Effect of alkyl chain length on the properties of triphenylamine-based hole transport materials and their performance in perovskite solar cells

Article abstract of DOI:10.1039/c7cp07682g

A new series of diacetylide-triphenylamine (DATPA) derivatives with five different alkyl chains in the para position, MeO, EtO, ⁿPrO, ⁱPrO and BuO, were synthesised, fully characterised and their function as hole-transport materials in perovskite solar cells (PSC) studied. Their thermal, optical and electrochemical properties were investigated along with their molecular packing and charge transport properties to analyse the influence of different alkyl chains in the solar cell parameters. The shorter alkyl chain facilitates more compact packing structures which enhanced the hole mobilities and reduced recombination. This work suggests that the molecule with the methoxy substituent (MeO) exhibits the best semiconductive properties with a power conversion efficiency of up to 5.63%, an open circuit voltage (V_oc) of 0.83 V, a photocurrent density (J_sc) of 10.84 mA cm^-2 and a fill factor of 62.3% in perovskite solar cells. Upon replacing the methoxy group with longer alkyl chain substituents without changing the energy levels, there is a decrease in the charge mobility as well as PCE (e.g. 3.29% for BuO-DATPA). The alkyl chain length of semiconductive molecules plays an important role in achieving high performance perovskite solar cells.

Full text of DOI:10.1039/c7cp07682g

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Effect of alkyl chain length on the properties of triphenylamine-
based hole transport materials and their performance in
perovskite solar cells†.
Rosinda Fuentes Pinedaa, Joel Troughtonb, Miquel Planellsa, Irene Sanchez-Molina Santosc, Farmin Muhitha,
Received 00th January 20xx,
Accepted 00th January 20xx
Gary S. Nichol,a Saif Haquec, Trystan Watsonb* and Neil Robertsona*.
DOI: 10.1039/x0xx00000x
A new series of diacetylide-triphenylamine (DATPA) derivatives with five different alkyl chains in the para position, MeO,
www.rsc.org/
n
i
EtO, PrO, PrO and BuO, was synthesised, fully characterised and their function as hole-transport materials in perovskite
solar cells (PSC) studied. Their thermal, optical and electrochemical properties were investigated along with their
molecular packing and charge transport properties to analyse the influence of different alkyl chains in the solar cells
parameters. The shorter alkyl chain facilitates more compact packing structures which enhanced the hole mobilities and
reduced recombination. This work suggests that the molecule with the methoxy substituent (MeO) exhibits the best
semiconductive properties with a power conversion efficiency of up to 5.63%, an open circuit voltage (Voc) of 0.83 V, a
photocurrent density (Jsc) of 10.84 mA cm-2 and a fill factor of 62.3% in perovskite solar cells. Upon replacing the methoxy
group by longer alkyl chain substituents without changing the energy levels, there is a decrease in the charge mobility as
well as PCE (e.g. 3.29% for BuO-DATPA). The alkyl chain length of semiconductive molcules plays an important role for
achieving
high
performance
perovskite
solar
cells.
1 Introduction
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Figure 2. Left: Normalized UV-Vis absorption (solid line) and emission (dashed line) of each DATPA derivative. EtO-DATPA (black line), iPrO-DATPA (red line), nPrO-DATPA (purple
line) and BuO-DATPA (green line). Right: Square-wave voltammetry of EtO-DATPA (black line), iPrO-DATPA (red line), nPrO-DATPA (purple line) BuO-DATPA (green line).
b)
In recent years, organic-inorganic halide perovskites have Spiro-OMeTAD and progress on the preparation methods and
generated tremendous interest from the academic cost. However, to date Spiro-OMeTAD remains the preferred
community, showing rapid improvements in their performance choice.
as absorbers in photovoltaic solar cells. Starting from 3.8% in Triarylamines (TAA)26 are widely used as a result of the easy
2009 by Miyasaka1 to a current certified record efficiency of oxidation of the nitrogen centre and their capacity to transport
22.1%2,3, perovskite-based solar cells have surpassed values of positive charges via radical cations. In previous work reported
the commercialised polycrystalline silicon solar cells and are by our group, a series of triphenylamine-based HTMs having
currently competitive with the established thin-film two diacetylide-triphenylamine (DATPA)27 moieties and
technologies. This rapid development is mainly the product of varying redox potentials was prepared by a simple synthetic
the remarkable properties of the perovskite absorber route which allows easy tuning of electrochemical and other
materials, e.g. MAPbX3 (MA= methylammonium; X = I, or properties through varying the substituent R groups. These
mixed I/Br, I/Cl), such as high panchromatic absorption, large materials were studied as HTMs in PSCs and showed promising
carrier diffusion length, low non-radiative recombination and properties in comparison with Spiro-OMeTAD. The chemical
easy processability, leading to great potential for low cost and structure of the HTMs can significantly affect the hole transfer
large scale technologies4–6
. Different configurations for kinetics between the HTM and the perovskite and the HTM
photovoltaic devices have been proposed including and the metal contact which affect the performance of the
mesoscopic structure, planar layers and HTM-free devices. solar cell28,29. The length of alkyl chain for instance, can
Nevertheless, the most-common standard architectures for influence the solubility, molecular ordering and charge
these devices comprise six main constituents: 1) a conductive transport of the HTMs thus affecting the performance of the
glass substrate FTO (fluorine tin oxide), 2) a compact TiO2 solar cell30,31
blocking layer, 3) mesoporous TiO2 (mp-TiO2), 4) a perovskite
layer, 5) a hole transport layer and 6) gold as counter
electrode. One of the main components in these devices is the
hole-transporting semiconductor either in the form of a π-
conjugated polymer7,8, an inorganic salt9–11 or small organic
molecule. Although many different materials have been
explored, most attention has focused on small molecules due
to their simplicity, ease of purification and batch-to-batch
consistency. Spiro-OMeTAD12 is the most common hole
transport material (HTM) in perovskite solar cells showing high
conversion efficiencies. Yet, Spiro-OMeTAD is expensive due to
its lengthy synthetic procedure and problematic purification.
To address these limitations, researchers have explored
alternative HTMs structures with different motifs such as
.
carbazole13,14
derivatives18–20
,
triarylamine derivatives15–17
,
spiro-based
Figure 1. Chemical structure of HTMs used in this study. Each molecule is
,
triazatruxenes21,22 azulenes23, and other
,
comprised of two bridged Ph3N moieties.
related examples24,25, leading to efficiencies comparable with
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Table 1 Summary of the optical and electrochemical properties
HTM
λmax (nm)
ε
(cm-1 M-1)
60300
69200
64280
65990
-
λem (nm)a
Egap (V)b
Eox
(V)c
EHOMO (eV)d
EtO-DATPA
nPrO-DATPA
iPrO-DATPA
BuO-DATPA
Spiro-OMETAD
389
389
389
389
385
504
504
508
495
424
2.85
2.85
2.84
2.85
3.05
+0.28
+0.26
+0.28
+0.26
+0.03
-5.38
-5.36
-5.38
-5.36
-5.13
aExcitation at λmax
.
bEgap is the optical gap determined from the intersection of absorption and emission spectra. cFrom SWV and CV measurements and referenced
42
to ferrocene.dEHOMO(eV) = -5.1- (Eox
) .
Although the large majority of triarylamine HTMs include greater systematic understanding of this effect for HTMs in
methoxy groups to tune the redox potential, little research has PSCs is important Accordingly, we report a series of new
been done to study the effect of the alkyl chain on the diacetylide-triphenylamine substituents HTM coded as EtO-
i
n
properties of the HTM. In previous studies Hagfeldt30 and co- DATPA, PrO-DATPA, PrO-DATPA and BuO-DATPA with similar
workers demonstrated the importance of the substituents by optical and electrochemical properties to the MeO–DATPA
tuning the position and length of the alkyl chain in four studied before27. We have systematically investigated the
triphenylamine-based organic HTMs. In a different study, effect of varying the alkyl chain in structurally-similar HTMs on
Nazeeruddin32 and co-workers studied the influence of alkyl material properties and solar cell performance parameters.
chain length on the power conversion efficiency of a series of
hole transport material. Such reports are rare however, and
2 Results and Discussion
there are still limited comprehensive studies of the effect of
alkyl chain length on material and device properties. In the
2.1 Synthesis
case of organic photovoltaics, studies have shown both
The synthetic route (Scheme 1) reported for MeO-DATPA in
our previous work27 was revised and optimised to improve
increases and decreases in power-conversion efficiency
through increasing the length of alkyl chains31,33–35, therefore
(b)
(a)
(c)
(d)
Figure 3. Crystal packing of all DATPA derivatives: (a) EtO-DATPA, (b) iPrO-DATPA, (c)nPrO-DATPA and (d) BuO-DATPA
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yields and reduce reactions times. Detailed experimental
procedures are described in the Supporting Information. Apart
from EtO-DATPA, the method of synthesis involved five stages.
The first step is a simple SN2 reaction, however for EtO-DATPA
1-Ethoxy-4-iodobenzene is commercially available, and this
step was omitted. The triarylamine moieties were formed with
HOMO
a
Copper Catalysed Ullmann36 coupling reaction, and
subsequently the acetylene was attached with silyl
a
protecting group via Sonogashira cross-coupling37. The silyl
group was removed using TBAF (tetrabutylammonium
fluoride), leaving the compound with a terminal alkyne. The
final product was obtained by a Copper-mediated Oxidative
Homocoupling38 reaction at room temperature. The analytical
and spectroscopic data for all HTMs are consistent with the
formulated structures
.
LUMO
2.2 Optical and electrochemical properties
The UV-Vis absorption spectra of the DATPA derivatives (EtO,
i
nPrO, PrO, BuO) measured in dichloromethane (DCM), are
shown in Figure 2 (left, solid line). All compounds exhibit an
absorption peak at 389 nm independent of the substituent.
Molar extinction coefficients were calculated for each
compound using the Lambert-Beer law and results are
displayed in Table 1. Additionally, photoluminescence (PL)
spectra were recorded (dashed line, Figure 2 left). The results
show similar PL spectra for all DATPA derivatives with a
maximum emission at 493 nm.
derivatives. The calculated trend of HOMO and LUMO matches
the experimental data, and no significant difference was found
among the DATPA series. The HOMO (Figure 4) is delocalized
over the π orbitals of the triphenylamine unit and the
diacetylene bridge. The alkyl groups do not contribute to the
HOMO energy level; therefore any change beyond the oxygen
does not affect the optoelectronic properties, which coincides
with the electrochemistry results. The LUMO electron density
is localised on the diacetylene bridge and is also not
significantly influenced by the substituents of the
Optical gaps were determined from the intersection of the
excitation and the emission spectra. Cyclic voltammograms
(CV) of the DATPA series are displayed in Figure S5 (Supporting
information). The redox peaks of all HTMs are chemically and
electrochemically reversible, indicating excellent stability and
rapid electron transfer. These materials present two oxidation
peaks corresponding to one electron oxidation for each
triarylamine unit. However, in cyclic voltammetry, the two
oxidation process are strongly overlapped because of their
similar potentials. To distinguish both processes, square-wave
voltammetry (SWV) was carried out (Figure 2, right). The
triphenylamine unit.
A summary of the optical and
electrochemical properties of all DATPA derivative is presented
in Table 1. As expected, different alkyl substituents do not
significantly modify the optical and electrochemical properties
of the hole transport materials. This confirms that this series is
appropriate to compare the structural, morphological and
interface effects of differing alkyl chains without any
similarity of the two redox potentials suggests
a low
interaction between the two triarylamines units. The HOMO
energy level is of most importance for hole transfer from the
perovskite and HTM and from molecule to molecule. The
HOMO energy levels were calculated from the CV data using
the following equation EHOMO=-5.1-(Eox), where Eox is the
oxidation potential of the HTM with reference to ferrocene.
The extracted values are listed in Table 1. Overall these results
indicate that the hole transfer from CH3NH3Pbl3 to the HTM is
energetically favourable. Spiro-OMeTAD has a HOMO of -5.13
eV, which it is at higher energy than the materials reported
here.
Table 2 Charge transport and thermal properties
HTM
Hole Mobility µFE Tg
(cm2/Vs)a
Tm
(oC) b
(oC)b
MeO-DATPA
EtO-DATPA
iPrO-DATPA
nPrO-DATPA
BuO-DATPA
1.90x10-4
1.45x10-4
x
1.11x10-4
8.81x10-5
76.9
69.5
70.7
x
197.3
149.4
160.8
148.0
118.9
Density Functional Theory (DFT) calculations were performed
to predict the electronic properties of all HTMs using Gaussian
09 with B3LYP 6-31G(d) level of theory. Figure S1 (Supporting
Information) shows the position of the HOMO–LUMO energy
levels and their electron density on the molecules of all DATPA
x
aEstimated from OFET measurements. bDetermined from differential
scanning calorimetry (DSC).
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complicating changes to the optoelectronic properties.
2.3 XRD analysis
Single crystals of the new molecules were grown by the slow
solvent evaporation method to determine the crystal packing
(Figure 3) by X-ray diffraction (XRD). The compounds were
found to crystallize in the monoclinic space group P21/n (EtO-
DATPA, nPrO-DATPA), monoclinic space group P21/c (BuO-
DATPA) with half of the formula unit present in the
asymmetric unit and the other half consisting of symmetry
i
equivalent atoms; PrO-DATPA crystallizes in the triclinic space
group P-1 showing two independent molecules in the
asymmetric unit (Figure S3, Supporting Information). In every Figure 5. J–V curves (light and dark) of the champion PSCs with the DATPA series and
Spiro-MeOTAD HTMs.
case the molecule crystallises without the presence of any
solvent within the unit cell. The EtO-DATPA molecules are
the aromatic moieties resulting in the absence of face to face
packed in
a layer-by-layer configuration with molecules
stacking. The crystal lattice shows C-H⋯O-R (2.665 Å), C-H⋯HC
(2.270 Å, 2.304 Å) formed between one hydrogen atom of the
arranged in a herringbone motif characterised by edge-to-face
contacts. The intersection angle between two adjacent
fragments is measured to be 72.18o. The distance between
alkyl chain and CH group of an aromatic ring. The molecules
also interact via C-H
⋯ π (2.720 Å) formed between an
parallel fragments is too large to induce co-facial π
interactions. The molecules interact mainly via C-H π (3.154
Å) and C-H HC formed by the CH groups of phenyls
(dCH HC=2.554 Å, 2.634 Å). The EtO-DATPA molecules also
⋯ π
aromatic centroid and the alkyl chain. The crystal packing of
MeO-DATPA was reported and discussed in more detail in our
previous work27, showing a herringbone pattern, with one
molecule in the asymmetric unit and a regular stacking
distance between the molecules. Overall, it was observed that
as the alkyl chain increases, the number of van der Waals
interactions between alkyl chains also increases while the H-
bonding contacts decrease. Among all of the examples, MeO-
DATPA and EtO-DATPA showed the strongest and highest
number of intermolecular interactions (H-bonding) while
fewer H-bonds were found in the crystal lattice of BuO-DATPA
but stronger van der Waals interactions within the alkyl chains.
Longer alkyl chains result in the poorer stacking of the
molecule and therefore weaker intermolecular interactions.
Furthermore, it is known that closer molecular arrangements
can facilitate charge transport in thin film devices39. The X-ray
powder diffraction patterns were experimentally obtained and
compared with those calculated from their single-crystal data
(Figure S6-7, Supporting Information). The powder pattern of
all DATPA derivatives showed some crystallinity, and were in
agreement with the same phase as the calculated pattern from
single-crystal data. A summary of the crystallographic data is
depicted in the supporting information Table S4.
⋯
⋯
⋯
form intermolecular H-bonding interactions in the crystal
lattice formed by the oxygen and the hydrogen atom of the
aromatic rings (dC-H⋯O-R=2.715 Å, 2.804 Å, 2.916 Å). The
crystal structure of nPrO-DATPA also exhibits a layer-by-layer
herringbone pattern. The T-shaped edge-to-face and the
parallel-displaced stacking arrangement predominate.
n
Although similar, the structure of PrO-DATPA is stabilised via
π⋯π interactions. The distance between two parallel
fragments and the intersection angle of adjacent molecules
were measured to be 3.44 Å and 46.63o respectively. The nPrO-
DATPA molecules also interact via C-H
⋯ π (3.086 Å) and C-
H
⋯
O-R (shortest distance found 2.725 Å). A more complex
packing is observed for iPrO-DATPA. The X-ray crystal structure
reveals three crystallographically unique molecules in the
asymmetric unit. Two lie on crystallographic inversion centres
while the third lies entirely in general positions. Each unit is
surrounded by six molecules arranged in an alternating layered
pattern with an intersection angle of 77.59o. The interactions
i
arising in the structure of PrO-DATPA are mainly C-H
⋯ π
(3.034 Å, 2.813 Å, and 2.875 Å) and C-H
⋯O-R (shortest
distance found 2.712 Å). Furthermore, C-H O-R contact is also
⋯
observed between the CH group of the alkyl chain and the
2.4 Thermal properties
i
oxygen atom (2.788 Å). The aromatic cores of the PrO-DATPA
Thermal properties of the HTMs were evaluated by Differential
Scanning Calorimetry (DSC), and the results are presented in
the Supporting information (Figure S9-10) The DSC results
showed a glass transition temperature (Tg) of 69.5oC and
70.7oC for EtO-DATPA and PrO-DATPA. PrO-DATPA and BuO-
DATPA however do not present Tg. On the other hand, lower
melting points were observed for longer alkyl chains. These
observations can be explained by the difference in the crystal
packing, and their intermolecular interaction observed in the
are aligned with a very strong slipped π-stacking configuration
such that no π⋯ π interactions are observed. In the case of
BuO-DATPA, the molecules are arranged in a zig-zag largely
i
slipped stack packing. Similar to PrO-DATPA, the aromatic
i
n
cores have a very strong slipped π-stacking with an average
distance of 4.820 Å which it is too large to induce π-orbital
overlap in the crystalline structure. The bulkiness of the alkyl
chain in the para position largely impacts the overlapping of
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XRD analysis. Longer alkyl chains presented poorer stacking as 2.6 Transient absorption spectroscopy
the molecules are more disordered which results in weaker
To further probe the performance in PSCs, the charge transfer
dynamics occurring in an mp-TiO2/CH3NH3PbI3/HTM
arrangement for all DATPA derivatives were obtained by
ultrafast absorption spectroscopy (TAS). Samples were
prepared by spin-coating of the perovskite onto a mesoporous
TIO2 layer and with the HTM on top. Details of the sample
preparation and measurements are described in the
experimental section. The corresponding steady-state
absorption spectra are shown in Figure S11b, where all
samples show the dominant spectral features of the perovskite
layer CH3NH3PbI3, with the onset at 800 nm and the bands
corresponding to the oxidised HTMs (at 1600 nm). Figure S11a
shows the TAS time profiles following 510 nm excitation
probed at 1600 nm. All signals have a negative feature that
interactions. This explains why the BuO group showed the
lowest melting point. The MeO and EtO groups have the
highest melting points and the strongest intermolecular
i
interactions. We can also observe that branched PrO has a
higher melting point than the straight-chain nPrO, although
nPrO-DATPA (2080 Å3/molecule) has a more compact structure
i
i
than PrO-DATPA (2213 Å3/molecule). The PrO chain typically
has a smaller surface area which leads to weaker Van der
Waals interactions. However as we can observe in the crystal
packing, iPrO shows shorter C-H⋯O-R interactions, which likely
explains the higher melting point in comparison to nPrO.
2.5 Charge Transport Characteristics
Charge mobility is an important parameter to evaluate the arises from some charge separation already happening in the
properties of hole-transport materials in photovoltaic devices. ground state, and the intensity of these bleaches correlates
To investigate the hole mobilities, OFETs (Organic Field-Effect with the intensity of the band of the oxidised HTMs observed
Transistors)40 were fabricated for four of the compounds in the steady-state absorption spectra. The analysis of the
reported here (MeO-DATPA, EtO-DATPA, nPrO-DATPA and normalised TAS time profiles also displays a hint of the
BuO-DATPA) and the mobilities were extracted from the lifetimes of the charge-separated states. From the results,
saturation regime of the transfer characteristic curves (Figure MeO-DATPA presents the most long-lived signal indicating a
S8, Supporting Information). Details of the device fabrication greater charge-pair separation. Among the other DATPA
and measurement are found in the Experimental section. It derivatives, no significant differences of the lifetimes of the
i
was not possible to fabricate a high-quality thin film for PrO- charge separation states could be resolved, albeit all were
DATPA. In general, the molecules with the shorter alkyl chains shorter than for MeO-DATPA.
presented higher mobilities which can be attributed to
stronger intermolecular interactions in the XRD analysis. A 2.7 Solar cell studies
summary of the estimated hole mobilities is presented in Table
To investigate the effect of the alkyl chain of these HTMs in
2.
perovskite solar cells, we prepared a set of PSCs in the
configuration FTO/bl-TiO2/mp-TiO2/CH3NH3PbI3/HTM/Au. All
Figure 6. Solar cells parameters over 7 repeats for each HTM
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In summary, we have presented the synthesis and
characterisation of a new series of diacetylide-triphenylamine
(DATPA) derivatives to study their function in PSCs. These
molecules have the same backbone structures but different
alkyl chain length in the para position, i.e., ethoxy (EtO),
propoxy (nPrO), isopropoxy (iPrO) and butoxy (BuO). Although
the length of the aliphatic side chain does not influence the
optoelectronic properties of the molecules such as the HOMO
levels, it strongly affects the charge transport properties and
transfer dynamics of the molecules as well as the morphology
and photovoltaic performance in PSCs. The molecules with
shorter alkyl chains have a more ordered and compact
structure which facilitates charge transport in the thin films.
These results are in agreement with the previous work found
by Hagfeldt41 and Nazeeruddin. The methoxy substituent
shows the best semiconductive properties and PCE of 5.63%
compared with the longer side chain. This is due to the
molecular packing and faster charge transport. This work
provides insight on the importance of the alkyl chain length in
the design of organic semiconductors.
Table 3 Summary of device performance for champion cells
HTM
PCE (%)
Jsc (mA cm-2)
Voc (V)
FF (%)
MeO-DATPA
5.63
10.84
0.83
62.25
EtO-DATPA
nPrO-DATPA
iPrO-DATPA
BuO-DATPA
Spiro-OMeTAD
1.02
4.83
3.24
3.29
15.34
4.10
10.33
9.89
0.77
0.79
0.80
0.77
1.00
32.19
59.22
41.11
61.19
78.13
7.02
19.67
DATPA HTMs were doped using two times the concentration
of additives typically used for Spiro-OMeTAD12 which include
bis(trifluoromethylsulfonyl)imide lithium salt (Li-TFSI), 4-tert-
butylpyridine
(tBP)
and
tris(2-(1H-pyrazol-1-yl)-4-tert-
butylpyridine)- cobalt(III)tris(bis(trifluoromethylsulfonyl)imide)
(FK209). Full details of the solar cell fabrication are given in the
experimental section. Current-voltage scans were recorded
using an AAA-rated solar simulator calibrated against a KG5
global (AM 1.5 G) solar irradiation. All devices were fabricated
in a single continuous study over 7 repeat cells for each HTM
to facilitate comparison. Figure 6 shows the box plots with the
mean and standard deviation of the solar cell parameters and
a summary of the data can be found in Table S12 (Supporting
information) . The J-V graphs of the champion cells are shown
in Figure 5 and the results are summarised in Table 3. Despite
the identical backbone structures and HOMO energy levels,
the difference in the alkyl chain length had an effect on the
photovoltaic device performances. Among the DATPA series,
MeO-DATPA shows the highest performance with a PCE of
5.63%. Upon increasing the length of alkyl chain the
performance of the devices decreases to 4.83% for nPrO-
DATPA, 3.24% for iPrO-DATPA and 2.54% for BuO-DATPA. Also
noticeable is the decrease in the short circuit current values
(Jsc) from 10.84 mA cm-2 for MeO-DATPA to 7.02 mA cm-2 for
BuO-DATPA. These trends are in broad agreement with
mobility and structural packing trends, with the exception of
EtO-DATPA which gives unexpectedly poor cells. Since the
HTMs in the device are doped, it is perhaps unsurprising to see
some deviation from the expected trend since the morphology
of the doped materials will also play a role in the final
properties. The methoxy substituent presents the best
semiconductive properties in comparison with the longer alkyl
chains. The variance in PCE values can be attributed to the
differences in the photocurrent density and fill factor which
may have resulted from the higher hole mobility, greater
charge-pair separation and optimal morphology. This is
sustained by the results from the hole mobility data, transient
absorption spectroscopy studies and XRD analyses discussed
previously.
4 Experimental details.
4.1 Chemical characterization
1H and 13C NMR spectra were recorded on a Brucker Advance
500 spectrometer (500 MHz). The deuterated solvents are
indicated in the synthesis description. Chemical shifts, δ, are
given in ppm, using the solvent residual as an internal
standard. MS were recorded on Micro-Tof using electrospray
ionisation (ESI) technique. Elemental analyses were carried out
by Stephen Boyer at London Metropolitan University
.
4.2 Optical characterization
Solution UV-Visible absorption spectra were recorded using a
Jasco V-670 UV/Vis/NIR spectrometer controlled with
SpectraManager software. Photoluminescence (PL) spectra
were recorded with a Fluoromax-3 fluorimeter controlled by
ISAMain software. All samples were measured in a 1 cm cell at
room temperature in dichloromethane as solvent.
Concentrations of 2.5 x 10-5 M and 1 x 10-6 M were used for
UV/Vis and PL respectively.
Crystallographic details
Powder diffraction was performed on a Bruker Discover D8
with CuKa1/2 source and a scintillation detectior 4860.
Single yellow block-shaped crystals of EtO-DATPA were
recrystallised from benzene by slow evaporation. Single
orange block-shaped crystals of iPrO-DATPA were
recrystallised from benzene by slow evaporation. Single yellow
plate-shaped crystals of nPrO-DATPA were recrystallised from
a mixture of DCM and hexane by slow evaporation. Single
orange block-shaped crystals of BuO-DATPA were
recrystallized from benzene by slow evaporation. A suitable
crystal of EtO-DATPA (0.24×0.16×0.14), iPrO-DATPA
3 Conclusions
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Journal Name
(0.51×0.23×0.10), nPrO-DATPA (0.55×0.29×0.09) and BuO-
DATPA (0.25×0.24×0.14) were selected and mounted on a
MITIGEN holder in Paratone oil on a Rigaku Oxford Diffraction
SuperNova diffractometerThe crystal was kept at T = 120.0 K
during data collection. Using Olex2 (Dolomanov et al., 2009),
the structure was solved with the ShelXS (Sheldrick, 2008)
structure solution program, using the Direct Methods solution
method. The model was refined with version of ShelXL
(Sheldrick, 2008) using Least Squares minimisation. A summary
of the data collection and structure refinement is reported in
Table S4.
ꢌ
ꢅꢆ
2ꢎ
ꢄ
ꢀꢁꢂ = ꢃ
ꢇꢈꢋ . ꢍ
ꢒ
ꢅꢉꢊ
ꢏꢐꢑ
Where Z is the channel width, L the channel length, Ci the
capacitance, VG the gate voltage and IDS is the drain current
and ꢅꢆꢇꢈꢓꢅꢉꢊ , the slope of the Transfer Characteristic Curves
ꢄ
in the saturation regime40.
4.7 Transient Absorption Spectroscopy
Sample preparation
4.3 Thermal characterization
A solution of mesoporous TiO2 in terpineol (Dyesol 30 nm TiO2,
weight ratio 1:2) was spin coated on a 10 mm by 10 mm glass
square, with an acceleration of 12,000 rpm for 30 s. The pieces
of glass were cleaned with isopropanol (IPA) in a sonicator for
5 minutes before spin coating. After spin coating, the
substrates were placed into a 450 °C oven for 1 hour and
cooled down to room temperature before deposition of the
perovskite layer. For the deposition of the perovskite layer 1M
solution of PbI2 and MeNH3I in DMSO was prepared. This
solution was spin-coated onto the glass slides covered with the
mesoporous oxide via a 3-step spin coating process: (i) 1000
rpm, 10s, 2000 acc; (ii) 5000 rpm, 20s, 2000 acc; (iii) 6000 rpm,
20s, 2000acc. Toluene (300 µL) was dropped on the substrates
by the end of the second step. The films were then annealed at
50˚C for 20 min and at 100˚C for 25-30 min and cooled down
to room temperature. A 20 mg/mL solution in chlorobenzene
of the corresponding HTM was spin-coated onto the
perovskite layer at 200 rpm for 30 s, with an acceleration of
2000.
Differential scanning calorimetry (DSC) was performed on
-1
NETZSCH STA 449F1 at a scan rate of 5 K min under a
nitrogen atmosphere in DSC/TG aluminium pan. The
measurement range was 25 o C to 250 oC.
4.4 Electrochemical characterization
All cyclic voltammetry measurements were carried out in
freshly distilled CH2Cl2 using 0.3 M [TBA][BF4] electrolyte in a
three-electrode system with each solution being purged with
N2 prior to measurement. The working electrode was a Pt disk,
the reference electrode was Ag/AgCl, and the counter
electrode was a Pt rod. All measurements were made at room
temperature using a µUTOLAB Type III potentiostat, driven by
the electrochemical software GPES. Cyclic voltammetry (CV)
measurements used scan rates of 100 mV/s: square wave
voltammetry (SWV) was carried out at a step potential of 4
mV, square wave amplitude of 25 mV, and a square wave
frequency of 10 Hz, giving a scan rate of 40 mV/s. Ferrocene
was used as the internal standard in each measurement.
Measurements
UV-Vis was performed on a PerkinElmer UV/VIS Spectrometer
Lambda 25. Photoluminescence spectra were recorded on a
Horiba Yobin-Ybon Fluorolog-3 spectrofluorometer, using an
excitation wavelength of 450 nm and slit widths of 10 nm. For
pump-probe micro to millisecond transient absorption
spectroscopy, films were excited by a dye laser (Photon
Technology International GL-301, sub-nanosecond pulse
width) pumped by a pulsed nitrogen laser (Photon Technology
International GL-3300). A quartz halogen lamp (Bentham IL1)
was passed through a monochromator and used to probe
changes in the absorption characteristics of the film as a
function of time after the laser excitation. The probe light was
detected using home-built silicon (≤ 1000 nm) or InxGa1-xAs (>
1000 nm) photodiodes and an oscilloscope. Unless otherwise
stated, films were kept under flowing N2 during the
measurements. All micro to millisecond transient absorption
spectroscopy measurements were conducted employing 450
nm laser pulses (25 µJ/cm2).
4.5 Computational details
All calculations were carried out using the Gaussian 09
program with Lee Yang–Parr correlation functional (B3LYP)
level of theory. All atoms were described by the 6-31G(d) basis
set. All structures were input and processed through the
Avogadro software package.
4.6 Organic Field-Effect Transistors
The Ossila low-density Pre-fabricated substrates (channel
length 0.1 mm width 0.03 mm) with a bottom gate/bottom
contact were used to fabricate OFET devices. The substrates
were treated with HMDS (Hexamethyldisilazane) to optimise
the silicon surface property. A solution of the HTM (5 mg/mL)
in dichlorobenzene was stirred at room temperature. The
Ossila substrate was covered with the solution by drop-casting.
The electric characteristics of the fabricated OFETs were
measured using
a Keithley 2612A System SourceMeter.
4.8 Perovskite Solar Cells and Characterisation
Mobilities were calculated using the following equation
Etched FTO glass substrates (NSG Pilkington, TEC7) were
cleaned sequentially in detergent, deionised water, acetone
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titanium diisopropoxide bis(acetylacetonate) in isopropanol at
450 °C.
Upon
cooling,
a
mesoporous
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CH3NH3PbI3 perovskite precursor solution was prepared by
dissolving 576mg PbI2, and 199 mg CH3NH3I in a 4:1 vol
solution of DMF:DMSO. 100 µl of the perovskite precursor
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dissolved in chlorobenzene with 2 times the volume of
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HOMO level of these compared with Spiro-OMeTAD. The
HTM solution was spin-coated onto perovskite films at
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Conflicts of Interest
There are no conflicts of interest to declare.
Acknowledgements
RFP thanks CONACYT, Mexico for a PhD studentship. We thank
EPSRC EP/H040218/1; EP/M023532/1 for financial support.
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