How is the PCBM stream

Chirvase, Dana (2004) Electrical characterization of organic devices: case study: polythiophene-fullerene based solar cells. PhD, University of Oldenburg.

Abstract

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 behavior 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 work function of the metallic electrode. The hypothesis of Fermi level pinning of the work function 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 (larger than 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.

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Polymer solar cells with a bulk heterojunction structure were produced, with P3HT acting as the donor and the fullerene derivative PCBM as the acceptor. The electrical and optical properties of these organic cells were mainly studied via the dependence of their current-voltage characteristics on temperature and light intensity. The increase in short-circuit current density with temperature is evidence of a thermally activated transport mechanism that is characteristic of disordered materials. This result explains the outstanding property of organic solar cells to function better in a warm climate than in low temperatures. In contrast, conventional inorganic solar cells show a completely different behavior. The origin of the open circuit voltage was investigated by varying the work function of the metal electrode. From this experiment it could be concluded that the open circuit voltage is sensitive to the work function of the metal electrode used. The hypothesis of the Fermi-level pinning of the work function of the metal electrode on the lumen level of the acceptor has not been confirmed in the case of the P3HT: PCBM solar cells. The thermal treatment of the P3HT: PCBM solar cells proved to be a very effective method for increasing the short-circuit current density and thus also very effective for improving the overall efficiency of the cell. Investigations of the morphology of the P3HT: PCBM solar cells using AFM show the presence of very large PCBM clusters (larger than 500 nm) as a result of the thermal treatment. Based on these results, the J - V curves and measurements of the external quantum yield, the proportion of the PCBM acceptor in the P3HT: PCBM mixture was optimized to a weight ratio between 1: 0.9 and 1: 1. The best P3HT: PCBM solar cell that was produced had a P3HT: PCBM weight ratio of 1: 1 and achieved the following results at room temperature and a light intensity of 100 mW / cm2: Jsc = 6.4mA / cm2, Voc = 0.59V, FF = 63.2%, h = 2.4% at room temperature and a light intensity of 100mW / cm2. In addition, it could be shown that the electrical properties of P3HT: PCBM solar cells are significantly influenced by the quality of the organic material used.

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