Self‐Organized Gradient Hole Injection to Improve the Performance of Polymer Electroluminescent Devices

Lee, Tae‐Woo; Chung, Youngsu; Kwon, Ohyun; Park, Jong-Jin

doi:10.1002/adfm.200600278

Cited by 205 publications

(221 citation statements)

References 29 publications

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“…[ 6 ] The anionic polyelectrolyte Nafi on, which is also an acid, is found to give the opposite effect compared to cationic polyelectrolytes. A thin fi lm of Nafi on on ITO is found to increase the work function by 0.5 eV, [21][22][23] which indicates that Nafi on can modify the metal work function. Another anionic polyelectrolyte is poly(4-styrenesulfonic acid) (H-PSS), a component of PEDOT:PSS formulations.…”

Section: Introductionmentioning

confidence: 99%

Origin of Work Function Modification by Ionic and Amine‐Based Interface Layers

Reenen

Kouijzer

Janssen

et al. 2014

Adv Materials Inter

131

138

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can evolve into a successful thin fi lm photovoltaic technology. To this end, further improvements in the sunlight-toelectricity conversion effi ciency are still needed. To do so, not only the optoelectronic, active layer should be considered, but also the connection between this layer and the electrodes. This connection is facilitated by so-called work function modifi cation layers (WMLs) that serve to align the transport levels in the organic semiconductor and the conductor by modifi cation of the work function. Materials that are often used are poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) for improving hole collection and injection in combination with an indium tin oxide (ITO) electrode and LiF for the electron contact in combination with an Al metal electrode. A new generation of WMLs fi rst introduced by Cao et al. is based on polyelectrolytes or tertiary aliphatic amines, such as poly [(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt -2,7-(9,9-dioctylfluorene)] (PFN) and the corresponding ethyl ammonium bromide. [ 1 ] Although these materials improve carrier injection or extraction in different organic diode devices, [1][2][3][4][5][6][7] the mechanism behind this improvement is not completely clear. This ambiguity hinders design, selection and optimization of new WMLs.Mechanisms that are generally considered to result in electric fi elds and concomitant work function shifts at metal-organic interfaces are doping, charge transfer, and dipole formation. [8][9][10] Regarding the work function modifi cation of amine side-groups, Lindell et al. [ 11 ] found by photoelectron spectroscopy and density functional calculations that the electron donor para-phenylenediamine chemisorbs onto atomically clean Ni in vacuum. [ 12 ] Due to partial electron transfer from the amine unit, a work function reduction of 1.55 eV is achieved, resulting in a less deep Fermi level. This explanation is not likely to hold for the technologically more relevant procedure on which we shall focus, being the deposition of WMLs from solution on a metal at atmospheric pressure: the metal is not atomically clean, which likely inhibits chemisorption. In other work by Zhou et al. [ 13 ] it is shown that the work function reduction, at atmospheric pressure, by the amine groups in poly(ethylenimine ethoxylated) (PEIE), is generally the same on different conductors. Work function modifi cation by polyelectrolytes and tertiary aliphatic amines is found to be due to the formation of a net dipole at the electrode interface, induced by interaction with its own image dipole in the electrode. In polyelectrolytes differences in size and side groups between the moving ions lead to differences in approach distance towards the surface. These differences determine magnitude and direction of the resulting dipole. In tertiary aliphatic amines the lone pairs of electrons are anticipated to shift towards their image when close to the interface rather than the nitrogen nuclei, which are sterically hindered by the alkyl side chains. ...

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Section: Introductionmentioning

confidence: 99%

Origin of Work Function Modification by Ionic and Amine‐Based Interface Layers

Reenen

Kouijzer

Janssen

et al. 2014

Adv Materials Inter

131

138

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“…PFI has been shown to be successful at creating high F HTLs for blue organic light emitting diodes (OLEDs). [21][22][23] Like low band-gap polymers, blue emitting polymers oen have a large IP. As an example, for poly(9,9-dioctyluorene) IP ¼ 5.9 eV.…”

Section: -12mentioning

confidence: 99%

High work-function hole transport layers by self-assembly using a fluorinated additive

Mauger

Özmen

et al. 2014

J. Mater. Chem. C

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The hole transport polymer poly (3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) derives many of its favorable properties from a PSS-rich interfacial layer that forms spontaneously during coating. Since PEDOT:PSS is only usable as a blend it is not possible to study PEDOT:PSS without this interfacial layer. Through the use of the self-doped polymer sulfonated poly(thiophene-3-[2-(2-methoxyethoxy) ethoxy]-2,5-diyl) (S-P3MEET) and a polyfluorinated ionomer (PFI) it is possible to compare transparent conducting organic films with and without interfacial layers and to understand their function. Using neutron reflectometry, we show that PFI preferentially segregates at the top surface of the film during coating and forms a thermally-stable surface layer. Because of this distribution we find that even small amounts of PFI increase the electron work function of the hole transport layer. We also find that annealing at 150 C and above reduces the work function compared to samples heated at lower temperatures. Using near edge X-ray absorption fine structure spectroscopy and gas chromatography we show that this reduction in work function is due to S-P3MEET being doped by PFI. Organic photovoltaic devices with S-P3MEET/PFI hole transport layers yield higher power conversion efficiency than devices with pure S-P3MEET or PEDOT:PSS hole transport layers. Additionally, devices with a doped interface layer of S-P3MEET/PFI show superior performance to those with un-doped S-P3MEET.

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“…For small organic molecules, graded structures can be fabricated by codeposition processes [4][5][6][7] and annealing [8], but for polymers it is difficult to control the gradient profile in the active layer due to dissolution by the solvent. There have been attempts to achieve polymer graded structures, such as molecular-scale interface engineering [9], self-organization [10], and thermal transfer process [11]; however, the processes involved are not simple.We have developed a vacuum spray (VS) method [12][13][14][15][16] …”

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confidence: 99%

Graded doped structure fabricated by vacuum spray method to improve the luminance of polymer light-emitting diodes

Mizokuro

Herk

et al. 2010

Nano-Micro Lett

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An increase in luminance of a polymer light-emitting diode (PLED) was obtained by fabricating a graded doping structure using a vacuum spray method. The small electron transport molecule, Tris(8-hydroxyquinolinato) aluminum(III)(Alq 3 ), was graded dispersed along the film in the direction of growth in the hole transport polymer poly(3-hexylthiophene-2,5-diyl) (P3HT, regiorandom) layer of the PLED, despite being dissolved in the same organic solvent as the polymer. The PLED reported here, which is composed of a graded structure, emitted brighter light than PLEDs composed of pure polymer or of a blend of active layers prepared by spin coating and/or vacuum spray methods. Good balance between electron and hole injection is very important for the functioning of both OLEDs and PLEDs. In PLEDs, the most common way to achieve this balance is by blending the electron and hole transporting materials. The electroluminescent (EL) efficiency is higher when the electron and hole transport materials are separated and attached to the cathode and anode layers, respectively. Another way to achieve this balance could be by producing a structure in which the two materials are graded along the film direction of growth. As there is no interface between the electron and hole transport materials in this structure, it is expected that there will be an increase in the lifetime of the devices. For small organic molecules, graded structures can be fabricated by codeposition processes [4][5][6][7] and annealing [8], but for polymers it is difficult to control the gradient profile in the active layer due to dissolution by the solvent. There have been attempts to achieve polymer graded structures, such as molecular-scale interface engineering [9], self-organization [10], and thermal transfer process [11]; however, the processes involved are not simple.We have developed a vacuum spray (VS) method [12][13][14][15][16]

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Self‐Organized Gradient Hole Injection to Improve the Performance of Polymer Electroluminescent Devices

Cited by 205 publications

References 29 publications

Origin of Work Function Modification by Ionic and Amine‐Based Interface Layers

Origin of Work Function Modification by Ionic and Amine‐Based Interface Layers

High work-function hole transport layers by self-assembly using a fluorinated additive

Graded doped structure fabricated by vacuum spray method to improve the luminance of polymer light-emitting diodes

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