Dopant-Free Hole-Transport Materials Based on
Methoxytriphenylamine-Substituted
Indacenodithienothiophene for Solution-Processed
Perovskite Solar Cells
Xiaoyuan Liu
+
,
[a, b]
Xiaolu Zheng
+
,
[c]
Yulong Wang,
[b]
Zhiliang Chen,
[c]
Fang Yao,
[c]
Qi Zhang,
[c]
Guojia Fang,*
[c]
Zhi-Kuan Chen,*
[a]
Wei Huang,*
[a]
and Zong-Xiang Xu*
[b]
Introduction
Hybrid lead-halide perovskite solar cells (PSCs) that can be pro-
cessed in solution have attracted considerable attention for
next-generation photovoltaic technology. PSCs offer numerous
advantages, including facile processing, very high power con-
version efficiency (PCE), and relatively low fabrication costs.
[1–4]
Recently, the PCE of PSCs has increased significantly with
numbers as high as 22.1 % being reported.
[5]
Further increases
in the performance of PSCs require improvements in hole-
transport materials (HTMs) to facilitate the collection of
transport-free carriers to the corresponding electrode. 2,2’,7,7’-
Tetrakis(N,N’-di-p-methoxyphenylamino)-9,9’-spirobifluorene
(spiro-MeOTAD) is a widely used HTM,
[6]
and although devices
that contain spiro-MeOTAD have achieved high PCEs they
must be doped with lithium bis(trifluoromethanesulfonyl)imide
(LiTFSI)
[7]
or cobalt complexes
[8]
to improve hole mobility and
conductivity. This doping leads to decreased long-term device
stability.
[9]
A number of organic HTMs have been developed re-
cently, which have yielded devices with excellent PCEs.
[9–13]
Un-
fortunately, most of these HTMs require complicated synthetic
routes, which is not favorable to industrial and large-scale
production. Therefore, dopant-free HTMs that are simple to
synthesis are highly desirable for stable and efficient PSCs.
Here, two novel dopant-free HTMs that contain an extended
fused-ring core [indacenodithiophene (IDT) or indacenodithie-
nothiophene (IDTT)] with methoxytriphenylamine (TPA) moiet-
ies substituted on either side were synthesized and incorporat-
ed into PSCs.
A material’s molecular structure and electronic properties
can significantly affect its aggregation and charge-transport
abilities. TPA has an ionization potential of 6.80 eV and is
a strong electron donor, which benefits hole injection and hole
transport. Furthermore, TPA is a nonplanar molecule, which
helps retain intermolecular aggregations, forms uniform and
smooth thin films with a stable morphology, and imparts iso-
tropic and homogeneous features, all of which are essential for
materials used in electronic devices.
[14]
Although many HTMs based on TPA have been reported,
their poor mobilities and low conductivities need to be ad-
dressed.
[15,16]
IDT and IDTT units are commonly used to con-
Solution-processed hole transporting materials (HTMs) that are
dopant-free show promise for use in low-cost, high-per-
formance perovskite solar cells (PSCs). The highest-efficiency
PSCs use organic HTMs, many of which have low mobilities
and therefore require doping, which lowers the device stability.
Additionally, these materials are not easily scaled because they
often requir e complicated synthesis. Two new HTMs (IDT–TPA
and IDTT–TPA) were synthesized, which contained either an
extended fused-ring indacenodithiophene (IDT) or indacenodi-
thienothiophene (IDTT) core and strong electron-donating
methoxytriphenylamine (TPA) groups as the end-capping units.
The extended conjugation in the backbone of IDTT–TPA re-
sulted in stronger p–p interactions (3.321 ) and a higher hole
mobility of 6.46 10
4
cm
2
V
1
s
1
when compared with that of
IDT–TPA (9.5310
5
cm
2
V
1
s
1
). A dopant-free, planar PSC
that contained IDTT–TPA was fabricated and exhibited a high
power conversion efficiency (PCE) of 15.7 %. This cell exhibited
a higher PCE and less hysteresis than devices that contained
IDT–TPA.
[a] X. Liu,
+
Prof. Z.-K. Chen, Prof. W. Huang
Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Mate-
rials (IAM)
National Jiangsu Synergetic Innovation Center for Advanced Materials
Nanjing Tech University
30 South Puzhu Road, Nanjing 211816 (P. R. China)
E-mail: iamzkchen@njtech.edu.cn
iamwhuang@njtech.edu.cn
[b] X. Liu,
+
Y. Wang, Dr. Z.-X. Xu
Department of Chemistry
South University of Science and Technology of China
Shenzhen (P. R. China)
E-mail: xu.zx@sustc.edu.cn
[c] X. Zheng,
+
Z. Chen , F. Yao, Q. Zhang, Prof. G. Fang
Shenzhen Institute of Wuha n University
Shen Zhen, Guangdong 518000 (P. R. China)
E-mail: gjfang@whu.edu.cn
[
] These authors contributed equally to this work.
Supporting Information, including experimental details, and the ORCID
identification number(s) for the author(s) of this article can be found
under https://doi.org/10.1002/cssc.201700197.
ChemSusChem 2017, 10,1–7 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim1
&
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Full Papers
DOI: 10.1002/cssc.201700197