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Increasing the Fluorine Substituent of Thieno[3,4‑c]pyrrole-4,6-dione Terthiophene Copolymers Progressively Narrows the Nanofibrils and Enhances the Efficiency of Fullerene-Based Polymer Photovoltaics
journal contribution
posted on 2020-08-14, 18:33 authored by Adane
Desta Fenta, Song-Fu Liao, Syuan-Wei Li, Chun-Fu Lu, Chin-Ti ChenFive thieno[3,4-c]pyrrole-4,6-dione (TPD)-terthiophene
copolymers, PHT, P1F3HT, P1F1HT, P3F1HT, and PFT, having 0, 25, 50, 75,
and 100% of fluorine substituents on the center of terthiophene exhibit
a progressive increase of open-circuit voltage (VOC), short-circuit current density (JSC), and hence a power conversion efficiency (PCE) up
to 9.68% of PC61BM-blended copolymer photovoltaics, which
is the highest PCE reported for TPD-based polymer solar cells with
fullerene derivatives as the electron acceptor. It is evident in atomic
force microscopy and transmission electron microscopy studies that
the width of copolymer nanofibrils formed in solar cells decreases
progressively with the increasing fluorine substituents of the copolymers.
A declining solubility, which is attributed to the fluorophobic effect,
narrows the nanofibril of copolymers with more fluorine substituents.
Grazing incidence wide angle X-ray scattering studies reveal the improved
crystallinity of lamellar stacking and π–π stacking
structures of copolymers with more fluorine substituents. Density
functional theory calculation indicates a virtually coplanar conformation
of terthiophene units with fluorine substituents. The increasing VOC is also associated with the increasing fluorine
substituents, which lower the highest occupied molecular orbital energy
level of the copolymers verified by the electrochemical and photoelectron
spectroscopic measurements.