Highly Bendable Large-Area Printed Bulk Heterojunction
Film Prepared by the Self-Seeded Growth of Poly(3-hexylthiophene)
Nanofibrils
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Abstract
Applying conventional printing technologies
to fabricate large-area
flexible bulk heterojunction (BHJ) solar cells is of great interest.
Achieving this task requires (i) large tolerance of the maximum photoconversion
efficiency (PCE) to the film thickness, (ii) fast hole transport in
both the thickness and lateral directions of the BHJ layer, and (iii)
improved stability against bending and heat. This paper demonstrates
that a P3HT:PCBM BHJ layer made of long P3HT nanofibrils of almost
100% crystallinity can be an excellent approach to achieve large-area
printed solar cells. We applied a cool-and-heat (C&H) process
with a P3HT/PCBM <i>m</i>-xylene solution to generate P3HT:PCBM
nanofibril composite films. We found that the hole transport of the
nanofibril composite was 2.6 times faster in the thickness direction
and 6.5 times more conductive in the in-plane direction compared with
conventionally annealed composites. The fast hole transport in the
thickness direction led to negligible dependence of the PCE on the
thickness of the composite layer. The improved conductivity in the
in-plane direction prevented the sharp drop of the PCE as the active
area increased. Taking advantage of the unique characteristics, we
employed a roll-printing method to fabricate large-area unit solar
cells in air. In addition, the curved contour path of the nanofibrils
provided excellent stability against large bending strains, allowing
the production of highly bendable organic solar cells