Oxygen doping in zinc phthalocyanine layers for photovoltaic applications

Harald Robert Kerp; E. E. van Faassen

doi:10.1117/12.416933

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5 February 2001 Oxygen doping in zinc phthalocyanine layers for photovoltaic applications

Harald Robert Kerp, E. E. van Faassen

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Proceedings Volume 4108, Organic Photovoltaics; (2001) https://doi.org/10.1117/12.416933
Event: International Symposium on Optical Science and Technology, 2000, San Diego, CA, United States

Abstract

Gas effusion measurements show that ZnPc layers absorb considerable quantities of oxygen and water if exposed to ambient air. Bulk concentrations of O₂ and H₂O reach levels of (1.1 +/- 0.3)*10²⁰ and (1.7 +/- 0.4)*10²⁰ molecules per cm³ respectively. O₂ is observed to maintain its molecular form when incorporated into the ZnPc matrix. At room temperature (296 K), the oxygen molecules are free to move diffusively in the layer with a diffusion coefficient of (3.0 +/- 0.4)*10^-8 cm²/s. Electrical analysis of ZnPc layers in controlled gas atmospheres show that O₂ establishes a p-type doping in the bulk. The space charge density increases linearly with the external oxygen pressure. In ambient conditions, the O₂ concentration equals (1.6 +/- 0.2)*10¹⁶ ions per cm³, which corresponds with a thermal activation energy of (0.23 +/- 0.05) eV for ionizing O₂. In contrast to O₂, H₂O does not affect the space charge density of the depletion layer. The performance of p-n type solar cells incorporating a ZnPc layer was investigated under different oxygen pressures. It was possible to raise the efficiency of the cell when applying higher O₂ pressures. The short-circuit current increased by a factor of 1.5 when raising the oxygen pressure threefold from atmosphere pressure under AM1.5 illumination. This can be attributed to a more efficient hole collection through the organic layer resulting from a higher internal electrical field strength. However, the exciton diffusion length was observed to decrease approximately 25% when raising the external oxygen pressure from 0.02 to 0.47 bar. We conclude that the exciton diffusion length in organic solar cells is limited by the ionic impurity density, which on the other hand is required for the effective collection of charge carriers.

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Harald Robert Kerp and E. E. van Faassen "Oxygen doping in zinc phthalocyanine layers for photovoltaic applications", Proc. SPIE 4108, Organic Photovoltaics, (5 February 2001); https://doi.org/10.1117/12.416933

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