1 research outputs found
<i>V</i><sub>oc</sub> from a Morphology Point of View: the Influence of Molecular Orientation on the Open Circuit Voltage of Organic Planar Heterojunction Solar Cells
The film morphology and device performance
of planar heterojunction
solar cells based on the molecular donor material α-sexithiophene
(6T) are investigated. Planar heterojunctions of 6T with two different
acceptor molecules, the C<sub>60</sub> fullerene and diindenoperylene
(DIP), have been prepared. The growth temperature of the 6T bottom
layer has been varied between room temperature and 100 °C for
each acceptor. By means of X-ray diffraction and X-ray absorption,
we show that the crystallinity and the molecular orientation of 6T
is influenced by the preparation conditions and that the 6T film templates
the growth of the subsequent acceptor layer. These structural changes
are accompanied by changes in the characteristic parameters of the
corresponding photovoltaic cells. This is most prominently observed
as a shift of the open circuit voltage (<i>V</i><sub>oc</sub>): In the case of 6T/C<sub>60</sub> heterojunctions, <i>V</i><sub>oc</sub> decreases from 0.4 to 0.3 V, approximately, if the
growth temperature of 6T is increased from room temperature to 100
°C. By contrast, <i>V</i><sub>oc</sub> increases from
about 1.2 V to almost 1.4 V in the case of 6T/DIP solar cells under
the same conditions. We attribute these changes upon substrate heating
to increased recombination in the C<sub>60</sub> case while an orientation
dependent intermolecular coupling seems to change the origin of the
photovoltaic gap in the DIP case