Quantification and effects of molecular oxygen and water in zinc phthalocyanine layers
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K.T. Westerduin and A.T. van Veen Interfaculty Reactor Institute, Reactor Physics Department, Mekelweg 15, 2629 JB Delft, The Netherlands
E.E. van Faassen Utrecht University, Debye Institute, Section Interface Physics, P.O. Box 80000, 3508 TA Utrecht, The Netherlands (Received 14 February 2000; accepted 6 November 2000)
Gas effusion measurements on zinc phthalocyanine (ZnPc) layers showed the presence of a significant amount of oxygen and water inside the material during exposure to ambient conditions. Of both species the bulk concentration lay in the range of 1020 molecules per cm3. Temperature-dependent analysis indicated that at 296 K all O2 molecules, and roughly one half of the H2O molecules, were mobilized and diffused with diffusion coefficients DO2 of 3 * 10−8 cm2/s and DH2O of 1.3 * 10−10 cm2/s. Electrical analysis of ZnPc layers in controlled atmospheres revealed that the electrical properties of the bulk were determined by O2, whereas H2O influences the surface conductivity. A space-charge density of (1.6 ± 0.2) * 1016 O2− ions per cm3 was measured in atmospheric conditions.
I. INTRODUCTION
Organic photovoltaic devices are currently receiving great interest because of the positive expectations of their role in the development of inexpensive solar cells. In order to reach enhanced efficiencies, a variety of cell concepts are investigated in ongoing research projects. Devices entirely made of molecular semiconductors often feature poor light-to-current efficiencies, although some relatively promising cells have been constructed.1,2 As the photophysical and electrical processes in these molecular materials are not so well understood as in inorganic semiconductors, a better conception of the materials is required before a systematic approach toward efficient future device architecture can be made. The dyes used as the active part of flat film organic solar cells exhibit strong absorption bands in the visible region of the solar spectrum. Ideally, each molecular photoexcitation leads to the collection of one electronhole pair by the external electrodes of the device. Two fundamental conditions determining the quantum yield of this process are efficient energy transfer inside the organic layer and charge-carrier separation at the active interface. Phthalocyanines (Pcs) are promising candidates for fulfilling these criteria, because they exhibit a characteristic structural self-organization,3,4 which is reflected in an efficient energy migration in the form of J. Mater. Res., Vol. 16, No. 2, Feb 2001
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exciton transport. The doping properties of these molecular semiconductors can be manipulated by adding acceptorlike molecules, like halogens, o-chloranil or trinitrofluorenone (TNF);5,6 e.g., by diffusion from the gas phase or coevaporation during physical vapor deposition of the layer. Using these techniques, the intrinsic electrical conductivity of 10−11 to 10−16 S per centimeter can be raised several orders of magnitude. By shifting the Fermi level to
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