Olaf Stenzel

Enhancement of the photovoltaic conversion efficiency of copper phthalocyanine thin film devices by incorporation of metal clusters

ITO-copperphthalocyanine-indium sandwich structures have been prepared by thermal evaporation in high vacuum. Different types of metal nanoclusters have been incorporated at the ITO-copperphthalocyanine interface. Significant differences in the photovoltaic conversion efficiency of the sandwich samples have been established as a function of the excitation wavelength. Copper clusters have been found to increase the conversion efficiency by a factor of nearly three for solar illumination. The observed effects are discussed assuming resonant light absorption in the metal clusters as the responsible physical mechanism for enhancement.

The incorporation of metal clusters into thin organic dye layers as a method for producing strongly absorbing composite layers: an oscillator model approach to resonant metal cluster absorption

We present data on the NIR/VIS/UV optical constants of thin composite layers, built up from a copper phthalocyanine matrix with embedded metal clusters. The metal clusters (copper, gold and silver) have diameters on a nanometre scale and act as strong absorption centres. In particular, local plasmon resonances and metal interband transitions could be observed from a fit of the experimentally determined dielectric functions by means of a Lorentzian multi-oscillator model. Average dipole transition matrix elements with respect to a single metal cluster have been estimated from the oscillator parameters.

Gaussian quadrature approach to the calculation of the optical constants in the vicinity of inhomogeneously broadened absorption lines

A fast numerical procedure to calculate the convolution of Gaussian and Lorentzian functions is introduced as well as its application to the approximation of experimental spectra. It is shown that these spectra can be well approximated with the method, especially in the case when the inhomogeneous and homogeneous linewidth contributions as parameters of a Voigt lineshape are of the same order of magnitude. Particularly, the method is applicable to refractive index modelling in the vicinity of inhomogeneously broadened absorption lines.

Computational manufacturing of optical interference coatings: method, simulation results, and comparison with experiment

Virtual deposition runs have been performed to estimate the production yield of selected oxide optical interference coatings when plasma ion-assisted deposition with an advanced plasma source is applied. Thereby, deposition of each layer can be terminated either by broadband optical monitoring or quartz crystal monitoring. Numerous deposition runs of single-layer coatings have been performed to investigate the reproducibility of coating properties and to quantify deposition errors for the simulation. Variations of the following parameters are considered in the simulation: refractive index, extinction coefficient, and film thickness. The refractive index and the extinction coefficient are simulated in terms of the oscillator model. The parameters are varied using an apodized normal distribution with known mean value and standard deviation. Simulation of variations in the film thickness is performed specific to the selected monitoring strategy. Several deposition runs of the selected oxide interference coatings have been performed to verify the simulation results by experimental data.

Flexible construction of error functions and their minimization: application to the calculation of optical constants of absorbing or scattering thin-film materials from spectrophotometric data

A flexible numerical procedure for the calculation of thin-film optical constants from specular transmittance and reflectance data is presented. The method is based on the minimization of a quadratic error function, which may be adapted to the specifics of the optical behaviour of the given sample (or set of samples), and the given wavenumber region. The flexibility in choosing an appropriate form of the minimized error function, in combination with the powerful minimization method of conjugated gradients, allowed us to investigate the optical constants of very different types of novel thin-film material with a complicated optical loss behaviour. In particular, the results concerning the investigation of single- and two-layer systems based on the following technologically interesting optical thin film materials are presented: (1) amorphous silicon as an example of-an anorganic solar cell material; (2) as-deposited (rough) CVD diamond layers as an example of a polycrystalline protective material; (3) hydrogenated amorphous carbon, applicable as a protective long-wavelength in antireflection coating as well as a spectrally selective solar absorber; (4) copper phthalocyanine layers as an example of a molecular solid, potentially applicable as an organic solar cell material; (5) rare-earth diphthalocyanine layers, interesting because of their electrochromic behaviour.

Plasma ion assisted deposition of hafnium dioxide using argon and xenon as process gases

Hafnium dioxide films have been produced by plasma ion assisted electron beam evaporation, utilizing argon or xenon as working gases. The optical constants of the layers have been investigated by spectrophotometry, while X-ray reflection measurements (XRR), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM) have been performed with selected samples. The correlation between structural and optical properties is discussed. With respect to optical quality, the application of xenon as working gas results in coatings with higher refractive index and smaller surface roughness than the application of argon. This effect is attributed to a more efficient momentum transfer from high energetic working gas ions or atoms to hafnium atoms during deposition.

Deposition and spectral performance of an inhomogeneous broadband wide-angular antireflective coating

Gradient index coatings and optical filters are a challenge for fabrication. In a round-robin experiment, basically the same hybrid antireflection coating for the visible spectral region, combining homogeneous refractive index layers of pure materials and linear gradient refractive index layers of material mixtures, has been deposited. The experiment involved three different deposition techniques: electron-beam evaporation, ion-beam sputtering, and radio frequency magnetron sputtering. The material combinations used by these techniques were Nb(2)O(5)/SiO(2), TiO(2)/SiO(2), and Ta(2)O(5)/SiO(2), respectively. The spectral performances of samples coated on one side and on both sides have been compared to the corresponding theoretical spectra of the designed profile. Also, the reproducibility of results for each process is verified. Finally, it is shown that ion-beam sputtering gave the best results in terms of deviation from the theoretical performance and reproducibility.

A model for calculating the effect of nanosized pores on refractive index, thermal shift and mechanical stress in optical coatings

A simple model is presented that relates optical and mechanical properties of optical coating materials deposited by typical physical vapour deposition techniques to the porosity of the layers. In terms of this model, a clear correlation between thermal shift, mechanical stress and refractive index could be simulated. As a practical conclusion, it is found that a certain fraction of small nanosized pores in the films is essential in order to get a valuable balance between optical and mechanical coating properties.

Tuning of the plasmon absorption frequency of silver and indium nanoclusters via thin amorphous silicon films

Silver and indium nanoclusters have been incorporated into thin sputtered amorphous silicon films. The optical properties of these films have been investigated and compared with the results of spectra simulations. The dependence of the metal cluster plasmon resonance position on the silicon film thickness has been established. Additionally, the inhomogeneous broadening of the plasmon line has been estimated. The data are discussed with respect to the results of transmission electron microscopy.

The position of the fundamental absorption edge and activation energies for thermally activated electrical conductivity in amorphous carbon layers

Abstract Amorphous carbon layers (a-C, a-C:H) with a hydrogen content between 3 at.% and 25 at.% were deposited by plasma decomposition processes, sputtering and evaporation. Their mass density values were obtained from a flotation method. The refractive index and absorption coefficient were calculated from spectrophotometric data. Special attention was paid to the Urbach tail and Taucs plot absorption regions. The electrical conductivity was investigated in the temperature range T = 80–350 K. The conductivity values of all types of layers are discussed in terms of thermally activated conduction processes. In this sense all layers behave like semiconductors. For interpreting the conductivity values of the high gap layers, a Davis-Mott model with broad band tails was applied. However, this model was insufficient for fitting the conductivity data of samples with vanishing gaps. Reproduction of the conductivity values of these layers was possible in terms of a band model considering a structureless band at a position of 10–50 meV above the Fermi level. The conductivity of low gap samples (optical gaps around 0.3 eV) could only be fitted by a superposition of the conductivity laws following from both models. Fortuitously, the superposition of these functions yields a temperature dependence very similar to Motts T − 1 4 law between 50 K and 300 K, which may be an explanation of this widely observed behaviour.