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Elastic Stretchability of Conjugated Polymer Blends

  • Author(s): Gao, Huier
  • Advisor(s): Pei, Qibing
  • et al.
Abstract

Fabricating fully stretchable thin film electronic devices entails innovation fundamentally new materials and modification of device architecture. My research has focused on employing polymer light emitting electrochemical cells (PLECs) as the device architecture, and exploring morphological control in conjugated polymer blends as a novel approach to obtain semiconducting materials with elastic stretchability.

PLECs are based on a conjugated polymer and solid electrolyte blend sandwiched between a pair of electrodes to form in-situ a light emitting p-i-n junction. The junction formation is an expedient way to reduce interfacial energy, leading to the insensitivity of the device operation with regard to the thickness of the active layer and work function of electrode materials. These features help simplify the device architecture and the fabrication process, and therefore, the PLECs are an attractive platform to investigate the stretchability of electronic materials and demonstrate stretchable light emitting devices.

An efficient PLEC has been demonstrated with a thin-film electroluminescent polymer sandwiched between two opposite electrodes. The electroluminescent layer is a blend of a soluble alkyloxy phenyl substituted poly(1,4-phenylene vinylene) (SY-PPV) with an ionically conductive medium exoxylated trimethylolpropanetriacrylate (ETPTA), and lithium trifluoromethanesulfonate (LiTf). The concentration of lithium salt plays an important role in the device performance, in terms of lifetime, and turn-on time. A maximum current efficiency of 5.9 cd/A and luminance stabilized at 1500 cd/m2 were acquired at 4 wt% salt and 15 wt% ionic conductor concentration. The addition of poly (ethylene oxide) (PEO) as ionic conductive source reduced the stability of PLEC operation due to the severe phase separation issue, but would be a benefit to enhance stretchability.

Conjugated polymers containing long-chain alkyl side groups for solubility are generally unstretchable: large strain induces crack formation, fracture, or plastic deformation. A soluble bright yellow light emission conjugated polymer, SY-PPV, admixed with ionically conductive mediums containing PEO and ETPTA, and LiTf can form an interpenetrating polymer network (IPN) to impart elastomeric deformability to a conjugated polymer. The spin-cast blend film formed an IPN morphology wherein SY-PPV forms a porous network with pores filled by the ionic medium. No global polarization of the SY-PPV chains was observed at strains up to 100% as the dichroic ratios of absorption and photoluminescence spectra remain close to 1. Light emitting devices based on the blend sandwiched between two stretchable transparent composite electrodes could be stretched by up to 140% strain. No electroluminescence polarization was observed.

Stimulated by the compliance of the stretchable PLEC devices, thin-film polymer solar cell comprising an elastically stretchable polymer blend was also demonstrated. The photovoltaic blend consists of [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) and poly(thieno[3,4-b]-thiophene/benzodithiophene) (PTB7) and is sandwiched between a pair of stretchable transparent electrodes. The use of a high boiling point additive, 1,8-diiodooctane (DIO) in the formation of the blend film not only improves the photovoltaic efficiency, but also renders the blend stretchable. The power conversion efficiency of the resulting solar cell device was 3.48%, and increased to 3.67% after one cycle of stretching to 50% strain. The cells could be stretched by as much as 100% strain. Microstructural analysis showed that the spin-cast blend film formed a uniform bulk heterojunction morphology where the PC71BM domain is present as small grains. The free volume left behind from DIO evaporation is crucial to the stretchability of the OPV blend. No global polarization of the PTB7 chains was observed when strain is released as the dichroic ratio remains close to 1.

Overall, this study shows that morphological control of polymer blends is an effective approach to render otherwise non-stretchable conjugated polymers into highly stretchable material while largely retaining the polymers electronic properties.

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