Minimization of Carrier Losses for Efficient Perovskite Solar Cells through Structural Modification of Triphenylamine Derivatives

Article abstract of DOI:10.1002/anie.201915022

Three hole transport materials (HTMs) based on a substituted triphenylamine moiety have been synthesized and successfully employed in triple-cation mixed-halide PSCs, reaching efficiencies of 19.4 %. The efficiencies, comparable to those obtained using spiro-OMeTAD, point them out as promising candidates for easily attainable and cost-effective alternatives for PSCs, given their facile synthesis from commercially available materials. Interestingly, although all these HTMs show similar chemical and physical properties, they provide different carrier recombination kinetics. Our results demonstrate that is feasible through the molecular design of the HTM to minimize carrier losses and, thus, increase the solar cell efficiencies.

Full text of DOI:10.1002/anie.201915022

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
International Edition Chemie
www.angewandte.org
Accepted Article
Title: Structural Modification of Triphenylamine Derivatives vs the
Minimization of Carrier Losses for Efficient Perovskite Solar Cells.
Authors: Emilio Jose Palomares, Cristina Rodríguez-Seco, Maria
Mendez, Cristina Roldan-Carmona, Ravi Pudi, and Md. K.
Nazeeruddin
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201915022
Angew. Chem. 10.1002/ange.201915022
Link to VoR: http://dx.doi.org/10.1002/anie.201915022
http://dx.doi.org/10.1002/ange.201915022

Angewandte Chemie International Edition
10.1002/anie.201915022
RESEARCH ARTICLE
Structural Modification of Triphenylamine Derivatives vs the
Minimization of Carrier Losses for Efficient Perovskite Solar Cells
C. Rodríguez-Seco [a,b], M. Méndez , C. Roldán-Carmona* , R. Pudi1, M. K. Nazeeruddin* and E.
[a]
[c]
[c]
Palomares*[a,d]
.
[
a]
Institute of Chemical Research of Catalonia, The Barcelona Institute of Science and Technology
(ICIQ-BIST), Avda. Països Catalans, 16, E-43007 Tarragona, Spain.
[
b]
Departament d’Enginyeria Electrꢀnica, Elꢁctrica i Automꢂtica, URV, Avda. Països Catalans, 26, E-
4
3007 Tarragona, Spain.
[
c]
Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and
Engineering, École Polytechnique Fédérale de Lausanne, CH-1951 Sion, Switzerland.
[
d]
ICREA, Passeig Lluís Companys, 23, E-08010 Barcelona, Spain.
Introduction.
[
1]
Abstract: Three hole transport materials (HTMs) based on
a
Since the first report in 2009 , hybrid lead-halide perovskites
rd
substituted triphenylamine moiety have been synthetized and
successfully employed in triple-cation mixed-halide PSCs, reaching
efficiencies of 19.4%. The efficiencies, comparable to those obtained
using spiro-OMeTAD, point them out as promising candidates for
easily attainable and cost-effective alternatives for PSCs, given their
facile synthesis from commercially available materials. Interestingly,
although all these HTMs present close chemical and physical
properties, they provide different carrier recombination kinetics. Our
results demonstrate that is feasible through the molecular design of
the HTM to minimize carrier losses and, thus, increase the solar cell
efficiencies.
(PSC’s) have become a new technology for the 3 generation of
photovoltaic devices. In few years, PSC’s have demonstrated a
rapid evolution in PCE from 9 % to over 25.2% . The most
efficient device configurations consists in a mesoporous layer of
TiO where the perovskite material is infiltrated, covered with a
uniform layer of solid HTM . To date the most studied organic
HTMs are polytriarylamine (PTAA) , and spiro-OMeTAD, the
later representing the best choice for obtaining highly efficient and
reproducible devices . Unfortunately, these molecules are made
through difficult synthetic steps that require expensive purification
processes, which currently represents a major limitation towards
low-cost commercialization. To solve the inconveniences, many
attempts have focused in the design and synthesis of alternative
HTMs. For instance, triphenylamine (TPA) is an electron donor
[2]
2
[3]
[4]
[5]
[7][8][9]
group widely used in optoelectronics
thanks to its redox-
[10] [11]
active properties
. Among the several advantages are
highlighted its easy synthesis and tuneable molecular structure
towards optimal solubility, band structure and absorption
[
a]
Dr. C. Rodríguez-Seco, Dr. M. Méndez, Dr. R. Pudi, Prof. Dr. E.
Palomares
[10a]
characteristics
. Because of its high solubility in common
Institute of Chemical Research of Catalonia, The Barcelona institute
of Science and Technology (ICIQ-BIST).
Avda. Països Catalans, 16, E-43007 Tarragona (Spain)
E-mail: epalomares@iciq.es
solvents, the deposition on top of the perovskite material can be
done through compatible solution methods. For this reason, in
the last years various examples of HTMs containing TPA
[
[
b]
c]
Dr. C. Rodríguez-Seco,
Departament d’Enginyeria Electrꢀnica I Automꢂtica
University Rovira I Virgili (URV).
Avda Països Catalans, 26, E-43007 Tarragona (Spain)
Dr. C. Roldán-Carmona, Prof. Dr. M. K. Nazeeruddin
Group of Molecular Engineering of Functional Materials
Institute of Chemical Sciences and Engineering
École Polytechnique Fédérale de Lausanne (EPFL)
Rue de l’Industry, 17, CH-1951 Sion (Switzerland)
Prof. Dr. E. Palomares
moieties have been reported. For example, Nazeeruddin and co-
[12]
workers
synthesised in 2014 a new TPA-based derivative
(
TPA-MeOPh) showing a PCE of 10.79 %. Few years later Kloo
[12b]
et al.
molecularly engineered two new structures named X1
and X14, yielding impressive PCE of 14.4 % and 16.4 %,
respectively, exceeding the performance of spiro-OMeTAD
[13]
(
15.6 %). Contemporaneously Ko et al. reported three small
molecules B(BMPHDPH) , DPEDOT-B(BMPDP) and DPBTD-
B(BMPDP) for efficient PSCs incorporating groups containing
[
d]
2
2
ICREA
2
Passeig Lluís Companys, 23, E-08010, Barcelona (Spain)
heteroatoms, such as EDOT (3,4-Ethylenedioxythiophene) and
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
benzothiadiazole. This resulted in PCEs of 13.81 %, 14.23 % and
[14]
1
4.55 %, respectively. Later on, a TPA-based HTM (X2) has
demonstrated efficiency values of 15.53 % when measured in
reverse scan, a benchmark for TPA-based organic structures.
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Angewandte Chemie International Edition
10.1002/anie.201915022
RESEARCH ARTICLE
However, despite their high potential as HTMs, low cost and
higher performance TPA based structures have not yet been
discovered for higher solar cell efficiency.
thermal decomposition, and a more stabilized amorphous phase
compared to the state-of-the art spiro-OMeTAD.
The normalized UV-Vis absorption and photoluminescence (PL)
-5
spectra of the new small molecules (10 M) in DCM are shown in
Figure S3, SI. The absorption bands located between 300 and
In this work, triphenylamine (TPA) units were chosen as electron
donor groups, having in common 1,3-dimethoxybenzene as
substituent (Scheme 1). Thanks to the weak interaction between
the donating moieties, we expect very low absorption in the visible
range, yielding colourless solids that will not interfere with the light
that perovskite collects. Note that the nitrogen atom in the TPA
acts as a doping site and can be stabilized through conjugation
with neighbouring phenyl rings. In addition, thanks to the non-
planar configuration it also helps to increase the intermolecular
distance, inducing an amorphous nature of the HTM layer.
Because it might also reduce the hole mobility, these two
properties must be balanced, so that a high mobility is combined
with an amorphous nature, ensuring a high glass transition
4
00 nm, which are present in the three derivatives, are assigned
to the π-π* transitions of the conjugated system. CS02 and CS04
have almost identical absorption bands in the UV region, with
peak maximum (λabs, max) located at 340.5 nm and 334.5 nm,
respectively. On the contrary, CS05 has its maximum wavelength
blue shifted, located at 302.5 nm, surely related to the
incorporation of carbazole units to the molecular structure.
Interestingly, none of the molecules absorb in the visible region of
the spectrum. A similar behaviour is observed for the emission
spectra, which exhibits PL maxima centred at 443 nm, 405 nm
and 379 nm for CS02, CS04 and CS05, respectively, when they
are excited at λabs, max
.
[6]
temperature .
We have synthetized three TPA-derivatives named CS02, which
contains a phenyl ring as a central core acting as a link between
the donor units; CS04, a simple TPA-based HTM surrounded with
1
,3-dimethoxybenzene; and CS05, which is a combination of the
TPA and a carbazole derivative. The general procedures for the
preparation of CS02, CS04 and CS05 are shown and described
in detail in the SI.
Figure 1: Scheme of the energy levels of the perovskite and the different HTMs
studied in this work. The oxidation potential values approximating the HOMO of
CS02, CS04, CS05 and spiro-OMeTAD have been estimated from cyclic
voltammetry in DCM (see Figure S4 and Table S3).
To have a better understanding of the energy levels, we also
performed cyclic voltammetry. The HOMO values are estimated
to be -5.18 eV and -5.20 eV for CS02 and CS05, and -4.73 eV
CS04 respectively (see Supplementary Information, Figure S4),
suggesting feasible hole-injection from the perovskite.
Interestingly, the introduction of carbazole units in CS05 leads to
a deeper HOMO energy level value to CS04. This is related to a
decreased electron density of the π-system induced by the
electron withdrawing substituents, thus stabilizing the system
towards increased oxidation potential and shift in the HOMO to
lower energy. Consequently, the HOMO values for CS02, CS04
and CS05 properly match with the valence band edge of the
perovskite, leading to an effective hole extraction from the HTM,
but also an efficient electron-blocking due to the high LUMO level
Scheme 1: Molecular structures of CS02, CS04 and CS05.
Results and Discussion
The thermal behaviour of the small molecules was analysed by
thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) measurements. The results, showed in Figure
S1 and Table S1, confirmed a very high thermal stability, with a
decomposition temperature (weight loss of only 5 %) up to
4
64.2 °C, 445.7 °C and 451.7 °C for CS02, CS04 and CS05,
[15]
respectively, superior to that of spiro-OMeTAD (400 °C) . In
addition, DSC revealed very high glass transition temperature (T
(see Figure 1).
g
)
during the second and third heating cycle for CS02 (192 °C) and
CS05 (170 °C) (Figure S2, SI), suggesting that they exist in both
crystalline and amorphous states. No melting was detected in
CS04, thus only existing in its amorphous phase. Interestingly, T
for spiro-OMeTAD (126 °C) is considerably lower compared to the
g
[16]
TPA molecules , which plausibly facilitates the formation of
uniform films for device applications. All the data are collected in
Table 1. These results confirm their higher stability towards
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Angewandte Chemie International Edition
10.1002/anie.201915022
RESEARCH ARTICLE
and the HTM thickness was optimized to obtain the best device
performance for each molecule (see SI for further details). The J-
V curves, shown in Figure 2, correspond to the champion cells
containing the three investigated HTMs, measured under AM 1.5
-1
G conditions at a scan rate of 10 mV s . The results summarized
in Table 1, show excellent performances for CS04 and CS05
molecules, with average PCE values over 18 % in both cases. As
observed in Figure 2, the three TPA-molecules present similar
and over 1 V open circuit voltages (VOC), and high fill factor (FF)
Figure 2. J-V curves (left) measured in reverse bias and IPCE spectra with
integrated JSC (right) for triple-cation mixed-halide perovskite devices using
CS02, CS04, CS05 and spiro-OMeTAD as HTMs.
values, differing mostly in the short circuit current densities (J ).
sc
Note that very high values are achieved in the case of CS04 and
-2
CS05 molecules (> 23 mA·cm ), while for CS02 it remains close
-
2
-
to 18 mA·cm . Interestingly, JSC obtained for CS05 (23.8 mA·cm
)
2
-2
is very close to that of spiro-OMeTAD (24.1 mA·cm ), leading
We also analysed the hole mobility (μ) of the three different HTMs
to a remarkable efficiency as high as 19.4 %. The scanning
electron microscopy (SEM) images of the cross-section devices
are included in Figure S9 and S10. We highlight the presence of
inhomogeneities and cracks observed within CS02 films, which is
due to the nature of the molecular structure and its difficulties to
form thin films. On the contrary, CS04 and CS05 form
homogeneous layers of ~ 100 nm, with a good morphology and
film coverage, leading to the improved FF values (> 70 %). A
closer inspection to the top- SEM images reveals the formation of
sphere-like aggregates for CS04, which correspond to
[17]
following the space-charge limited current (SCLC) method
.
Films of the molecules were prepared in equivalent conditions as
those employed for device fabrication (see SI for more details),
obtaining for all the molecules very good hole-mobilities in the
range of spiro-OMeTAD, when containing 50 mol% of bis(trifluo-
romethane)sulfonimide lithium salt (LiTFSI) and 330 %mol of 4-
tert-butylpyridine (tBP) as dopants. The J-V curves and related μ,
including spiro-OMeTAD for comparison, are presented in Figure
S5 and Table S4.
[11b, 18]
amorphous domains
. Note that CS05 has a more rigid
molecular structure, which induces a better π- π stacking of the
HTM, favouring improved film coverage and better performance.
In addition, the incident photon-to-current conversion efficiency
(IPCE) of the PSCs, included in Figure 2, reveal high current
generation over the whole range of visible spectra for CS04 and
CS05. This results in an integrated photocurrent (Jint) of 20.5
mA/cm , 22.78 mA/cm , and 22.20 mA/cm for CS02, CS04,
CS05 respectively, further supporting those obtained from the J-
V measurements. The statistic distribution of the PV parameters
obtained from more than 12 devices are included in Figure 3.
Table 1: PV parameters showing the performance of the best PSCs devices by
using CS02, CS04, CS05 as HTMs.
V
OC (V)
J
SC (mA/cm2)
FF (%)
0.81
PCE (%)
HTM
CS02
CS04
CS05
1
1
1
5.40
8.05
9.38
1
1
1
.04
.04
.05
18.30
23.09
0.75
23.80
0.78
2
2
2
With the aim to investigate the origin of efficiency losses under
operating conditions, we analysed them by using transient photo
current (TPC), transient photo voltage (TPV) and differential
capacitance (DC) techniques. These techniques have been
frequently employed in the past few decades to analyse the
carrier losses and the carrier recombination order in dye-
[19]
sensitized solar cell (DSSC) and OPV . Understanding the
carrier losses mechanisms is key to increase the solar cell
efficiency. All the above-mentioned techniques, are based on the
application of a laser pulse that generates a small perturbation of
the Voc. When the solar cell is open-circuited (TPV) the “extra”
photo-generated carriers are forced to recombine with an
associated lifetime (τΔn). When the solar cell is short-circuited
(TPC), the extra carriers are extracted before recombination
Figure 3. Statistical distribution of the cell parameters of more than 12 devices
of PSCs using TPA-based HTMs.
process. Differential Capacitance (DC) is the combination of the
above-mentioned techniques, taking Δq from the TPC and the
small perturbation in voltage created by the laser pulse at every
Voc (ΔV) (Equation 1).
To evaluate the TPA-derivatives PSCs based on a triple-cation
perovskite absorber were fabricated. In brief, an optimized device
dC (VOC) = Δq / (ΔV (VOC))
Equation 1
architecture consisting on FTO/ TiO -blocking/ TiO
2
2
-mesoporous/
SnO / Cs0.1(MA0.15FA0.85)Pb(I0.85Br) / HTM / Au was employed,
2
3
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Angewandte Chemie International Edition
10.1002/anie.201915022
RESEARCH ARTICLE
show faster recombination than CS05. Therefore, the
recombination is faster in the following order CS02 > CS04 >
CS05.
It is also possible to obtain the recombination order (δ) as δ = λ +
1
, in which λ is a parameter that describes the slope of the power
law and can be obtained from Equation 2. From Figure 5 (b), we
obtained the recombination orders of the different devices
fabricated by using the different HTMs: δ(CS02) = 1.3, δ(CS04) =
1
.8, and δ(CS05) = 2.1. As can be observed, the slopes for the
different devices follow the same trend than the recombination
kinetics. The deduced recombination orders confirm that devices
are mostly ruled by first- (δ = 1) and second-order (δ = 2)
dynamics independent of the hole transporting material, but
clearly CS02 shows the fastest recombination kinetics and
different slope.
Figure 4. Differential Capacitance under different light bias with Cgeo and
without Cgeo for the devices with different hole transporting materials as
selective contacts. Dashed lines correspond to the linear fitting of the
geometrical capacitance. The solid lines at the bottom represent only the
Cx
experimental part of the fits: y=Be (chemical capacitance) after subtraction of
the geometrical capacitance.
-λ
ꞇ
Δn = ꞇΔn0 (n/n
0
)
Equation 2
To carry on the comparison between PSCs with the different
molecules used as HTM, we have taken care that the film
thickness is equal in all devices within the measurement error and
with an HTM thickness close to 70 nm (see Figure S9, SI). Figure
4
shows the photo-generated charge stored in the cell at
equilibrium at different VOC values, achieved by tuning the
background illumination from dark to 1 Sun equivalent. Each
experimental curve can be fitted to a linear plus exponential
dependence law. The linear component is caused by the
geometric capacitance of free charges accumulating in the
selective contacts. Solid lines in the bottom represent only the
carriers in the bulk of the perovskite, once the geometric
capacitance (Cgeo) is extracted (dashed lines) from the total
charge obtained (symbol). The capacitive component leads to a
[20]
misinterpretation of the data collected . The carriers in the bulk
solid lines) clearly show a much steeper slope at lower voltages
(
for CS02 and CS04 molecules and over 1 V for CS05, meaning
that the voltage at which the chemical capacitance becomes
relevant for each cell follows the trend CS02 < CS04 < CS05.
These changes in the accumulated charge as a function of
voltage indicates a difference in the energy offsets with respect to
the perovskite valence band (VB), with the most favourable
[21]
alignment for CS05. As reported previously , when using
different HTMs for fabricating a PSC, the resulting VOC will mainly
[
22]
relate to the respective HOMO energy level
as well as the
[23]
recombination constant and density of states disorder
.
In order to analyse the influence of the recombination when using
different HTMs, we carried out TPVmeasurements on the devices
under open circuit conditions. Since the carrier recombination
kinetics depends on the charge (n), it is convenient to plot the data
extracted from the TPV measurements versus n, as shown in
Figure 5 (a). Taking into account the charge accumulated in the
solar cells under the same light bias, indicated with a black solid
line in the bulk dynamics region, there are significant differences
between the HTMs. Nonetheless, we considered more correctly
to subtract the geometrical capacitance again as can be seen in
Figure 5 (b). The most significant result is that CS02 and CS04
Figure 5. Charge (n) measured from DC versus the carrier lifetime obtained via
TPV (a) and after subtracting the geometrical capacitance (b) for the different
HTMs.
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
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Conclusion
In this work, three novel molecules (CS02, CS04 and CS05) have
been synthetized based on triphenylamine derivative substituted
with dimethoxyphenyl rings as HTM in PSCs. The energy levels
show good alignment with those of the triple-cation mixed-halide
perovskite and the values of hole mobility are in the same order
of magnitude than that of spiro-OMeTAD. CS05 and CS04
exhibited an outstanding PCE of 19.4 % and 18.0 %, respectively.
We demonstrated that the HTMs structure can be tuned to
minimize the carrier recombination kinetics that control the solar
cell efficiency. For instance, CS02 shows poorer film formation
with respect CS04 and CS05 that affects negatively the solar cell
efficiency and the carrier recombination order. In contrast, for
CS05 the introduction of a carbazole moiety leads to a significant
reduction of the carrier losses and a notable increase of the solar
cell efficiency, which proof that careful design of the HTM
molecular structure is key to match or overpass the efficiency of
current HTM used in record efficiency perovskite solar cells such
as spiro-OMeTAD and poly-TPA.
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Authors contribution.
CRS, and EP designed the molecules and CRS and RP
synthesised CS02, CS04, CS05. MM carried out the photo-
physical characterization under the supervision of EP. CRC and
CRS fabricated the solar cells under the supervision of MN. All
authors participated in the discussion of the results and the
manuscript writing and, finally, approved the submission.
[
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CRS, MM, RP and EP thank MINECO (projects CTQ2013-47183
and CTQ 2017-89814-P) and SGR-AGAUR 2017SGR00978. EP
is also thankful to ICIQ and ICREA for economical support. CRC
and MKN thank the Swiss National Science Foundation
forfinancial support.
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Jankauskas, M. K. Nazeeruddin, Angewandte Chemie International
Edition 2016, 55, 7464-7468.
Keywords: Hole Transporting Material • Perovskite Solar Cells •
Recombination kinetics • Energy levels • Triphenylamines
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