Elsevier

Polymer

Volume 46, Issue 13, 17 June 2005, Pages 4654-4663
Polymer

Synthesis, optical and morphological characterization of soluble main chain 1,3,4-oxadiazole copolyarylethers—potential candidates for solar cells applications as electron acceptors

https://doi.org/10.1016/j.polymer.2005.03.080Get rights and content

Abstract

New copoly(arylether)s containing substituted terphenyl, quinquephenyl, fluorene and anthracene moieties with aromatic 1,3,4-oxadiazole units were prepared and the resulting copolymers are soluble in common organic solvents. Investigation of their optical properties revealed that they emit blue and yellow light. Moreover, their photovoltaic response was studied in blends with poly(3-hexylthiophene) (P3HT) as the electron donor. Despite the low power conversion efficiencies it was shown that photo-induced electron transfer does take place and the performances are higher than a single layer P3HT device. In addition, an anthracene–fluorene–oxadiazole main chain copolymer (PAFOXD) was also examined in a single layer photovoltaic device and gave one of the highest reported open-circuit voltage (Voc) values in the literature (0.89 V). Finally, a detailed morphological study of the blends and the PAFOXD surface using the atomic force microscopy (AFM) technique, revealed the effect of solvent selection to the preparation of thin films exhibiting the desired performance characteristics.

Introduction

Solar energy offers great potential as a renewable source for conversion to electrical power. The demand for renewable energy sources is the driving force behind new approaches in the development of low-cost photovoltaic devices. The prospect of roll-to-roll processing makes organic semiconducting materials intriguing alternatives for future generations of photovoltaic devices [1]. However, power conversion efficiencies are not yet high enough for organic based devices to be commercially viable. An encouraging breakthrough in realizing higher efficiencies has been achieved by mixing electron donor type polymers with suitable electron acceptors. Efficient organic solar cells have been developed based on the bulk heterojunction concept of a conjugated polymer and a soluble derivative of the fullerene (C60 or C70, PCBM) with power conversion efficiencies of approximately 3% [2]. The power conversion efficiency is determined, in part, by the short-circuit current and the open-circuit voltage of the device. Improvement of the energy conversion efficiency of these devices relies on optimizing: (1) the charge carrier generation; (2) the selective transport and collection of charges at the electrodes; (3) the absorption of light.

The photovoltaic effect involves the generation of electron and hole pairs and their subsequent collection at the opposite electrodes. Studying photovoltaic efficiency in relation to morphology and mesoscopic ordering in the active layer is of intense interest [3]. One of the factors that plays an important role is the low entropy of mixing between two polymers. When a polymer blend is spin-coated from a well-mixed solution, the faster the solvent evaporates, the less time the polymer has to rearrange itself into discrete domains. Phase separation is then taking place and domain sizes of a few nanometers to several microns are created. By controlling the morphology of the phase separation into an interpenetrating bi-continuous network of donor and acceptor phases on a nanoscopic scale (∼10 nm), one can achieve a high interfacial area within a bulk heterojunction blend. However, the actual charge collection efficiency has not reach yet such values that will enable the commercialization of the device because the perfect charge transfer reaction is only effective at the interface, not in the bulk. The development of certain techniques such as annealing the film after spin-coating, exposure it to solvent vapors or controlling the rate of solvent evaporation could alter the morphology of the blend and a phase separation on the scale of ∼10 nm can be achieved.

Charge transport efficiency is an essential parameter capable of improving the performance of such optoelectronic devices. Well known hole transport (p-type) conjugated polymers are the poly(p-phenylenevinylene)s (PPVs), poly(p-phenylene)s, polythiophenes as well as their derivates. Poly(3-hexylthiophene) (P3HT) is one of the best candidates due to low energy band gap suitable for red light absorption, the high charge carrier mobility and the excellent solubility in organic solvents. In addition, electron accepting optoelectronic materials (n-type) with high electron affinity have recently been synthesized and investigated. Main chain polyquinoxalines, [4] polyquinolines [5] and main chain as well as side chain polyoxadiazoles [6] are representative class of materials which combine the high electron affinity with high thermal and oxidative stability, outstanding mechanical properties and good film-forming ability [7]. Furthermore several oxadiazole-containing polymers have been employed as electron transporting materials in organic light emitting diodes (OLEDs) [8]. This ability to carry electrons is believed to arise from the high electron affinity of the oxadiazole ring in the molecule. However, due to their low solubility, these compounds possess poor solution processability. Therefore, a lot of effort has been consumed to incorporate π-conjugated oligomers into polymers either as polymer backbones or as side chains in order to improve their solubility [9].

In this work, we present the synthesis of four new main chain 1,3,4-oxadiazole copolyethers (IIV) by the nucleophilic polycondensation reaction and their optical characterization. Moreover, a fluorene copolymer containing 1,3,4-oxadiazole unit (PFOXD) was also synthesized by Suzuki coupling reaction. In addition, a detailed AFM study of the surface morphology of the blends consisted of the regioregular poly(3-hexyl)thiophene with TSTPOXD and PFOXD as thin films was performed. Finally, the photovoltaic response of the TSTPOXD or PFOXD with the P3HT as bulk heterojunction mixtures and a single layer device of the PAFOXD were investigated.

Section snippets

Materials and measurements

Regioregular poly(3-hexylthiophene) (P3HT) was received from Aldrich and all the other chemicals and solvents were purchased from Aldrich or Acros and used without purification unless otherwise noted. TSTDIOL, TSQDIOL, ANTDIOL, and (FPh)2OXD were prepared according to literature procedures (Scheme 1, Scheme 2) [10]. The structures of the synthesized monomers (FLUDIOL and (BrPh)2OXD) as well as the soluble oxadiazole polyethers were clarified by high-resolution 1H NMR spectroscopy with a Bruker

Preparation and solubility of the copolymers

Polymers (IIV) were synthesized by nucleophilic substitution of the aromatic bisfluoro compound ((FPh)2OXD) with bisphenols (TSTDIOL, TSQDIOL, ANTDIOL and ANTDIOL/FLUDIOL (70/30)) as shown in Scheme 2. The polymerization procedure runs in an azeotropic mixture of the aprotic polar solvents of dimethylformamide and toluene at 140 °C for 12 h and at 180 °C for 6 h in a presence of K2CO3 and KOH as base. The molecular weights of the resulting copolymers are depicted in the Table 1. The TSTPOXD,

Conclusions

New copoly(arylethers) containing 1,3,4-oxadiazole units in the main chain were synthesized. These polymers are soluble in common organic solvents. Blends from those polymers with P3HT were prepared and investigated with optical and AFM techniques. All the prepared mixtures made by P3HT and TSTPOXD or PFOXD showed photoluminescence quenching compared to the emission spectra of P3HT. This finding confirms a charge transfer from the phase of the electron donor (P3HT) to the phase of the electron

Acknowledgements

The authors are grateful to Konarka Technologies of Lowell, MA, USA and the Greek Ministry of Development, under the grant EPAN E13 for the financial support of the project. In addition, we would like to thank Dr Russell Gaudiana of Konarka Technologies for the helpful discussions. Finally, we also thank Dr Vasilis Dracopoulos of FORTH-ICEHT for the help in acquiring the AFM images.

References (17)

  • J.F. Eckert et al.

    J Am Chem Soc

    (2000)
    A.M. Ramos et al.

    J Am Chem Soc

    (2001)
    T. Gu et al.

    Chem Phys Chem

    (2002)
  • C.J. Brabec et al.

    Adv Funct Mater

    (2001)
  • M.M. Wienk et al.

    Angew Chem Int Ed

    (2003)
    F. Padinger et al.

    Adv Funct Mater

    (2003)
  • L. Ouali et al.

    Adv Mater

    (1999)
  • J.K. Lee et al.

    Chem Mater

    (1999)
    Y. Cui et al.

    Macromolecules

    (1999)
    T. Yamamoto et al.

    J Am Chem Soc

    (1996)
    X. Zhang et al.

    Macromolecules

    (1999)
    Y. Dai et al.

    J Organomet Chem

    (1997)
  • S.A. Jenekhe et al.

    J Phys Chem B

    (2000)
    W.Y. Huang et al.

    Macromolecules

    (1999)
    L. Lu et al.

    Macromolecules

    (2001)
    C.L. Chiang et al.

    Chem Mater

    (2002)
    X. Zhang et al.

    Macromolecules

    (2002)
  • M. Strukelj et al.

    Science

    (1995)
    F. Cacialli et al.

    Synth Met

    (1995)
    X.C. Li et al.

    Adv Mater

    (1995)
    M. Ueda et al.

    Polym J

    (1989)
  • B. Schulz et al.

    Adv Mater

    (1997)
    W. Huang et al.

    Macromolecules

    (1999)
    Z. Peng et al.

    Chem Mater

    (1998)
There are more references available in the full text version of this article.

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