Highly efficient multilayer organic pure-blue-light emitting diodes with substituted carbazoles compounds in the emitting layer

Bright blue organic light-emitting diodes (OLEDs) based on 1,4,5,8,N-pentamethylcarbazole (PMC) and on dimer of N-ethylcarbazole (N,N'-diethyl-3,3'-bicarbazyl) (DEC) as emitting layers or as dopants in a 4,4'-bis(2,2'-diphenylvinyl)-1,1'-biphenyl (DPVBi) matrix are described. Pure blue-light with the C.I.E. coordinates x = 0.153 y = 0.100, electroluminescence efficiency \eta_{EL} of 0.4 cd/A, external quantum efficiency \eta_{ext.} of 0.6% and luminance L of 236 cd/m2 (at 60 mA/cm2) were obtained with PMC as an emitter and the 2,9-dimethyl-4,7-diphenyl-1,10-phenantroline (BCP) as a hole-blocking material in five-layer emitting devices. The highest efficiencies \eta_{EL.} of 4.7 cd/A, and \eta_{ext} = 3.3% were obtained with a four-layer structure and a DPVBi DEC-doped active layer (CIE coordinates x = 0.158, y=0.169, \lambda_{peak} = 456 nm). The \eta_{ext.} value is one the highest reported at this wavelength for blue OLEDs and is related to an internal quantum efficiency up to 20%

High-gain AlGaAs/GaAs double heterojunction Darlington phototransistors for optical neural networks

High-gain MOCVD-grown (metal-organic chemical vapor deposition) AlGaAs/GaAs/AlGaAs n-p-n double heterojunction bipolar transistors (DHBTs) and Darlington phototransistor pairs are provided for use in optical neural networks and other optoelectronic integrated circuit applications. The reduced base doping level used results in effective blockage of Zn out-diffusion, enabling a current gain of 500, higher than most previously reported values for Zn-diffused-base DHBTs. Darlington phototransitor pairs of this material can achieve a current gain of over 6000, which satisfies the gain requirement for optical neural network designs, which advantageously may employ neurons comprising the Darlington phototransistor pairs in series with a light source

An air-stable DPP-thieno-TTF copolymer for single-material solar cell devices and field effect transistors

Following an approach developed in our group to incorporate tetrathiafulvalene (TTF) units into conjugated polymeric systems, we have studied a low band gap polymer incorporating TTF as a donor component. This polymer is based on a fused thieno-TTF unit that enables the direct incorporation of the TTF unit into the polymer, and a second comonomer based on the diketopyrrolopyrrole (DPP) molecule. These units represent a donor–acceptor copolymer system, p(DPP-TTF), showing strong absorption in the UV–visible region of the spectrum. An optimized p(DPP-TTF) polymer organic field effect transistor and a single material organic solar cell device showed excellent performance with a hole mobility of up to 5.3 × 10–2 cm2/(V s) and a power conversion efficiency (PCE) of 0.3%, respectively. Bulk heterojunction organic photovoltaic devices of p(DPP-TTF) blended with phenyl-C71-butyric acid methyl ester (PC71BM) exhibited a PCE of 1.8%

Electroluminescence in ion gel gated organic polymer semiconductor transistors

This thesis reports the light emission in ion gel gated, thin film organic semiconductor transistors and investigates the light emission mechanism behind these devices. We report that ion gel gated organic polymer semiconductor transistors emit light when the drain source voltage is swept slightly beyond the energy gap of the polymer divided by the elementary charge (Vds > Eg/e). In particular, the light emission in poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT) polymer semiconductor, with 1-ethyl-3-methylimidazoliumbis (trifluoromethylsulfonyl) imide/ poly(styrene-block-ethylene oxide-block-styrene (EMIM TFSI/ SOS) ion gel as dielectric material is reported. The current-voltage characteristics corresponding to the light emission, where the systematic increase of the drain current, correlated with light emission is reported. In low voltage regime, (Vds < Eg/e), well saturated transistor characteristics are observed. By charge modulation spectroscopy (CMS) study we show that there is a prominent electrochemical doping occurring with gate voltages. Further, owing to the movement of ions with voltages, irrespective of the location of electrodes, we show that the ion gel, bilayer planar devices emit light in Vds > Eg/e regime (without any gate voltages), at room temperature. Based on the location of the recombination zone in the proximity of electron injecting electrode and CMS results showing prominent di ffusion of negative ions into the polymer layer, we conclude that the light emitting mechanism is akin to light emitting electrochemical cells (LECs). Even in the the transistor regime, where Vds << Eg/e, with the signatures of increasing drain current for fixed Vg and Vds values, we show that the transistor can not be of purely electrostatic operation alone. We study the fluorescence quenching of an operating bilayer device under a constant bias over a period of time and compare the results with the electroluminescence of the device and show that the formation of the p-n junction within the polymer layer due to the penetrated ions from the gel dielectric into the polymer semiconductor layer on the application of the voltage is the cause behind the light emission. We show that diffusivity of the cation (EMIM) is very low compared to the anion (TFSI). This is consistent with the fact that the recombination zone is near the electron injecting electrode in these devices. We have developed a theoretical model for the ions movement within the semiconductor polymer matrix governed by both diffusion and drift independently, for the bilayer, polymer ion gel planar, light emitting electrochemical cells. We have further developed a 2- dimensional numerical model based on the theoretical model and have compared the results of the numerical model with the results of a fluorescence probing of the bilayer device with time, at constant potential across the bilayer LEC and report that the drift coefficient of 1x10^-13 cm^2/V.s and a diffusion coefficient of 1 x 10^-15cm2/V.s for TFSI ions in F8BT matrix.Gates Cambridge Trust and Cambridge Overseas Trus

Unforeseen high temperature and humidity stability of FeCl3_3 intercalated few layer graphene

We present the first systematic study of the stability of the structure and electrical properties of FeCl$_3$ intercalated few-layer graphene to high levels of humidity and high temperature. Complementary experimental techniques such as electrical transport, high resolution transmission electron microscopy and Raman spectroscopy conclusively demonstrate the unforeseen stability of this transparent conductor to a relative humidity up to $100 \%$ at room temperature for 25 days, to a temperature up to 150\,^\circC in atmosphere and up to a temperature as high as 620\,^\circC in vacuum, that is more than twice higher than the temperature at which the intercalation is conducted. The stability of FeCl$_3$ intercalated few-layer graphene together with its unique values of low square resistance and high optical transparency, makes this material an attractive transparent conductor in future flexible electronic applications.Comment: Scientific Reports, volume 5, article no. 760

Recent advances in solid-state organic lasers

Organic solid-state lasers are reviewed, with a special emphasis on works published during the last decade. Referring originally to dyes in solid-state polymeric matrices, organic lasers also include the rich family of organic semiconductors, paced by the rapid development of organic light emitting diodes. Organic lasers are broadly tunable coherent sources are potentially compact, convenient and manufactured at low-costs. In this review, we describe the basic photophysics of the materials used as gain media in organic lasers with a specific look at the distinctive feature of dyes and semiconductors. We also outline the laser architectures used in state-of-the-art organic lasers and the performances of these devices with regard to output power, lifetime, and beam quality. A survey of the recent trends in the field is given, highlighting the latest developments in terms of wavelength coverage, wavelength agility, efficiency and compactness, or towards integrated low-cost sources, with a special focus on the great challenges remaining for achieving direct electrical pumping. Finally, we discuss the very recent demonstration of new kinds of organic lasers based on polaritons or surface plasmons, which open new and very promising routes in the field of organic nanophotonics

A novel DR/NIR T-shaped aiegen: Synthesis and x-ray crystal structure study

We developed a new benzodifuran derivative as the condensation product between 2,6-diamino-4-(4-nitrophenyl)benzo[1,2-b:4,5-b’]difuran-3,7-dicarboxylate and 3-hydroxy-2-naphthaldehyde. The intramolecular hydrogen-bond interactions in the terminal half-salen moieties produce a sterically encumbered highly conjugated main plane and a D-A-D (donor-acceptor-donor) T-shaped structure. The novel AIEgen (aggregation-induced enhanced emission generator) fulfils the requirement of RIR (restriction of intramolecular rotation) molecules. DR/NIR (deep red/near infrared) emission was recorded in solution and in the solid state, with a noteworthy photoluminescence quantum yield recorded on the neat crystals which undergo some mechanochromism. The crystal structure study of the probe from data collected at a synchrotron X-ray source shows a main aromatic plane π-stacked in a columnar arrangement