Enhancing Optoelectronic Properties of Conjugated Block Copolymers through Crystallization of Both Blocks
2020
Microphase-separated
block copolymers composed of electron donor and acceptor blocks may
provide morphology control to address many challenges in organic electronics.
Crucial to controlling the self-assembly of fully conjugated block
copolymers is tuning the interplay between crystallization of the
individual blocks and microphase separation between the donor and
the acceptor. Thus, we have examined the kinetics of the morphological
evolution in P3HT-b-PFTBT block copolymer films during
two processes: solution casting and thermal annealing. We use in situ
wide-angle and small-angle grazing incidence X-ray scattering to monitor
the crystallization of P3HT and microphase separation between the
two blocks. We find that during film drying, initial P3HT crystallization
happens quickly, before phase separation of the two blocks. However,
crystallization is significantly suppressed with respect to neat materials,
enabling microphase separation to proceed at time scales after some
initial crystallization of the donor block takes place. This enables
a mesoscale structure to develop during processes such as thermal
annealing because self-assembly of the lamellar structure takes place
before the crystallization of the donor block is complete. We also
find that significant crystallization of PFTBT blocks after P3HT crystallization
is possible at elevated temperatures. Crystallization of both blocks
is important to maximize the performance of solar cells and transistors
with block copolymer active layers. As a consequence, we exceed 3%
average power conversion efficiencies in P3HT-b-PFTBT
photovoltaic devices.
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