Research
Fundamental studies of disorder
Soft semiconductors are materials that are held together by van der Waals forces rather than the stronger covalent bonds that are more typical of conventional semiconductors such as Si. Examples of soft semiconductors are molecular crystals and thin films of molecules and polymers. Due to weak intermolecular interactions, disorder is found in all soft semiconductors. Even molecular crystals possess thermodynamically-stable molecular vacancies that cannot be removed by annealing. We examine the influence of disorder on charge injection and charge transport in disordered semiconductors. Charge injection dominates the performance of many organic devices. For example, the operating voltage of OLEDs based on small-molecular weight materials is typically determined by charge injection. We have proposed that interfacial disorder is a critical determinant of charge injection into soft semiconductors.
Understanding the electric field dependence of charge-carrier mobility is central to the rational design of organic semiconductor devices. We present an analytic description of mobility by considering non-equilibrium carrier distributions within a percolation framework. The theory is compared to measurements by Brütting, et al [Organic Electronics 2, 1 (2001)] of the current-voltage and mobility of the archetype small molecule tris(8-hydroxyquinoline) aluminum. The theory accurately reproduces the temperature, carrier density, and electric field dependences of the experimental data. Finally, we have pursued direct measurements of energetic disorder in soft semiconductors. We measure the density of states as a function of energy in thin films of copper phthalocyanine (CuPC).The density of states is an important factor in understanding charge transport in organic semiconductors. Using Kelvin probe force microscopy we find an exponential density of states with a characteristic energy of 0.11eV over a 0.5eV range of the highest occupied molecular orbital of CuPC. We also find that the technique is limited by charge trapping and hysteresis at low densities of states, and non-uniform potential profiles within the CuPC film at high densities of states.
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