We analyzed the experimental time-of-flight data for photoinjected holes in two smectic liquid crystals, the first consisting of a phenylnaphthalene derivative 8PNPO12, and the second consisting of a biphenyl derivative 6OBP6. We fit the time of flight transients for different electric field strengths to a multiple trapping model (MTM). From these fits we determined the distribution of trap depths, under the assumption that (i) linear response is valid, and (ii) the trap release rates are independent of field.
An effective method for detecting a trace amount of chemical impurity, e.g., a few ppm or less, that degrades the charge carrier transport properties in smectic liquid crystalline (SmLC) semiconductors was investigated with a model system i.e., a 2-phenylnaphthalene derivative of 2-(dodecyloxy)-6-(4-octylphenyl)naphthalene (8-PNP-O12) and a terthiophene derivative of 2,5-bis(5-hexylthiophene-2-yl)thiophene (6-TTP-6). A transient photocurrent measurement could detect a chemical impurity of a few ppm or less that conventional analytical methods such as high-performance liquid chromatography (HPLC) and gas chromatography (GC) failed to detect: the slow transit induced by drift of ionized impurity molecules allowed us to detect it, which was clearly distinguished from the fast transit induced by the electronic conduction in the host SmLC semiconductor. This systematic study provided a semiquantitative basis for evaluating the contamination of chemical impurity.
We have investigated the hole transport in smectic mesophases by Monte Carlo simulation based on a 2D hopping transport in Gaussian-distributed density of states and time-of-flight experiments. We found that their unique carrier transport properties such as non-Poole-Frenkel type of behavior i.e., field-and-temperature independent mobility, is well explained by the 2D disorder model with a small Gaussian width of 50-60 meV. Furthermore, we found the Pool-Frenkel type of behavior in a biphenyl derivatives and at a low temperaure range below ambient temperature in a therthiphene derviative and determined the Gaussian width to be 100-120meV and 50 meV, respectively. We came to a conclusion that the charge carrier transport in smectic mesophases can be explained by a 2D disorder model with a small Gaussian width of the density of states σ, where a value of σ/kT plays important role to determien its behavior at a given temperature.