Reduced efficiency roll-off in light-emitting diodes enabled by quantum dot–conducting polymer nanohybrids

Bae, Wan Ki; Lim, Jaehoon; Zorn, Matthias; Kwak, Jeonghun; Park, Young-Shin; Lee, Donggu; Lee, Seonghoon; Char, Kookheon; Zentel, Rudolf; Lee, Changhee

doi:10.1039/c4tc00232f

Cited by 42 publications

(53 citation statements)

References 27 publications

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“…Nevertheless, CdSe/CdS/ZnSe nanorods have been recently shown to boost both the efficiency and brightness of QD-LEDs with respect to spherical QDs, possibly due to favorable band offset and enhanced light outcoupling 41 , thus making elongated structures promising candidates for high efficiency QD-LEDs. Using deep-red-emitting (650 nm) CdSe/CdS DiRs together with a polyelectrolytic ETL with mobile Br -counter anions, we obtain all-solution-based QD-LEDs with low turn-on voltage (V ON~3 V), high brightness (1200 cd/m 2 ) and EQE of 6,1%, which is over one order of magnitude higher than literature record EQE for QD-LEDs with all-solution-processed organic CTLs 42-44 and exceeds by ~200% the highest EQE of QD-LEDs embedding vacuum evaporated organic interlayers [45][46][47][48] . Importantly, QD-LEDs incorporating polyelectrolytic ETL show no roll-off of the EQE at high current densities, which further emphasizes the great applicative potential of the proposed strategy.…”

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confidence: 81%

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High-Efficiency All-Solution-Processed Light-Emitting Diodes Based on Anisotropic Colloidal Heterostructures with Polar Polymer Injecting Layers

et al. 2015

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Colloidal quantum dots (QDs) are emerging as true candidates for light-emitting diodes with ultrasaturated colors. Here, we combine CdSe/CdS dot-in-rod heterostructures and polar/polyelectrolytic conjugated polymers to demonstrate the first example of fully solution-based quantum dot light-emitting diodes (QD-LEDs) incorporating all-organic injection/transport layers with high brightness, very limited roll-off and external quantum efficiency as high as 6.1%, which is 20 times higher than the record QD-LEDs with all-solution-processed organic interlayers and exceeds by over 200% QD-LEDs embedding vacuum-deposited organic molecules.

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confidence: 81%

“…The black squared refer instead to QDLEDs with thermally evaporated CTLs of small organic molecules (refs. 10,17,18,[45][46][47][69][70][71][72][73][74] ). The coloured dots indicate the EQE values obtained in this work.…”

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confidence: 99%

High-Efficiency All-Solution-Processed Light-Emitting Diodes Based on Anisotropic Colloidal Heterostructures with Polar Polymer Injecting Layers

et al. 2015

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“…This approach was successfully applied to spherical, rod‐like, and tetrapod shaped nanoparticles using different side chain conjugated semiconducting block copolymers as ligands. Dynamic light scattering (DLS) in dilute solution revealed that the NCs were individually dispersed and not aggregated …”

Section: Compatibilization Of Nanoparticles and Polymersmentioning

confidence: 99%

“…This reduced efficiency roll‐off could be explained by the suppression of exciton quenching, as well as an improved charge balance within the active layer. This resulted in devices with stable external quantum efficiencies of EQE = 1.38% and a standard deviation of SD(σ) = ± 0.03% (compared to EQE = 1.20% and SD(σ) = ± 0.11% in case of unfunctionalized QDs which showed a drastic efficiency roll‐off at current densities above 50 mA cm −2 ) …”

Section: Compatibilization Of Nanoparticles and Polymersmentioning

confidence: 99%

“…In this context, devices employing polymers with various HOMO levels were prepared and the role of the polymer HOMO level in the device performance was investigated . Additionally, devices with active layers of polymer/QD nanocomposites were shown to have improved charge carrier balance as well as the necessary charge carrier distribution …”

Section: Applications Of Semiconducting Nanocompositesmentioning

confidence: 99%

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Morphology Control in Biphasic Hybrid Systems of Semiconducting Materials

Mathias

Fokina

Landfester

et al. 2015

Macromol. Rapid Commun.

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Simple blends of inorganic nanocrystals and organic (semiconducting) polymers usually lead to macroscopic segregation. Thus, such blends typically exhibit inferior properties than expected. To overcome the problem of segregation, polymer coated nanocrystals (nanocomposites) have been developed. Such nanocomposites are highly miscible within the polymer matrix. In this Review, a summary of synthetic approaches to achieve stable nanocomposites in a semiconducting polymer matrix is presented. Furthermore, a theoretical background as well as an overview concerning morphology control of inorganic NCs in polymer matrices are provided. In addition, the morphologic behavior of highly anisotropic nanoparticles (i.e. liquid crystalline phase formation of nanorod-composites) and branched nanoparticles (spatial orientation of tetrapods) is described. Finally, the morphology requirements for the application of inorganic/organic hybrid systems in light emitting diodes and solar cells are discussed, and potential solutions to achieve the required morphologies are provided.

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Enhancing Quantum Dot LED Efficiency by Tuning Electron Mobility in the ZnO Electron Transport Layer

Kirkwood

Singh

Mulvaney

2016

Adv Materials Inter

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nanoparticle film, [7,11] the postdeposition film annealing temperature, [12] and the density of dopant ions in the ZnO NPs, [13,14] these works do not explain how the tunable electrical properties of ZnO nanoparticles themselves [15,16] may be exploited to enhance QLED performance.An important challenge arising from the architecture of QLEDs is balancing electron and hole injection into the thin QD layer. Due to the deep valence band energy of core-shell CdSe QDs, hole injection into a QD is expected to be less facile than electron injection. This causes a build-up of electrons in the QD layer, [4,7,17] reducing the device efficiency due to rapid nonradiative Auger recombination [18] or injection of electrons into the HTL. [19,20] Approaches taken to reduce the build-up of electrons in the QD film include selecting hole transport materials with smaller highest-occupied-molecular-orbital (HOMO) energies, [9] engineering the shell of the QDs to hinder electron injection, [17,[21][22][23] inserting the QD layer inside the HTL, [19,24] or inserting a thin layer of insulating polymer [4] or lower-mobility TiO 2[25] between the QD and ETL to hinder charge injection. Despite these advances, the development of alternative approaches to balance charge injection, which do not require modification of the energy levels or architecture of the device, are needed to diversify the range of materials and device architectures available for use in QLEDs.To this end, we have developed a method for balancing charge injection in QLED devices by tuning the electron mobility in the ZnO nanoparticle ETL without modification of the device architecture. This is achieved by employing a solution-phase annealing step prior to film deposition which, unlike postdeposition annealing steps, does not change the physical properties of the ZnO film such as thickness or density. In this way changes in the QLED performance can be unambiguously attributed to the measurable electrical properties of the ZnO film. The focus on the ZnO electron mobility is inspired by models of OLED devices which have demonstrated that the location of electron-hole recombination in a device is determined by the relative mobilities of holes and electrons. [26,27] In a QLED the QD film is typically 2-3 monolayers thick and so the position of recombination must be centered on this small region for maximum luminescence efficiency. Here, we show that the position of recombination and hence charge balance in the QD layer, varies with both electron mobility and device luminance. At the same time we address the need for Quantum-dot (QD) light-emitting diodes (QLEDs) are an important new class of optoelectronic device. Despite the ubiquity of ZnO as the electrontransport material in QLEDs, little is known about how its properties influence QLED performance. Here, it is demonstrated that the defect density and electron mobility of the ZnO nanoparticle electron-transport layer strongly affect QLED device efficiency and can be used to balance electron and hole injection into the QD l...

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Reduced efficiency roll-off in light-emitting diodes enabled by quantum dot–conducting polymer nanohybrids

Abstract: Hybridization of colloidal quantum-dots and conducting polymers improves the efficiency roll-off of quantum-dot light-emitting diodes.

Cited by 42 publications

References 27 publications

High-Efficiency All-Solution-Processed Light-Emitting Diodes Based on Anisotropic Colloidal Heterostructures with Polar Polymer Injecting Layers

High-Efficiency All-Solution-Processed Light-Emitting Diodes Based on Anisotropic Colloidal Heterostructures with Polar Polymer Injecting Layers

Morphology Control in Biphasic Hybrid Systems of Semiconducting Materials

Enhancing Quantum Dot LED Efficiency by Tuning Electron Mobility in the ZnO Electron Transport Layer

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