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Title: Operating organic light-emitting diodes imaged by super-resolution spectroscopy

Abstract

Super-resolution stimulated emission depletion (STED) microscopy is adapted here for materials characterization that would not otherwise be possible. With the example of organic light-emitting diodes (OLEDs), spectral imaging with pixel-by-pixel wavelength discrimination allows us to resolve local-chain environment encoded in the spectral response of the semi-conducting polymer, and correlate chain packing with local electroluminescence by using externally applied current as the excitation source. We observe nanoscopic defects that would be unresolvable by traditional microscopy. They are revealed in electroluminescence maps in operating OLEDs with 50 nm spatial resolution. We find that brightest emission comes from regions with more densely packed chains. Conventional microscopy of an operating OLED would lack the resolution needed to discriminate these features, while traditional methods to resolve nanoscale features generally cannot be performed when the device is operating. As a result, this points the way towards real-time analysis of materials design principles in devices as they actually operate.

Authors:
 [1];  [2]
  1. Univ. of Illinois, Urbana, IL (United States)
  2. IBS Center for Soft and Living Matter, UNIST, Ulsan (South Korea)
Publication Date:
Research Org.:
Univ. of Illinois at Urbana-Champaign, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1252188
Grant/Contract Number:
FG02-02ER46019
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Nature Communications
Additional Journal Information:
Journal Volume: 7; Journal ID: ISSN 2041-1723
Publisher:
Nature Publishing Group
Country of Publication:
United States
Language:
English
Subject:
30 DIRECT ENERGY CONVERSION; 36 MATERIALS SCIENCE; 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 77 NANOSCIENCE AND NANOTECHNOLOGY; LED; super-resolution; light-emitting; imaging; in situ; 47 OTHER INSTRUMENTATION; conjugated polymers; charge-transport; fluorescence microscopy; interchain interactions; stimulated-emission; morphology; resolution; poly(p-phenylenevinylene); electroluminescence; conformation

Citation Formats

King, John T., and Granick, Steve. Operating organic light-emitting diodes imaged by super-resolution spectroscopy. United States: N. p., 2016. Web. doi:10.1038/ncomms11691.
King, John T., & Granick, Steve. Operating organic light-emitting diodes imaged by super-resolution spectroscopy. United States. doi:10.1038/ncomms11691.
King, John T., and Granick, Steve. 2016. "Operating organic light-emitting diodes imaged by super-resolution spectroscopy". United States. doi:10.1038/ncomms11691. https://www.osti.gov/servlets/purl/1252188.
@article{osti_1252188,
title = {Operating organic light-emitting diodes imaged by super-resolution spectroscopy},
author = {King, John T. and Granick, Steve},
abstractNote = {Super-resolution stimulated emission depletion (STED) microscopy is adapted here for materials characterization that would not otherwise be possible. With the example of organic light-emitting diodes (OLEDs), spectral imaging with pixel-by-pixel wavelength discrimination allows us to resolve local-chain environment encoded in the spectral response of the semi-conducting polymer, and correlate chain packing with local electroluminescence by using externally applied current as the excitation source. We observe nanoscopic defects that would be unresolvable by traditional microscopy. They are revealed in electroluminescence maps in operating OLEDs with 50 nm spatial resolution. We find that brightest emission comes from regions with more densely packed chains. Conventional microscopy of an operating OLED would lack the resolution needed to discriminate these features, while traditional methods to resolve nanoscale features generally cannot be performed when the device is operating. As a result, this points the way towards real-time analysis of materials design principles in devices as they actually operate.},
doi = {10.1038/ncomms11691},
journal = {Nature Communications},
number = ,
volume = 7,
place = {United States},
year = 2016,
month = 6
}

Journal Article:
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  • Super-resolution stimulated emission depletion (STED) microscopy is adapted here for materials characterization that would not otherwise be possible. With the example of organic light-emitting diodes (OLEDs), spectral imaging with pixel-by-pixel wavelength discrimination allows us to resolve local-chain environment encoded in the spectral response of the semi-conducting polymer, and correlate chain packing with local electroluminescence by using externally applied current as the excitation source. We observe nanoscopic defects that would be unresolvable by traditional microscopy. They are revealed in electroluminescence maps in operating OLEDs with 50 nm spatial resolution. We find that brightest emission comes from regions with more densely packedmore » chains. Conventional microscopy of an operating OLED would lack the resolution needed to discriminate these features, while traditional methods to resolve nanoscale features generally cannot be performed when the device is operating. As a result, this points the way towards real-time analysis of materials design principles in devices as they actually operate.« less
  • Vapor-deposited Alq{sub 3} layers typically possess a strong permanent electrical polarization, whereas NPB layers do not. (Alq{sub 3} is tris(8-quinolinolato)aluminum(III); NPB is 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl.) The cause is a net orientation of the Alq{sub 3} molecules with their large dipole moments. Here we report on consequences for an organic light-emitting diode (OLED) with an NPB hole-transport layer and Alq{sub 3} electron-transport layer. The discontinuous polarization at the NPB|Alq{sub 3} interface has the same effect as a sheet of immobile negative charge there. It is more than compensated by a large concentration of injected holes (NPB{sup +}) when the OLED is running. Wemore » discuss the implications and consequences for the quantum efficiency and the drive voltage of this OLED and others. We also speculate on possible consequences of permanent polarization in organic photovoltaic devices. The concentration of NPB{sup +} was measured by charge-modulation spectroscopy (CMS) in the near infrared, where the NPB{sup +} has a strong absorption band, supplemented by differential-capacitance and current-voltage measurements. Unlike CMS in the visible, this method avoids complications from modulation of the electroluminescence and electroabsorption.« less
  • We investigate the properties of N,N′-[(Diphenyl-N,N′-bis)9,9,-dimethyl-fluoren-2-yl]-benzidine (BF-DPB) as hole transport material (HTL) in organic light-emitting diodes (OLEDs) and compare BF-DPB to the commonly used HTLs N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD), 2,2′,7,7′-tetrakis(N,N′-di-p-methylphenylamino)-9,9′-spirobifluorene (Spiro-TTB), and N,N′-di(naphtalene-1-yl)-N,N′-diphenylbenzidine (NPB). The influence of 2,2′-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6-TCNNQ p-dopant) concentration in BF-DPB on the operation voltage and efficiency of red and green phosphorescent OLEDs is studied; best results are achieved at 4 wt. % doping. Without any light extraction structure, BF-DPB based red (green) OLEDs achieve a luminous efficacy of 35 .1 lm/W (74 .0 lm/W) at 1000 cd/m{sup 2} and reach a very high brightness of 10 000 cd/m{sup 2} at a very low voltage of 3.2more » V (3.1 V). We attribute this exceptionally low driving voltage to the high ionization potential of BF-DPB which enables more efficient hole injection from BF-DPB to the adjacent electron blocking layer. The high efficiency and low driving voltage lead to a significantly lower luminous efficacy roll-off compared to the other compounds and render BF-DPB an excellent HTL material for highly efficient OLEDs.« less
  • High efficiency/high luminance small-molecule organic light-emitting diodes (OLEDs) are fabricated by combining thin, covalently bound triarylamine hole injection/adhesion interlayers with hole- and exciton-blocking/electron transport interlayers in tris(8-hydroxyquinolato)aluminum(III) (Alq) and tetrakis(2-methyl-8-hydroxyquinolinato)borate (BQ{sub 4}{sup -})-based OLEDs. Green-emitting OLEDs with maximum luminance {approx}85 000 cd/m{sup 2}, power and forward external quantum efficiencies as high as 15.2 lm/W and 4.4{+-}0.5%, respectively, and turn-on voltages {approx}4.5 V are achieved in devices of the structure, ITO/N,N(prime)-diphenyl-N,N(prime)-bis(p-trichlorosilylpropylphenyl)(1,1(prime)-biphenyl)-4,4(prime)-diamine (TPD-Si2)/1,4-bis(1-naphthylphenylamino)biphenyl (NPB)/Alq doped with N,N(prime)-di(3-heptyl)quinacridone (DIQA)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/Li/AgMg. Also, bright and efficient blue-emitting OLEDs with turn-on voltages {approx}5.0 V, maximum luminance {approx}30 000 cd/m2, and {approx}5.0 lm/W and 1.6{+-}0.2% power andmore » external forward quantum efficiencies, respectively, are achieved in devices of the structure, ITO/TPD-Si2/NPB/BQ{sub 4}{sup -}/BCP/Li/Al. TPD-Si2 interlayers are fabricated by spin casting N,N(prime)-diphenyl-N,N(prime)-bis(p-trichlorosilylpropylphenyl)(1,1(prime)-biphenyl)-4,4(prime)-diamine onto the ITO surface, while BCP interlayers are introduced by thermal evaporation. The excellent OLED performance is attributed to the differing functions of the above two interlayers: (1) The TPD-Si2 layer has a direct impact on hole injection by reducing the injection barrier and improving interfacial cohesion, and an indirect but strong effect on electron injection by altering internal electric fields. (2) The BCP layer, doped with lithium, directly reduces the electron injection barrier. Incorporation of both interlayers in OLED structures affords synergistically enhanced hole/electron injection and recombination efficiency. The results demonstrate a strategy to enhance OLED performance and an alternative strategy to increase electron density in electron-limited devices.« less
  • We report that organic light-emitting diodes (OLEDs) have received a significant attention over the past decade due to their energy-saving potential. We have recently synthesized two novel carbazole-based donor-acceptor compounds and analyzed their optical properties to determine their suitability for use as blue emitters in OLEDs. These compounds show remarkable photo-stability and high quantum yields in the blue region of the spectrum. In addition, they have highly solvatochromic emission. In non-polar solvents, bright, blue-shifted (λmax ≈ 398 nm), and highly structured emission is seen. With increasing solvent dielectric constant, the emission becomes weaker, red-shifted (λmax ≈ 507 nm), and broad.more » We aim to determine the underlying cause of these changes. Electronic structure calculations indicate the presence of multiple excited states with comparable oscillator strength. These states are of interest because there are several with charge-transfer (CT) character, and others centered on the donor moiety. We theorize that CT states play a role in the observed changes in emission lineshape and may promote charge mobility for electrofluorescence in OLEDs. In the future, we plan to use Stark spectroscopy to analyze the polarity of excited states and transient absorption spectroscopy to observe the dynamics in the excited state.« less