ABSTRACT
Transport in Amorphous Semiconductors and Semiconducting Organic Polymers ......................................................................................... 176 5.3.1 Amorphous Semiconductors ............................................................ 177 5.3.2 Semiconducting Organic Polymers ................................................. 182
5.4 Polaron Formation and Dynamics in Molecular Electronic Materials ........ 184 5.4.1 Dynamics of Polaron Formation: Quasi-One-Dimensional
Systems ............................................................................................ 185 5.4.2 Carrier Transport in Pentacene ........................................................ 187
5.5 Charge Transport in Nanoscale Materials: Nanocrystalline Semiconductors and Quantum Dots............................................................. 188 5.5.1 Conductivity and Dielectric Screening in Nanoporous TiO2 .......... 189 5.5.2 Excitons in Semiconductor Quantum Dots...................................... 190
5.6 Extending into Mid-Infrared Spectral Regime: Carrier Dynamics in Graphite...................................................................... 193
5.7 Summary ...................................................................................................... 196 Acknowledgments .................................................................................................. 196 References .............................................................................................................. 196
Terahertz (THz) spectroscopy based on femtosecond laser techniques1-12 has emerged as a powerful probe of charge transport and carrier dynamics. The technique makes use of ultrashort pulses of propagating electromagnetic radiation to measure conductivity in the THz spectral regime. (A frequency of 1 THz 1012 Hz, and corresponds to an energy of 4.2 meV 33 cm 1 48 K and a wavelength of 300 Mm.) In particular, when combined with a time-synchronized femtosecond excitation pulse, THz spectroscopy is suitable for the investigation of electronic charge transport under nonequilibrium conditions.13-17 These attributes permit THz spectroscopy to circumvent many of the constraints of conventional transport measurement techniques. Conventional electrical characterization techniques do not generally allow access to material response at frequencies as high as those of carrier-scattering processes, information that is of great value in examining the fundamentals of charge transport mechanisms, and are not suited for probing very rapid changes in the material response, so that they typically cannot explore the interesting and important regime of nonequilibrium electronic excitation. In addition, in conventional measurements of charge transport, one must have samples amenable to good electrical contact, an issue that can be a severe constraint, especially in studies of nanoscale materials. The THz probing technique, used in conjunction with optical pump excitation, can overcome all of these limitations. In this chapter, we describe the application of this approach to studies of the a number of aspects of fundamental properties of charge transport in novel electronic materials and highlight new insights into the properties and dynamics of charge carriers that have resulted from the distinctive measurement capabilities of time-resolved THz spectroscopy. We focus in particular on carrier dynamics made accessible by recent ultrafast time-resolved techniques; measurements of steady-state properties in this important spectral region using both recently developed time-domain THz and well-established far-infrared/submillimeter techniques have been applied in a wide range of studies of materials, as reviewed extensively elsewhere.1-12
In Section 5.2, we discuss time-resolved THz studies of charge transport in insulators. Unlike in metals and semiconductors, because of difculties in doping or thermally exciting the system, charge carriers are often present only under nonequilibrium conditions. Insulators, therefore, provide an ideal laboratory to demonstrate the unique capabilities of time-resolved THz spectroscopy. In addition, the subpicosecond timescale of the probe electric eld transient implies that we have access to frequencies up to the THz range, frequencies comparable to charge carrier scattering rates in typical crystalline materials or transfer rates between neighboring sites in disordered systems. In Section 5.3, we present examples of how THz spectroscopy can yield important information about charge transport in disordered electronic materials, focusing in particular on dispersive transport in amorphous semiconductors and photoconducting polymers. The short-pulse nature of the THz probe also implies that we have access to the dynamics of rapidly evolving systems. In Section 5.4, we describe time-resolved THz studies of fast photocarrier dynamics in molecular electronic materials, including studies of the dynamics of polaron formation in quasi-one-dimensional systems and of photoconductivity in molecular crystals. Since the THz measurements are made with traveling electromagnetic waves,
no electrical contacts are needed. In Section 5.5, we describe how one can take advantage of this property to perform measurements of nanoscale materials, systems for which formation of electrical contacts may be difcult and, further, may signicantly modify the properties of the material. Finally in Section 5.6, we indicate how the technique of THz spectroscopy can be extended from the far-infrared to the mid-infrared regime and can be used to obtain both amplitude and phase information about material response in this spectral regime after photoexcitation. These ultrabroadband THz sources can also provide pulse durations well below 100 fs, permitting higher time resolution for probing ultrafast processes. Studies of the impact of strongly coupled optical phonons on the electronic charge transport properties in graphite provide an example of this approach.
