Hopping and band mobilities of holes in organic semiconductors at room temperature were estimated from first principle calculations. Relaxation times of charge carriers were evaluated using the acoustic deformation potential model. It is found that van der Waals interactions play an important role in determining accurate relaxation times. The hopping mobilities of pentacene, rubrene, and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) in bulk single crystalline structures were found to be smaller than 4 cm(2)∕Vs, whereas the band mobilities were estimated between 36 and 58 cm(2)∕Vs, which are close to the maximum reported experimental values. This strongly suggests that band conductivity is dominant in these materials even at room temperature.

The formation of a two-dimensional electron gas at oxide interfaces as a consequence of polar discontinuities has generated an enormous amount of activity due to the variety of interesting effects it gives rise to. Here we study under what circumstances a similar processes can also take place underneath ferroelectric thin films. We use a simple Landau model to demonstrate that in the absence of extrinsic screening mechanisms a monodomain phase can be stabilized in ferroelectric films by means of an electronic reconstruction. Unlike in the LaAlO3/SrTiO3 heterostructure, the emergence with thickness of the free charge at the interface is discontinuous. This prediction is confirmed by performing first principles simulations of free standing slabs of PbTiO3. The model is also used to predict the response of the system to an applied electric field, demonstrating that the two-dimensional electron gas can be switched on and off discontinuously and in a non-volatile fashion. Furthermore, the reversal of the polarization can be used to switch between a two-dimensional electron gas and a two-dimensional hole gas, which should, in principle, have very different transport properties. We discuss the possible formation of polarization domains and how such configuration competes with the spontaneous accumulation of free charge at the interfaces.

C 60 fullerene is widely used in organic devices; however, there are still unaccountable phenomena, such as the strong light absorption around 2.8 eV of C 60 films and favorable device performance of organic photovoltaics (OPVs) using C 60rich bulk heterojunctions. We studied the excited states of C 60 fullerene clusters using large-scale time-dependent density functional theory taking thermal vibrations into account. A strong absorption peak around 2.8 eV appeared because of aggregation and we found that Wannier-like delocalized excitons spread over multiple molecules were generated in this energy region. This is contrary to the accepted theory that only Frenkel or charge transfer (CT) excitons are generated in organic materials. It is considered that the delocalized excitons with energies greater than electrical gap (E g ) become CT excitons after thermalization whereas those with energies lower than E g become either CT excitons by obtaining thermal energy or Frenkel excitons after thermalization. In the exciton dissociation process, there is an essential channel in which the delocalized excitons are generated first and then subsequently become CT excitons. The delocalized exciton generation can explain both the strong absorption around 2.8 eV and enhanced OPV performance using C 60 -rich bulk heterojunctions.

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