Carrier mobility in organic charge transport materials: Methods of measurement, analysis, and modulation

The measurement and control of charge carrier mobility in organic semiconductors are two prevalent issues in the growing field of organic electronics. The mobility is a measure of the speed of net charge movement per unit of applied field. This quantity determines how fast circuits and elements can respond and how much current they can support at a given voltage. While there are many methods to measure the hole and electron mobilities of organic materials, each has its own limitations and requirements. There is room for new methods to allow the measurement of additional materials and in different circumstances. In addition, exploring the capability of molecular systems to modulate these mobilities provides opportunities for improvements in the performance of organic electronic devices.;This thesis has focused on developing new methods of measuring the charge carrier mobility in organic semiconductors and on evaluating the capability of a series of hybrid compounds to modulate the emitting layer's mobilities for application in organic light-emitting diodes. Key results are summarized as follows:;(1) The charge-retraction time-of-flight technique for carrier mobility measurements was explained, explored, and validated with two well know hole transport materials producing retraction transients nearly identical to those of photocurrent time-of-flight, while amenable to thinner samples and utilizing a simple, all-electrical experimental setup. In addition, a method to determine the transition voltage more accurately in these devices was developed.;(2) The electron mobilities of a known electron transport material and an unknown polycrystalline electron transport material were measured by both charge-retraction time-of-flight and photocurrent time-of-flight. The results for the known compound were found to match within error for both techniques, and various measures of how dispersive the transport was also matched very closely, validating the charge-retraction time-of-flight technique for dispersive and electron transport.;(3) The integrating-mode photocurrent time-of-flight technique was described in detail, and an analysis method extending Scher and Montroll's procedures to integrating-mode transients was derived and explored. The mobility values determined by this analysis, as well as three other methods (two of them from the literature) were compared and contrasted for nondispersive hole transport and dispersive electron transport, and the analysis developed here was found to be the only one to agree with traditional, current-mode time-of-flight for both cases.;(4) Three compounds were synthesized to complete a series of hybrid materials designed to modulate the carrier mobilities in the emitter layer of organic light-emitting diodes. The hole and electron mobilities of these compounds were measured by photocurrent time-of-flight, in both current- and integrating-modes, as functions of field and temperature. It was found that the mobilities in these compounds spanned over four orders of magnitude, with the ratios of the hole to the electron mobility in neat layers ranging from 59:1 to 1:180. The trends in these mobilities were discussed using the disorder formalism for charge transport.