Composites of conjugated poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C 61 butyric acid methyl ester (PCBM) demonstrate an efficient photogeneration of mobile charge carriers. Thermal annealing of P3HT:PCBM based devices gives rise to a significant increase of the photovoltaic efficiency, as follows from measurements of the external quantum efficiency and the current-voltage characteristics. Upon annealing, the absorption spectrum of the P3HT:PCBM composite undergoes a strong modification, whereas in the pure components it remains unchanged. The absorption of the annealed blends becomes stronger and red shifted in the wavelength region ascribed to P3HT, while the absorption due to the PCBM contribution does not change. Atomic force microscope measurements on P3HT:PCBM disclose some variation in morphology due to the crystallization of PCBM. The concentration of the PCBM clusters and their size (up to 500 nm) were found to be correlated with the amount of PCBM in the blend. We have studied the performance of photovoltaic devices with different weight ratios of P3HT:

The current–voltage characteristics of ITO/PEDOT:PSS/OC1C10‐PPV:PCBM/Al solar cells were measured in the temperature range 125–320 K under variable illumination, between 0.03 and 100 mW cm–2 (white light), with the aim of determining the efficiency‐limiting mechanism(s) in these devices, and the temperature and/or illumination range(s) in which these devices demonstrate optimal performance. (ITO: indium tin oxide; PEDOT:PSS: poly(styrene sulfonate)‐doped poly(ethylene dioxythiophene); OC1C10‐PPV: poly[2‐methoxy‐5‐(3,7‐dimethyl octyloxy)‐1,4‐phenylene vinylene]; PCBM: phenyl‐C61 butyric acid methyl ester.) The short‐circuit current density and the fill factor grow monotonically with temperature until 320 K. This is indicative of a thermally activated transport of photogenerated charge carriers, influenced by recombination with shallow traps. A gradual increase of the open‐circuit voltage to 0.91 V was observed upon cooling the devices down to 125 K. This fits the picture in which the open‐circuit voltage is not limited by the work‐function difference of electrode materials used. The overall effect of temperature on solar‐cell parameters results in a positive temperature coefficient of the power conversion efficiency, which is 1.9 % at T = 320 K and 100 mW cm–2 (2.5 % at 0.7 mW cm–2). The almost‐linear variation of the short‐circuit current density with light intensity confirms that the internal recombination losses are predominantly of monomolecular type under short‐circuit conditions. We present evidence that the efficiency of this type of solar cell is limited by a light‐dependent shunt resistance. Furthermore, the electronic transport properties of the absorber materials, e.g., low effective charge‐carrier mobility with a strong temperature dependence, limit the photogenerated current due to a high series resistance, therefore the active layer thickness must be kept low, which results in low absorption for this particular composite absorber.

Cu(2)S-CuInS(2) hybrid nanostructures as well as pure CuInS(2) (CIS) nanocrystals were synthesized by methods of colloidal chemistry. The structure, the shape and the composition of these nanomaterials were investigated with transmission electron microscopy (TEM), powder X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX). By changing the reaction conditions, CuInS(2) nanorods with different aspect ratio, dimeric nanorods as well as hexagonal discs and P-shaped particles could be synthesized. Under our reaction conditions, CIS nanoparticles crystallize in the hexagonal wurtzite structure, as confirmed by Rietveld analysis of the X-ray diffraction patterns. The formation of Cu(2)S-CuInS(2) hybrid nanostructures turned out to be an essential intermediate step in the growth of CIS nanoparticles, the copper sulphide part of the hybrid material playing an important role in the shape control of the CIS nanocrystals. By a treatment of Cu(2)S-CuInS(2) with 1,10-phenanthroline, Cu(2)S parts of the hybrid nanostructures could be removed, and pure CIS nanoparticles with shapes not accessible with other methods can be obtained. Our synthetic procedure turned out to be suitable to synthesize also other compounds, like CuInS(2)-ZnS alloys, and to modify, in this way, the optical properties of the nanocrystals.

We present measurements of the near-field heat transfer between the tip of a thermal profiler and planar material surfaces under ultrahigh vacuum conditions. For tip-sample distances below 10 −8 m our results differ markedly from the prediction of fluctuating electrodynamics. We argue that these differences are due to the existence of a material-dependent small length scale below which the macroscopic description of the dielectric properties fails, and discuss a corresponding model which yields fair agreement with the available data. These results are of importance for the quantitative interpretation of signals obtained by scanning thermal microscopes capable of detecting local temperature variations on surfaces.PACS numbers: 44.40.+a, 03.50.De, 78.20.Ci Radiative heat transfer between macroscopic bodies increases strongly when their spacing is made smaller than the dominant wavelength λ th of thermal radiation. This effect, caused by evanescent electromagnetic fields existing close to the surface of the bodies, has been studied theoretically already in 1971 by Polder and van Hove for the model of two infinitely extended, planar surfaces separated by a vacuum gap [1], and re-investigated later by Loomis and Maris [2] and Volokitin and Persson [3,4]. While early pioneering measurements with flat chromium bodies had to remain restricted to gap widths above 1 µm [5], and later studies employing an indium needle in close proximity to a planar thermocouple remained inconclusive [6], an unambiguous demonstration of near-field heat transfer under ultrahigh vacuum conditions and, thus, in the absence of disturbing moisture films covering the surfaces, could be given in Ref. [7].The theoretical treatment of radiative near-field heat transfer is based on fluctuating electrodynamics [8]. Within this framework, the macroscopic Maxwell equations are augmented by fluctuating currents inside each body, constituting stochastic sources of the electric and magnetic fields E and H. The individual frequency components j(r, ω) of these currents are considered as Gaussian stochastic variables. According to the fluctuationdissipation theorem, their correlation function reads [9]where E(ω, β) = ω/ exp(β ω) − 1 , with the usual inverse temperature variable β = 1/(k B T ); the angular brackets indicate an ensemble average. Moreover, ǫ ′′ (ω) denotes the imaginary part of the complex dielectric function ǫ(ω) = ǫ ′ (ω) + iǫ ′′ (ω). It describes the dissipative properties of the material under consideration, which is assumed to be homogeneous and non-magnetic. Thus, Eq. (1) contains the idealization that stochastic sources residing at different points r, r ′ are uncorrelated, no matter how small their distance may be. Applied to a material occupying the half-space z < 0, facing the vacuum in the complementary half-space z > 0, these propositions can be evaluated to yield the electromagnetic energy density in the distance z above the surface, giving [10]dκ ρ E (ω, κ, β, z) + ρ H (ω, κ, β, z)

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Colloidal CdSe quantum dots (QDs) are suitable as electron acceptors in polymer/nanoparticle bulk heterojunction hybrid solar cells. For this application, a thick organic ligand shell which is typically surrounding the QDs after synthesis needs to be removed. Ligand exchange with pyridine is the most widely used method for this purpose. Although this approach is already 15 years old, detailed studies on the effectiveness of ligand exchange with pyridine for solar cell applications are still missing. In the present work hybrid solar cells were prepared from CdSe QDs initially capped with oleic acid (OA), and the impact of single and multiple pyridine treatment was thoroughly investigated. NMR was applied to determine the composition of the ligand shell as well as to distinguish the bound and free ligands before and after ligand exchange. It is shown that after a single pyridine treatment some amount of OA is still present in the samples. By using thermal gravimetric analysis (TGA) we could obtain also quantitative information about the effectiveness of subsequent pyridine treatments. In a series of one-, two-, and threefold ligand exchange, the estimated surface coverage by OA decreased from 26% to 12%, whereas that of pyridine increased from 54% to 80%. Laboratory solar cells with pyridine-capped CdSe QDs and poly(3-hexylthiophene) (P3HT) were characterized by current-voltage (I-V) measurements, and in order to get deeper insight into charge carrier generation and recombination processes, CdSe/P3HT blends were studied by light-induced electron spin resonance (l-ESR). Although repeated pyridine treatment was found to have a beneficial effect in the sense that more complete ligand exchange was achieved, which in turn enabled more efficient charge transfer, the performance of the solar cells was found to be reduced. This fact correlates with increased aggregation tendency of repeatedly pyridine-treated particles, negatively influencing the morphology of the blends, as well as with a larger amount of surface defects in particles stabilized by the weak pyridine ligand shell.

Many physical and chemical properties of semiconducting nanocrystals strongly depend on their spatial dimensions and crystallographic structure. For these reasons, achieving a high degree of size and shape control plays an important role with respect to their application potential. In this report we present a facile route for the direct colloidal synthesis of copper(I) sulfide nanorods. A high reactivity of the starting materials is essential to obtain nanorods. We achieve this by using a thiol that thermally decomposes easily and serves as the sulfur source. The thiol is mixed in a noncoordinating solvent, which acts as the reaction medium. Adjustment of the nucleation temperature makes it possible to tailor uniform nanorods with lengths from 10 to 100 nm. The nanorods are single crystalline, and the growth direction is shown to occur along the a-axis of djurleite. The growth process and character of the nanorods were investigated through UV-vis and NIR absorption spectroscopy, transmission electron microscopy, and powder X-ray diffraction measurements.

The addition of small amounts of dodecylamine-capped Au nanoparticles into the active layer of organic bulk heterojunction solar cells consisting of poly(3-octylthiophene) (P3OT) and C(60) was recently suggested to have a positive impact on device performance due to improved electron transport. This issue was systematically further investigated in the present work. Different strategies to incorporate colloidally prepared Au nanoparticles with a narrow size distribution into organic solar cells with the more common donor/acceptor system consisting of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C(61)-butyric acid methyl ester (PCBM) were pursued. Au nanoparticles were prepared with either P3HT or dodecylamine as ligands. Additionally, efforts were undertaken to incorporate nearly ligand-free Au nanoparticles into the system. Therefore, a procedure was successfully developed to remove the dodecylamine ligand shell by a postpreparative ligand exchange with pyridine, a much smaller molecule that can later partly be removed from solid films by annealing. However, for all types of nanoparticles studied here, the performance of the P3HT/PCBM solar cells was found to decrease with the Au particles as an additive to the active layer, meaning that adding Au nanoparticles is not a suitable strategy in the case of the P3HT/PCBM system. Possible reasons are discussed on the basis of detailed investigations of the structure, photophysics and charge transport in the system.