n‑Type
Semiconducting Naphthalene Diimide-Perylene Diimide Copolymers: Controlling
Crystallinity, Blend Morphology, and Compatibility Toward High-Performance
All-Polymer Solar Cells
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Abstract
Knowledge
of the critical factors that determine compatibility, blend morphology,
and performance of bulk heterojunction (BHJ) solar cells composed
of an electron-accepting polymer and an electron-donating polymer
remains limited. To test the idea that bulk crystallinity is such
a critical factor, we have designed a series of new semiconducting
naphthalene diimide (NDI)-selenophene/perylene diimide (PDI)-selenophene
random copolymers, <i>x</i>PDI (10PDI, 30PDI, 50PDI), whose
crystallinity varies with composition, and investigated them as electron
acceptors in BHJ solar cells. Pairing of the reference crystalline
(crystalline domain size <i>L</i><sub>c</sub> = 10.22 nm)
NDI-selenophene copolymer (PNDIS-HD) with crystalline (<i>L</i><sub>c</sub> = 9.15 nm) benzodithiophene-thieno[3,4-<i>b</i>]thiophene copolymer (PBDTTT-CT) donor yields incompatible blends,
whose BHJ solar cells have a power conversion efficiency (PCE) of
1.4%. However, pairing of the new 30PDI with optimal crystallinity
(<i>L</i><sub>c</sub> = 5.11 nm) as acceptor with the same
PBDTTT-CT donor yields compatible blends and all-polymer solar cells
with enhanced performance (PCE = 6.3%, <i>J</i><sub>sc</sub> = 18.6 mA/cm<sup>2</sup>, external quantum efficiency = 91%). These
photovoltaic parameters observed in 30PDI:PBDTTT-CT devices are the
best so far for all-polymer solar cells, while the short-circuit current
(<i>J</i><sub>sc</sub>) and external quantum efficiency
are even higher than reported values for [70]-fullerene:PBDTTT-CT
solar cells. The morphology and bulk carrier mobilities of the polymer/polymer
blends varied substantially with crystallinity of the acceptor polymer
component and thus with the NDI/PDI copolymer composition. These results
demonstrate that the crystallinity of a polymer component and thus
compatibility, blend morphology, and efficiency of polymer/polymer
blend solar cells can be controlled by molecular design