Floris B. Kooistra

Effect of traps on the performance of bulk heterojunction organic solar cells

The effect of electron traps on the performance of bulk heterojunction organic solar cells made of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is investigated. By deliberately introducing 7,7,8,8-tetracyanoquinodimethane, which is a stronger electron accepting molecule than PCBM, the electron transport in the solar cells becomes trap limited. We demonstrate that the open circuit voltage and its light-intensity dependence are strongly affected by recombination of holes with trapped electrons. Depending on the amount of traps, their recombination strength, and the light intensity, the losses due to trap-assisted recombination can even dominate over the intrinsic bimolecular recombination.

Low-voltage organic transistors based on solution processed semiconductors and self-assembled monolayer gate dielectrics

Reduction in the operating voltage of organic transistors is of high importance for successful implementation in low-power electronic applications. Here we report on low-voltage n-channel transistors fabricated employing a combination of soluble organic semiconductors and a self-assembled gate dielectric. The high geometric capacitance of the nanodielectric allows transistor operation below 2V. Solution processing is enabled by analysis of the surface energy compatibility of the dielectric and semiconductor solutions. Electron mobilities in the range of 0.01–0.04cm2∕Vs and threshold voltages ⩽0.35V are demonstrated. The present work paves the way toward solution processable low-voltage/power, organic complementary circuits.

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-C(61)-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 approximately 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.

Charge dissociation in polymer:fullerene bulk heterojunction solar cells with enhanced permittivity

The dissociation efficiency of bound electron-hole pairs at the donor-acceptor interface in bulk heterojunction solar cells is partly limited due to the low dielectric constant of the polymer:fullerene blend. We investigate the photocurrent generation in blends consisting of a fullerene derivative and an oligo(oxyethylene) substituted poly(p-phenylene vinylene) (PPV) derivative with an enhanced relative permittivity of 4. It is demonstrated that in spite of the relatively low hole mobility of the glycol substituted PPV the increase in the spatially averaged permittivity leads to an enhanced charge dissociation of 72% for these polymer:fullerene blends.

Fluorine containing C60 derivatives for high-performance electron transporting field-effect transistors and integrated circuits

We report on electron transporting organic transistors and integrated ring oscillators based on four different solution processible fluorine containing C60 derivatives. Electron mobilities up to 0.15cm2∕Vs are obtained from as-prepared bottom-gate, bottom-contact transistors utilizing gold source-drain electrodes. Despite the high mobility, no long-range structural order could be identified with the semiconductor films exhibiting amorphouslike characteristics. The good electron transport is attributed to the structural symmetry of the fullerene derivatives and the enhanced π-π interactions between C60 units even in the case of amorphouslike films. These advantageous characteristics make fluorine containing C60 derivatives attractive for application in high-performance, large-area organic electronics.

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

Diazirine addition reactions to C(60), followed by an HCl elimination step, yielded [6,6] and [5,6] sp(2) 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.

Soluble fullerene derivatives : The effect of electronic structure on transistor performance and air stability

The family of soluble fullerene derivatives comprises a widely studied group of electron transporting molecules for use in organic electronic and optoelectronic devices. For electronic applications, electron transporting (n-channel) materials are required for implementation into organic complementary logic circuit architectures. To date, few soluble candidate materials have been studied that fulfill the stringent requirements of high carrier mobility and air stability. Here we present a study of three soluble fullerenes with varying electron affinity to assess the impact of electronic structure on device performance and air stability. Through theoretical and experimental analysis of the electronic structure, characterization of thin-film structure, and characterization of transistor device properties we find that the air stability of the present series of fullerenes not only depends on the absolute electron affinity of the semiconductor but also on the disorder within the thin-film.

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.

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)]