Terahertz Time-Domain Spectroscopy Study of the Conductivity of Hole-Transporting Materials

Some redox-active ionic liquids, organic amorphous solids containing electron-donating moieties, and conductive polymers can efficiently transport positive electrical charges. These hole-conducting media find increasing applications in unconventional solar cells and organic light-emitting diodes. Appropriate methods are required to unravel the detailed conduction and trapping mechanisms in these materials as well as to fully understand the interplay of molecular vibrations and charge transport processes. Here we present terahertz time-domain spectroscopy (THz-TDS) as a powerful technique, that allows the direct determination of the complex conductivity of hole transporting materials in a contactless, purely optical manner. Beyond the measurement of the conductivity of solid and liquid materials, THz (far infrared) spectroscopy also provides direct information of the librational and vibrational modes coupled to charge transport processes and therefore is invaluable in the study of the mechanism of polaronic transport in matter. Application of this technique is illustrated by examples provided by the study of the ionic liquid 3-methyl-1-propylimidazolium bis(trifluoromethane) sulfonimide (PMIT FSI) and that of the molecular liquid hole conductor 10-methylphenoxazine. Both systems are of particular interest, as both types of holetransporting media are successfully used as alternatives to solvent-based electrolytes in dye sensitized solar cells