Langmuir–Blodgett Films of Porphyrins for Applications in Photovoltaics - Applications of Porphyrinoids - page 125

Fig. 8 Structure of zinc porphyrin-fullerene dyad used in [ 24 ]

For these molecules, octadecylamine was used as the film formation promoter,
and it was demonstrated that LB deposition induces the generation of J aggre-
gates, as measured by means of UV-vis spectroscopy. The relative position of the
hydrophilic groups of the donor-acceptor molecules allowed to transfer the dyads
without any variation in the orientation, and the charge recombination in the
dyads in Fig. 9a, b occurred only in one definite direction, unlike what happened
in the dyads in Fig. 9c, d where the recombination process took place in all
directions.
Managing the direction of charge transfer and charge recombination, the effi-
ciency of the photovoltaic device can be improved, since such an approach makes
possible to reduce the charge-trapping and exciton recombination.

4.1 Role of the Interlayer in the Kinetics of Photoinduced
Electron Transfer in LB Films

In addition to the preparation of porphyrin-fullerene monolayers, LB method was
used to deposit one or more semiconducting organic interlayers. Lemmetyinen and
coworkers reported many examples of such device architecture, with the aim to
obtain long-living charge separated states. The well-known poly(3-hexylthiophene)
(P 3 HT) was used as interlayer in porphyrin-fullerene covalent dyads aligned
according to the strategy reported in the previous paragraph. Lifetimes of
porphyrin-fullerene LB film and porphyrin-fullerene/P 3 HT LB layer were com-
pared by means of transient absorption [ 26 ]. The observations carried out by flash
photolysis measurements demonstrated an electron transfer from the polymer layer
to the porphyrin-fullerene LB film and a final transient state with a positive charge
on P 3 HT and the negative one on the fullerene moiety of the covalently bounded
dyad. The lifetime of the interlayer is about 150

ʼ s, an order of magnitude longer

than that of the porphyrin-fullerene monolayer.
The longer distance between the separated charges reduces the charge recombi-
nation process, and a longer living charge separated state was observed for the triad
if compared with the dyad. The study of the action spectra of the triad showed a
well-pronounced Soret band, although a very low absorption in correspondence of
the P 3 HT absorption was observed [ 27 ]. Such a phenomenon demonstrated that the