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The optical properties of chemical vapor deposition grown graphene were measured with high accuracy by spectroscopy ellipsometry in the energy range of 0.7 eV–9.0 eV, which is spectrally broader compared with those reported in literature. The refractive index (n) and extinction coefficient (k) of graphene were accurately obtained and compared with directly measured transmittance data. The absorption of graphene follows the well-known fine structure constant in the visible range, becomes lower below 1 eV, and shows a strong absorption peak around 4.8 eV. The latter was attributed to the resonant excitons near the van Hove singularity at the M point of the Brillouin zone. A higher energy absorption peak was observed at 6.4 eV resulting from the excitonic effect of the σ-to-σ∗ transition at the Γ point of the Brillouin zone. The multi-layer graphene, fabricated by repeated transfer, exhibit similar optical properties to mono-layer graphene and the 4.8 eV absorption peak exhibits a layer-number dependent peak shift, similar to the exfoliated AB-stacked multi-layer graphene. The broadband optical properties reported in this letter are believed to enhance the understanding of the optical properties of graphene and will benefit the development of graphene-based optoelectronic devices.

Modulated photocurrent spectroscopy was used to investigate the dynamic response of charge carrier transport in thin film CdTe/CdS solar cells. The impact of light bias and temperature over a broad excitation frequency range were measured. The observed features of the data, including a photocurrent ‘phase-lead’ and a ‘phase-lag’ over different regions of the frequency spectrum, were explored in the context of an equivalent circuit model. Comparisons between the model's predicted performance and the measured data suggest that charge carrier recombination at the cell's back metal/semiconductor contact is the main source of photocurrent loss in these devices.

The threshold voltage and the turn-on voltage of pentacene thin film transistors (TFT) can be shifted by covering the gate insulator with a self assembled monolayer (SAM) of organosilane molecules. In this article we present experimental evidence identifying the SAM-induced modifications of the surface potential as the main cause for the shifted characteristics. To this end, FETs have been produced both on thin films and on single crystals. In the “flip-crystal” method for fabricating FETs, the single crystals of rubrene were placed onto prefabricated structures comprising a gate electrode, gate insulator and source/drain contacts. Prior to attaching the crystals, the SiO 2 gate insulator was treated with different organosilane molecules that form SAMs on the gate insulator. Depending on the molecule’s dipole moment, shifts of the characteristics by up to 40 V in V g are measured. This behavior can be explained in a simple energy level diagram where the surface potential of the gate insulator is changed by the built in electric dipole field of the self assembled monolayer.

Systematic studies are a prerequisite for a detailed understanding of the internal processes in organic semiconductors and devices, which is of great importance for optimizing organic light-emitting diode performance. Devices based on small molecules are especially well-suited for introducing thin layers (

We report on the contact resistances for pentacene thin film transistors with two different designs: top and bottom contact configurations (referred to as TC and BC TFTs, respectively) for two different contact metals (gold and palladium). The extraction was done based on the dependencies of the channel resistances on the gate length and gate voltage. The extracted gold TC TFT contact resistance depends on VGS, but shows no dependence on the drain bias. The TC TFT contact resistance is comparable to or exceeds the channel resistance for channels shorter than approximately 10 μm. The contact resistance of BC TFTs depends both on gate and drain bias. We propose a circuit simulating the BC TFT contact resistance and verify the circuit applicability by extracting and comparing the TFT channel resistances at different drain voltages. Our results reveal an important role played by contact resistances and provide an accurate model of the contact phenomena suitable for implementation in Spice or other circuit simulators.