Role of LiTFSI in high: T g triphenylamine-based hole transporting material in perovskite solar cell

Article abstract of DOI:10.1039/c6ra12574c

A hole transporting material based on triphenylamine with a high glass transition temperature (T_g) of 99 °C, coded as BT41, was synthesized and applied to a perovskite solar cell. The pristine BT41 showed a low power conversion efficiency (PCE) of 1.1% due to a low photocurrent density (J_sc) of ca. 6 mA cm^-2 and an almost negligible fill factor of less than 0.2, which was significantly improved to 9.0%, however, owing mainly to the 3-fold improved J_sc of 17.6 mA cm^-2, by adding both tert-butylpyridine (tBP) and lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) as additives. The oxidation of BT41 was dominated by LiTFSI, which was responsible for the hole mobility increasing by one order of magnitude. The addition of the additive also reduced the recombination resistance, which correlates with the higher fill factor. Although both additives in BT41 contributed cooperatively to the improvement of the photovoltaic performance, LiTFSI played the major role in the enhancement.

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Page 1 of 19
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DOI: 10.1039/C6RA12574C
Role of LiTFSI in High Tg Triphenylamine-based Hole Transporting
Material in Perovskite Solar Cell
AnꢀNa Cho,a HuiꢀSeon Kim,a ThanhꢀTuân Bui,b* Xavier Sallenave,b Fabrice Goubard,b Namꢀ
Gyu Parka*
aSchool of Chemical Engineering and Department of Energy Science, Sungkyunkwan
University, Suwon 440ꢀ746, Korea
bLaboratoire de Physicochimie des Polymères et des Interfaces, Université de CergyꢀPontoise,
5 mail Gay Lussac, 95000 NeuvilleꢀsurꢀOise, France
*Corresponding authors
T.ꢀT.B.: Eꢀmail: tbui@uꢀcergy.fr
N.ꢀG.P.: Eꢀmail: npark@skku.edu
Authors Eꢀmail Addresses:
AnꢀNa Cho (an.cho@skku.edu), HuiꢀSeon Kim (hseon.kim@skku.edu), Fabrice Goubard
(fabrice.goubard@uꢀcergy.fr), Xavier Sallenave (xavier.sallenave@uꢀcergy.fr)
1

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Abstract
A hole transporting material based on triphenylamine with high glass transition temperature
o
(Tg) of 99 C, coded as BT41, was synthesized and applied to perovskite solar cell. The
pristine BT41 showed low power conversion efficiency (PCE) of 1.1% due to low
photocurrent density (Jsc) of ca. 6 mA/cm2 and almost negligible fill factor of less than 0.2,
which however significantly improved to 9.0% owing to mainly 3ꢀtimes improved Jsc of 17.6
mA/cm2 by adding both tertꢀbutylpyridine (tBP) and lithium bis(trifluoromethane sulfonyl)
imide (LiTFSI) as additives. Oxidation of BT41 was dominated by LiTFSI, which is
responsible for the one order of magnitude increased hole mobility. Additive addition also
reduced recombination resistance, which correlates to the higher fill factor. Although both
additives in BT41 contributed cooperatively to improvement of photovoltaic performance,
LiTFSI played major role in the enhancement.
Keywords: perovskite solar cell, high Tg, triphenylamine, HTM, additive, LiTFSI
1. Introduction
Organicꢀinorganic halide perovskite solar cell (PSC) has received spotlight since the report
on the solidꢀstate perovskite solar cell employing spiroꢀMeOTAD as a hole transporting
material (HTM) in 20121 following the reports on perovskite sensitized liquid junction solar
cells in 20092 and 2011.3 The report on solidꢀstate PSC in 2012 eventually triggered off
researches on PSCs, as a result power conversion efficiency (PCE) reached 22.1% in 2016.4
In perovskite solar cell, needless to say, perovskite layer is most important component.
However, equally important components are materials for selective contacts since selective
contacts serve electron and hole extraction. TiO2 has been commonly used for electron
collecting, whereas various materials have been proposed as candidate for hole transporting
layer. Among the proposed HTMs spiroꢀMeOTAD has been normally adapted for perovskite
solar cell since its successful use in solidꢀstate dyeꢀsensitized solar cell in 1998.5 Compared
to polymeric HTMs, molecular HTM is beneficial to reproducibility. Nevertheless, relatively
low conductivity and a highly complicated synthetic process are still issues to be addressed in
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spiroꢀMeOTAD. A donorꢀacceptor type DOR3TꢀTBDT6 and tetrathiafulvalene derivative7
have been proposed as an alternative to spiroꢀMeOTAD because those materials
demonstrated PCEs of 14.9% and 11.03%, respectively, even without additives thanks to high
hole mobility.
Triphenylamine (TPA), an important constituent in spiroꢀMeOTAD, is regarded as basic unit
for HTM. For instance, TPA linked with diphenyl led to linear πꢀconjugated small molecule
showing better pore filling and efficient hole mobility.8 Modification of MeOTAD structure
demonstrated comparable performance for the device with spiroꢀMeOTAD.9 Flattering of
core TPA was found to increase hole transport owing to effective πꢀπ interaction, which
showed PCE of 12.8% without adding additives to this HTM.10
Recently Bui et al., reported 5ꢀ(4ꢀ(bis(4ꢀ(5ꢀ(bis(4ꢀmethoxyphenyl)amino)thiophenꢀ2ꢀ
yl)phenyl)amino)phenyl)ꢀN,Nꢀbis(4ꢀmethoxyphenyl)thiophenꢀ2ꢀamine (coded as BT41)
based on triphenylamine core that is expected to be a candidate for HTM for perovskite solar
o
cell because of high glass transition temperature (Tg = 99 C) allowing good contact with
perovskite layer due to amorphous state.11,12 In addition, BT41 is expected to be less
expensive than spiroꢀMeOTAD because of facile synthetic process and relatively high yield.
For the case of spiroꢀMeOTAD, its low conductivity13,14 can be overcome by addition of
additives such as lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) and 4ꢀtertꢀ
butylpyridine (tBP). Addition of both additives was found to improve significantly perovskite
solar cell performance. However, the role of LiTFSI or tBP has not been clarified. In this
work, effects of additives in BT41 on photovoltaic performance of perovskite solar cell are
investigated. Photovoltaic performance of pristine BT41 is compared with those of BT41
including additives. BT41 with only LiTFSI is much superior to that with only tBP, which is
further improved by combining two additives. Additive effects are analyzed by hole mobility,
photoluminescence and impedance spectroscopy.
2. Experimental Section
Synthesis of materials. BT41 was synthesized according to the method described
elsewhere.11 Typical procedure: in a dry schlenk equipped with a magnetic stir bar, tris(4ꢀ(5ꢀ
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bromothiophenꢀ2ꢀyl)phenyl)amine (628 mg, 0.8622 mmol), bis(4ꢀmethoxyphenyl)amine (890
mg, 3.88 mmol), palladium(II) acetate (19 mg, 0.08622 mmol), sodium tertꢀbutoxide (497 mg,
5.1732 mmol), triꢀtertꢀbutylphosphine (35 mg, 0.1724 mmol, dissolved in 3 mL of dry
toluene), and freshly distillated toluene (30 mL) were charged under argon stream. The
mixture was then degassed by repeated vacuum evacuation/argon refill cycle (5 times) and
was heated at 100°C for 80 h. After cooling to room temperature, the mixture was diluted
with chloroform (100 mL) and was filtered through a short plug of silica to yield a clear
yellow liquid. This solution was concentrated to give a viscous oil (ca. 3 – 4 mL) to which
was added methanol (250 mL) while stirring. The precipitate was then filtered, rinsed with
methanol, and dried under vacuum to yield the product as a yellow solid (787 mg, 78 %
yield). 1H NMR (DMSOꢀd6, δ/ppm): 7.48 (d, 6 H, J = 8.5 Hz), 7.18 (d, 3 H, J = 3.8 Hz), 7.11
(d, 12 H, J = 9.0 Hz), 7.00 (d, 6 H, J = 8.5 Hz), 6.93 (d, 12 H, J = 9.0 Hz), 6.42 (d, 3 H, J =
4.3 Hz), 3.76 (s, 18 H). 13C NMR (1,2ꢀdichlorobenzenꢀd4, δ/ppm): 156.30, 152.92, 146.05,
141.66, 135.13, 122.99, 125.99, 124.90, 124.61, 121.41, 118.00, 114.82, 55.23. HRMS
(ESI+): calculated for M+: 1172.3675, found 1172.3711. CH3NH3I was synthesized by
reacting 27.8 ml of CH3NH2 (40wt% in methanol, TCI) with 30 ml of HI (57wt% in water,
Aldrich) in a round bottomed flask with vigorous stirring for 2 h in an ice bath. The
o
precipitated CH3NH3I was collected using a rotary evaporator at 50 C for 1 h, washed with
diethyl ether four times and finally dried in vacuum for 24 h. PbI2 (0.461g), CH3NH3I
(0.159g) and N,Nꢀdimethyl sulfoxide (DMSO) (0.078g) in 0.4943g of dimethylformamide
(DMF) were prepared at room temperature for spinꢀcoating of MAPbI3 (MA = CH3NH3)
(corresponding to 52wt% solution).
Solar cell fabrication. The compact blocking TiO2 layer (blꢀTiO2) was formed on the cleaned
FTO glasses (Pilkington, TECꢀ8, 8ꢁ/sq) using the 0.15 M solution of titanium diisopropoxide
bis(acetylacetonate) (75 wt % in 2ꢀpropanol, Aldrich) in 1ꢀbuthanol (99.8%, Aldrich) as spin
o
coated at 2800rpm for 20s, which was followed by drying at 125 C for 5min. The homeꢀ
made nanocrystalline TiO2 (diameter of about 50 nm) paste was diluted (0.1 g paste/ml 1ꢀ
buthanol), which was spinꢀcoated on the blꢀTiO2 at 2000 rpm for 20 s and annealed at 550 oC
for 1 h to form the mesoporous TiO2 (mꢀTiO2) layer. The mꢀTiO2 layer was further treated
o
with 0.02 M aqueous TiCl4 (99.9%, Aldrich) solution at 70 C for 10 min, followed by
annealing at 500 °C for 30 min. The adduct method was used to prepare perovskite layer,15
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where a precursor solution containing CH3NH3I, PbI2 and DMSO that were dissolved in
DMF was spinꢀcoated on the mesoporous TiO2 film at 4000 rpm for 25 s and diethyl ether
o
was dripped while spinning. The perovskite layer was formed by drying at 65 C for 1 min
and 100 oC for 9 min. BT41 was spinꢀcoated on the top of perovskite layer at 4000 rpm for 30
s using the BT41 solutions with or without additives, where 59.04 mM BT41 was first
prepared by dissolving 69.3 mg BT41 in 1 ml of chlorobenzene, to which 28.8 ꢂl tBP (96%,
Aldrich) was added and then 25 ꢂl LiTFSI (99.95%, Aldrich) was finally added. LiTFSI
solution was prepared by dissolving 520 mg LiTFSI in 1 ml acetonitrile. Finally, gold counter
electrode was thermally deposited on the BT41 HTM layer at 1×10ꢀ6 torr.
Characterization. Toluene was dried by distillation from sodium metal just prior to use.
NMR spectra were recorded on a Bruker DPXꢀ250 FTꢀNMR spectrometer with deuterated
solvent in all cases. Chemical shifts (δ) are given in ppm using the residual solvent signal as
internal reference. Highꢀresolution mass spectrometry was performed by the Small Molecule
Mass Spectrometry platform of the CNRS IMAGIF in GifꢀsurꢀYvette. Thermogravimetric
analysis (TGA) was carried out on a TA Instrument Q50 TGA under argon flow at a heating
rate of 20 °C/min. The temperature of thermal degradation (Td) was measured at the point of
5% weight loss. Differential scanning calorimetry (DSC) was performed on a TA Instruments
Q100 calorimeter, calibrated with indium and flushed with argon. Samples were scanned
from ꢀ50°C to 280°C at a heating rate of 20°C/min then rapidly cooled to ꢀ50°C (quenching)
and heated at the same rate to 280°C. Absorbance was measured by using a UV/Vis
spectrometer (lamda35, PerkinElmer). Timeꢀintegrated photoluminescence (PL) spectra were
collected using
a
QuantaurusꢀTau compact fluorescence lifetime spectrometer
(HAMAMATSU) with a 450 W CW Xenon lamp and a photomultiplier tube (PMT) detector.
(Excitation wavelength 464nm, Repetition rate 10MHz) Cyclic voltammetry (CV) was
measured using an Autolab (AUT128N, FRA2) electrochemical analyzer. BT41 thin film was
prepared by dropping and drying the BT41 chlorobenzene solution on a glassy carbon, which
was used as a working electrode, Pt and Ag/AgCl were used for a counter and a reference
electrode, respectively. These threeꢀelectrode were immersed in CH2Cl2 solution with 0.1 M
tetrabutylammonium hexafluorophosphate (NBu4PF6) and oxidation potential was evaluated
with respect to ferrocene (Fc) at a scan rate 100 mV/s. Hole mobility (µh) was estimated from
the current density (J)ꢀvoltage (V) characteristics using the space charge limited current
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(SCLC) method. The MottꢀGurney equation, J = (9/8)ε0εrꢁh(V2/L3), was applied to obtain
µh,16 where ε0 is the vacuum permittivity (8.857ⅹ10ꢀ12 F/m), εr is the dielectric constant of
the film (ε
r = 3 was assumed17), and L is the thickness of the active layer. The device structure
for SCLC measurement was ITO/BT41/Au and the BT41 film thickness was determined by
an alphaꢀstep IQ surface profiler system (KLA Tencor).
Photocurrent density (J)ꢀvoltage (V) was measured with a solar simulator (Oriel Sol 3 A class
AAA) equipped with a 450W Xenon lamp (Newport 6279NS) and a Keithly 2400 source
meter. Light intensity was adjusted with the NRELꢀcalibrated Si solar cell having KGꢀ2 filter
for approximating one sun light intensity (100 mW/cm2). Active area was covered with
aperture mask (0.125 cm2) during the measurement.
Impedance spectroscopy (IS) characterization was carried out with PGSTAT 128N (Autolab,
EcoꢀChemie). IS under illumination (80 mW/cm2) was measured by applying a small
perturbation of AC 20 mV over a DC bias voltage with a frequency ranging from 1 MHz to 1
Hz where 1 s of equilibrium time was given between each scan. DC bias voltage ranging
from 0 V to 0.7 V was applied to the device with a potential step of 0.1 V. The obtained
Nyquist plots were fit using an equivalent circuit composed of a series resistance (Rs) and
two or three RꢀC components (resistance and capacitance in parallel) depending on the HTM
composition. In high frequency region impedance raw data were fit with two RꢀC
components in series where the sum of two resistance values represents the resistance in high
frequency region (RHF). The last arc in low frequency region was fit with one RꢀC circuit
indicating the recombination resistance (Rrec).
3. Results and discussion
The molecular structure of BT41 is shown in Figure 1. Studied compound was synthesized
from commercially available tris(4ꢀbromophenyl)amine in three steps with an overall yield of
52%. Briefly,
a
Stille coupling between 2ꢀtributylstannylthiophene and tris(4ꢀ
bromophenyl)amine gave tris(4ꢀ(thiophenꢀ2ꢀyl)phenyꢀl)amine which was then brominated
leading to tris(4ꢀ(5ꢀbromothiophenꢀ2ꢀyl)phenyl)amine. The later was then subjected to
threefold Pdꢀcatalyzed Buchwald–Hartwig amination with di(4ꢀmethoxyphenyl)amine
leading to BT41. The structures of intermediate and final compounds were confirmed by
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NMR and HRMS. As expected, BT41 has good solubility in common organic solvents,
highly thermal stability (Td = 428°C) with relative high glass transition temperature (Tg =
99°C).
Figure 1. The molecular structure of Tris(4ꢀ(5ꢀ(4,4’ꢀdimethoxydiphenylaminyl)ꢀ2ꢀ
thiophenyl)phenyl)amine (BT41)
Absorption and emission properties of BT41 are measured, where BT41 in CH2Cl2 solution
solution and in thin solid film are compared in Figure 2a. The maximum absorption appears
similarly at 405 nm for the solution and 406 nm for the thin film, while the peak for the thin
film is broader than that for the solution due to enhanced overlap between molecules, leading
to splitting of HOMO and LUMO level.18 Photoluminescence (PL) spectrum for the solution
shows asymmetric single peak with maximum at 466 nm, whereas the maximum shifts to 508
nm leaving shoulder peak at 471 nm for the thin film case. The redꢀshift in the thin film form
is related to the intermolecular πꢀπ interaction that can be enhanced by tilting
triphenylamine.19
C
yclic voltammogram (CV) is measured to determine the oxidation and reduction potential of
BT41.20,21 Figure 2b shows the CV result, where oxidation potential of BT41 is determined
to be ꢀ0.14 V from the onset potential. Highest occupied molecular orbital (HOMO) of BT41
is calculated to be ꢀ4.96 eV from the relation of EHOMO = ꢀ5.1 ꢀ (Eox,HTM vs. Fc/Fc+).20
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Figure 2. (a) Normalized absorption and photoluminescence (PL) spectra for the BT41
solution dissolved in CH2Cl2 (black) and the BT41 film formed on plain glass (gray). (b)
Cyclic voltammogram of BT41 with ferrocene as the reference (scan rate = 100mV/s).
From the combined results of optical spectroscopy and CV, HOMO and LUMO level of BT
41 are determined, which is displayed in Figure 3 together with conduction band (CB) and
valence band (VB) positions of MAPbI3 and CB of TiO2. The HOMOꢀLUMO gap of BT41 is
obtained by onset wavelength (467.9 nm) in absorption spectrum, which results in 2.65 eV.
LUMO level is then estimated to be ꢀ2.31 eV from HOMO and gap energy. HOMO of BT41
is well suited for hole transfer from MAPbI3.
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Figure 3. Energy level diagram of TiO2, MAPbI3 and BT41. CB, VB, HOMO and LUMO
represent conduction band, valence band, highest occupied molecular orbital and lowest
unoccupied molecular orbital, respectively.
Figure 4 compares photocurrentꢀvoltage curves of perovskite solar cells employing pristine
BT41, BT41 with tBP, BT41 with LiTFSI and BT41 with both tBP and LiTFSI. The device is
composed of FTO/blꢀTiO2/mꢀTiO2/MAPbI3/BT41/Au, where thickness of mꢀTiO2 layer and
perovskite capping layer is 100 nm and 400 nm, respectively. Photovoltaic parameters are
listed in Table 1. Pristine BT41 without additive shows low performance due to low
photocurrent density (Jsc) of 6.28 mA/cm2 and almost negligible FF of 0.2. Addition of tBP to
BT41 deteriorates Jsc but improves Voc. Jsc and FF are significantly improved to 13.25
mA/cm2 and 0.48, respectively, by addition of LiTFSI, which indicates that LiTFSI
dominates the enhancement of photovoltaic performance rather than tBP. Jsc is further
improved to 17.64 mA/cm2 and FF to 0.49 upon coꢀexistence of tBP and LiTFSI in BT41.
This indicates that although tBP itself have little impact on the performance, cooperative
effect is demonstrated when tBP is combined with LiTFSI. tBP indirectly increases power
conversion efficiency by enabling LiTFSI to be dissolved in the solution. Chlorobenzene is
used as a solvent for BT41. Nevertheless, LiTFSI itself is insoluble in chlorobenzene due to
its low polarity. tBP is required to reveal the effect of LiTFSI. PCE is significantly improved
from 1.1% to 9.0% by additive effect.
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Figure 4. Current density (J)ꢀvoltage (V) curves of BT41ꢀbased perovskite solar cells
depending on additives. Data are collected at reverse scan (from Voc to Jsc) at scan rate 200
ms (voltage settling) under 1.5G one sun illumination.
Table1. Photovoltaic parameters of BT41ꢀbased perovskite solar cells depending on additives.
Jsc, Voc, FF and PCE represent shortꢀcircuit photocurrent density, openꢀcircuit voltage, fill
factor and power conversion efficiency. Data were collected at reverse scan (from Voc to Jsc)
at scan rate of 200 ms.
BT41
Pristine
Jsc (mA/cm2)
6.28
Voc (V)
0.89
FF
PCE (%)
1.1
0.196
0.216
0.483
0.491
w/ tBP
2.64
1.04
0.6
w/ LiTFSI
w/ tBP + LiTFSI
13.25
0.90
5.8
17.64
1.04
9.0
Timeꢀintegrated photoluminescence (PL) is measured to investigate effect of additive on
charge separation between MAPbI3 and hole transport. Figure 5 shows the normalized PL
with respected to bare MAPbI3. Upon contacting MAPbI3 with the pristine BT41, PL
intensity is significantly reduced. PL intensity is also decreased for the MAPbI3 with BT41
containing both tBP and LiTFSI, however, PL quenching is less pronounced compared to the
MAPbI3 with pristine BT41 without additive. This implies that additive has little effect on
charge separation. The PL peaks of MAPbI3 are slightly blueꢀshifted upon contacting BT41,
which is attributed to PL of BT41. A small difference in PL quenching between the pristine
BT41 and the BT41 with additives is indicative of difference in contact morphology at
MAPbI3/BT41 interface.
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Figure 5. Timeꢀintegrated PL spectra for the glass/MAPbI3/PMMA (MAPbI3), the
glass/MAPbI3/pristine BT41 (Pristine BT41) and the glass/MAPbI3/BT41 with tBP and
LiTFSI (tBP + LiTFSI). The data are normalized with respect to the PL data for MAPbI3.
Crossꢀsectional scanning electron microscopy (SEM) images are compared in Figure 6.
Pinholeꢀfree HTM layer is formed from the pristine BT41 layer, whereas large pinholes in the
HTM layer and small pinholes at interface are developed upon adding the additives to BT41.
The interfacial pinholes is likely to hinder charge separation, which might be responsible for
the less PL quenching for the BT41 including additives than for the pristine BT41, as
observed in Figure 5. Nevertheless, better photovoltaic performance achieved after addition
of additives may be related to change in carrier mobility in BT41.
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Figure 6. Crossꢀsectional SEM images of the full cell layout with (a) pristine BT41 and (b)
BT41 with tBP and LiTFSI additives. Scale bar is 100 nm.
We have carefully examined absorption spectra of BT41 depending on additives. In Figure
7a, the BT41 solutions with and without additives are compared. Strong absorption at around
400 nm appears regardless of additives. The BT41 solution containing LiTFSI shows
additional small peak at around 590 nm, while no addition peak appears for the BT41 with
tBP. For the case of spiroꢀMeOTAD, a sharp peak at around 400 nm is characteristics of
neutral spiroꢀMeOTAD and a broad peak at around 500 is newly developed due to oxidation
of spiroꢀMeOTAD.22ꢀ25 The peak appeared at longer wavelength after oxidation is due to that
fact that the destabilized bonding orbital by removal of electron leads probably to decrease
the transition energy. Thus, a small and broad peak at around 590 nm, which is absent in the
pristine BT41, is indicative of oxidation of BT41. On the other hand, tBP does not have
ability to oxidize BT41. Evolution of oxidation of BT41 is monitored in the presence of
LiTFSI in Figure 7b. Gradual evolution of peak at around 590 nm is indicative of oxidation
of BT41 by LiTFSI. Compared to very low performance in the presence of only tBP as
observed in Figure 4, much higher photocurrent for the presence of only LiTFSI indicates
strongly that oxidation of BT41 plays important role in hole conduction in HTM.26
Figure 7. (a) Absorption spectra of BT41 with and without additives. Materials dissolved in
chlorobenzene in glove box in argon atmosphere. Measurements were performed for the
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samples aged for 80 min in air atmosphere. Inset shows absorbance in the 500ꢀ700 nm. (b)
Evolution of oxidation of BT41 in the presence of LiTFSI. Absoprtion spectra were measured
with chlorobenzene solution of 0.031 mM BT41 and 0.0037 mM LiTFSI every 5 min in dark
at scan speed of 480 nm/min.
Space charge limited current (SCLC) is measured to estimate hole mobility. Hole only
devices with ITO/BT41/Au layout are prepared for measuring SCLC.27 Hole mobility is
obtained by fitting the MottꢀGurney equation (see Experimental section) to the SCLC data in
Figure 8. The film thickness was 366 nm for pristine BT41, and was 893 nm for BT41 with
tBP and LiTFSI. Hole mobility of pristine BT41 is estimated to be 3.06 × 10ꢀ4 cm2/Vs, which
is 20 times increased to 7.19 × 10ꢀ3cm2/Vs upon addition of tBP and LiTFSI to BT41. Such a
dramatic improvement of hole mobility correlates to oxidation of BT41 that is mainly
dominated by LiTFSI. It leads to increase the hole mobility because the injected hole fills the
deepest trap sites, which induces smooth potential landscape. It results in an increase of intraꢀ
molecular charge transfer.22,28 Along with improved hole mobility, the increased density by
oxidation consequently enhances the conductivity which is proportional to the hole density
and hole mobility (σ = n×e×ꢁ, σ = electrical conductivity, n = the number density of charge
carriers, e = the elementary charge, ꢁ = charge carrier mobility).13,28
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Figure 8. Current as a function of applied voltage obtained by SCLC method. HTM thickness
in ITO/HTM/Au was 366 nm for pristine BT41, and 893 nm for BT41 with tBP and LiTFSI.
Impedance spectroscopy was measured to analyze the effect of additives on the resistance.
Figure 9a represents Nyquist plots obtained at 0.2 V of applied voltage where solid line
indicates the fit results using an equivalent circuit as described in experimental section. In
Figures 9b and 9c, Nyquist plots obtained in the applied voltage ranging from 0 V to 0.7 V
were used to determine the resistances as a function of bias voltage in high and low frequency
range. Figure 9b shows resistance observed in high frequency region (RHF) which is
attributed to the resistance of selective contacts such as compact TiO2 and hole transport
material (HTM).29,30 Because the only difference between the devices was a composition of
HTM where additives were artificially added to the pristine BT41, it is expected that the RHF
is dominantly determined by HTM layer. Moreover, it is noted that the almost biasꢀ
independent RHF features are obtained, which is frequently observed when the charge
transport resistance governs RHF.31 As can be seen in Figure 9b, RHF of BT41 containing
additives show overall lower RHF in the region of low applied voltage (0ꢀ0.4 V) compared to
the pristine BT41. Therefore, the addition of additives improves charge transport in BT41
resulting in higher photocurrent density especially in the low applied voltage region where
recombination process is not evident. Figure 9c represents recombination resistance (RLF)
obtained in low frequency region. RLF was maintained constantly in low applied voltage
region and began to decrease in high applied voltage region due to increased Fermi level.
BT41 with additives (tBP + LiTFSI) shows higher recombination resistance compared to the
pristine BT41, which is well consistent with the increased Voc for the BT41 with additives.
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Figure 9. (a) Nyquist plots of the devices employing pristine BT41 and BT41 with additives
(tBP + LiTFSI) under 0.8 sun. Nyquist plots measured at DC bias voltage of 0.2 V were
shown as an example. (b) Resistances obtained in high frequency range (RHF) and (c)
recombination resistance obtained in low frequency range (RLF) with respect to the applied
voltage.
Conclusions
High Tg tryphenylamine based hole transporting material, BT41, was synthesized and applied
to perovskite solar cell. High photovoltaic performance was hardly achieved using pristine
BT41, which was however dramatically improved in the presence of tBP and LiTFSI in BT41.
tBP itself had little effect but played cooperative role when combining with LiTFSI. LiTFSI
played major role in oxidation of BT41 and enhancing hole mobility, which was responsible
for the improved photovoltaic performance in BT41ꢀbased perovskite solar cell.
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Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grants funded
by the Ministry of Science, ICT & Future Planning (MSIP) of Korea under contract No.
NRFꢀ2015K1A3A1A21000315 (International Research & Development Program) and in part
NRFꢀ2012M3A6A7054861 (Global Frontier R&D Program on Center for Multiscale Energy
System), NRFꢀ2015M1A2A2053004 (Climate Change Management Program) and NRFꢀ
2012M3A7B4049986 (Nano Material Technology Development Program). The authors also
thank the French Ministry of Foreign Affairs, the French Ministry of National Education and
Ministry of Higher Education and Research, the Korean Ministry of Science, ICT & Future
Planning for financially support this FrenchꢀKorean research collaboration through the
Hubert Curien Partnership (PHC STAR) under the project number 34301QH.
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TOC
Perovskite solar cell employing triphenylamineꢀbased HTM (BT41) showed improved
photovoltaic performance in the presence of lithium salt as an additive in BT41 due to
increased hole mobility by oxidation of BT41.
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