A Thermally Stable Semiconducting Polymer

A bs or pt io n RT 120 170 200 250 300 350 400 Early research on polymer electronic devices successfully demonstrated function and performance adequate for specific applications. As a result, the performance of devices fabricated from semiconducting polymers has improved to the point where ‘‘plastic’’ electronics are now expected to develop into a significant industry with a large market opportunity. However, the limited stability of polymer-based devices continues to hinder the path toward commercialization. Because stability in air is critical to the commercialization of polymer electronic devices, discussions concerning the stability of semiconducting polymers have focused on degradation caused by reaction with oxygen and water vapor. Conjugated polymers are, however, generally believed to be incapable of withstanding high temperatures (i.e., temperatures well above the glasstransition temperature, Tg), [6,7] thus, stability at high temperatures has received less attention. The availability of semiconducting polymers that can survive exposure to elevated temperatures would open a variety of new possibilities. For example, since inorganic electronic devices typically require process steps that must be carried out at high temperature (often over 300 8C), semiconducting polymers capable of withstanding high temperatures will enable the fabrication of novel organic–inorganic hybrid devices. Here, we report the remarkable stability of the poly(2,7carbazole) derivative, poly[N-900-hepta-decanyl-2,7-carbazole-alt5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)], (PCDTBT; see the inset of Fig. 1a). Prior to this report, there was no known example of a semiconducting polymer that is both stable in air at (and above) room temperature and capable of withstanding high temperatures for extended periods of time. PCDTBT is one of a relatively large class of ‘‘donor–acceptor’’ polycarbazole co-polymers. Recently, polymer bulkheterojuction solar cells fabricated with phase-separated blends of PCDTBT and PC71BM were demonstrated with internal quantum efficiency approaching 100%, power conversion efficiency of 17% in response to monochromatic radiation within the absorption band, and power conversion efficiency of 6.1% in response to solar radiation. To investigate the stability of PCDTBT, we have carried out spectroscopic studies on PCDTBT thin films and transport studies using the field-effect transistor (FET) architecture with PCDTBTas the semiconductor material in the channel. Figure 1 shows UV–visable (UV–vis) absorption spectra of PCDTBT thin films annealed for 15 minutes at various temperatures in air (Fig. 1a) and under N2 atmosphere (Fig. 1b). In air, the p–p* absorption spectrum is not affected after exposure to temperatures up to 150 8C. Under N2 atmosphere (Fig 1b), the electronic band structure of PCDTBT is stable after exposure to temperatures as high as 350 8C.

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