Highly efficient phenothiazine 5,5-dioxide-based hole transport materials for planar perovskite solar cells with a PCE exceeding 20%

Article abstract of DOI:10.1039/c9ta00654k

Two novel phenothiazine 5,5-dioxide (PDO) core building block-based hole transport materials (HTMs), termed PDO1 and PDO2, were designed and synthesized. The introduction of a sulfuryl group in a core unit can deeply influence the energy levels and charge carrier mobilities of relative HTMs. The combined suitable energy level alignment, higher hole mobility and conductivity, as well as highly efficient hole transfer of PDO2 enable the perovskite solar cell (PSC) to achieve an impressive power conversion efficiency (PCE) of 20.2% and good stability when aged under ambient conditions. These results demonstrate the potential versatility of the PDO building block for further development of cost-effective and highly efficient HTMs for PSCs.

Full text of DOI:10.1039/c9ta00654k

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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DOI: 10.1039/C9TA00654K
Highly Efficient Phenothiazine 5, 5-Dioxide-Based Hole Transport
Materials for Planar Perovskite Solar Cells with PCE Exceeding 20%
a
Xingdong Ding, Cheng Chen,a* Linghao Sun,a Hongping Li,a Hong Chen,c Jie Su,d
Huaming Li,a* Henan Li,b Li Xu,a Ming Cheng,a*
a
Institute for Energy Research, Key Laboratory of Zhenjiang, Jiangsu University,
Zhenjiang 212013, PR China
* E-mail: chencheng@ujs.edu.cn, lihm@ujs.edu.cn, mingcheng@ujs.edu.cn.
b
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang
212013, P. R. China
c
School of Environmental Science and Engineering, Southern University of Science
and Technology, Shenzhen 518055, P. R. China
d College of Chemistry and Molecular Engineering, Peking University, Beijing100871,
P. R. China
Keywords: planar structure, perovskite solar cell, no hysteresis, hole transport material,
phenothiazine 5, 5-dioxide,
Abstract
Two novel phenothiazine 5,5-dioxide (PDO) core building block-based hole transport
materials (HTMs), termed PDO1 and PDO2, were designed and synthesized. The
introduction of sulfuryl group on core unit can deeply influence the energy levels and
charge carrier mobilities of relative HTMs. The combined suitable energy level
alignment, higher hole mobility and conductivity, as well as highly-efficient hole
transfer for PDO2 make the perovskite solar cell (PSC) reach an impressive power
conversion efficiency (PCE) of 20.2% and good stability when aged in ambient
condition. These results demonstrate the potential versatility of the PDO building block
for further development of cost-effective and highly-efficient HTMs for PSCs.

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In the past few years, the power conversion efficiencies (PCEs) of PSCs have been
rapidly boosted from the initial 3.8% in 2009 to over 23%.1, 2 Such a large increase in
PCEs is inseparable from the excellent hole transport property and the effective
suppression of charge recombination of hole transport material (HTM) in PSCs. At
present, the most commonly used HTMs are 2,2',7,7'-Tetrakis-(N,N-di-4-methoxy-
phenylamino)-9,9'-spiro bifluorene (Sprio-OMeTAD) and poly (triarylamine) (PTAA).
3-5
However, because of the complicated synthetic routes of the Sprio-OMeTAD and
PTAA, their synthetic costs are prohibitively high, limiting their large-scale application
in future commercialization. 6, 7 Therefore, it is highly appreciated to develop or search
for low-cost and highly efficient alternatives to Spiro-OMeTAD and PTAA, aiming at
8-29
reaching state-of-the-art photovoltaic performance.
Phenothiazine 5,5-dioxide
(PDO) is a heterocyclic conjugated unit, which can be easily obtained by one step
oxidation from phenothiazine (PTZ) (6$/kg). The 3-, 7- and N- position of PDO
monomer can be easily tailored, providing easy access to a wide library of HTMs.
Oxidizing PTZ to PDO would also have significant impacts on molecular conformation,
which will in turn affect HTM stacking behavior in film state and charge carrier
transport properties. Furthermore, the conversion of electron-donating sulfur atom to
electron-withdrawing sulfone group would change the charge affinity of the core unit.
To the best of our knowledge, the HTM incorporating PDO core unit has not been
reported. The possible impacts of sulfone group on charge carrier transport property
and photovoltaic performance are very intriguing to us and thus motivated our studies.
O
O
O
N
O
N
O O
S
N
N
O O
S
O
N
O
O
N
O
N
O
O
O
PDO1
PDO2
Figure 1 Molecular structures of HTMs PDO1 and PDO2

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Based on these concerns, herein, we report two facilely synthesized PDO core building
block based HTMs (see Figure 1). PDO has a symmetrical nonplanar structure. By
substituting its 3- and 7-positions with 4,4'-dimethoxydiphenylamine, and substituting
the N-position with anisole or 4,4'-dimethoxytriphenylamine as the peripheral groups
afforded HTMs PDO1 and PDO2, respectively. With the conversion of electron-
donating sulfur atom to electron-withdrawing sulfone group (from PTZ to PDO), the
charge affinity of core unit is increased, making the designed HTMs PDO1 and PDO2
possess a donor-acceptor-donor (D-A-D) structure. According to molecular orbital
hybridization theory, the formation of D–A–D structure can rationally decrease the
highest occupied molecular orbital (HOMO) energy level and improve the intrinsic
30, 31
charge carrier transport properties.
The introduction of two 4,4'-
dimethoxydiphenylamine groups on PDO unit are supposed to further adjust the energy
level and charge carrier mobility, while the N-substitutions are to tune the spatial
configuration of HTMs. The specific impact of sulfone group and different N-
substituent groups on the material properties and photovoltaic performance were
systematically studied. These two PDO-based HTMs show high hole mobility, high
conductivity and suitable HOMO and lowest unoccupied molecular orbital (LUMO)
levels. Through optimization, the PSCs employing PDO2 as HTM show PCEs up to
20.2%, and good stability when aged in ambient condition in the dark for 500 h.
PDO1 and PDO2 were facilely synthesized through four steps, involving Buchwald-
Hartwig reaction, radical substitution, and oxidation reaction. The detailed synthetic
routes for PDO1 and PDO2 are depicted in Supporting Information. The synthetic costs
of PDO1 and PDO2 are calculated to be 62.1791$/g and 61.2761$/g, respectively,
6, 7
which are much lower than that of Spiro-OMeTAD.
The synthesized HTMs were
fully characterized by nuclear magnetic resonance (NMR), high-resolution mass
spectrum (HRMS) and single crystal X-ray diffraction technologies.
From the crystal structures of PDO1 and PDO2, significant geometry difference can be
observed on the configuration of PDO core unit in PDO1 and PDO2. The PDO unit in
PDO1 is a twisted butterfly configuration, while it becomes more planar in PDO2 (see
Figure 2a and Figure S7). PDO1 crystallizes in the monoclinic space group of C2/c with

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four crystallographic independent molecules in the unit cell. The PDO1 molecules form
alternately zig-zag chain along the crystallographic c axis through π-π stacking
interaction (see Figure 2b). In contrast, PDO2 crystallizes in the monoclinic space
group I2/a. As shown in Figure 2d, different than PDO1, because of the more planar
configuration of PDO core unit in PDO2, the two adjacent molecules are tightly packed
together related with a mirror operation or inversion center because of the strong steric-
hinerance effect of N-substitution. The tight packing behavior is beneficial for
intermolecular hole hopping. For both PDO1 and PDO2, each molecule has multiple
short contacts distance with adjacent molecules from the same and surrounding unit
cells including CH/π hydrogen bonds and oxygen-carbon contacts, providing path for
hole hopping.
Figure 2 a) Crystal structure of HTM PDO1, b) Molecular stacking of PDO1 view along
the b axis, c) Crystal structure of HTM PDO2 and d) Molecular stacking of PDO2 along
the a axis.

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Figure 3 a) UV-vis absorption spectra of PDO1 and PDO2, b) CV of HTMs PDO1 and
PDO2 in chloroform solution, c) Schematic energy level diagram of PSC, d) Cross-
section SEM of PSC
To evaluate whether these two new HTMs possess appropriate energy levels for
application in PSCs, UV-vis absorption and cyclic voltammetry (CV) were measured
in chloroform solution. The test results are shown in Figure 3a and Figure 3b. The
energy-levels of the main device components used in this study are illustrated in Figure
3c. The HOMO energy levels of PDO1 and PDO2 are calculated to be -5.25 and -5.24
eV vs vacuum, respectively, which are much more positive than the valence band (VB)
of the mixed perovskite used here, suggesting hole extraction is favorable from energy
level point. 32, 33 , Obviously, the almost invariable HOMO energy levels of PDO1 and
PDO2 indicate the N-substitution has negligible influence on HOMO levels. Compared
with HTM PTZ1 (-4.77 eV) with PTZ as core building block reported by Grisorio, 10
the oxidation of sulfur atom to sulfuryl in our case renders more negative HOMO levels,
which is particularly appropriate for application in PSCs. By combining the CV and the
UV-vis absorption test data, the LUMO energy levels of PDO1 and PDO2 were roughly

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estimated to be -2.67 and -2.35 eV vs vacuum, respectively, which are high enough for
electron blocking. 32, 33
Density functional theory (DFT) calculations were performed by using Gaussian
program at the B3LYP/6-31G (+d) level to gain insight of the charge distribution of
PDO1 and PDO2. The calculated results are shown in Figure 4. The optimized
molecular structures are well consistent with crystal structures. From the results we can
find that the N-substitution has nearly no effects on charge distribution, which agrees
well with that from CV measurements. For PDO1 and PDO2, the HOMO delocalizes
on the whole molecular backbone, while the LUMO levels shift to the PDO core unit.
The overlaps between HOMO and LUMO suggest the existence of strong Coulomb
interaction, which is beneficial for hole extraction and transport. 10 The calculated hole
reorganization energies of PDO1 and PDO2 are172 and 126 meV, respectively. The
smaller reorganization energy suggests that PDO2 may have a higher hole mobility than
PDO1. The hole mobilities of HTM PDO1 and PDO2 were calculated to be 1.76×10-4
and 5.93×10-4cm2·V-1·s-1 (see Figure S8a), respectively, By using space-charge-limited
current (SCLC) model.12 The result is exactly consistent with the DFT calculation
results.
Figure 4 DFT calculation results of PDO1 and PDO2

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Figure 5 J-V characteristic curves of the PSCs containing a) PDO1, b) PDO2
and c) Spiro-OMeTAD as HTMs, d) IPCE spectra of PSCs containing PDO1,
PDO2 and Spiro-OMeTAD as HTMs, e) steady-state power outputs at
maximum point, f) statistics of PCE for PSCs containing PDO1 and PDO2 as HTMs
Table1. The best photovoltaic performances of PSCs employing PTDO1, PTDO2 and
Spiro-OMeTAD as HTMs

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Scan
Jsc /
PCE hysteresis
HTM
Voc / V
FF / %
direction
mA·cm-2
/ %
index / %
From OC
to SC
1.08
1.07
1.15
1.15
1.14
1.14
22.6
22.7
23.9
23.9
24.2
24.1
68.3
67.7
73.6
72.1
71.1
69.0
16.7
16.4
20.2
19.8
19.8
19.3
PDO1
0.025
0.016
0.020
From SC
to OC
From OC
to SC
PDO2
From SC
to OC
From OC
to SC
Spiro-
OMeTAD
From SC
to OC
Note: scan forward: from open circuit (OC) to short circuit (SC); scan
backward: from short circuit (SC) to open circuit (OC).
We proceeded to test the synthesized HTMs in state-of-the-art planar PSCs with
a detailed device structure of FTO/c-TiO2/SnO2/Perovskite/HTM/Au (see
Figure 4d), and mixed halide perovskite (FAPbI3)0.85(MAPbBr3)0.15 was
employed as light-harvesting material. Figure 5a-5c display the current density-
voltage (J-V) characteristic curves of the PSCs containing PDO1, PDO2 and
Spiro-OMeTAD as HTMs. The devices were measured under a simulated AM
1.5 G (100 Mw.cm-2) sunlight with a slow scan rate of 10 mV·s-1. The detailed
parameters are showed in Table1. The PDO1- and PDO2-based devices show
the best-performing PCE of 16.7% and 20.2%, respectively, especially the
PDO2 even achieved higher champion PCE than reference HTM Spiro-
OMeTAD (19.8%). From the IPCE spectra (Figure 5d), the estimated integrated

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−2
current densities are 22.0, 23.4 and 23.5 mA.cm for PDO1, PDO2 and Spiro-
OMeTAD, respectively, which are agree well with the Jsc values acquired from
the J-V curves (error values lower than 5%). The stabilized power outputs at
maximum point for PDO1- and PDO2-based are 16.4% and 19.5% (see Figure
5f), respectively, consisting with the J-V measurements. The hysteresis index
(HI) were calculated to be 0.025, 0.016 and 0.020 for PDO1-, PDO2- and Spiro-
OMeTAD-based PSCs, respectively. The negligible hysteresis behavior
indicates highly efficient charge carriers transport at the interfaces with the electron
and hole-selective contacts. Compared with PDO1-based devices, the PSCs based
on PDO2 exhibited higher open-circuit voltages (Voc), short-circuit current
densities (Jsc), and fill factors (FF). As is well known, the Jsc is closely related
to light-harvesting efficiency, charge separation efficiency and charge
collection efficiency. Considering the same fabrication process and components
except HTM, the higher Jsc for PDO2-based PSCs should mainly result from
higher hole extraction efficiency, which is confirmed by PL decay results. As
shown in Figure S9, the steady-state and time-resolved PL decay measurements
indicate the perovskite/PDO2 system showed much higher hole extraction
efficiency and hole extraction rate than PDO1-based one, indicating hole transport
in PDO2 based PSCs is more efficient. This is consistent with the higher PCE for PDO2
based device. The similar PL decay behavior for PDO2- and Spiro-OMeTAD-based
system suggests the hole transport properties of PDO2 and Spiro-OMeTAD are
comparable with each other. The higher IPCE values in entire response region for
PDO2-based PSCs are also agreed perfectly with the J-V and steady-state PL
decay measurements discussed above. The improved FF can mainly be ascribed
to the higher conductivity of PDO2-based hole transport layer (HTL) (see
Figure S8b). To gain further explanations of higher Voc for PDO2-based PSCs,
the light intensity dependence of Jsc and Voc were performed, as shown in Figure
6. It is worth noticing that PDO2-based PSCs exhibit stronger law dependence
value (α = 0.905) of the Jsc on light intensities, while weaker dependence of Voc
on light intensities (1.23kT/q). The results indicate the improved Voc mainly

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arises from highly restricted Shockley−Read−Hall (SRH) recombination during
working process. 34 As shown in Figure 5f, PDO1- and PDO2-based PSCs also
show excellent reproducibility with average PCEs of 15.3±0.79 and 19.3±0.62,
respectively, and the majority of devices obtained above-average PCEs. To
better demonstrate the important effects of sulfuryl group, PTZ1 based PSCs
were also evaluated for comparation. Expectedly, PDO1-based PSCs exhibit
better photovoltaic performance (see Figure S10). The test results clearly show
that except the peripheral groups adjustments, the oxidation sulfur atom to
sulfone is also an effective molecular engineering mean to enhance the charge
carrier mobility and their photovoltaic performance.
Figure 6 a) The dependence of Jsc on different light intensities, b) the dependence of
Voc on different light intensities

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Figure 7 Aging test results a) PCE, b) Jsc, c) Voc and d) FF of PSCs containing
PDO1, PDO2 and Spiro-OMeTAD as HTMs
Figure 7a-7d shows the stability test results of the corresponding PSCs in air
condition (relative humidity of 30-45%, room temperature) without
encapsulation. The PDO1-, PDO2- and Spiro-OMeTAD based PSCs
maintained 84.6%, 84.7% and 80.1% of the initial PCEs after 480 hours aging.
The thermogravimetric analysis results (see Figure S11) show that PDO1 and
PDO2 begin to decompose at 420 oC. Considering the good thermal stability of
PDO1 and PDO2, the PCE decay might mainly cause by the dopants in HTL
and the degradation of perovskite film. 35, 36 Therefore, our further research will
focus on developing PDO core unit-based dopant-free HTMs.
In conclusion, we demonstrated that the oxidation of sulfur atom to sulfuryl and
molecular engineering on N-substitution dramatically influenced the molecular
configuration and photovoltaic performance of HTMs PDO1 and PDO2. Through the
introduction of sulfuryl group on core building block, the electron density distribution,
as well as HOMO levels, can be effectively adjusted. Importantly, PDO1 and PDO2
with the introduction of sulfuryl, possess particularly appropriate energy level
alignment, excellent charge carrier mobility and the dramatically improved
photovoltaic performance, in comparison with previously reported PTZ1. A PCE as
high as 20.2% was achieved by PDO2-based PSC, which can be mainly attributed to
the more planar geometry of PDO core unit and much stronger molecular stacking
behavior. PDO2 is among the very few HTMs that can rival with Spiro-OMeTAD. This
work reveals a great potential of the PDO building block for further development of

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low-cost and highly-efficient HTMs for PSCs.
Acknowledgements
This work was financially supported by the Natural Science Foundation of
Jiangsu province (BK20180867, BK20180869), National natural science
foundation of China (Grants 21805114), the Jiangsu University Foundation
(17JDG032, 17JDG031), High-tech Research Key laboratory of Zhenjiang
(SS2018002), the high-performance computing platform of Jiangsu University,
the Priority Academic Program Development of Jiangsu Higher Education
Institutions
Supporting Information
The experimental details, characterization of HTMs, hole mobility and
conductivity of HTMs, PL decay and thermogravimetric analysis results,
supplied as Supporting Information.
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Journal of Materials Chemistry A
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DOI: 10.1039/C9TA00654K
16730.

Journal of Materials Chemistry A
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View Article Online
DOI: 10.1039/C9TA00654K
Two novel highly efficient and low-cost phenothiazine 5, 5-dioxide core building block
based hole transport materials are reported, achieving power conversion efficiency as
high as 20.2%.