Degradation pathways in standard and inverted DBP-C 70 based organic solar cells

Achieving long-term stability in organic solar cells is a remaining bottleneck for the commercialization of this otherwise highly appealing technology. In this work, we study the performance and stability differences in standard and inverted DBP/C70 based organic solar cells. Differences in the charge-transfer state properties of inverted and standard configuration DBP/C70 solar cells are revealed by sensitive external quantum efficiency measurements, leading to differences in the open-circuit voltages of the devices. The degradation of standard and inverted solar cell configurations at ISOS aging test conditions (ISOS-D-3 and ISOS-T-3) was investigated and compared. The results indicate that the performance drop in the small molecule bilayer solar cells is less related to changes at the D-A interface, suggesting also a pronounced morphological stability, and instead, in the case of inverted cells, dominated by degradation at the electron transport layer (ETL) bathocuproine (BCP). Photoluminescence measurements, electron-only-device characteristics, and stability measurements show improved exciton blocking, electron transport properties and a higher stability for BCP/Ag ETL stacks, giving rise to inverted devices with enhanced performance and device stability

Fragmentation pathways of nanofractal structures on surface

We present a detailed systematical theoretical analysis of the post-growth processes occurring in nanofractals grown on surface. For this study we developed a method which accounts for the internal dynamics of particles in a fractal. We demonstrate that particle diffusion and detachment controls the shape of the emerging stable islands on surface. We consider different scenarios of fractal post-growth relaxation and analyze the time evolution of the island's morphology. The results of our calculations are compared with available experimental observations, and experiments in which the post-growth relaxation of deposited nanostructures can be probed are suggested.Comment: 34 pages, 11 figure

Classification of Light-Induced Desorption of Alkali Atoms in Glass Cells Used in Atomic Physics Experiments

We attempt to provide physical interpretations of light-induced desorption phenomena that have recently been observed for alkali atoms on glass surfaces of alkali vapor cells used in atomic physics experiments. We find that the observed desorption phenomena are closely related to recent studies in surface science, and can probably be understood in the context of these results. If classified in terms of the photon-energy dependence, the coverage and the bonding state of the alkali adsorbates, the phenomena fall into two categories: It appears very likely that the neutralization of isolated ionic adsorbates by photo-excited electron transfer from the substrate is the origin of the desorption induced by ultraviolet light in ultrahigh vacuum cells. The desorption observed in low temperature cells, on the other hand, which is resonantly dependent on photon energy in the visible light range, is quite similar to light-induced desorption stimulated by localized electronic excitation on metallic aggregates. More detailed studies of light-induced desorption events from surfaces well characterized with respect to alkali coverage-dependent ionicity and aggregate morphology appear highly desirable for the development of more efficient alkali atom sources suitable to improve a variety of atomic physics experiments.Comment: 6 pages, 1 figure; minor corrections made, published in e-Journal of Surface Science and Nanotechnology at http://www.jstage.jst.go.jp/article/ejssnt/4/0/4_63/_articl

Quasiclassical trajectory calculations of integral cross sections for highly vibrationally excited Li<SUB>2</SUB>---He, Kr systems

On the basis of recently published, bond-distance dependent semiempirical potentials total cross sections have been calculated for Li2---He and Li2---Kr collisions as functions of collision energy and initial vibrational state &#957;1= 0-20 of the molecule, using either the vibrational adiabatic quantum-mechanical infinite-order-sudden (IOS) approximation or vibration-dependent quasiclassical trajectories (QCT). Via the QCT calculations the experimentally observed non-monotonic change of total cross section with changing initial vibrations state could be reproduced and explained. For Li2 (&#957;1)---He at Etr=0.08 eV the vibrational and rotational inelastic cross sections at &#957;1=20 are significantly larger than for &#957;1=0

Quasiclassical trajectory study of Li<SUB>2</SUB> (v&#8804;25,j&#8804;100)-Na exchange reaction

The effect of high initial vibrational (v=0-25) and rotational (j=0-100) excitation of the reactant molecule on the cross-section &#963;<SUP>R</SUP> for the reaction Li<SUB>2</SUB>+Na&#8594;LiNa+Li at a relative translational energy (E<SUB>trans</SUB>) of 0.5 eV has been investigated using the three-dimensional quasiclassical trajectory method on the recently proposed Varandas-Morais-Pais (VMP) potential. &#963;<SUP>R</SUP> increases almost linearly with increase in v, but it remains unaltered, within the statistical error, with increase in j for j=0-30. For the higher j states, there is an increase in &#963;<SUP>R</SUP> with increase in j, and the rotational enhancement gets larger with increase in v, becoming comparable to the vibrational enhancement at v=20. The dependence of &#963;<SUP>R</SUP> on E<SUB>trans</SUB> over the range 0.18-0.85 eV has been examined for v=0, j=0. For E<SUB>trans</SUB>&#8804;0.25 eV, all trajectories get trapped. For higher E<SUB>trans</SUB>, &#963;<SUP>R</SUP> rises to a maximum and then levels off. Comparison of our results with those on the Whitehead-Grice (WG) potential reveals that the reactivity is larger on the VMP potential than on the WG for the low v while the opposite is true for the higher v

Observation of an anomalous increase in total cross sections with high vibrational excitation in the Li2 (V = 0-21) + Na reactive system

The effect of initial vibrational excitation on Li2(n,J) + Na collisions was studied exptl. and theor. The relative integral cross sections s(n,J) were measured in a crossed beam expt. for vibrational states n populated thermally or by optical pumping, and for final states detected by laser induced fluorescence. It is found that s(n,J) increases by 35 +- 7% between n = 0 and n = 20, while the dependence on the initial rotational state J is insignificant, within +-5%. The effect of vibrational enhancement is about twice as large as that found for the Li2-rare gas systems for the same amt. of vibrational excitation. The effect is about an order of magnitude larger than predicted by the authors' quasiclassical trajectory calcns. on a LEPS potential energy surface, thus suggesting that addnl. long-range potential terms ought to be included. The authors also attempted to measure state-selectively the LiNa (n',J') products from the thermal reaction and found an upper limit of the reactive cross section of 1 .ANG.2. This contrasts with the trajectory calcns. that predict a 20 times larger cross section; the authors thus conclude that the reactive part of the potential also needs to be re-evaluated

Resonant effects in optical second-harmonic generation from alkali covered

Polarized second-harmonic generation (SHG) coverage dependencies for alkali (\chem{Na, Li}) adsorption on \chem{Si(111)}$7\times7$ both at room and at low temperatures are obtained for fundamental wavelengths of 497\un{nm}, 570\un{nm} and 1067\un{nm}, showing characteristic and reproducible non-monotonic changes of SHG efficiency. At submonolayer coverage the SHG intensities are qualitatively different for visible vs. near-resonant IR radiation. In the coverage regime $\theta < 1/3$, low-symmetry \chem{Na}-\chem{Si} bonds, resulting from a \chem{Na}-induced surface reconstruction, are formed, which are resonant with 1067\un{nm} radiation. By comparing parallelly and perpendicularly polarized coverage dependencies, we deduce that the resonant contribution in the parallel configuration is due to the $\chi^{(2)}_{zzz}$-component