cm6b04828_si_001.pdf (1.48 MB)
Chlorination of Low-Band-Gap Polymers: Toward High-Performance Polymer Solar Cells
journal contribution
posted on 2017-03-08, 14:48 authored by Daize Mo, Huan Wang, Hui Chen, Shiwei Qu, Pengjie Chao, Zhen Yang, Leilei Tian, Yu-An Su, Yu Gao, Bing Yang, Wei Chen, Feng HeHalogenation
is an effective way to tune the energy levels of organic
semiconducting materials. To date, fluorination of organic semiconducting
materials to fabricate polymer solar cells (PSCs) has been used far
more than chlorination; however, fluorine exchange reactions suffer
from low yields and the resulting fluorinated polymer always comes
with a higher price, which will greatly hinder their commercial applications.
Herein, we designed and synthesized a series of chlorinated donor–acceptor
(D-A) type polymers, in which benzo[1,2-b:4,5-b]dithiophene and chlorinated
benzothiadiazole units are connected by thiophene π-bridges
with an asymmetric alkyl chain. These chlorinated polymers showed
deep highest occupied molecular orbital (HOMO) energy levels, which
promoted the efficiency of their corresponding PSCs by increasing
the device open circuit voltage. The asymmetric alkyl chain on the
thiophene moieties gave the final polymer sufficient solubility for
solution processing and strong π–π stacking in
films allowed for high mobility. Although the introduction of a large
Cl atom increased the torsion angle of the polymer backbone, the chlorinated
polymers maintained a favorable backbone orientation in blend films
for efficient PSC application. These factors contributed to respectable
device performances from thick-film devices, which showed PCEs as
high as 9.11% for a 250-nm-thick active layer. These results demonstrate
that chlorination is a promising method to fine-tune the energy levels
of conjugated polymers, and chlorinated benzothiadiazole may be a
versatile building block in materials for efficient solar energy conversion.