Partial Energy Transfer Research Articles

Photoluminescence of conjugated polymer blends at the nanoscale

Here we report on a combined photoluminescence and morphological study of a polymer-polymer blend composed of a copolymer of derivatives of polyspirobifluorene and polyfluorene (PBFF) and a derivative of polyphenylene vinylene (MDMO-PPV). Evidence of partial Forster energy transfer from PBFF to MDMO-PPV is revealed by steady-state and time-resolved photoluminescence. Atomic force microscopy (AFM) on blends with different weight ratios of host (PBFF) and guest (MDMO-PPV) polymers shows that in blends with a MDMO-PPV concentration higher than 10% two phases are present. The nature of the phases is identified by means of scanning near-field optical microscopy (SNOM). Using this technique the topography and the local photoluminescence of the thin film are simultaneously mapped with a spatial resolution of ∼100 nm. The local measurements reveal that the PL signal of MDMO-PPV is present in the whole surface, but with variations of the PL intensity and spectral shape. The correlation of the SNOM, time-resolved and steady-state photoluminescence measurements allows us to identify the two phases, one rich in PBFF and the other rich in MDMO-PPV. © 2011 The Royal Society of Chemistry.

White and Red Electroluminescence from a Single Copolymers of Fluorene and 4-Thienyl-2,1,3-Benzothiadiazole

We have developed efficient pure white- and red-light-emitting polymers through incorporating of low-band gap orange-light-emitting 4-thienyl-2,1,3-benzothiadiazole (BTT) moieties into the backbone of a blue-light-emitting poly(9,9-dioctylfluorene) (PFO). Partial or complete energy transfer from the blue electroluminescent PFO backbone to the orange- light-emitting unit occurred by carefully controlling the relative content of the low-band gap unit BTT in the resulting polymers. Pure white- and red-light electroluminescence from a single polymer has been obtained in a device with a configuration of ITO/PEDOT:PSS/polymer/Ca/Al. The highest external quantum efficiency is 2.4% with luminous efficiency 2.20 cd/A at the luminance of 793 cd/m2 for the red-light emitting copolymer PFO-BTT with emission peak at 607 nm for 15mol% BTT content, the highest external quantum efficiency is 0.5% at the luminance of 81 cd/m2 for the white emitting polymer with color coordinates (0.34, 0.34) very close to the CIE coordinates for National Television System Committee (NTSC) standard white-light emission (0.33,0.33).

White Light-Emitting Devices Based on Star-Shape Polymers with a Bisindolylmaleimide Core

A new type of star-shape polymers employing bisindolylmaleimide dye (2a−c) as the core and poly(2,7-fluorene) (PF) and/or poly(2,7-carbazole) (PC) as the arms were synthesized. These materials exhibited dual emissions consisting of an intensive blue luminescence from PF or PC and an orange emission from maleimide a result of partial energy transfer between the two. Highly efficient white light emitting devices were fabricated using a single emitting film made by spin-coating method. The electroluminescence (EL) properties of the devices were investigated from several directions, such as the loading amount of maleimide core, the concentration dependent spectral changes, the difference in the composition of arms, and the substituent effect in the indole segment, etc. A typical device based on the star-shape polymer MF001 containing 0.01 mol % of core exhibited a maximal luminous efficiency of 7.2 cd/A and external quantum efficiency of 3.2%. The device based on MMF001 with a methyl substituent on the indole...

Multicolour Self‐Assembled Fluorene Co‐Oligomers: From Molecules to the Solid State via White‐Light‐Emitting Organogels

Five fluorene-based co-oligomers have been prepared to study their self-assembly in a wide range of concentrations, from dilute solutions to the solid state. Subtle changes to the chemical structures, introduced to tune the emission colours over the entire visible range, induce strong differences in aggregation behaviour. Only two of the fluorescent co-oligomer derivatives self-assemble to form soluble fibrils from which fluorescent organogels emerge at higher concentrations. In contrast, the other compounds form precipitates. Mixed fluorescent co-oligomer systems exhibit partial energy transfer, which allows the creation of white-light-emitting gels. Finally, a mechanism for the hierarchical self-assembly of this class of materials is proposed based on experimental results and molecular modelling calculations.

RGB Emission through Controlled Donor Self‐Assembly and Modulation of Excitation Energy Transfer: A Novel Strategy to White‐Light‐Emitting Organogels

A white-light-emitting organogel is designed by the controlled self-assembly of a bischolesterol-functionalized OPV donor that allows slow energy migration and partial energy transfer in the gel state to an encapsulated acceptor. The white-light emission occurs because of the combination of blue-light emission from the OPV monomers, green-light emission from the OPV self-assembly, and red-light emission from the acceptor.

Eu2 + , Ce3 + Luminescence and Ce3 + → Eu2 + Energy-Transfer Studies on Sr2LiSiO4F : A White Light-Emitting Phosphor

luminescence is studied in , and the emission spectrum shows an intense and broad green emission band with two peak maxima (475 and 505 nm). The two emission bands are due to present in the two crystallographic distinct Sr sites in the host lattice. luminescence is studied in and shows a broad and bright blue emission band at around 430 nm under 350 nm excitation. The effect of co-doping is studied in phosphor. Thus, the co-doping of also enhances the absorption of in the near-UV region, where the light-emitting diode emission occurs. co-doped samples show broad-band emission ranging from 360 to 620 nm (blue to green–yellow), which results in white emission. This indicates partial energy transfer from to . Selected compositions of this phosphor material can find potential application in solid-state lighting.

Phosphorescent White OLEDs for Solid-state Lighting

AbstractWhite OLEDs (WOLEDTMs) fabricated using energy efficient phosphorescent OLED (PHOLEDTM) technology open up exciting new ways to develop efficient white lighting. WOLEDs have the potential to transform the lighting industry. In this presentation, phosphorescent WOLEDs with high conductivity transport layers will be discussed. White light can be generated by partial energy transfer from blue to green and red. Single WOLED stacks are demonstrated that match the Energy Star® lighting color criteria for 2700K and 3000K with high efficiency (˜80 lm/W) and high color rendering indices (˜80). Both devices had operational lifetimes (LT70%) over 30,000 hours measured from an initial luminance of 1,000 cd/m2. Different techniques to improve optical outcoupling will also be discussed.

Carbazole-Based Organogel as a Scaffold To Construct Energy Transfer Arrays with Controllable Fluorescence Emission

A diaryldiketopyrrolopyrrole derivative functionalized with phenothiazine moieties (DPPP) was synthesized and introduced into the ordered 4-(3,6-di-tert-butyl-9H-carbazol-9-yl)benzamide (TBCB) organogel system. It was found that TBCB-based gel became a scaffold to make DPPP molecules line up along the gel fibers, resulting in new self-assembled arrays, whose XRD patterns were quite different from those of the neat TBCB gel and DPPP crystal. In the composite gel, the occurrence of a partial energy transfer from the excited light-harvesting antenna of TBCB to the DPPP acceptor was confirmed on the basis of time-dependent and time-resolved fluorescence investigations. Remarkably, the composite gel could emit intense red light or purplish white light by tuning the excitation wavelength. Such ordered soft materials with color-tunable emission may possess potential applications in sensor and photonic devices.

Novel White Electroluminescent Single Polymer Derived from Fluorene and Quinacridone

A novel series of white light emitting single polymers are prepared by incorporating low contents of quinacridone into the main chain of polyfluorene. This is the first report of quinacridone-containing conjugated polymer. Single layer devices (ITO/PEDOT:PSS/polymer/Ca/Al) are fabricated with these polymers. Energy transfer from fluorene segments to quinacridone unit is observed. Moreover, in the EL process, quinacridone unit can trap electrons and cannot trap holes from fluorene segments. Electroluminescence (EL) spectra of these polymers exhibit simultaneous blue emission (λmax = 425 nm/445 nm) from the fluorene segments and yellow emission (λmax = 540 nm/580 nm) from the quinacridone unit. The latter one comes from the partial energy transfer and charge trapping from the fluorene segments to the quinacridone unit. With the increase of the quinacridone unit's content in the copolymers, the relative intensity of the orange emission band in the EL spectra becomes stronger owing to the more complete energy...

Color Tuning of Doped PLEDs by Energy Transfer from PVK to Dopant Constituted with Biphenyl and Butadienyl Unit

A dopant, 4,4′-bis(dimethyl[4-(2-phenyl-buta-1,3-dienyl)phenyl]amine) biphenyl (BPAB), was synthesized, and single layered polymer light emitting diode (PLED)s, ITO/Host:Dopant/Al, were fabricated with blends of poly(vinyl carbazole) (PVK) as host and BPAB as dopant. The electroluminescence (EL) was observed in doped PLEDs. From the EL and photoluminescence (PL) spectra, the existence of energy transfer from host to dopant was verified. With increasing content of dopant (BPAB), blue emission originated from PVK reduced, but didn't completely disappear, suggesting inefficient energy transfer from PVK to BPAB. When the content of BPAB was 15 wt%, yellow green emission from PLED was observed due to partial energy transfer from PVK to BPAB.

White electroluminescence from a single polymer: A blue‐emitting polyfluorene incorporating orange‐emitting benzoselenadiazole segments on its main chain

AbstractWe have developed efficient white‐light‐emitting polymers through the incorporation of low‐bandgap orange‐light‐emitting benzoselenadiazole (BSeD) moieties into the backbone of a blue‐light‐emitting bipolar polyfluorene (PF) copolymer, which contains hole‐transporting triphenylamine and electron‐transporting oxadiazole pendent groups. By carefully controlling the concentrations of the low‐energy‐emitting species in the resulting copolymers, partial energy transfer from the blue‐fluorescent PF backbone to the orange‐fluorescent segments led to a single polymer emitting white light and exhibiting two balanced blue and orange emissions simultaneously. Efficient polymer light‐emitting devices prepared using this copolymer exhibited luminance efficiencies as high as 4.1 cd/A with color coordinates (0.30, 0.36) located in the white‐light region. Moreover, the color coordinates remained almost unchanged over a range of operating potentials. A mechanistic study revealed that energy transfer from the PF backbone to the low‐bandgap segments, rather than charge trapping, was the main operating process involved in the electroluminescence process. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2938–2946, 2007

A systematic doping strategy to control the emission spectrum of ternary luminescent polymer blends for white emission

In order to tune the emission color of a well-mixed ternary blend of organic luminescent materials with different bandgap energies, one should practice trial and error of varying the guest concentrations because the Förster-type energy transfer among the materials takes place so easily. In this work, we suggest a systematic doping strategy to control the emission color in ternary organic/polymer luminescent blends for white emission. The strategy comprises two steps: finding out the doping concentration of each guest in its binary blend with the host at which the emission intensities of the two components are comparable and then applying the predetermined doping concentrations of the two guests to the ternary blend system. The doping concentrations were as low as below 0.1wt% of the host. Doping of a host chromophore with a very small amount of two emitting guests did not allow the excitation energy transfer among the guests although the partial energy transfer between the host and each of the guests occurred to give white electroluminescence.

Energy transfer in organic quantum well structures

Different types of organic quantum well structures have been grown by organic molecular beam deposition. Energy level and optical property of the quantum well are determined by cyclic voltammetry, optical absorption and photoluminescence. Cyclic voltammetry and optical absorption results indicate that PBD/ Alq3 multilayer can be classified as type Ⅰ, NPB/Alq3 and BCP/ Alq3 as type Ⅱ. The PL experimental results indicate that the structures of PBD/ Alq3 and BCP/ Alq3 exhibit complete energy transfer from PBD or BCP to Alq3, and the structures of NPB/Alq3 exhibit partial energy transfer from NPB to Alq3. The reason for energy transfer in organic quantum well structures is discussed in light of the theoretical and experimental results published in the literature.

Bright-White Light-Emitting Devices Based on a Single Polymer Exhibiting Simultaneous Blue, Green, and Red Emissions

We have developed efficient white-light-emitting polymers through the incorporation of low band gap green-light-emitting benzothiadiazole and red-light-emitting bisthiophenylbenzothiadiazole moieties into the backbone of a blue-light-emitting bipolar polyfluorene copolymer. Partial energy transfer from the blue-fluorescent polyfluorene backbone to the green- and red-fluorescent components resulted in individual emissions from the three emissive species. By carefully controlling the concentrations of the low-energy-emitting species in the resulting copolymers, the emission of white light, with contributions from each of the three primary colors, was achieved. Efficient polymer light-emitting devices prepared using these copolymers exhibited luminance efficiencies as high as 4.87 cd/A with color coordinates (0.37, 0.36) that were very close to the ideal CIE chromaticity coordinates for pure white light (0.33, 0.33). In addition, the color coordinates remained almost unchanged over a range of operating poten...

Metal-Enhanced Fluorescence from Gold Surfaces: Angular Dependent Emission

The first observation of Metal-Enhanced Fluorescence (MEF) from large gold colloids is presented. Gold colloids, 40 and 200 nm diameter, were deposited onto glass substrates in a homogeneous fashion. The angular-dependent fluorescence emission of FITC-HSA, adsorbed onto gold colloids, was measured on a rotating stage which was used to evaluate MEF at all spatial angles. The emission intensity of FITC-HSA was found to be up to 2.5-fold brighter than the emission on bare glass substrates at an angle of 270 degrees. This is explained by the Radiating Plasmon Model, whereby the combined system, composed of the fluorophore and the metal colloids, emits with the photophysical characteristics of the fluorophore, after the excitation and the partial radiationless energy transfer between the excited states of the fluorophore and the surface plasmons of the gold colloids. The fluorescence enhancement was found to be higher with 200 nm gold colloids as compared to 40 nm colloids due to the increased contribution of the scattering portion of the 200 nm gold colloid extinction spectrum. These observations suggest that gold colloids could be used in MEF applications, offering more stable surfaces than the commonly used silvered surfaces, for applications requiring longer term storage and use.

Improving the power efficiency of white light-emitting diode by doping electron transport material

Highly efficient white light emission was realized via the partial energy transfer from blue host polyfluorene (PF) to orange light emission dopant rubrene. A more balanced charge transport was achieved by adding an electron transport material, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), into the PF-rubrene system to enhance the electron transportation. Efficiency improvement by as much as a factor of 2 has been observed through the addition of PBD. These devices can easily reach high luminance at low driving voltages, thus achieving high power efficiency at high luminance (14.8, 13.5, and 12.0lm∕W at the luminances of 1000, 2000, and 4000cd∕m2, respectively). Therefore, this performance is an important approach toward solid-state lighting application. The enhancement is mainly attributed to three factors: increased electron transport property of the host material, increased photoluminescence quantum efficiency, and the shifting of emission zone away from cathode contact. The reported efficiency is among the highest values reported in the white emission polymer light-emitting diodes.

WHITE-ELECTROLUMINESCENCE DEVICE BASED ON POLYMER/QUANTUM DOT NANOCOMPOSITES

White light emission was obtained from a light-emitting diode prepared from polymer/quantum dot nanocomposites consisting of poly(9,9′-dihexylfluorene-2,7-divinylene-m-phenylenevinylene-stat-p-phenylenevinylene) (PDHFPPV) and CdSe nanoparticles. Blue emission from PDHFPPV and red emission from the nanoparticles, which is triggered by partial excitation energy transfer from PDHFPPV, jointly contribute to white emission of the organic/inorganic hybrid device. Also, the blue-emitting matrix polymer makes the device preparation process simpler due to its high processability. By controlling the blend ratio, we could obtain a pure white color from the hybrid device.

Synthesis and properties of polyfluorenes containing 1,8-naphthalimide moieties for white electroluminescence

Two polyfluorenes containing 1,8-Naphthalimide units were synthesized by Yamamoto polycondensation reaction. The 1,8-naphthalimide moieties act as both electron and hole trap in the resulting polymers, leading to the EL spectra different from the PL spectra in film. White electroluminescence with a maximum brightness of 11100 cd/m 2, an external quantum efficiency of 2.0% and CIE coordinates of (0.26, 0.36) has been realized through partial energy transfer from polyfluorene to 1,8-naphthalimide moieties.

Synthesis and properties of polyfluorenes containing 1,8-naphthalimide moieties for white electroluminescence

Two polyfluorenes containing 1,8-Naphthalimide units were synthesized by Yamamoto polycondensation reaction. The 1,8-naphthalimide moieties act as both electron and hole trap in the resulting polymers, leading to the EL spectra different from the PL spectra in film. White electroluminescence with a maximum brightness of 11100 cd/m 2, an external quantum efficiency of 2.0% and CIE coordinates of (0.26, 0.36) has been realized through partial energy transfer from polyfluorene to 1,8-naphthalimide moieties.

White-Light Generation in Sr2SiO4 : Eu2 + , Ce3 + under Near-UV Excitation: A Novel Phosphor for Solid-State Lighting

White-light emission is generated by combining blue and blue green to yellow emissions of and , respectively, in a single host lattice of . The excitation is in the near-UV region . The role of concentration of on the photoluminescence emission intensity in [, 0.005, 0.0075, and 0.01] is studied, and it is found that the critical concentration is . Energy migration over sites occurs, resulting in concentration quenching. exhibits a high absorption in the near-UV region. Energy transfer from to occurs in , . Optimization of concentration of to produce white light in shows that the optimum concentrations of and are 0.01 and , respectively. Partial energy transfer from to is responsible for the white-light generation. The results reveal that , is an efficient “single host lattice phosphor” for solid-state lighting technology using UV light-emitting diode (LED) to generate white light.