Soft Nondamaging Contacts Formed from Eutectic Ga-In for the Accurate Determination of Dielectric Constants of Organic Materials

A method for accurately measuring the relative dielectric constant (εr) of thin films of soft, organic materials is described. The effects of the bombardment of these materials with hot Al atoms, the most commonly used top electrode, are mitigated by using electrodes fabricated from eutectic gallium-indium (EGaIn). The geometry of the electrode is defined by injection into microchannels to form stable structures that are nondamaging and that conform to the topology of the organic thin film. The εr of a series of references and new organic materials, polymers, and fullerene derivatives was derived from impedance spectroscopy measurements for both Al and EGaIn electrodes showing the specific limitations of Al with soft, organic materials and overcoming them with EGaIn to determine their dielectric properties and provide realistic values of εr

Two new types of π-conjugation between a fullerene sphere and an addend

Diazirine addition reactions to C60, followed by an HCl elimination step, yielded [6,6] and [5,6] sp2 carbon bridged fullerenes. The [5,6] adducts (alkylidenehomofullerenes) are the first examples of fullerene structures where the addend is attached to the fullerene cage in a true, albeit highly bent, alternating single–double bond fashion.

1064-nm Sensitive Organic Photorefractive Composites

Organic photorefractive composites with sub-second response time and complete internal diffraction efficiency at low-intensity 1064 nm illumination are presented. Direct sensitization of the composites is provided by the C84 fullerene derivative [84]PCBM or the organic/inorganic hybrid Ni-dithiolene complex TT-2324. Holographic measurements on blends with varying contents of sensitizer are demonstrated.

Charge Transfer Dynamics in Polymer-Fullerene Blends for Efficient Solar Cells

Blends of poly(3-hexylthiophene) (P3HT) and the bis-adduct of [6,6]-phenyl-C61-butyric acid methyl ester (bisPCBM) show enhanced performances in bulk-heterojunction solar cells compared to P3HT:PCBM thin films due to their higher open-circuit voltage. However, it is not clear whether the decrease of the short-circuit current observed in P3HT-bisPCBM blends originates from the 100 mV reduction of the offset between the lowest unoccupied molecular orbitals of the donor and the acceptor or from a change in the morphology. The analysis of the photoluminescence dynamics of the various bulk heterojunctions provides information on the dependence of the electron transfer process on their microstructure. We find that in solution, where the donor-acceptor distribution is homogeneous, the photoluminescence dynamics is the same for the bis- and PCBM-based blends, while in thin films the first shows a slower dynamics than the second. This result indicates that the reduction of the LUMO offset of ~100 meV does not influence the electron transfer efficiency but that the diversity between the photoluminescence dynamics in thin films should be ascribed to the different microstructure of the bulk heterojunctions fabricated with the two acceptors.

Fullerene Bisadducts for Enhanced Open-Circuit Voltages and Efficiencies in Polymer Solar Cells

A fullerene bisadduct can enhance the efficiency of polymer:fullerene bulk heterojunction solar cells. The bisadduct has a LUMO that is 100 meV higher compared to that of [6,6]-phenyl C-61 butyric acid methyl ester (PCBM). This increases the open-circuit voltage of polymer: fullerene bulk heterojunction solar cells based on poly(3-hexylthiophene) and bisadduct PCBM to 0.73 V, while maintaining high fill factors and currents

Air-Stable n-Channel Organic Transistors Based on a Soluble C84 Fullerene Derivative

Air-stable n-channel organic transistors are fabricated using a newly synthesized soluble fullerene derivative. The airstable nature of this molecule allows the realization of complementary circuits under ambient conditions without encapsulation. As shown in the figure, the I-V characteristics of the devices are retained even after exposure to air for a week. To the best of our knowledge, this is the first demonstration of an air-stable electron-transporting fullerene-based molecule

Influence of the sensitizer reduction potential on the sensitivity of photorefractive polymer composites

We report on a series of near infrared (NIR)-sensitive photorefractive polymer composites (PPCs) based on the hole-conducting polymer PF6-TPD, which are sensitized by soluble fullerene-derivatives as electron-accepting agents. We demonstrate a direct correlation between the electron accepting capability of the sensitizer and the holographic response time. The holographic recording speed is found to improve by one order of magnitude when lowering the reduction potential of the sensitizer by approx. 400 mV, while all other physical parameters of the materials remain essentially identical. Furthermore, the lifetime of the mobile charge carriers is found to correlate linearly with the reduction potential, thus indicating a decrease in recombination rates for stronger accepting capability of the sensitizer. Finally, we found that pre-illumination enhanced the holographic sensitivity. The effect is found to be most pronounced for the strongest acceptor due to reduced recombination of the preformed carriers. Overall, the PPCs reported here feature the currently highest sensitivity in the NIR spectral region.

Increasing the Open Circuit Voltage of Bulk-Heterojunction Solar Cells by Raising the LUMO Level of the Acceptor

We report the synthesis, characterization, and electrochemical properties of ten new fullerene derivatives for usage in organic solar cells. The phenyl ring of PCBM was substituted with electron-donating and electron-withdrawing substituents to study their influence on the LUMO level of the parent fullerene. We varied the LUMO level over a range of 86 mV and show a small but significant change of the open circuit voltage upon application in MDMO-PPV:methanofullerene bulk-heterojunction photovoltaic cells.

Solution processed self-assembled monolayer gate dielectrics for low-voltage organic transistors.:Section Title: Electric Phenomena

Low-voltage org. transistors are sought for implementation in high vol. low-power portable electronics of the future. Here we assess the suitability of three phosphonic acid based self-assembling mols. for use as ultra-thin gate dielecs. in low-voltage soln. processable org. field-effect transistors. In particular, monolayers of phosphonohexadecanoic acid in metal-monolayer-metal type sandwich devices are shown to exhibit low leakage currents and high geometrical capacitance comparable to previously demonstrated self-assembled monolayer (SAM) type dielecs. [1, 2] but with a higher surface energy. The improved surface energy characteristics enable processing of a wider range of org. semiconductors from soln. Transistors based on a no. of soln.-processed org. semiconductors with operating voltages below 2 V are also demonstrated. [on SciFinder(R)