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44294-G7
Terahertz Spectroscopy of Photo-Conducting Liquid Crystals

Jie Shan, Case Western Reserve University

The goal of this PRF G project is to investigate the electronic charge transport properties of organic discotic materials using the ultrafast technique of terahertz time-domain spectroscopy. Organic photoconductors are promising candidates for electronic and optoelectronic applications. And the electronic charge transport properties of the material are one of the key parameters for the performance of the electronic and optoelectronic devices. The THz time-domain spectroscopy measures the complex conductivity at the THz (10¹² Hz) frequencies without the need of electric contacts. The method is ideal for the study of intrinsic transport properties of organic materials, high quality large crystalline samples of which are often difficult to obtain.

 As a model system, we investigated Zn phthalocyanine (Pc) derivatives that belong to a family of discotic materials. These disk-like molecules self-assemble into columns with significant overlap of the delocalized π-electrons of the neighboring molecules, thereby creating quasi-one-dimensional channels for efficient charge transport. High carrier mobilities (> 100 cm²/Vs) have been previously reported in single crystal Pc’s [Hellmeier and Harrison, Phys. Rev. 132, 2010 (1963)]. Discotic materials are also known to form a liquid crystalline phase, in which the core molecules remain well ordered while the side chains are free to move. Such an intermediate state provides an interesting system to investigate the relation between charge transport properties and the microscopic order of the material. Liquid crystalline materials could also be advantageous from the standpoint of fabrication and processing.

 In the first year of the project we have successfully tested the feasibility of the THz technique in probing charge transport in Pc derivatives in both crystalline and liquid crystalline phase. THz photoconductivity was observed within 1 ps of the femtosecond optical excitation (limited by the instrumental time resolution) and to persist for 100’s ps.

 The main focus of our study in the second year is to understand the nature of charge transport in discotic materials, a model quasi-1D system. Both the temperature and frequency dependence of the complex photoconductivity have been studied. We first considered the crystalline phase for simplicity. In the temperature range of 80 – 370 K (below the transition temperature to the liquid crystalline phase) the conductivity in ZnPc was found to increase slowly with temperature. A detailed analysis of the frequency dependence of the complex conductivity showed that in the range of 0.2 – 1 THz the real part of the complex conductivity is positive and the imaginary part is negative. The absolute values of the real and imaginary part of the photoconductivity are comparable and they increase with frequency sublinearly.

 We compared the experimental results with several charge transport models. The observed frequency dependence of the conductivity is drastically different from that of the Drude model, behavior often observed in band transport in crystalline inorganic and organic materials. The observed conductivity resembles some of the features of ac hopping conductivity in disordered systems that was observed in photoconductive polymers [Hendry et al., Phys. Rev. Lett. 92, 196601 (2004)]. Many models based on dielectric relaxation have been developed for ac conductivity in disordered systems [S. Elliott, Adv. Phys. 36, 135 (1987)].  However, it is unclear that they are applicable to the conductivity at THz frequencies, which are approaching the time scale of a typical relaxation time. The plan for the coming year is to develop a suitable model to describe transport in a quasi-1D system with defects and electron-phonon interactions and compare with the experimental result of the THz complex conductivity.

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