Grazing emitted light from films of derivative polymer of polyfluorene

doi:10.1016/S0379-6779(99)00314-8

Synthetic Metals

Volumes 111–112, 1 June 2000, Pages 583-586

https://doi.org/10.1016/S0379-6779(99)00314-8 Get rights and content

Abstract

Emission patterns from the films of polyfluorene derivative on silica substrates are found to show light emission at grazing angles to the substrate. This light is found to be polarised and to have a narrowed emission, as well as enhanced intensity with respect to the light collected in the forward direction. Both the wavelengths of the peaks and their polarisation (s or p) are thickness-dependent. The peak's wavelength positions are used to characterise the spectrum of the refractive indices of the film both in and out of plane. The relative intensity of s- to p-polarised peaks serves as an independent indication of the dipole orientation. The deduced dipole in-plane alignment ratio (88%) and the magnitude of the birefringence (Δn=0.2 at λ=600 nm) suggest that the main chains of this polymer lie preferentially in plane of the film. These findings seem to be consistent with the rod structure of the polymer studied.

Introduction

The studies of electroluminescent organic and polymeric material have been vigorously purposed in the development of light emitting diodes. There has also been much progress in device configurations, resulting in LEDs with achieved a low driving voltage and high efficiency. In particular, the modification of the spontaneous emission by a cavity [1] is one interesting approach: emission from the active material placed within a cavity or a waveguide [2] couples to the confined field (mode) causing enhancement at the mode wavelength, and suppression elsewhere. The microcavity consisting of a distributed Brugg reflector and a metal mirror has been used for the cavity in many studies [3], [4], [5]. These report that emission from the LEDs with a microcavity is characterised by enhanced intensity, narrow line width, and a narrow divergence angle. Although there have been many studies of stimulated emission using waveguide configuration [6], [7], [8], [9], there are fewer in the field of spontaneous emission.

In this paper, we describe how a substrate radiation mode at grazing angles can enhance the spontaneous emission at the mode wavelength. This substrate radiation mode has angle of incidence θ, which is just a below the critical angle at the interface between the polymer layer and the substrate as shown in Eq. (1) and Fig. 4: $sin θ= n_{3} n_{2} sin (90−δ)≅ n_{3} n_{2}$ where n₂ and n₃ are the refractive indices of the polymer layer, substrate, respectively.

At this angle, the reflectivity at the boundary between the polymer layer and the substrate is below 100% but still large enough to confine light to the polymer layer. In this way, part of the confined light can be emitted to the substrate at grazing angles.

We also describe the optical properties of the polyfluorene derivative deduced from the results of the grazing angle photoluminescence measurement. Polyfluorene [10], [11] is soluble in organic solvents, and displays extremely high photoluminescence efficiencies in the blue spectral region [12]. Many derivatives of polyfluorene have also been studied extensively, and have shown promising characteristics for polymer LEDs. In this work, we used a green-light-emitting polyfluorene derivative, whose PL efficiency is more than 60%. We discuss the anisotropic optical properties of this polymer.

Section snippets

Experimental

The experimental setup for grazing angle photoluminescence (GPL) measurement is shown in Fig. 1. The samples were excited by an argon-ion laser (457 nm) at 40° incident angle. A polariser was placed between the sample and the detector of an Oriel CCD-array spectrograph to measure the s- and p-component of the luminescence. The GPL measurement detects rays that are emitted into the transparent substrate from the polymer layer at a grazing angle, and emitted from the edge of the substrate. For

Results and discussion

The spectra of conventional PL and GPL for polymer films with thicknesses of 62 and 184 nm are shown in Fig. 2, Fig. 3, respectively. For the 62-nm-thick sample, the s-component of the GPL indicates both a narrowed spectrum and enhanced intensity. These features were obviously observed only within about 5° from the horizontal plane. The p-component is found to be suppressed very well and does not show the features of the s-component. On the other hand, at the thickness of 184 nm, a peak is

Conclusions

We found enhanced emission from films of the derivative polymer of polyfluorene at grazing angles. This emission has narrow peaks whose wavelength and polarization are dependent on the film thickness, and also shows a narrow divergence angle. We derived the refractive indices and the distribution of dipole orientation in this derivative polymer from the wavelengths and intensities of the peaks. These results indicate that the film has both a large birefringence, and that dipoles are oriented

Acknowledgements

We thank N. Tessler, R.W. Gymer and P. Ho for useful suggestions and discussions, J.H. Burroughes and Carl Towns of Cambridge Display Technology for providing the polyfluorene derivative used in this work, and S. Nebashi of Epson Cambridge Laboratory for much help.

References (20)

R. Burzynski et al.
Polymer
(1990)
N. Takada et al.
Appl. Phys. Lett.
(1993)
M. Hiramoto et al.
Appl. Phys. Lett.
(1993)
T. Nakayama et al.
Appl. Phys. Lett.
(1993)
A. Dodabalapur et al.
Appl. Phys. Lett.
(1994)
H.F. Wittmann et al.
Adv. Mater.
(1995)
F. Hide et al.
Science
(1996)
M. McGehee et al.
Bull. Am. Phys. Soc.
(1997)
C. Zenz et al.
Appl. Phys. Lett.
(1997)
G.J. Denton et al.
Adv. Mater.
(1997)

There are more references available in the full text version of this article.

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Synthetic Metals

Grazing emitted light from films of derivative polymer of polyfluorene

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

Polymer

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

Adv. Mater.

Science

Bull. Am. Phys. Soc.

Appl. Phys. Lett.

Adv. Mater.

Unveiling photophysical and photonic phenomena by means of optical gain measurements in waveguides and solutions

Horizontal orientation of linear-shaped organic molecules having bulky substituents in neat and doped vacuum-deposited amorphous films

Leaky-mode emission of luminescent thin films on transparent substrates

Emitter Orientation as a Key Parameter in Organic Light-Emitting Diodes

Highly luminescent π-conjugated dithienometalloles: Photophysical properties and their application in organic light-emitting diodes

Interplay of processing, morphological order, and charge-carrier mobility in polythiophene thin films deposited by different methods: Comparison of spin-cast, drop-cast, and inkjet-printed films