Low-cost synthesis of heterocyclic spiro-type hole transporting materials for perovskite solar cell applications

Article abstract of DOI:10.1039/c8nj01082j

Four heterocyclic spiro-type hole transporting materials (HTMs) carrying a spiro[fluorene-9,9′-xanthene] (SFX) (SFX-TPAM and SFX-TPA) or spiro[fluorene-9,9′-thioxanthene] (SFT) unit (SFT-TPAM and SFT-TPA) were synthesized through a low-cost facile route with high yields for perovskite solar cell (PSC) applications. In terms of absorption, these four compounds in the film state are all transparent at wavelengths longer than 430 nm, which is beneficial for allowing visible light to reach the perovskite active layer without being absorbed by the hole transporting layer (HTL). The photovoltaic performance of the inverted PSCs based on these small molecular HTMs with the device architecture of glass/ITO/HTL/CH₃NH₃PbI₃/C₆₀/BCP/Ag was tested. Only SFX-TPAM had its highest occupied molecular orbital (HOMO) level matched with the valence band of CH₃NH₃PbI₃. The inverted PSC based on a dopant-free SFX-TPAM HTL achieves a power conversion efficiency of 10.23% under the illumination of standard one sun lighting, which is better than that (8.17%) of the cell based on dopant-free spiro-OMeTAD. The better photovoltaic performance of SFX-TPAM compared to spiro-OMeTAD may be due to the MAPbI₃ film deposited on it having better quality. These results indicate that the facilely synthesized, low-cost SFX based small molecules can be used as the HTMs for PSCs.

Full text of DOI:10.1039/c8nj01082j

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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Low-cost synthesis of heterocyclic spiro-type hole transporting materials for
perovskite solar cell applications
a
a
a
a,b
Venkatesan Govindan, KaiꢀCheng Yang, Yung Seng Fu and ChunꢀGuey Wu*
a
Department of Chemistry, National Central University, JhongꢀLi, 32001 Taiwan, ROC
b
Research Center for New Generation Photovoltaics, National Central University, JhongꢀLi, 32001 Taiwan, ROC
,
a,b
*
Professor: Chun Guey Wu,
Department of Chemistry, National Central University, JhongꢀLi, Taiwan 32001, ROC
Fax: 886ꢀ3ꢀ4227664
Eꢀmail: t610002@cc.ncu.edu.tw
1

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Abstract:
Four Heterocyclic spiroꢀtyped hole transporting materials (HTMs) carrying spiro[fluoreneꢀ
’
9
,9’ꢀxanthene] (SFX) (SFXꢀTPAM and SFXꢀTPA) or spiro[fluoreneꢀ9,9 ꢀthioxanthene] (SFT) unit
(SFTꢀTPAM and SFTꢀTPA) were synthesized through low cost facile route with high yields for
perovskite solar cell (PSC) applications. The absorption of these four compounds in film state are
all transparent at the wavelength longer than 430 nm, which is beneficial for allowing the visible
light to reach the perovskite active layer without being absorbed by hole transporting layer (HTL).
The photovoltaic performance of the inverted PSCs based on these small molecular hole
transporting materials with the device architecture of glass/ITO/HTL/CH NH PbI / C /BCP/Ag
3
3
3
60
was test. Only SFXꢀTPAM having its highest occupied molecular orbital (HOMO) level matched
with the valence band of CH NH PbI . Inverted PSC based on dopantꢀfree SFXꢀTPAM HTL
3
3
3
achieves a power conversion efficiency of 10.23% under the illumination of standard one Sun
lighting, which is better than that (8.17%) of the cell based on dopantꢀfree spiroꢀOMeTAD. The
better photovoltaic performance of SFXꢀTPAM compared to spiroꢀOMeTAD may be due to the
MAPbI film deposited on it has better quality. These results indicate that the facile synthesis, low
3
cost SFX based small molecules can be used as the hole transporting materials for perovskite solar
cell.
Keywords: perovskite solar cell, organic hole transporter, Lowꢀcost synthesis.
2

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1
. Introduction
Lead halide perovskite based solar cells (PSC) have attracted a great attention in the past 5
1
ꢀ3
4
years. To date, the powder conversion efficiency (PCE) of PSC has reached more than 22%. The
inverted (pꢀiꢀn) perovskite solar cells exhibiting a PCE of 20% have drown much more attention in
8
ꢀ10
recent years.
Inverted PSCs are attractive, due to the advantages of low temperature process,
hysteresisꢀfree and simple architecture. The charge transporting materials play an important role in
PSC as they match the frontier orbital energy levels of the absorber and transport the free carriers to
5
ꢀ7
the corresponding electrodes.
materials (HTMs), such as poly(3,4ꢀethylenedioxythiophene):poly(styrenesulfonic acid)
It was known that several polymer based hole transporting
9,10ꢀ13
14,15
(PEDOT:PSS),
polyꢀ(bis(4ꢀphenyl)(2,4,6ꢀtrimethylphenyl) amine) (PTAA),
and
5
,16
poly(N,N′ꢀbis(4ꢀbutylphenyl)ꢀN, N′ꢀbis(phenyl)benzidine) (polyꢀTPD) have been used in PSC to
achieve high PCE. On the other hand, small molecule based HTMs are also very attractive due to
their easy purification, high purity, and no batchꢀtoꢀbatch variations. SpiroꢀOMeTAD is the mostly
used small molecular HTM but it needs a multistep for the synthesis, as a result, high price
hindering further progress. Moreover, SpiroꢀOMeTAD with low hole mobility makes it is necessary
to be doped with lithium salt and tꢀBP. These additives are hydrophilic, which causes the instability
of the devices. Furthermore, the doped HTL may lower the device reproducibility due to the
inhomogeneous distribution of the dopant. Therefore, new small molecule based HTMs used in the
1
7ꢀ21
3
PSCs were developed.
Among of them spirobifluorene having a spiro structure with a sp
carbon connecting two orthogonal π systems together is a good choice because of the rigid spiroꢀ
2
2,23
conjugation can enhance the hole transporting ability by hopping process.
Spiro[fluoreneꢀ9,9’ꢀ
xanthene] (SFX) core has received increasing attention due to its competitive facile synthesis by
2
4
oneꢀpot method. The perpendicular spiro structure in the SFXꢀbased building units can effectively
inhibit the intermolecular πꢀπ stacking and thus improve the solubility but still keep the good hole
mobility. SFX based small molecules thus were widely used in the high performance organic
2
5
26ꢀ28
electronics and organic light emitting devices.
electronꢀdonating bis(4ꢀmethoxyphenyl) aniline or bis(4ꢀmethoxyphenyl)amine group were used in
SFXꢀbased HTMs functionalized with an
2
9ꢀ32
the regular type perovskite solar cells, showing an impressive PCE over 20%.
Thus, the high
performance and lowꢀcost synthesis of SFX based HTMs provide as a promising alternative HTM
’
for applying in PSCs. Spiro[fluoreneꢀ9,9 ꢀthioxanthene] (SFT) is another interesting building block
3

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with the structure similar to SFX, in which the bridge atom is sulfur instead of oxygen in SFT, is
3
3
34, 35
widely used as a hole transporting materials in OPV and OLED.
In this work, we report the synthesis and photovoltaic performance of two SFX and two SFT
based small molecules in which the SFX or SFT unit was functionalized with 4ꢀmethylꢀNꢀphenylꢀ
Nꢀ(pꢀtolyl)aniline (TPAM) or triphenylamine (TPA), respectively. The chemical structure of these
four new small molecular HTMs: (4,4',4'',4'''ꢀ(spiro[fluoreneꢀ9,9'ꢀxanthene]ꢀ2,2',7,7'ꢀ
tetrayl)tetrakis(N,Nꢀdiꢀpꢀtolylaniline) (SFXꢀTPAM), 4,4', 4'',4'''ꢀ(spiro[fluoreneꢀ9,9'ꢀxanthene]ꢀ
2
,2',7,7'ꢀtetrayl)tetrakis (N,Nꢀdiphenylaniline) (SFXꢀTPA), 4,4',4'',4'''ꢀ(spiro[fluoreneꢀ9,9'ꢀthioꢀ
xanthene]ꢀ2,2',7,7'ꢀtetrayl) tetrakis(N,Nꢀdiꢀpꢀtolylaniline) (SFTꢀTPAM) and 4,4',4'',4'''ꢀ(spiroꢀ
fluoreneꢀ9,9'ꢀthioxanthene]ꢀ2,2',7,7'ꢀtetrayl)tetrakis(N,Nꢀdiphenylaniline) (SFTꢀTPA) are
[
displayed in Figure 1.
Figure 1: The chemical structures of four new spiroꢀbased HTMs
These four new HTMs were designed to increase the mobility of the spiroꢀbased HTM to avoid
the sideꢀeffect of the dopants used in SpiroꢀOMeTAD and to tune the surface properties for
depositing high quality perovskite film. The propellerꢀlike triphenylamine (TPA) serves as a
3
6
building block to construct the molecules with high hole mobility. The electron donating TPAM
and TPA on the central core unit were used to promote the πꢀ π stacking for efficient charge
4

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transporting and increase the surface hydrophobility. The nonꢀplanar spiroꢀbased architecture
makes the molecule soluble in common organic solvents, which is compatible to the fabrication of
the devices using the wet chemical process such as spinꢀcoating. Moreover, the cost effective and
easy synthesis are the merits for these spiroꢀbased small molecules to be used as the HTM in PSCs
to reach the low cost devices.
2
2
. Experimental section
.1 Materials and physicochemical studies
All chemicals, unless otherwise specified, were purchased from the commercial resources and
used as received. PbI (99.999% from Aldrich), CH NH (40% from TCI), HI(aq) (57% from
2(aq)
2
3
Aldrich), BCP (Aldrich) and C (Solenne B. V., Netherlands) were used in this study. CH NH I
6
0
3
3
was synthesized from CH NH
and HI(aq) with the method similar to that reported in the
3
2(aq)
3
7 1
13
literature. H NMR and C NMR spectra were recorded with a Bruker 300 MHz NMR
spectrometer using CDCl as a solvent. FABꢀMS spectra were obtained using JMSꢀ700 HRꢀMS.
3
UV/Vis absorption spectra were measured using a Cary 300 Bio spectrometer. Emission spectra
were measured using FluoroMaxꢀ4 spectrofluorometer (HORIBA JOBIN YVON) at an exciting
wavelength of 500 nm. HPLC grade CH Cl was purchased from ACROS Co. and was distilled
2
2
over CaH under an argon (Ar) atmosphere for electrochemical study. Tetraꢀnꢀbutylammonium
2
hexafluorophosphate (TBAPF ) was recrystallized from ethanol twice and then stored in a
6
desiccator to be an electrolyte. The glassy carbon working electrode was polished with 0.05 mm
alumina on Buehler felt pads and was ultrasonicated for 2 min to remove the residue contamination.
A platinum wire is used as the auxiliary electrode and a homeꢀmade Ag/AgCl, KCl (sat'd) is the
reference electrode. The supporting electrolyte is 0.1 M tetrabutylhexafluorophosphate (TBAPF6)
dissolved in CH Cl and the amount of the HTMs is ca. 5 mg. The cyclic and squareꢀwave
2
2
voltammograms (frequency: 25 Hz; step potential: 0.0051 V; amplitude: 0.01995 V) were recorded
using a potentiostat/galvanostat (PGSTAT 30, Autolab, EcoꢀChemie, the Netherlands) and
ferrocene was used as a calibration standard (0.54 V versus Ag/AgCl reference electrode). Scanning
electron microscopy (SEM) images were obtained from a Hitachi Sꢀ800 microscope at 10 kV.
Samples for SEM imaging were mounted on a metal stub with a piece of conducting tape and then
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coated with a thin layer of platinum film to avoid charging. The thickness of the films was
measured from the cross section SEM images. Hole mobility was measured according to a method
3
8,39
based on the space charge–limited current (SCLC) model described in the literature,
using a
diode configuration of ITO/PEDOT:PSS/HTL/Au The IꢀV curve was measured under dark
conditions using a Keithley Model 2400 unit. The slope of the logJ vs logV curve was used to
evaluate the SCLC region and the mobility of the HTL was calculated according to the Mottꢀ
Gurney’s law. The detailed analysis method can be found in the supporting information (SI).
2
.2 Synthesis of the four new HTMs
The detailed synthetic procedures and structural analysis of the four new small molecular
HTMs can be found in the supporting information (SI).
2
.3 Device fabrication and characterization
Prior to the fabrication, the ITO coated glass substrates were cleaned in an ultrasonic bath with
DI water, followed by acetone and isopropanol. Subsequently the substrates were dried and UVꢀ
Ozone treating for 10 min. The substrates were then transferred into the glove box. The HTM
ꢀ1
solution (5 mg mL in chlorobenzene) was spin coated onto the ITO and annealed at different
temperature for 5 min. The perovskite precursor solution (1.25 M) was prepared by mixing
CH NH I and PbI (molar ratio 1:1) in dimethylsulfoxide (DMSO)/ γꢀbutyrolactone (GBL) mixed
3
3
2
solvent (volume ratio 3:7). The perovskite precursor solution was then spinꢀcoated on the hole
transporting layer (HTL) using oneꢀstep spinꢀcoating combined with the antiꢀsolvent washing
method (1000 rpm for 35s, 5000 rpm for 15s and toluene was dropped on the film at the last 5
seconds of the spinning) to form a full coverage perovskite film. The substrate was thermally
o
annealed at 100 C for 5 min and subsequently the electron transporting layer (50 nm of C ), buffer
6
0
layer (5 nm of BCP) and Ag electrode (100 nm) were deposited by highꢀvacuum thermal
evaporation. The current densityꢀvoltage (JꢀV) curves of the cells were measured using a Keithley
2
400 sourceꢀmeasurement unit under the AM1.5G solar simulator light (Oriel, 91 150 V, using
ꢀ2
Xenon lamp) calibrated to 100 mW cm with a KGꢀ5 silicon reference cell. The external quantum
6

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efficiency (EQE) or incident photoꢀtoꢀcurrent conversion efficiency (IPCE) was measured in air
after sealing the device with a silica sealant and measuring immediately. A chopper and lockꢀin
amplifier were used for the phase sensitive detection with the QEꢀR3011 measurement system
designed by Enlitech Inc., Taiwan.
3
3
. Results and discussion
.1 Synthesis
The spiro[fluoreneꢀ9,9’ꢀxanthene] (SFX) core was prepared by oneꢀpot, nonꢀsolvent synthetic
4
0
method using ready available starting materials such as phenol, methane sulfonic acid with the
yield of 75%. Brominated SFX (SFXꢀBr ) was prepared by reacting SFX with Br at room
4
2
temperature to get a yield of 78%. The SFXꢀTPAM and SFXꢀTPA were synthesized by Stille
coupling using Pd(PPh ) as a catalyst and toluene as the solvent as illustrated in Figure 2.
3
4
7

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Figure 2: The synthetic scheme of the four new spiroꢀbased HTMs
Stille coupling between SFXꢀBr4 and stannated TPAM afforded SFXꢀTPAM with the yield of
8
5%. Similar method was used for reacting SFXꢀBr4 with stannated TPA to obtain SFXꢀTPA with
’
yield of 78%. The spiro[fluoreneꢀ9,9 ꢀthioxanthene] (SFT) core was synthesized by oneꢀstep from
dibenzoꢀ [b,d]thiophene 5,5ꢀdioxide and 9Hꢀthioxanthene in the presences of KHMDS with the
4
1
yield of 76% using the method reported in literature. SFT was brominated with Br at room
2
temperature to form SFTꢀBr4 (73% yield). The Stille coupling reactions similar to that used for
preparing SFXꢀTPAM and SFXꢀTPA mentioned above were used to obtain the final products SFTꢀ
TPAM and SFTꢀTPA with the yields of 71% and 73%, respectively. All new compounds were
1
13
characterized by H NMR, C NMR and HRꢀMS as described in SI. These four compounds have
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good solubility in common organic solvents such as dichloromethane, ethyl acetate, toluene,
chloroform and chlorobenzene.
3
.2 Optical properties
The UVꢀVis absorption spectra of these new spiroꢀbased molecules in dichloromethane and
solid film form are shown in the Figure 3(a) and (b). The absorption peak at 350 nm ~ 420 nm was
assigned to the πꢀπ* transition of the entire molecules. Substitution of a sulfur atom by oxygen
atom on the spiroꢀcore, the absorption profile has no significant change. In dichloromethane, the
λmax values of SFTꢀTPAM (399 nm) and SFTꢀTPA (395 nm) are slightly longer than those of SFXꢀ
TPAM (385 nm) and SFXꢀTPA (391 nm), respectively. The absorption spectra of the films are
quite similar to those of the corresponding solutions except the bands are broader due to the
molecular aggregation in the solid state films. All new HTMs show very low absorption at the
wavelength longer than 430 nm in the film state, which is beneficial for using as a HTL in the
inverted PSC. The optical band gaps of the HTMs were calculated from the onset of the absorption
spectra (in CH Cl ) and the optical properties of the new HTMs as well as spiroꢀOMeTAD are
2
2
summarized in Table 1.
1
400
SFX-TPAM
SFX-TPA
SFT-TPAM
SFT-TPA
SFX-TPAM
SFX-TPA
SFT-TPAM
SFT-TPAM
SFX-TPAM
SFX-TPA
SFT-TPAM
SFT-TPA
1
1
0
0
0
0
0
.2 (a)
(b)
0
0
.20
.15
(c)
1200
1000
.0
.8
.6
.4
.2
.0
8
6
4
00
00
00
0.10
0
.05
200
0
0.00
300
3
00
350
400
450
500
350
400
450
500
550
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Figure 3: UV/Vis spectra of the four new HTMs (a) in CH Cl . (b) in film state. (c) PL spectra of
2
2
ꢀ5
the four new HTMs in CH Cl . (concentration 1 x 10 M)
2
2
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Table 1: The optical and electrochemical data of the four new HTMs
HTMs
SFX-
TPAM
385
SFX-
TPA
391
SFT-
TPAM
399
SFX-
TPA
395
Spiro-
OMeTAD
385
λmax in CH Cl (nm)
2
2
a
λEm in CH Cl (nm)
452
3850
0.51
ꢀ5.21
2.92
ꢀ2.29
441
2900
0.87
456
3130
0.99
ꢀ5.69
2.88
ꢀ2.81
442
2690
1.01
ꢀ5.71
2.93
ꢀ2.78
429
2660
ꢀ0.06
ꢀ5.0
2
2
ꢀ
1 b
Stoke shift (cm )
c
E1/2 (V vs Ag/AgCl)
d
HOMO (eV)
ꢀ5ꢀ57
2.94
e
E0ꢀ0 (eV)
2.95
ꢀ2.05
f
LUMO (eV)
ꢀ2.63
a
.
Excitation wavelength: the absorption maxima (λmax).
b. The difference between the maximum of absorption and maximum of emission spectra.
+/0
c. FeCp2 = +0.50 V (vs Ag/AgCl) in CH Cl /TBAPF
2
2
6
d. The HOMO energy level was calculated from the half –wave potential of oxidation according to the
empirical EHOMO = ꢀ(E +4.5) V by assuming the energy level of ferrocene to be ꢀ4.5 eV below vacuum
1/2
level. (E (V) vs. NHE)
1
/2
e. The energy gap level (E ) was calculated from the onset wavelength of UVꢀVis spectrum according to the
0
-0
empirical Egap = 1240/λonset
f. LUMO = HOMO + E0-0
The emission of the HTMs in dichloromethane at room temperature is shown in Figure 3(c). All
HTMs have a broad photo luminescence band in the range of 400ꢀ600 nm which comes from the
π*ꢀπ transition. The λEm is in the order of: SFXꢀTPA (441 nm) ≒ SFTꢀTPA (442 nm) < SFXꢀ
TPAM (452 nm) ≒ SFTꢀTPAM (456 nm). The SFXꢀTPAM and SFTꢀTPAM have a redꢀshift in the
PL peaks compared to SFXꢀTPA and SFTꢀTPA, respectively. These could be attributed to that 4ꢀ
methylꢀNꢀphenylꢀNꢀ(pꢀtolyl) aniline (TPAM) has stronger electronꢀdonating ability than
triphenylamine (TPA), similar tendency was observed in the absorption spectra. Stokes shifts (the
difference between the λmax of the absorption and emission peaks) of SFXꢀTPA and SFTꢀTPA
ꢀ1
(
2900 and 2690 cm respectively) are smaller than those of SFXꢀTPAM and SFTꢀTPAM (3850 and
ꢀ1
3
130 cm respectively), which is due to the latter couple have larger geometry differences between
the ground and excited states arising from the increasing number of nonhindered interꢀring torsional
2
2
angles.
3
.3 Electrochemical properties
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The electrochemical behaviors of the HTMs were investigated by cyclic voltammetry (CV) and
square wave voltammetry (SWV) studies. All four molecules show quasiꢀreversible in the first
oxidation/reduction couple at the potential of 0.5 ~ 1.1 V (vs Ag/AgCl), similar potentials were
observed in SWV curves as the CV and SWV diagrams displayed in Figure 4.
SFX-TPA
SFX-TPAM
SWV
CV
SWV
CV
0
.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
0
.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Potential (V) Vs Ag/AgCl
Potential (V) vs Ag/AgCl
SWVSFT-TPAM
SFT-TPA
SWV
CV
CV
0
.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
0
.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
Potential (V) Vs Ag/AgCl
Potential (V) Vs Ag/AgCl
Figure 4: The cyclic voltammograms (CV) and square wave voltammograms of the four new
HTMs.
SFXꢀTPAM containing the electron rich 4ꢀmethylꢀNꢀphenylꢀNꢀ(pꢀtolyl)aniline shows the lowest
first oxidation potential of 0.51 eV. On the other hand, triphenylamine attached SFXꢀTPA is
’
oxidized at more positive oxidation of 0.87 eV. The spiro[fluoreneꢀ9,9 ꢀthioxanthene] based SFTꢀ
TPAM, SFTꢀTPA are oxidized at even more positive potential of 0.99 and 1.01 eV, respectively.
The presence of a sulfur atom (instead of an oxygen atom) on the heterocyclic spiro core makes the
significant change in the oxidation potential, although their absorption profiles are close with each
other. This is due to the interaction between the 3P orbital of S and the π orbitals of carbons is
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1

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weaker than that between 2P orbital of O and the π orbitals of carbons. The same characteristic has
’
26
been observed in the spiro[fluoreneꢀ9,9 ꢀthioxanthene] core based small molecules.
The HOMO energy levels of SFXꢀTPAM and SFXꢀTPA calculated from the oxidation
potentials are ꢀ5.21 and ꢀ5.57 eV, respectively whereas SFTꢀTPAM and SFTꢀTPA have the HOMO
energy levels of ꢀ5.69 and ꢀ5.71 eV, respectively. The optical band gaps estimated from the onset of
the absorption spectra (in CH Cl solution) are 2.29 eV (SFXꢀTPAM), 2.94 eV (SFXꢀTPA), 2.88
2
2
eV (SFTꢀTPAM) and 2.93 eV (SFTꢀTPA). The LUMO energy levels were calculated from the
optical band gap and HOMO levels, all data are also summarized in Table 1 and Figure 5. Figure 5
reveals clearly that only SFXꢀTPAM having its HOMO energy level matches with the HOMO of
CH NH PbI (ꢀ5.4 eV) used in this paper for providing sufficient hole extraction capabilities
3
3
3
although all new HTMs can block the electrons transferred from the CH NH PbI photoactive layer
3
3
3
to ITO electrode within the PSC.
Figure 5: The frontier orbital energy levels of the four new HTMs and spiroꢀOMeTAD as well as
the components used in the fabrication of the inverted perovskite solar cell
3
.4 Photovoltaic performance
The photovoltaic performance of all four new HTMs used in the inverted perovskite solar cells
was verified even the HOMO levels of three of them do not match the HOMO energy of MAPbI3.
The structure of the cell is ITO/HTL/CH NH PbI /C /BCP/Ag (see Figure S1, SI)) and the
3
3
3
60
corresponding photovoltaic parameters are summarized in Table 2. The data clearly reveal that cell
1
2

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based on dopantꢀfree SFXꢀTPAM HTL has much higher power conversion efficiency (PCE)
compared to those used the other three HTMs. For the comparison, cells without HTL or using
dopantꢀfree spiroꢀOMeTAD HTL were also tested and the best PCE of 5.87% and 8.17%,
respectively were achieved (see Table 2). The frontier orbitals energy levels of the new HTMs and
MAPbI illustrated in Figure 5 reveal that besides sprioꢀOMeTAD, only SFXꢀTPAM has a suitable
3
HOMO (ꢀ5.21 eV) level for MAPbI (HOMO: ꢀ5.4 eV) to transport its photo excited holes. That is
3
the reason that SFXꢀTPAM has the best photovoltaic performance. After optimizing the spin rate
for depositing perovskite film and film post thermal annealing temperature the highest PCE of
1
0.23% was achieved for cell based on SFXꢀTPAM HTL. Cells based on SFXꢀTPA, SFTꢀTPAM
and SFTꢀTPA HTLs, which having HOMO levels lower than that of MAPbI , show the PCE even
3
lower than the cell without HTL, due to the hole blocking HOMO level and insulating nature of the
organic molecules. Moreover, the inverted PSC based on SFXꢀTPAM HTL exhibits higher
efficiency compared to that used dopantꢀfree spiroꢀOMeTAD HTL under the same devices
fabrication processes.
The IꢀV and IPCE curves of the cells based on dopantꢀfree SFXꢀTPAM and spiroꢀOMeTAD
HTLs displayed in Figure 6 clear disclose that the inverted PSC based on the former HTL has
larger Jsc (the Jsc integrated from IPCE spectra is consistent with that obtained from the IꢀV
measurements). Interesting, Figure 6 (b) reveal that PSC based on SFXꢀTPAM HTL has lower
IPCE value at short wavelength but higher at the wavelength longer than 635 nm compared to the
cell based on spiroꢀOMeTAD HTL. This may due to the difference in perovskite film quality
which will be discussed in more detail later. SFXꢀTPAM and spiroꢀOMeTAD have the right
HOMO level to transport hole from MAPbI to the electrode but show differentiated photovoltaic
3
performance. It is valuable to know some properties, such as the surface hydrophobicity, hole
mobility and charge extraction ability, of these two HTMs to learn how to design the small HTMs
for inverted PSC by.
Table 2: The photovoltaic parameter of the inverted perovskite based on various HTLs and
no HTL.
HTM
2
J (mA/cm )
V (V)
Fill Factor
Efficiency (%)
sc
oc
SFXꢀTPAM
19.12
1.01
0.53
10.23
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SFXꢀTPA
SFTꢀTPAM
SFTꢀTPA
15.35
12.17
11.32
15.20
17.02
0.64
1.04
0.68
0.99
1.00
0.39
0.25
0.37
0.39
0.48
3.83
3.16
2.85
5.87
8.17
none
spiroꢀOMeTAD
(
a)
80 (b)
20
15
10
5
0
60
40
20
Spiro-OMeTAD
SFX-TPAM
Spiro-OMeTAD
SFX-TPAM
0
300
0.0
0.2
0.4
0.6
0.8
1.0
400
500
600
700
800
Voltage (V)
Wavelength (nm)
Figure 6: (a) IꢀV and (b) IPCE curves of the PSC based on SFXꢀTPAM and spiroꢀOMeTAD HTLs.
3
.5 Surface hydrophilicity, hole mobility and charge extraction ability of SFX-TPAM and
spiro-OMeTAD HTLs and the quality of the perovskite films deposited on the two HTLs.
The surface wetting capability of HTLs is an important factor that affects the morphology of
the perovskite films deposited on them and therefore the photovoltaic performance of the
o
corresponding PSC. The water contacting angles of SFXꢀTPAM and siproꢀOMeTAD are 97 and
o
8
5 , respectively (the H O wetting pictures are shown in the Figure S2, SI). SFXꢀTPAM has a
2
4
2
slightly hydrophobic surfaces but the surface of siproꢀOMeTAD is weakly hydrophilic according
4
2
to the definition by Quere although the hydrophobicity has no significant difference. Recent
4
3
study suggests that a higher hydrophobic surface will facilitate the formation of the perovskite
films with larger grains although inverted PSC based on very hydrophilic PEDOT:PSS HTL
9
exhibits the highest efficiency of 20 % was proved. We found that the provskite films deposited
on more hydrophobic SFXꢀTPAM HTL has much larger grains compared to that coated on siproꢀ
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OMeTAD surface as the SEM images displayed in Figure 7. Therefore the superior photovoltaic
performance of SFXꢀTPAM HTL may due to MAPbI film deposited on SFXꢀTPAM has larger
3
grains. However, there are so many holes on the large perovskite grains. As a result PSC based on
SFXꢀTPAM HTL exhibits only 10% efficiency. Uv/Vis absorption spectra of the provskite films
on both HTLs are illustrated in Figure S3, SI. It was shown that MAPbI deposited on SFXꢀTPAM
3
has weaker absorption at short wavelength but stronger at long wavelength compared to that coated
on spiroꢀOMeTAD HTL.This is due to the difference in the light scattering of the perovskite films
which have different morphology.
IPCE curves displayed in Figure 6 (b).
Nevertheless, the absorption spectra are consistent with the
Figure 7: SEM images of MAPbI3 films spinꢀcoated on SFXꢀTPAM and spiroꢀOMeTAD films.
The Space‐Charge‐Limited Current (SCLC) was widely used to estimate the carrier mobility of
4
4
thin semiconductor film without knowing the conductivity (or carrier intensity) of the smaple.
The hole mobility of these new HTMs were also investigated by SCLC according to the Mottꢀ
4
5
Gurney’s law. The hole only device with the structure of ITO/ PEDOT:PSS/HTL/Au was
fabricated and its I-V curve under dark was measured. The interception of the log V vs log J curve
4
6
(
at the slope of 2) was used to calculate the hole mobility as reported previously. The I-V curves
of the holeꢀonly devices based on SFXꢀTPAM and spiroꢀOMeTAD are displayed in Figure S4, SI
1
5

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and the corresponding hole mobility data are also displayed in the insert of the Figure (the principle
of SCLC theory used to calculate the charge mobility is also described under Figure S4, SI). SFXꢀ
ꢀ4
2
ꢀ1 ꢀ1
ꢀ4
2
ꢀ1 ꢀ
TPAM film has the hole mobility of 2.0×10 cm V S which is lower than that (5.2×10 cm V S
1
)
of spiroꢀOMeTAD film. Therefore the difference in the photovoltaic performance is not due to
the hole mobility of the HTLs, mainly due to the quality of the provskite films coated on them. PL
spectra can provide the information regarding to the quality of the perovskite film and the charge
extraction capability at the CH NH PbI /HTL interface. The PL intensity (shown in Figure 8) of
3
3
3
CH NH PbI /SFXꢀTPAM is weaker than that of CH NH PbI /spiroꢀOMeTAD, and both are lower
3
3
3
3
3
3
than CH NH PbI /ITO which has no HTL indicating that both SFXꢀTPAM and spiroꢀOMeTAD
3
3
3
HTLs can extract hole from perovskite absorber. CH NH PbI /SFXꢀTPAM has weaker PL peak
3
3
3
but lower hole mobility than CH NH PbI /spiroꢀOMeTAD, suggesting that the hole extraction is
3
3
3
not totally determined by the hole mobility, probably because of the two HTLs have same order of
ꢀ2
magnitude in mobility. PSC based on SFXꢀTPAM HTL has higher Jsc value (19.12 mA cm ) than
ꢀ2
that used sprioꢀOMeTAD HTL (17.02 mA cm ) is due to MAPbI film with bigger grains was
3
formed on SFXꢀTPAM HTL as revealed by the SEM images displayed in Figure 7.
MAPbI /SFX-TPAM
3
MAPbI /Spiro-OMeTAD
3
MAPbI /ITO
3
600
650
700
750
800
850
Wavelength (nm)
Figure 8: PL spectra of MAPbI films deposited on SFXꢀTPAM film or spiroꢀOMeTAD film or
3
ITO/glass
4
. Conclusion:
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In conclusion, four heterocyclic spiroꢀtypes HTMs, namely SFXꢀTPAM, SFXꢀTPA, SFTꢀ
TPAM and SFTꢀTPA were synthesized by few synthetic steps with cost effective upscale. The
structureꢀphysicochemical property relationship of these four molecules were wellꢀstudied and their
application as HTLs in the inverted PSC was investigated. It was found that when sulfur was
introduced onto spiroꢀcore (replaced oxygen) will lower the HOMO and LUMO energy levels of
the molecules due to the weak interaction between the 3p orbital of sulfur atom and πꢀorbital of the
phenyl ring. On the other hand adding electronꢀdonating alkyl groups will raise the HOMO level
(such as SFXꢀTPAM/SFTꢀTPAM pair). As a result only SFXꢀTPAM with the HOMO level
compatible to that of MAPbI perovskite to perform good photovoltaic properties. Inverted PSC
3
based on dopantꢀfree SFXꢀTPAM HTL achieves a high PCE of 10.23% which is higher than that
(8.17%) of the cell using dopantꢀfree spiroꢀOMeTAD as a HTL. The better performance of the
SFXꢀTPAM HTL is due to that it has high mobility, therefore have extract hole more efficiently
compared to dopantꢀfree spiroꢀOMeTAD.
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The new spiro based PSC used HTM SFX-TPAM with good photovoltaic
performance was prepared through low cost facile route.