Elsevier

Organic Electronics

Volume 4, Issues 2–3, September 2003, Pages 165-177
Organic Electronics

Fluorene-based conjugated polymer optical gain media

https://doi.org/10.1016/j.orgel.2003.08.009Get rights and content

Abstract

We report a detailed investigation of the optical gain properties of a series of semiconducting polyfluorenes with photoluminescence emission spectra that span the entire visible spectrum. Stimulated emission was demonstrated at low pump pulse energies (⩾0.1 μJ for 10 ns, 10 Hz pulses) in the blue, green and red parts of the spectrum via amplified spontaneous emission (ASE) measurements on asymmetric slab waveguides. These structures exhibit large net gains g⩽74 cm−1, and corresponding gain cross-sections σ⩾7×10−16 cm2, together with very low loss coefficients, α⩾3.2 cm−1. The spectral location of the maximum waveguide amplification can be widely tuned (Δλ⩾30 nm) by controlling the allowed propagating modes or by blending the active emitter with a second larger optical gap polyfluorene. Our results confirm that fluorene-based polymers are attractive gain media for use in highly tuneable solid-state polymer lasers and amplifiers.

Introduction

The demonstration of stimulated emission in conjugated polymers makes these materials promising as high gain media for solid-state lasers [1], [2], [3]. Some of the unique optical properties that make conjugated polymers attractive candidates for such applications are their high photoluminescence quantum efficiencies (PLQE), large stimulated emission cross-sections and chemically tuneable emission wavelengths [4], [5], [6]. To date, optically pumped lasers based on conjugated polymers have been demonstrated in both solution and thin films, and in resonators based on simple waveguides or more sophisticated structures using distributed feedback geometries, photonic crystals and microrings [7], [8], [9], [10], [11]. Considerable efforts are being made to improve the intrinsic carrier mobilities in these materials and lower their lasing thresholds in order to progress towards electrically pumped polymer laser diodes [12], [13]. Such organic laser sources may eventually compete with inorganic semiconductor lasers in a number of fields, especially if the benefits of low-cost, low-temperature, high throughput manufacture on a variety of substrates can be attained. Semiconducting conjugated polymers exhibit optical gain over broad spectral ranges, offering potential applications in highly tuneable lasers and broadband amplifiers [14], [15].

In this paper, we report a detailed investigation of the optical gain properties of four semiconducting polyfluorenes with chemically tuned emission characteristics that span the spectral range from 400 to 800 nm. We demonstrate low threshold light amplification in the blue, green and red via stripe-pumped, amplified spontaneous emission (ASE) measurements on asymmetric slab waveguides (formed by spin coating polymer films on top of synthetic quartz substrates). ASE leads to a rapid growth of the output light intensity at the maximum gain position and a collapse in the total linewidth of the emission from ∼100 nm to a few nm. The wavelength at which amplification occurs can be widely tuned: this can be done by altering the slab waveguide thickness to control the allowed modes and/or by modifying the refractive index, emission spectrum and PLQE of the active material via blending with a second, larger optical gap “host” polyfluorene. Our observations highlight the broad spectral range for gain that is accessible with these materials. We report the net gain, g, and loss, α, characteristics of the polymer waveguides that we have studied and calculate the corresponding effective gain cross-sections, σ. Finally, we calculate the refractive indices of the active layers using the slab waveguide cut-off thicknesses for light propagation at the maximum gain wavelength.

We note that polyfluorenes also offer excellent charge transport properties [16] and are thus of strong potential interest for electrically pumped devices [17], [2]. However, it is important to recognise that the fabrication of laser diodes based on thin films of organic semiconductors still represents a major challenge for research and development. Amongst the identified issues that lead to additional problems in structures that are suitable for electrical pumping, are optical losses due to: (i) the proximity of the emissive region to the electrodes, (ii) the intra-gap absorption peaks of charge states, and (iii) the intra-gap absorption peaks of triplet states [4], [6], [18], [19]. Low charge carrier mobilities play an important rôle here as they make lateral separation of the carrier injection and optical gain regions very difficult: Carriers do not transport effectively across distances that are large compared to the optical confinement dimensions. In addition, low mobilities limit the inter-electrode separation for acceptable driving voltages (transport is field dependent), with the result that there are relatively strong interactions between in-plane propagating modes and the electrodes. Intra-gap absorption reduces net gain and potentially leads to instability (via excitation of higher lying states). Charged states can also be stabilized by the same disorder that gives rise to low mobilities, leading to lifetimes of tens to hundreds of μs. Triplet states, expected to be more significant under electrical- than optical-pumping (due to the non-geminate pair recombination that dominates in the former case), have similarly long lifetimes and can, via energy transfer, lead to the excitation of molecular oxygen into its highly reactive, singlet excited state. The lesson, in this context, from the development of dye lasers is that, ideally, sufficient time needs to be allowed to elapse between sequential excitation events in order that the triplet states are able to decay. For a dye laser this equates with the use of a circulator that takes the dye around an extended flow path before re-entering the pumping region. This is not necessarily so easy to achieve for a solid-state laser. However, how closely these, reportedly, general features of organic lasers map onto specific conjugated polymer systems and device structures remains to be established. The above issues have not yet been addressed in detail for the materials that are the subject of this paper. Thus, whilst better characterization and understanding of the limitations of optically pumped structures, as described below, represents an important move in the right direction, it does not present a solution to the problem of achieving an electrically pumped laser. Additional work will be needed to carefully address the factors that play a specific rôle in electrically pumped structures: This is the subject of on-going research.

Section snippets

Sample fabrication and experimental procedures

The conjugated polymers that we have investigated were synthesized and carefully purified at The Dow Chemical Company. In particular, we have studied the blue emitting poly(9,9-dioctylfluorene) [PFO] and poly(9,9-dioctylfluorene-co-9,9-di(4-methoxy)phenylfluorene [F8DP], the green emitting poly(9,9-dioctylfluorene-co-benzothiadiazole) [F8BT], and the Dow proprietary red emission copolymer known as Dow Red F. Planar waveguides were made by spin-coating 35–350 nm thick films from 20 mg/ml toluene

Experimental results

Fig. 1 shows (a) the absorption and (b) the photoluminescence spectra of the four polymers that we have studied. The measurements were made on 120 nm (for PFO and F8DP), 180 nm (for F8BT) and 250 nm (for Red F) thickness films spin-coated on polished spectrosil B discs. The PL spectra were obtained by exciting the samples at 340 nm for PFO, F8DP and F8BT, and at 440 nm for Dow Red F. The absorption and PL spectra of F8DP (solid lines) and PFO (dash-dotted lines) are very similar, exhibiting a

Discussion

The ease with which ASE can be achieved for the polymers we have studied and their good performance in DFB lasers is a consequence of a combination of attractive features that are characteristic for conjugated polymers:

  • (i)

    They have large absorption coefficients, α≈105 cm−1, and consequently large cross-sections for stimulated emission,

  • (ii)

    They have high PLQE values in the solid state unlike many laser dyes for which there is strong concentration quenching,

  • (iii)

    There is a spectral separation between the

Conclusions

We have presented a detailed study of the gain properties of four polyfluorenes, namely poly(9,9-dioctylfluorene), poly(9,9-dioctylfluorene-co-9,9-di(4-methoxy)phenylfluorene), poly(9,9-dioctylfluorene-co-benzothiadiazole), and a Dow proprietary red emission copolymer, Dow Red F. The emission spectra of these four materials span the range from 400 to 800 nm, covering the full visible spectrum. Efficient light amplification was demonstrated at 452 nm, 466 nm, 576 nm and 685 nm via ASE in

Acknowledgments

The authors thank Mark Bernius, Rob Fletcher, Mike Inbasekaran, and Jim O’Brien of The Dow Chemical Company for providing the four polymers that we have studied in our experiments. We are also grateful to the United Kingdom Engineering and Physical Sciences Research Council (Ultrafast Photonics Collaboration GR/R55078) for financial support. Finally, we acknowledge experimental assistance from Dr. Mattijs Koeberg and useful discussions with Mariano Campoy-Quiles.

References (40)

  • U. Scherf et al.

    Curr. Opin. Solid State Mat. Sci.

    (2001)
  • B.J. Schwartz et al.

    Chem. Phys. Lett.

    (1997)
  • Y. Hou et al.

    Synth. Met.

    (2003)
  • K.L. Shaklee et al.

    Appl. Phys. Lett.

    (1971)
  • G. Heliotis et al.

    Synth. Met.

    (2003)
  • R. Xia, G. Heliotis, Y. Hou, D.D.C. Bradley, Chem. Phys. Lett. (2003) in...
  • R. Gupta et al.

    Appl. Phys. Lett.

    (1998)
  • F. Hide et al.

    Science

    (1996)
  • R.H. Friend et al.

    Nature

    (1999)
  • S.V. Frolov et al.

    Phys. Rev. Lett.

    (1997)
  • N. Tessler

    Adv. Mater.

    (1999)
  • M.D. McGehee et al.

    Adv. Mater.

    (2000)
  • D. Moses

    Appl. Phys. Lett.

    (1992)
  • W. Holzer et al.

    Adv. Mater.

    (1996)
  • S.V. Frolov et al.

    Appl. Phys. Lett.

    (1998)
  • N. Tessler et al.

    Nature

    (1997)
  • M.D. McGehee et al.

    Appl. Phys. Lett.

    (1998)
  • G. Heliotis et al.

    Appl. Phys. Lett.

    (2002)
  • T.-W. Lee et al.

    Appl. Phys. Lett.

    (2002)
  • G.A. Turnbull et al.

    Phys. Rev. B

    (2001)
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