R. Signerski

Photoelectric properties of heterojunctions formed from di-( pyridyl ) -perylenetetracarboxylic diimide and copper phthalocyanine or pentacene

The origin of conductivity in di-(pyridyl)-perylenetetracarboxylic diimide (Py-PTC) is investigated. The obtained results allow us to classify Py-PTC as n-type material. Heterojunctions formed from Py-PTC and copper phthalocyanine or pentacene reveal reasonable photovoltaic properties.

Electroluminescence in Tetracene Crystals

Abstract dc and pulsed electroluminescence (EL) in tetracene crystals have been studied at room temperature. The crystals are provided with Na/K alloy electron injecting contact and Au semitransparent layer as a hole injecting contact. dc EL intensity shows a superlinear relationship with crystal current. EL decay indicates that the electroluminescence in tetracene consists of two components: one prompt and a second delayed component. The results can be understood if the delayed component depends upon crystal current and upon some characteristics of the active traps in the crystal. The spectral distribution of the electroluminescence as compared to photoluminescence spectrum reveals the intensive long wavelength emission. It is presumably due to trapped excitons.

Photoenhanced current in thin organic layers

Experimental dependences of photoenhanced current (PEC) on wavelength, voltage, light intensity and temperature carried out on polycrystalline tetracene, pentacene and perylene layers are presented. As it is shown, the description of PEC in thin organic layers requires introducing the Gaussian trap distribution as well as diffusion of both excitons and charge carriers into the classical model of PEC.

Effect of buffer layers on performance of organic photovoltaic devices based on copper phthalocyanine-perylene dye heterojunction

The work presents the results of research on the systems formed from thin films of copper phthalocyanine (CuPc), N-N′-dimethylperylene-3,4,9,10-dicarboximide (MePTCDI), electrodes of ITO and Ag, and from buffer layers: MoO3 at ITO and BCP at Ag. We have observed the effect of each buffer layer on voltage dependence of dark current and photocurrent, and on open circuit voltage-light intensity relationship. The system with both buffer layers exhibited the highest values of open circuit voltage and fill factor. The buffer layers improve transport of charge carriers within near-electrode regions, reduce dissociation of excitons on electrodes and reveal processes of charge carrier generation and recombination within the CuPc/MePTCDI junction.

Photoenhanced currents in pentacene layers

Photoenhanced current measurements carried out on polycrystalline pentacene layers are presented. A new approach to the description of the photoenhanced current is proposed. The inclusion of generation and recombination processes induced by oxygen centres enables us to explain the experimental current-voltage and current-light intensity relations.

Diffusion length of singlet excitons in copper phthalocyanine films

The paper presents the way that colour can serve solving the problem of calibration points indexing in a camera geometrical calibration process. We propose a technique in which indexes of calibration points in a black-and-white chessboard are represented as sets of colour regions in the neighbourhood of calibration points. We provide some general rules for designing a colour calibration chessboard and provide a method of calibration image analysis. We show that this approach leads to obtaining better results than in the case of widely used methods employing information about already indexed points to compute indexes. We also report constraints concerning the technique. Nowadays we are witnessing an increasing need for camera geometrical calibration systems. They are vital for such applications as 3D modelling, 3D reconstruction, assembly control systems, etc. Wherever possible, calibration objects placed in the scene are used in a camera geometrical calibration process. This approach significantly increases accuracy of calibration results and makes the calibration data extraction process easier and universal. There are many geometrical camera calibration techniques for a known calibration scene [1]. A great number of them use as an input calibration points which are localised and indexed in the scene. In this paper we propose the technique of calibration points indexing which uses a colour chessboard. The presented technique was developed by solving problems we encountered during experiments with our earlier methods of camera calibration scene analysis [2]-[3]. In particular, the proposed technique increases the number of indexed points points in case of local lack of calibration points detection. At the beginning of the paper we present a way of designing a chessboard pattern. Then we describe a calibration point indexing method, and finally we show experimental results. A black-and-white chessboard is widely used in order to obtain sub-pixel accuracy of calibration points localisation [1]. Calibration points are defined as corners of chessboard squares. Assuming the availability of rough localisation of these points, the points can be indexed. Noting that differences in distances between neighbouring points in calibration scene images differ slightly, one of the local searching methods can be employed (e.g. [2]). Methods of this type search for a calibration point to be indexed, using a window of a certain size. The position of the window is determined by a vector representing the distance between two previously indexed points in the same row or column. However, experiments show that this approach has its disadvantages, as described below. * E-mail: A.Nowakowski@ire.pw.edu.pl Firstly, there is a danger of omitting some points during indexing in case of local lack of calibration points detection in a neighbourhood (e.g. caused by the presence of non-homogeneous light in the calibration scene). A particularly unfavourable situation is when the local lack of detection effects in the appearance of separated regions of detected calibration points. It is worth saying that such situations are likely to happen for calibration points situated near image borders. Such points are very important for the analysis of optical nonlinearities, and a lack of them can significantly influence the accuracy of distortion modelling. Secondly, such methods may give wrong results in the case of optical distortion with strong nonlinearities when getting information about the neighbouring index is not an easy task. Beside this, the methods are very sensitive to a single false localisation of a calibration point. Such a single false localisation can even result in false indexing of a big set of calibration points. To avoid the above-mentioned problems, we propose using a black-and-white chessboard which contains the coded index of a calibration point in the form of colour squares situated in the nearest neighbourhood of each point. The index of a certain calibration point is determined by colours of four nearest neighbouring squares (Fig.1). An order of squares in such foursome is important. Because the size of a colour square is determined only by the possibility of correct colour detection, the size of a colour square can be smaller than the size of a black or white square. The larger size of a black or white square is determined by the requirements of the exact localisation step which follows the indexing of calibration points [3]. In this step, edge information is extracted from a blackand-white chessboard. This edge information needs larger Artur Nowakowski, Wladyslaw Skarbek Institute of Radioelectronics, Warsaw University of Technology, Nowowiejska 15/19, 00-665 Warszawa, A.Nowakowski@ire.pw.edu.pl Received February 10, 2009; accepted March 27, 2009; published March 31, 2009 http://www.photonics.pl/PLP

Small signal admittance spectroscopy applied for extraction of charge carrier mobility in thin organic layers

The work presents new way of extraction of charge carrier mobility from small signal spectra of admittance. It can be a useful method for thin organic layers in a sandwich arrangement. It is better than the version of small signal admittance currently applied for getting charge carrier mobility, since the final result is not charged with uncertainty of geometric capacitance and the upper limit of measurable charge carrier mobility is higher.

The analysis of photoenhanced current in organic films: the influence of charge carrier diffusion

This paper examines the influence of charge carrier diffusion on photoenhanced current characteristics. The results of our analysis indicates that in the case of thin layers the diffusion of charge carriers leads to the following type of photoenhanced current–voltage relationship: j∼Un with 1≤n<2. Experimental results of photoenhanced current obtained on tetracene and pentacene layers confirming the role of charge carrier diffusion in thin samples are presented.

X-Ray-Excited Photocurrent in Anthracene Crystals

Abstract The mechanism of charge carrier generation by X-rays in anthracene crystals has been studied. We conclude that charge carrier pairs are generated by electrons due to the scattering of X-rays and the separation mechanism is in disagreement with the Onsager model. Recombination of charge carriers determines the slope of the photocurrent-electric field characteristics. The experimental current-electric field characteristics have been described theoretically and the role of photoinjection of charge carriers by X-rays from metallic electrodes has been established.

Temperature and light intensity dependence of photoenhanced current in thin organic layers

Abstract Experimental dependences of photoenhanced current (PEC) on temperature and light intensity carried out on tetracene and pentacene layers are presented. Performed analysis of the results confirms the necessity of inclusion of the Gaussian trap distribution and charge carrier diffusion into theoretical model describing PEC. Moreover, temperature characteristics of the current indicate that exciton–charge carrier rate constant depends on temperature.