33 research outputs found
Reading the Number of Extra Dimensions in the Spectrum of Hawking Radiation
After a brief review of the production and decay of Schwarzschild-like
(4+n)-dimensional black holes in the framework of theories with Large Extra
Dimensions, we proceed to derive the greybody factors and emission rates for
scalars, fermions and gauge bosons on the brane. We present and discuss
analytic and numerical methods for obtaining the above results, and demonstrate
that both the amount and type of Hawking radiation emitted by the black hole
can help us to determine the number of spacelike dimensions that exist in
nature.Comment: 8 pages, Latex file, 1 figure, to appear in the proceedings of the
String Phenomenology 2003 Conference, Durham, UK, 29th July-4th August, 200
Characterization of the fullerene derivative [60]PCBM, by high-field carbon, and two-dimensional NMR spectroscopy, coupled with DFT simulations
High-resolution (600 MHz) 1H and 13C chemical shift and 2D HETCOR NMR spectra of [60]PCBM were recorded. Resonances from every carbon atom of the ester, phenyl and cyclo-fullerenyl groups, were fully accounted. Assignments of the fullerene cyclopropa-ring, and all phenyl and ester carbons to their respective resonances were based on a HETCOR 2D NMR spectrum. Remaining fullerene assignments were made to a high level of confidence with the aid of an ÏB97X hybrid HF/DFT simulation of the 13C NMR spectrum employing a triple zeta Dunning-type basis set. The best result was obtained with the range-separation parameter Ï set effectively to zero. This indicates that the fraction of HF in the HF/DFT hybrid at very short range is the dominant factor in achieving good NMR results, that ÏB97X with its 15.77% HF fraction at rij = 0 seems very well suited, and that allowing the HF fraction to increase with range is not particularly beneficial. The resulting spectrum had a remarkable qualitative agreement with experiment with a very low mean absolute error for fullerene carbons of 0.09 ppm, which was considerably lower than the 0.28 ppm of the more commonly used B3LYP/6-31G(d,p) method
Capillary-Written Single-Crystalline All-Inorganic Perovskite Microribbon Arrays for Highly-Sensitive and Thermal-Stable Photodetectors
Understanding Charge Transport in Endohedral Fullerene Single-Crystal Field-Effect Transistors
Hybridization of Local Exciton and Charge-Transfer States Reduces Non-Radiative Voltage Losses in Organic Solar Cells
A number of recent studies have shown that the non-radiative voltage losses in organic solar cells can be suppressed in systems with low energetic offsets between donor and acceptor molecular states, but the physical reasons underpinning this remain unclear. Here, we present a systematic study of 18 different donor:acceptor blends to determine the effect that energetic offset has on both radiative and non-radiative recombination of the charge transfer (CT) state. We find that for certain blends, low offsets result in hybridization between charge-transfer and lowest donor or acceptor exciton states, which leads to a strong suppression in the non-radiative voltage loss to values as low as 0.23V associated with an increase in the luminescence of the CT state. Further, we extend a two-state CT-state recombination model to include the interaction between CT and first excited states, which allows us to explain the low non-radiative voltage losses as an increase in the effective CT to ground state oscillator strength due to the intensity borrowing mechanism. We show that low nonâradiative voltage losses can be achieved in material combinations with a strong electronic coupling between CT and first excited states, and where the lower band gap material has a high oscillator strength for transitions from the excited state to the ground state. Finally, from our model we propose that achieving very low non-radiative voltage losses may come at a cost of higher overall recombination rates, which may help to explain the generally lower FF and EQE of highly hybridized systems
Antisolvent-assisted controllable growth of fullerene single crystal microwires for organic field effect transistors and photodetectors
Organic Single-Crystalline p-n Heterojunctions for High-Performance Ambipolar Field-Effect Transistors and Broadband Photodetectors
Organic semiconducting
single crystals are ideal building blocks
for organic field-effect transistors (OFETs) and organic photodetectors
(OPDs) because they can potentially exhibit the best charge transport
and photoelectric properties in organic materials. Nevertheless, it
is usual for single-crystal OFETs to be built from one kind of organic
material in which the dominant transport is either electron or hole;
such OFETs show unipolar charge transport. Furthermore, single-crystal
OPDs present high performance only in restricted regions because of
the limited absorption of one-component single crystals. In an ideal
situation, devices which comprise both electron- and hole-transporting
single crystals with complementary absorptions, such as single-crystalline
pân heterojunctions (SCHJs), can permit broadband photoresponse
and ambipolar charge transport. In this paper, a solution-processing
crystallization strategy to prepare an SCHJ composed of C60 and 6,13-bisÂ(triisopropylsilylethynyl)Âpentacene (TIPS-PEN) was shown.
These SCHJs demonstrated ambipolar charge-transport characteristics
in OFETs with a balanced performance of 2.9 cm2 Vâ1 sâ1 for electron mobility and 2.7 cm2 Vâ1 sâ1 for hole mobility. This
demonstration is the first of single-crystal OFETs in which both electron
and hole mobilities were over 2.5 cm2 Vâ1 sâ1. OPDs fabricated upon as-prepared SCHJs exhibited
highly sensitive photoconductive properties ranging from ultraviolet
to visible and further to near-infrared regions as a result of complementary
absorption between C60 and TIPS-PEN, thereby attaining
photoresponsivities that are among the highest reported values within
the OPDs. This work would provide valuable references for developing
novel SCHJ systems to achieve significant progress in high-performance
ambipolar OFETs and broadband OPDs
Relationship between molecular properties and degradation mechanisms of organic solar cells based on bis-adducts of phenyl-C61 butyric acid methyl ester
Environmental stability remains a major challenge for the commercialisation of organic solar cells and degradation pathways remain poorly understood. Designing materials for improved device stability requires an understanding of the relationship between the properties of the donor or acceptor molecule and different degradation mechanisms. Here we study the correlations between various molecular parameters of the fullerene derivative bis-PCBM and the degradation rate of polymer:bis-PCBM organic solar cells, based on the same carbazole-alt-benzothiadiazole polymer, in aerobic and anaerobic conditions. We compare eight high purity bis-PCBM isomers with different electronic, chemical and packing properties along with PCBM and the mixture of bis isomers. In the case of aerobic photodegradation, we find that device degradation rate is positively correlated to the LUMO energy of the bis-PCBM isomer and to the degree of crystallinity of the isomer, while the correlation of degradation with driving force for epoxide formation is unclear. These results support the idea that in these samples, aerobic photodegradation proceeds via superoxide formation by the photogenerated polaron on the fullerene, followed by further chemical reaction. In the absence of air, photodegradation rate is correlated with molecular structure, supporting the mechanism of microstructural degradation via fullerene dimerization. The approach and findings presented here show how control of specific molecular parameters through chemical design can serve as a strategy to enhance stability of organic solar cells