Supramolecular Coordination of Pb2+ Defects in Hybrid Lead Halide Perovskite Films Using Truxene Derivatives as Lewis Base Interlayers

Article abstract of DOI:10.1002/cphc.201900068

Truxene derivatives, due to their molecular structure and properties, are good candidates for the passivation of defects when deposited onto hybrid lead halide perovskite thin films. Moreover, their semiconductor characteristics can be tailored through the modification of their chemical structure, which allows-upon light irradiation- the interfacial charge transfer between the perovskite film and the truxene molecules. In this work, we analysed the use of the molecules as surface passivation agents and their use in complete functional solar cells. We observed that these molecules reduce the non-radiative carrier recombination dynamics in the perovskite thin film through the supramolecular complex formation between the Truxene molecule and the Pb²⁺ defects at the perovskite surface. Interestingly, this supramolecular complexation neither affect the carrier recombination kinetics nor the carriers collection but induced noticeable hysteresis on the photocurrent vs voltage curves of the solar cells under 1 sun illumination.

Full text of DOI:10.1002/cphc.201900068

Accepted Article
Title: Supramolecular Coordination of Pb2+ Defects in Hybrid Lead
Halide Perovskite Films Using Truxene Derivatives as Lewis
Base Interlayers
Authors: Emilio Jose Palomares, Ece Aktas, Jesús Jiménez-López,
Cristina Rodríguez‐Seco, Rajesh Pudi, Manuel A. Ortuño, and
Núria López
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemPhysChem 10.1002/cphc.201900068
Link to VoR: http://dx.doi.org/10.1002/cphc.201900068
A Journal of
www.chemphyschem.org

ChemPhysChem
10.1002/cphc.201900068
2
+
Supramolecular Coordination of Pb Defects in Hybrid Lead Halide Perovskite
Films Using Truxene Derivatives as Lewis Base Interlayers.
Ece Aktas1,2,§, Jesús Jiménez-López1,3,§, Cristina Rodríguez‐ Seco , Dr. Rajesh Pudi ,
1,3
1
Dr. Manuel A. Ortuño , Prof. Núria López and Prof. Emilio Palomares1–4,*
1
1
1
2
3
4
. Institute of Chemical Research of Catalonia-The Barcelona Institute of Science
and Technology (ICIQ-BIST), Avda. Països Catalans 16, 43007 Tarragona, Spain
. Department of Chemical Science and Technology, Avda. Països Catalans 16,
4
3007, Universitat Rovira i Virgili, 43007 Tarragona, Spain
. Department d’Enginyeria Electrònica, Elèctrica i Automàtica, Univeristat Rovira
i Virgili, Avda. Països Catalans 16, 43007 Tarragona, Spain
. ICREA, Passeig Lluís Companys 23, 08010 Barcelona, Spain
§
*
: Both authors contributed equally to this work.
To whom the correspondence should be addressed: epalomares@iciq.es
Page 1 of 25
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ChemPhysChem
10.1002/cphc.201900068
Abstract.
Truxene derivatives, due to their molecular structure and properties, are good candidates
for the passivation of defects when deposited onto hybrid lead halide perovskite thin films.
Moreover, their semiconductor characteristics can be tailored through the modification of
their chemical structure, which allows-upon light irradiation- the interfacial charge
transfer between the perovskite film and the truxene molecules. In this work, we analysed
the use of the molecules as surface passivation agents and their use in complete functional
solar cells. We observed that these molecules reduce the non-radiative carrier
recombination dynamics in the perovskite thin film through the supramolecular complex
2
+
formation between the Truxene molecule and the Pb defects at the perovskite surface.
Interestingly, this supramolecular complexation neither affect the carrier recombination
kinetics nor the carriers collection but induced noticeable hysteresis on the photocurrent
vs voltage curves of the solar cells under 1 sun illumination.
Page 2 of 25
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ChemPhysChem
10.1002/cphc.201900068
INTRODUCTION.
The passivation of defects, in inorganic or hybrid photoactive thin films, results key to
increase the solar cell efficiency[1–3]. The defects induce a change in the solar cell voltage
reducing the energetic difference between the quasi-Fermi levels as a result of an increase
carrier recombination and the solar cell photocurrent, because less photo-generated
carriers are extracted[4–6]
.
In dye sensitized solar cells (DSSC), where mesoporous TiO
dyes that act as light harvesters, the passivation of TiO results key to achieve remarkable
efficiencies[4,7,8]. For instance, the use of conformally deposited Al
onto the
nanocrystalline particles reduces the back-electron transfer to the oxidized dye but also
decreases the number of defects on the TiO nanocrystals surface, which act as carrier
2
is used as scaffold to anchor
2
2
O
3
2
[
9]
recombination centres .
Recently, in hybrid lead halide perovskite materials, many research groups have started
to study the effect of molecules, as additives, to reduce the presence of surface defects
[2,10,11].
oxide leads to an increase in the solar to energy conversion efficiency due to the
suppression of carrier recombination centres associated to surface defects[12–14]
As an example, the recent work of Di Carlo’s group shows that the use of graphene
.
Organic molecules with truxene structure (see Scheme 1, 1a) have been explored[15] in
organic solar cells[16–18], organic light emitting diodes[19], and as a hole transport material
in perovskite solar cells[20–22] due to their good optical and semiconductor properties. In
addition, truxene molecules have good solubility in most common organic solvents,
which improves film morphology. Moreover, the truxene scaffold has an excellent
thermal stability; a must when incorporated into organic electronic devices[23,24]. Their
molecular structure and their molecular shape allow their deposition at the surface of the
perovskite in face-to-surface configuration with strong interaction with the perovskite
semiconductor surface. A similar approach has been observed for graphene oxide
layers[25]. Furthermore, the introduction of 3-fluoropyridine substituents will act as a
Lewis base to passivate the non-coordinated Pb2+ ions present at the surface of the
perovskite.
In this work, we have synthesised a truxene derivative, 4,4’,4’’-(5,5,10,10,15,15-
hexahexyl-10,15-dihydro-5H-diindeno[1,2-a:1’,2’2-c]fluorine-2,7,12-triyl)tris(3-
Page 3 of 25
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ChemPhysChem
10.1002/cphc.201900068
fluoripyridine) (Trux-FPy), Scheme 1, and we have measured its optical and electronic
properties. Moreover, we do have deposited a thin film of Trux-FPy on top of the methyl
ammonium lead iodide (MAPI) hybrid perovskite and analysed its role as a Lewis base
to passivate perovskite defects. Finally, we have measured the performance of complete
MAPI perovskite solar cells and the effect of the Trux-FPy thin film as interfacial layer
between the perovskite and the hole transport material (HTM) spiro-OMeTAD.
Scheme 1. Synthetic route for the Trux-FPy.
EXPERIMENTAL SECTION.
Synthesis of Trux-FPy.
The tetrahydrofuran (THF) used as the solvent for the synthesis of 1b, and Trux-FPy was
dried through standard procedure. The other reagents and solvents and used were
purchased form Sigma-Aldrich© and TCl and used as received without further
purification. The syntheses of 1a, 1b, and 1 was carried out following the scientific
1
13
1
literature. Our H and C{ H} NMR data was in good agreement with those values
previously reported[18,26,27]
.
Synthesis of 10,15-dihydro-5H-diindeno(1,2-a;1',2'-c)fluorene (1a)
-Indanone (0.5 g, 10.6 mmol) was dissolved in acetic acid (10.0 mL), and then
1
concentrated hydrochloric acid (5.0 mL) was added. The solution was heated to 120 °C
and refluxed overnight. The hot mixture was poured into saturated sodium carbonate
aqueous solution (100.0 mL) with ice and stirred for 1 h. The yellow precipitate was
filtered and washed with acetone (50.0 mL) and ethanol (50.0 mL) to give an off-white
Page 4 of 25
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ChemPhysChem
10.1002/cphc.201900068
1
powder 1a. (0.8 g, 64% isolated yield). H NMR (500 MHz, CDCl
3
) δ 7.97 (d, J = 7.6
Hz, 3H), 7.71 (d, J = 7.4 Hz, 3H), 7.50 (t, J = 6.8, 3H), 7.40 (t, J = 7.4, 3H), 4.29 (s, 6H).
1
3
1
C{ H} NMR (101 MHz, CDCl
3
) δ 143.8, 141.7, 137.1, 135.3, 126.9, 126.3, 125.1,
1
21.9, 36.6.
Synthesis of 5,5,10,10,15,15-Hex(1-hexyl)10,15-dihydro-5H-diindeno(1,2-a;1',2'-
c)fluorene (1b)
t
To a suspension of 1a (0.5 g, 1.5 mmol) and BuOK (5.7 g, 51.0 mmol) in dry THF (50.0
mL) 1-bromohexane (3.2 mL, 22.5 mmol) was added at room temperature under argon
atmosphere. The resulting suspension was heated at 70 °C and stirred overnight. The solid
material in the reaction mixture was removed through filtration and washed with hexane
(
100.0 mL). The filtrate part was concentrated down under reduced pressure and the
resulting oil was dissolved in hexane (50.0 mL). The mixture was washed with 0.2 M
HCl (25.0 mL) and saturated NaHCO (50.0 mL). The organic layer was dried over
MgSO and concentrated in vacuo. Then, the residue was purified by silica column
3
4
chromatography using hexane as eluent to afford the target compound 1b as off-white
1
powder (0.85 g, 67% isolated yield). H NMR (400 MHz, CDCl
3
)) δ 8.42 (d, J = 7.5 Hz,
3
1
H), 7.54 – 7.48 (m, 3H), 7.46 – 7.35 (m, 6H), 3.19 – 2.89 (m, 6H), 2.27 – 1.98 (m, 6H),
13
1
.09 – 0.78 (m, 36H), 0.64 (t, J = 7.1 Hz, 18H), 0.61 – 0.50 (m, 12H). C{ H} NMR
) δ 153.6, 144.8, 140.3, 138.4, 126.3, 125.9, 124.6, 122.1, 55.6, 37.0,
1.5, 29.5, 23.9, 22.3, 13.9.
(101 MHz, CDCl
3
3
Synthesis
of
2,7,12-tribromo-5,5,10,10,15,15-hexahexyl-10,15-dihydro-5H-
diindeno[1,2-a:1',2'-c]fluorene (1)
To a solution of compound 1b (0.83 g, 0.98 mmol) in chloroform (8.0 mL) FeCl (2.0
3
mg, 0.012 mmol) was added as catalyst. A solution of bromine (0.2 mL, 3.43 mmol) in
chloroform (2.0 mL) was added dropwise under stirring at 0 °C. The mixture was allowed
to warm to room temperature and stirred overnight. Then, a saturated Na
solution (20.0 mL) was added to remove excess bromine. The mixture was extracted with
chloroform (3x50.0 mL), and the combined organic phases were dried over MgSO . After
2
SO aqueous
3
4
the solvent was removed, the yellow residue was recrystallized from ethanol to yield the
1
compound 1 as an off-white powder (0.99 g, 93% isolated yield). H NMR (400 MHz,
CDCl
3
) δ 8.17 (d, J = 8.5 Hz, 3H), 7.56 (d, J = 2.0 Hz, 3H), 7.51 (dd, J = 8.4, 2.0 Hz,
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ChemPhysChem
10.1002/cphc.201900068
3
1
1
H), 2.93 – 2.72 (m, 6H), 2.10 – 1.92 (m, 6H), 0.99 – 0.77 (m, 36H), 0.62 (t, J = 7.1 Hz,
1
3
1
8H), 0.51 – 0.36 (m, 12H). C{ H} NMR (101 MHz, CDCl
3
) δ 155.9, 144.9, 138.9,
37.6, 129.4, 125.9, 125.5, 121.0, 56.0, 36.8, 31.4, 29.4, 23.9, 22.2, 13.9.
Synthesis of 4,4',4''-(5,5,10,10,15,15-hexahexyl-10,15-dihydro-5H-diindeno[1,2-
a:1',2'-c]fluorene-2,7,12-triyl)tris(3-fluoropyridine) (Trux-FPy)
In a 50 mL two-neck round bottom flask 1 (0.25 g, 0.23 mmol), 3-fluoro-4-pyridine
boronic acid pinacol ester (0.30 g, 1.38 mmol) and the system was purged with argon for
3
0 minutes. Then, freshly dried THF (15.0 mL) and Na
added to the medium. Finally, Pd(PPh (20.0 %, 53.0 mg) was added as catalyst and the
reaction temperature was arranged to 80 °C and stirred for overnight. After that the
solvent was removed in vacuo, chloroform (CHCl ) (50.0 mL) was added to the crude
product and the mixture was washed with brine (saturated) (2x50.0 mL) and water
2x50.0 mL) until a clear solution was obtained. The organic layer was dried over
anhydrous MgSO , filtered and concentrated in vacuo. Finally, the residue was purified
2
CO
3
solution (2 M, 2.0 mL) were
)
3 4
3
(
4
by silica column chromatography using Hexane:Ethyl acetate (2:1) as elution solvent.
Precipitation from methanol yielded Trux-FPy as off-white powder (0.12 g, 46 %
1
isolated yield). H NMR (400 MHz, CDCl
3
) δ 8.61 (d, J = 2.6 Hz, 3H), 8.54 (dd, J = 4.9,
0
7
3
1
2
.8 Hz, 3H), 8.49 (d, J = 8.3 Hz, 3H), 7.76 (d, J = 1.7 Hz, 3H), 7.72 (d, J = 8.2 Hz, 3H),
.58 (dd, J = 6.8, 5.0 Hz, 3H), 3.06 – 2.93 (m, 6H), 2.25 – 2.11 (m, 6H), 1.02 – 0.80 (m,
1
3
1
6H), 0.68 – 0.51 (m, 30H). C{ H} NMR (101 MHz, CDCl
3
) δ 154.1, 146.4, 146.0,
41.2, 139.3, 139.0, 137.9, 136.2, 131.0, 127.0, 124.9, 124.1, 122.7, 56.0, 37.0, 31.4, 29.4,
+
+
4.0, 22.2, 13.8. Calcd for C78
H
96
F
3
N , (M ): 1131.7551; found: 1131.7544 (0.6 ppm).
3
Anal. Calcd for C78
H
96
F N
3 3
: C, 82.71; H, 8.54; N, 3.71. Found: C, 82.62; H, 9.00; N, 3.65.
Electrochemical measurements.
All electrochemical measurements were examined in a one-component cell under
Nitrogen gas and equipped with a glassy-carbon working electrode, a platinum counter-
+
electrode, and an Ag/Ag reference electrode in acetonitrile solution at a concentration of
0
.5 mM. The supporting electrolyte was a 0.1 M solution of tetrabutyl ammonium
+
hexafluorophosphate (TBAPF
6
). All potentials were corrected against Fc/Fc . The cyclic
voltammogram (CV) was measured with a scan rate of 100 mV/s at room temperature.
UV-Visible absorption spectra.
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10.1002/cphc.201900068
UV-Vis measurements were carried out on an Agilent Cary 60 UV-Vis
spectrophotometer equipped with two silicon diode detectors, double beam optics and
Xenon pulse light.
Steady state fluorescence emission
Steady state fluorescence emission measurements were carried out on a Fluorolog Horiba
Jobin Yvon spectrofluorimeter equipped with photomultiplier detector, double
monochromator and Xenon light source.
Time correlated single photon counting measurements.
Lifetime measurements were carried out on a Edinburgh Instruments LifeSpec-II based
on the time-correlated single photon counting (TCSPC) technique, equipped with a PMT
detector, double subtractive monochromator and 470 nm picosecond pulsed diode lasers
source.
Solar cells preparation.
FTO substrates are cleaned first with a cloth, brushing without damaging the FTO layer.
Then, they are ultrasonicated with water and Hellmanex, isopropanol, and acetone, for 15
minutes for each one. Before depositing the TiO
UV/Ozone for 15 minutes.
2
layer, the substrates are treated with
To deposit the compact TiO
2
layer, 40 l of a 0.3 M Ti(iPrO)
2
(acac) solution in isopropyl
2
alcohol (IPA) are spin coated at 4000 rpm for 25 s using an acceleration of 1000 rpm/s.
Right after the spin coating process, the substrates are dried at 125 °C for 5 minutes. Then,
o
they are transferred into a hot plate and calcined at 450 C for 30 minutes.
Once the substrates are at room temperature, they are immersed in a 40 mM TiCl solution
4
at 70 °C for 30 minutes. Then, they are rinsed with water and ethanol consecutively and
dried with a strong air flow.
Finally, a mesoporous TiO
2
layer is deposited. We used a commercially available 30 nm
particle size TiO paste (30NR-T, Dyesol) diluted in ethanol in a ratio 1:7 (w:w). 40 l
2
of this paste were spin coated at 6000 rpm for 30 s, using an acceleration of 1000 rpm.
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10.1002/cphc.201900068
The films are dried 5 minutes at 125 °C and after that they are calcined at 450 °C for 30
minutes. The substrates are then placed into a glovebox when they are still warm to avoid
humidity.
The perovskite solution is prepared in a Nitrogen filled glove-box. In this case, we use
MAPbI
3
perovskite. The concentration of the MAPbI
3
solution is 1.25 M. To prepare the
solution, first, the required amount of PbI
2
is weighted and dissolved in DMSO. To help
the solution of the lead salt, we heat up the solution to a temperature of 150 °C for 10
minutes. Then, the solution is cooled down to room temperature and the methyl
ammonium iodide is added.
It is very important to control the atmosphere and the solvent vapours inside the glove-
box. Hence, the work is carried out with a continuous N
sulfoxide (DMSO) vapours.
2
flow that removes the dimethyl
The deposition of the perovskite solution is carried out using anti-solvent treatment. First,
0 l of the perovskite solution are spin coated with a two-step programme. First, 1000
4
rpm for 10 s using an acceleration of 500 rpm, and then 4000 rpm for 30 s with an
acceleration of 500 rpm. 10 seconds before the spinning process ends, 100 l of
chlorobenzene is spin coated right on the centre of the spinning substrate. The films are
annealed for 45 minutes at 100 °C.
A 60 mM spiro-OMeTAD solution in chlorobenzene containing 38.7 l/ml solution of
tert-butylpyridine and 23.7 l/ml solution of LiTFSI (stock solution 520 mg/ml in
acetonitrile. The solution is stirred until spiro-OMeTAD is completely dissolved. Then,
3
0 l are deposited dynamically by spin coating at 4000 rpm for 30 s.
For the substrates containing the Trux-FPy interlayer, 40 l of a 5 mg/ml Trux-FPy
solution in chlorobenzene are deposited at 4000 rpm for 30 s using an acceleration of
2
000 rpm/s.
Then, the substrates are kept overnight in dry air and in the dark. Then, they are placed in
the thermal evaporator and 80 nm of gold is evaporated on top as metal contact.
JV curves
The J-V curves were measured using a solar simulator (ABET 11000) and a source meter
Page 8 of 25
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10.1002/cphc.201900068
2
(
Keithley 2400). The curves were registered under 1 Sun conditions (100 mW/cm , AM
1
.5G) calibrated with a Si-reference cell. The active area of the devices was 0.09 cm2.
The scan rate employed was 0.01 V/s. For the measurements with different light
intensities, different optical filters were employed.
Photo-induced transient photovoltage, Transient photocurrent and charge extraction.
Photo-induced transient photovoltage (TPV), photo-induced transient photocurrent
(TPC) and photo-induced charge extraction (CE) measurements were carried out using a
white LED controlled by a programmable power supply and a control box that switches
from open to short-circuit states. All the signals are recorded in an oscilloscope
Yokogawa DLM2052 registering drops in voltage. Light perturbations pulses for TPV
and TPC were provided by a nanosecond PTI GL-3300 nitrogen laser.
Electrostatic potential surface
The electrostatic potential surface was computed at Density Functional Theory on a
model structure of Trux-FPy, where hexyl chains were replaced by methyl groups. The
PBE density functional[28] was used together with Grimme’s D3 dispersion scheme[29] as
implemented in Gaussian 09[30]. The 6-31G** basis set was employed for all atoms[31,32]
and diffuse functions were added to the electronegative F atoms[33]. The optimized
,
structure and computational details can be consulted in the ioChem-BD repository[34,35]
.
RESULTS AND DISCUSSION.
The Trux-FPy show reversible oxidation and reduction processes (Figure 1). The
oxidation waves of Trux-FPy were determined at +1.20 V and +0.95 V vs Ag/Ag+
reference electrode in the oxidation process (Figure 1). Moreover, the Trux-FPy showed
+
reversible reduction waves at -1.92 V and -1.85 V vs Ag/Ag reference electrode.
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10.1002/cphc.201900068
Figure 1. Single scan cyclic voltammogram of Trux-FPy on glassy-carbon electrode in
.1 M TBAPF6/ACN solution. On the left the oxidation waves and on the right the
0
reduction waves.
+
Using Fc/Fc as internal reference electrode we have estimated the energies for the
Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular
Orbital (LUMO) energy levels[36]. The results are -5.37 eV and -2.42 eV, respectively.
All the relevant electrochemical parameters are listed in Table 1.
Table 1: Summary of the most relevant parameters for the electrochemical measurements
of Trux-FPy.
1/2E
1/2Eox2
1/2Ered1
1/2Ered2
HOMO
(eV)
LUMO
(eV)
ox1
(
V)
(V)
(V)
(V)
Trux-FPy
1.20
0.95
1.92
1.85
-5.37
-2.42
Attending to the energy values given for the MAPI conduction band (CB) and valence
band (VB) of -5.43eV and -3.90eV the Trux-FPy has a very low energy offset (Eoffset
)
for the hole transfer, Eoffset= 0.07eV. Such Eoffset will suffice, as show later on, to allow
carrier transport to the HTM, the spiro-OMeTAD.
Once the electrochemical characteristics of the Trux-FPy were measured, we turned onto
the optical measurements. Figure 2 illustrates the UV-Visible absorption spectra for the
Trux-FPy in solution and in thin film.
Figure 2. Left, normalized UV-Visible spectra (grey curve) for the Trux-FPy in
chloroform ([Trux-FPy]=0.05 mM) and deposited in a cover slide glass (black curve) as
Page 10 of 25
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10.1002/cphc.201900068
thin film (film thickness of 90 nm). Right, fluorescence emission spectra upon excitation
at ex= 340 nm measured at room temperature.
As can be seen in Figure 2, the Trux-FPy film does not show a noticeable new absorption
band that may correspond to intermolecular interactions. Nonetheless, the main band in
the UV region (max= 340 nm) is slightly red shifted, which indicates the formation of
molecular aggregates[37]. Notwithstanding the featureless UV-Visible spectra, the
fluorescence emission spectra, on the contrary, shows remarkable fine structure with a
visible shoulder centred em=460 nm, which is in agreement with the presence of
intermolecular interactions accounting for the existence of molecular aggregates.
The deposition of Trux-FPy, using the spin-coating technique, on top of a MAPI
perovskite thin film shows modest quenching of the fluorescence emission (Figure 3),
which indicates that due to the rather small Eoffset between MAPI and Trux-FPy the
interfacial hole transfer process is not efficient. Importantly, when Trux-FPy is used as
an interfacial layer between MAPI and the HTM spiro-OMeTAD, the quenching process
approaches unit yield, which implies outstanding interfacial charge transfer between the
MAPI film and the HTM spiro-OMeTAD through the interfacial layer of Trux-FPy. It
is important to highlight that the interfacial layer of Trux-FPy between MAPI and the
HTM spiro-OMeTAD has a thickness of 5 nm approximately, which is thin enough to
allow charge transport through it.
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10.1002/cphc.201900068
Figure 3. Luminescence emission band upon excitation at ex= 435 nm at room
temperature for the MAPI/PMMA (total thickness of 450-500 nm), the MAPI/Trux-FPy
thickness ≃ 5 nm) and MAPI/Trux-FPy/spiro-OMeTAD (thickness = 150-200 nm)
(
Also, we focus in the role of TruxFPy in the passivation of the Lewis acid sites that act
as traps for free carriers at the MAPI film surface, as we have previously hypothesised.
Figure 4 shows the luminescence emission decays recorded at room temperature, using
the time correlated single photon-counting technique, using the same films as in Figure
3
. Moreover, we have also added the MAPI/spiro-OMeTAD thin film for comparison
purposes. The MAPI films were coated with PMMA with encapsulating purposes[38]
.
Time Resolved Photoluminescence (TRPL) decays show two different decay profiles.
The faster decay is being assigned to trap filling whereas the slower decay corresponds
to the bimolecular recombination[39–41]. TRPL decays are shown in Figure 4 and they
have been fitted to a biexponential decay, y = A
1
exp(-t/
1
) + A
2
exp(t/
2
). The results of
= 53 ns for
the fitting are shown in Table 2, obtaining a lifetime 
1
= 69 ns and 
1
MAPI/PMMA and MAPI/TruxFPy samples, respectively. These kinetics, associated
with trap filling, shows us the role of TruxFPy, passivating traps on the perovskite
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10.1002/cphc.201900068
surface, as it takes shorter times to be filled. It is also very interesting the analysis of the
lifetime  , which we already have assigned to bimolecular recombination in the
2
perovskite. With values of 268 ns and 346 ns for MAPI/PMMA and MAPI/TruxFPy, it
represents a direct evidence of the passivation effect of TruxFPy layer, indeed.
The passivation of Lewis acid sites at the surface of the MAPI perovskite leads to an
improvement of the carrier’s lifetime. This improvement is not seen, however, in the
steady state luminescence emission represented in Figure 2 as an increase in the
perovskite emission quantum yield due to the effective but not efficient interfacial charge
transfer between the MAPI film and the Trux-FPy film. Moreover, in good agreement
with Figure 2, the luminesce decay for the sample MAPI/Trux-FPy/spiro-OMeTAD
shows efficient quenching and much faster decay kinetics.
Figure 4. Normalized luminescence emission decays (ex=470 nm) measured at room
temperature for MAPI/PMMA, MAPI/spiro-OMETAD, MAPI/Trux-FPy, and
MAPI/Trux-FPy/spiro-OMETAD on glass substrate.
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ChemPhysChem
10.1002/cphc.201900068
Table 2. Fitting values obtained from the de-convolution of the luminescence decays in
Figure 4.
Films
τ
1
(ns)
69
5
τ (ns)
2
MAPI/PMMA
268
21
MAPI/spiro-OMETAD
MAPI/Trux-FPy
53
5
346
16
MAPI/Trux-FPy/spiro-OMETAD
We moved one-step further and fabricated solar cells to proof if the passivation of the
surface defects using the Trux-FPy interfacial layer results also beneficial in a complete
device.
Figure 5 illustrates the measured IV (photocurrent vs voltage) curves for the best
MAPI/Trux-FPy/spiro-OMeTAD and the MAPI/spiro-OMeTAD used as a reference.
Figure 5. IV curves for the MAPI/spiro-OMeTAD (left, blue colour) and the MAPI/Trux-
FPy/spiro-OMeTAD (right, green colour) solar cells when illuminated under sun
2
simulated 1 sun conditions (100mW/cm 1.5 AM G).
At first glance, it is clear that the use of Trux-FPy as interfacial layer, although achieved
the passivation of the MAPI surface as shown in Figure 4, does not improve noticeably
the solar cell efficiency. A more detailed statistical study also supports this observation
(Figure 6).
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ChemPhysChem
10.1002/cphc.201900068
Figure 6. Statistics for different sets of devices employing MAPI/Trux-FPy/spiro-
OMeTAD (green colour) and MAPI/spiro-OMeTAD (blue colour) sun simulated
irradiated conditions at 1 sun.
We would like to highlight that even in the best case for the MAPI/Trux-FPy/spiro-
OMeTAD solar cells there are minor differences between the forward (measured from
short-circuit to open circuit) and the reverse (measured from open circuit to short circuit)
IV curve. This difference accounts for the hysteresis process that has been largely
discussed in the scientific literature for hybrid lead halide perovskites[42,43]
.
In this particular case the reference sample, measured under identical conditions, shows
negligible hysteresis, which leads us to think that the observed differences are due to the
presence of the Trux-FPy interlayer. A first hypothesis is that due to the low Eoffset
between the MAPI film and the Trux-FPy film, the latter results in a practical barrier
that hampers the efficient transport of charges and will result in the accumulation of
electronic holes and ionic species at the interface between the MAPI film and the Trux-
FPy film. Hence, further studies, in complete devices under operando conditions, were
carried out to analyse if the losses in efficiency were related to carrier losses due to
Page 15 of 25
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Equation 1
푉표푐 ∝ 푛 푞 ln(퐿퐼)
ChemPhysChem
10.1002/cphc.201900068
interfacial recombination processes. Figure 7 illustrates the changes in open circuit
voltage (Voc) and short-circuit current upon illumination.
Figure 7. Left, Voc vs light intensity and right, Jsc vs light intensity.
As can be seen in Figure 7, there is a substantial difference between the values for the
MAPI/Trux-FPy/spiro-OMeTAD and the MAPI/spiro-OMeTAD. Those values
correspond to the fitting of Equation 1, where n is the ideality factor, k is the Boltzmann’s
constant, T is the temperature in Kelvin, q is the elementary charge and LI stands for light
intensity.
푘푇
In non-ionic solar cells, values close to unity (kT/q) indicate that the bimolecular
recombination is the dominant process. However, for values higher than 1 (1.7-1.8 for the
MAPI/spiro-OMeTAD and 2.4 for MAPI/Trux-FPy/spiro-OMeTAD) it indicates that
there are other parallel processes that occur during illumination. One of these is the
reorganization of ions at the perovskite solar cell, which has been demonstrated to play a
role in the final Voc of the solar cell[44,45]. The major dependence of Voc on irradiance and
the higher slope value for MAPI/Trux-FPy/spiro-OMeTAD based perovskite solar cells
implies that these processes have a greater impact when the Trux-FPy layer is present,
which agrees with the greater hysteresis observed in Figure 5. Moreover, the analysis of
the slope of the Jsc versus light illumination intensity gives similar values close to unity,
which implies that carrier recombination at short circuit is negligible[46,47]. The JV curves
measured to obtain these values are shown in Figures S12 and S13.
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10.1002/cphc.201900068
We have shown previously that techniques such as photo-induced charge extraction (CE)
and photo-induced transient photovoltage (TPV), developed to study DSSC (dye
sensitized solar cells) and OSC (organic solar cells), can be very useful to understand
carrier recombination and ion migration in perovskite solar cells. In this work, we do have
used CE and TPV to measure the above mentioned solar cell properties. Figure 8 shows
the CE and TPV decays obtained under 1 sun illumination conditions for two of the best
solar cells fabricated in this work.
Figure 8. Top-left (spiro-OMeTAD), the decays at 1 sun for the CE and the TPV. Top-
right (Trux-FPy/spiro-OMETAD) the decays at 1 sun for the CE and the TPV. Bottom-
left, the normalized TPV decays comparison for both solar cells. Bottom-right, the
normalized CE decays comparison for both solar cells.
As can be observed in Figure 8, in both perovskite solar cells the photo-induced carrier
recombination at 1 sun measured using TPV and the charge extraction decays are very
similar. Hence, we can conclude that the differences in the device performance are not
due to the carrier recombination or the carrier extraction. To further confirm this
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ChemPhysChem
10.1002/cphc.201900068
experimental observation, we measured the interfacial carrier recombination kinetics at
different charge obtained at different light bias (different Voc because of different light
irradiation intensities) in Figure 9. In Figures S14 and S15 we show the dependence of
carrier lifetime at different light bias and the carrier density at different light bias.
Figure 9. Carrier lifetime vs charge obtained for different light bias for MAPI/Trux-
FPy/spiro-OMeTAD (green) and MAPI/spiro-OMeTAD (blue).
As illustrated in Figure 9, for charge values corresponding to light irradiation intensity
close to 1 sun the carrier life time are alike and, moreover, the slope of the curves are also
very close in units, which implies that the interfacial carrier recombination order is very
much close. Thus, our first hypothesis that the possible accumulation of ions was
responsible for the hysteresis resulted was incorrect. Nonetheless, taking into account the
spectroscopic data, it results evident that the Trux-FPy do passivate the defects in the
perovskite thin film.
We decided to focus more in depth on the Lewis base properties of the Trux-FPy and
carried out a titration experiment using PbI and the Trux-FPy molecule in solution. As
2
can be seen in Figure 10, upon addition of increasing amounts of Trux-FPy an isosbestic
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ChemPhysChem
10.1002/cphc.201900068
point appears at =350nm, which is indicative of a supramolecular interaction between
the Trux-FPy and the Pb2+ ions. Moreover, the new supramolecular complex has a
maximum absorption band at 325nm, which is 10nm blue shifted with respect to the
Trux-FPy main absorption band in the UV.
Figure 10. UV-Visible spectra of the titration experiment using 3mL of a 0.1 mM solution
of PbI in dimethylformamide (DMF) in a quartz cuvette and increasing concentration of
2
Trux-FPy from a stock solution of 0.05 mM in chloroform.
Figure 11 shows the electrostatic potential (ESP) surface calculated at DFT level for a
methyl derivative of Trux-Fpy. The molecule shows a planar -conjugated core where
the largest negative charges are localised on the pyridinic N atoms (density in red) and
the F atoms (density in yellow). Hence, those N atoms are expected to coordinate the Pb2+
uncoordinated atoms at the perovskite surface through Lewis acid–Lewis base
supramolecular interactions, which is in good agreement with the experiment shown in
Figure 10.
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ChemPhysChem
10.1002/cphc.201900068
Figure 11. Electrostatic potential surface for a methylated model of the Trux-FPy
molecule.
CONCLUSIONS.
We have designed and synthesised an organic semiconductor molecule with truxene core.
The molecule, Trux-FPy, was intended to contain peripheral moieties that can work as
Lewis bases to passivate surface defects in lead halide perovskite as a result of the non-
coordinated lead contained in methyl ammonium lead iodide perovskite thin films. Those
surface defects act as traps for carriers and increase the carrier recombination, which, in
overall, limits the solar cell efficiency. The Trux-FPy was fully electrochemically and
optically characterized and it was found that, upon deposition on top of the MAPI thin
film, the Trux-FPy thin film decreases the number of defects at the MAPI surface
increasing its luminescence lifetime. Moreover, the Trux-FPy thin film was capable of
carrying out interfacial charge transfer processes with the MAPI thin film upon
illumination, which leads us to incorporate the Trux-FPy as interfacial layer.
Once incorporated as an interfacial layer between the MAPI film and the HTM spiro-
OMeTAD film, the best solar cells matched the efficiency of those standards prepared
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ChemPhysChem
10.1002/cphc.201900068
using only spiro-OMeTAD. Nonetheless, the presence of hysteresis in the IV curves for
the Trux-FPy containing solar cells was noticed.
An analysis in depth of the MAPI/Trux-FPy/spiro-OMeTAD solar cells using CE and
TPV techniques determines that the interfacial carrier recombination processes in these
devices are not affected by the presence of the Trux-FPy interfacial layer. Nevertheless,
the Trux-FPy interfacial layer does have supramolecular interactions with the
uncoordinated lead ions. Our results show the potential of surface supramolecular
interactions between the perovskite semiconductor and intermediate layers to decrease
the uncoordinated site defects, which are the cause negative effects in the perovskite solar
cells performance.
Acknowledgements.
All the authors would like to acknowledge economic support from MICINN (project
CTQ2016-80042-R) and Generalitat de Catalunya for the AGAUR funding (project 2017
SGR 00978). M.A.O. also acknowledges the “Juan de la Cierva–Incorporación” program
(
IJCI-2016-29762) for financial support. Prof. Palomares also thanks the reviewers for
their critical review of the work, which indeed has been very useful.
Keywords: Lewis base, perovskite, solar energy, truxene
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Table of contents
2
+
Supramolecular Coordination of Pb Defects in Hybrid Lead Halide Perovskite
Films Using Truxene Derivatives as Lewis Base Interlayers.
Ece Aktas, Jesús Jiménez-López, Cristina Rodríguez‐ Seco, Dr. Rajesh Pudi, Dr. Manuel
A. Ortuño, Prof. Núria López, and Prof. Emilio Palomares*
In this work, we synthesized a pyridine functionalized truxene core that is able to form a
Lewis acid/Lewis base supramolecular interaction (see picture). This small molecule
works as a passivation layer on top of the perovskite thin film, reducing carrier
recombination in perovskite solar cells.
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