Polymer/Polymer Blend Solar Cells Using Tetraazabenzodifluoranthene
Diimide Conjugated Polymers as Electron Acceptors
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Abstract
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