Synthesis and characterization of triphenylamine modified azobenzene dyes

Article abstract of DOI:10.1016/j.dyepig.2016.10.018

A series of triphenylamine modified azobenzenes were synthesized and studied theoretically and experimentally. D-A-D structures of these dyes were revealed with an intramolecular charge transfer (ICT) from both triphenylamine or indoline units (donor) to

Full text of DOI:10.1016/j.dyepig.2016.10.018

Accepted Manuscript
Synthesis and characterization of triphenylamine modified azobenzene dyes
Chuanwu Zhao, Tianyang Wang, Dongmei Li, Ting Lu, Dongzhi Liu, Qingbo Meng,
Qianqian Zhang, Fengqing Li, Wei Li, Wenping Hu, Lichang Wang, Xueqin Zhou
PII:
S0143-7208(16)30950-0
10.1016/j.dyepig.2016.10.018
DYPI 5534
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 6 July 2016
Revised Date: 19 September 2016
Accepted Date: 13 October 2016
Please cite this article as: Zhao C, Wang T, Li D, Lu T, Liu D, Meng Q, Zhang Q, Li F, Li W, Hu W, Wang
L, Zhou X, Synthesis and characterization of triphenylamine modified azobenzene dyes, Dyes and
Pigments (2016), doi: 10.1016/j.dyepig.2016.10.018.
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ACCEPTED MANUSCRIPT
Synthesis and characterization of Triphenylamine modified
azobenzene dyes
Chuanwu Zhaoa, b, Tianyang Wanga ,b, *, Dongmei Li c, Ting Lua, b, Dongzhi Liua, d
Qingbo Mengc, Qianqian Zhanga, b, Fengqing Lia, b, Wei Lia, b,d, Wenping Hub,
Lichang Wanga, b, e,** and Xueqin Zhoua, b,d ***
a
School of Chemical Engineering and Technology, Tianjin University, Tianjin
300072, China
b
Collaborative Innovation Center of Chemical Science and Engineering, Tianjin
300072, China
cKey Laboratory for Renewable Energy, Chinese Academy of Sciences,
Beijing100190,China.
dTianjin Engineering Research Center of Functional Fine chemicals, Tianjin, 300072,
China;
e
Department of Chemistry and Biochemistry and the Materials Technology Center,
Southern Illinois University Carbondale, IL 62901, United States;
Corresponding author:
*Tianyang Wang, School of Chemical Engineering and Technology, Tianjin
University, Tianjin 300072, China. E-mail: tianyangwang@tju.edu.cn;
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**Lichang Wang, Department of Chemistry and Biochemistry and the Materials
Technology Center, Southern Illinois University Carbondale, IL 62901, United States.
E-mail: lwang@chem.siu.edu;
***Xueqin Zhou, School of Chemical Engineering and Technology, Tianjin
University, Tianjin 300072, China, E-mail: zhouxueqin@tju.edu.cn
ꢀElectronic Supplementary Information (ESI) available: Experimental; Synthesis of
4a-d; 1H NMR, HRMS. Additional figures of UV,CVand SCLC of 1a-d measurement
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ABSTRACT: A series of triphenylamine modified azobenzenes were synthesized and studied
theoretically and experimentally. D-A-D structures of these dyes were revealed with an
intramolecular charge transfer (ICT) from both triphenylamine or indoline units (donor) to the
central azobenzene unit (acceptor). Both computational and experimental results indicate that a
stronger electron donating ability of the donor enhances the degree of ICT and improve their
HOMO energies. The absorption spectra indicate an increased light absorption by enhanced ICT
effect. The intermolecular interaction also affects their absorption ability. The strongest
intermolecular interaction results in the best light absorption ability in films. An obviously
increased hole mobility was also found as the ICT was enhanced. These dyes were used as hole
transport materials (HTM) of perovskite solar cells to investigate their feasibility. It is revealed
that the solubility and filming ability are significant for the photovoltaic performance of devices.
KeywordsꢁDyes, triphenylamine, intramolecular charge transfer, hole transport
material, solar cells
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1. Introduction
Triphenylamine derivatives are one kind of significant dyes and have been widely investigated
because of their strong emissions and excellent hole transport mobility and therefore potential
applications have been paid attention to the fields of organic light emitting diodes[1-5],
sensors[6-8], solar cells[9-13], and photochromophore[14-16]. Specially the important
dye-2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene(spiro-OMeTAD) has
been widely used as hole transport material (HTM) in various solar cells and the power conversion
efficiencies (PCE) exceeding 19 % based on organic-inorganic halide CH3NH3PbI3 perovskites
have been reported[17]. However, Spiro-OMeTAD requires multi-step synthesis and very
expensive. Therefore high hole-transportation ability of cheaper dyes remain a challenge for
highly efficient solar cells.
Numerous novel compounds have been developed by altering the electron donor modules and
conjugation bridge of spiro-OMeTAD. As-constructed spiro-bridged[18-20], linear π-conjugated
structured[21, 22] and sheet-shaped structured[23] compounds together with star-π-systems[24]
have good film morphological stability and reduced molecule aggregation ability. High hole
mobility can be achieved by doping metal complex[25-28] and then a relatively satisfactory
performance (PCE of 11-17 %) was attained by employing these compounds as the HTMs of solar
cells. However the PCE was not higher than that of spiro-OMeTAD. The synthesis is complicate
yet. The hole mobility is only about 10-4 cm2ꢀV-1ꢀS-1 for spiro-OMeTAD in pristine form[29].
Additional dopants are required for applications. The dopants bring the device instability,which
are unfavorable for practical applications[30]. Thus a novel structure is expected for high
performance HTMs to overcome above disadvantages.
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Conductive polymers such as poly(triarylamine) (PTAA) and P3HT are alternative HTMs
because of their excellent filming ability and stability. Moreover, better hole mobility could be
achieved by donor–acceptor (D-A) structure design, such as benzo[1,2,5]thiadiazole based
polymer, of which higher hole mobility (6.68×10-4 cm2ꢀV-1ꢀS-1) and deeper HOMO energy level
(-5.24 eV) than those of P3HT (2.45×10-4 cm2ꢀV-1ꢀS-1and -5.10 eV, respectively) result in a better
performance in the application of perovskite solar cells (6.64 % vs 4.24 % of PCE )[31]. Kwon[32]
reported a D-A polymer, Poly[2,5-bis(2-decyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-
(E)-1,2-di(2,2’-bithiophen-5-yl)ethane with a hole mobility of 10-3 cm2ꢀV-1ꢀS-1. Therefore, a PCE
of 9.2 % was achieved in the perovskite solar cell based on this polymer which was higher than
that of the doped Spiro-OMeTAD (7.6 %). Recently a photoconversion efficiency of 12.32 % was
reported with diketopyrrolopyrrole (DPP) based D-A polymer as the HTM in perovskite solar
cells[33]. D–A structure has been revealed to be able to enhance the photoinduced intramolecular
charge transfer, which facilitates the transfer of photogenerated cations, and thus attain long
charge separated state lifetime and high hole mobility[34]. However, the polymeric HTMs have
the disadvantages of poor reproducibility and complex synthesis process.
Above discussions indicate that D-A conjugated organic molecules may be the most promising
candidate for high performance HTMs. Sun licheng et al[35] designed a benzodithiophene
based D-A molecule BDT-C1 with the high hole mobility (3.18×10-4 cm2ꢀV-1ꢀS-1) and
conductivity (1.16×10-4 Sꢀcm-1) in pristine form. The average PCE of 13.4 % for BDT-C1 was
only slightly lower than the Spiro-OMeTAD (13.9 %). Yang yang reported two dopant-free D-A
molecules, DERDTS–TBDT [36] and DOR3T-TBDT[37], of which the devices showed a PCE of
16.2 % and 14.9 %, respectively. They are comparable to or even better than that of the devices
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using the doped Spiro-OMeTAD (14.0 %) as HTMs under equivalent conditions. Moreover, the
D-A structure may also reduce the HOMO-LUMO gap, and thus enhance the light harvesting
ability of HTMs in the long wavelength range, making it possible to participate as light absorbers
together with perovskite in the low energy region[38]. Recently Michael Grätzel[39] developed
A-D-A type molecular HTMs with which the devices achieved a PCE of 10.3-11.4 % without any
dopant, though the hole mobilities are around 4×10-5 cm2ꢀV-1ꢀS-1. In this paper, we report a new
kind of D-A-D type molecular dyes, triphenylamine-modified azobenzene (1a-d). The structures
are shown in scheme 1. The synthesis was discussed. Then the computation and electrochemical
results together with absorption spectra were investigated in detail and it was found that these
novel dyes may be applied in organic solar cells. Finally we report their application in Perovskite
Solar Cells.
2. Experimental section
2.1. Materials
Nitrobenzyl-triphenylphophonium-bromide(NTB), Benzyl triphenylphosphonium bromide
(BTB),N,N’-di(4-octyloxyphenyl)aniline, 4-(p-tolyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole
were prepared according to the literature[40-42]. Triphenylamine (AR, 98 %) and
4-formyl-4’,4”-dimethyltriphenylamine (2b) (AR, 98 %) were obtained from TCI, PbI2 was
purchased from Aldrich. Hydroiodic acid (AR, 45 wt% in water) and methylamine (AR, 27 % in
methanol) were bought from Sinopharm Chemical Reagent Co. Ltd. Other reagents were
commercially available and used without further purification.
2.2. Instrumentation and measurement conditions
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1HNMR spectra were obtained on a Varian INOVA-500MHZ superconducting spectrometer.
The ESI mass and HR-ESI mass spectra were obtained on a Thermo Fisher LCQ Deca XP MAX
mass spectrometer. MALDI-TOF mass and HR-MALDI-TOF mass spectra were obtained on a
microOTOF-Q II mass spectrometer respectively. Absorption spectra were taken on a NicoIet
Evolution 300 UV-visible spectrometer. Electrochemical properties were measured using a BAS
100W electrochemical analyzer and performed in a three-electrode cell. A glassy carbon electrode
was used as working electrode, Ag/AgNO3 electrode as reference electrode and platinum as
auxiliary electrode. Scan rates were 100 mV/s. The dichloromethane containing 0.05 mol/L
tetrabutylammonium hexafluorophosphate (TBAPF6) was employed as the medium for all the
cyclic voltammetric experiments. All solutions were purged using nitrogen gas prior to the
electrochemical measurement.
2.3. Computational details
Geometry optimizations of the molecules were carried out using the three-parameter exchange
functional of Becke and correlation functional of Lee, Yang and Parr (B3LYP)[43-45]with the
6-31G(d) basis set in in a dichloromethane environment without any symmetry constraints. In
order to confirm the optimized geometry as a global minimum, frequency calculations at the same
level of theory were performed, All the calculations were performed using Gaussian09
package[46]. Detailed description of computational methods can also be found in our previous
studies[47].
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2.4. Synthesis
2.4.1ꢂGeneral synthetic step of the aldehyde derivatives
Aldehyde derivatives were synthesized by a revised method of the literature[48]. Phosphorous
oxychloride (POCl3) (10 mL, 107 mmoL, 1.0 equiv.) was added dropwise to DMF (8 mL, 107
mmoL, 1.0 equiv.) at 0 °C. The mixture was stirred at 25 °C for 1 h. Triphenylamine or its
derivatives (Ar) (107 mmol, 1.0 equiv.) was added and the reaction mixture was stirred and heated
at 90 °C for 3 h. After cooling down to room temperature, the mixture was poured into 50 mL cold
distilled H2O and extracted with dichloromethane (3×20 mL). The combined organic layer was
dried with anhydrous Na2SO4 and then filtered. The filtrates were concentrated by rotary
evaporation. The target compounds were purified by column chromatography on silica gel using
petroleum:ethyl aceate (v:v = 4:1) as the eluent.
4-formyltriphenylamine(2a)
Yield 76% as orange solid, Rf = 0.44, m.p.=132 °C; ESI-MS: m/z =273.3(M)+. 1H NMR
(CDCl3, 500 MHz) δꢁ9.81 (s, 1H, -CHO), 7.68 (d, J = 8.8 Hz, 2H), 7.34 (t, J = 9.0 Hz, 4H), 7.17
(d, J = 9.0 Hz, 4H), 7.15 (d, J = 9.0 Hz, 2H), 7.12-6.98 (m, 2H), 13C NMR (CDCl3, 500 MHz) δ:
190.29, 153.30, 146.10, 131.26, 129.68, 129.10, 126.27, 125.10, 119.32.
4-formyl-4’,4”-dioctyloxyltriphenylamine (2c)
Yield 80 % as orange oil liquid, Rf = 0.57, HR-ESI-MS: m/z = 530.3630[M+H]+ (calcd for
+
1
C35H48NO3 : m/z = 530.3629). H NMR (CDCl3, 500 MHz) δꢁ9.75 (s, 1H, -CHO), 7.63 (d, J =
9.0 Hz, 2H), 7.12 (d, J = 9.0 Hz, 4H), 6.88 (d, J = 9.0 Hz, 4H), 6.85 (d, J = 9.0 Hz, 2H), 3.95 (t, J
= 6.5 Hz, 4H), 1.82-1.76 (m, 4H), 1.50-1.44 (m, 4H), 1.38-1.26 (m, 16H), 0.90 (t, J = 6.5 Hz, 6H),
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13C NMR (CDCl3, 500 MHz) δ: 190.15, 157.09, 154.35, 138.84, 131.60, 128.20, 128.00, 116.92,
116.94, 68.40, 32.00, 29.60, 29.54, 29.47, 26.31, 22.90, 14.26.
4-(p-tolyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole-7-carbaldehyde (2d)
Yield 83% as orange solid, Rf = 0.37, m.p.= 79 °C, ESI-MS: m/z = 277.1(M)+, HR-ESI-MS:
m/z = 278.1538[M+H]+ (calcd for C19H20NO+: m/z = 278.1539).1HNMR(CDCl3, 500MHz) δ:
9.69 (s, 1H.-CHO), 7.63 (s, J = 8.5 Hz, 1H), 7.50 (d, J = 8.5 Hz, 1H), 7.16-7.23 (m, 4H), 6.71 (d, J
=8.8 Hz, 1H), 4.91 (m, 1H), 3.81 (m, 1H), 2.36 (s, 3H), 1.49-2.14 (m, 6H). 13C NMR (CDCl3, 500
MHz) δ: 189.90, 154.42, 138.54, 135.70, 134.34, 133.89, 130.33, 127.99, 125.26, 122.68, 105.97,
70.32, 44.80, 35.62, 33.41, 24.42, 21.18.
2.4.2ꢂGeneral synthetic step of Nitro-derivatives
This compound was prepared by a revised method of the literature[49]. NaH (1.00 g, 4.0
mmoL) were added into an anhydrous tetrahydrofuran (100 mL) of NTB (4.20 g, 8.8 mmoL) and
the reaction mixture was stirred at room temperature for 3 h. The colour of solution changed from
white to deep red, Afterwards compound 2 (2.40 g, 8.0 mmoL) was added to the solution and
refluxing at 80 °C. After the reaction was complete, the mixture was slowly poured into cold
distilled H2O (50 mL) and extracted with dichloromethane (3×20 mL). The combined organic
layer was dried with anhydrous Na2SO4 and then filtered. The combined filtrates were
concentrated by rotary evaporation. The products were purified by column chromatography on
silica gel using cyclohexane:dichloromethane (v:v= 2:1) as the eluent.
(E)-4-(4-nitrostyryl)-N,N-diphenylaniline (3a)
Yield 75% as red solid, Rf = 0.31, m.p.= 149 °C, HR-ESI-MS: m/z = 415.1417[(M+Na)+]
1
(calcd for C26H20N2O2Na+: m/z = 415.1417). H NMR (CDCl3, 500 MHz)δ: 8.21 (d, J = 8.5 Hz,
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2H), 7.60 (d, J = 8.5 Hz, 2H), 7.42 (d, J = 8.5 Hz, 2H), 7.30 (m, 4H), 7.23 (d, J = 16.5 Hz, 1H),
7.14 (d, J = 7.5 Hz, 4H), 7.10-7.06 (m, 4H), 7.02 (d, J = 16.5 Hz, 1H). 13C NMR (CDCl3, 500
MHz) δ: 148.81, 147.46, 146.50, 144.55, 133.13, 130.03, 129.66, 128.25, 126.73, 125.22, 124.39,
123.84, 122.93.
(E)-4-(4-nitrostyryl)-N,N-di(4-(methyl)phenyl)aniline (3b)
Yield 75% as red solid, Rf = 0.32, m.p.= 162 °C, HR-ESI-MS: m/z = 421.1911[(M+H)+] (calcd
for C28H25N2O2 : m/z = 421.1911). 1H NMR (CDCl3, 500 MHz): δ 8.20 (d, J = 8.5 Hz, 2H), 7.59
+
(d, J = 9.0 Hz, 2H), 7.38 (d, J = 8.5 Hz, 2H), 7.21 (d, J = 16.0 Hz, 1H), 7.11 (d, J = 8.5 Hz, 4H),
7.04 (d, J = 8.5 Hz, 4H), 7.00 (m, 3H), 2.35(s, 6H). 13C NMR (CDCl3, 500 MHz) δ: 149.23,
146.56, 144.92, 144.63, 133.60, 133.30, 130.29, 129.06, 128.15, 126.62, 125.46, 124.37, 123.85,
121.65, 21.18.
(E)-4-(4-nitrostyryl)-N,N-di(4-(octyloxyl)phenyl)aniline (3c)
Yield 73% as orange oil liquid, Rf = 0.31, HR-ESI-MS: m/z = 648.3923 (M)+(calcd for
+
C42H52N2O4 : m/z = 648.3922). 1H NMR (CDCl3, 500 MHz) δꢁ8.20 (d, J = 8.5 Hz, 2H), 7.58 (d,
J = 8.5 Hz, 2H), 7.35 (d, J = 8.5 Hz, 2H), 7.21 (d, J = 16.0 Hz, 1H), 7.08 (d, J = 8.5 Hz, 4H), 6.95
(d, J = 16.0 Hz, 1H), 6.90 (d, J = 8.5 Hz, 2H), 6.85(d, J = 9.0 Hz, 4H), 3.94 (t, J = 6.5 Hz, 4H),
1.81-1.76 (m, 4H), 1.49-1.43 (m, 4H), 1.38-1.26 (m, 16H), 0.90 (t, J = 6.5 Hz, 6H). 13C NMR
(CDCl3, 500 MHz) δ: 155.94, 151.93, 149.04, 140.74, 140.57, 130.25, 129.18, 127.71, 127.03,
127.00, 125.12, 123.56, 120.26, 115.48, 68.52, 32.00, 29.98, 29.60, 29.48, 26.24, 22.89, 14.29.
(E)-7-(4-nitrostyryl)-4-p-tolyl-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole (3d)
Yield 50 % as black solid, Rf = 0.30, m.p. = 157 °C, HR-ESI-MS: m/z = 419.1727[(M+Na)+]
(calcd for C26H24N2O2Na+: m/z = 419.1730). 1H NMR (CDCl3, 500 MHz) δ: 8.18 (d, J = 8.5 Hz,
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2H), 7.56 (d, J = 8.5 Hz, 2H), 7.37 (s,1H), 7.15-7.25 (m, 6H), 6.93 (d, J = 16.5 Hz, 1H), 6.86 (d, J
= 8.0 Hz, 1H), 4.85 (m, 1H), 3.84 (m, 1H), 2.36 (s, 3H), 1.65-2.13 (m, 6H). 13C NMR (CDCl3, 500
MHz) δ: 149.35, 146.06, 145.31, 139.98, 135.98, 134.12, 132.36, 130.12, 128.43, 126.84, 126.24,
124.37, 123.06, 121.53, 120.83, 107.47, 69.62, 45.45, 35.43, 33.86, 24.67, 21.09.
2.4.3ꢂGeneral synthetic step of azobenzene derivtives
LiAlH4 (1.90 g, 50 mmoL) was added to the anhydrous tetrahydrofuran (200 mL) of Compound
3 (13 mmoL) and the reaction mixture was stirred and refluxed for 2 h under N2. After the reaction
was complete. Then the solvent was removed, The residue was dispersed in CH2Cl2, filtered off
the suspension and washed with distilled H2O. The organic phase was concentrated by rotary
evaporation. Then the products were obtained by column chromatography on silica eluting with
petroleum : dichloromethane(v:v = 3:1) as the eluent.
(E)-4,4’-bis(4-N,N-diphenylaminostyryl)azobenzene (1a)
Yield 80% as red solid, Rf = 0.25, m.p. = 219 °C, MALDI-TOF: m/z= 720.210 (M+),
+
HR-MALDI-MS: m/z = 721.3325 [(M+H)+] (calcd for C52H42N4 : m/z = 721.3326). 1H NMR
(CDCl3, 500 MHz) δ:7.94-7.92 (d, J = 8.5 Hz, 4H), 7.64 (d, J = 8.5 Hz, 4H),7.43 (d, J = 8.5 Hz,
4H), 7.30-7.27 (m, 8H), 7.19 (d, J =16.5 Hz, 2H), 7.14 (d, J = 7.5 Hz, 8H), 7.08-7.04 (m, 10H).
13C NMR (CDCl3, 500 MHz) δ: 150.75, 149.08, 144.01, 140.50, 133.00, 130.10, 130.00, 129.95,
128.00, 126.75, 125.60, 125.00, 123.40, 122.00.
(E)-4,4’-bis(4-N,N-di(p-tolyl)aminostyryl)azobenzene (1b)
Yield 85% as red solid, Rf = 0.26, m.p. = 238 °C, MALDI-TOF: m/z = 776.261(M+),
+
HR-MALDI-MS: m/z = 777.3952 [(M+H)+] (calcd for C56H49N4 : m/z = 777.3951).1H NMR
(CDCl3, 500 MHz) δ: 7.92 (d, J = 8.5 Hz, 4H), 7.63-7.612 (d, J = 8.5 Hz, 4H), 7.38 (d, J = 8.5 Hz,
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4H), 7.16 (d, J = 16.5 Hz, 2H), 7.09 (d, J = 8.0 Hz, 8H), 7.04-7.02 (m, 14H), 2.33(s, 12H). 13C
NMR (CDCl3, 500 MHz) δ: 151.80, 148.30, 145.00, 140.50, 133.00, 130.10, 130.00, 129.90,
127.50, 126.80, 125.60, 125.00, 123.40, 122.0, 20.80.
(E)-4,4’-bis(4-N,N-di(p-octyloxylphenyl)aminostyryl)azobenzene (1c)
Yield 87% as red oil liquid, Rf = 0.24, MALDI-TOF: m/z=1232.746 (M+), HR-MALDI-MS:
m/z = 1232.8051 (M+) (calcd for C84H104N4O4: m/z = 1232.8052). 1H NMR (CDCl3, 500 MHz) δ:
7.92 (d, J = 8.5 Hz, 4H), 7.61 (d, J = 8.5 Hz, 4H),7.35 (d, J = 8.5Hz, 4H), 7.17-6.95 (m, 12H),
6.92 (d, J =16.5 Hz, 4H), 6.84 (d, J = 7.5 Hz, 8H), 3.94 (t, J = 5.5Hz, 8H), 1.82-1.76 (m, 8H),
1.51-1.23 (m, 40H), 0.95-0.84 (m, 12H). 13C NMR (CDCl3, 500 MHz) δ: 148.11, 144.80, 144.04,
138.69, 134.71, 132.82, 131.14, 128.84, 127.13, 125.52, 124.98, 123.15, 121.75, 120.28, 119.59,
106.59, 68.34, 44.15, 34.14, 32.56, 28.68, 23.36, 19.80, 14.35.
(E)-4,4’-bis(4-p-tolyl-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indol-7-yl)vinyl)azobenzene (1d)
Yield 88 % as deep red solid, Rf = 0.21, m.p. = 226 °C, MALDI-TOF: m/z=728.843(M+),
HR-MALDI-MS: m/z = 728.3873 (M+) (calcd for C52H48N4: m/z = 728.3873). 1H NMR (CDCl3,
500 MHz) δ: 7.91 (d, J = 8.0 Hz 4H), 7.61 (d, J = 8.0 Hz, 4H), 7.38 (s, 2H), 7.21-7.16 (m, 12H),
6.98 (d, J = 16.5 Hz, 2H), 6.89 (d, J = 8.0 Hz, 2H), 4.83 (m, 2H), 3.85 (m, 2H), 2.35 (s,6H),
2.12-1.50 (m,12H). 13C NMR (CDCl3, 500 MHz) δ: 151.56, 148.27, 140.77, 140.13, 137.28,
135.54, 133.36, 130.74, 129.80, 126.54, 126.14, 123.45, 122.39, 120.69, 107.38, 69.40, 45.24,
35.02, 33.70, 24.28, 20.94.
2.5. Solar cell fabrication
Methyl ammonium iodide (CH3NH3I) was prepared according to the reported procedure[50].
Substrates are fluorine-doped tin oxide conducting glass (FTO, Pilkington, thickness: 2.2 mm,
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sheet resistance 14 ꢁ/square). Patterned FTO glass was first cleaned with mild detergent, rinsed
with distilled water for several times and subsequently with ethanol in an ultrasonic bath, finally
dried under air stream.
For fabricating the device, 30 nm-thickness TiO2 compact layer was spin-coated on the
pre-cleaned FTO glass, then sintered at 500 °C. Subsequently, 200 nm-thickness mesoporous TiO2
anatase compact layer (dyesol 18NR-T paste) were deposited according to the literature[51]
.
CH3NH3PbI3 was deposited on the mesoporous TiO2 film by using a sequential method as
reported[52]. Azobenzene derivatives chlorobenzene solutions with a concentration of 5 mg mL-1
were spin-coated on the mesoporous TiO2/CH3NH3PbI3 film at 3000 rpm for 20 s. Finally, 80
nm-thickness Au electrode was thermally evaporated under the vacuum of 10-7 Torr.
2.6. Device measurement
For J-V characteristics, the cells were illuminated under 100 mW cm-2 (AM 1.5) by an Oriel
solar simulator 91160A while the data were recorded on Keithley 2602. A mask with a window of
0.10 cm2 was used to define the active area of the cell. The monochromatic incident
photon-to-electron conversion efficiency (IPCE) spectrum was obtained on a lab-made IPCE
testing system[53, 54].
2.7. Instrumentation and Mobility measurements
The hole-only devices were fabricated by spin-coating PEDOT:PSS (Baytron PVP Al4083)
onto pre-cleaned, patterned indium tin oxide (ITO) substrates. Films of azobenzene derivatives
were spin-coated on top from the chloroform solution with a concentration of 5 mg mL-1. The film
thickness was controlled at about 100 nm. As a counter electrode Al was deposited on top by
vacuum evaporation. The current density–voltage curves of the devices were recorded with a
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Keithley 2400 source. For the hole-only devices, space charge limitation of current (SCLC) is
described as
2
9εε ꢂ V
0
0
3
J =
(1)
8L
where ε is the dielectric constant of the transport material, ε0 is the permittivity of the vacuum, ꢀ0
is the zero-field mobility, J is the current density, and L is the thickness of the film[55]. Then the
square root of J was plotted against the voltage and the hole mobility can be calculated from the
slope.
3. Results and discussion
3.1. Synthesis and characterization
The triphenylamine modified azobenzenes 1a-d were synthesized from various donors,
triphenylamine or indoline derivatives, by Vilsmer formylation reaction, Wittig and reduction
reaction (Scheme 1). Aldehydes 2a, 2c and 2d were first obtainedin good yields (76-83 %) via
Vilsmer formylation reaction. The vilsmer reagent, which was prepared from the POCl3 and DMF
(molar ratio 1:1) at 0 °C under the N2, was the key to the yields. Low temperature may be benefit
to the stability of vilsmer reagents. For triphenylamine, the same nucleophilic reactivity of three
para-benzene positions may produce dialdehydes[56], leading to relatively low yield of 2a (76 %);
while only one active-site in indoline and octyloxyl-triphenylamine together with their higher
electron density in comparison to triphenylamine result in better yield of 2c (80 %) and 2d (83 %).
Then Wittig reactions were performed by reacting 2a-d with NTB Wittig reagent to yield
compounds 3a-d. The electron withdrawing ability of nitro group can stabilize the Wittig reagent
o
anion, and benefit the Wittig reaction. Though a high temperature (80 C) was required for the
synthesis of 3a-d, a relatively high yield was reached as about 70 %.
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Nitro groups may be reduced to amino or azo groups depending on the reducers. Previously we
used SnCl2 as reducing reagents and 3a, 3b and 3d were converted into amine derivatives[57].
Herein LiAlH4 was employed to reduce 3a-d in tetrahydrofuran. The reduction reaction starts
from the attack of hydrogen anion on nitrogen cation, resulting in an intermediate N-nitroso
derivatives. N-nitroso derivatives may be further reduced to amine derivatives. Since N-aryl group
can stabilize N-nitroso derivatives, a coupling reaction is allowed between N-nitroso and amine
derivatives, which finally produces the azo derivatives 1a-d. Moreover the electron-rich
substituents(methyl, alkoxyl) and indoline-nitroaromatics benefit the stability of the intermediate
N-nitroso derivatives, and thus boost their coupling reaction with amine derivatives, leading to a
higher yield of 1b-d (~90 %) than nonsubstituent-nitroarene substrate 1a (80 %). The structure of
1
all compounds was confirmed by ESI-MS, HR-ESI-MS, MALDI-TOF, HR-MALDI-TOF, H
NMR and 13C NMR. The single crystal of 1a (Fig.1) further confirm its trans-structure.
3.2. Computational results
As the trans-structure of compounds 1a-d was identified, we conducted density functional
theory (DFT) computation to investigate their molecular orbitals. As shown in Fig.2, the electron
distribution of the HOMO is homogeneously delocalized over the triphenylamine or indoline
group (donor) and the styryl-linear conjugated part, and the electron distribution of the LUMO is
confined to the styryl and the azobenzene unit (acceptor), which is similar to those of 4a-d[58].
The intramolecular charge transfer (ICT) from both donor groups to the central acceptor is found
based on the molecular orbitals of 1a-d. The large differences in their Stokes shifts in polar and
non-polar aprotic solvents (Table S1) confirm the presence of ICT. Therefore compounds 1a-d
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have a typical D-A-D structure. As we further study HOMO and LUMO energies of 1a-c, it is
discovered that both energies are improved as introducing electron donating groups such as
methyl and alkoxyl onto the triphenylamine unit. This effect is more significant for HOMO,
leading to a reduced Eg cal (calculated HOMO-LUMO energy gap) in 1b and 1c.
3.3. Electrochemical Properties
Fig.3 gives the electrochemical curves of 1a-d in CH2Cl2 solutions at 298K. All compounds
show one quasi-reversible oxidation wave, corresponding to the electron loss process of the
triphenylamine or indoline modules. This is comparable with the oxidation wave of 4a-d (See Fig
S1). But the oxidation potentials are clearly improved in 1a-d (0.83, 0.81, 0.37 and 0.48 V vs
Ag/AgNO3 in CH2Cl2, respectively) in comparison with those of 4a-d (0.60, 0.58, 0.35 and 0.24 V
vs Ag/AgNO3 in CH2Cl2, respectively). Previous studies reveal a D-A stucture for compounds
4a-d and the triphenylamine or indoline module plays as a donor[58]. Herein the azo group is
electron withdrawing, and thus enhances the charge transfer degree from the triphenylamine or
indoline module (donor) to the styryl module (acceptor), leading to an increase of stability for
1a-d when compared to that of the reference compounds 4a-d. Moreover, as we compare the
oxidation potential of 1a-c, it is found that the electron donating groups at the donor such as
methyl in 1b and octyloxyl in 1c may reduce their stability. This result agrees with that of 4a-c.
The HOMO energy was deduced from the oxidation onset potential and calculated by the
equation: EHOMO = -Eoneset-4.93 (eV) as -5.30, -5.28, -5.21 and -5.20 eV for 1a-d, respectively.
A slight improvement of HOMO energies from 1a to 1b and 1c are consistent with above
computation results, verifying the donor effects on HOMO.
3.4. Absorption Abilities
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Fig.4(a) plots the absorption spectra of 1a-d in dichloromethane solutions. The absorption band
between 260 and 400 nm can be ascribed to π-π* electronic transitions of aryl groups and
azobenzne bands [59]. The absorption maxima of 1a-d locates at around 464 nm, 489 nm, 491 nm
and 506 nm, respectively. This absorption is attributed to HOMO-LUMO transition according to
the computation results and influenced by the degree of ICT. Decreased stokes shifts of 1a-d in
cyclohexane and benzene (Table S1) suggest that the ICT degree reduces from 1a to 1d. Since
the acceptor unit is almost the same for four compounds, the maxima absorption wavelength
changes with the donor units. 1d has the longest absorption band at 506 nm. Methyl group is
electron donating, leading to a 15 nm bathochromic shift of 1b in comparison with 1a. A further 2
nm bathochromic shift is found in 1c by substitution of both methyl groups in 1b for octyloxyl
groups. The optical energy gap (Eg) is 2.19, 2.12, 2.11 and 2.04 eV for 1a-d, respectively. This
agrees with computation results and declares that enhanced ICT may reduce the energy gap.
The absorption spectra of 1a-d films were also measured by spin-coating the chlorobenzene
solution onto a glass substrate at a speed of 1500 rpm. As shown in Fig.4(b), the absorption peaks
especially the longest band become wider for films in comparison with those obtained in solution,
indicative of intermolecular interactions. An obvious red-shift by 47 nm and a clearly reduced Eg
by 0.23 eV were observed for 1b in films while the red-shift and Eg variation were within 20 nm
and 0.20 eV for other compounds (1a, 1c-d, 4a-d and sprio-OMeTAD) as shown in Table S2.
Since the films are prepared in similar thickness, a great improve of the absorption ability is
discovered for 1b for the absorbance of 1b in films is similar as that of 1c and larger than that of
1a and 1b in films whereas the absorbance of 1b in solutions is distinctly below that of 1c and 1d
in solutions. All these phenomena demonstrate that, among these compounds, 1b has the strongest
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intermolecular interaction, which enhances its absorption ability especially for the longer
wavelength.
3.5. Application in perovskite solar cells
The electrochemical diagram of TiO2, CH3NH3PbI3 and compounds 1a-d are shown in Fig.5(a).
The HOMOs of 1a-d are between that of CH3NH3PbI3 (-5.44 eV) and Au (-5.10 eV), indicative of
the possibility of 1a-d as the hole transfer materials for perovskite solar cells. The LUMOs of
1a-d are also higher than that of CH3NH3PbI3 (-3.93 eV), showing a possible transfer electron
from 1a-d to CH3NH3PbI3. Then we fabricated perovskite solar cells using 1a-d as the HTM. The
cross-sectional SEM image is shown in Fig.5(b). The J-V curves are shown in Fig.6(a) and the
photovoltaic parameters are listed in Table 1.
Perovskite solar cells with compounds 1a-d as HTM show an adequate photovoltaic
performance with a PCE of 7-9 %. The highest Voc, Jsc and PCE are achieved with compound 1b
as 917.1 mV, 15.96 mAꢀcm-2 and 9.02 %, respectively. The Voc depends on the gap (Eg oc)
between the HOMO of HTMs and the quasi-Fermi level of the TiO2. Therefore the relatively low
Eg oc leads to a relatively low Voc for compound 1c and 1d. The difference of Jsc may be relative
to the absorption ability and hole mobility of HTMs as well as the charge transportation ability at
interfaces. The IPCE spectra in Fig.6(b) record higher efficiency in the range of 400-550 nm,
corresponding to the absorption peaks of compounds 1a-d. This indicates that light absorption
from compound 1a-d also contribute to the photocurrent. Above discussions reveal that compound
1b has the highest absorption ability in films, corresponding to its highest Jsc. And here the hole
mobility, which was obtained with the film processed in same procedure and same film thickness
as the preparation of solar cells, is found to increase from 1a to 1d (Table 1 and Fig S3). However,
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the Jsc of 1c is higher than that of 1d. Therefore we assume that the charge transportation ability at
interfaces is significant for high performance pervoskite solar cells. The higher HOMO of
compounds 1a-d than that of CH3NH3PbI3 allows hole injection from compounds 1a-d to
CH3NH3PbI3, thus contributing to Jsc. A match of HOMO potentials between CH3NH3PbI3 and the
HTM can achieve relatively high Jsc. Results by Karl Leo et al. indicate that the most appropriate
ionization potential of HTMs is around 5.3 eV for triphenylamine modified spiro-derivatives[60].
Herein similar HOMO energy of 1b may also contribute to the higher Jsc of 1b than 1a.
Furthermore, the solubility of 1a-d is low in common organic solvents. Chlorobenzene is a
relatively good solvent for them. Thus, during the device fabrication, chlorobenzene was
employed to dissolve 1a-d and the concentration is as low as 5 mgꢀmL-1. Even so, films of 1a-d
exhibit many cracks, which is adverse to the exciton separation at the interface of CH3NH3PbI3
and HTMs as well as the hole transportation within HTM layer. For comparison, under the same
fabrication process, we fabricated perovskite solar cells using compound 4b as the HTM and
results are also given in Table 1. Compound 4b shows similar HOMO energy to 1b, but better
solubility and filming ability than 1a-d. It is found that their contrast device has a comparable
PCE (7.61 %) to that of compound 1a and 1d, but obviously less than that of 1b. As comparing
with that of 4b, 1b shows obviously higher hole mobility and better light absorption ability.
However, the Jsc is found only a bit higher with 1b than 4b. All these suggest the significance of
filming ability for this kind of HTMs.
Spiro-OMeTAD was also employed as the HTM without any dopants to fabricate perovskite
solar cells. As shown in Table 1, the Voc of spiro-OMeTAD is a bit lower than that of 1b due to its
a bit higher HOMO (-5.22 eV). However, devices of spiro-OMeTAD present an obviously
19

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improved Jsc (18.25 mAꢀcm-2), and thus a higher PCE (11.46 %). This is attributed to a higher hole
mobility and better light absorption ability of spiro-OMeTAD. In experiments, we found the hole
mobility and photovoltaic parameters change little as we improved the film thickness by
increasing its concentration to 20 mgꢀmL-1. It can be reasonably attributed to its good solubility
and filming ability, which reduce the effect of charge transportation at interfaces. This further
declares the importance of filming ability for this kind of HTMs.
4. Summary
In conclusion, a novel series of triphenylamine modified azobenzenes 1a-d have been
synthesized and studied theoretically and experimentally. An intramolecular charge transfer(ICT)
from both triphenylamine or indoline units (donor) to the central azobenzene unit (acceptor) is
revealed, suggesting a D-A-D structure for these dyes. Both computational and experimental
results indicate that stronger electron donating ability of the donor may enhance the degree of ICT ,
and improve their HOMO energies. An electron donating groups at the donor such as methyl in 1b
and octyloxyl in 1c may reduce their electrochemical stability and optical energy gap, but improve
their light absorption ability and hole mobility. The intermolecular interaction also affects their
absorption ability. The strongest intermolecular interaction in 1b results in the best light
absorption ability in films. But as they are applied to perovskite solar cells, their solubility and
filming ability are significant for the photovoltaic performance of devices. Low solubility and
poor filming ability of 1a-d lead to an inadequate PCE of respect solar cells. The highest PCE was
obtained as 9.02 % with perovskite solar cells using 1b as the HTM. High performance is
expected as the solubility and filming ability of triphenylamine-based azobenzene are improved.
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Acknowledgements
This work was supported by National Natural Science Foundation of China (No.
21576195 and No. 21506151), Tianjin Science and Technology Support Program
(14TXGCCX001).
Appendix A. Supplementary data
Supplementary data related to this article can be found at
http://dx.doi.org/10.1016/j.dyepig.
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