The critical role of additives in binary halogen-free
solvent systems for the general processing of highly
efficient organic solar cells†
Kai Yao,
*
a
Yun-Xiang Xu,
b
Xiaofeng Wang,
c
Fan Li
c
and Jiren Yuan
a
With the rapid progress of organic solar cells (OSCs), avoiding processing with halogenated solvents has
become an urgent task for the practical utilization of OSC technology. In this work, a non-halogenated
solvent combination of methylbenzene and methylnaphthalene was developed to replace the use of the
halogenated binary solvents chlorobenzene and 1-chloronaphthlene. We have systematically studied the
effects of the binary solvents on the morphology, charge carrier mobility, and photovoltaic properties.
Importantly, we have found that the halogen-free mixture solvent showed a wider applicability to various
polymer:fullerene systems than that of the halogenated ones and realized a PCE of over 8% in the
PTB7:PC
71
BM based OSC. Our work demonstrated that the use of the additive strategy for halogen-free
solvent systems may provide a feasible route to address the critical environmental issues associated with
large-scale manufacturing.
Introduction
Organic solar cells (OSCs) have the potential to reduce the costs
and environmental impacts associated with other solar cells,
while having the considerable advantages of being lightweight,
exible, and easily processed.
1
As the power conversion effi-
ciency (PCE) of solution processed OSCs based on bulk-
heterojunction (BHJ) structures goes steadily over 10%,
2
the
development of OPV is approaching the requirement for prac-
tical application. The attention gained by organic photovoltaics
will then be focused on fullling the commercial platforms
necessary for deployment on a large-scale through roll-to-roll
printing techniques.
3
As is well known, conjugated polymers
and fullerene derivatives have limited solubility in common
solvents, and so halogenated solvents with a strong salvation
effect,
4
such as chlorobenzene (CB), 1,2-dichlorobenzene (o-
DCB), diiodooctane (DIO), and 1-chloronaphthalene (1-CN), are
used in solution processing. From the perspective of
manufacturing, these halogenated solvents are generally not
acceptable due to safety concerns and the need for
environmentally-friendly processing conditions.
5
Therefore, the
search for non-halogenated solvents is urgently required in this
transition toward future large-scale production.
A BHJ layer consists of a phase-separated electron-donor
(typically a conjugated polymer) and an electron-acceptor
(typically a fullerene derivative) with ideal interpenetrating
domains (10 nm in length), and the morphology of the BHJ
layer has a huge inuence on the charge generation, transfer,
and transport.
6
Such an ideal morphology can be difficult to
obtain via simple solution processing because the thermo-
dynamically metastable state is extremely sensitive to the
solvent system used for processing.
7
Thus, lots of physical
properties of the solvents, including the boiling point (BP),
toxicity, viscosity and solubility for the blended compounds, etc.
need to be considered for processing.
8
Hence, alternative non-
halogenated solvents should have comparable physical prop-
erties to those of halogenated solvents, and a decent solubility
for conjugated polymers and fullerene derivatives is essential to
achieve high-performance OSCs.
9
Compared to halogenated solvents, non-halogenated ones,
such as toluene
10
and isomers of xylene,
11
usually lead to less-
than-ideal morphologies in the BHJ layers due to their infe-
rior physical properties for the drying process and solubility for
conjugated polymers and fullerene derivatives. There are
several reports that use binary solvent mixtures of differing
compositions to provide one facile route to modulate the
solvent quality. Even in halogenated solvents, the CB (DCB) host
solvent has been widely used in many state-of-the-art photo-
voltaic OSC systems with 1-CN
12
or DIO
13,14
additives. In an
attempt to rationally select candidate solvent mixtures, Hansen
solubility parameters (HSP) have been utilized to describe the
total cohesion energy, which is directly related to the solubility
a
Institute of Photovoltaics, Nanchang University, 999 Xuefu Avenue, Nanchang
330031, China. E-mail: yaokai@ncu.edu.cn
b
College of Polymer Science & Engineering, State Key Laboratory of Polymer Materials
Engineering, Sichuan University, Chengdu 610065, China
c
Department of Materials Science and Engineering, Nanchang University, Nanchang
330031, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra19850j
Cite this: RSC Adv.,2015,5, 93689
Received 25th September 2015
Accepted 20th October 2015
DOI: 10.1039/c5ra19850j
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This journal is © The Royal Society of Chemistry 2015 RSC Adv.,2015,5,93689–93696 | 93689
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