125 research outputs found
Local density of states of electron-crystal phases in graphene in the quantum Hall regime
We calculate, within a self-consistent Hartree-Fock approximation, the local
density of states for different electron crystals in graphene subject to a
strong magnetic field. We investigate both the Wigner crystal and bubble
crystals with M_e electrons per lattice site. The total density of states
consists of several pronounced peaks, the number of which in the negative
energy range coincides with the number of electrons M_e per lattice site, as
for the case of electron-solid phases in the conventional two-dimensional
electron gas. Analyzing the local density of states at the peak energies, we
find particular scaling properties of the density patterns if one fixes the
ratio nu_N/M_e between the filling factor nu_N of the last partially filled
Landau level and the number of electrons per bubble. Although the total density
profile depends explicitly on M_e, the local density of states of the lowest
peaks turns out to be identical regardless the number of electrons M_e. Whereas
these electron-solid phases are reminiscent to those expected in the
conventional two-dimensional electron gas in GaAs heterostructures in the
quantum Hall regime, the local density of states and the scaling relations we
highlight in this paper may be, in graphene, directly measured by spectroscopic
means, such as e.g. scanning tunneling microscopy.Comment: 8 pages, 7 figures; minor correction
Enhancing electron affinity and tuning band gap in donorāacceptor organic semiconductors by benzothiadiazole directed CāH borylation
Electrophilic borylation using BCl3 and benzothiadiazole to direct the CāH functionalisation of an adjacent aromatic unit produces fused boracyclic materials with minimally changed HOMO energy levels but significantly reduced LUMO energy levels. In situ alkylation and arylation at boron using Al(alkyl)3 or Zn(aryl)2 is facile and affords boracycles that possess excellent stability towards protic solvents, including water, and display large bathochromic shifts leading to far red/NIR emission in the solid state with quantum yields of up to 34%. Solution fabricated OLEDs with far red/NIR electroluminescence are reported with EQEs > 0.4%
Defect-mediated metastability and carrier lifetimes in polycrystalline (Ag,Cu)(In,Ga)Se-2 absorber materials
Using a combination of optical and electrical measurements, we develop a model for metastable defects in Ag-alloyed Cu(In,Ga)Se-2, one of the leading thin film photovoltaic materials. By controlling the pre-selenization conditions of the back contact prior to the growth of polycrystalline (Ag,Cu)(In,Ga)Se-2 absorbers and subsequently exposing them to various stresses (light soaking and dark-heat), we explore the nature and role of metastable defects on the electro-optical and photovoltaic performance of high-efficiency solar cell materials and devices. Positron annihilation spectroscopy indicates that dark-heat exposure results in an increase in the concentration of the selenium-copper divacancy complex (V-Se-V-Cu), attributed to depassivation of donor defects. Deep-level optical spectroscopy finds a corresponding increase of a defect at E-v+0.98eV, and deep-level transient spectroscopy suggests that this increase is accompanied by a decrease in the concentration of mid-bandgap recombination centers. Time-resolved photoluminescence excitation spectroscopy data are consistent with the presence of the V-Se-V-Cu divacancy complex, which may act as a shallow trap for the minority carriers. Light-soaking experiments are consistent with the V-Se-V-Cu optical cycle proposed by Lany and Zunger, resulting in the conversion of shallow traps into recombination states that limit the effective minority carrier recombination time (and the associated carrier diffusion length) and an increase in the doping density that limits carrier extraction in photovoltaic devices.Peer reviewe
Hole injection and transport in organic semiconductors
EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Usage of Gradient Masks for Rapid Determination of Images Centers of Laser Beams Spots with Improved Accuracy
P-181: Device Parameters Determination by Numerical Model Fitting for Organic Light-Emitting Diodes (OLEDs) using Impedance Spectroscopy Measurement
Ambipolar charge transport in films of methanofullerene and poly(phenylenevinylene)/methanofullerene blends
Herein, we report experimental studies of electron and hole transport in thin films of [6,6]-phenyl C61 butyric acid methyl ester (PCBM) and in blends of poly[2-methoxy-5-(3ā²,7ā²-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) with PCBM. The low-field hole mobility in pristine MDMO-PPV is of the order of 10ā7ācm2āVā1āsā1, in agreement with previous studies, whereas the electron mobility in pristine PCBM was found by current-densityāvoltage (JāV) measurements to be of the order of 10ā2ācm2āVā1āsā1, which is about one order of magnitude greater than previously reported. Adding PCBM to the blend increases both electron and hole mobilities, compared to the pristine polymer, and results in less dispersive hole transport. The hole mobility in a blend containing 67āwt.-% PCBM is at least two orders of magnitude greater than in the pristine polymer. This result is independent of measurement technique and film thickness, indicating a true bulk property of the material. We therefore propose that PCBM may assist hole transport in the blend, either by participating in hole transport or by changing the polymer-chain packing to enhance hole mobility. Time-of-flight mobility measurements of PCBM dispersed in a polystyrene matrix yield electron and hole mobilities of similar magnitude and relatively non-dispersive transport. To the best of our knowledge, this is the first report of hole transport in a methanofullerene. We discuss the conditions under which hole transport in the fullerene phase of a polymer/fullerene blend may be expected. The relevance to photovoltaic device function is also discusse
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