs) with fluorescent emitters are realized exploiting p-i-n structure. White light is obtained by two complementary colors system, in which the yellow and the blue emitting components are based on 5,6,11, 12-tetraphenylnaphthacene (rubrene) and 4,4-bis-2, 2-diphenylvinyl-1, 1-spirobiphenyl (Spiro-DPVBi), respectively. The effects on the device performances of various electron blocking layers and hosts for rubrene are discussed. The best device shows a luminance of 1000 cd/m(2) at bias of as low as 2.9 V, and 10 000 cd/m(2) at 4.7 V with a maximum power efficiency of 8.7 lm/W. The Commission Internationale de l'Eclairage chromaticity coordinates change from (0.36, 0.45) at 1000 cd/m(2) to (0.33, 0.42) at 10 000 cd/m(2) showing high color stability. (C) 2008 American Institute of Physics.

We demonstrate efficient red phosphorescent organic light emitting diodes with a bipolar emission structure (D-EML) formed by two different layers doped with a red phosphorescent dye. Due to its self-balancing character, the recombination zone is shifted far from the emission/carrier-blocking-layer interfaces. This prevents the accumulation of carriers at the interfaces and reduces the triplet-triplet annihilation, resulting in an improved efficiency of the D-EML device compared with the standard single-EML architecture. However, a current efficiency of 8.4 cd A(-1) at 10 mA cm(-2) is achieved in the D-EML device compared with 3.7 cd A(-1) in the single-EML device.