We consider a dilute hard-sphere Bose gas in random external potentials at low temperatures, in Z) = 3, using the technique of pseudopotentials and the Bogliubov transformation. At absolute zero, the random potentials can deplete the Bose condensate, though not completely. On the other hand, they generate an amount of normal fluid equal to j the condensate depletion. This is a localization effect that can destroy superfluidity at absolute zero. General features of the superfluid density in the neighborhood of this transition point agree qualitatively with experimental results on helium in porous media.PACS numbers: 67.40. Bz, 05.30.Jp, 64.60.Cn We report on some results concerning the low-temperature properties of a dilute hard-sphere Bose gas in random external potentials in three dimensions. Such a model is a crude simulation of superfluid helium in porous media [1]. The spongelike media are here idealized as random distributions of hard-sphere potentials. To make the problem tractable, we further assume that the randomness is sufficiently dilute, and the temperature sufficiently low, that the hard spheres can be approximated by delta-function pseudopotentials [2]. We also assume that the potentials are distributed with uncorrelated randomness. Thus, the very large pores that are apparently present in the experimental media are not taken into account here. The purpose of this study is not to construct a quantitative model for the experiments, but to illuminate some qualitative features. We are able to show, for example, that at absolute zero superfluidity can where y/Cx) is the field operator for nonrelativistic boson of mass ra, N=fd 3 xy/^y/ is the number operator, U(x) is the external potential, and v^^Ana/m, where a is the hard-sphere diameter. The ground-state energy is rendered finite by subtracting an appropriate divergent constant [3].The external potential U(x) may be pictured as a sum of randomly located scattering centers of random strengths, either attractive or repulsive. We assume (U(x)U(y)) av oz8 3 (x-y), and characterize the potentials by a single parameter Ro,where V is the total volume of the system, £/* is the Fourier transform of U(x), and the subscript av denotes a quenched average over potentials. It has dimension (energy) 2 (length) 3 , and is (average density)x(meansquare strength) of the individual scatterers, the strength be destroyed by the randomness, through an effect suggestive of boson localization. From a theoretical point of view, the interparticle interactions are necessary to prevent a total condensation into a single localized orbital in the external potential. Thus, unlike the much-studied case of fermions, one does not have the luxury of treating the potentials as perturbations on a free-particle Hamiltonian. Here we do the next best thing, namely, start with the simplest soluble problem involving interparticle interactions-a dilute hard-sphere gas at low temperatures [2,3]. This approach differs from previous efforts on this subject [4,5], in that ours is a microscopic...

Interface carrier recombination currently hinders the performance of hybrid organic-silicon heterojunction solar cells for high-efficiency low-cost photovoltaics. Here, we introduce an intermediate 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) layer into hybrid heterojunction solar cells based on silicon nanowires (SiNWs) and conjugate polymer poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS). The highest power conversion efficiency reaches a record 13.01%, which is largely ascribed to the modified organic surface morphology and suppressed saturation current that boost the open-circuit voltage and fill factor. We show that the insertion of TAPC increases the minority carrier lifetime because of an energy offset at the heterojunction interface. Furthermore, X-ray photoemission spectroscopy reveals that TAPC can effectively block the strong oxidation reaction occurring between PEDOT:PSS and silicon, which improves the device characteristics and assurances for reliability. These learnings point toward future directions for versatile interface engineering techniques for the attainment of highly efficient hybrid photovoltaics.

With atomic-layer-deposition grown zinc oxide as the electron selective layer, we developed plastic substrate compatible processing for organic photovoltaic devices and demonstrated flexible inverted organic solar cells on poly(ethylene naphthalate) with a power conversion efficiency of 4.18%.

In this work, hybrid heterojunction solar cells are demonstrated based on a conjugate polymer poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) directly spun-cast on micro-textured n-type crystalline silicon wafers. The fabrication conditions suggest that the organic coverage on the micro-textured surface is excellent and key to achieve high efficiency, leading to an average power conversion efficiency of 9.84%. A one-dimensional drift-diffusion model is then developed based on fitting the device characteristics with experimentally determined PEDOT:PSS parameters and projects an ultimate efficiency above 20% for organic/inorganic hybrid photovoltaics. The simulation results reveal the impacts of defect densities, back surface recombination, doping concentration, and band alignment.

An effective approach to reduce defects and increase electron mobility in a-IGZO thin-film transistors (a-IGZO TFTs) is introduced. A strong reduction layer, calcium, is capped onto the back interface of a-IGZO TFT. After calcium capping, the effective electron mobility of a-IGZO TFT increases from 12 cm(2) V(-1) s(-1) to 160 cm(2) V(-1) s(-1). This high mobility is a new record, which implies that the proposed defect reduction effect is key to improve electron transport in oxide semiconductor materials.

Multilayer polymer light-emitting diodes fabricated by blade coating are presented. Multilayer of polymers can be easily deposited by blade coating on a hot plate. The multilayer structure is confirmed by the total thickness and the cross section view in the scanning electron microscope. The film thickness variation is only 3.3% in 10cm scale and the film roughness is about 0.3nm in the micron scale. The efficiency of single layer poly(para-phenylene vinylene) copolymer Super Yellow and poly(9,9-dioctylfluorene) (PFO, deep blue) devices are 9 and 1.7cd∕A, respectively, by blade coating. The efficiency of the PFO device is raised to 2.9cd∕A with a 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD) hole-blocking layer and to 2.3cd∕A with a poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl))diphenylamine)] elec-tron-blocking layer added by blade coating.

We perform comprehensive long-time monitoring of the p-doping and de-doping of poly͑3-hexyl thiophene͒ under changing external conditions of oxygen, light, and temperature. They are shown to be controlled by the complex adsorption and desorption process with time scales ranging from seconds to weeks. The oxygen doping at atmospheric pressure takes several hours in the dark. The doping is dramatically accelerated to be within seconds with light of wavelength of 500-700 nm. Even at low oxygen pressure of 10 −4 torr doping occurs within minutes with light. The de-doping by oxygen desorption takes as long as weeks at room temperature and vacuum of 10 −4 torr, but when the temperature is raised to near the polymer glass temperature of 370 K, the de-doping is accelerated to minutes as the enhanced chain motion releases the trapped oxygen. Even though visible and near infrared light causes very efficient doping within seconds or minutes depending on vacuum level, such light-induced doping is not a chemical reaction and is fully reversible by thermal annealing at the end without sacrificing the mobility. For the polymer field-effect transistors, only the carrier density is changed while the mobility remains roughly a constant for all the conditions.

This work demonstrates a real-time visible-light phototransistor comprised of a wide-band-gap amorphous indium-gallium-zinc-oxide ͑a-IGZO͒ thin-film transistor ͑TFT͒ and a narrow-band-gap polymeric capping layer. The capping layer and the IGZO layer form a p-n junction diode. The p-n junction absorbs visible light and consequently injects electrons into the IGZO layer, which in turn affects the body voltage as well as the threshold voltage of a-IGZO TFT. The hysteresis behavior due to the charges at IGZO back interface is also discussed.