Diketopyrrolopyrrole or benzodithiophene-arylamine small-molecule hole transporting materials for stable perovskite solar cells

Article abstract of DOI:10.1039/c6ra18823k

Two simple small-molecular arylamine derivatives 4-methoxy-N-(4-methoxyphenyl)-N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline (OMeTPA-DPP) and 4,4′-(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (OMeTPA-BDT) linked with diketopyrrolopyrrole or benzodithiophene moieties have been synthesized. The new compounds show better thermal stability than spiro-OMeTAD. The steady-state and time-resolved photoluminescence demonstrate that the new compounds have good hole extraction ability. The perovskite solar cells employing OMeTPA-BDT show a comparable power conversion efficiency with that of spiro-OMeTAD. After more than 200 hours of aging under one sun illumination, the residual efficiencies of the PSCs based on OMeTPA-DPP, OMeTPA-BDT and spiro-OMeTAD are 8.69%, 11.15% and 9.08%, respectively. The results demonstrate that the newly-developed compounds can act as efficient hole transporting materials for stable perovskite solar cells.

Full text of DOI:10.1039/c6ra18823k

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DOI: 10.1039/C6RA18823K
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Diketopyrrolopyrrole or benzodithiophene-arylamine small-molecular hole
transporting materials for stable perovskite solar cells
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Xuepeng Liu,a,b Fantai Kong,*a Zhan’ao Tan,c Tai Cheng,c Wangchao Chen,a,b Ting
Yu,a,b Fuling Guo,a Jian Chen,a Jianxi Yaoc and Songyuan Dai*c,a
a Key Laboratory of Novel Thin-film Solar Cells, Institute of Applied Technology, Hefei
Institutes of Physical Science, Chinese Academy of Sciences, Hefei, Anhui, 230088, P.
R. China
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b University of Science and Technology of China, Hefei 230026, P. R. China
c Beijing Key Laboratory of Novel Thin-Film Solar Cells, North China Electric Power
University, Beijing, 102206, P. R. China
Corresponding authors:
E-mail: kongfantai@163.com. (F. Kong). Tel: +86 0551-65593222.
Email: sydai@ncepu.edu.cn. (S. Dai). Tel: +86 010-61772268.
Two simple small-molecular arylamine derivatives 4-methoxy-N-(4-methoxyphenyl)-
N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline (OMeTPA-DPP)
and 4,4'-(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(N,N-
bis(4-methoxyphenyl)aniline) (OMeTPA-BDT) linked with diketopyrrolopyrrole or
benzodithiophene moieties have been synthesized. New compounds show better
thermal stability than spiro-OMeTAD. The steady-state and time-resolved
photoluminescence demonstrate that new compounds have good hole extraction
ability. The perovskite solar cells employing OMeTPA-BDT show a comparable
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power conversion efficiency with that of spiro-OMeTAD. After more than 200 hours
DOI: 10.1039/C6RA18823K
of aging under one sun illumination, residual efficiency of the PSCs based on
OMeTPA-DPP, OMeTPA-BDT and spiro-OMeTAD are 8.69%, 11.15% and 9.08%,
respectively. The results demonstrate that the newly-developed compounds can act as
efficient hole transporting materials for stable perovskite solar cells.
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1. Introduction
Recently, organometal halide perovskite (CH3NH3PbX3, X = Cl, Br, I) have
drawn great attention as light absorbing material for their direct band gaps, large
optical absorption, excellent ambipolar charge mobility and small exciton binding
energy.[1-4] The breakthrough of perovskite solar cells (PSCs) is that Kim et al
reported solid-state devices using organic 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-
amine) 9,9′-spirobifluorene (spiro-OMeTAD) as hole transporting materials (HTMs)
to replace the liquid electrolyte, leading to a promising power conversion efficiency
(PCE) of 9.7%.[1] Since then, the solid-state PSCs lead to a remarkable PCE of
exceeding 20%.[5,6] Now HTMs are an important part of PSCs. Inorganic and organic
HTMs have been applied in PSCs. However, inorganic HTMs usually exhibit
relatively low efficiency in PSCs and are limited by materials selection (mainly Ni,
Cu compounds).[7-11] Organic HTMs are widely used in PSCs for their superior
performance. They are classified into small molecules and polymers.[12] The former
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mainly includes phenylamine derivatives,[13-19] thiophene derivatives,
[20-24]
phthalocyanine derivatives,[25-28] etc. The latter mainly includes tertiaryarylamine
polymer (PTAA),[5] aniline polymer,[29] diketopyrrolopyrrole and benzodithiophene
polymers.[30-35] Among these compounds, spiro-OMeTAD is the most widely used
HTM in PSCs. Nevertheless, it is expensive due to its complicated multi-step
synthesis and difficult purification process. Moreover, the devices based on it show
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inferior stability. These defects limit the large-scale industrial production DoOfI: 1c0o.1s03t-9/C6RA18823K
effective PSCs. Hence, developing new organic HTMs is necessary for cost reduction
and improving the performance of the devices.
To develop cost-efficient and stable HTMs for PSCs, herein, we introduce low-
cost DPP and BDT moieties as bridge into small-molecular compounds (OMeTPA-
DPP, OMeTPA-BDT) and find these two compounds can act as HTM when applied
in PSCs. The structures of them are shown in Fig. 1. Two molecules show better
thermal stability than spiro-OMeTAD. The charge dynamics at the HTMs/perovskite
interface are investigated by steady-state photoluminescence (PL) and time-resolved
PL measurements. And the long-term stability of the PSCs based on new HTMs are
superior to that of spiro-OMeTAD.
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Fig. 1 Molecular structures of OMeTPA-DPP and OMeTPA-BDT.
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2. Results and discussion
2.1 Optical electrochemical and thermal properties of the compounds
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Fig. 2a shows the UV-Vis absorption and fluorescence emission spectra of
OMeTPA-DPP and OMeTPA-BDT dissolved in dichloromethane (DCM), and the
characteristic of them are listed in Table 1. OMeTPA-DPP and OMeTPA-BDT
exhibit absorption peaks at 642 nm and 414 nm, respectively. The absorption
spectrum of OMeTPA-DPP shows stronger red shift than OMeTPA-BDT due to
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more powerful absorbing capacity of functionalized diketopyrrolopyrrolVeiew Article Online
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configurations than benzodithiophene configurations. The fluorescence spectra of
both the new compounds show large stokes shifts (64 nm and 69 nm), which suggest
that they can occur relatively large structural change upon excitation. The absorption
spectra confirm that the compounds don’t significantly absorb the visible light. The
optical band gap (Eg) of HTMs estimated from the intersection of the corresponding
normalized absorption and fluorescence emission spectra are 1.82 eV and 2.74 eV,
respectively.
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Fig. 2 (a) Normalized UV-vis absorption and photoluminescence, (b) cyclic
voltammogram with ferrocene as the reference in DCM and (c) thermogravimetric
analysis (TGA) of OMeTPA-DPP, OMeTPA-BDT and spiro-OMeTAD.
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Table 1. Summary of the optical, electrochemical and thermal properties of new
compounds
[a]
[a,b]
[c]
[f]
[g]
λmax
nm
/
λPL
nm
/
Eg / HOMO[d]/ LUMO[e]/ Td / Tg[g]/ Tm
/
HTM
eV
eV
eV
oC
oC
oC
OMeTPA-
DPP
642
414
706
483
1.82
-5.13
-3.31
305 157
399 158
169
215
OMeTPA-
BDT
2.74
-5.19
-2.45
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[a] UV-vis absorption spectra and fluorescence spectra were measured in CH2Cl2
solution. [b] Excitation at λmax. [c] From the intersection of absorption and emission
spectra. [d] From CV measurement and referenced to ferrocene. [e] ELUMO = EHOMO
+Eg. [f] Decomposition temperature, from TGA. [g] Glass-transition temperature and
melting temperature, from DSC.
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Cyclic voltammograms (CV) measurements (Fig. 2b) were carried out to get the
electrochemical properties of new compounds and spiro-OMeTAD. The pair of redox
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peaks of OMeTPA-DPP and OMeTPA-BDT is highly reversible, indicating that
DOI: 10.1039/C6RA18823K
they have excellent electrochemical stability. The HOMO energy levels are calculated
by the following equation[36]: EHOMO=-5.1-(Eox.HTM vs. Fc/Fc+) (eV), where Eox.HTM vs.
Fc/Fc+ is onset of oxidation potential with reference to ferrocene as standard. The
highest occupied molecular orbital (HOMO) energy levels of them calculated from
CV are -5.13 eV and -5.19 eV. Interesting, electron-withdrawing diketopyrrolopyrrole
bridge do not lower HOMO level of OMeTPA-DPP compared to OMeTPA-BDT,
and repeated CV data are shown in Fig. S2. HOMO levels of new compounds are
more positive than the conductive band of perovskite, indicating a favorable driving
force that may generate for electron-hole separation at the HTMs/perovskite interface.
And both of them are deeper than spiro-OMeTAD (-5.08 eV). It indicates that the
compounds have appropriate HOMO levels relating to the valence band of perovskite
and lead to higher open circuit voltage than that of spiro-OMeTAD. The lowest
unoccupied molecular orbital (LUMO) levels calculated from HOMO and Eg are
found to be -3.31 eV and -2.45 eV for OMeTPA-DPP and OMeTPA-BDT,
respectively.
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The thermal properties of the compounds are investigated by thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC). As demonstrated in Fig.
o
2c and Fig. S1, the results show that these two HTMs exhibit more than 150 C of
glass-transition temperature (Tg) and more than 300 oC of decomposition temperature,
respectively. The spiro-OMeTAD show a low Tg of 122 oC (shown in Fig. S1), which
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is similar with the reported value (Tg = 126 C).[37] The introduction of DPP or BDT
moieties into these new molecules as cores can lower the twisting force and increase
the π-conjugation ability than spiro-core. Moreover, the existence of heterocyclic
aromatic DPP or BDT into molecular main chain could decrease mobility of chain
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segment and molecular flexibility of these two new compounds than spiro-OMeTAD,
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which lead to high Tg than spiro-OMeTAD.38,39 And high thermal stability is
beneficial to the long-term durability of PSCs.
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2.2 Charge dynamics
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To investigate the charge transfer dynamics at the HTM/perovskite interface, we
measure charge generation of the perovskite film via steady-state and time-resolved
photoluminescence (PL). Theoretically, the quenching of steady-state PL and
reduction of PL lifetime indicate the efficient charge separation. From the steady-state
PL spectra in Fig. 3a, we can find that both of OMeTPA-DPP and OMeTPA-BDT
showed almost completely quenched (quenching efficiency 85% of OMeTPA-DPP
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and 96% of OMeTPA-BDT) compared with a perovskite film without any HTM,
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which indicate efficient charge transfer at HTM/perovskite interface.
The higher
quenching efficiency of OMeTPA-BDT may be more beneficial for effective PCEs
than that of OMeTPA-DPP. Moreover, the steady-state PL spectra of perovskite
films coated with HTMs reveal a hypochromatic shift, which may result from the
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optical properties of the perovskite/HTM interface
between perovskite surface and HTM.43
and the chemical interaction
Fig. 3b shows the time-resolved PL decay spectra of perovskite film coated with
OMeTPA-DPP, OMeTPA-BDT and one without any HTM layer. In the absence of
HTM layer, the excited perovskite has no other possibility than recombining with
electron within perovskite layer, indicating a longer decay time. Pristine perovskite
exhibits a lifetime of 3.77 ns, illustrates relatively slow carrier recombination in
perovskite layer. However, when the HTMs exhibit, the perovskite can be quenched
by the HTMs layer, resulting in a faster decay process. All the perovskite coated with
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HTMs show significantly reduced lifetime, meaning faster decay rates (1.9D9OIn: 1s0.1f0o3r9/C6RA18823K
OMeTPA-DPP and 0.96 ns for OMeTPA-BDT). Combined with the steady-state PL
spectra, the faster decay rates of perovskite coating with new HTMs show the
dynamical possibility of the charge transfer at HTM/perovskite interface.
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Fig. 3 (a) Time-integrated PL spectra. Excitation wavelength: 600 nm. (b) Time-
resolved PL spectra. Monitored at 765 nm, excitation at 445 nm.
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2.3 Device performance
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To prove the potential of OMeTPA-DPP and OMeTPA-BDT acting as HTMs iVniew Article Online
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PSCs, we compare the devices with new compounds and spiro-OMeTAD as HTM,
together with a cell without HTM. The cross-sectional field-emission scanning
electron microscopy (SEM) image of a conventional mesoscopic PSC (shown in Fig.
4a) shows typical structural configuration with clear interfaces. The thickness of
mesoporous TiO2 composed of perovskite and the HTM layer are about 500 nm and
150 nm. The device configuration of hybrid solar cells is shown in Fig. 4b. And Fig.
4c shows the energy level diagram of the materials used in our devices according to
the positions of HOMO and LUMO levels of the new HTMs.
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Fig. 4 (a) Cross-sectional SEM image of the perovskite solar cells. (b) Scheme of the
PSCs configuration. (c) Energy level diagram of the corresponding materials used in
devices.
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Fig. 5a shows the current-density-voltage (J-V) curves of fore-mentioned devices.
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The photovoltaic parameters of the PSCs are summarized in Table 2. As the PSCs
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have exhibited a greatly improvement in PCE by increasing the hole-conductivity of
HTMs via doping the additives such as lithium bis(trifluoromethanesulfonyl)imide
(Li-TFSI) and 4-tert-butylpiridine (TBP). We fabricate the devices by doping the two
additives into OMeTPA-DPP and OMeTPA-BDT, spiro-OMeTAD HTMs to
enhance cells performance.
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After preliminary optimization of devices, the PCEs of the PSCs with OMeTPA-
DPP, OMeTPA-BDT and spiro-OMeTAD as HTM are 8.63%, 10.89% and 13.45%,
respectively. The device fabricated without any HTM shows a PCE of only 5.41%.
Obviously, the improved PCEs of devices with OMeTPA-DPP or OMeTPA-BDT
than a device without HTM demonstrate that the new compounds have the potential to
be efficient HTMs in PSCs. The short circuit current (Jsc) of the devices fabricated
with OMeTPA-DPP, OMeTPA-BDT and spiro-OMeTAD are 14.87, 16.60, and
19.93 mA cm-2. The incident photon to current efficiency (IPCE) spectra represents
the ratio of extracted electrons to incident photons at the electrode surface at a given
wavelength. And corresponding IPCE spectra of the devices with OMeTPA-DPP,
OMeTPA-BDT, spiro-OMeTAD and without HTMs is shown in Fig. 5b. Compared
with hole-conductor-free device, the IPCE spectra of the PSCs with HTMs are
significantly improved in 400-800 nm. The devices with new compounds show lower
IPCE response in 400-800 nm than that of spiro-MeOTAD, which may result from
lower HOMO level of them than spiro-OMeTAD. So there is lower driving force for
electron injection efficiency between the perovskite layer and the OMeTPA-
DPP/OMeTPA-BDT layer. In addition, the integrated IPCE spectra of the devices
are in good agreement with the Jsc from each device. In perovskite solar cells, the
open circuit voltage (Voc) mainly depends on the HOMO energy level of HTMs and
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the Fermi level of TiO2. The HOMO energy level of OMeTPA-BDT is lower thaVniew Article Online
DOI: 10.1039/C6RA18823K
that of spiro-OMeTAD by about 150 mV. Therefore, the difference of 110 meV
between them lead to the average Voc of device based on OMeTPA-BDT are higher
than that of spiro-OMeTAD. The hysteresis characteristics of the PSC devices with
new HTMs and spiro-OMeTAD were evaluated from the forward and reverse scan
directions, as shown in Fig. S3 and table S1. There are similar hysteresis behavior
among the devices with new HTMs and spiro-OMeTAD
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Table 2. The current−voltage (J−V) characteristics of CH3NH3PbI3-based devices
with OMeTPA-DPP, OMeTPA-BDT and spiro-OMeTAD as HTM and with any
HTM for comparison measured under simulated AM 1.5G irradiation.
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HTM
Voc (V)
0.75
Jsc (mA cm-2)
11.21
FF(%)
63.9
63.0
68.5
71.1
PCE(%)
5.41
no HTM
OMeTPA-DPP
OMeTPA-BDT
spiro-OMeTAD
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14.87
8.63
0.96
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Fig. 5 (a) Current density-voltage of cells based on OMeTPA-DPP, OMeTPA-BDT,
spiro-OMeTAD and a cell without any HTM. (b) Corresponding incident photo-to-
current conversion efficiency (IPCE) spectrum of the devices.
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To study the stability of devices base on different HTMs, we further optimize the
device fabrication and gain a PCE of 10.05%, 12.81% and 14.20% for OMeTPA-
DPP, OMeTPA-BDT and spiro-OMeTAD, respectively (shown in Fig. S5). Then we
put these cells without encapsulation under one sun illumination. As shown in Fig. 6
and Table S5, we noted that the devices with OMeTPA-DPP and OMeTPA-BDT
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show a 14% and 13% loss of PCE, however, the spiro-OMeTAD-based shoDwOeI:d10.t1h03Ve9ie/wCA6rRtiAcl1e8O8n2li3nKe
most significant deterioration (36% loss of PCE) after 10 days of aging. The excellent
stability of the OMeTPA-DPP and OMeTPA-BDT based cells than that of spiro-
OMeTAD are attributable to excellent thermal stability and the existence of
hydrophobic alkyl chains on them.
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Fig. 6. Stability test for devices based on OMeTPA-DPP (■), OMeTPA-BDT (●),
and spiro-OMeTAD (▲).
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3. Conclusions
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In summary, we develop two simple small-molecular hole transporting materials with
high thermal stability by introducing DPP or BDT moieties into molecules. These two
HTMs can be synthesized by short route with high-yielding. Photoluminescence study
indicate that OMeTPA-DPP and OMeTPA-BDT can act efficient HTM in PSCs.
After 10 days of aging, residual efficiency of the PSCs based on OMeTPA-DPP,
OMeTPA-BDT and spiro-OMeTAD are 8.69%, 11.15% and 9.08%, respectively.
These HTMs are thus promising HTMs with the potential to replace spiro-OMeTAD
due to their good performance and simple synthesis route. This work also provide a
strategy for designing and developing new efficient and stable HTMs for PSCs.
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4. Experimental section
4.1 Materials and reagents
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All reagents and chemicals were purchased from Sigma-Aldrich, Alfa, Sinopharm
Chemical Reagent Co., Ltd., TCI and Derthon Optoelectronic Materials Science
Technology Co., Ltd. without further purification. FTO coated glass substrates with a
sheet resistance of 15 Ω·sq−1 were purchased from TEC company.
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4.2 Hole transporting materials synthesis
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The synthesis of OMeTPA-DPP and OMeTPA-BDT are simple with high yields by
well-know reaction (Suzuki cross-coupling reactions). The synthesis route of them is
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shown in Scheme 1. And the molecular structures were confirmed by H and 13C
NMR spectroscopy and mass spectrometry (MALDI-TOF). Detailed synthesis
procedures for both HTMs are shown as follows.
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Scheme 1. Synthetic routes to OMeTPA-DPP and OMeTPA-BDT.
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Synthesis
of
4-methoxy-N-(4-methoxyphenyl)-N-(4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenyl)aniline (compound 2). Under the protection of Ar,
compound 1 (3 mmol, 1.728 g), bis(pinacolato)diboron (3.75 mmol, 0.96 g), KOAc
(9.36 mmol, 0.9 g), dry 1,4-dioxane were added into a 50 mL flask. Then Pd2dba3 (14
o
mg) and X-Phos (30 mg) were added to the system and stirring at 80 C overnight.
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The reaction mixture was then cooled down to room temperature and extracDteOdI: 1w0.1i0t3Vh9ie/wCA6rRtiAcl1e8O8n2li3nKe
CH2Cl2, remove the solvent. The residue mixture was purified by column
chromatography (CH2Cl2/hexane=1:1) to obtain the white solid (product 2, 1.10 g,
74%). 1H NMR (400 MHz, DMSO) δ 7.47 (d, 2H), 6.93 (d, 4H), 6.70 (d, 4H), 6.68 (d,
2H), 3.76 (s, 6H), 1.26 (s, 12H). 13C NMR (100 MHz, CDCl3) δ 156.23, 151.44,
140.43, 135.80 , 127.20, 119.47, 118.65, 114.77, 83.41, 55.48, 24.89 .
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Synthesis of 3,6-bis(5-(4-(bis(4-methoxyphenyl)amino)phenyl)thiophen-2-yl)-2,5-
bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione
(OMeTPA-DPP).
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Compound 2 (1.2 mmol, 0.517 g), compound 3 (0.5 mmol, 0.34 g), 2M solution of
K2CO3 (8 mmol, 1.1 g) in H2O, [Pd(PPh3)4] (0.05 mmol, 58 mg), DMF (20 mL) were
added into a 100 mL Ar-protected flask. The reaction solution was kept with stirring
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at 90 C overnight. The reaction mixture was then cooled down to room temperature
and poured into cool water, extracted with CH2Cl2. The organic layer was dried with
MgSO4, and the solvent was removed by rotary evaporator. The residue mixture was
purified by column chromatography (CH2Cl2/hexane=1:1) to obtain the product as
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atropurpureus solid (OMeTPA-DPP, 415 mg, 73%). H NMR (400 MHz, CDCl3) δ
7.44 (d, 4H), 7.29 (d, 4H), 7.10 (d, 8H), 6.87 (t, 12H), 4.06 (m, 4H), 3.81 (s, 12H),
1.95 (s, 2H), 1.47 – 1.27 (m, 16H), 0.95 – 0.81 (m, 12H). 13C NMR (100 MHz, CDCl3)
δ 161.69, 156.38, 150.18, 149.40, 140.06, 139.49, 137.07, 127.34, 127.05, 126.78,
124.70, 122.87, 119.69, 114.84, 107.71, 55.49, 45.93, 40.93, 39.19, 30.30, 28.48,
23.08, 14.07, 10.60. HRMS (MALDI-TOF): [M-H] m/z calcd for C70H73N4O6S2:
1130.4898; found: 1130.4796.
Synthesis
of
4,4'-(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b']dithiophene-2,6-
diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (OMeTPA-BDT).
Compound 4 (0.6 mmol, 0.36 g), compound 2 (1.5 mmol, 0.65 g), 2M solution of
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K2CO3 (8 mmol, 1.1 g) in H2O, [Pd(PPh3)4] (0.05 mmol, 58 mg), DMF (20 mL) werVeiew Article Online
DOI: 10.1039/C6RA18823K
added into a 50 ml Ar-protected flask and following the same procedure to make
OMeTPA-DPP. The OMeTPA-BDT was obtained as a green solid (554 mg, 88%).
1H NMR (400 MHz, CDCl3) δ 7.60 – 7.46 (m, 6H), 7.10 (d, 8H), 6.95 (d, 4H), 6.85 (d,
8H), 4.18 (s, 4H), 3.81 (s, 12H), 1.82 (dd, 2H), 1.75 – 1.65 (m, 2H), 1.58 (dd, 4H),
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13
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1.53 (d, 2H), 1.40 (s, 8H), 1.02 (t, 6H), 0.93 (d, 6H). C NMR (100 MHz, CDCl3) δ
7
156.22, 148.93, 143.96, 143.36, 140.50, 132.60, 129.17, 127.08, 126.99, 126.21,
120.16, 114.83, 113.95, 75.92, 55.52, 40.71, 30.52, 29.26, 23.93, 23.19, 14.21, 11.38.
HRMS (MALDI-TOF): [M-H] m/z calcd for C66H71N2O6S2: 1052.4177; found:
1052.4655.
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4.3 Device Fabrication
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The fluorine-coated SnO2 glass substrate (FTO) with high transparency in the
visible range (2 cm × 1.5 cm) were etched in specific pattern with zinc powder and 1
M aqueous solution of HCl for 20 seconds. Then glass substrates were washed using
an ultrasonic bath for 30 min, rinsed with ethanol, annealed at 510 oC for 30 min.
The compact TiO2 layer was deposited on the etched FTO substrate by aerosol
o
spray-pyrolysis using O2 as the carrier gas at 450 C from a precursor solution of 1.2
ml titanium diisopropoxide and 0.8 ml bis(acetylacetonate) in 14 ml anhydrous
o
isopropanol, and then sintered at 510 C for 30 min. The mesoporous TiO2 film was
deposited by spin coating at 4000 rpm for 30 s with a ramp of 2000 rpm s-1 from a
diluted commercial TiO2 paste in ethanol (Dyesol 18NR-T, 2:7, weight ratio),
gradually heated to 510 oC, keep 15 min and cooled to room temperature in air. Then
the TiO2 films were treated in a 0.04 M aqueous solution of TiCl4 at 70 oC for 30 min,
rinsed with deionized water and following annealed at 510 oC for 30 min.
The PbI2 in DMF solution (461 mg mL-1) was dropped on the TiO2/FTO
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substrate by spin-coated at 3000 rpm for 30 s, put in room temperature for 5 min,
DOI: 10.1039/C6RA18823K
annealed on a hot plate at 70 °C for 30 min. After cooling down, the film was dipped
in 9 mg mL-1 CH3NH3I (prepared according to the previously reports 44) in 2-propanol
solution for 30 s, spin coating at 3000 rpm for 30 s and dried at 70 oC for 30 min. The
HTM layer was coated by spin-coating at 5000 rpm for 30 s. The concentration of
new HTMs and spiro-OMeTAD are 70 mg mL-1 in 1 mL of chlorobenzene with
heating to70oC overnight, then 28.8 μL of 4-tert-butylpyridine, 17.5 μL of lithium
bis(trifluoromethylsulphonyl)imide (520 mg Li-TFSI in 1 mL of acetonitrile) were
added to the solution. The HTMs were spin-coated on the CH3NH3PbI3/TiO2/FTO
substrate at 4000 rpm for 30 s. All device fabrications were performed below 15% of
relative humidity.
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Finally, Au layer with a thickness of 60 nm was deposited on HTM layer by
thermal evaporator to form the back contact. The active area of the device was
defined by a black mask with a size of 0.09 cm2 for all measurement.
4.4 Analytical measurement
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1H NMR and 13C NMR spectra were measured by a Bruker DPX 400 MHz
spectrometer with the chemical shifts against tetramethylsilane (TMS).
HRMS(MALDI-TOF) experiments were performed using a MS Bruker Daltonik
Reflex III and Bruker solariX spectrometer. The absorption spectra of the compounds
were recorded by a UV-Vis spectrophotometer (U-3900H, Hitachi, Japan). Cyclic
voltammograms (CV) were carried out on a CHI660D (Shanghai Chenhua Device
Company, China) in a three-electrode electrochemical cell with a Pt disk working
electrode, an SCE reference electrode and a platinum wire sphere with supporting
electrolyte of 0.1 mol L-1 tetrabutylammonium hexafluorophosphate (TBAPF6) in
CH2Cl2 at a scan rate of 50 mV s-1 using ferrocene as inner standard at the end of each
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measurement. The cross-sectional morphologies of the devices were observeDdOwI: 1i0t.h103Va9ie/wCA6rRtiAcl1e8O8n2li3nKe
field emission scanning electron microscopy (FE-SEM, sirion200, FEI Corp.,
Holland). The photocurrent-voltage (J-V) characteristics of the PSCs were measured
under AM 1.5 (100 mW cm-2) illumination which was provided by an AAA grade
solar simulator (Newport, USA, 94043A, calibrated with a standard crystalline silicon
solar cell). The incident photon to current conversion efficiency (IPCE) was
performed on QE/IPCE measurement kit (Newport, USA, CA). The steady-state
photoluminescence (PL) emission and time-resolved PL decay measurements were
carried out on a Fluorescence Detector (QM400 and LaserStrobe, Photo Technology
International, USA). Time-integrated PL emission spectra of film were recorded by a
standard 450 W xenon CW lamp. Time-resolved PL decay measurements were
carried out on a pulsed nitrogen/dye laser. Differential scanning calorimetry of HTMs
was recorded with scan rate of 20 oC min-1 (DSC, TA Instruments-Waters LLC, USA,
Q2000). The decomposition temperature of the compounds were carried out on a
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Thermal Gravimetric Analyzer (TGA, Q5000IR) at a heating rate of 10 C min-1 in a
temperature range of 0 oC to 800 oC under nitrogen atmosphere.
17
Acknowledgements
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This work was financially supported by the National Basic Research Program of
China (No. 2015CB932200), Natural Science Foundation of Anhui Province (No.
1508085SMF224).
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Table of Contents
Two new hole transporting materials with diketopyrrolopyrrole or benzodithiophene
moieties were developed for stable perovskite solar cells.