Effect of Dipolar Dopants on Charge Carrier Transport in Ordered Molecular Semiconductors with Order Parameters

We have investigated the orientational and translational order effect of dipolar molecules on carrier transport. Random orientation of dipolar or polar molecules interacting with carriers provides static disorder in organic semiconductors. We examined the doping effect of dipolar dopants using liquid crystalline organic semiconductors, which provides us with a useful model system for examining the effect in both experiments and simulations: we doped molecules with electrostatic dipoles or molecular polarization in smectic liquid crystal, e.g., phenylnaphthalene derivatives, which show the layered structure and resulting 2D carrier transport. The dopants such as organic cyanides and ketones are controlled in their orientation and translational ordering in the smectic liquid crystals where they are either mixed in intralayer or microphase separated in the interlayer, while the others are inserted into the interlayer and behave in a three-dimensional random orientation and coordination in the alkyl chain of liquid crystal.<br/>Obtained mobilities by time-of-flight experiments have characteristic temperature and field dependence and can be analyzed by Gaussian disorder formalism. These effects are successfully analyzed by using our theoretical model based on calculating the N-th moment of the total Coulomb potential contribution of dipoles for an ordered molecular alignment. We found that the dipole dopants degrade the carrier transport in the 2D-ordered layer structure and gave the additional energetic disorder <i>σ_d</i> proportional to a square root of the dipole concentration ~<i>C</i>^1/2. Therefore, it should be noted that the carrier transport is less sensitive to the dipoles compared with that in the 3D conduction system. We also confirmed the phenomena by carrier transport simulation of kinetic Monte Calro for a snap-shot of hopping sites decided by MD simulation with generated energetic disorder landscape.<br/>This dependence shows us that the effect on carrier transport is limited in highly ordered materials, even if there is an origin that causes a large static disorder. This gives us an insight into why the behavior of carrier transport is different between crystal and semi-ordered phases such as liquid crystals or polymers. Intrinsic mobilities measured in various liquid crystalline (LC) materials reported in the last few decades, do not change much from material to material but do change much from mesophase to mesophase: for example, typical mobility is on the order of ~10^-2-10^-3 cm²/Vs in crystal B and E phases, of ~10^-2cm²/Vs in hexatic B phase, and 10^-3cm²/Vs in smectic A and C phases [1] regardless chemical structures of liquid crystal. On the other hand, a wide variation of mobilities from 10^-3 to 10 cm²/Vs depending on chemical structures is often observed in crystals. It is true even for liquid crystalline molecules, benzothienobenzothiophene (BTBT) derivatives, such as Cn-BTBT and Ph-BTBT-Cn in crystalline phase: in fact, high mobility of over 10 cm²/Vs reported in their crystalline phase. [2]<br/>These facts indicate a clear contrast in carrier transport between the ordered and semi-ordered phases. For investigating the origin of differences, we introduced the Gaussian disorder model restricted by the order parameters as static disorder into the charge transport model with the dynamic disorder. This model can be consistently applied for both crystalline and liquid crystalline phases and made it possible to discuss the difference in carrier transport between crystal and semi-ordered materials. These fundamental studies are very important for the functional design of organic semiconductors, especially for the operation of printable electronics.<br/><br/>Referencies<br/>[1] Jun-ichi Hanna, Akira Ohno, Chap.3 in Self-Organized Organic Semiconductors: From Materials to Device Applications, edited by Quan Li, WILEY, (U.S.) 2011.<br/>[2]H. Iino, T. Usui, and J. Hanna, Nature Comm., 6, 6828 (2015).