Frequency-Modulated Gigahertz Complex Conductivity of TiO2 Nanoparticles: Interplay of Free and Shallowly Trapped Electrons

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• 作者：Akinori Saeki ; Yoshihiro Yasutani ; Hikaru Oga ; Shu Seki
• 刊名：Journal of Physical Chemistry C
• 出版年：2014
• 出版时间：October 2, 2014
• 年：2014
• 卷：118
• 期：39
• 页码：22561-22572
• 全文大小：601K
• ISSN：1932-7455

文摘

In a quest to explore the prominent electronic properties of titanium dioxide (TiO₂), precise knowledge on the electron mobility and trapping process is an immensely important part of organic photovoltaic applications. We have investigated the frequency dispersion of complex photoconductivity of TiO₂ nanoparticles by laser-flash frequency-modulated time-resolved microwave conductivity (FM-TRMC) in the 9鈥?4 GHz region. The absolute ratio of imaginary to real transient conductivities was analyzed by the combination of Drude鈥揝mith and Drude鈥揨ener models, which allows for determining the trap depth (E_T) and population of trapped electrons (f_trap). We have identified that the shallowly trapped electrons (E_T = 70鈥?10 meV, f_trap > 0.8) have a large impact on the negative imaginary conductivity, while the free electron confined in TiO₂ nanoparticle significantly contributes to the positive real part in spite of its small population (<0.2). The kinetic mismatch of real and imaginary conductivities suggests the involvement of different decay processes, which is a sharp contrast to a blend film of poly(3-hexylthiophene) (P3HT) and methanofullerene (PCBM), where both real and imaginary transients were identical over the microsecond time range. The proposed analysis method was found applicable to the wide frequency range of alternating current photoconductivity from gigahertz (GHz) to terahertz (THz) regions. Accordingly we suggest that gigahertz complex photoconductivity contains rich fingerprints regarding interplay between free and trapped electrons, which can facilitate the rational design of high-performance optoelectronic devices.