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Towards solution-processible semiconducting polymer-based photonic devices
ELSEVIER
Synthetic Metals 102 (1999) 1020
Towards Solution-Processible Semiconducting Polymer-Based Photonic Devices Peter K. H. Ho, Nir Tessler and Richard H. Friend Cavendish
Laboratory,
University
of Cambridge,
Madingley
Road,
Cambridge,
CB3
OHE,
United
Kingdom
Abstract We demonstrate that the refractive indices of semiconducting polymer thin films can be tuned over significant ranges by dispersing nanosized dielectric particles into the polymer matrix. We have dispersed up to 30 vol% 5OA SiOz or 55 ~01% IOA TiO? particles into the conjugated polymer poly@-phenylenevinylene) (PPV) and found no significant Mie scattering. A maximum decrease An=-0.75 at 550nm is measured for nanocomposites with 5.5 ~01% Ti02. We have also fabricated semiconducting polymer photonic structures such as distributed Bragg reflectors and Fabry-Perot resonators by successive solution-deposition of alternate high- and low-index layers. Keywords:
1.
Refractive Index, Conjugated
Polymers, Nanoparticles,
Introduction
Organic semiconductor devices based on n-conjugated materials are attracting interest because of the tailorability and processing advantages of organic semiconducting materials [ 11. Although many of their properties could be tuned over impressive ranges by clever chemical design, their refractive indices are much less readily manipulated [2]. We introduce here a general approach to modify the refractive index using effective medium effects, in which inorganic particles in the l- 10 nm size range are dispersed into the semiconducting polymer matrix. Such films are unaffected by Mie scattering when the particles are well-dispersed, but they can retain the desirable solution processibility and semiconducting transport properties of the polymer matrix. 2.
Experimental
The SiOz and TiOz colloids were prepared by the hydrolysis of SiC14 and TiC14 respectively in a water-in-cyclohexane reverse microemulsion with dioctylsulfosuccinate as surfactant, and purified by dialysis against pH-adjusted MeOH [3]. Poly@-xylylene tetrahydrothiophenium chloride) (precursor PPV) in MeOH was blended with the SiOz or TiOz nanoparticle dispersion, centrifuged to clear (for SiO& and then spun-coated onto glass substrates. These precursor films were then converted at 170-180°C under ~~10.” mbar to give PPV nanocomposites containing selected volume fractions of the nanoparticles [3]. Refractive indices of the thin films were determined by microcavity resonance transmission and spectroscopic ellipsometry. 3.
Results
and Discussion
The composition-dependent refractive index dispersions of PPV and its nanocomposites are shown in Fig. 1. The dispersion curves can evidently be tuned over significant ranges. For the SiOz composites, both electronic and vibration spectroscopies
Nanocomposites show that x-conjugation is largely preserved. This contrasts with the TiOz composites, in which conjugation-shortening is observed. (A)
(B)
3.5 m
3.5 t‘j
.s 2.5 I 2.0 1.5 -
600
700
600
“m
1.5 -
600
700
600
“m
Fig. 1. Composition-dependent refractive index dispersion of PPVnanocomposites with (A) SOA SiO2 or(B) IOA TiOz. The refractive index function of neat PPV varies strongly with conversion conditions.
With this approach, semiconducting polymer waveguides and other photonic structures can be fabricated by solution-processing. The reflectance spectra of two PPV-based distributed Bragg reflectors constructed from quarter-wave stacks of high-index PPV and low-index PPV-Ti02 layers are shown in Fig. 2.
Fig. 2. Reflectance polymer quarter-wave from three or six pairs of PPV (H) and its nanocomposite wtth 50 v/v% 10-A TiOz (L). The stack periodicity could be varied to give different peak reflectance wavelengths as shown
[l] .I. A. Rogers, Z. Bao and V. R. Raju, Appl. Phys. Lett. 72,2716 (1998) [2]
[3]
0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)01274-O
D. W. van Chemical Additive Details to
Krevelen, Properties of Polymers: Their Correlation with Structure, Their Numerical Estimation and Prediction from Group Contributions (Elsevier, Amsterdam, ed. 3, 1990). be published elsewhere.
Synthetic Metals 102 (1999) 1020
Towards Solution-Processible Semiconducting Polymer-Based Photonic Devices Peter K. H. Ho, Nir Tessler and Richard H. Friend Cavendish
Laboratory,
University
of Cambridge,
Madingley
Road,
Cambridge,
CB3
OHE,
United
Kingdom
Abstract We demonstrate that the refractive indices of semiconducting polymer thin films can be tuned over significant ranges by dispersing nanosized dielectric particles into the polymer matrix. We have dispersed up to 30 vol% 5OA SiOz or 55 ~01% IOA TiO? particles into the conjugated polymer poly@-phenylenevinylene) (PPV) and found no significant Mie scattering. A maximum decrease An=-0.75 at 550nm is measured for nanocomposites with 5.5 ~01% Ti02. We have also fabricated semiconducting polymer photonic structures such as distributed Bragg reflectors and Fabry-Perot resonators by successive solution-deposition of alternate high- and low-index layers. Keywords:
1.
Refractive Index, Conjugated
Polymers, Nanoparticles,
Introduction
Organic semiconductor devices based on n-conjugated materials are attracting interest because of the tailorability and processing advantages of organic semiconducting materials [ 11. Although many of their properties could be tuned over impressive ranges by clever chemical design, their refractive indices are much less readily manipulated [2]. We introduce here a general approach to modify the refractive index using effective medium effects, in which inorganic particles in the l- 10 nm size range are dispersed into the semiconducting polymer matrix. Such films are unaffected by Mie scattering when the particles are well-dispersed, but they can retain the desirable solution processibility and semiconducting transport properties of the polymer matrix. 2.
Experimental
The SiOz and TiOz colloids were prepared by the hydrolysis of SiC14 and TiC14 respectively in a water-in-cyclohexane reverse microemulsion with dioctylsulfosuccinate as surfactant, and purified by dialysis against pH-adjusted MeOH [3]. Poly@-xylylene tetrahydrothiophenium chloride) (precursor PPV) in MeOH was blended with the SiOz or TiOz nanoparticle dispersion, centrifuged to clear (for SiO& and then spun-coated onto glass substrates. These precursor films were then converted at 170-180°C under ~~10.” mbar to give PPV nanocomposites containing selected volume fractions of the nanoparticles [3]. Refractive indices of the thin films were determined by microcavity resonance transmission and spectroscopic ellipsometry. 3.
Results
and Discussion
The composition-dependent refractive index dispersions of PPV and its nanocomposites are shown in Fig. 1. The dispersion curves can evidently be tuned over significant ranges. For the SiOz composites, both electronic and vibration spectroscopies
Nanocomposites show that x-conjugation is largely preserved. This contrasts with the TiOz composites, in which conjugation-shortening is observed. (A)
(B)
3.5 m
3.5 t‘j
.s 2.5 I 2.0 1.5 -
600
700
600
“m
1.5 -
600
700
600
“m
Fig. 1. Composition-dependent refractive index dispersion of PPVnanocomposites with (A) SOA SiO2 or(B) IOA TiOz. The refractive index function of neat PPV varies strongly with conversion conditions.
With this approach, semiconducting polymer waveguides and other photonic structures can be fabricated by solution-processing. The reflectance spectra of two PPV-based distributed Bragg reflectors constructed from quarter-wave stacks of high-index PPV and low-index PPV-Ti02 layers are shown in Fig. 2.
Fig. 2. Reflectance polymer quarter-wave from three or six pairs of PPV (H) and its nanocomposite wtth 50 v/v% 10-A TiOz (L). The stack periodicity could be varied to give different peak reflectance wavelengths as shown
[l] .I. A. Rogers, Z. Bao and V. R. Raju, Appl. Phys. Lett. 72,2716 (1998) [2]
[3]
0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)01274-O
D. W. van Chemical Additive Details to
Krevelen, Properties of Polymers: Their Correlation with Structure, Their Numerical Estimation and Prediction from Group Contributions (Elsevier, Amsterdam, ed. 3, 1990). be published elsewhere.