The elastic moduli of polythiophenes, regioregular poly(3-hexylthiophene) (P3HT) and poly-(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) (pBTTT), are compared to their field effect mobility showing a proportional trend. The elastic moduli of the films are measured using a buckling-based metrology, and the mobility is determined from the electrical characteristics of bottom contact thin film transistors. Moreover, the crack onset strain of pBTTT films is shown to be less than 2.5%, whereas that of P3HT is greater than 150%. These results show that increased long-range order in polythiophene semiconductors, which is generally thought to be essential for improved charge mobility, can also stiffen and enbrittle the film. This work highlights the critical role of quantitative mechanical property measurements in guiding the development of flexible organic semiconductors.

It has recently been shown that surface energy effects can cause selective segregation at the active layer interfaces of a bulk heterojunction (BHJ) organic photovoltaic device. The active layer interface composition has been suggested to impact device performance. In this study changes in the BHJ vertical composition profile of BHJ active layers cast on two hole transport layers (HTL) with significantly different surface energies (γ) are characterized using spectroscopic ellipsometry and near-edge X-ray absorption fine structure spectroscopy. Changes in the HTL γ are shown to significantly affect the BHJ interfacial segregation at the buried interface near the HTL while the composition near the free surface (air) of the BHJ is unaffected. Despite the significant differences in vertical segregation at the HTL interface, the performances of the resulting organic photovoltaic devices were relatively similar.

Electron transport through Si-C bound alkyl chains, sandwiched between and Hg, is characterized by two distinct types of barriers, each dominating in a different voltage range. At low voltage, the current depends strongly on temperature but not on molecular length, suggesting transport by thermionic emission over a barrier in the Si. At higher voltage, the current decreases exponentially with molecular length, suggesting transport limited by tunneling through the molecules. The tunnel barrier is estimated, from transport and photoemission data, to be approximately 1.5 eV with a 0.25m(e) effective mass.

The buried interface composition of polymer-fullerene blends is found by near-edge x-ray absorption fine structure spectroscopy to depend on the surface energy of the substrate upon which they are cast. The interface composition determines the type of charge transport measured with thin film transistors. These results have implications for organic photovoltaics device design and the use of transistors to evaluate bulk mobility in blends.

Current transport by tunneling through molecular devices is thought to be dominated by the height and width of the barrier(s) resulting from the presence of molecules between the electrodes. To a first approximation, the barrier height in metal/molecule junctions is given by the energy difference between the Fermi level of the electrode and the closest molecular energy levels, the highest occupied molecular orbital (HOMO) and/or the lowest unoccupied molecular orbital (LUMO). For semiconductor/molecule junctions, the corresponding barrier height is the energy difference between the edge of the conduction or valence band and the LUMO or HOMO, respectively, depending on the semiconductor doping type, and can be tuned by changing the semiconductor doping type.[1] Experimentally the position of the molecules' HOMO and LUMO relative to the electrodes' Fermi level or band edges can be determined using ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES) measurements. Therefore, the tunneling-barrier height through a molecular layer can, in principle, be deduced by using this method.[2]Here, we compare and analyze the electronic transport through alkyl chains, C n H 2n+1 with n = 12, 14, 16, and 18, bound directly to p-or n-Si, via C-Si bonds, and contacted by Hg to form Si-alkyl/Hg junctions. In these molecular junctions the alkyl chains are connected via the same Si-C bonds to either n-or p-Si, with presumably the same amount of charge transfer between the molecule and the electrode as a result of this bond formation. This feature allows us to isolate and ascertain the effect of the electrode Fermi-level position on charge transport through the junction. [3,4] As carried out earlier for the n-Si system only, [5] we now deduce the barriers for charge transport through the alkane monolayers, both from transport through the junctions and from spectroscopic measurements of the corresponding p-and n-Si-C n H 2n+1 interfaces. The main differences in analyses with our earlier n-Si work [5] are that we use a more complete model for transport analysis and that we can now interpret the photoemission data with the help of complementary theoretical computations. [6] In this way, we find that whereas the spectroscopic measurements show a tunnel barrier of approximately 3-4 eV (rather than the smaller one derived earlier [5] without the help of theory to interpret the IPES and UPS data), fitting the current-voltage (I-V) curves to transport by tunneling yields a barrier of only approximately 0.7-1 eV. We show that this difference, which is ascribed to the presence of states at the interface caused by Si molecule interactions beyond Si-C bond formation, forces us to revise our view of tunneling through such molecular junctions. As shown earlier, in a semiconductor/saturated-molecule/ metal junction, two transport barriers can exist simultaneously, a Schottky barrier inside the semiconductor, caused by band bending near the interface, and a tunnel barrier formed by the insulating, r-bonded molecu...