Electrical characterization of organic devices: case study: polythiophene-fullerene based solar cells

Polymer based solar cells were fabricated by using Poly(3-hexylthiophene-2.5diyl)(P3HT) as a donor, combined with the fullerene derivative [6,6]-phenyl -C61 butyric acid methyl ester, (PCBM) as an acceptor, in a bulk heterojunction structure. Electrical and optical properties of these organic devices were studied together with the dependence of current-voltage characteristics on temperature and illumination intensity. The increase of the short circuit current density with temperature is evidence of a thermally activated transport mechanism, characteristic to disordered materials. This result explains the specific feature of organic solar cells to operate better in a warm climate, rather than at low temperatures, a totally different behaviour compared to conventional inorganic solar cells. The origin of open circuit voltage was investigated by varying the work function of the metallic electrode. From this experiment it was concluded that the open circuit voltage is very sensitive to the workfunction of metallic electrode. The hypothesis of Fermi level pinning of the workfunction of the metallic electrode to the LUMO level of the acceptor could not be confirmed in the case of P3HT:PCBM based solar cells. The thermal annealing applied to P3HT:PCBM based solar cells, was found to be a very effective method to increase the short current density, and therefore, the overall power conversion efficiency of the device. Morphology investigations (by using the Atomic Force Microscopy) for P3HT:PCBM based solar cells show the presence of large PCBM clusters (größer als 500nm) built as a result of thermal annealing. Based on AFM results, as well as on current density-voltage (J-V) curves and external quantum efficiency measurements, the amount of the PCBM acceptor was optimized in the blend to a P3HT:PCBM weight ratio between 1:0.9 and 1:1. The best P3HT:PCBM based solar cell was fabricated with a P3HT:PCBM weight ratio of 1:1 and gave the following results: Jsc= 6.4mA/cm2, Voc= 0.59V, FF = 63.2%, h = 2.4% at room temperature and a white light intensity of 100mW/cm2. Finally, it was shown that the electrical performance of P3HT:PCBM based solar cells can be considerably influenced by the quality of the organic material used for the fabrication.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Oldenburg

Year

2004

Author dc:creator

• Chirvase, Dana

• [Keine Schlagwörter von Autor/in vergeben.]

Repository record source_url

http://oops.uni-oldenburg.de/168

OAI identifier oai:identifier

oai:oops.uni-oldenburg.de:168