Abstract

The course of photovoltaic phenomenon in organic photovoltaic cells utilizing thin layers of perfluorozincphthalocyanine was studied in few single and planar bilayer configurations. Mechanisms of photogeneration and recombination of free charge carriers were determined and active interfaces were localized. In case of single layer cells these interfaces were located at the front electrode/F16ZnPc junctions, whereas photogeneration of free charge carriers in all investigated bilayer systems took place at the donor/acceptor junctions. Theoretical models describing the current-voltage behavior of single layer excitonic cells and planar bilayer cells (termed the CFA models) were developed. These models show how free charge carrier mobilities, electrode work functions, dielectric constants and thickness of donor and acceptor layers along with mono and bimolecular recombination constants affect the shape of the current-voltage curves. Knowing this dependence, such important physical quantities as the open-circuit voltage and short-circuit current were correlated with the aforementioned parameters. Subsequently, experimental data collected for all investigated systems were analyzed on the basis of the developed models and the Shockley equivalent circuit model commonly used for organic solar cells. Comparison of the derived CFA models and the SEC model brought some insight on the relation between physical properties of the layers forming the modeled cells and parameters characterizing performance of a cell, such as series and shunt resistances, diode ideality factor or photogeneration and saturation currents. Inter alia, it was possible to explain the illumination dependence of the shunt resistance commonly observed in case of organic solar cells. Moreover, it was shown that one of the layers of the D/A systems may govern the current flow and that the free charge carrier mobility is not necessarily the most important factor influencing the free charge carrier transport through the cell. Surprisingly, the best photovoltaic performance was shown by the single layer CuI/F16ZnPc/BCP/Ag cell. Further increase of the performance of this system is possible if optimization of the phthalocyanine layer is made. Relatively high values of fill factors and open-circuit voltage obtained in the course of this study lead to the conclusion, that F16ZnPc may be a promising material for the applications in the field of organic photovoltaics.

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