Molecular engineering of phenothiazine-based monomer and dimer hole transport materials and their photovoltaic performance

Article abstract of DOI:10.1016/j.dyepig.2021.109340

Through molecular engineering, phenothiazine-based monomer hole transport material (HTM) PTZT and dimer HTM D-PTZT were designed and synthesized. The photoelectrochemical properties are significantly influence by the adjustment of the molecular structure. Applied in perovskite solar cells (PSCs), monomer HTM PTZT-based device achieves the highest efficiency of 18.74% while only 15.45% for dimer HTM D-PTZT-based device under 100 mW cm^?2 AM 1.5G solar illumination.

Full text of DOI:10.1016/j.dyepig.2021.109340

Dyes and Pigments 191 (2021) 109340
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Short communication
Molecular engineering of phenothiazine-based monomer and dimer hole
transport materials and their photovoltaic performance
,
,
,
Mengde Zhaia, Yawei Miaoa b, Cheng Chena **, Haoxin Wanga, Xingdong Dinga b, Cheng Wua,
Xichuan Yangc, Ming Chenga
,
*
a Institute for Energy Research, Jiangsu University, Zhenjiang 212013, China
b School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China
c State Key Laboratory of Fine Chemicals, Dalian University of Technology (DUT), Dalian 116024, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Through molecular engineering, phenothiazine-based monomer hole transport material (HTM) PTZT and dimer
HTM D-PTZT were designed and synthesized. The photoelectrochemical properties are significantly influence by
the adjustment of the molecular structure. Applied in perovskite solar cells (PSCs), monomer HTM PTZT-based
device achieves the highest efficiency of 18.74% while only 15.45% for dimer HTM D-PTZT-based device under
100 mW cmꢀ 2 AM 1.5G solar illumination.
Perovskite solar cell
Hole transport material
Dimer
Molecular engineering
1. Introduction
2. Results and discussion
Perovskite solar cells (PSCs) have achieved an impressive power
conversion efficiency (PCE) of 25.5% due to their excellent optoelec-
tronic properties, and this remarkable PCE breakthrough makes them
comparable to silicon solar cells [1–3]. Hole transport materials (HTMs),
which are responsible for the hole extraction and transport, play a
crucial role in acquiring highly efficient PSCs [4]. Apart from the typical
HTMs such as 2,2′,7,7′-tetrakis (N,N-bis(p-methoxyphenyl)amino)-9,
9′-spirobifluorene (Spiro-OMeTAD) and poly[bis(4-phenyl) (2,4,6-tri-
methylphenyl)amine (PTAA), various small molecular organic HTMs
have been widely developed, due to their variable molecular structures,
low synthesis cost and good batch-to-batch reproducibility [5–15].
Thereinto, phenothiazine (PTZ), which is an electron-rich heterocycle
with a butterfly-like configuration, is widely employed as a core build-
ing block to construct HTMs, achieving excellent performance [16,17].
The reported PTZ-based HTMs are almost monomers, and the previous
research work mainly focused on the optimization of peripheral sub-
stituents on the PTZ ring to tune the highest occupied molecular orbital
(HOMO) energy levels. Moreover, the molecular geometries of HTMs
have also proved to deeply influence their physicochemical and photo-
electronic properties [18]. Nevertheless the in-depth impact of mono-
mer and dimer configurations of PTZ-based HTMs on their properties
and the photovoltaic performance have never been investigated.
In this regard, we designed and synthesized a PTZ dimer 4,4′,4′′,4′′′-
((9,9-dimethyl-9H-fluorene-2,7-diyl)bis(10H-phenothiazine-10,3,7-
triyl))tetrakis(N,N-bis(4-methoxyphenyl) aniline) and a PTZ monomer
4,4’-(10-(p-tolyl)-10H-phenothiazine-3,7-diyl)bis(N,N-bis(4-methox-
yphenyl)aniline), termed D-PTZT and PTZT (see Scheme 1a), respec-
tively, allowing us to study the effects of dimer and monomer
configurations on their properties and the photovoltaic performance.
The new HTMs were easily synthesized in 3-steps and the detailed
synthetic routes were depicted in Scheme 1b and supporting information
(SI). Both of the HTMs D-PTZT and PTZT were characterized by 1H NMR
and high-resolution mass spectrometry (HRMS) to verify their chemical
structures. According to previous reported calculation model [19], the
synthetic cost of HTMs D-PTZT and PTZT were evaluated to be 92.5 $/g
and 48.3 $/g, respectively, which are much lower than that of
Spiro-OMeTAD (200 $/g), most notably PTZT, as shown in SI.
Cyclic voltammetry (CV) and UV–vis absorption spectroscopy were
used to evaluate the energy of D-PTZT and PTZT. The results presented
in Fig. 1a and b, and the corresponding optical and electrochemical data
were summarized in Table 1. The detailed calculation method of energy
levels was described in the SI. The HOMO energy levels of D-PTZT and
PTZT were estimated to be ꢀ 5.16 and ꢀ 5.17 eV vs. vacuum, respec-
tively, which were more negative than the valence band (VB) of
** Corresponding author.
* Corresponding author.
E-mail addresses: chencheng@ujs.edu.cn (C. Chen), mingcheng@ujs.edu.cn (M. Cheng).
https://doi.org/10.1016/j.dyepig.2021.109340
Received 27 February 2021; Received in revised form 22 March 2021; Accepted 22 March 2021
Available online 31 March 2021
0143-7208/© 2021 Elsevier Ltd. All rights reserved.

M. Zhai et al.
Dyes and Pigments 191 (2021) 109340
Scheme 1. (a) The chemical structure of HTMs D-PTZT and PTZT. (b) The detailed synthetic routes of HTMs PTZT and D-PTZT.
perovskite (ꢀ 5.45 eV), facilitating the hole extraction and transport at
the perovskite/HTM interface. Combining with the UV–vis absorption
and photoluminescence test results, the lowest unoccupied molecular
orbital (LUMO) energy levels of D-PTZT and PTZT were estimated to be
ꢀ 2.37 and ꢀ 2.35 eV vs. vacuum, respectively, which is more positive
than the conduction band (CB) of perovskite (ꢀ 3.90 eV), effectively
blocking the backward electron transfer from perovskite. It is found that
there were no significant changes occurring in the HOMO and LUMO
energy levels from PTZ monomer to dimer, considering the test error
(see Fig. 3a).
the PTZ core unit. The substantial overlap between HOMO and LUMO of
PTZT testified the existence of a Coulomb interaction, which is favorable
for the exciton formation and hole migration. Totally different from
PTZT, the HOMO of D-PTZT is mainly distributed on either of the two
PTZ units, the LUMO transfers to the fluorene core unit. The extreme
variation in the charge delocalization between HOMO and LUMO of D-
PTZT implies the poor hole extraction. As expected, the hole mobility
and conductivity of the monomer HTM PTZT are measured to be 2.62 ×
10ꢀ 4 cm2 Vꢀ 1 sꢀ 1 and 3.00 × 10ꢀ 4 S cmꢀ 1, which are much higher than
those of the dimer HTM D-PTZT (1.62 × 10ꢀ 4 cm2 Vꢀ 1 sꢀ 1 and 1.52 ×
10ꢀ 4 S cmꢀ 1), as shown in Fig. 1c and d and Table 1. Furthermore, it is
detected that the S and O atoms on the PTZ unit have strong negative
electrostatic potential from the electrostatic potential maps, indicating
the HTMs may interact with uncoordinated Pb2+ thereby further
passivating surface defects due to the Lewis base nature of S and O [20].
Considering the suitable energy levels, the excellent hole mobility
and conductivity, and good thermal stability of D-PTZT and PTZT, they
were further employed as HTMs to fabricate PSCs with the device
structure of FTO/compact- TiO2/mesoporous-TiO2/perovskite/HTM/
Au (see Fig. 3a and b). The typical HTM Spiro-OMeTAD was chosen as
the reference HTM and the light harvesting layer perovskite adopted in
this work is (FAPbI3)0.85(MAPbBr3)0.15. The detailed device fabrication
process is described in SI. The thickness of HTM layer is measured to
around 80 nm according to the cross-sectional SEM. From the top-view
SEM images of D-PTZT and PTZT films coating on the perovskite (see
Fig. S8), we can see that D-PTZT film exhibits more intensive pinholes,
The thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) measurements were also conducted to analyze the
thermal stability of D-PTZT and PTZT (see Fig. S7 and Table S10). The
decomposition temperature (Td) of D-PTZT and PTZT were detected to
be 443 ◦C and 424 ◦C, respectively, manifesting the good thermal sta-
bility of these two PTZ-based HTMs. Meanwhile, no glass transition
temperatures (Tg) were found in the DSC measurement for D-PTZT and
PTZT, demonstrating their intrinsic amorphous phase which is helpful
for obtaining good film formation property.
To take deep insight of the molecular configuration and the charge
distribution, the density functional theory (DFT) calculation was per-
formed by using the Gaussian program at the B3LYP/6-31G* level. As
shown in Fig. 2, both D-PTZT and PTZT exhibit a twisted butterfly
configuration. Due to the orthorhombic insertion and steric hindrance of
fluorene linker, D-PTZT shows weaker intramolecular force. For PTZT,
HOMO is delocalized along the whole backbone, while LUMO shifts to
2

M. Zhai et al.
Dyes and Pigments 191 (2021) 109340
Fig. 1. (a) Cyclic voltammograms of D-PTZT and PTZT in THF. (b) UV–vis absorption (solid line) and photoluminescence spectra (dashed line) of D-PTZT and PTZT
in THF (1 × 10ꢀ 5 M). (c) Hole mobility and (d) Conductivity of the doped D-PTZT and PTZT.
Table 1
Optical and electrochemical properties of HTMs.
HTMs
λab,max (nm)
λph,max (nm)
E0-0 (eV)
HOMO (eV)
LUMO (eV)
Hole mobility (cm2 Vꢀ 1 Sꢀ 1
)
Conductivity (S cmꢀ 2
)
D-PTZT
PTZT
307
329
461
461
2.79
2.82
ꢀ 5.16
ꢀ 5.17
ꢀ 2.37
ꢀ 2.35
1.62 × 10ꢀ 4
1.52 × 10ꢀ 4
2.62 × 10ꢀ 4
3.00 × 10ꢀ 4
Fig. 2. DFT calculations of HTMs D-PTZT and PTZT.
which would become the charge recombination center and accelerate
the non-radiative recombination at the interface. The electrochemical
impedance spectroscopy (EIS) measurement further confirmed the
conclusion. As shown in Fig. S9, the D-PTZT based PSC showed lower
recombination resistance (59 kΩ) than that of PTZT based device (97
kΩ), suggesting the serious non-radiative recombination in D-PTZT
3

M. Zhai et al.
Dyes and Pigments 191 (2021) 109340
Fig. 3. (a) Energy level diagrams of PSCs. (b) Cross-section SEM image of PSC device containing HTM PTZT. (c) Current-Voltage (J-V) curves of PSC device based on
HTMs D-PTZT, PTZT and Spiro-OMeTAD. (d) IPCE spectra of PSCs based on HTMs D-PTZT, PTZT and Spiro-OMeTAD.
photoluminescence (TRPL) spectroscopy were further conducted to
Table 2
characterize the charge transfer kinetics at perovskite/HTM interface
Optimal photovoltaic performance parameters of PSCs applying various HTMs.
(see Fig. 4a and b and Table S14). Significant PL quenching were
Jsc (mA cmꢀ 2
)
FF (%)
PCE (%)
J
sc
(mA cmꢀ 2
)
cal
HTMs
Voc (V)
detected for the cases of perovskite film coating with different HTMs.
Furthermore, perovskite film coating with PTZT exhibited higher PL
quenching efficiency than that of D-PTZT, indicating the more efficient
hole extraction of PTZT. The PL decay lifetime (τave) of D-PTZT, PTZT
and Spiro-OMeTAD coated perovskite film were calculated to be 24.99,
9.72 ns and 12.35 ns, respectively, illustrating the faster hole extraction
rate for PTZT. The results are consistent with the hole mobility and
conductivity measurements mentioned above.
D-PTZT
PTZT
1.02
1.06
1.05
21.90
22.78
23.82
68.88
77.69
73.57
15.45
18.74
18.57
21.45
23.05
22.99
Spiro-
OMeTAD
based PSC. Therefore, a higher open-circuit voltage (Voc) can be ex-
pected for PTZT-based PSC.
We also evaluated the long-term stability of the unencapsulated PSCs
containing D-PTZT and PTZT as HTMs in ambient atmosphere (60%
humidity, room temperature) and the test results were shown in Fig. 4c.
It is revealed that the PTZT based device exhibits much better stability
than D-PTZT and Spiro-OMeTAD, maintaining over 62% of its initial
efficiency after 1000 h storage. Considering the effect of moisture on the
stability of the device, we carried out the contact angle (θ) measurement
of D-PTZT and PTZT films coating on the perovskite (see Fig. 4c, illus-
tration). The PTZT film showed larger water contact angle of 90.20◦
than D-PTZT (85.20◦) and Spiro-OMeTAD (89.10◦), suggesting the
better hydrophobic property of PTZT. Furthermore, the PTZT film with
less pinholes observed above also decreases the active site of the
perovskite decomposition.
Fig. 3c presents the current density–voltage (J–V) curves of the
optimized devices containing D-PTZT and PTZT as HTMs, and the spe-
cific photoelectric parameters are presented in Table 2. The device
adopted PTZT as HTM achieved a power conversion efficiency (PCE) of
18.74%, with a short-circuit current density (Jsc) of 22.78 mA cmꢀ 2, a
V
oc of 1.06 V, and a fill factor (FF) of 0.78, which is much higher than
that of D-PTZT (15.45%) and the reference Spiro-OMeTAD (18.57%).
From the incident photo-to-electron conversion efficiency (IPCE) mea-
surement shown in Fig. 3d, the IPCE values of PTZT were much higher
than that of D-PTZT, and comparable with that of Spiro-OMeTAD. This
is mainly attributed to the higher hole extraction and transport effi-
ciency of PTZT because of higher hole mobility and conductivity of
PTZT. The integrated Jsc of PTZT, D-PTZT and Spiro-OMeTAD were
estimated to be 23.05, 21.45 and 22.99 mA cmꢀ 2, matching well with
the J–V test results. Moreover, 20 cells of each species were fabricated
(see Tables S11–13) and the boxplots of Jsc, Voc, FF and PCE values were
depicted in Fig. S10. All devices exhibited good reproducibility.
The steady-state photoluminescence (PL) and time-resolved
3. Conclusion
In conclusion, we synthesized PTZ-based dimer and monomer HTMs
D-PTZT and PTZT, and further investigated the impact of the spatial
4

M. Zhai et al.
Dyes and Pigments 191 (2021) 109340
Fig. 4. (a) steady-state photoluminescence (PL) spectra; (b) the time-resolved photoluminescence (TRPL) curves of the bare perovskite film and with different HTMs.
(c) Long-term stability of device in ambient environment. Inset, Contact angle measurement of the water droplet on the films of pristine perovskite and with
different HTMs.
molecular structures of D-PTZT and PTZT on their physicochemical and
photoelectrochemical properties. The results demonstrated the mono-
mer structure is more favorable for hole extraction and transport.
Applied in PSCs as HTMs, the monomer HTM PTZT based device ach-
ieved a higher PCE of 18.74% under 100 mW cmꢀ 2 AM 1.5G solar
illumination than that of D-PTZT-based one. The poor performance of D-
PTZT may result from the more twisted molecular structure inducing by
Writing-review & editing.
Declaration of competing interest
The authors declare no competing financial interest.
Acknowledgments
the non-linear π-linker. The dimer with linear π-linker, such as biphenyl
and alkyne will be synthesized and investigated. This work is in
progress.
This work was financially supported by the National Natural Science
Foundation of China (grants 21905119 and 21805114), Natural Science
Foundation of Jiangsu province (BK20180867 and BK20180869), China
Postdoctoral Science Foundation (2019M651741), Six Talent Peaks
Project in Jiangsu Province (XNY066), High-tech Research Key Labo-
ratory of Zhenjiang (SS2018002), the State Key Laboratory of Fine
Chemicals (KF1902).
Associated content
The Supporting Information is available free of charge on the
Elsevier Publications website. Synthetic routes, synthesis cost calcula-
tion, device fabrication procedure, and other characterization data are
supplied as Supporting Information.
Appendix A. Supplementary data
Credit authorship contribution statement
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2021.109340.
Mengde Zhai: Investigation, Methodology, Writing-original draft.
Yawei Miao: Investigation, Methodology, Writing-original draft. Cheng
Chen: Project administration, Supervision, Writing-review & editing.
Haoxin Wang: Methodology, Formal analysis. Xingdong Ding: Formal
analysis. Cheng Wu: Formal analysis. Xichuan Yang: Software, Formal
analysis. Ming Cheng: Conceptualization, Data curation, Resources,
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