Wernfried Haas

Investigation of the formation of CuInS2 nanoparticles by the oleylamine route: comparison of microwave-assisted and conventional syntheses.

The formation of copper indium disulfide nanoparticles via the oleylamine route using copper iodide, indium chloride, and elemental sulfur has been investigated by applying conventional thermal heating as well as microwave irradiation. Oleylamine thereby acts as a capping ligand as well as a solvent. In an initial set of experiments, the onset of the reaction was determined to be around 115 °C by an in situ X-ray study using Synchrotron radiation. Using comparatively low synthesis temperatures of 120 °C, it is already possible to obtain nanoparticles of 2-4 nm with both heating methods but with irregular shape and size distribution. By applying higher temperatures of 220 °C, more crystalline and larger nanoparticles were obtained with slight differences in crystallite size and size distribution depending on the synthesis route. The size of the nanoparticles is in the range of 3-10 nm depending on the heating time. Using microwave irradiation, it is possible to obtain nanoparticles in only 90 s of total synthesis time. Control experiments to probe a nonthermal microwave effect were carried out ensuring an identical experimental setup, including the heating profile, the stirring rate, and the volume and concentration of the solutions. These experiments clearly demonstrate that for the preparation of CuInS(2) nanoparticles described herein no differences between conventional and microwave heating could be observed when performed at the same temperature. The nanoparticles obtained by microwave and thermal methods have the same crystal phase, primary crystallite size, shape, and size distribution. In addition, they show no significant differences concerning their optical properties.

Bismuth sulphide–polymer nanocomposites from a highly soluble bismuth xanthate precursor

Bismuth sulphide nanocrystal–polymer hybrid layers are of interest for various optoelectronic, thermoelectric or sensing applications. In this work, we present a ligand-free in situ route for the formation of Bi2S3 nanorods directly within a polymer matrix. For this purpose, we introduce a novel bismuth xanthate (bismuth(III) O-3,3-dimethylbutan-2-yl dithiocarbonate), which is highly soluble in non-polar organic solvents. The analysis of the crystal structure revealed that the prepared bismuth xanthate crystallises in the monoclinic space group C2/c and forms dimers. The bismuth xanthate can be converted into nanocrystalline Bi2S3 with an orthorhombic crystal structure via a thermally induced solid state reaction at moderate temperatures below 200 °C. In combination with the high solubility in non-polar solvents this synthetic route for Bi2S3 is of particular interest for the preparation of Bi2S3–polymer nanocomposites as exemplarily investigated on Bi2S3–poly(methyl methacrylate) and Bi2S3–poly(3-hexylthiophene-2,5-diyl) (P3HT) nanocomposite layers. Atomic force and transmission electron microscopy revealed that Bi2S3 nanorods are dispersed in the polymer matrix. Photoluminescence experiments showed a quenching of the P3HT fluorescence with increasing Bi2S3 content in the hybrid layer.

Influence of morphology and polymer:nanoparticle ratio on device performance of hybrid solar cells?an approach in experiment and simulation

We present a thorough study on the various impacts of polymer:nanoparticle ratios on morphology, charge generation and device performance in hybrid solar cells, comprising active layers consisting of a conjugated polymer and in situ prepared copper indium sulfide (CIS) nanoparticles. We conducted morphological studies through transmission electron microscopy and transient absorption measurements to study charge generation in absorber layers with polymer:nanoparticle weight ratios ranging from 1:3 to 1:15. These data are correlated to the characteristic parameters of the prepared solar cells. To gain a deeper understanding of our experimental findings, three-dimensional drift-diffusion-based simulations were performed. Based on elaborate descriptions of the contributions of polymer and nanoparticle phase to device performances, our results suggest that a polymer:CIS volume ratio of 1:2 (weight ratio 1:9) is necessary to obtain a balanced hole and electron percolation. Also at higher CIS loadings the photocurrent remains surprisingly high due to the contribution of the CIS phase to the charge carrier generation.

Investigation of CuInS2 thin film formation by a low-temperature chemical deposition method.

Copper indium disulfide (CuInS(2), CIS) thin films were prepared by an alternative solution-based coating process adapted from the well-established aqueous metal salt/thiourea precursor system. The temperature for the decomposition of the precursors and the formation of CIS was lowered significantly to 130 °C by using the strongly coordinating solvent pyridine instead of the commonly used water. In addition, the influence of different annealing temperatures and concentrations of thiourea (TU) in the precursor solution on the obtained CIS samples was investigated. The films possess highly beneficial properties for photovoltaic applications, showing a chalcopyrite crystal structure, a high optical absorption (>10(4) cm(-1)) and an optical band gap between 1.45 and 1.51 eV. Chemical and morphological changes during the thin film formation were detected and explained by time-resolved simultaneous grazing incident small- and wide-angle X-ray scattering (GISAXS, GIWAXS) measurements, scanning electron microsccopy (SEM) and simultaneous thermogravimetry/mass spectroscopy (TG/MS).

Polymer - CuInS 2 hybrid solar cells obtained by an in-situ formation route

Bulk heterojunction hybrid solar cells with a CuInS2-poly(p-phenylene vinylene) active layer are presented in this contribution. The acceptor phase is obtained in-situ within the matrix of the polymer by the reaction of the metal ions with the decomposition products of a sulfur source (thioacetamide) using a moderate heating step. As prepared donor-acceptor nanocomposites were used as active layers in hybrid solar cells. Structural characterization was performed using XRD and TEM methods, solar cell performance and optical properties were explored by j-V-, absorbance- and IPCE-measurements.

Comparing photovoltaic parameters of conventional cathodes with a novel silver nanoparticle/aluminum cathode in polymer based solar cells

Besides active materials the choice of suitable cathodes is crucial for a good overall performance in solar cells. A cathode modification utilizing a thin silver interlayer followed by an aluminum layer can largely enhance the fill factor of polymer: CIS (copper indium sulfide) solar cells in comparison to pure Al or Ag cathodes. In this work, we investigate if this concept can also be applied to polymer: fullerene solar cells to enhance their overall power conversion efficiency. The organic solar cells examined consisted of blends of low band-gap donor polymers (PCDTBT and PSiF-DBT) as well as PCBM as acceptor.