A novel AIE molecule as a hole transport layer enables efficient and stable perovskite solar cells

Article abstract of DOI:10.1039/d1cc00484k

A low-cost and efficient hole transport layer (HTL) material (TPE-CZ) with the aggregation-induced emission (AIE) effect has been synthesized. Due to the AIE effect, perovskite solar cells with TPE-CZ as the HTL deliver a higher power conversion efficiency (PCE) of 18% with better stability than those with the reference HTL (Spiro-OMeTAD).

Full text of DOI:10.1039/d1cc00484k

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A novel AIE molecule as a hole transport layer
enables efficient and stable perovskite solar cells†
Cite this: Chem. Commun., 2021,
7, 4015
5
ab
a
a
c
c
Bin Huang,‡ Yujun Cheng,‡ Hui Lei,‡ Lin Hu, * Zaifang Li,
a
a
ad
Received 31st January 2021,
Accepted 8th March 2021
Xuexiang Huang, Lie Chen * and Yiwang Chen
DOI: 10.1039/d1cc00484k
rsc.li/chemcomm
6
7
A low-cost and efficient hole transport layer (HTL) material (TPE-CZ) amine) (PTAA) and poly(3-hexylthiophene) (P3HT), and the
0
0
with the aggregation-induced emission (AIE) effect has been synthe- small molecule 2,2 ,7,7 -tetrakis(N,N-di-p-methoxyphenylamine)-
sized. Due to the AIE effect, perovskite solar cells with TPE-CZ as the 9,9 -spirobifluorene (Spiro-OMeTAD) (Fig. 1a). It is well known
HTL deliver a higher power conversion efficiency (PCE) of 18% with that batch-to-batch variation and difficulties in purification are
0
8
better stability than those with the reference HTL (Spiro-OMeTAD).
unavoidable problems for polymers, while organic small
molecules can relieve these problems and their structures are
Organic–inorganic lead halide hybrid perovskite solar cells well defined. Currently, state-of-the-art PeSCs are predominantly
PeSCs) have attracted great attention owing to the perovskite constructed by employing the small molecule Spiro-OMeTAD as
(
light absorber with exceptional intrinsic properties such as a the HTL. Nevertheless, the synthesis of Spiro-OMeTAD is
high extinction coefficient, long electron–hole diffusion lengths, prohibitively expensive since it includes reaction steps that
1
a direct band gap and high charge carrier mobilities, etc. With require low temperature (ꢀ78 1C) and sensitive (n-butyllithium
2
the continuous effort of the scientific community, the power or Grignard reagents) and aggressive (Br ) reagents. Moreover,
conversion efficiency (PCE) of PeSCs has increased from the high-purity sublimation-grade Spiro-OMeTAD is required to
2
,3
8
initial 3.8% in 2009 to over 25% in 2021. And to date, a PCE of obtain high device performance. On the other hand, stability
over 24% has been achieved for commonly prepared PeSCs with is also crucial for the commercialization of PeSCs. Extensive
4
regular (n-i-p type) mesoporous structures.
efforts have been made to enhance the stability via optimizing
the morphology of the perovskite films. Meanwhile, the HTL
3
In regular PeSCs, a compact n-type metal oxide (TiO , SnO
2
2
or ZnO) is always firstly deposited on the substrates as an also plays an important role as a strong protective layer on the
electron transport layer (ETL). Then, the perovskite light absorber
is deposited on top of the ETL, and subsequently coated with an
organic hole transport layer (HTL) and evaporated metal as a
back electrode. The organic HTL plays a critical role in
efficiently extracting holes separated from the perovskite layer
5
and simultaneously transporting them to the top metal anode.
Numerous organic materials have been strategically designed
and synthesized to improve the performance of PeSCs in recent
years. Representatives include the organic polymers poly(triaryl
a
College of Chemistry/Institute of Polymers and Energy Chemistry (IPEC),
Nanchang University, 999 Xuefu Avenue, Nanchang, 330031, China
b
School of Metallurgical and Chemical Engineering, Jiangxi University of Science
and Technology, 156 Ke Jia Road, Ganzhou, 341000, China
c
China–Australia Institute for Advanced Materials and Manufacturing (IAMM),
Jiaxing University, 56 Yuexiu South Avenue, Jiaxing, 314001, China
d
Institute of Advanced Scientific Research (iASR), Jiangxi Normal University,
9
9 Ziyang Avenue, Nanchang, 330022, China
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1cc00484k
Equal contribution.
†
Fig. 1 (a) Molecular structures of TPE-CZ and Spiro-OMeTAD. (b) CV
measurements of TPE-CZ and Spiro-OMeTAD. The ferrocene/ferrocenium
+
‡
(Fc/Fc ) couple was used as an internal reference.
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perovskites against moisture penetration and UV light to
To investigate the potential application of TPE-CZ as a HTL
material in PeSCs, density functional theory calculations at the
suppress the intrinsic degradation, ultimately making
a
contribution to the long-term stability of PeSCs. Thus, these B3LYP/6-31G(d,p) level are applied to give insights into its
concerns motivate a more facile and reasonable design of HTLs highest occupied molecular orbital (HOMO) and lowest unoccupied
that can offer high efficiency with low cost production and molecular orbital (LUMO) energy levels (Fig. S4, ESI†). From the
9
superior stability compared to commonly used Spiro-OMeTAD.
optimized molecular geometries, the HOMO mainly locates on
The phenomenon of aggregation-induced emission (AIE) the flanking carbazole-based triphenylamine functional group,
was reported by Tang and co-workers in 2001 and is the exact while the LUMO spreads over part of the central TPE core. The
1
0,11
opposite of the aggregation-caused quenching (ACQ) effect.
calculated energy level is ꢀ4.26 eV for the HOMO and ꢀ1.52 eV
AIE molecules are generally non-fluorescent in dilute solutions for the LUMO. The energy levels are also determined through
because of active intramolecular motions but are induced to emit cyclic voltammetry (CV) measurements. As shown in Fig. 1b
intensely in an aggregated state due to the restriction of such and Table 1, the HOMO energy level of TPE-CZ (ꢀ5.52 eV) is
motions. The efficient solid-state emission of AIE molecules gives higher than that of Spiro-OMeTAD (ꢀ5.22 eV). The ferrocene/
+
them great potential in fabricating optoelectronic devices. It has ferrocenium (Fc/Fc ) couple was used as an internal reference.
been reported that tetraphenylethylene (TPE), a classic AIE core,
The optical properties of organic semiconductors have been
functioned with a methoxyphenylamine (OMe) group can serve as an proved to be closely related to their aggregation behaviour, espe-
efficient HTL material. Owing to the TPE-based HTL with excellent cially for this novel molecule with a twisted structure. The normal-
charge extraction and favorable energy alignment, these PeSCs ized ultraviolet-visible (UV-Vis) absorption spectrum of the TPE-CZ
deliver a comparable performance with those based on doped PTAA film is shown in Fig. 2a. The absorption band locating in the region
or Spiro-OMeTAD as HTLs. Although several AIE-based interlayer of 300–400 nm is assigned to a high-energy p–p* transition of the
materials have been reported, researchers have mainly focused on conjugated system in the molecule. When the film is excited by a
improving interfacial contact, diminishing the interfacial energy light with an excitation wavelength of 352 nm, it delivers a strong
barrier and facilitating charge extraction and transfer. The utilization fluorescence. The fluorescence locates in the region around 450–
of the unique AIE effect of these novel HTL materials to improve the 700 nm with a peak at 510 nm. The phenomenon is shown
device performance has rarely been examined or reported.
directly in Fig. 2b, where the film emits a strong white light when
In this work, a novel low-cost and efficient HTL material exposed to an ultraviolet lamp. To further explore the origins, a
(TPE-CZ), based on a TPE core and the p-methoxydiphenylamine- series of photoluminescence (PL) measurements are performed.
substituted carbazole (CZ) functional group, is developed for The TPE-CZ is firstly dissolved in THF at a concentration of 3 ꢁ
ꢀ5
PeSCs (Fig. 1a). When TPE-CZ is employed as the HTL, the AIE 10 M. As shown in Fig. 2c, the pristine TPE-CZ THF solution is
effect of the TPE-CZ layer can convert the ultraviolet light in the not fluorescent when exposed to an ultraviolet lamp. However,
incident light to visible light that is reabsorbed by the perovskite when deionized water is added into the TPE-CZ solution to form
layer, leading to an improvement in the photocurrent. This AIE THF/H
effect also reduces the instability of the device caused by the fluorescence intensities are gradually enhanced with the increase
ultraviolet light in the incident light. Therefore, PeSCs based on of the water fraction (f ) in the resultant THF/H O mixture. Here,
2
O mixtures, the samples could fluoresce. Moreover, the
w
2
TPE-CZ as the HTL achieve a higher PCE (PCE = 18.0%) than the water is poor solvent for the TPE-CZ molecule. The TPE-based
reference devices based on the Spiro-OMeTAD HTL (PCE = twisted structure tends to aggregate and emission is induced
1
6.7%) and maintain a superior stability.
For the new HTL material, we select TPE as the core and CZ enon indicates that TPE-CZ is a typical AIE molecule. The
equipped with OMe as the functional group. To ensure that the corresponding PL spectra of the solutions are shown in Fig. 2d.
resulting compound possess a good AIE effect, two CZ TPE-CZ is then employed as the HTL in the PeSCs. The device
functional groups flank both sides of the TPE core. The target structure is FTO/compact TiO /mesoporous TiO /CH NH PbI
TPE-CZ molecule is synthesized in only three steps via the HTL/Ag (Fig. 3a). A compact and mesoporous layer of TiO is
when poor solvents are added into the solution. This phenom-
2
2
3
3
3
/
2
Buchwald–Hartwig cross coupling reaction. The corresponding deposited on top of FTO for electron collection. The organic HTL
synthetic route is shown in Scheme 1 in the ESI.† The nuclear (TPE-CZ or Spiro-OMeTAD) is successively deposited on top of
magnetic resonance (NMR) spectra and time of flight mass
spectrometry (MALDI-TOF-MS) results (with an isotopic cluster
+
peaking at a m/z of 1570.24, calculated for M ) synergistically
Table 1 Summary of the electrochemical and physical properties of TPE-
confirmed the structure of TPE-CZ (Fig. S1, ESI†). Roughly
CZ and Spiro-OMeTAD
calculating the cost of TPE-CZ, it is obvious that its cost is
much lower than that of the commonly used Spiro-OMeTAD
c
9
Hole mobility
a
b
2
ꢀ1 ꢀ1
HTLs
E
ox
E
HOMO (eV)
(cm V
s )
(Table S1 and Fig. S2, ESI†). Because of the special twisted
ꢀ
5
TPE-CZ
Spiro-OMeTAD
0.81
0.51
ꢀ5.52
ꢀ5.22
8.57 ꢁ 10
structure, TPE-CZ can easily dissolve in common organic
solvents such as chloroform, chlorobenzene, toluene, etc.
Thermo gravimetric analysis (TGA) measurements reveal that the
TPE-CZ film shows good thermal stability. The TPE-CZ film starts
to decompose at a temperature of around 370 1C (Fig. S3, ESI†).
ꢀ5
5.39 ꢁ 10
a
b
E
ox obtained from the cyclic voltammograms. The ferrocene/ferro-
cenium (Fc/Fc ) couple was used as an internal reference and its
vacuum level was set to 4.71 eV. Determined by the space-charge-
+
c
limited current (SCLC) method.
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PeSCs based on Spiro-OMeTAD as the HTL with a PCE of 16.7%,
ꢀ
2
a VOC of 1.09 V, a JSC of 20.17 mA cm and a FF of 75.9%. After
applying a bias at the maximum output voltage, a more stable
power output is also obtained (Fig. S6, ESI†). It is obvious that
the improvement of the device’s performance is mainly owed to
the enhanced JSC and FF, indicating the more efficient hole
extraction and collection from the TPE-CZ HTL to the top
electrode. The external quantum efficiency (EQE) spectra are
consistent with the results from the J–V characteristics (Fig. 3c).
Interestingly, we note that an additional spectrum response in
the near ultraviolet region is found for the PeSCs based on TPE-
CZ as the HTL. Combined with the aforementioned novel optical
properties (Fig. 2), this can be presumably attributed to TPE-CZ
absorbing the incident ultraviolet light and emitting fluores-
cence that can be absorbed again by the perovskite layer. This
point is in agreement with the phenomenon that the sample of
Fig. 2 (a) UV-Vis absorption spectrum and PL emission spectrum of TPE-
CZ on quartz substrates. (b) Pictures of the TPE-CZ film exposed to an
ꢀ5
2 2 3 3 3
FTO/compact TiO /mesoporous TiO /CH NH PbI possesses a
ultraviolet lamp. (c) The TPE-CZ THF solution (3 ꢁ 10 M) with various
water contents exposed to an ultraviolet lamp and (d) PL spectra of the
corresponding TPE-CZ solutions.
lower transmittance in the near ultraviolet region after coating
with a TPE-CZ layer (Fig. S7, ESI†). Therefore, the enhanced JSC
confirms the positive effect caused by the TPE-CZ HTL with the
novel AIE characteristic.
To further investigate the improvement of the device
performance, steady state PL and time-resolved PL (TRPL)
measurements are employed to evaluate the hole extraction in the
the perovskite layer, followed by a layer of evaporated Ag. The
CH NH PbI perovskite serves as the light absorber. The detailed
3 3 3
device fabrications are described in the ESI.† A high-resolution
scanning electron microscopy (SEM) cross-section image of the
prepared PeSCs is shown in Fig. 3b. PeSCs with the commonly
used Spiro-OMeTAD are prepared for comparison. As shown in
Fig. 3c and Table 2, the optimized PeSC based on TPE-CZ as the
3 3 3
PeSCs (as seen in Fig. 4a). The pristine CH NH PbI film possesses a
strong photoluminescence centered at 760 nm. The PL spectrum is
obviously quenched after depositing a HTL on top of the perovskite
film. Moreover, the PL quenching is more pronounced for
HTL yields a high PCE of 18.0%, an open-circuit voltage (V ) of
OC
ꢀ
2
CH
3
NH
3
PbI
3
/TPE-CZ than for CH
3
NH
3
PbI
3
/Spiro-OMeTAD, suggest-
NH PbI to TPE-CZ.
1
.08 V, a short-circuit current density (JSC) of 21.47 mA cm and
ing the more efficient hole extraction from CH
3
3
3
a fill factor (FF) of 77.5%. The hysteresis effect of the PeSCs is
negligible when the device is scanned in the reverse and forward
directions. J–V characteristics of the PeSCs based on TPE-CZ with
various mass concentrations are shown in Fig. S5 (ESI†). The
detailed device performance is summarized in Table S2 (ESI†).
The optimized device performance is superior to that of the
The result agrees with the point that TPE-CZ possesses a better
matched energy alignment with the perovskite layer for hole extrac-
tion. The corresponding TRPL decay of the samples is shown in
Fig. 4b. A bi-exponential decay model is used to fit the PL decay
curves to obtain an average exciton lifetime (tave) (Table S3, ESI†).
The pristine CH NH PbI layer presents a t of 26.1 ns. After
3
3
3
ave
depositing a Spiro-OMeTAD HTL on top of the CH NH PbI layer,
3
3
3
the exciton lifetime decays rapidly and the tave decreases to 4.7 ns.
When TPE-CZ serves as the HTL, the tave decreases more signifi-
cantly to 3.3 ns, indicating more efficient charge transfer. The
better charge transfer between the perovskite layer and the TPE-
CZ layer plays a considerable role in improving the device
performance. In addition, the carrier mobility is determined
by the space-charge limited currents (SCLCs) to elucidate
the charge transportation in the devices. The structure
3 3 3
of the hole-only devices is ITO/PEDOT:PSS/CH NH PbI /
TPE-CZ or Spiro-OMeTAD/Ag. Fig. S8 (ESI†) shows their J–V
characteristics measured in the dark. According to the Mott–
Gurney equation, the obtained hole mobility of TPE-CZ is
ꢀ
5
2
ꢀ1 ꢀ1
8
.57 ꢁ 10 cm V
s , which is higher than that of Spiro-
ꢀ
5
2
ꢀ1 ꢀ1
OMeTAD (5.39 ꢁ 10 cm V
s ). The improved hole mobility
would reduce the carrier’s loss when they are transported through
the interlayer. The PeSCs based on the TPE-CZ HTL thus obtain a
superior device performance, especially maintaining a high FF.
The stability of a device is an issue of increasing concern for
the scientific community. It is well known that the decomposition
Fig. 3 (a) Schematic of the device structure. (b) SEM cross-section image
of the prepared PeSCs. (c) J–V characteristics of the optimized PeSCs based
on TPE-CZ as the HTL. The device-based on Spiro-OMeTAD as the HTL is
used for comparison. (d) The corresponding EQE spectra for the PeSCs.
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Table 2 The device parameters based on TPE-CZ and Spiro-OMeTAD as the HTLs
ꢀ2
a
HTLs
JSC (mA cm
)
VOC (V)
FF (%)
PCEmax/PCEave (%)
TPE-CZ (reverse)
TPE-CZ (forward)
Spiro-OMeTAD (reverse)
Spiro-OMeTAD (forward)
21.47 ꢂ 0.3
21.07 ꢂ 0.3
20.17 ꢂ 0.1
18.81 ꢂ 0.2
1.08 ꢂ 0.01
1.08 ꢂ 0.01
1.09 ꢂ 0.01
1.09 ꢂ 0.01
77.5 ꢂ 1.0
75.7 ꢂ 1.0
75.9 ꢂ 0.6
73.8 ꢂ 0.4
18.0/17.8 ꢂ 0.20
17.2/16.9 ꢂ 0.3
16.7/16.5 ꢂ 0.3
15.1/15.0 ꢂ 0.1
a
Average PCE values are calculated from ten individual cells.
In summary, the twisted AIE molecule TPE-CZ has been
designed and synthesized by a facile straightforward synthetic
route. TPE-CZ can be employed as an efficient HTL to improve the
PCE and stability of PeSCs. Careful investigations demonstrate
that TPE-CZ can convert the ultraviolet light in the incident light
into visible light. The visible light is absorbed by the perovskite to
improve the photocurrent. Moreover, this AIE characteristic
eliminates the notorious detrimental effect to the device stability
caused by the ultraviolet light. Therefore, an excellent PCE of 18%
and much better stability are achieved for the PeSCs based on
TPE-CZ as the HTL, which is higher than the reference PeSCs
based on Spiro-OMeTAD as the HTL (PCE = 16.7%). This work
provides a new route to develop PeSCs with high efficiency as well
as long-term stability.
Fig. 4 (a) The steady state PL spectra and (b) TRPL spectra of pristine
3 3 3 3 3 3 3 3 3
CH NH PbI , CH NH PbI /TPE-CZ and CH NH PbI /Spiro-OMeTAD.
of perovskite crystals usually occurs, and that the devices degrade
faster when exposed to UV light, moisture, oxygen, etc. We firstly
investigated the UV light stability of the devices. The devices were
stored in a N -filled glove box under UV light illumination
2
(365 nm, 8 W) without encapsulation. As shown in Fig. S9a (ESI†),
the device based on TPE-CZ as the HTL maintains B60% of its Conflicts of interest
initial PCE after 6 h of UV light irradiation, while the reference
device based on the Spiro-OMeTAD HTL only delivers B30%
There are no conflicts to declare.
under the same exposure conditions. The enhanced UV stability
presumably originates from the TPE-CZ molecule with a novel
optical property. During the test of the UV light stability, the
Notes and references
1
M. Liu, M. B. Johnston and H. J. Snaith, Nature, 2013, 501, 395–398.
ultraviolet light is incident from the FTO side into the device while
it will be reflected by the top opaque Ag electrode. When the
reflected ultraviolet light passes through the HTL, the TPE-CZ
molecules will absorb the ultraviolet light and convert it into
visible light, thereby reducing the damage to the perovskite layer.
The humidity and air stability of the PeSCs are also measured.
The PeSCs without encapsulation were stored in a Petri dish in
ambient conditions (25 1C, humidity B50%). To reduce the
impact caused by the indoor light, the Petri dish was wrapped
with opaque tin foil. As shown in Fig. S9b (ESI†), the reference
PeSC exhibits extremely inferior stability. Its PCE drops to less
than 20% after being stored in ambient conditions for 10 days.
The decline of the PCE is presumably attributed to the damage
caused by the penetration of moisture or oxygen into the functional
layers, especially for the perovskite layer. However, the TPE-CZ layer
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