Abstract:
Solar cell comprising conjugated polymer with porous metal
oxides that could serve as electron transporter is the theme of several
recent researches because of the desirable properties possessed by the
metal oxide such as its stability, better electron transport properties, ease
of fabrication, low cost and environmental friendly as well as possibilities
for controlling surface morphology. Although the hybrid metal oxide /
polymer solar cells provide several merits, its photovoltaic power
conversion efficiency is still poor compared to polymer / fullerene solar
cells due to interfacial charge recombination. Suppressing recombination
relative to charge transport is therefore a key issue in improving the device
performance. This study focuses on strategies to control charge
recombination kinetics by introducing a series of self-assembled
monolayers (SAMs) of para-substituted benzoic acids with varying dipoles
at the metal oxide-polymer interface in hybrid conjugated polymer /
titanium dioxide (TiO2) photovoltaic devices. The effects of all monolayers
on current densities are in accordance with expected effect of the self assembly monolayers. However, the effect of monolyers on open circuit
voltage is quite unexpected from the interfacial energetics as all the
monolayers improve the open circuit voltage suggest that the monolayer
has an additional function. Overall device performance is enhanced by over
100% using a SAM with permanent dipole pointing towards the TiO2
surface, compared to a control device with no interface modification. This
study concludes that the SAM layer has two functions: (i) to shift the
position of the conduction band of the porous TiO2 relative to the polymer
HOMO level so as to influence interfacial charge separation and (ii) to act
as a barrier, insulating back electron transfer from the TiO2 to the polymer.
Both effects can benefit the performance of hybrid polymer metal oxide
solar cells.
