Photoabsorption and photoconductivity in C₆₀ layers

A complex investigation of the photoconductivity of fullerene films, prepared by thermal evaporation in vacuum, was carried out. The investigated films contain predominantly C₆₀ in various phases as shown elsewhere. The multiple photocurrent spectra analysis (MPSA method) has been used in the current study. The MPSA method utilizes a family of routine photocurrent spectra, measured at various intensities generated by the light source. A suitable data processing of such a family allowed several basic characteristics of the films to be obtained – the optical bandgap, subbandgap optical absorption spectra, features of shallow and deep defect states as well as the spectra of the power index in the dependence of the photocurrent on the photon flux. These characteristics were interpreted in terms of structural features of the investigated samples. Conclusions about applications of such C₆₀ films were made

Evaluation of bottom-up and top-down strategies for aggregated forecasts: state space models and arima applications

Abstract. In this research, we consider monthly series from the M4 competition to study the relative performance of top-down and bottom-up strategies by means of implementing forecast automation of state space and ARIMA models. For the bottomup strategy, the forecast for each series is developed individually and then these are combined to produce a cumulative forecast of the aggregated series. For the top-down strategy, the series or components values are first combined and then a single forecast is determined for the aggregated series. Based on our implementation, state space models showed a higher forecast performance when a top-down strategy is applied. ARIMA models had a higher forecast performance for the bottom-up strategy. For state space models the top-down strategy reduced the overall error significantly. ARIMA models showed to be more accurate when forecasts are first determined individually. As part of the development we also proposed an approach to improve the forecasting procedure of aggregation strategies

Effect of different technological approaches on the optical properties of ZnO sol-gel thin films

peer reviewedThe work presents a sol-gel approach with two types of solvents for ZnO films deposition in view of varying the films' structural and optical properties. The ZnO films were characterized by X-Ray diffraction (XRD) and found to have a polycrystalline wurtzite phase structure. The grain sizes and the lattice parameters were determined; it was established that the ZnO crystallites' size increases with the annealing temperatures from 25 nm to 36 nm. The ZnO films possess high transmittance in the visible spectral range. Further, the optical band gap values were estimated. The material's vibration properties were analyzed by Fourier Transform Infrared (FTIR) spectroscopy. The two technological sol-gel approaches for deposition of ZnO films, using two different solvents, proved to be successful in producing structures with different structural and optical properties. The thin films fabricated were very smooth and uniform and exhibited a high transparency in the visible spectral range

Conductivity and photoconductivity peculiarities observed in C₆₀ layers

Thin layers of various thickness prepared from C60 with traces of C70 were studied. They were deposited by thermal evaporation on quartz, glass, p-Si or n-Si substrates. An apparatus fixing current values every 3 ms was used to measure and register the kinetics of layer conductivity and photoconductivity. Series of peculiarities were observed in the kinetics of the current when an electrical field was applied to illuminated or darkened samples. For example, when voltage is applied to the sample the current immediately rises to a certain value and then falls to a different quasi-stationary value. These peculiarities depend on the particular state of the samples. After analyzing the phenomena, a scheme explaining these peculiarities was proposed which relates mainly to intrinsic polarization. The influence, which these processes might exert on the photoconductivity and data accuracy, was discussed. A substantial influence of ambient humidity was determined. Our study of the observed changes showed that humidity did not substantially affect the generation-recombination processes in the bulk material, but predominantly the carrier transport mechanism