Optically induced dielectric changes in organic semiconductor heterostructures

PhD ThesisThis work includes studies into optically induced dielectric changes in organic heterostructure devices. The goal is experimentally measure the changes in the dielectric constant under illumination in organic devices. The organics used in this study is a 95:5 ratio of Poly(3-hexylthiophene) : Phenyl-C61-Butyric-acid-Methyl-ester, P3HT:PCBM blend. The permittivity change under illumination is studied at both high (GHzTHZ) frequencies as well as low (sub-MHz and DC) frequencies. To facilitate this study into permittivity, charge transport properties also need to be investigated. The charge carrier mobility is measured by transient time of ight method, which resulted in typical mobilities of 2 104 cm2V1s1. This is then used, together with I-V characteristics of the blended organic devices, to calculate charge carrier concentrations upon illumination of 10141015 cm3. This is then compared to unblended devices which give a lower charge density of 10121013 cm3. This increase of carrier concentration is further investigated upon by calculating the photogenerated charge carrier e ciency which we report to be 1 104 and 7 102 for the pure and blended samples respectively. This is a substantial improvement of two orders of magnitude with only the inclusion of 5% acceptor, which denotes that this donor acceptor ratio has viability in non-photovoltaic applications which rely on the polarisability of the material. Quasi-Optic Transmissometry is used to measure the phase and magnitude change under illumination for encapsulated organic devises. iv We report that these (non-contact electrode based) devices result in a change in the imaginary dielectric constant of "Im = 0:6 to 1:2 at 60 mWcm2 illumination and GHz frequency. This corresponds with real permittivity change of "Re = 0:05 to 0:55. Low frequency changes under illumination were also investigated via impedance spectroscopy. No conclusions can be drawn about the viability of the organic devices tested for optically tunable dielectric applications at low frequencies. This is due to practical limitations in impedance spectroscopy not being able to measure geometrical capacitance, due to the system being dominated by the chemical capacitance of the organic-metallic interface. The measurement of this interfacial capacitance is further masked by a leakage current at low illumination. From interpretation of Nyquist plots we are able to report a photocapatience of 0:8 nF under 60mWcm2. The outcome of this study is that 95% P3HT:5%PCBM blends are viable as optically tuneable dielectric devices for use in radio frequency applications. This is due to the relatively large dielectric changes reported and the substantial increase in photocarrier generation e ciency of the blend compared to pure P3HT devices