Polymeric hole-transport materials with side-chain redox-active groups for perovskite solar cells with good reproducibility

Article abstract of DOI:10.1039/c8cp04162h

Two monomers, M:OO and M:ON, and their corresponding polymers, P:OO and P:ON, were prepared from styrene derivatives N,N-diphenyl-4-vinyl-aniline with different substituents (-OCH₃ and -N(CH₃)₂) in the N-phenyl para positi

Full text of DOI:10.1039/c8cp04162h

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Polymeric hole-transport materials with side-
chain redox-active groups for perovskite solar
Cite this: Phys.Chem.Chem.Phys.,
cells with good reproducibility†
2
018, 20, 25738
a
a
b
Rosinda Fuentes Pineda,
Benjamin R. M. Lake,
Joel Troughton,
c
a
c
b
Irene Sanchez-Molina, Oleg Chepelin, Saif Haque,
Trystan Watson,
*
a
a
Michael P. Shaver
and Neil Robertson
*
Two monomers, M:OO and M:ON, and their corresponding polymers, P:OO and P:ON, were prepared
from styrene derivatives N,N-diphenyl-4-vinyl-aniline with different substituents (–OCH and –N(CH
3
3 2
) )
in the N-phenyl para positions. The polymers were synthesised and fully characterised to study their
function as hole transport materials (HTMs) in perovskite solar cells (PSCs). The thermal, optical and
electrochemical properties and performance of these monomers and polymers as HTMs in PSCs were
compared in terms of their structure. The polymers form more stable amorphous glassy states
and showed higher thermal stability than the monomers. The different substituent in the para position
influenced the highest occupied molecular orbital (HOMO) level, altering the oxidation potential. Both
monomers and polymers were employed as HTMs in perovskite solar cells with a device configuration
Received 1st July 2018,
Accepted 1st September 2018
2 2 3 3 3
FTO/bl-TiO /mp-TiO /CH NH PbI /HTM/Au resulting in power conversion efficiencies of 7.48% for
M:OO, 5.14% for P:OO, 5.28% for P:ON and 3.52% for M:ON. Although showing comparatively low efficiencies,
the polymers showed much superior reproducibility in comparison with Spiro-OMeTAD or the monomers,
suggesting further optimisation of polymeric HTMs with redox side groups is warranted.
DOI: 10.1039/c8cp04162h
rsc.li/pccp
(
mp-TiO
2
), (4) a perovskite layer, (5) a hole transport layer and
1
Introduction
(6) gold as a counter electrode. Furthermore, despite offering
1
In 2009, Miyasaka introduced the hybrid organic–inorganic high efficiencies, typical perovskite solar cells suffer from stability
lead halide perovskite as the light-absorbing material in liquid and durability problems. One promising approach for stabilising
dye-sensitised solar cells (DSSCs) reaching 3.8% power conver- PSCs is by modifying the hole transport material (HTM) with an
7,8
9–12
sion efficiency (PCE). To date, perovskite solar cells (PSCs) have inorganic material, a small organic molecule
or a polymer.
2
,3
13
achieved PCEs of over 20%, opening the possibility to further Spiro-OMeTAD is the most broadly used HTM in highly efficient
reduce the cost of solar cell modules. This rapid improvement solid-state devices. Nevertheless, the lengthy synthesis and
over a short period is due to the unique properties of the expensive purification are barriers to its commercialization.
perovskite material including high panchromatic absorption, large The structure of the hole transport material plays an important
4–6
carrier diffusion length and low non-radiative recombination.
role for achieving high efficiency and also helps to protect the
1
4
Based on the perovskite properties, there have been several sensitive perovskite layer from air and moisture. Hence some
developments in the cell configuration for perovskite solar cells, researchers have focussed on replacing Spiro-OMeTAD with a
yet the most common device structure comprises six mains cheaper material that can give similar efficiencies and improve
1
5
components: (1) a conductive glass substrate (fluorine tin oxide the stability of these devices. Triarylamine derivatives are
FTO), (2) a compact TiO
2
blocking layer, (3) mesoporous TiO
2
widely used as hole-transporting materials due to their electron
donating and hole transporting capabilities. Polymeric triaryl-
amines may offer improved properties compared to their
monomeric or oligomeric derivatives, such as easy film formation
to allow for low-cost manufacture of large scale devices. One of
the most notable advantages is the higher glass transition
a
EaStCHEM School of Chemistry, The University of Edinburgh, King’s Buildings,
David Brewster Road, Edinburgh, EH9 3FJ, UK. E-mail: neil.robertson@ed.ac.uk
b
c
SPECIFIC, Swansea University Bay Campus, Fabian Way, Swansea, SA1 8EN, UK.
E-mail: T.M.Watson@swansea.ac.uk
Department of Chemistry, Imperial College London, London, SW7 2AZ, UK.
E-mail: s.a.haque@imperial.ac.uk
g
temperature (T ) and amorphous nature of the polymers,
suggesting advantageous stability and reproducibility of end
devices. Furthermore, the hydrophobic properties of polymers
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8cp04162h
2
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N-( p-(dimethylamino)phenyl-N-(p-methoxyphenyl)-4-vinyl-aniline)
(
M:ON), and their corresponding side-chain polymers (P:OO and
23
P:ON, Fig. 1). In a previous study, J ¨a ger and co-workers synthe-
sized similar styrenic triarylamines with different substitutes
which were later polymerized by nitroxide-mediated poly-
merisation (NMP) to prepare block copolymers for directional
charge transfer. In this work, two different substituted styrenic
triarylamines were prepared followed by inexpensive free radical
polymerization using AIBN to study and compare their proper-
ties and function as HTMs in perovskite solar cells.
2
Results and discussion
The synthetic route to both monomers and polymers is shown
in Scheme 1 and detailed synthetic procedures can be found in
the Experimental section. P:OO and P:ON were prepared by the
free radical polymerization of M:OO and M:ON, respectively.
Fig. 1 Chemical structure of the HTMs used in this study. Each monomer
with its corresponding polymer.
may act as a protecting layer for the perovskite film to external Free radical polymerization was chosen to provide a cost-
atmosphere or contaminants, which may also enhance the solar effective route to the polymeric HTMs, avoiding complications
cell stability. Previous studies of polymers as promising HTMs from controlled radical polymerization methods to directly
1
6
in PSCs include: poly(3-hexylthiophene) (P3HT), poly[2,6-(4,4- compare monomers and polymers. The target triarylamines
0
bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b ]dithiophene)-alt- containing a styrene unit were synthesized by the palladium
1
7
0
24
4
,7(2,1,3-benzothiadiazole)] (PCPDTBT),
poly[N-9 -hepta- catalysed Buchwald–Hartwig coupling due to its scalability.
0
0
0
0
0
decanyl-2,7-carbazole-alt-5,5-(4 ,7 -di-2-thienyl-2 ,1 ,3 -benzo- All materials were characterised in detail by nuclear magnetic
thiadiazole)] (PCDTBT), poly-triarylamine (PTAA) and others.
18
19
12
1
13
resonance (NMR) ( H and C) spectroscopy, elemental analysis
In incorporating HTMs into polymers, they can be and mass spectrometry (MS). We sought to determine polymer
embedded either within the backbone (main-chain) or pendant molecular weight by gel permeation chromatography, however
to the chain (side-chain). Most polymers used in perovskite we encountered column blockages. By MALDI TOF mass spectro-
cells are main-chain polymers. Side-chain polymers have been metry (Fig. S2, ESI†) we could observe the lower molecular
2
0
studied before in organic electroluminescent devices (EL) and weight components and confirm the repeat units, however this
2
1,22
organic field effect transistors (OFET)
because they are technique is not able to determine average molecular weight.
The optical properties of monomers and polymers were
comparable with their low molecular weight analogs in terms
of the electronic properties while also having high solubility investigated by UV/Vis and photoluminescence (PL) spectro-
and good thermal properties. To our knowledge, side- scopy in dichloromethane solution. All compounds exhibit a
chain polymers have not been studied before in perovskite strong absorption around 300 nm almost independent of the
solar cells. Furthermore, there is no systematic comparison of substituents, with no significant absorption in the visible
polymers and their parent monomers in solar cells. Here region. Transparent HTMs with no absorption in the visible
we investigate two different monomers, namely the styrene region provide additional advantages and flexibility as they can
derivatives N,N-di(p-methoxyphenyl)-4-vinyl-aniline (M:OO) and be used in PSC cell architectures such as inverted and tandem
Scheme 1 Schematic representation of the synthesis of triarylamines by Hartwig–Buchwald coupling and preparation of the polymers by radical
t
polymerization. Reaction conditions: (i) Pd
2
(dba)
3
, phosphine ligand, NaO Bu, toluene, 18 h, 80 1C, N
2
; (ii) Pd
2
3
(dba) , phosphine ligand, bromostyrene,
t
NaO Bu, toluene, 12 h, 110 1C; (iii) AIBN, toluene, 120 1C (see Experimental section for details).
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Fig. 2 Left: Normalized emission spectra of monomers and polymers in the solid state thin film. Right: Square-wave voltammetry of the monomers and
polymers. M:OO (blue line), M:ON (pink line), P:OO (green line) and P:ON (purple line).
structures where it is important that there is no competition the compounds were estimated from the half-wave potential
with the perovskite layer. The monomers present two adjacent using ferrocene/ferrocenium as an internal standard in square-
bands corresponding to p–p* and n–p* absorption, while for the wave voltammetry experiments (Fig. 2, right). The four com-
polymers one single band from n–p* absorption is observed. pounds show an oxidation process assigned to the oxidation of
Photoluminescence spectra of the materials as thin films are the side-chain redox-active group triarylamine moiety. The
illustrated in Fig. 2, with solution data show in Fig. S3b (ESI†). influence of the substituents is reflected by a shift in the
In contrast to UV-Vis, the emission energies are significantly potential of the redox couple. The oxidation of M:OO and
influenced by the substituents. The thin-film results show a clear P:OO occurs around +0.22 V and +0.19 V, whereas the electron
redshift of the PL spectrum in the series with increasing donating Me N-substituent of M:ON and P:ON causes a shift to
2
electron-donating character of the substituents. This trend could lower potentials (ꢀ0.1 V and ꢀ0.15 V) thus increasing their
be explained as an effective S1 energy stabilisation due to the HOMO energy level to ꢀ5.0 eV and ꢀ4.95 eV respectively. This
presence of electron-donating groups of various strengths, likely makes M:ON and P:ON significantly stronger donor molecules
linked with rotation around the N-aryl bond after excitation. than M:OO, P:OO and Spiro-OMeTAD. These observations were
Moreover, a hypsochromic shift is observed upon polymeriza- explained with density functional theory calculations using
tion, which can be attributed to both the loss of the conjugated Gaussian 09 with B3LYP6-31(d) level of theory in dichloro-
double bond and the steric shielding effect (solvent exclusion). methane (DCM). For the polymers, a model of the monomer
The polymers have a more rigid and compact structure, which fragment with saturated alkyl chain was used to calculate their
may lead to steric shielding of the interior units, such that lower electronic properties. The calculated trend of HOMO energy
interaction with the solvent molecules leads to less stabilisation levels matches the experimental data, and they were shown to
of the emissive state explaining the observed blue-shifted emis- delocalise over the p orbitals of the triphenylamine unit and the
sion. The optical band gaps, shown in Table 1, were determined peripheral substituents. The delocalisation of the HOMO onto
from the intersection of the excitation and the emission spectra. the peripheral substituents (Fig. 3) explains the large shift in
The oxidation potential and derived energy levels of the the oxidation potential upon changing the substituent from
HTMs are fundamental parameters for constructing high- MeO- to Me N-. A summary of the optical and electrochemical
2
performance PSCs. The electrochemical properties were investi- properties of these materials is presented in Table 1. These
gated by cyclic voltammetry (CV) and square-wave voltammetry. results indicate an energetically-favourable hole transfer from
From the CV measurements (Fig. S3a, ESI†), it can be noted the perovskite (CH
3 3 3
NH PbI ) to the HTM.
that the redox peaks of all the HTMs are highly chemically Thermal properties of the monomers and polymers were
and electrochemically reversible, indicating excellent chemical estimated by Differential Scanning Calorimetry (DSC), and the
stability and rapid electron transfer. The HOMO energy levels of results are displayed in the ESI† (Fig. S5 and S6). Both polymers
Table 1 Optical, electrochemical and thermal properties
ꢀ1
ꢀ1
a
b
c
d
HTM
lmax (nm)
e (cm
M
)
l
em (nm)
E
gap (V)
E
ox (V)
E
HOMO (eV)
g
T (1C)
M:OO
M:ON
P:OO
P:ON
308*, 336
309*, 334
300*
307*
385
22 500
23 500
19 000
23 000
—
457
424
395
511
424
3.15
3.29
3.39
2.99
3.05
+0.22
ꢀ0.10
+0.19
ꢀ0.15
+0.03
ꢀ5.32
ꢀ5.00
ꢀ5.29
ꢀ4.95
ꢀ5.13
X
X
252.8
252.8
—
Spiro-OMeTAD
a
b
c
Excitation at lmax*. Optical gap, determined from the intersection of the excitation and emission spectra. From CV measurements and referenced
d
25
to ferrocene.
E
HOMO (eV) = ꢀ5.1 ꢀ Eox
.
2
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are the photoluminescence lifetimes of the perovskite in the
absence and presence of the HTM layer, respectively (Table S1,
ESI†). These were found to be 0.88 ꢁ 0.03 (M:OO), 0.85 ꢁ 0.05
(
P:OO), 0.61 ꢁ 0.01 (M:ON), 0.53 ꢁ 0.02 (P:ON). It was found
that monomers reduce the lifetime of the perovskite emission
more effectively than polymers, indicating more effective
charge extraction. Surprisingly, M:ON and P:ON showed lower
hole-extraction yields than M:OO and P:OO, despite their lower
oxidation potential, suggesting this is not the key factor in hole
extraction efficiency.
We also sought to investigate charge mobility in the
polymers by FET measurements by depositing the materials
onto prepatterned chips with gold electrodes. We were unable
however to observe significant gate effect, possibly due to
mismatch of energy levels or electrode contacts. This may also
indicate that mobility in the polymers is not ideal, which would
Fig. 3 Molecular orbital distribution of HOMO of monomers and polymer
model derivatives at B3LYP/6-31G(d) level of theory.
affect the resulting solar cell performance, particularly Jsc
vide infra) and remains an aspect for further optimisation in
future studies.
We prepared a set of perovskite solar cells in the configu-
of 252.8 1C. On the other hand monomers M:OO and M:ON, do ration FTO/bl-TiO /mp-TiO /CH NH PbI /HTM/Au (Fig. 4). All
not present T
. M:OO presented a melting point of 74.7 1C and HTMs were doped using similar concentrations of additives
,
(
(P:OO and P:ON) showed the same glass transition temperature
2
2
3
3
3
g
no melting point was found for M:ON. These results confirms [LiTFSi] and tBP (tert-butylpyridine) as described in the ESI.†
that the polymers form a more stable amorphous glassy state The current density–voltage ( J–V) characteristics were mea-
with higher thermal stability.
sured under simulated air mass 1.5 global (AM 1.5G) solar
To investigate the ability of the monomers and polymers to irradiation. Fig. 5 shows the J–V curves characteristic of the
extract holes, we measured steady-state and transient photo- champion devices and the results are summarised in Table 2.
luminescence (PL) decay. Samples were prepared by spin-coating All devices were fabricated in a single continuous study over 15
2 3
of the perovskite onto a mesoporous Al O
layer with the HTM repeat cells for each HTM to facilitate comparison between the
on top. Details of the sample preparation and measurement are reported HTMs and Spiro-OMeTAD. Spiro-OMeTAD presented
described in the Experimental section. The perovskite exhibited the highest efficiency of 15.09%. The corresponding values for
a strong PL peak near 760 nm as shown in Fig. S7 (ESI†) and PL M:OO, P:OO and P:ON of 7.48%, 5.28% and 5.14% are reasonable
is largely quenched when any of the HTMs (M:OO, P:OO, M:ON
and P:ON) was coated onto the perovskite, indicating an effective
charge extraction into the HTM. From the transient PL decays
(
Fig. S8, ESI†), photoluminescence lifetimes were obtained by
fitting the decays with exponentials. We calculated the efficiency
of hole transfer, calculated as 1 ꢀ t /(t + t ), where t and t
q
0
q
0
q
Fig. 5 J–V curves of the champion PSCs of the new HTMs and Spiro-
OMeTAD.
Table 2 Device parameters of the champion cells
ꢀ2
HTM
PCE (%)
Jsc (mA cm
)
Voc (V)
FF (%)
M:OO
P:OO
M:ON
P:ON
7.48
5.14
3.52
5.28
15.09
12.81
10.61
6.79
12.49
20.56
0.89
0.85
0.68
0.65
0.95
65.54
57.17
76.19
64.73
63.81
Fig. 4 Top: Energy diagram for perovskite (CH
and polymers. Bottom: Device structure in the configuration FTO/bl-TiO
mp-TiO /CH NH PbI /HTM/Au.
3 3 3
NH PbI ), monomers
2
/
2
3
3
3
Spiro-OMeTAD
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Fig. 6 Solar cells parameter over 15 repeats for each HTM.
considering that this work represent the first solar cells study with 3 Conclusions
these HTMs and that Spiro-OMeTAD has gone through extensive
In conclusion, we present the synthesis and characterisation of
two styrenic triphenylamine-based monomers and their corres-
ponding side-chain polymers for use as HTMs in perovskite solar
cells. The structures contained different electron-donating groups
in the para-position, leading to significantly different redox
potentials and HOMO energies. The optical, electrochemical,
thermal properties and device performance of the monomers
and polymers were compared in terms of their structure. From
this study, it was found that despite offering lower efficiencies in
comparison with Spiro-OMeTAD and monomers, the polymers
presented much higher reproducibility in their solar cell perfor-
mance parameters. The polymers have significantly smaller stan-
dard deviation in the power conversion efficiency and fill factor
data which we attribute to better and more stable amorphous
character resulting in a more homogeneous film. The results
found in this work may encourage further efforts on polymeric
HTMs with redox-active side groups for perovskite solar cells
which may have advantages in manufacture and scale up due to
enhanced reproducibility.
optimisation of doping and processing procedures for many
years. For the polymer materials, the champion cells show
significant hysteresis, although less so in the average Jsc (Fig. S9,
ESI†). The most significant observation is that in comparison with
both monomers and Spiro-OMeTAD, the polymers exhibit photo-
voltaic parameters with significantly smaller standard deviation,
leading to average PCE 3.94% ꢁ 0.68 for P:OO and 4.22% ꢁ 0.48
whereas for Spiro-OMeTAD, M:OO and M:OO average values are
11.7% ꢁ 2.96, 4.24% ꢁ 2.03 and 1.03% ꢁ 1.23. The difference in FF
values are even greater with polymers showing 51.31% ꢁ 6.41 for
P:OO and 63.24% ꢁ 1.71 and Spiro-OMeTAD, M:OO and M:ON
presenting 62.92% ꢁ 14.67, 44.67% ꢁ 15.54 and 43.67% ꢁ 19.98.
These result can be attributed to the difference in the morphology.
The polymers form a more stable amorphous state and have
higher thermal stability which result in a more homogenous film
deposition during the device fabrication. Accordingly, when
comparing performance parameters with molecular design of
the new materials, it is clearly more meaningful to make
comparisons between the two polymers where the cells were
very reproducible. Although PCE values in the polymers are very
similar, we note the increase in the average short circuit current
4
Experimental details
(Jsc) values upon increasing the electron donating character from
2
ꢀ2
4.1 Materials and synthesis
9
.58 mA cm for P:OO to 10.68 mA cm for P:ON. On the other
hand, the HOMO level of P:ON is higher (Fig. 4, top) than P:OO All reagents were purchased from either Sigma-Aldrich or Alfa-
leading to smaller open circuit voltage (V ) value; 0.78 V for Aesar and they were used as received without further purifica-
oc
P:OO and 0.62 V for P:ON. Differences in fill factor are more tion unless otherwise stated.
difficult to interpret and may relate to film morphology as well as
4
.2 Chemical characterization
inherent electronic factors. Fig. 6 shows the box plots with the
mean and standard deviation of the solar cell parameters and
the results are summarised in Table S2 (ESI†).
1
13
H and C NMR spectra were recorded on a Brucker Advance
1
13
500 spectrometer (500 MHz for H and 124 MHz for C) relative
2
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to deuterated solvents as indicated in individual synthesis 2.88 (s, 6H). C NMR (126 MHz, DMSO-d
PCCP
1
3
6
) d 148.93, 148.11,
description. MS were recorded on Bruker ESI Micro-Tof equipped 140.62, 136.72, 136.47, 128.92, 127.48, 127.35, 126.79, 118.83,
with LC using electrospray ionization (ESI). Elemental analyses 115.30, 114.02, 111.27, 55.71, 40.82, 39.77. Anal. calcd for
were carried out by Stephen Boyer at London Metropolitan C H N O: C, 80.20; H, 7.02; N, 8.13; found: C, 80.05; H,
2
3
24 2
+
University.
7.16; N, 8.05): [M] calcd 344.46 found 345.1961.
P:OO was synthesised via free radical polymerisation (FRP).
An ampoule was charged with M-OO (1.0 g, 3.0 mmol), AIBN
(5.0 mg, 30 mmol) and anhydrous toluene (2 mL). The resulting
4
.3 Synthesis and characterisation
4
2 3
-Ethenyl-N,N-bis(4-methoxyphenyl)benzenamine Pd (dba)
(
4
1.31 mmol, 1.2 g), tri-o-tolylphosphine (6.5 mmol, 2 g), solution was heated at 120 1C for 20 hours. After this time, the
,4 -dimethoxydiphenylamine (43.62 mmol, 10 g) and NaOtBu reaction mixture was cooled to ambient temperature. The
0
(
50 mmol, 4.8 g) were added into a Schlenk tube and dried reaction mixture was added dropwise to methanol (75 mL),
under vacuum for 30 minutes. 4-Bromostyrene (5.8 mmol, inducing the precipitation of the polymer, which was collected
.7 g) and toluene (30 mL) were all degassed and added to the by filtration. It was necessary to re-dissolve (in a minimum of
reaction mixture and the contents heated at 110 1C overnight THF) and re-precipitate (in methanol) the collected polymer to
under N . The crude material was purified first by an extrac- ensure that all remaining monomer was removed. Finally, the
tion with water following by a silica plug (70 : 30 Hex/EtA) of purified polymer was dried in vacuum, yielding an off-white
0
2
1
the organic phase. Solvent was removed from the solution solid. Yield: 0.55 g. H NMR (500 MHz, DMSO-d
under vacuum and the product purified by flash column (chain, 3H), 3.5–3.8 (OMe, 6H), 6.3–7.0 (aromatic, 12H).
chromatography (SiO , hexanes up to hexanes/EtOAc 80 : 20) P:ON was synthesised by an analogous route, yielding an off-
6
) d 1.4–1.6
2
1
1
to afford a yellow powder (5.3 g, 52.5% yield). H NMR white solid. H NMR (500 MHz, DMSO-d ) d 1.5–1.7 (chain, 3H),
6
(
500 MHz, DMSO-d ) d 7.31–7.24 (m, 2H), 7.05–6.98 (m, 4H), 2.7–3.2 (NMe , 6H), 3.5–3.8 (OMe, 3H), 6.5–7.3 (aromatic, 12H).
6
2
6
1
1
.96–6.88 (m, 4H), 6.76–6.69 (m, 2H), 6.61 (dd, J = 17.6,
4
.4 Thermal characterisation
1.0 Hz, 1H), 5.61 (dd, J = 17.6, 1.2 Hz, 1H), 5.08 (dd, J =
1
3
0.9, 1.3 Hz, 1H), 3.75 (s, 6H). C NMR (126 MHz, DMSO-d
6
)
Differential scanning calorimetry (DSC) was performed on
ꢀ
1
d 119.65, 115.43, 111.73, 55.72, 39.77, 39.59. Anal. calcd for NETZSCH STA 449F1 at a scan rate of 5 K min under nitrogen
: C, 79.73; H, 6.39; N, 4.23; found: (C, 79.81; H, 6.50; atmosphere in DSC/TG aluminium pan. The measurement
22 2
C H21NO
+
N, 4.34): [M] calcd 331.42 found 331.1571.
range was 25 1C to 300 1C.
0
4
-Methoxy-4 -(dimethylamino)diphenylamine 4-bromo-N,N-
4
.5 Optical characterization
(115 mg, 0.125 mmol), JohnPhos ligand (74 mg, Solution UV-visible absorption spectra were recorded using a Jasco
.250 mmol) and NaOtBu (3.35 g, 35 mmol) were minutes. V-670 UV/Vis/NIR spectrometer controlled with SpectraManager
Previously degassed dry toluene (66 mL) was added and the software. Photoluminescence (PL) spectra were recorded with a
mixture was stirred at 80 1C for 48 h under N . The crude Fluoromax-3 fluorimeter controlled by ISAMA software. All
material was purified first by an extraction with water/DCM and samples were measured in a 1 cm cell at room temperature
dimethylaniline (5 g, 25 mmol), p-anisidine (3.7 g, 30 mmol),
Pd (dba)
2
3
0
2
ꢀ
5
a silica plug (70 : 30 Hex/EtOAc). Solvent was removed from the with dichloromethane as solvent. Concentrations of 5 ꢂ 10
M
ꢀ6
solution under vacuum and the product was purified by flash and 2 ꢂ 10 M were used for UV/Vis and PL respectively.
column chromatography (SiO , hexane/EtOAc 9.9 : 0.10) to afford
2
1
4.6 Electrochemical characterization
a yellow powder (5.3 g, 52.5% yield). H NMR (500 MHz,
benzene-d ) d 6.97–6.90 (m, 2H), 6.83–6.74 (m, 4H), 6.65–6.60 All cyclic voltammetry measurements were carried out in
m, 2H), 4.73 (s, 1H), 3.37 (s, 3H), 2.56 (s, 6H). freshly distilled CH Cl using 0.3 M [TBA][BF ] electrolyte in a
-Ethenyl-N,N-bis(4-methoxy-4 (dimethylamino)diphenylamine three electrode system, with each solution being purged with
6
(
2
2
4
0
4
Pd
2
(dba)
3
(0.25 mmol, 0.23 g), tri-o-tolylphosphine (1.24 mmol,
2
N prior to measurement. The working electrode was a Pt disk.
0
0.38 g), 4-methoxy-4 -(dimethylamino)diphenylamine (8.26 mmol, The reference electrode was Ag/AgCl and the counter electrode
2
g) and NaOtBu (9.5 mmol, 0.9) were added into a Schlenk tube and was a Pt rod. All measurements were made at room temperature
dried under vacuum for 30 minutes. 4-Bromostyrene (5.8 mmol, using a mAUTOLAB Type III potentiostat driven by the electro-
.7 g) and toluene (15 mL) were all degassed and added to the chemical software GPES; square wave voltammetry (SWV) was
reaction mixture and the contents heated at 110 1C overnight carried out at a step potential of 2 mV, square wave amplitude of
under N . The crude material was purified by an extraction with 25 mV, and a square wave frequency of 25 Hz. Ferrocene was
0
2
water following by a silica plug (70 : 30 Hex/EtA). Solvent was used as the internal standard in each measurement.
removed from the solution under vacuum and the product
4.7 Computational details
purified by flash column chromatography (SiO , hexanes up
2
to hexanes/EtOAc 80 : 20) to afford a thick yellow oil (1.6 g, All calculations were carried out using the Gaussian 09 program
1
8
7
6
0% yield). H NMR (500 MHz, DMSO-d ) d 7.28–7.21 (m, 2H), with Lee Yang–Parr correlation functional (B3LYP) level of
6
.05–6.97 (m, 2H), 7.00–6.91 (m, 2H), 6.94–6.86 (m, 2H), theory. All atoms were described by the 6-31G(d) basis set. All
.77–6.66 (m, 4H), 6.60 (dd, J = 17.6, 11.0 Hz, 1H), 5.58 (dd, structures were input and processed through the Avogadro
J = 17.6, 1.2 Hz, 1H), 5.05 (dd, J = 10.9, 1.2 Hz, 1H), 3.74 (s, 3H), software package.
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.8 Perovskite solar cells and characterisation
Paper
4
wavelength of 450 nm and slit widths of 10 nm. Time-
correlated single-photon counting was recorded using a Deltaflex
spectrometer (Horiba Yobin-Tbon), using an excitation of
ꢀ1
FTO substrates (7 O sq ) were etched with zinc powder and
HCl (2 M aqueous solution) to give the desired electrode
patterning. The substrates were cleaned in a solution of deter-
gent and deionised water before sequential sonication in
deionised water, acetone and isopropanol and a 10 minute
oxygen plasma treatment to remove the last traces of organics.
4
04 nm and measuring the emission of the perovskite at 770 nm.
Conflicts of interest
The FTO substrates were subsequently coated with a compact There are no conflicts of interest to declare.
layer of TiO2 (50 nm) by spray pyrolysis deposition using
titanium diisopropoxide bis(acetylacetonate) in anhydrous
ethanol as precursor solution (volume ratio 1 : 9). After cooling
Acknowledgements
from 450 1C, a dilute suspension of TiO
Dyesol 30NR-D: ethanol) was deposited by spin coating
4500 rpm, 30 seconds). The samples were then heated at
2
nanoparticles (2 : 7 wt,
RFP thanks CONACYT, Mexico for a PhD studentship. We thank
EPSRC EP/H040218/1; EP/M023532/1 for financial support.
(
150 1C for 10 minutes, followed by sintering at 550 1C for
3
0 minutes. Upon cooling, samples were immediately trans- Notes and references
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2
2
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NH
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PbI
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2
3
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hole transporters materials were dissolved in chlorobenzene
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1
(75 mg mL ) with the standard additives 5-tert-butylpyridine
(32 mL) and lithium bis(trifluoromethanesulfonyl) imide (20 mL,
ꢀ
1
5
20 mg mL solution in acetonitrile). Hole transport solutions
5
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measuring the performance of the solar cells, simulated sunlight
was generated using an AAA-rated solar simulator (Newport)
calibrated with KG-5 filtered Si reference cell (Newport).
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.9 Time resolved photoluminescence
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2
3
2
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ꢀ
1
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