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Research News


09/12/2007

Peaking inside an organic transistor

Recent Research from the group of Professor Xiaoyang Zhu.

Charge transport at or across interfaces is central to the operation of a wide variety of molecule-based devices, including organic light-emitting diodes, organic thin film transistors (OTFT), organic photovoltaic cells. In each of these devices, the critical charge transporting interfaces are buried interfaces, which are not readily accessible to conventional structural or spectroscopic probes. Though there have been tremendous advancements in molecule-based electronics in the last a few years, the difficulty in determining structure-property relationships at buried interfaces has produced a knowledge gap that is a key obstacle to future development. Gaining rigorous and verifiable knowledge of the molecular states involved during the build up and movement of charge would help to close that gap.

A recent JACS paper by graduate student Loren Kaake, postdoc Ying Zou, and chemistry professor Xiaoyang Zhu, in collaboration with Dr. Matt Panzer and Prof. Dan Frisbie of Chemical Engineering & Materials Science, demonstrated an exciting approach to probe buried interfaces (http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/2007/129/i25/abs/ja070615x.html). These authors applied attenuated-total-internal-reflection Fourier transform infrared (ATR-FTIR) spectroscopy to directly probe active layers in organic thin film transistors (OTFTs) fabricated on top of IR waveguides. The OTFT studied uses the n-type organic semiconductor, N-Nfdioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8) and a polymer electrolyte gate dielectric made from polyethylene oxide (PEO) and LiClO4. FTIR spectroscopy of the device shows signatures of anionic PTCDI-C8 species and broad polaron bands when the organic semiconductor layer is doped under positive gate bias (VG). The authors discovered two distinctive doping regions: a reversible and electrostatic doping region for VG 2V and an irreversible and electrochemical doping regime for VG > 2V. Based on intensity loss of vibrational peaks attributed to neutral PTCDI-C8, the authors reported a quantitative charge carrier density of 2.9x1014/cm2 at VG = 2 V; this charge injection density corresponded to the conversion of slightly over one monolayer of PTCDI-C8 molecules into anions. At higher gate bias voltage, electrochemical doping involving the intercalation of Li+ into the organic semiconductor film was found to convert all PTCDI-C8 molecules in a 30 nm film into anionic species. For comparison, when a conventional gate dielectric (polystyrene) was used, the maximum charge carrier density achievable at VG = 200 V was ~4.5x1013/cm2, which corresponds to the conversion of 18% of a monolayer of PTCDI-C8 molecules into anions. The success of this study opened the door to exciting research opportunities in quantitative study of organic electronics.

 
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