Elsevier

Synthetic Metals

Volume 158, Issue 16, September 2008, Pages 643-653
Synthetic Metals

Photoacid cross-linkable polyfluorenes for optoelectronics applications

https://doi.org/10.1016/j.synthmet.2008.02.016Get rights and content

Abstract

Polyfluorene copolymers are functionalised with oxetane side groups which make them cross-linkable upon exposure to an acid. Upon addition of a photoacid they become photo-cross-linkable if exposed to UV light. Optical and electrochemical properties of these copolymers, both in the soluble and cross-linked forms are compared. We further show that the use of this type of copolymer allows the fabrication of multilayer structures without restriction of the organic solvent used to deposit an upper layer. In particular, we fabricate multilayer LEDs with enhanced luminous efficiency using one of the copolymers. The possibility to photo-cross-link these materials is also used to prepare micrometer size patterns, using a transmission electron microscope grid as a shadow mask. Furthermore, the use of a scanning near-field optical microscope allows submicron patterns to be prepared. We propose that this approach, which can be extended to a wide range of conjugated polymers, can be useful for the fabrication of optical, electronic and optoelectronic structures.

Introduction

The molecular design and supramolecular control of the organic active layers is a key parameter in the performance optimisation of organic electronic and optoelectronics devices, such as light-emitting diodes (LEDs), photovoltaic cells and field-effect transistors. To fabricate integrated and/or pixelated systems, we need to be able to pattern the organic materials at various length scales, using techniques compatible with sequential processing. While dimensions on the order of tens of micrometers are required for display pixels, achieving lower dimensions may not only allow higher integration density of electronic/optoelectronic circuits but also lead to a better control of the optical properties of the materials, for example in photonic structures. This aim is particularly attractive for solution processable polymers. For this purpose, various techniques have been developed, such as screen printing [1], [2] and inkjet printing [3], [4] to name two of the most commonly used. Another approach lies in the ability to carry out photo-induced patterning, which, due to its simple application protocol, has a high potential of technological success in the fabrication of polymer-based devices.

To develop photo-lithographic patterning techniques, the most common approach is the insertion of photoactive moieties in the organic materials, commonly soluble conjugated polymers, which, upon illumination, turn the material into an insoluble form, while having little or no detrimental effects on the materials optoelectronics properties. Various reports have been addressing this approach, either applied to the formation of hole-transporting layers or insoluble polymeric layers [5], [6], [7], [8], [9], [10], [11], [12], [13], using various photoactive moieties, such as acrylates and oxetanes. In particular, the use of a thermally cross-linkable polyfluorene in the fabrication of multilayer LEDs was previously reported [14]. The work reported by Müller et al. [10] was particularly relevant to the field of organic electronics since they used photoacid cross-linkable polyfluorenes to fabricate a 100 μm pixel colour display. While this work was based on polymers containing reactive oxetanes, Tang and co-workers have used acrylate groups instead. They have reported on the fabrication of two-dimensional patterns, with micrometric resolution, of disubstituted polyacetylenes containing photo-cross-linkable acrylate moieties, using Cu-negative photomasks [15], [16], [17]. The availability of photo-induced cross-linkable electroluminescent polymers is therefore important not only for the fabrication of multilayer devices but also for the fabrication of pixelated displays and possibly even for the fabrication of photonic structures if they could be patterned at dimensions close to the visible wavelengths. In particular, as the use of scanning-near field optical microscope was demonstrated to allow the fabrication of patterns with submicrometer dimensions based on a poly(p-phenylene vinylene) precursor [18], we demonstrate the use of the same tool to pattern this broader range of conjugated polymers down to similar length scales.

In a previous publication [19] we presented preliminary results on the preparation of insoluble networks of a fluorene-based copolymer containing benzothiadiazole as a low energy gap comonomer (identified then as BTBOx, and which we now name BTOx), where, similar to the work by Müller et al. [10], the network formation was due to the polymerization of oxetane side groups. We have prepared two new other similar cross-linkable polymers, F8Ox and 3TOx (see Scheme 1). F8Ox emits in the blue region of the visible spectrum and 3TOx in the orange. Further, we made some modifications in the preparation of BTOx. In particular, we have now used an aqueous solution of an organic base (tetraethylammonium hydroxide, Et4NOH) for the polimerisation reaction, instead of K2CO3(aq.), which yielded a polymer with higher molecular weight, showing higher efficiency in light-emitting diodes [19].

These copolymers are prepared by the Suzuki coupling of three comonomers: 9,9-dioctylfluorene units, phenylene units carrying oxetane side groups and a third monomer (terthiophene, benzothiadiazole or 9,9-dioctylfluorene) which tunes the emission colour. The identification of the materials as 3TOx, BTOx, and F8Ox, respectively, specifies the colour tuning unit (3T, BT, or F8, respectively) and the presence of cross-linkable oxetanes (Ox).

The selection of oxetane as cross-linkable unit is justified by their high rates of polymerization (via cationic ring-opening reaction resulting in acyclic polyethers) and low shrinkage during the cross-linking polymerization [20]. As shown in Scheme 1, a flexible 6-carbon alkyl chain spacer separates the oxetane ring from the conjugated chain, in order to increase the mobility of the oxetane groups, enabling high conversions in the cross-linking reaction. Upon the cationic polymerization of the oxetane groups, a polymeric network is formed, with the conjugated chains cross-linked by oligoether chains.

As shown in Scheme 1, the use of the Suzuki coupling [21] allows the preparation of statistical copolymers made of two units: [(9,9-dioctylfluorene)-(oxetane-functionalised phenylene comonomer] and [(9,9-dioctylfluorene)-(color tuning X comonomer)]. To promote an equal content of these units, the molar feed ratio of the comonomers [2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-9,9-dioctylfluorene]:Ox:X, was 2:1:1. The Suzuki reaction was left to occur for 48 h in Et4NOH(aq.)/toluene (1:2, v/v) in the presence of 1–2% of Pd[PPh3]4 as catalyst. The resulting copolymers were then end-capped with phenyl groups upon addition of phenylboronic ester and bromobenzene to the reaction mixture.

The polymerization of the oxetane units of these copolymers was performed in solid films containing a photoacid generator, PAG, in 1–2.5% content, upon UV-irradiation and curing at 100–150 °C. The thermal, electrochemical and optical properties of these polymers were first studied in the solution form (non-cross-linked structures). The cross-linked films were characterised by UV–visible absorption, fluorescence and FTIR. We have also fabricated patterns of these copolymer networks upon implementation of a reticulation protocol using a transmission electron microscope, TEM, grid as a shadow mask and using scanning near-field optical lithography, SNOL [18], [22]. Light-emitting diodes based on single and multilayers made of cross-linked polymers were prepared and characterised.

Section snippets

Synthesis and properties of the copolymers

The molecular weight of the copolymers BTOx and 3TOx is moderate (as typically found for polymers prepared via Suzuki coupling), which is not detrimental for UV-induced patterning applications [18], [22], while F8Ox was obtained with high molecular weight (see Table 1). This difference in molecular weight is likely due to the higher solubility of the F8 monomer (due to the presence of long alkyl chains), with respect to both BT and 3T comonomers, which avoids precipitation, maintaining the

Conclusions

The oxetane-functionalisation of polyfluorenes is shown to be a valuable tool to allow both the fabrication of multilayer devices, without restrictions in terms of solvent orthogonality, and the fabrication of micron- down to submicron- (eventually nano-) patterns using a photoacid assisted photopatterning. This chemical modification, while not disturbing the conjugated backbone, may be extended to other conjugated polymers. We believe this is a valuable approach not just to fabricate pixelated

Materials

The palladium (0) tetrakis(triphenylphosphine) catalyst was purchased from Aldrich and handled under inert atmosphere. The tetraethylammonium hydroxide (Et4NOH) used in the polymerization reactions was purchased from Merck as aqueous solution (20%) and it was degassed prior to use by vacuum-freeze cycles. All the solvents used for synthesis were purified according to standard methods and were degassed prior to use. Bromobenzene was obtained from Aldrich and used as received.

Acknowledgments

This work was partially supported by FCT-Portugal and FEDER under the contract no. POCTI/CTM/40063/2001. HA thanks FCT for a post-doctoral grant. The collaboration between IST and UCL was partially supported by a British Council/CRUP bilateral agreement. We also thank the Royal Society and the European Commission (THREADMILL—MRTN-CT-2006-036040) for financial support. This work, as part of the European Science Foundation EUROCORES Programme SONS, was also supported from funds by the EPSRC and

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