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DOI: 10.1039/A707485I (Paper) Reference Section for: J. Mater. Chem., 1998, 8, 945-954

Influence of surface reactions and ionization gradients on junction properties of F16PcZn

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Stefan Hiller, Derck Schlettwein, Neal R. Armstrong and Dieter Wöhrle

Abstract

Compared to unsubstituted phthalocyaninatozinc(ii) (PcZn), electron withdrawing fluorine atoms in hexadecafluorophthalocyaninatozinc(ii) (F16PcZn) cause a stabilization of the frontier orbitals of about 1.6 eV. This is concluded from photoelectron spectroscopy (UPS) at thin films on Au surfaces. From experiments at thin films [physical vapor deposition (PVD)] of PcZn deposited on top of F16PcZn under UHV conditions it is seen that a closed film of PcZn is formed at least within 5 nm average film thickness, that thermodynamic equilibrium between the films is achieved by charge transfer in redox reactions at the interface which, however, do not lead to a macroscopic space-charge layer. To study electrical device properties thin films of F16PcZn and PcZn were prepared in a range between 90 nm and 240 nm. Changes in electrical properties of ITO, Au|F16PcZn|metal (metal=In, Au) and ITO|F16PcZn|PcZn|Au devices have been studied in the dark and under illumination. Results of current–voltage characteristics and short-circuit photocurrent spectra of devices as prepared and measured under high vacuum (HV, 10–5–10–6 mbar) and after exposure to air are presented. In vacuum symmetrical I(U) characteristics were found for ITO|F16PcZn|Au devices. After exposure to air a decrease in dark conductivity, unsymmetrical I(U) characteristics and a considerable photovoltage (UOC) was measured under illumination. The magnitude of UOC as well as its direction can be clearly correlated with the exposure to atmosphere. This observation leads to a discussion based on a local asymmetry in O2 content as caused by slow diffusion into F16PcZn. O2 would lead to a decrease in the local majority carrier density as typically expected for organic n-type conductors. Rectification found in F16PcZn|In devices can be explained by a chemical reaction between the distinct electron acceptor F16PcZn and In as a donor. Photocurrent action spectra of devices with different thicknesses of F16PcZn layers in ITO|F16PcZn|PcZn|Au revealed detailed information about the site of charge carrier generation (photoactive area). The junction properties are discussed in detail based on the frontier orbital positions of PcZn and F16PcZn, and the work functions of the corresponding electrode materials.

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