Introduction
Thin film organic solar cells containing a photoactive dye-layer on a non-corrugated TiO₂ semiconductor require a light-harvesting antenna for sufficient light absorption [1]. For antenna structures consisting of octyl-substituted tetraphenylporphyrins use has been made of their self-organising properties resulting in stacks favouring exciton migration [2]. For a preferential direction of energy transfer and trapping of the antenna excitation at the photoactive porphyrin- TiO₂ interface, the orientation of the porphyrin antenna molecules w.r.t. those in the photo-active dye layer are shown to affect the energy transfer. An intimate contact of the dye mole-cules with the photoactive porphyrin- TiO₂ interface has been achieved by annealing the organised antenna.

Fig. 1. Molecular structures of functionalised porphyrins. (N): 3- or 4-substituted pyridyl sidegroup.

Results and Discussion
Preferentially directed energy transfer has been demonstrated using zinc-tetra(n-octyl)phenylporphyrin (ZnTOPP) as the antenna and tetra-(3-pyridyl)porphyrin free base (H₂T3PyP) as the photoactive dye at the TiO₂ interface. After spincoating a TiO₂ substrate covered with H₂T3PyP and annealing, resulting in a liquid crystalline (LC) phase, a parallel heterodimer at the TiO₂ interface is formed with ZnTOPP ligated to the 3-pyridyl nitrogen of H₂T3PyP (Fig. 2a) as demonstrated by the absorption spectrum of ZnTOPP. The fluorescence of ZnTOPP is completely quenched due to energytransfer to the free base, whereas the photocurrent action spectrum as a result of electron injection from the free base into the TiO₂/ITO substrate contains maxima of ligated ZnTOPP.L (L = 3-pyridyl) as well as of H₂T3PyP. Charge separation at the ZnTOPP/H₂T3PyP interface may also contribute to the ZnTOPP.L maxima in the photocurrent action spectrum. On the other hand, no parallel hetero-dimer is formed for the ZnTOPP/H₂T4PyP combination with the nitrogen at the 4-pyridyl position (Fig. 2b).

Fig. 2.(a). Energy- and electron transfer in a parallel heterodimer; (b). decrease of energy transfer in a non-parallel dimer.

For this combination the photocurrent action spectrum only contains weak maxima of non-ligated ZnTOPP. Now fluorescence is observed from traps in the ZnTOPP antenna.
In ordered films of ZnTOPP doped with its free base analogue H₂TOPP the latter serves as acceptor for energy transfer from the S1 state of ZnTOPP. Doping with 1% w/w H₂TOPP quenches already half of the ZnTOPP S1 fluorescence, demonstrating that the excitation extends over a number of ZnTOPP molecules before being trapped. The results can be explained by assuming that the ZnTOPP to H₂TOPP energy transfer is diffusion-limited exciton migration in the ZnTOPP stack. From the H₂TOPP concentration dependence of the ZnTOPP fluorescence quenching an exciton path length of 6 nm is derived [2]. Fluorescence decay measurements indicate a one-dimensional exciton migration within ZnTOPP stacks, with a rate constant of 10¹² s^-1.
The importance of a good contact between the antenna molecules and the photo-active TiO₂ interface deposited on ITO is demonstrated by the effect of annealing/melting on the photocurrent response of a spin-coated film of stacked ZnTOPP on a TiO₂/ITO substrate. For ZnTOPP layers spincoated from toluene onto a 30 nm TiO₂ layer which are thin as compared to the ITO roughness (50 nm) onto which the TiO₂ is deposited, the photocurrent response is very low before annealing. Annealing at 250 °C results in a >20 fold increase of the photocurrent with a maximum at 430 nm in the action spectrum typical for monomeric ZnTOPP. On the other hand, for ZnTOPP layers on thick (100 nm) TiO₂/ITO the photocurrent response is relatively high, but the spectral maximum at 450 nm, typical for stacked ZnTOPP does not shift to 430 nm upon annealing. Evidently, for thin TiO₂ layers ZnTOPP stacks, which before annealing only make contact with photo-inactive ITO, are partly broken up by annealing into smaller units and/or monomers, resulting in interaction with the photo-active TiO₂ substrate.
For non-functionalised porphyrins, such as Zn-tetraphenyl porphyrins (ZnTPP), no self-organised films are expected. The observed effects of annealing are indeed quite different from those of ZnTOPP. Annealing up to 150 °C results in a small but distinct blue-shift of the Q-band at 550 nm and a pronounced broadening of the Soret-band at its blue side. Consistent with results from triplet state magnetic resonance experiments, ZnTPP in untreated films may ligate to water molecules penetrating into the film, resulting in red-shifted absorption bands in the Soret- and Q-band regions and fluorescence from ligated ZnTPP.L traps with L = H₂O. Heating of the film removes most of the ligands, causing the ZnTPP molecules to move closer together, thereby increasing their excitonic interaction. By contrast, polar ligands, such as water, cannot penetrate into in ZnTOPP films, due to the presence of the octyl substituents at the porphyrin moiety.

References

[1]	T.J. Schaafsma, Organic solar cells using porphyrin assemblies on semiconductor substrates. Sol. Energy Mat. Sol. Cells 38 (1995) 349-351.
[2]	H.R. Kerp, H. Donker, R.B.M. Koehorst, T.J. Schaafsma, and E.E. van Faassen, Exciton transport in organic dye layers for photovoltaic applications. Chem. Phys. Lett. 298 (1998) 302-308.

Acknowledgement
This work was supported by the Netherlands Agency for Energy and Environment under contract # 146.100-024.4.

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