In this study, we investigate the lasing properties of 4,4′-bis[(N-carbazole)styryl]biphenyl thin films under electrical pumping. The electroluminescent devices incorporate a mixed-order distributed feedback SiO2 grating into an organic light-emitting diode structure and emit blue lasing. The results provide an indication of lasing by direct injection of current into an organic thin film through selection of a high-gain organic semiconductor showing clear separation of the lasing wavelength from significant triplet and polaron absorption and design of a proper feedback structure with low losses at high current densities. This study represents an important advance toward a future organic laser diode technology.
Organic semiconductor laser operating in the quasi-CW regime at 80 MHz and under 30 ms long pulse photoexcitation is demonstrated.
Near-infrared (NIR) organic light-emitting devices have aroused increasing interest because of their potential applications such as informationsecured displays, photodynamic therapy, and optical telecommunication. While thermally activated delayed fluorescent (TADF) emitters have been used in a variety of high-performance organic light-emitting diodes (OLEDs) emitting in the visible spectral range, efficient NIR TADF materials have been rarely reported. Herein, we designed and synthesized a novel solution-processable NIR TADF dimeric borondifluoride curcuminoid derivative with remarkable photophysical, electroluminescence and amplified spontaneous emission properties. This dye was specifically developed to shift the emission of borondifluoride curcuminoid moiety toward longer wavelengths in the NIR region while keeping a high photoluminescence quantum yield. The most efficient OLED fabricated in this study exhibits a maximum external quantum efficiency of 5.1% for a maximum emission wavelength of 758 nm, which ranks among the highest performance for NIR electroluminescence. In addition, this NIR TADF emitter in doped thin films displays amplified spontaneous emission above 800 nm with a threshold as low as 7.5 μJ/cm 2 , providing evidence that this material is suitable for the realization of high-performance NIR organic semiconductor lasers.
High-performance non-volatile memory that can operate under various mechanical deformations such as bending and folding is in great demand for the future smart wearable and foldable electronics. Here we demonstrate non-volatile solution-processed ferroelectric organic field-effect transistor memories operating in p-and n-type dual mode, with excellent mechanical flexibility. Our devices contain a ferroelectric poly(vinylidene fluoride-cotrifluoroethylene) thin insulator layer and use a quinoidal oligothiophene derivative (QQT(CN)4) as organic semiconductor. Our dual-mode field-effect devices are highly reliable with data retention and endurance of 46,000 s and 100 cycles, respectively, even after 1,000 bending cycles at both extreme bending radii as low as 500 mm and with sharp folding involving inelastic deformation of the device. Nano-indentation and nano scratch studies are performed to characterize the mechanical properties of organic layers and understand the crucial role played by QQT(CN)4 on the mechanical flexibility of our devices.
A study of triplet-triplet exciton annihilation and nonradiative decay in films of iridium(III)-centered phosphorescent dendrimers is reported. The average separation of the chromophore was tuned by the molecular structure and also by blending with a host material. It was found that triplet exciton hopping is controlled by electron exchange interactions and can be over 600 times faster than phosphorescence quenching. Nonradiative decay occurs by weak dipole-dipole interactions and is independent of exciton diffusion, except in very thin films (<20 nm) where surface quenching dominates the decay. DOI: 10.1103/PhysRevLett.100.017402 PACS numbers: 78.66.Qn, 73.50.Gr, 78.40.Me, 78.55.Kz Excitation energy transfer is an important process in organic semiconductors and has to be taken into account when designing new optoelectronic materials and devices. In photovoltaic devices, the neutral excited state is generated by light absorption and must diffuse to a heterojunction with another material to be separated into charge carriers and so provide photocurrent. In organic light emitting diodes (OLEDs), exciton diffusion can lead to a decrease of the electroluminescence efficiency due to quenching of the emitting state by intermolecular interactions and defects [1,2], exciton interactions with charge carriers [2,3], and exciton-exciton annihilation [4,5]. These quenching effects are more pronounced in phosphorescent OLEDs than in fluorescent devices because of the longer excited state lifetime. Nevertheless, phosphorescent OLEDs show much higher internal quantum efficiencies due to their ability to convert both singlet and triplet excitons into light [6 -8]. A photoluminescence (PL) study of iridium(III) complexes dispersed into a wide energy gap host suggested that intermolecular quenching of phosphorescence in films is controlled by the dipole-dipole interactions between emitters [9]. However, the impact of exciton migration on phosphorescence quenching has not been considered.In this Letter, we compare the dynamics of triplet exciton diffusion and quenching in several fac-tris(2-phenylpyridyl)iridium(III) [Ir ppy 3 ]-cored phosphorescent dendrimers. Dendrimers provide a convenient way of changing the spacing of the core chromophores in the solid state and hence of studying the effect of spacing on the physics of exciton diffusion and light emission. The triplet exciton diffusion rates are extracted from the measurements of triplet-triplet annihilation and have an exponential dependence on chromophore spacing. This shows that diffusion is controlled by nearest-neighbor electron exchange interactions [10]. Nonradiative decay in 180 nm thick films is governed by much weaker dipole-dipole interactions and does not depend on the triplet diffusion rate. In much thinner films (<20 nm), phosphorescence quenching is found to depend on exciton diffusion and can be modeled using the diffusion equation together with quenching at the film surface.Five green Ir ppy 3 -cored dendrimers were used in this study and their chemical st...
Fabrication of p-n structures is a key issue in a number of electronic devices, including rectifying diodes, solar cells, and bipolar transistors. Complementary metal oxide semiconductor (CMOS) technology based on the integration of discrete p-and n-channel field-effect transistors (FETs) on a same substrate has been widely used in a variety of electronic applications and enables the fabrication of integrated circuits with low-power dissipation and high operational stability. In the growing field of organic electronics, efficient large-scale organic CMOS integrated circuits [1,2] as well as lateral p-n diodes [3] have already been realized by thermally evaporating p-and n-type semiconducting organic molecules separately through shadow masks. Alternatively, fabrication of organic inverters by inkjet printing, self-assembly, or spin-coating has also been reported and has demonstrated the enormous potential of solution processing for low-cost, large-area, flexible electronics. [4][5][6][7][8][9][10] However, in spite of remarkable recent advances in high-mobility organic FET materials [11][12][13][14] and nanostructuration techniques, [15] there has still been no reliable approach for effectively patterning p-n bipolar and CMOS microstructures made from solutionprocessible organic semiconductors. We have addressed this issue by exploiting the unique charge-transport properties of the solution-processible dicyanomethylene-substituted quinoidal quaterthiophene [QQT(CN)4], which can be converted from an ambipolar p-type-dominant to n-type semiconductor by either thermal annealing or direct laser writing.The chemical structure of the QQT(CN)4 oligomer is shown in the inset of Figure 1a. Quinoidal oligothiophene derivatives generally show a low highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy bandgap and are very promising candidates for n-type or single-component ambipolar organic FETs. [16][17][18][19][20][21] We have fabricated top-contact bottom-gate organic FETs under ambient conditions by spin-coating a QQT(CN)4 thin film on top of a polyimide gate-dielectric layer and using chromium drain/source electrodes. The transfer and output characteristics displayed in Figure 1 show that the device based on as-prepared QQT(CN)4 thin film exhibits an ambipolar behavior with a predominant hole-transport character. The charge-carrier field-effect mobilities were extracted from the transfer characteristics in the saturated regions. Hole and electron mobilities were measured to be 2 AE 1 Â 10 À3 and 4 AE 1 Â 10 À4 cm 2 V À1 s À1 , respectively. Compared to the QQT(CN)4-based device performance obtained using octadecyltrichlorosilane-treated SiO 2 as gate-dielectric layer and gold top-contact electrodes, [16] these mobility values are substantially lower. However, the use of a polyimide gate dielectric presents the significant advantage of strongly reducing COMMUNICATION www.advmat.de www.MaterialsViews.com
wileyonlinelibrary.com COMMUNICATIONoverlap between emission and triplet absorption in BN-PFO. [ 10 ] Therefore, the development of organic laser dyes with less spectral overlap between the excited-state absorption and emission is crucial for the realization of cw and quasi-cw lasing with low threshold.In our group, we have continuously investigated the optical and amplifi ed spontaneous emission (ASE) characteristics of many organic materials with the aim of realizing electrically pumped organic laser diodes. [21][22][23][24][25][26][27] Among them, 4,4′-bis[( Ncarbazole)styryl]biphenyl (BSBCz) is one of the most promising candidates because a vacuum-deposited fi lm of the host material 4,4′-bis( N -carbazolyl)-1,1′-biphenyl (CBP) blended with 6 wt% BSBCz, the chemical structures of which are shown in Figure 1 a, possesses outstanding optical and ASE characteristics, such as a high photoluminescence quantum yield ( Φ PL ) of nearly 100% and short PL lifetime ( τ PL ) of about 1.0 ns, leading to a large radiative decay constant ( k r ) of about 10 9 s −1 and low ASE threshold energy of about 0.3 µJ cm −2 . [ 23,26 ] In this paper, we report quasi-cw surface-emitting lasing in a distributed feedback (DFB) device based on this BSBCz:CBP blend fi lm. In this laser device, we obtained the highest repetition rate (up to 8 MHz) and the lowest threshold (on the order of 0.25 µJ cm −2 ) ever reported for a quasi-cw laser based on organic thin-fi lm systems. The incorporation of triplet quenchers is not necessary in our blend fi lm because of its high Φ PL and no signifi cant spectral overlap between the emission and triplet absorption of BSBCz. [ 24 ] In a DFB structure, a laser oscillation takes place when the following Bragg condition is satisfi ed: mλ Bragg = 2 n eff Λ , where m is the order of diffraction, λ Bragg is the Bragg wavelength, n eff is the effective refractive index of the gain medium, and Λ is the period of the grating. [ 28,29 ] When considering a second-order mode ( m = 2), the grating period is calculated to be Λ = 280 nm using the reported n eff and λ Bragg for BSBCZ. [ 21,22 ] A grating with Λ = 280 nm provides surface-emitting lasing in a direction normal to the substrate plane as shown in Figure 1 b. Although a second-order grating typically leads to a higher lasing threshold compared to a fi rst-order grating, surface-emitting lasing using a second-order grating is suitable for the fabrication of electrically pumped organic laser diodes having an organic light-emitting diode structure showing the same surface emission. [ 30,31 ] Using electron beam lithography and reactive ion etching, such gratings were directly engraved onto silicon dioxide surfaces over a 5×5 mm 2 area. Figures 1 c,d shows scanning electron microscopy (SEM) images of a representative grating fabricated in this study. We obtained Λ = 280 ± 2 nm and a grating depth of d = 70 ± 5 nm from the SEM images, which are in perfect Since the discovery of organic solid-state lasers, [1][2][3][4][5][6] great efforts have been devoted to th...
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