2 research outputs found
n‑Type Semiconducting Naphthalene Diimide-Perylene Diimide Copolymers: Controlling Crystallinity, Blend Morphology, and Compatibility Toward High-Performance All-Polymer Solar Cells
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-thienoÂ[3,4-<i>b</i>]Âthiophene 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
Polymer/Polymer Blend Solar Cells Using Tetraazabenzodifluoranthene Diimide Conjugated Polymers as Electron Acceptors
Two n-type semiconducting polymers
with alternating arylene (thiophene
or selenophene)–tetraazabenzodifluoranthene diimide (BFI) donor–acceptor
architecture have been investigated as new electron acceptors in polymer/polymer
blend solar cells. The new selenophene-linked polymer, PBFI-S, has
a significantly smaller optical band gap (1.13 eV) than the thiophene-linked
PBFI-T (1.38 eV); however, both polymers have similar HOMO/LUMO energy
levels determined from cyclic voltammetry. Blends of PBFI-T with the
thiazolothiazole–dithienylsilole donor polymer (PSEHTT) gave
a 2.60% power conversion efficiency (PCE) with a 7.34 mA/cm<sup>2</sup> short-circuit current. In contrast, PBFI-S:PSEHTT blends had a 0.75%
PCE with similarly reduced photocurrent and external quantum efficiency.
Reduced free energy for charge transfer and reduced bulk electron
mobility in PBFI-S:PSEHTT blends compared to PBFI-T:PSEHTT blends
as well as significant differences in bulk film morphology are among
the reasons for the large loss in efficiency in PBFI-S:PSEHTT blend
solar cells