Tris(4-(1-phenyl-1H-benzo[d]imidazole)phenyl)phosphine oxide for enhanced mobility and restricted traps in photovoltaic interlayers

Article abstract of DOI:10.1039/d0tc06049f

Among small molecule organic materials, tris(4-(1-phenyl-1H-benzo[d]imidazole)phenyl)phosphine oxide (TIPO) and 2,4,6-tris(4-(1-phenyl-1H-benzo[d]imidazol)phenyl)-1,3,5-triazine were newly synthesised and introduced into an n-type interlayer in planar per

Full text of DOI:10.1039/d0tc06049f

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Materials Chemistry C
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Tris(4-(1-phenyl-1H-benzo[d]imidazole)phenyl)-
phosphine oxide for enhanced mobility and
restricted traps in photovoltaic interlayers†
Cite this: J. Mater. Chem. C, 2021,
, 3642
9
a
b
a
b
Jihyun Lim,‡ Do-Yeong Choi, Woongsik Jang, Hyeon-Ho Choi,
c
a
Yun-Hi Kim * and Dong Hwan Wang
*
Among small molecule organic materials, tris(4-(1-phenyl-1H-benzo[d]imidazole)phenyl)phosphine oxide
TIPO) and 2,4,6-tris(4-(1-phenyl-1H-benzo[d]imidazol)phenyl)-1,3,5-triazine were newly synthesised and
(
introduced into an n-type interlayer in planar perovskite solar cells for effective electron transport. The small
molecule materials contain phenyl benzimidazole, which is combined with a phosphine oxide core or a
triazine ring core and contributes to the improvement of charge extraction and stability. As the constituent
molecules—phosphine oxide and benzimidazole—have strong polarity properties and p-electrons, the
molecules induce passivating defects towards improving charge transport and flattening the surface
morphology. Moreover, the stability of the device was increased due to the introduction of the TIPO
material as the passivation and protection layer. In this electron extraction analysis, electrical resistance and
surface morphology investigations were carried out via space charge-limited current, photoluminescence,
impedance, and atomic force microscopy analyses.
Received 26th December 2020,
Accepted 8th February 2021
DOI: 10.1039/d0tc06049f
rsc.li/materials-c
structure. In particular, various attempts have been made to
apply newly designed ETLs of planar PSCs by directly controlling
1
. Introduction
1
7–19
The organic–inorganic hybrid perovskite-based solar cell is one the material to improve the electron extraction efficiency.
In
of the representative solar cells in photovoltaic applications various studies, semiconductor materials with small organic
1
,2
20–24
and has been actively studied due to its enhanced efficiency.
Recently, a high efficiency of 25.2% for perovskite solar cells organic molecules are applied to a device, the performance and
PSCs) has been reported, which indicated a photovoltaic stability of the device usually increase through enhanced charge
molecules have often been used as interlayers.
When small
(
3
application with excellent potential values. In PSCs, the p–i–n mobility.
structure is of particular interest based on its relative ease of Small molecule organic compounds can acquire strong
fabrication and hysteresis suppression effect. For enhancing chemical properties and excellent solubility by adjusting the
the application of p–i–n structured PSCs, studies to improve functional groups of the molecules. Therefore, we envisage that
their efficiency and stability by modifying the active layer of the new small molecules will improve the performance and stability
4
–6
7
–10
11–16
perovskite material
or the interlayer
are often conducted. of the device in the planar PSC.
We synthesise small molecules, such as bis(1-phenyl-1H-
Adjusting the interlayer of the device is a simpler method for
efficiency improvement. It is important to enhance the trans- benzo[d]imidazole) phenyl-phosphine oxide (BIPO), tris(4-(1-
port capacity of generated charges by modifying the hole phenyl-1H-benzo[d]imidazole)phenyl)phosphine oxide (TIPO), and
transport layer (HTL) and the electron transport layer (ETL), 2,4,6-tris(4-(1-phenyl-1H-benzo[d]imidazol)phenyl)-1,3,5-triazine
as well as to reduce the resistance and trap density in the device (TBIT), which are advantageous for electron extraction. Imidazole,
phosphine oxide, and triazine units are well-known electron-
a
b
c
accepting units. To compare the effect of electron extraction, we
prepared BIPO, TIPO, and TBIT. Notably, BIPO was composed of a
highly polar phosphine oxide core and two phenyl benzimidazole
side units, while TIPO was composed of highly polar phosphine
oxide core and three phenyl benzimidazole side units; TBIT had a
highly electron acceptor triazine core unit and three phenyl benzi-
midazole side units. The core polarity and electron-accepting ability
effects were systematically compared for the planar PSC.
School of Integrative Engineering, Chung-Ang University, 84 Heukseok-ro,
Dongjak-gu, Seoul, 06974, Republic of Korea
Department of Materials Engineering and Convergence Technology and ERI,
Gyeongsang National University, Jinju 660-701, South Korea
Department of Chemistry and RINS, Gyeongsang National University,
5
01 Jinju-daero, Jin-ju, 52828, Republic of Korea
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0tc06049f
‡
J. Lim, D.Y. Choi, and W. Jang contributed equally to this work
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The previously reported small molecule material, BIPO, is
25,26
known as an excellent electron-transporting organic material.
The newly designed TIPO small molecule, synthesised with the
phenyl benzimidazole group in BIPO, has an electron-withdrawing
property of phosphine oxide with high polarity in the core, thereby
27,28
improving the electron transport.
Furthermore, benzimidazole
(
BIZ) has rich p-electrons with strong bases, which are expected to
29
extract higher charges from electron donors and acceptors.
However, we synthesised another small organic molecule, a TBIT
small molecule, wherein the triazine ring was substituted on the
TIPO core, and the p-electron deficiency of the triazine ring-
11
enabled efficient extraction of charge. These small molecules
are suitable for interlayer applications based on their good solu-
bility in isopropyl alcohol (IPA) solvents. In addition, the number
of phenyl BIZ groups around the core was compared to investigate
the effect of passivating Lewis acidic and basic defects of perovskite,
thereby reducing the roughness of the surface when introduced into
29
a bilayer. Moreover, the amine group of the molecule reduces the
trap density at the ([6,6]-phenyl-C71-butyric acid methyl ester)
PC BM/Ag interface and inhibits the Ag-iodide ion bonds of
7
0
the perovskite, thereby contributing to the improvement in
device stability. Thus, the efficiency of devices comprising a
bilayer of these small organic molecules increases, and in
particular, the inverted perovskite device with the TIPO inter-
layer applied to the PC70BM layer has been demonstrated to have
a 17% increase in the power conversion efficiency (PCE) com-
pared to the device without the interlayer.
Scheme 1 Synthesis of BIPO, TIPO, and TBIT. Details related to the
detailed synthesis method will be displayed in the ESI.†
annealed at 140 1C for 10 min to remove the solvent. The
CH NH PbI solution (CH NH I and PbI (1.06 : 1 mol%)) was
3 3 3 3 3 2
prepared by using a g-butyrolactone (GBL) solvent and dimethyl
sulfoxide in a 7 : 3 volume ratio, with a molar concentration of
ꢀ
1
1
.4 mol L for 12 h. The perovskite photoactive layer material was
2
. Experimental
spin-coated on a PEDOT:PSS layer in one step to form a thickness
of approximately 300 nm (5000 rpm and 30 s); a chlorobenzene
washing solvent was used. After spin coating, the substrate was
dried for 5 min on a hot plate kept at 100 1C. The PC BM was
2.1. Materials
All organic synthetic reactions were performed under a nitrogen
atmosphere, and the corresponding starting materials were
purchased from commercial suppliers. Solvents applied to the
reactions were used after drying by well-established methods.
Molecule isolation was conducted via flash column chromato-
graphy using 230–400 mesh-sized silica gel, and the final products
were purified by means of sublimation steps. For compound and
device preparation, the appropriate materials, if available in the
market, were obtained reasonably (Fig. S1–S9, ESI†). Also, BIPO
was synthesized by the process reported in previous research of
70
3 3 3
spin-coated onto a CH NH PbI substrate with which an approxi-
mately 40 nm-thick layer was produced (2000 rpm and 40 s).
Furthermore, TIPO was dispersed in IPA at a concentration of
ꢀ1
1
mg mL using an ultrasonic processor (show Fig. S10, ESI†).
This solution was spin-casted on top of the PC70BM layer after the
filter (10 nm or less). A 90 nm-thick Ag layer, which was employed
ꢀ
6
as the cathode, was then deposited under a pressure of 1.8 ꢁ 10
Torr through thermal evaporation. All the devices were encapsu-
lated using a resin and cover glass.
25,26
our group
(Scheme 1).
2.2. Device fabrication
2.3. Characterisation
Inverted PSCs with a TIPO interlayer were fabricated as shown The current density–voltage (J–V) characteristics and impe-
in Fig. 1. The device structure was indium tin oxide (ITO)- dance spectra of the organic photovoltaic device were obtained
3
sputtered glass substrate/PEDOT:PSS as HTL/MAPbI as active using a ZIVE SP1 and a solar simulator under illuminated AM
ꢀ2
layer/PC70BM as ETL/interlayer/Ag (90 nm). The glass/ITO was 1.5 global conditions at an intensity of 100 mW cm and a cell
2
ultrasonically cleaned with detergent, distilled water, acetone, area of 0.15 cm . The short-circuit current verification of JSC
and IPA for at least 20 min. After cleaning, the substrates were related to the J–V curve was evaluated by recording the incident
kept in an oven (100 1C) for at least 20 min before use. The photocurrent efficiency spectrum of the solar cell after power
surface properties of the ITO substrate were changed to hydrophilic calibration (ABET technologies, Inc., LS150, USA) using a
surfaces by UV-ozone treatment for 15 min. An HTL material monochromatic chromatograph (Dongwoo OPTRON Co., Ltd,
of PEDOT:PSS (Al4083) was stacked on ITO by spin coating at MonoRa-500i, Korea). The surface morphology of the interlayer
5000 rpm for 40 sec (B40 nm). The PEDOT:PSS layer was of PC70BM/(BIPO, TIPO, or TBIT) was observed via atomic force
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Fig. 1 (a) Schematic structure of BIPO, TIPO, and TBIT. (b) Energy-level diagram of inverted perovskite devices.
microscopy (AFM) (Park NX10) in non-contact mode. The in the molecule has a larger p-conjugation system according to
27
photoluminescence spectra were recorded via fluorescence richer p-electrons, which enhances the charge flow. As the
spectroscopy (F-7000 fluorescence spectrophotometer, HITA- highest occupied molecular orbital (HOMO) levels of BIPO, TIPO,
CHI, Tokyo, Japan) for determining the quenching rate of and TBIT are lower than that of PC70BM, the holes created in the
3
ITO/PEDOT:PSS/MAPbI /PC70BM/(BIPO, TIPO, or TBIT). The perovskite layer are blocked (Fig. 1(b)). Therefore, when BIPO,
excitation wavelength was 514 nm, while the photomultiplier TIPO, and TBIT molecules are applied as an interlayer between
voltage was 700 V. Raman spectra were obtained using a Raman PC70BM and the electrode, they contribute to effective charge
microscope (Xperam200 (Nanobase.inc)) with the laser (the transport and suppress opposite charge movement, leading to an
25,26
wavelength was 532 nm and the power was set at 5 mW). The increased current density of the device.
magnification of the object lens was 40ꢁ. I 3d XPS spectra were
The structure of the synthesised TIPO, BIPO, and TBIT was
ꢀ
5
recorded by using a KAlpha X-ray photoelectron spectrometer confirmed via spectroscopy analysis. The solution (1 ꢁ 10
M
operating with an AleKa radiation source.
using a chloroform) and film UV-vis absorption spectra and PL
spectra of BIPO, TIPO, and TBIT are compared (Fig. 2 and Fig.
S14 (ESI†)). From the UV-vis analysis of BIPO, TIPO, and TBIT,
the absorption peaks were observed at 311, 312, and 342 nm in
3. Results and discussion
The new TIPO molecule was synthesised via the nucleophilic solution and 318, 315, and 349 nm in film states, respectively,
3
0–32
substitution of lithiated phenyl BIZ following oxidation and which could be due to the p–p* transitions.
The p–p*
compared with reported BIPO. In addition, TBIT was also newly transitions frequently appear in compounds with aromatic rings,
obtained via the cyclotrimerisation of cyanophenyl BIZ. These
small molecule structures were introduced as a bilayer in the
inverted perovskite device, as shown in Fig. 1(a) and Fig. S11
(ESI†). In the BIPO and TIPO molecules, phosphine oxide is
located in the core, which has electron-withdrawing characteristics
based on high polarity. Another molecule, TBIT, has a triazine ring
located at the core, which is advantageous for efficiently extracting
electrons according to p-electron-deficient properties. The calcu-
lated dipole moments of BIPO, TIPO, and TBIT via density
functional theory were compared in Fig. S12 and S13 (ESI†).
The BIZ group is covalently bonded to the side of the phosphine
oxide and triazine ring core, and BIPO has two, whereas TIPO and
Fig. 2 UV-vis absorption and PL spectra of (a) TIPO and (b) TBIT with
TBIT have three BIZ functional groups. The binding of BIZ groups solutions and films, respectively.
3644 | J. Mater. Chem. C, 2021, 9, 3642ꢀ3651
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because non-bonding and p-bonding electrons are largely
3
3
distributed in the aromatic rings. Thus, phenyl benzimidazole
with an aromatic ring forms p–p* transitions, which are
observed by the red shift of the peak (Fig. 2 and Fig. S14, ESI†).
Moreover, BIPO, TIPO, and TBIT showed PL at 373, 374, and
418 nm in solution and 389, 392, and 446 nm in film states,
respectively, (Table S1, ESI†). This indicates that the absorption
region is more red-shifted in TIPO and TBIT than BIPO, and a
larger magnitude of light absorption is achieved as the wave-
length of the absorbed light becomes wider. Additionally, as it is
confirmed that all of the film samples are red-shifted compared
to the solution samples, the intermolecular p–p* interactions
3
4,35
occur strongly, which improves the electron transport ability.
In addition, an excellent dopant, BIZ, passivates both the
Lewis acid and base defects on the surface to smoothly form
the surface, thereby enhancing the performance and stability
2
9
of the device. Therefore, higher performance is expected,
especially in devices with TIPO and TBIT molecules with a
large number of BIZ groups. In addition, TGA of BIPO, TIPO,
and TBIT materials was performed (Fig. S15, ESI†), and the
detailed values of T of each material are shown in Table S1
g
(ESI†). Based on TGA analysis, the TBIT material has the best
thermal stability of 188 1C.
This is because it is a molecular and structurally stable
substance, which balances the N atom in the triazine with the N
atom in the benzyl imidazole bound to the centre in the TBIT
molecule.
Nonetheless, BIPO and TIPO are phosphine oxides with high
polarity in the core, resulting in an unbalanced difference in
polarity of the molecule. Notably, TIPO is more stable than
BIPO by dispersing the force of the phosphine oxide attracting
electrons relatively uniformly because the number of bound BIZ
groups is one more TIPO than BIPO. The intramolecular
stability is also demonstrated by means of thermal stability
tests, where BIPO degrades at a lower temperature than TIPO.
Raman analysis was conducted to confirm the chemical
bond structure of BIPO, TIPO and TBIT small molecules. As
shown in Fig. 3, the Raman spectra of BIPO and TIPO showed
that there is not a distinguished change owing to structural
similarity. The spectra revealed two major peaks; one is
assigned to P–O stretching modes from phosphine oxide, and
the other is to aromatic rings and imidazole functional groups
3
6,37
of a branch, respectively.
Interestingly, as the number of
branches bound to phosphine oxide increased in TIPO, peaks
of the branches appeared sharper than those in BIPO and
Fig. 3 Raman spectrum of (a) BIPO, (b) TIPO and (c) TBIT layers.
ꢀ
1
ꢀ1
downshifted from 1604 cm to 1602 cm , which indicates
3
8
more p–p* stacking in the molecular structure.
On the other hand, the Raman spectra of TBIT revealed a for application in PSCs (Fig. S16 and Table S1, ESI†). The highest
ꢀ
1
broad peak between 1500 and 1600 cm attributed to triazine occupied molecular orbital (HOMO) level was calculated from
rings. In particular, the peak of the branch in TBIT shifted up to the oxidation potential. The lowest unoccupied molecular orbital
ꢀ
1
1
607 cm . Therefore, the strong electron-withdrawing charac- (LUMO) level was obtained from the HOMO level and UV-edge.
teristics of phosphine led to the interaction with three As shown in Fig. 1(b), the HOMO levels of BIPO, TIPO, and TBIT
branches, which contributed to efficient p–p* interactions in are 7.07, 7.01, and 6.73 eV, respectively. Therefore, since the
the TIPO structure.
HOMO energy levels of the newly used small molecules are deep,
Cyclic voltammetry (CV) measurements of BIPO, TIPO, and they have effective hole-blocking abilities as an ETL. The effect of
TBIT were performed to evaluate their electronic energy levels hole blocking and the use of organic semiconductor materials
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with high electron-transporting properties enabled their use as Table 1 Parameters of the device with BIPO, TIPO, and TBIT interlayers
bilayers of inverted PSC ETLs, leading to improved charge
ꢀ2
ꢀ2
ETL interlayer VOC (V) JSC (mA cm ) EQE (mA cm ) FF (%) PCE (%)
3
9
extraction.
Fig. 4(a and b) show the J–V curves of the inverted perovskite
device in the PC BM/BIPO, PC BM/TIPO, and PC BM/TBIT
Pristine
BIPO
TIPO
0.927 17.25
0.894 18.74
0.890 19.18
0.940 16.78
16.35
18.76
19.01
17.05
69.17 11.07
75.25 12.61
76.14 13.00
72.81 11.49
7
0
70
70
interlayers. The structure of the device is ITO/PEDOT:PSS/ TBIT
MAPbI /PC70BM/(BIPO, TIPO or TBIT)/Ag, and the related
photovoltaic parameters are summarized in Table 1 and Table
3
S2 (ESI†). The highest performance of 13% is observed in the external quantum efficiency (EQE) and integrated current density
TIPO interlayer devices. The JSC and fill factor (FF) values of the analysis presented Fig. 4(b), the current density of the device was
ꢀ2
ꢀ2
optimised TIPO device were 19.18 mA cm
and 76.14%, 19.01 mA cm , which proved an increase of 16% compared to
respectively, and the current density and FF were increased that of the pristine device. As BIPO has weaker electron extraction
by 11% and 10% compared to the pristine device. Furthermore, properties of BIZ than TIPO, a less increased current flow
29,40
the increase in JSC and FF parameters in the average distribu- occurred in the BIPO interlayer device
than the device with
tion graph of the TIPO interlayer device shows excellent reliable TIPO. However, the efficiency of the TBIT interlayer devices slightly
performance (Fig. S17, ESI†). This is because the TIPO inter- increased relative to that of the pristine devices due to the amine
layer contributed to efficient charge extraction based on groups of BIZ with many p-electrons and the core material of the
electron-withdrawing properties and morphology passivation. triazine ring that induce electron extraction from the TBIT inter-
Thus, in the device with a TIPO interlayer, the current flow was layer. Moreover, from the dark current density data (Fig. S18, ESI†),
accelerated and the FF was most optimised, while in the the shunt resistance of the TIPO device increased, indicating that
the leakage current of the device reduced. Both the BIPO and TIPO
interlayer devices have lower series resistance compared to pristine
41
devices due to the morphology passivation effect (Table S3, ESI†).
The TBIT-based device exhibited slightly higher series resistance
because TBIT is an electron- deficient triazine ring core covalently
bonded to the BIZ, which improves the extraction ability of
electrons, but provides reduced electron transport compared to
the phosphine oxide-centred TIPO due to the difference in polarity.
Accordingly, the device with the most improved performance is
that with the TIPO interlayer, and the efficiency increases by 17%
or more compared to the pristine device.
To confirm the morphology of the surface, AFM analysis was
performed (Fig. 5). The samples for AFM analysis were prepared
by forming them on the PC70BM layer to observe the morphology
of the ETL. The root-mean-square (RMS) roughness values of the
PC BM/BIPO, PC BM/TIPO and PC BM/TBIT are 0.783, 0.631,
7
0
70
70
and 1.10 nm, respectively. Compared to the roughness of the
PC70BM layer (RMS value: 1.703 nm), these RMS values were
reduced, which indicated the smoother surface under the elec-
trode by forming interlayers (BIPO, TIPO, and TBIT), because the
interlayers had an amorphous structure. (see Fig. S11 and S19,
ESI†). The smooth morphology provides efficient passivation of
the interface between PC BM and the electrode, thereby indu-
7
0
cing uniform electron movement and FF improvement. In
particular, the PC70BM/TIPO exhibited the most reduced surface
roughness, which was advantageous for higher electron extrac-
tion and collection, contributing to an FF value of over 75%. To
analyse the optical characteristics of the devices, PL measure-
ments were performed as depicted in Fig. 6. As a result of PL
3
analysis on the ITO/PEDOT:PSS/MAPbI /PC70BM/(BIPO, TIPO or
TBIT) structure, the PL intensity of the samples with BIPO, TIPO,
and TBIT materials as bilayers was reduced compared to that of
the pristine sample.
This is because the electrons generated in the active layers
are rapidly transported by introducing small organic molecules
Fig. 4 (a) Current density–voltage (J–V) curves of devices of pristine and
with BIPO, TIPO and TBIT interlayers were added onto the electron trans-
port layer (PC70BM). (b) EQE and integrated photocurrent graphs of devices. into the interlayer. Meanwhile, a remarkably quenched PL
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Fig. 7 Space-charge-limited current (SCLC) electron mobility and trap
density for electron-only devices with (a) pristine, (b) BIPO, (c) TIPO, and
(
d) TIBT interlayers.
Fig. 5 Topographic height atomic force microscopy (AFM) images (5 mm ꢁ
directions. In addition, surface treatment of TiCl is required to
deposit the perovskite layer on top. The J–V characteristics at a
4
5
mm) of (a) ITO/PC70BM, (b) ITO/PC70BM/BIPO, (c) ITO/PC70BM/TIPO and
(d) ITO/PC70BM/TBIT.
high voltage and in a dark room can be expressed according to
4
4,45
Mott–Gurney’s law:
emission is observed for the PC70BM/TIPO, indicating that the
photoinduced electrons can be efficiently extracted by the TIPO
interlayer. TIPO facilitates the separation of excitons generated
9
V2
L3
JSCLC ¼ ere0m
;
0
8
in the photoactive layer, and electrons are rapidly transported where e is the dielectric constant (MAPbI ), e is the dielectric
r
3
0
4
2
and extracted based on its excellent optical properties.
3
constant (vacuum), and L is the thickness (MAPbI ). As a result of
Therefore, the TIPO interlayer has phosphine oxide and amine measuring the space-charge-limited current (SCLC) device, a
groups of BIZ that attract electrons, thereby enhancing the efficient graph was obtained, as shown in Fig. 7, and the electron mobility
43
extraction of electrons and suppression of recombination.
of devices with the BIPO and TIPO interlayers was higher than
Additionally, electron-only devices were fabricated to calcu- that of the pristine device. In particular, a slightly higher electron
ꢀ
4
2
ꢀ1 ꢀ1
late the mobility of each device (Fig. 7). An electron-only device mobility of 9.68 ꢁ 10 cm V
s
was calculated in the device
with an ITO/ZnO/TiCl /MAPbI /PC BM/(BIPO, TIPO, or TBIT)/ with the TIPO interlayer (Table 2). The device with the highest
4
3
70
Ag structure was fabricated and zinc oxide was introduced as a electron mobility by the TIPO effect has the best electron-
hole-blocking layer to prevent electrons from leaking in different extraction properties according to the high polarity of phosphine
oxide and the p–p interaction effect between molecules due to
the rich p-electrons of the BIZ and enhanced electrical
2
7
46,47
conductivity, thereby improving electron extraction.
The
devices using BIPO and TIPO interlayers had higher electron
mobility, which proved to be relatively correlated with the
enhanced integrated JSC and PCE (%).
The current exhibited a rapid non-linear increase, and the
voltage is known as the trap-filled limit voltage (VTFL), which is
Table 2 Electron mobility and trap density factors of the pristine (pure
PC70BM) device, device with PC70BM/BIPO, PC70BM/TIPO and PC70BM/
TBIT electron transport layers
2
ꢀ1 ꢀ1
ꢀ3
ETL interlayer Electron mobility (cm V
s
)
Trap density (cm )
ꢀ4
ꢀ4
ꢀ4
ꢀ4
16
Pristine
BIPO
1.52 ꢁ 10
9.47 ꢁ 10
9.68 ꢁ 10
7.67 ꢁ 10
3.72 ꢁ 10
16
2.76 ꢁ 10
16
Fig. 6 (a) Photoluminescence (PL) spectra (excitation at 514 nm) of devices TIPO
with BIPO, TIPO, and TBIT interlayers introduced.
3.33 ꢁ 10
16
TBIT
3.36 ꢁ 10
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determined from the trap state filled limit. Hence, the trap
4
4,45
density (ntrap or N
t
) can be calculated as follows:
entrapL2
VTFL ¼
2e0e
where e is the elementary charge, e
and 28.8 is the relative dielectric constant of MAPbI
is calculated as follows:
0
is the vacuum permittivity,
48
3
.
Therefore,
N
t
2
ere0VTFL
eL2
Nt ¼
In the trap density analysis, the low trap density value is calculated
1
6
ꢀ3
as 3.33 ꢁ 10 cm
for the TIPO device because the TIPO
interlayer inhibits charge recombination by passing traps in the
PC70BM layer. On the one hand, using the TIPO interlayer
increases the current density and improves the FF of the device
by efficiently transferring the charge generated in the active layer to
the electrode. On the other hand, the device with the BIPO interlayer
ꢀ4
2
ꢀ1 ꢀ1
has an electron mobility value of 9.47 ꢁ 10 cm V
s , which is
similar to that of the device with the TIPO, but exhibits a higher trap
density, resulting in lower electron transport ability than the device
with TIPO. The device with the TBIT interlayer has a relatively low
16
ꢀ3
electron mobility, but the trap density is 3.36 ꢁ 10 cm , which is
lower than that of the pristine device and has the effect of being
passivated on the PC70BM layer, thereby eliminating traps that
interfere with electron extraction under the Ag electrode. The amine
groups of BIPO, TIPO, and TBIT can effectively reduce the interface
trap density. Therefore, as the device with TIPO interlayer showed
the highest electron mobility improvement and the lowest value in
the trap density at the PC70BM/Ag interface, it exhibited excellent
49
characteristics in suppressing the resistance and increasing the Fig. 8 (a) Internal quantum efficiency (IQE) profile measurements of
50
BIPO, TIPO and TBIT interlayers in perovskite solar cells, (b) impedance
of devices with BIPO, TIPO and TBIT interlayers in the light.
current density and FF, thereby improving the performance of
the PSCs. This improvement in current density suggests that the
modification with TIPO reduces the injection barrier at the PC70BM/
Ag interface.
In the reflectance measurement depicted in Fig. S20 (ESI†), the the IQE response close to 98% at approximately 500 nm showed
low reflectance of the device with TIPO was confirmed from 350 to a 94% IQE current flow between 680 and 750 nm. The TIPO
400 nm, indicating that it has high light absorption compared to interlayer device with a high current density value indicated
the absorbance data (Fig. S21, ESI†). As confirmed by PL and that almost all the absorbed photons were extracted and
SCLC analyses, the electron carriers generated in the active layer converted into a photocurrent value. The IQE value is improved
were easily transported to the electrode through the TIPO inter- by extracting the generated electrons wherein phosphine
5
1
layer. Therefore, the internal quantum efficiency (IQE) of the oxide, the core of the TIPO interlayer, and strong p–p stacking
device was quantified to confirm the improved charge transport are formed between BIZ molecules to induce high electron
and extraction by introducing the TIPO interlayer. Notably, the mobility. The combined BIZ molecule and the phenyl group
IQE entails the calculation of the number of charge carriers induce intermolecular charge-transfer transitions, and the elec-
34,35
extracted from the device versus the number of photons absorbed trons are effectively extracted.
In addition, Ag–amine bonds at
in the active layer. The IQE value in Fig. 8(a) is calculated as the metal interface inhibit the formation of insulating Ag–I bonds
follows, through which the current flow of the device and the on the Ag electrode surface, thereby leading to smooth electron
4
0
extraction of charge from the electrode can be confirmed.
flow inside the TIPO interlayer device. Therefore, improved
electron mobility, excluding optical properties, was most optimized
IQE = EQE/(1 ꢀ reflectance)
52
in the device with TIPO.
Additionally, the resistances of the devices were measured
The average IQE of pristine devices shows a response of 85% at
400–750 nm, and the devices with the BIPO or TBIT interlayer through electrochemical impedance spectroscopy (EIS) analysis.
showed a similar percentage response rate. However, the IQE The charge transport resistance exhibited upon the introduction of
graph of the device with the TIPO interlayer showed an average BIPO, TIPO, and TBIT interlayers in the light and 0 V environment
of more than 91% response from 400 to 600 nm. In particular, was confirmed. The device with the TIPO interlayer had a reduced
3648 | J. Mater. Chem. C, 2021, 9, 3642ꢀ3651
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Journal of Materials Chemistry C
semi-circular radius compared to the pristine device, indicating a showed a 100% decrease in efficiency after 24 days, and the PCE
decrease in the electron transport resistance (Fig. 8(b)). In addi- of the device with the BIPO interlayer also showed a steady
tion, the improvement in the flow of charges is demonstrated decrease of 49% after 16 days. Nonetheless, the devices with
because devices with the BIPO interlayer exhibit reduced resistance TIPO and TBIT showed relatively high stability, and in particular,
compared to the pristine device. However, TBIT exhibits a slightly the former maintained excellent stability consistently for over
reduced charge transport resistance compared to the pristine 24 days. According to a previous report, PSC decomposition
device, resulting in a higher electron transport resistance than under an Ar atmosphere is mainly caused by the immigration
ꢀ
53
the devices with the TIPO and BIPO interlayers, resulting in a of I ions, which gradually corrodes the Ag cathode. Never-
reduced JSC of the device with the TBIT interlayer. Therefore, the theless, it has been demonstrated that some N-containing mate-
ꢀ
device with the TIPO interlayer reduces the electron transport rials might be able to capture the immigrated I anions, thereby
29,40
resistance by 52% compared to the pristine device, thereby inhibiting the corrosion of Ag cathodes.
The I 3d signal was
suppressing the recombination of charges and improving the confirmed through XPS analysis on the structures of ITO/PED-
electron transport. At the PC BM/Ag interface, the high polarity OT:PSS/MAPbI /PC BM depending on TIPO interlayers after
70
3
70
properties combined with the amine groups of the TIPO molecule aging for 1 week (see Fig. 9(b)). In the structure without TIPO,
are due to the rapid transfer of the generated charge to the the I 3d signals showed 620.3 eV and 631.9 eV peaks, which are
0
electrode before recombination. The improved electron transport identified as peaks of I , not ion state, indicating the fast
5
4–56
capability has been demonstrated to increase the device perfor- degradation of the pristine device.
mance by increasing the current density.
On the other hand, the
52
618.3 eV peak observed in the structure with the TIPO interlayer
The long-term stability of the device incorporating small indicates negatively ionized iodide (I ),
molecules containing the BIZ group was predicted to be the initial state of MAPbI3. Thus, the I ions were captured by
improved and verified through stability tests. The PCE decay the N atoms of the BIZs before reaching the electrode.
ꢀ
55,56
which is related to
5
7
ꢀ
2
9,41,56
The
ꢀ
versus storage time is plotted in Fig. 9. The pristine device
I
peak implied that the TIPO interlayer suppressed the for-
mation of insulating Ag–I bonds by blocking the ion migration.
41
Furthermore, the high stability of the device with TIPO is due to
the effect of three functional groups of phenyl BIZ, and the TIPO
provides an efficient passivation effect with not only the Lewis
2
+
ꢀ
acid defects (Pb ) but also the Lewis base defects (I ) of the
perovskite device. Furthermore, the devices with TIPO and TBIT
ꢀ
improved the stability by delaying the I migration from the
perovskite to the Ag cathode. In addition, the flat molecular
shape of the TBIT makes the surface hydrophobic on the PC70BM
layer, thereby suppressing the penetration of external moisture
58,59
into the perovskite layer (Fig. S22, ESI†).
Therefore, it was
confirmed that the use of PC70BM/TIPO as the ETL has an
advantage in the operational stability of the device.
Conclusions
The improvement in the performance and stability of the
devices was demonstrated by developing new small molecule
materials that were advantageous for electron extraction. The
TIPO, BIPO, and TBIT materials were synthesised by controlling
the core and the surrounding group and compared as the
interlayer of the device, respectively. The highest efficiency
was confirmed in the device with TIPO, which indicated an
improvement of over 17%. The introduction of a phosphine
oxide core and phenyl benzimidazole induced efficient electron
extraction of the ETL and enhanced electron transport of the
device by forming strong p–p stacking between molecules. In
addition, TIPO contributed to smooth morphology and defect
passivation, which were related to the improved FF and stability.
The improvement of the JSC and FF increased the PCE of the
device, the effect of the device was verified through optical and
electrical analyses, and the surface of the film was confirmed
through the AFM images.
Fig. 9 (a) Stability test of devices with BIPO, TIPO, and TBIT interlayers, (b)
XPS analysis of the I 3d region on the surface of the structures with ITO/
3 3
PEDOT:PSS/MAPbI /PC70BM and ITO/PEDOT:PSS/MAPbI /PC70BM/TIPO.
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Author contributions
14 C. Ma and N.-G. Park, ACS Energy Lett., 2020, 5, 3268–3275.
1
5 Y. Liu, S. Akin, L. Pan, R. Uchida, N. Arora, J. V. Mili ´c ,
A. Hinderhofer, F. Schreiber, A. R. Uhl, S. M. Zakeeruddin,
A. Hagfeldt, M. I. Dar and M. Gr ¨a tzel, Sci. Adv., 2019,
J. Lim, D. Y. Choi, and W. Jang contributed equally to this work.
5
, eaaw2543.
Conflicts of interest
1
6 H. Zhou, Q. Chen, G. Li, S. Luo, T.-B. Song, H.-S. Duan,
There are no conflicts to declare.
Z. Hong, J. You, Y. Liu and Y. Yang, Science, 2014, 345,
542–546.
1
7 H.-L. Yip, S. K. Hau, N. S. Baek, H. Ma and A. K.-Y. Jen, Adv.
Mater., 2008, 20, 2376–2382.
Acknowledgements
This research was supported by the Basic Science Research Pro- 18 L.-L. Jiang, S. Cong, Y.-H. Lou, Q.-H. Yi, J.-T. Zhu, H. Ma and
gram, through the National Research Foundation of Korea (NRF),
G.-F. Zou, J. Mater. Chem. A, 2016, 4, 217–222.
funded by the Ministry of Science, ICT (MSIT) 2019R1A2C1087653 19 C. Kuang, G. Tang, T. Jiu, H. Yang, H. Liu, B. Li, W. Luo,
and 2018R1A2A1A05078734. This work was also supported by the
X. Li, W. Zhang, F. Lu, J. Fang and Y. Li, Nano Lett., 2015, 15,
Technology Innovation Program (no. 10062269, ‘Self-assembled
2756–2762.
One-dimensional Nanofiber for High Performance Optoelec- 20 J.-S. Yeo, R. Kang, S. Lee, Y.-J. Jeon, N. Myoung, C.-L. Lee,
tronics’) funded by the Ministry of Trade, Industry & Energy
D.-Y. Kim, J.-M. Yun, Y.-H. Seo, S.-S. Kim and S.-I. Na, Nano
Energy, 2015, 12, 96–104.
(MOTIE, Korea).
21 M. Sakomura, Y. Yokokura, Y. Takagi and K. Ueda, AIP Adv.,
2
020, 10, 015144.
Notes and references
2
2 A. Fakharuddin, L. Schmidt-Mende, G. Garcia-Belmonte,
R. Jose and I. Mora-Sero, Adv. Energy Mater., 2017,
7, 1700623.
1
2
3
Best Research-Cell Efficiency Chart, Photovoltaic Research,
NREL; https://www.nrel.gov/pv/cell-efficiency.html.
A. Rajagopal, K. Yao and A. K. Y. Jen, Adv. Mater., 2018, 23 F. Wu, W. Gao, H. Yu, L. Zhu, L. Li and C. Yang, J. Mater.
0, 1800455.
Chem. A, 2018, 6, 4443–4448.
R. Wang, J. Xue, K.-L. Wang, Z.-K. Wang, Y. Luo, D. Fenning, 24 L. Zhu, W. Gao, F. Wu, L. Li and C. Yang, J. Mater. Chem. A,
G. Xu, S. Nuryyeva, T. Huang, Y. Zhao, J. L. Yang, J. Zhu,
2018, 6, 18044–18049.
M. Wang, S. Tan, I. Yavuz, K. N. Houk and Y. Yang, Science, 25 Y. R. Cheon, H.-J. Yun, K. Thangaraju and Y.-H. Kim, Synth.
019, 366, 1509–1513.
Met., 2014, 190, 39–43.
H. Yang, J. Zhang, C. Zhang, J. Chang, Z. Lin, D. Chen, 26 S.-J. Yoo, H.-J. Yun, I. Kang, K. Thangaraju, S.-K. Kwon and
Y.-H. Kim, J. Mater. Chem. C, 2013, 1, 2217–2223.
27 Z. He, H. Wu and Y. Cao, Adv. Mater., 2014, 26, 1006–1024.
J. You, Y. Yang, Z. Hong, T.-B. Song, L. Meng, Y. Liu, 28 S. O. Jeon, K. S. Yook, B. D. Chin, Y. S. Park and J. Y. Lee, Sol.
3
2
4
5
6
7
8
9
X. Sun, H. Xi, G. Han and Y. Hao, Sol. Energy, 2016, 139,
90–198.
1
C. Jiang, H. Zhou, W.-H. Chang, G. Li and Y. Yang, Appl.
Phys. Lett., 2014, 105, 183902.
F. Li, X. Deng, F. Qi, Z. Li, D. Liu, D. Shen, M. Qin, S. Wu,
F. Lin, S.-H. Jang, J. Zhang, X. Lu, D. Lei, C.-S. Lee, Z. Zhu
Energy Mater. Sol. Cells, 2010, 94, 1389–1392.
29 Y. Wang, Y. Yang, F. Uhlik, Z. Slanina, D. Han, Q. Yang,
Q. Yuan, Y. Yang, D.-Y. Zhou and L. Feng, Org. Electron.,
2020, 78, 105573.
and A. K. Y. Jen, J. Am. Chem. Soc., 2020, 142, 20134–20142. 30 W. Feng, A. Fujii, M. Ozaki and K. Yoshino, Carbon, 2005,
S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou,
43, 2501–2507.
M. J. P. Alcocer, T. Leijtens, L. M. Herz, A. Petrozza and 31 Y.-T. Wang, Y.-S. Wu and G.-M. Tang, Opt. Mater., 2019,
H. J. Snaith, Science, 2013, 342, 341–344.
98, 109432.
W. Nie, H. Tsai, R. Asadpour, J.-C. Blancon, A. J. Neukirch, 32 S. Song, S. H. Park, Y. Jin, J. Kim, J. Y. Shim, Y. Goo,
O.-S. Jung, I. Kim, H. Lee, E. D. Jeong, J. S. Jin, K. Lee and
H. Suh, Synth. Met., 2011, 161, 307–312.
A. H. Slavney, R. W. Smaha, I. C. Smith, A. Jaffe, 33 M. Keiluweit and M. Kleber, Environ. Sci. Technol., 2009, 43,
G. Gupta, J. J. Crochet, M. Chhowalla, S. Tretiak, M. A. Alam,
H.-L. Wang and A. D. Mohite, Science, 2015, 347, 522–525.
D. Umeyama and H. I. Karunadasa, Inorg. Chem., 2017, 56,
6–55.
0 R. Fan, Y. Huang, L. Wang, L. Li, G. Zheng and H. Zhou,
Adv. Energy Mater., 2016, 6, 1600460.
1 J. Lim, M.-S. Kim, W. Jang, D. H. Wang and J. K. Park, Dyes
Pigm., 2020, 178, 108332.
2 W.-C. Lai, K.-W. Lin, T.-F. Guo, P. Chen and Y.-T. Wang,
Appl. Phys. Lett., 2015, 107, 253301.
3421–3429.
4
34 O. R. Shehab and A. M. Mansour, J. Mol. Struct., 2021,
1223, 128996.
35 M. J. Kamlet and R. W. Taft, J. Chem. Soc., Perkin Trans.,
1979, 2, 349–356.
36 G. B. Deacon and J. H. S. Green, Spectrochim. Acta, Part A,
1968, 24(7), 845–852.
37 D. S. Caswell and T. G. Spiro, J. Am. Chem. Soc., 1986,
108(21), 6470–6477.
1
1
1
1
3 E. M. Sanehira, B. J. Tremolet de Villers, P. Schulz,
M. O. Reese, S. Ferrere, K. Zhu, L. Y. Lin, J. J. Berry and 38 A. A. Profit, V. Felsen, J. Chinwong, E.-R. E. Mojica and
J. M. Luther, ACS Energy Lett., 2016, 1, 38–45.
R. Z. B. Desamero, Proteins, 2013, 81(4), 690–703.
3650 | J. Mater. Chem. C, 2021, 9, 3642ꢀ3651
This journal is The Royal Society of Chemistry 2021

View Article Online
Paper
Journal of Materials Chemistry C
39 J. F. Butscher, Q. Sun, Y. Wu, F. Stuck, M. Hoffmann, 50 D. Shi, V. Adinolfi, R. Comin, M. Yuan, E. Alarousu, A. Buin,
A. Dreuw, F. Paulus, A. S. K. Hashmi, N. Tessler and
Y. Vaynzof, J. Phys. Mater., 2020, 3, 025002.
0 M. Imran, H. Cos-kun, F. H. Isikgor, L. Bichen, N. A. Khan
and J. Ouyang, J. Mater. Chem. A, 2018, 6, 22713–22720.
1 H. Back, G. Kim, J. Kim, J. Kong, T. K. Kim, H. Kang, H. Kim,
J. Lee, S. Lee and K. Lee, Energy Environ. Sci., 2016, 9,
Y. Chen, S. Hoogland, A. Rothenberger, K. Katsiev,
Y. Losovyj, X. Zhang, P. A. Dowben, O. F. Mohammed,
E. H. Sargent and O. M. Bakr, Science, 2015, 347, 519–522.
51 Y. Zhao, C. Liang, H. Zhang, D. Li, D. Tian, G. Li, X. Jing,
W. Zhang, W. Xiao, Q. Liu, F. Zhang and Z. He, Energy
Environ. Sci., 2015, 8, 1256–1260.
4
4
1
258–1263.
52 D. Yang, R. Yang, K. Wang, C. Wu, X. Zhu, J. Feng, X. Ren,
G. Fang, S. Priya and S. Liu, Nat. Commun., 2018, 9, 3239.
4
4
2 W. Qiu, M. Buffi `e re, G. Brammertz, U. W. Paetzold,
L. Froyen, P. Heremans and D. Cheyns, Org. Electron., 53 E. Bi, H. Chen, F. Xie, Y. Wu, W. Chen, Y. Su, A. Islam,
2
015, 26, 30–35.
M. Gr ¨a tzel, X. Yang and L. Han, Nat. Commun., 2017,
8, 15330.
3 K. Rakstys, A. Abate, M. I. Dar, P. Gao, V. Jankauskas,
G. Jacopin, E. Kamarauskas, S. Kazim, S. Ahmad, M. Gr ¨a tzel 54 M. Sun, J. Hu, C. Zhai, M. Zhu and J. Pan, Electrochim. Acta,
and M. K. Nazeeruddin, J. Am. Chem. Soc., 2015, 137,
6172–16178.
4 Q. Dong, Y. Fang, Y. Shao, P. Mulligan, J. Qiu, L. Cao and
2017, 245, 863–871.
1
55 H. Tian, T. Gao, X. Li, X. Wang, C. Luo, X. Fan, C. Yang,
L. Suo, Z. Ma, W. Han and C. Wang, Nat. Commun., 2017,
8, 14083.
4
4
4
4
J. Huang, Science, 2015, 347, 967–970.
5 H. Wang, F. Yang, Y. Xiang, S. Ye, X. Peng, J. Song, J. Qu and 56 W. Chen, G.-n. Zhang, L.-m. Xu, R. Gu, Z.-h. Xu, H.-j. Wang
W.-Y. Wong, J. Mater. Chem. A, 2019, 7, 24191–24198. and Z.-b. He, Mater. Today Energy, 2016, 1-2, 1–10.
6 L. Duan, Y. Chen, J. Jia, X. Zong, Z. Sun, Q. Wu and S. Xue, 57 C. Rocks, V. Svrcek, P. Maguire and D. Mariotti, J. Mater.
ACS Appl. Energy Mater., 2020, 3, 1672–1683. Chem. C, 2017, 5, 902–916.
7 J. Yang, Q. Zhang, J. Xu, H. Liu, R. Qin, H. Zhai, S. Chen and 58 Y. Bai, Q. Dong, Y. Shao, Y. Deng, Q. Wang, L. Shen,
M. Yuan, Nanomaterials, 2019, 9, 1666.
8 A. Poglitsch and D. Weber, J. Chem. Phys., 1987, 87, 6373–6378.
D. Wang, W. Wei and J. Huang, Nat. Commun., 2016,
7, 12806.
4
4
9 X. Meng, C. H. Y. Ho, S. Xiao, Y. Bai, T. Zhang, C. Hu, H. Lin, 59 Z. Zhu, D. Zhao, C.-C. Chueh, X. Shi, Z. Li and A. K. Y. Jen,
Y. Yang, S. K. So and S. Yang, Nano Energy, 2018, 52, 300–306. Joule, 2018, 2, 168–183.
This journal is The Royal Society of Chemistry 2021
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