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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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