Simple 3,6-disubstituted Carbazoles as Potential Hole Transport Materials: Photophysical, Electrochemical and Theoretical Studies

Article abstract of DOI:10.1111/php.13337

Developing effective and low-cost organic hole-transporting materials (HTMs) is crucial for the construction of high-performance perovskite solar cells (PSCs) and to promote their production in commercial ventures. In this context, we herein report the mo

Full text of DOI:10.1111/php.13337

PROF. AIRODY VASUDEVA ADHIKARI (Orcid ID : 0000-0002-0141-4454)
Article type : Research Article
Simple 3,6-disubstituted Carbazoles as Potential Hole-Transport Materials:
Photophysical, Electrochemical and Theoretical Studies
Kavya S. Keremane a, Rathnamala Rao b, and Airody Vasudeva Adhikari a,c,
*
a
Organic Materials Laboratory, Department of Chemistry, National Institute of Technology Karnataka, Surathkal,
Mangalore-575025, India
b Department of Electronics and Communication Engineering, National Institute of Technology Karnataka, Surathkal,
Mangalore-575025, India
c Yenepoya Research Centre, Yenepoya deemed to be University, Deralakatte, Mangalore-575 018, India
*Corresponding author e-mail: avachem@gmail.com ; avachem@yenepoya.edu.in (A. Vasudeva Adhikari)
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through the copyediting, typesetting, pagination and proofreading process, which may lead to
differences between this version and the Version of Record. Please cite this article as doi:
10.1111/PHP.13337
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ABSTRACT
Developing effective and low-cost organic hole-transporting materials (HTMs) is crucial
for the construction of high-performance perovskite solar cells (PSCs) and to promote their
production in commercial ventures. In this context, we herein report the molecular design,
synthesis, and characterization of two novel D-A-D-A-D architectured 9-(2-ethylhexyl)-9H-
carbazoles, connecting the mono/dimethoxy phenyl substituted cyanovinylene side arms
symmetrically at 3rd and 6th positions of the carbazole heterocycle (CZ1-2), as potential hole-
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transporting materials (HTMs). The current work highlights their structural, photophysical,
thermal, electrochemical, and theoretical investigations, including their structure-property
correlation studies. Evidently, the optical studies showcased their excellent fluorescence ability
due to their push-pull natured structure with extended π-conjugation. Further, in-depth
solvatochromic studies demonstrated their intramolecular charge-transfer (ICT) dominated
optoelectronic behavior, supported by various correlation studies. Also, the optical results revealed
that CZ1 and CZ2 display λabs and λemi in the order of 410-430 nm and 530-560 nm, respectively,
with a bandgap in the range of 2.5-2.6 eV. Finally, their quantum chemical simulations have
provided an insight into the predictions of their structural, molecular, electronic, and optical
parameters. Conclusively, the study furnishes a deeper understanding of the intricacies involved in
the structural modification of carbazole-based HTMs for achieving better performance.
Keywords: Hole-transport materials; donor-acceptor; carbazoles; electrooptical properties; low-
cost; density functional theory
INTRODUCTION
Designing new organic molecules as efficient charge extraction as well as transport
materials in the optoelectronic based devices remains a great challenge for many photovoltaic
technologies like electrophotographic photoreceptors, photovoltaics (OPVs), organic light-
emitting diodes (OLEDs), and photorefractive components.[1–4] In fact, HTM is considered as an
important active component in perovskite solar cells (PSCs), which is highly responsible for
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efficient photogenerated hole extraction mainly at the perovskite/HTM interface and avoiding
unwanted charge transfer processes, thereby bringing about an enhanced device-performance.[5,6]
Generally, the device architecture of PSCs comprises a light-absorbing (perovskite) layer
sandwiched between an electron transport material (ETM) and an HTM. The device may be of the
type either p-i-n or n-i-p planar hybrid structure.[7] To achieve a high device-efficiency, their well-
aligned energy levels with perovskite are critical: HOMO energy level must be less negative than
that of perovskite material for efficient hole injection, whereas the high-lying LUMO energy level
would ensure low electron affinity enabling sufficient electron blocking characteristics.[8,9] An
ideal hole-transporting material must possess enhanced hole mobility to minimize losses during
hole transport to the contact, as well as superior conductivity.[10] Further, it must be easily soluble
and it should not require strong polar solvents for dissolution, since such solvents may dissolve the
perovskite material of the cell also.[11] Furthermore, as perovskite is very sensitive to humidity and
atmospheric oxygen attack, the HTM layer should function as protection from the moist air and
inhibit the diffusion of external species.[12] So, very high thermal and photochemical stability, as
well as superior hydrophobicity, are desirable characteristics of HTM to resist degradation factors
and ensure long-standing durability. On the other hand, for ensuring the low-cost of PSC
production, an ideal HTM is to be easily affordable for a reasonable cost.[13, 14]
Consequently, a vast research interest has raised for the development of new organic
HTMs and also to investigate a better understanding of the correlation between the HTM and the
performance of PSC.[15] A significant number of organic HTMs have been introduced so far, to
achieve the requirements of an ideal candidate. The most commonly used up-to-date organic
HTMs, viz. 2,2',7,7'-tetrakis[N,N-di-(p-methoxyphenyl)amino]-9,9'-spirobifluorene (Spiro-
OMeTAD) and even further interesting macromolecule poly[bis(p-phenyl)2,4,6-
trimethylphenyl)amine] (PTTA) were shown to deliver the PCE of >20% in the device.[1,16]
However, their limitations such as tedious multi-step synthesis, complicated purification steps,
instability to humid air, and difficulty in controlling conductivity, stimulate further research to
investigate more cost-effective, stable, and dopant-free materials for easy commercialization.[17] In
this respect, many efforts have been devoted to exploring better alternatives to the existing
materials like spiro-OMeTAD.[18,19]
Recently, organic small molecules like branched methoxy diphenylamine substituted
fluorene derivatives were shown to yield the power conversion efficiency (PCE) up to 20%, but
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their synthetic methods are expensive.[20] Moreover, they need dopants like lithium salt of
bis(trifluoromethylsulfonyl)imide (LiTFSI) and 4-tert-butylpyridine (TBP) for adjusting the
conductivity.[21] As an important class of organic HTMs, carbazole-based compounds have been
widely investigated in optoelectronic applications owing to their excellent charge-transporting
properties apart from interesting photoconductive, photo-refractive, and light-emitting
behaviour.[14] Small molecule carbazoles possess many advantages as optoelectronic materials
mainly due to their predictable HOMO-LUMO energy levels, which can be easily tuned through
chemical substitution, and high thermal stability, rendering them suitable candidates for thermal
evaporation.[22] In addition, carbazole, being a cheap raw material, can easily be substituted
through simple routes to yield derivatives of high photochemical stability and also it has the ability
to quickly form relatively stable radical cations.[23] Furthermore, the insertion of bulkier branched
alkyl chain at N-heteroatom of the ring promotes significant tilting and packing distortion and
tends to reduce the aggregation as well as solubility problems causing improved open-circuit
voltage (VOC) in the fabricated device. Also, it was reported that the length of alkyl chains with
significant branching influences the HTM solubility in common organic solvents, and thereby
influences the film quality.[24] Additionally, the length of alkyl chains significantly affects charge
delocalization property, structural planarity, and the hole transport properties of HTM.[25]
Encouraged by this, in the present study, we report the molecular design, synthesis, and
characterization of two novel carbazole-based small molecules, CZ1-2 comprising
mono/dimethoxyphenyl and carbazole units as the electron-donating moieties, cyanovinylene
groups as the electron-accepting systems with extended π-conjugation. Figure 1 portrays the
chemical structures of CZ1-2. These molecules were designed with the possible application in
PSCs as HTMs. By adopting the simple synthetic strategy, these carbazole derivatives containing
methoxy end-capping groups were synthesized from inexpensive starting materials. Because of
their easy three-step synthesis as well as facile availability of starting materials, these newly
synthesized molecules are expected to be cost-effective HTMs and hence, they are attractive for
commercial prospects of PSCs. We used the well-known Knoevenagel condensation method
which offers an extremely simple route for their synthesis. Also, the product separated as a
precipitate in the reaction mixture can easily be purified using a simple recrystallization technique.
Scheme 1 displays the synthetic schemes adopted for the lab-preparation of the target molecules.
Structures of all the new intermediates as well as final compounds were determined by
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spectroscopy tools and elemental analysis. Their optical properties and FMO levels were evaluated
by UV-Visible spectroscopy and photoluminescence (PL) studies. Further, cyclic voltammetric
(CV) studies were performed to assess their electrochemical data along with parameters related to
thermodynamic driving factors of the processes involved during energy conversion. Furthermore,
the molecular geometry, electron distributions, energetic properties and absorption spectra of CZ1-
2 were theoretically studied by means of Density Functional Theory (DFT), and Time-Dependent
Density Functional Theory (TD-DFT) using Turbomole 7.2V software. Also, their structure-
property relationships were discussed.
<Figure 1>
MATERIALS AND METHODS
The commercially available starting materials such as carbazole, sodium hydride, 2-ethylhexyl
bromide,
phosphorus
oxychloride,
(3,5-dimethoxyphenyl)acetonitrile,
and
4-
methoxyphenylacetonitrile were procured from Sigma-Aldrich, Alfa aesar and Spectrochem
companies. All the solvents used in the reactions were of synthetic grade (Merck, Loba Chemie
and Spectrochem companies) and they were purified by further drying and distillation process. All
the reactions were performed under an inert atmosphere and their completion was monitored by
the thin-layer chromatography (TLC) technique. The impure products were separated using
column chromatography technique by using silica gel (100-200 and 230-400 mesh). The melting
points of synthesized molecules were recorded using Stuart SMP10 digital melting point
apparatus. The structural elucidation of all the intermediates and final molecules were done by
1
using spectral techniques such as FTIR, H NMR, 13C NMR, Mass (ESI) spectroscopy, and
1
elemental analysis. H NMR and 13C NMR spectra were recorded on a Bruker AM 400 MHz
spectrometer, in CDCl3 and DMSO-d6 solvents with tetramethylsilane (TMS) as an internal
standard. The LCMS and elemental analysis of the synthesized molecules CZ1-2 were obtained
from LC-MS6410Q (Agilent Technologies) and Flash EA1112 CHNS elemental analyser (Thermo
Scientific), respectively. FT-IR spectra were obtained using Bruker FTIR Alpha spectrometer. The
UV-Vis absorption spectra and photoluminescence spectra of CZ1-2 in chloroform solvent were
recorded at room temperature by using Analytik Jena SPECORD S 600 and Jasco FP 6200
spectrophotometers, respectively. Furthermore, in order to assess their experimental GSOP and
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ESOP values, the CV (cyclic voltammetry) measurements were performed in anhydrous
acetonitrile solution with 0.1M tetrabutylammonium hexafluorophosphate [TBA] [PF6] as a
supporting electrolyte at a scan rate of 100 mVs-1. The theoretical simulations, viz. density
functional theory (DFT) and time-dependent density functional theory (TD-DFT), were performed
for all the final molecules using the Turbomole V7.2 software package.[61-64]
Synthetic methods
The required intermediates and final target molecules were synthesized using standard
synthetic methods and their detailed procedures along with structural characterization data are
given below.
Synthesis of 9-(2-ethylhexyl)-9H-carbazole (1). A mixture of carbazole (0.1 g, 0.598 mmol), NaH
(0.07 g, 2.99 mmol) was dissolved in a minimum amount of DMF (5 mL) and stirred at room
temperature for half an hour under an inert atmosphere. Later, 2-ethylhexyl bromide (0.34 g, 1.79
mmol) was added into the reaction mixture and continued stirring at room temperature for 12 h.
The reaction progress was monitored using thin-layer chromatography (TLC). After completion of
the reaction, the reaction mixture was cooled and poured into crushed ice (100 mL) and
neutralization was done using a saturated solution of ammonium chloride. The precipitate formed
was filtered, washed with ice-cold water and finally, it was recrystallized using ethanol to get
colourless liquid as a product. Yield: 92%.
1H NMR (400 MHz, DMSO-d6, δ ppm): 8.47 (s, 1H), 8.00-7.98 (t, 3H), 7.61 (s, 1H), 7.13-7.11
(d, 3H), 4.07-4.05 (d, 2H), 1.73 (s, 1H), 1.24-1.11 (m, 8H), 0.86-0.84 (t, 6H). Anal. Calcd. for
C20H25N: C, 85.97; H, 9.02; N, 5.01; and found C, 85.89; H, 8.99; N, 5.06. FT-IR (ATR), υ cm-1:
3063, 2956 (C-H), 1589, 1533, 1482 (C=C), 1191 (C-N).
Synthesis of 9-(2-ethylhexyl)-9H-carbazole-3,6-dicarbaldehyde (2). In a cleaned two neck round
bottom flask, freshly distilled DMF (1.22 mL, 15.75 mmol) was taken and cooled it at –3 to 4 oC.
Then, phosphorous oxychloride, POCl3 (1.67 mL, 17.90 mmol) was added drop-wise with
constant stirring at the same temperature under argon atmosphere to obtain a glassy white salt. To
this mixture, 9-(2-ethylhexyl)-9H-carbazole (1, 0.2 g, 0.71 mmol) dissolved in dichloroethane (2
mL) was added. The reaction mixture was refluxed at 95 ºC for 12 h. After completion of the
reaction, the reaction mass was cooled to room temperature and poured into 100 mL crushed ice
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and subsequently basified by using 5 M NaOH solution. The product formed was extracted with
ethyl acetate (50 mL x 3) and the organic layer was dried over sodium sulphate and evaporated
under reduced pressure. The impure residue was later purified by column chromatography on
silica gel (100-200 mesh) to yield a light brown solid (2). Yield: 69%. Melting point: 104-106 °C.
1H NMR (400 MHz, DMSO-d6, δ ppm): 10.09 (s, 2H), 8.89-8.88 (d, 2H), 8.06-8.04 (q, 2H), 7.82-
7.81 (d, 2H), 4.39-4.37 (d, 2H), 1.32-1.29 (d, 1H), 1.14-1.11 (m, 8H), 0.85-0.82 (t, 3H), 0.75-0.72
(t, 3H). Anal. Calcd. for C22H25NO2: C, 78.77; H, 7.51; N, 4.18; and found C, 78.02; H, 7.49; N,
4.13. FT-IR (ATR), υ cm-1: 3051, 2956 (C-H), 1681 (C=O), 1587, 1482 (C=C), 1193 (C-N).
Synthesis of (2Z, 2´Z)-3, 3´-(9-(2-ethylhexyl)-9H-carbazole-3,6-diyl)bis(2-(3,5-dimethoxy
phenyl)acrylonitrile) (CZ1). A mixture of 9-(2-ethylhexyl)-9H-carbazole-3,6-dicarbaldehyde (2,
0.05 g, 0.14 mmol) and potassium tert-butoxide (0.27 g, 2.47 mmol) was dissolved in dry
methanol (10 mL) and stirred at room temperature under an argon atmosphere for 15 min. Later,
(3,5-dimethoxy phenyl) acetonitrile (0.1 g, 0.62 mmol) was added while stirring and the reaction
mixture was refluxed for 6 h. The precipitate formed was filtered, washed with absolute methanol
and finally, it was recrystallized from chloroform to obtain bright yellow color solid. Yield: 84%.
Melting point: 192-194 °C.
1H NMR (400 MHz, DMSO-d6, δ ppm): 8.72 (s, 2H), 8.18-8.15 (t, 4H), 7.80-7.78 (d, 2H), 7.39
(s, 2H), 7.31-7.28 (d, 2H), 7.10-7.08 (d, 2H), 4.37-4.36 (d, 2H), 3.89 (s, 12H), 1.40-1.31 (m, 9H),
1.30-1.26 (t, 6H). 13C NMR (400 MHz, DMSO-d6, δ ppm): 148.66, 148.29, 141.32, 140.28,
126.92, 126.27, 124.80, 122.06, 121.49, 118.11, 117.76, 110.31, 108.83, 107.55, 107.01, 76.32,
76.20, 76.00, 75.68, 55.08, 55.03, 46.79, 38.49, 29.98, 28.68, 27.78, 23.39, 21.99, 12.99, 9.87.
Anal. Calcd. for C42H43N3O4: C, 77.16; H, 6.63; N, 6.43; and found C, 77.54; H, 6.89; N, 6.41.
FT-IR (ATR), υ cm-1: 2921 (C-H), 2207 (C≡N), 1592, 1513, 1472 (C=C), 1212 (C-N). Mass
(m/z): 653.81; Obtained (M+Na)+: 676.31.
Synthesis of (2Z, 2´Z)-3, 3´-(9-(2-ethylhexyl)-9H-carbazole-3,6-diyl)bis(2-(4-methoxy phenyl)
acrylonitrile) (CZ2). A mixture of 9-(2-ethylhexyl)-9H-carbazole-3,6-dicarbaldehyde (2, 0.05 g,
0.14 mmol) and potassium tert-butoxide (0.27 g, 2.47 mmol) was dissolved in dry methanol (10
mL) and stirred at room temperature under an argon atmosphere for 15 min. Later, (4-
methoxyphenyl) acetonitrile (0.09 g, 0.62 mmol) was added while stirring and the reaction mixture
was refluxed for 6 h. The precipitate formed was filtered, washed with absolute methanol and
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finally, it was recrystallized using chloroform to obtain pale orange-yellow color solid. Yield:
79%. Melting point: 178-180 °C.
1H NMR (400 MHz, DMSO-d6, δ ppm): 8.72 (s, 2H), 8.19-8.15 (m, 5H), 7.81-7.79 (d, 3H), 7.39
(s, 2H), 7.31-7.28 (d, 2H), 7.11-7.09 (d, 2H), 4.38-4.36 (d, 2H), 3.89 (s, 3H), 3.82 (s, 3H), 2.04-
2.03 (d, 1H), 1.18-1.31 (m, 8H), 0.90-0.86 (t, 3H), 0.80-0.77 (t, 3H). 13C NMR (400 MHz,
DMSO-d6, δ ppm): 160.06, 142.27, 141.08, 127.61, 127.13, 127.08, 125.89, 123.06, 122.55,
119.14, 114.43, 109.81, 107.84, 77.35, 77.24, 77.04, 76.72, 55.46, 47.80, 39.51, 31.01, 28.82,
24.42, 23.02, 14.03, 10.90. Anal. Calcd. for C40H39N3O2: C, 80.91; H, 6.62; N, 7.08; and found C,
80.76; H, 6.70; N, 7.10. FT-IR (ATR), υ cm-1: 3018 (C-H), 2209 (C≡N), 1605, 1510, 1488
(C=C), 1214 (C-N). Mass (m/z): 593.30; Obtained: 593
RESULTS AND DISCUSSION
Synthesis and characterization
The synthetic methods of two new products, viz. 3,3´-(9-(2-ethylhexyl)-9H-carbazole-3,6-
diyl)bis(2-(mono/di-methoxy substituted phenyl) acrylonitrile) (CZ1-2) are portrayed in Scheme 1.
The requisite intermediate 9-(2-ethylhexyl)-9H-carbazole (1) was obtained from 9H-carbazole by
treating it with 2-ethylhexyl bromide in presence of a strong base, i.e. sodium hydride. This was
converted to the key precursor 9-(2-ethylhexyl)-9H-carbazole-3,6-dicarbaldehyde (2) through
Vilsmeier-Hack reaction in good yield.
Finally, compound 2 on reaction with (3,5-
dimethoxyphenyl)acetonitrile and (4-methoxyphenyl) acetonitrile by following the Knoevenagel
condensation protocol, yielded CZ1 and CZ2 respectively. All the synthesized compounds were
purified further by using a simple recrystallization technique from chloroform solvent. The
molecular structures of these target compounds and their intermediates were established by using
different spectroscopy tools. Their spectra are displayed in S1-S11 of ESI.
<Scheme 1>
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Theoretical studies
To optimize the ground state geometry of molecules and to obtain an insight into the
electronic distribution behavior of compounds CZ1-2 quantum chemical calculations / Density
Functional Theory (DFT) simulations were carried out by employing the Turbomole 7.2v software
package. In fact, these computational studies play a key role in designing novel semiconducting
materials with proper energy levels as well as good charge mobility. In the present study, we have
optimized the ground-state geometries of the newly synthesized CZ1-2 in a gas phase using a semi-
empirical AM1 basis with MOPAC in Tmolex.[26] The further optimization of aforesaid geometries
was done with the help of software-generated C1 point group symmetry via B3LYP program and
here, the basic set def-TZVPP was used for all the calculations.[27] The obtained electronic cloud
distributions in the frontier molecular orbitals (FMOs) levels of CZ1-2 are shown in Figures 2 and
S12 and the related data are displayed in Table 1. Here, the LUMO+1, and HOMO-1 levels
correspond to second-lowest unoccupied molecular orbitals and second-highest occupied
molecular orbitals, respectively. The effective charge separation between the ground state and
excited state of the molecule can be clearly observed in 3-D visions of the molecules.
It has been well-documented that the electron-donating ability of carbazole donor has a
greater impact on energy levels and spatial charge distribution of the HOMOs, which further
influences the interfacial charge transport. In addition, FMOs are the most widely used to
demonstrate the charge transport behaviour of the molecules, as more delocalized FMOs can
undergo quicker charge transport by increasing the electron coupling between adjoining molecules
and reducing nuclear reorganization energy. As seen in Fig. 2, in case of HOMO levels, the total
electron densities of CZ1-2 are primarily distributed with the donor skeleton, which clearly
indicates the electron-donating ability of 9-(2-ethylhexyl)-9H-carbazole. In addition to this,
electron density also slightly distributed over pendant groups, i.e. methoxy phenyl groups.
Nevertheless, the electron density distribution in LUMO is mainly localized over electron-
withdrawing cyanovinylene moiety. Further, it can be observed that as we move from HOMO to
HOMO-1, the electron density is slightly away from donor carbazole to methoxyphenyl groups.
Interestingly, the complete shift of electron density towards electron-withdrawing cyanovinylene
group has been observed in the LUMO level. The major difference between CZ1 and CZ2, lies in
their substituent groups, as CZ1 has an additional methoxy group suggesting stronger electron-
donating capability. The analysis of FMO shows that the excitation of an electron from lower
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HOMO to LUMO level results in electron density to surge from electron-donating carbazole and
methoxyphenyl units to electron-withdrawing cyanovinylene group, which ultimately influence
the charge carrier transport inside the molecule and can create effective charge separation at the
donor-acceptor interface. Besides, CZ1 has higher HOMO energy as compare to CZ2 due to the
existence of additional electron-pushing methoxy functionality. Structurally, the location and
number of methoxy groups significantly affect the electron-donating ability, thereby increasing the
donor strength in CZ1; here, the presence of two electron-releasing methoxy groups at the meta-
positions with respect to the core (9-(2-ethylhexyl) carbazole-3,6-diyl)diacrylonitrile moiety could
facilitate the molecule to achieve higher donor-acceptor interactions than CZ2. Hence, CZ1 can be
considered as better HTM than CZ2 due to the enhanced electron-releasing effect of methoxy
groups.
In PSCs, the occurrence of appropriate energy levels between the perovskite and the HTM
is absolutely necessary for facilitating effective interfacial charge separation and thereby
maximizing the open-circuit voltage (VOC) of the devices. A deeper HOMO level of the molecules
usually indicates the larger VOC, since the voltage value is dependent on the difference between
HOMO levels of the HTM and quasi-Fermi level of the TiO2 semiconductor. The theoretical
HOMO energy levels obtained for CZ1 and CZ2 are –5.48, and –5.33 eV, respectively confirming
these compounds can undergo a quick ground state regeneration process. Generally, for promising
HTMs, the value of the LUMO levels must be greater than the conduction band of the perovskite
(–3.93 eV for CH3NH3PbI3) in order to block the recombination of electrons and holes between
perovskite absorber and the metal electrode.[28] The theoretical LUMO energy levels obtained for
CZ1 and CZ2 are –2.01 and –1.92 eV, respectively, demonstrating their fast electron injection
ability. When compared to the conduction edge of most commonly used perovskite CH3NH3PbI3,
the LUMOs of CZ1 and CZ2 give rise to a large energy barrier of 1.92-2.01 eV, indicating that the
unwanted electron-hole recombination process could be effectively prohibited. From the results, it
is clear that both the molecules possess a capability of photo-induced electron transfer from the
HOMO-LUMO excitations. The theoretical bandgaps obtained for CZ1 and CZ2 are 3.464 and
3.408 eV respectively, which may be accredited to the different donating ability in the molecules.
Although it is difficult to obtain accurate FMO energies by DFT studies, the changing tendencies
between theoretical and experimentally obtained energy values are consistent.
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From the energy level alignments and frontier orbital distribution we emphasize that the
carbazole donor with branched alkyl chain has been thoughtfully tailored as the side arms of the
molecules. On comparing the HOMO levels of the CZ1-2 with the reported HOMO energy level of
spiro-OMeTAD (–5.08 eV), our new molecules show comparatively stabilized HOMOs, and
hence better hole injection and enhanced VOC values can be expected.[3] Conclusively, the FMO
levels of the CZ1-2 can promise a faster charge-transport between the electron-donor and acceptor
units and the well-organized interfacial injection of electrons from the excited level of the HTM
into the conduction band (CB) of the semiconductor. Further, the electron distribution pattern in
the FMO levels clearly indicates that the molecules CZ1-2 possess good hole-transporting
properties, and hence they may turn out to be promising candidates of HTMs.
<Figure 2>
<Table 1>
In order to gain insights into the excitation properties as well as electronic transition, the
light absorption properties were explored using Time-Dependent Density Functional Theory (TD-
DFT) in the presence of time-dependent perturbations. According to adiabatic approximation, TD-
DFT temporal nonlocality is neglected, which postulates at any point of time, the exchange-
correlation (xc) factor depends exclusively on instantaneous density.[29,30] Consequently, it can be
utilized to the xc functional derived for ground-state DFT, i.e. BP (Beck-Perdew) and hybrid
functional (B3LYP). In TD-DFT calculations, the basis set used for the calculations generally
decides the accuracy of assimilated results.[31] Figure S13 (see Supporting Information) shows the
simulated absorption spectra of CZ1-2, obtained by employing B3LYP functional and def-TZVP
basis set.[32] The theoretically calculated absorption maxima (λmax) of the investigated molecules
CZ1-2 are summarized in Table 1. The energy values related to long-range charge-transfer states
can be predicted precisely using TD-DFT calculations. Further, as shown in Figure S13 the
investigated molecules CZ1-2 show two distinct peaks, corresponding to mixed π-π* transition and
charge-transfer process within the molecules in the visible light region. In all the molecules, there
occurs a strong intra-molecule electronic excitation, owing to the HOMO→LUMO and
HOMO→LUMO+1 transition. Here, the excitations at the longer wavelengths are due to the
electronic transition from the HOMO to LUMO+1 level. Accordingly, the lower-energy
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excitations are ascribed to intramolecular charge-transfer (ICT) transitions from CZ1 and CZ2 to
TiO2. Furthermore, λmax of CZ1 (420 nm) shows a noticeable redshift when compared to that of
CZ2 (400 nm). This may be ascribed to the higher electron-donating strength, as it contains two
pendent methoxy groups attached to carbazole via cyanovinylene unit. In addition, the oscillator
strength (ƒ) of CZ1 was found to be larger than that of CZ2, which corresponds to the appearance
of a larger experimental absorption coefficient or stronger fluorescence intensity. Accordingly,
CZ1 has a higher absorption intensity than the molecule CZ2. These exactitudes of TD-DFT results
demonstrate that the functional and basis set selected for the studies are well in agreement with
experimentally obtained results. Theoretically simulated IR-absorption spectra of CZ1-2 are
displayed in Figure S14. As expected, both the simulated and experimentally obtained results are
well in accordance with each other.
The distribution of total charge density of an organic molecule at any point in space around
the molecule produces a net electrostatic effect. This phenomenon is known as molecular
electrostatic potential (ESP). Computationally, molecular ESP simulations were carried out in
order to investigate the charge populations of CZ1-2 and the corresponding plotted ESP maps are
shown in Fig.3. The cavity boundaries in the molecules resemble the density isosurface in the ESP
plot, which is coupled with total charge distribution including electro-negativity, dipole moment as
well as site of chemical reactivity of the molecule.[33–35] The electrostatic potential values are
denoted by different colors and they increase in the order, blue > green > yellow > orange > red.
The electron-rich and electron-deficient regions are represented by red and blue colors on the plot,
which induces positive and negative charges on the cavity surface, further giving rise to local
electric fields inside the cavity. From Fig.3, it is evident that in molecules CZ1-2, the total electron
density is mainly concentrated on cyanovinylene, and pendent methoxy groups. Thus, the ESP
plots clearly demonstrate the movement of electron density from the donor moiety to the acceptor
units, thereby favoring the useful electron transfer in both CZ1-2. These results also reveal that
introduction of different end-capping methoxy groups lightly affects the stability of the molecules.
Further, Laplacian of electron density and ground-state total density of the molecules CZ1-2 are
shown in Figures S15-S16. The structure of the molecule is a mono/dimethoxy phenyl substituted
cyanovinylene side arms symmetrically placed at 3rd and 6th positions of a 9-(2-ethylhexyl)-9H-
carbazole, located at the centre. Studies were made to analyse the charge density and Laplacian of
charge density of the molecule, with a little much importance given to the central ring. From these
studies, we can figure out the position of bond critical point (BCP), which provides the knowledge
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of electronegative nature of atoms and its electron density distribution based on bonding nature.[36]
Laplacian of total density is considered as a useful tool for characterizing bonding interactions
inside the molecules. In-depth studies of these plots also provide a greater understanding of the
electronic properties bounded inside atoms and molecules, delocalization nature, and allow for
useful predictions of chemical reactivity.[25]
<Figure 3>
According to quantum mechanics, in a system, the energy levels per unit energy increment
correspond to the density of states (DOS). It determines the number of states per specific interval
of energy at each energy level, and are free to be occupied.[37-38] In a simpler way, the higher the
DOS corresponds to the availability of many states for occupation at a particular energy level,
whereas zero DOS indicates the availability of no free states. In general, the DOS is an average
over the time and space domains occupied by the system, and it describes the contribution of
different orbitals over a different range of energy. The DOS also describes the favorable charge-
transfer from a molecule to the TiO2 semiconductor, which further builds the dipolar fields at the
interface of TiO2/Material to tune the band energy levels.[39] The smaller the bandgap of the
material facilitates the faster electron transport from it and thus, increasing the driving force for
rapid charge injection at the interface. The DOS plots of CZ1-2 are depicted in Figures S17-S18.
Here, the figures indicate the contribution of different orbitals as energy increases. It is observed
from the figure that as the energy increases, the contribution of p-orbitals predominates over the s-
orbitals. It can also be seen that the p-orbital contribution of O and N atoms facilitates the binding
of dye with TiO2. Both the new molecules show a similar pattern of DOS maps indicating that
they meet the prerequisites of a good HTM. Further, DOS spectra of dyes did not show a
prominent energy gap but the obtained results are well in accordance with literature reports.[40]
Both the molecules exhibit significant charge-transfer from donor to acceptor unit confirming that
they meet the basic requirements of an ideal HTM.
Photophysical properties
The UV-vis absorption spectra of newly synthesized CZ1-2 were obtained in chloroform
(CHCl3) solution with a concentration of 10-5 M at room temperature. Their generated spectra are
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displayed in Fig.4a and the corresponding data are tabulated in Table 2. These spectra exhibit two
distinct absorption peaks, of which the lower maximum represents π-π* electronic excitation
occurring in the range of 300-350 nm, whereas the higher maximum, appearing at 400-430 nm
corresponds to the electronic transition from HOMO to LUMO and HOMO-1 to LUMO. Both the
molecules show absorption bands comparable to that of spiro-OMeTAD (388-390 nm),[41-43] and
hence it can be inferred that their optical properties are matching with that of commercially
available HTM.[44,45] As shown in Fig.4a, both CZ1-2 exhibit similar type of absorption profiles and
the observed major λmax of CZ1 is 428 nm, and that of CZ2 is 417 nm. Here, the peaks are due to
mixed intra-molecular charge transfer (ICT) / π-π*transition of the compounds, which was further
confirmed by TD-DFT calculations.[46] From the results, we infer that CZ1 shows a bathochromic
shift when compared to the spectral data of CZ2, which may be due to the presence of an
additional electron-donating pendent group that further extends the conjugation of CZ1. It is also
well-known that the number and position of the methoxy groups make a significant influence on
the geometrical structure and subsequently on the electronic properties of the molecules, which
means that the presence of more pendant methoxy groups augments the electron-donating ability
of the molecule, and thus providing superior performance.[47] Also, its position significantly affects
the planarity and hence, geometries of such molecules. In our molecules, positioning of the two
methoxy groups at the meta-positions will cause a lower steric hindrance than positioning them at
the ortho-ortho/ortho-para/ortho-meta positions.[48,49] For instance, in CZ1, the two methoxy
groups are placed at the meta-positions showing smaller conformational changes in the excited
state. Hence, we have chosen (3,5-dimethoxyphenyl) acetonitrile as dual pendant group for CZ1
rather than (3,4-dimethoxyphenyl)acetonitrile or (2,4-dimethoxyphenyl)acetonitrile moieties.
Further, from the literature, it is evident that the position of methoxy groups can control the
oxidation potential of the materials, and hence their electronic properties are affected by the
position of methoxy substituents.[50] The change in the substitution from para to meta results in
increased oxidation potential of the material due to the less electron-donating nature of m-OMe
groups when compared to that of p-OMe group. Also, the meta-substituted derivatives could
decrease the HOMO energy level of the HTM, and thus may contribute to an increase in open-
circuit voltage and fill factor of the resulting PSCs.[51]
Further, the molar extinction coefficients of molecules CZ1-2 were determined according to
Lambert-Beer law (the detailed procedure is given in the ESI). The molar extinction coefficients
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of CZ1-2 were found to be 21,300 M-1cm-1 (CZ1) and 20,400 M-1cm-1 (CZ2) indicating their good
light-harvesting capability. Among both, CZ1 shows a higher molar extinction coefficient than
CZ2 which may be owing to the stronger electron-donating ability of dual methoxy pendent units
towards carbazole scaffold. So, from the results, we can conclude that CZ1 shows an improved
light-harvesting ability and hence it can be considered as a better HTM.
The fluorescence emission spectra of molecules were measured in 10-5 M CHCl3 solution
by exciting at their respective absorption maxima. Figure 4b portrays the normalized emission
spectra of CZ1-2 and their resulting spectral parameters are tabulated in Table 2. Both the
molecules exhibit a characteristic single emission band in the region of 530-560 nm. The molecule
CZ1 displays λemi of 557 nm, which is slightly more red-shifted than that of CZ2 (536 nm), which
may be due to the strong electron-donating ability of dimethoxyphenyl group. Further, we
calculated Stokes shift values as well as the optical bandgap from the experimentally obtained
UV-PL spectral data. Indeed, the Stokes shift plays a key role in deciding whether the synthesized
HTMs would undergo any geometrical change on excitation. The intersection of absorption and
emission spectra of CZ1-2 is shown in Figure S19. The calculated bandgaps are 2.52 eV (CZ1), and
2.58 eV (CZ2), whereas Stoke shifts are in the order CZ2 (5323cm-1) < CZ1 (5410cm-1). Here,
CZ1 shows a higher Stokes shift value with appropriate bandgap and thus, it plays a pivotal role in
the light-harvesting phenomenon and it is considered to be better hole-transporting material.
<Figure 4>
<Table 2>
Further, the solvatochromic behavior of CZ1-2 in solvents of different polarities was
investigated in order to understand the deeper insight of intramolecular charge transfer (ICT)
phenomenon inside the molecules.[52] The UV-vis absorption and fluorescence emission spectra of
synthesized compounds were measured in five different solvents with varying solvent polarities
such as toluene, THF, DCM, DMF, and acetonitrile medium at a concentration of 10-5 M at room
temperature. The solvatochromic absorption spectra of CZ1 and CZ2 in five solvents having
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different polarity ratio starting from highly non-polar toluene to extremely polar DMF and
acetonitrile at micro-molar concentration are manifested in Figures 5a-b. It has been noticed that
there is noticeable variation in the absorption spectra of the molecules with increasing polarity,
indicating an effective dipole moment and non-polar ground state associated with it. Yet, all the
molecules exhibit a monotonic bathochromic shift with the increasing polarity of solvents. As seen
in the spectra, the molecule CZ1 (429 nm) shows an enhanced positive solvatochromic absorption
in all the solvents, particularly in the highest polar acetonitrile, when compared to CZ2 (420 nm).
In addition, from Fig.5, it is clear that both CZ1-2 display minor hypsochromic shift in less polar
and higher vibrational-resolution solvents such as toluene, THF, DCM when compared to CHCl3.
The largest blue-shift is observed for THF and toluene (4 and 3 nm respectively for CZ1, and 7
and 4 nm respectively for CZ2). Such behavior is characteristic of the donor-acceptor conjugated
systems.[53] Further, they display a slight bathochromic shift in polar solvents such as DMF and
acetonitrile when compared to CHCl3. This unusual redshift in polar solvents arises from the
effective solvation of the molecules, induced by the instant stabilization of polarizable electrons
during the electronic excitation.[54] This observation clearly indicates that with increasing the
solvent polarity, the energy of the ground state is lowered less than that of the excited state.[55]
From the results, it is worth noting that there exists a prominent intramolecular charge transfer
from the donor moiety towards the electron acceptor unit in the excited state of the molecule,
demonstrating CZ1-2 exhibit a polar excited state with a largely associated dipole moment.
<Figure 5>
Figure 6a-b displays the fluorescence emission spectra of CZ1-2 in five solvents owing to
different polarity ratio. With increasing solvent polarity, there is a considerable change in the
emission spectra of all the molecules at the same conditions. Interestingly, the molecule CZ1 (565
nm) shows a higher emission intensity than that of CZ2 (542 nm), which can be pragmatic through
the naked eyes with the colours changing from dark yellow (CZ1) to pale yellow (CZ2) as shown
in Fig.6c. The increased emission in CZ1 is likely due to the presence of an additional methoxy
group which provides an additional donating effect towards carbazole donor that further boosts the
conjugation to a greater extent. The Stokes shift values of CZ1-2 in all the five solvents were
calculated and the corresponding results are tabulated in Table 3. Hence, the results demonstrate
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that both the compounds display satisfactory positive solvatochromism and can be used as an
efficient light-harvesting source in perovskite solar cells. Figure 6d shows the emission behavior
of CZ1-2 solutions under 360 nm UV light.
<Figure 6>
<Table 3>
The solvatochromism can be well-explained by establishing the correlation between the
emission nature of CZ1-2 with solvent variables by using Lippert-Mataga plots (Fig.7) and graphs
of ET (30) index of solvent vs. Stokes shift values (Figure S20). Evidently, these two graphs
explain progressive linear relationships, which demonstrate the presence of a single excited state
with a little deviation in Stokes shift values of these HTMs. Moreover, the observed large Stokes
shift values, high positive slope, and linear correlation coefficient (R) of CZ1 clearly indicate the
comparably higher dipole moment of the excited state than that of the ground state. Hence, these
results confirm that the CZ1 exhibits stronger intramolecular charge transfer from donor to
acceptor moieties than CZ2.
<Figure 7>
Thermal properties
The thermal properties of the compounds CZ1-2 were investigated by thermogravimetric
analysis (TGA). The samples were heated at the uniform heating rate of 10 °C min-1 under an
inert (N2) atmosphere. The TGA plots of CZ1-2 are manifested in Fig.8. Both the synthesized
molecules exhibit satisfactory thermal stability with high reasonably thermal decomposition (Td)
temperatures at 245 °C (CZ1), and 236 °C (CZ2). The quantity of carbonized residue was found to
be less than 1% at 800 °C in a nitrogen atmosphere, due to their moderate aromatic content
(Figure S21). The CZ1 shows higher thermal stability than CZ2, making it more relevant material
for electronic/photonic applications. Thus, because of their promising thermal property, their
application as HTM would impart enhanced stability to the fabricated devices.
<Figure 8>
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Electrochemical characterization
The feasibility of thermodynamically allowed transfer of electrons from the excited state of
any HTM to the conduction band (CB) of semiconductor and its regeneration by blocking the
charge recombination between oxidized materials and photo-injected electrons in CB of TiO2 can
be effectively studied using cyclic voltammetry studies (CV).[56,57] The CV studies are performed
for CZ1-2 using a conventional three-electrode system, in which platinum disc, glassy carbon, and
Ag/AgCl serve as passive, working, and reference electrodes, respectively.[58] The potentials are
measured at a scan rate of 100 mVs-1 using supporting electrolyte, tetra n-butylammonium
hexafluorophosphate in acetonitrile solution in an ambient atmosphere. The generated
voltammograms of CZ1-2 are depicted in Figure S22. From the experimental CV data, we have
calculated ground state oxidation potential (GSOP)/HOMO energy levels from the onset oxidation
potential of the oxidation peak using equation (1) and the values obtained for both the molecules
are summarized in Table 4. Accordingly, they were found to be –5.37eV (CZ1), and –5.39eV
(CZ2), respectively.
표푥푑
(1)
퐻푂푀푂 = ― [퐸
+4.7 푒푉]
표푛푠푒푡
퐿푈푀푂 = 퐸0 ― 0 + 퐻푂푀푂
(2)
The experimental GSOP/HOMO values of CZ1-2 are in between that of CH3NH3PbI3 (–
5.43 eV) and Au counter electrode (–5.1 eV), suggesting that both the molecules can provide a
driving force for their quick ground state regeneration. Further, the exited state oxidation potential
(ESOP)/LUMO values were calculated from their GSOP values, and optical band gaps E0-0 and
respective values in volts (V) against NHE were converted into electron volt (eV) using the
equation (2). Table 4 depicts the estimated ESOP values of CZ1-2 and Fig.9 shows the pictorial
representation of their energy level diagram. As seen from the table, their ESOPs are –
2.85eV(CZ1), and –2.81eV(CZ2), which are greater than the conduction band edge of
CH3NH3PbI3(–3.93eV), and hence they can block electron transport from perovskite to the Au
counter electrode easily.[59,60] Thus, both the molecules satisfy the stringent requirement which is
mandatory for the affirmative transition of charges throughout the photo-electronic conversion
cycle. The generated radical cation of the carbazole moiety is stabilized due to the di-substitution
in the 3,6-positions of the molecule which ultimately leads to the first quasi-reversible system.
Based on these results, we can conclude that both CZ1-2 are potential hole-transporting candidates
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in CH3NH3PbI3 based solar cells. Here, CZ1 has more appropriate energy levels than that of CZ2,
which mediates holes from CH3NH3PbI3 perovskite to Au counter electrode due to the higher
donating capacity of dimethoxy groups on CZ1. Also, the introduction of pendant groups improves
the oxidation potential of the compounds due to the inductive effect caused by the terminal
methoxy groups. Thus, CZ1-2 satisfies the basic requirements of potential HTM, which further
facilitate the affirmative transition of charges throughout the photo-electronic conversion cycle.
<Figure 9>
<Table 4>
CONCLUSION
In summary, we designed two new push-pull type 9-(2-ethylhexyl)-9H-carbazole-based
CZ1-2 bearing mono/dimethoxyphenyl substituted cyanovinylene units in its 3rd and 6th positions
symmetrically, as potential HTMs for PSC application. The simplicity of their synthesis from
available commercial products and easy purification steps offer a cost-effective and scalable route.
Their photophysical, thermal as well as electrochemical properties were probed systematically and
the effect of structural modification on their properties has been discussed in-depth. Our results
demonstrate that their absorption and emission are in the range of 410-430 nm and 530-560 nm,
respectively with a bandgap of 2.5-2.6 eV, nearly matching to that of perovskite. Further, they
have been comprehensively evaluated using quantum chemistry simulations on the basis of DFT
and TD-DFT studies and the results strongly support their high charge transfer efficiency. The
results clearly indicate that CZ1-2 meet the basic prerequisites of an ideal HTM making them
highly suitable for PSC application. Thus, our outcomes suggest new design strategies for low-
cost and effective hole transport materials, and also provide the guiding principle on the structural
design of small organic molecules.
Acknowledgements
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The authors are grateful to the Director, NITK, Surathkal, India, for providing necessary
laboratory facilities.
Declaration of Competing Interest
The authors declare that they do not have known competing financial interests or personal
relationships that could have appeared to affect the work reported in this paper.
SUPPORTING INFORMATION
Additional supporting information may be found online in the Supporting Information section at
the end of the article:
Figure S1. FT-IR spectrum of intermediate 2
Figure S2. FT-IR spectrum of CZ1
Figure S3. FT-IR spectrum of CZ2
Figure S4. 1H-NMR spectrum of intermediate 1 recorded in DMSO-d6
Figure S5. 1H-NMR spectrum of intermediate 2 recorded in DMSO-d6
Figure S6. 1H-NMR spectrum of CZ1 recorded in DMSO-d6
Figure S7. 1H-NMR spectrum of CZ2 recorded in DMSO-d6
Figure S8. 13C-NMR spectrum of CZ1 recorded in DMSO-d6
Figure S9. 13C-NMR spectrum of CZ2 recorded in DMSO-d6
Figure S10. LCMS spectrum of CZ1
Figure S11. LCMS spectrum of CZ2
Figure S12. Optimized geometries and frontier molecular orbitals (HOMO and LUMO) of CZ1-2
Figure S13. Theoretical and experimental electronic excitation spectra of CZ1 and CZ2
Figure S14. Theoretical and experimental FT-IR spectra of CZ1 and CZ2
Figure S15. Theoretically obtained ground state total density plots of CZ1 and CZ2
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Figure S16. Theoretically obtained Laplacian of total density plots for CZ1 and CZ2
Figure S17. DOS plot of CZ1
Figure S18. DOS plot of CZ2
Figure S19. Intersection of UV-Vis absorption and fluorescence emission spectra of CZ1-2
recorded in 10-5 M CHCl3 solution under ambient atmosphere
Figure S20. Plots showing ET (30) index of solvent vs. Stokes shift of (a) CZ1 and (b) CZ2
Figure S21. Thermograms of CZ1-2 obtained under N2 atmosphere at 10 °C min-1 heating rate
Figure S22. CV traces of CZ1 and CZ2
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FIGURE CAPTIONS
Scheme 1. Synthetic routes for the compounds CZ1 and CZ2: (i) 2-Ethylhexyl bromide, NaH,
DMF, RT, 12 hours (ii) POCl3, DMF, 95 °C, 12 hours (iii) (3,5-Dimethoxyphenyl) acetonitrile,
potassium tert-butoxide, methanol, 60 °C, 6 hours (iv) (4-Methoxyphenyl) acetonitrile, potassium
tert-butoxide, methanol, 60 °C, 6 hours
Figure 1. Chemical structures of CZ1 and CZ2
Figure 2. The electronic cloud distributions in the FMO levels of CZ1 and CZ2
Figure 3. ESP mapping on isodensity surface of molecules (a) CZ1, (b) CZ2
Figure 4. (a) Normalized UV-Vis absorption and (b) fluorescence emission spectra of CZ1-2
recorded in 10-5 M CHCl3 solution under ambient atmosphere
Figure 5. Normalized UV-vis absorption spectra of (a) CZ1, and (b) CZ2 in solvents with varying
polarity index
Figure 6. Normalized emission spectra of (a) CZ1, (b) CZ2 in solvents with varying polarity
index, (c) Emission under visible spectrum, and (d) Emission under 360 nm UV light
Figure 7. Lippert-Mataga correlation plots of (a) CZ1 and (b) CZ2
Figure 8. Thermograms of CZ1-2 obtained under N2 atmosphere at 10 °C min-1 heating rate
Figure 9. Molecular energy level diagram showing experimental HOMO, LUMO, and bandgap
values of CZ1 and CZ2
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Table 1. Theoretical electrochemical data of CZ1 and CZ2
ELUMO
(eV)
ELUMO+1
(eV)
aEg
λmax
Oscillator
strength
(f)
EHOMO EHOMO-1
Molecules
(eV)
(nm)
(eV)
(eV)
–5.48
–5.33
–5.85
–5.68
–2.01
–1.92
–1.78
–1.70
3.464
3.408
420
400
0.40
CZ1
CZ2
0.33
aThe values obtained in DFT calculations to vacuum
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Table 2. Photophysical characterization data of CZ1and CZ2
a
b
Molecules
λabs
λemi
Stokes shift
(cm-1)
ɛ (M-1cm-1) E0-0c, Optical
(nm)
428
(nm)
557
at λabs (nm)
21,300
(eV)
2.52
2.58
5410
CZ1
CZ2
417
536
5323
20,400
a Absorption spectra measured in CHCl3 (at concentration of 10-5 M) at room temperature
b Emission spectra measured in CHCl3 (at concentration of 10-5 M) at room temperature
c
Optical band gap E0-0 is the voltage of intersection point between absorption and emission
spectra
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