Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
Organic solar cell modules are prospectively an inexpensive technology because they can be produced by roll-to-roll coating. This printing technique allows that the costs of modules will mostly be determined by the cost of material, but on the other hand and this production type precludes that sub-cells can be chosen for matching photocurrents. In case of varying film thicknesses in series connected modules, this leads to situations where one sub-cell limits the overall short-circuit current of the module. Krebs et al. have shown that in all but the best modules laser beam induced current mapping under forward bias conditions is unable to measure the local photocurrents[1]. In laser beam induced current measurements with bias illumination conditions we were able to reproduce these results. In order to understand this behavior we performed an approach where the photocurrent of each sub-cell is individually reduced below the threshold of the module short-circuit current while the remaining cells are kept under one sun illumination. In a module with sub-cells showing perfect blocking behavior we would directly be able to identify the current limiting cell as well as measure the sub-cells photocurrent, yet in more realistic conditions sub-cells have a lower parallel resistance which reduces the capability of individual cells to block the short-circuit current of the module under bias conditions. We investigated these conditions by electronic circuit simulations using SPICE and as a result we identified the prerequisites for which the filtering approach delivers reliable information on the photocurrent. We are now able to reliably distinguish between cells with low parallel resistances and cells for which we can determine their photocurrent by filtering. Further, for laser beam induced current measurements, we can show that the competition between the global series and the individual parallel resistances reduces the measured external quantum efficiency of the mapping laser beam.
[1] David M. Tanenbaum, H. F. Dam, R. Rösch, M. Jørgensen, H. Hoppe, F. C. Krebs, Edge sealing for low cost stability enhancement of roll-to-roll processed flexible polymer solar cell modules, Solar Energy Materials and Solar Cells 2012, 97, 157-163