Moritz Riede

Abstract:

One key problem in optimizing organic solar cells is to maximize the absorption of incident light and to keep the charge carrier transport paths as short as possible in order to minimize recombination losses during the charge carrier extraction. The large versatility of organic semiconductors and compositions requires specific optimization of each system. Due to the small thickness of the functional layers in the order of several ten nanometres, coherent optics has to be considered and therefore interference effects play a dominant role. Here we present and discuss concepts for light trapping in organic solar cells. These are wide gap layers in planar solar cells, folded solar cell architectures benefiting from the illumination under inclined incident angles and multiple reflections and absorptions as well as diffraction gratings embossed into the photoactive layer. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA.

Abstract:

In this paper we present a high throughput testing setup for organic solar cells that is necessary for an efficient analysis of their behaviour. The setup comprises process parameter logging, automated measurement data acquisition and subsequent data management and analysis. Utilising this setup the reproducibitity of solar cells and the effect of production parameter variations has been tested with a set of 360 solar cells based on thepoly-3- hexylthiophene:1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6)C61 bulk heterojunction. Variations in power conversion efficiency between 1 and 3% were observed on varying production parameters hardly mentioned in literature. The conditions during the vacuum deposition of the aluminium cathode turned out to have a significant effect. The key solar cell parameter affecting the performance was the fill factor (FF). As such the work exemplifies the necessity for a combined approach to analyse the complex behaviour of organic solar cells. The developed high throughput testing setup provides a basis for an efficient testing of production parameter variations and materials and additionally opens the door for statistical analysis. Copyright © 2008 John Wiley & Sons, Ltd.

Abstract:

We compare between two derivatives of dicyanovinyl-quinquethiophenes with different alkyl side chain lengths. Both materials show comparable open circuit voltages Voc in organic solar cells with fullerene C60 as acceptor, as expected since they have the same highest occupied molecular orbital energy. However, differences in the current-voltage-characteristics, particularly in the fill factor, are observed. We analyze both derivatives in hole-only devices and find a difference in the hole injection between the doped hole transport layer and the oligothiophenes. Additionally, we determine the hole mobility of the two materials and explain the different behaviors of the two materials in solar cells. © 2008 American Institute of Physics.

Abstract:

In this paper we focus on the current status of organic solar cells based on small molecules. Since their discovery, much progress has been made, and the main steps are highlighted that led to the current state-of-the-art devices. However, organic solar cells still need to be improved further, and the main strategies for improving the power conversion efficiency, namely raising the open circuit voltage V(oc) and increasing the short circuit current density J(sc), are discussed. In theory, power conversion efficiencies of around 15% should be possible with a single heterojunction; for higher efficiencies, stacked solar cell concepts have to be employed.