In this study the charge dissociation at the donor/acceptor heterointerface of thermally evaporated planar heterojunction merocyanine/C 60 organic solar cells is investigated. Deposition of the donor material on a heated substrate as well as post-annealing of the complete devices at temperatures above the glass transition temperature of the donor material results in a twofold increase of the fill factor. An analytical model employing an electric-fielddependent exciton dissociation mechanism reveals that geminate recombination is limiting the performance of as-deposited cells. Fourier-transform infrared ellipsometry shows that, at temperatures above the glass transition temperature of the donor material, the orientation of the dye molecules in the donor films undergoes changes upon annealing. Based on this finding, the influence of the dye molecules' orientations on the charge-transfer state energies is calculated by quantum mechanical/molecular mechanics methods. The results of these detailed studies provide new insight into the exciton dissociation process in organic photovoltaic devices, and thus valuable guidelines for designing new donor materials.

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In this paper we investigate the performance and stability of small-molecule organic solar cells with respect to the indium tin oxide (ITO)/organic interface. Different zinc-phthalocyanine (ZnPc)/fullerene (C 60 ) cell architectures with and without ITO O 2 -plasma treatment are compared and tested with respect to their degradation behavior under illumination in inert atmosphere. Photoelectron spectroscopy (UPS and XPS) shows that the O 2 -plasma treatment increases the ITO work function from 4.3 eV up to 5.6 eV. We find that both the increased ITO work function as well as the introduction of an electron blocking layer between ITO and the mixed donor/acceptor layer increases the open-circuit voltage V oc by more than 200 mV. For both cases our continuum approach device simulation quantitatively relates the increase of V oc to a reduced contact recombination and thus a reduced dark current. For cells built on ozone treated ITO we find a fast cell degradation caused by the UV part of the AM 1.5 spectrum. We identify the degradation, which manifests itself in a decrease of V oc of up to 25%, as a partial reversion of the plasma induced ITO work function increase. Additionally, we demonstrate that the degradation can be reduced by structural changes in the cell architecture, leading to improved cell stability. We present a comprehensive study of the recombination at the ITO/organic interface and its influence on the open-circuit voltage and the cell stability.

In organic heterojunction solar cells, the generation of free charge carriers takes place in a multistep process which involves charge transfer (CT) states, that is, bound electron-hole pairs at the interface between donor and acceptor molecules. Past efforts to model the CT-state dissociation during solar cell operation were not able to consistently reproduce the experimentally observed field and temperature dependence. This discrepancy between model and experiment was partly due to the field-dependent free charge carrier collection process, which plays an important role in the widely used bulk heterojunction cell configuration and superimposes a possible field-dependent charge carrier generation process. In order to distinguish between generation and collection of free charge carriers, we propose the planar heterojunction cell configuration as a model system to study the field-dependent charge carrier generation process in organic heterojunction solar cells. We apply this model system to check current CT-state dissociation models against experimental data. Although the models can quantitatively account for the photocurrent's dependence on the applied voltage and the device thickness, they fail to account for the virtually negligible temperature dependence of the field-dependent charge-generation process. This discrepancy is traced back to a common feature of the models: an Arrhenius-like temperature dependence, distinctive of all processes involving a thermally activated jump over an energy barrier. As a solution to the problem, we introduce an exciton dissociation model based on a field-dependent tunnel process and demonstrate its consistency with the experimental observations. Our results indicate that the current microscopic picture of the charge-generation process in organic heterojunction solar cells being limited by the CT-state dissociation process needs to be reconsidered.

Ajoint experimental and theoretical study ofthe double-differential ~r o ~~~e f t i o n s (DOCS) for ejection of electrons from helium by 1.00 and 1.84 MeVamu-' pi, Hez*, C6* and On+ is presented. Electron-emission angles from 20' to 160' are surveyed, and the electron energies range from 4 to 25OOeV. Single-differential and total cross sections, obtained hy integrating the DDCS over angle and electron energy, are also presented. Large departures from the q 2 dependence (where q is the projectile charge) predicted by the first Born approximation are observed in the cross sections. The deviations, which are seen to increase with projectile charge. are discussed and compared with calculations, using the first Born approximation and a caotinuum-distorted-wave eik~nal-ioitial-itate model. It is concluded that the deviations are due to the simultaneous effect of the projectile and residual target fields, thus indicating that two-centre effects are important in the electron emission.0953.4075/91/ 184001+ 16P03.50 0 1991 IOP Publishing Ltd for On+ impact (Knudsen et af 1984), and for 1.84 MeV amu-', it varies from 0.003 for pt impact to 0.006 for Os+ impact.We have measured DDCS for ejection of electrons from helium by p+, He'+, C"' and Oaf of 1.00 and 1.84 MeVamu-' energy, so that the comparison between protons and multiply charged ions can be made directly. Electron spectra at seven angles between 20" and 160" with respect to the incoming projectile beam were measured with electron energies ranging from 4 eV to 2.5 keV. This includes both the ECC region and the energy corresponding to the binary-encounter peak. The measurements are compared with calculations using the FBA and CDW-EIS model. J 0 P Pedersen et of Experimental methodThe measurements were performed using ions from the Aarhus Tandem van de Graaff accelerator. The higher charge states, Ch' and 0". were reached by poststripping in