IMEC's
associated laboratory IMOMEC, located on the campus of the Hasselt University,
developed a method to stabilize the nanomorphology of organic solar cells resulting
in a lifetime improvement of at least a factor 10. With these stabilized solar
cells, efficiencies were achieved comparable to state-of-the-art organic solar
cells. This breakthrough paves the way to commercial organic solar cells with
an operational lifetime of over 5 years and efficiencies of over 10%.
Transmission Electron Microscopy results for polymer/PCBM 1:1 active layers after degradation at 100åm for 2 h, showing phase segregation for the Rieke P3HT polymer (left), but a stable morphology for the novel conjugated polymer (right)
The efficiency and operation of organic solar cells strongly depends on the
nanomorphology of the active layer, i.e. on a stable mix of organic compounds
that can trap the light's energy and transport it to an electric contact.
IMEC already reported such cells based on P3HT:PCBM with efficiencies near 5%.
But to date, the lifetime of these cells is far too short for commercial applications,
for which 5 years is seen as a minimum. Under long term operation, all solar
cells based on an intimate mixing of organic semiconductors deteriorate. This
is due to segregation of the mixture whereby the compounds tend to separate
into different phases and consequently reduce the efficient conversion of light
into electricity. IMEC has shown before that this phase segregation is related
to the mobility of the organic polymer and that fixation of the nanomorphology
of the polymers could result in a prolonged operational lifetime.
IMEC/IMOMEC has now introduced a new method and new conjugated polymers to
stabilize the nanomorphology of the active layer making it far more robust to
phase segregation under prolonged operation. Experiments on bulk heterojunction
organic solar cells based on this new material showed no degradation of the
efficiency after more than 100 hours whereas reference cells degraded already
after a few hours. This means that a lifetime improvement of at least a factor
10 can be obtained. And the cells achieved efficiencies near 4% which is comparable
to state-of-the-art.
Future research targets further refinement of the method by optimizing the
chemical structures of the conjugated polymers.