Precise Control of Thermal and Redox Properties of Organic Hole-Transport Materials

Article abstract of DOI:10.1002/anie.201810809

We report design principles of the thermal and redox properties of synthetically accessible spiro-based hole transport materials (HTMs) and show the relevance of these findings to high-performance perovskite solar cells (PSCs). The chemical modification of an asymmetric spiro[fluorene-9,9′-xanthene] core is amenable to selective placement of redox active triphenylamine (TPA) units. We therefore leveraged computational techniques to investigate five HTMs bearing TPA groups judiciously positioned about this asymmetric spiro core. It was determined that TPA groups positioned about the conjugated fluorene moiety increase the free energy change for hole-extraction from the perovskite layer, while TPAs about the xanthene unit govern the T_g values. The synergistic effects of these characteristics resulted in an HTM characterized by both a low reduction potential (≈0.7 V vs. NHE) and a high T_g value (>125 °C) to yield a device power conversion efficiency (PCE) of 20.8 % in a PSC.

Full text of DOI:10.1002/anie.201810809

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Accepted Article
Title: Precise Control of Thermal and Redox Properties of Organic
Hole-Transport Materials
Authors: Valerie A Chiykowski, Yang Cao, Hairen Tan, Daniel P.
Tabor, Edward H. Sargent, Alán Aspuru-Guzik, and Curtis
Berlinguette
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201810809
Angew. Chem. 10.1002/ange.201810809
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10.1002/anie.201810809
Angewandte Chemie International Edition
COMMUNICATION
Precise Control of Thermal and Redox Properties of Organic
Hole-Transport Materials
Valerie A. Chiykowski,†[a] Yang Cao,†[a] Hairen Tan,[b] Daniel P. Tabor,[c] Edward H. Sargent,*[b] Alán
Aspuru-Guzik,*[c][d] and Curtis P. Berlinguette*[a][e][f]
Spiro-OMeTAD
HTM Series
Abstract: We report design principles of the thermal and redox
properties of synthetically accessible spiro-based hole transport
materials (HTMs) and show the relevance of these findings to high-
performance perovskite solar cells (PSCs). The chemical modification
of an asymmetric spiro[fluorene-9,9′-xanthene] core is amenable to
selective placement of redox active triphenylamine (TPA) units. We
therefore leveraged computational techniques to investigate five
HTMs bearing TPA groups judiciously positioned about this
asymmetric spiro core. It was determined that TPA groups positioned
about the conjugated fluorene moiety increase the free energy change
for hole-extraction from the perovskite layer, while TPAs about the
xanthene unit govern the Tg values. The synergistic effects of these
characteristics resulted in an HTM characterized by both a low
reduction potential (~0.7 V vs NHE) and a high Tg value (>125 °C) to
yield a device power conversion efficiency (PCE) of 20.8% in a PSC.
xanthene
fluorene
RX
RX
RX
RX
O
affect Tg
TPA
TPA
TPA
affect
HOMO
RF
RF
TPA
fluorene
fluorene
RF
RX
RXʹ
O
HTM-FX TPA TPA
H
H
TPA
HTM-F
HTM-X
HTM-Xʹ
TPA
H
H
TPA
H
N
H
H
TPA
TPA
HTM-FXʹ TPA
H
O
Figure 1. Molecular representations of Spiro-OMeTAD[15] and the HTM series
under investigation.
Hole-transport materials (HTMs) are a key component of organic light-
emitting diodes (OLEDs)[1–3] and solid-state solar cell devices.[4,5] High
performance HTMs for perovskite solar cells (PSCs) should exhibit
high glass transition temperatures (Tg) to suppress the formation of
grain boundaries in glassy films which can impede effective charge
transfer.[6,7] It is imperative that the HTM contains an appropriately
positioned and reversible redox couple to mediate charge extraction
from the photo-oxidized perovskite material, and be capable of
conducting charge through the charge transport layer.[8,9] These
materials should also have high temporal stability under elevated
temperatures and humidity.[10–12]
Organic HTMs are amenable to low-temperature solution
processing while also being capable of mediating high power
conversion efficiencies (PCEs).[13,14] Spiro-OMeTAD[15] (Figure 1)
is a widely used organic HTM and is capable of reaching PCEs
greater than 20% when integrated in PSCs.[16,17] However, Spiro-
OMeTAD is expensive to manufacture because the tetrahedral
spiro carbon that bridges the two perpendicular fluorene moieties
(9,9′-spirobifluorene) can only be accessed through a succession
of substitution, lithiation, condensation and bromination steps.[18]
Notwithstanding, the molecule teaches how a spiro carbon center
can impart a rigid 3D structure that leads to a high Tg (125 °C).[19]
The redox-active TPA substituents functionalized with methoxy
groups also yield reversible redox couples at potentials
appropriate for use in a PSC.[20] This combination of structural and
electrochemical properties contributes to the high hole mobilities
(2 × 10-4 to 5 × 10-5 cm2 V-1s-1) that have proven so effective for
Spiro-OMeTAD.[21]
[a]
[b]
V. Chiykowski, Dr. Y. Cao, Prof. Dr. C. P. Berlinguette
Department of Chemistry, The University of British Columbia, 2036
Main Mall, Vancouver, British Columbia, V6T 1Z1, Canada
E-mail: cberling@chem.ubc.ca
Dr. H. Tan, Prof. Dr. E. H. Sargent
Department of Electrical and Computer Engineering, University of
Toronto, 10 King’s College Road, Toronto, Ontario, M5S 3G4,
Canada.
E-mail: ted.sargent@utoronto.ca
[c]
Dr. D. P. Tabor, Prof. Dr. A. Aspuru-Guzik
Hagfeldt and Sun,[22] and Robertson[23] and their respective
coworkers recently independently disclosed that the alternative
spiro-based HTM “X60” exhibits performance parameters
competitive to Spiro-OMeTAD. X60, denoted herein as HTM-FX
(Figure 1), is an important discovery because it can be
Department of Chemistry and Chemical Biology, Harvard University,
12 Oxford St., Cambridge, Massachusetts, 02138, USA.
Email: alan@aspuru.com
[d]
[e]
[f]
†
Department of Chemistry and Department of Computer Science,
University of Toronto, Toronto, Ontario M5S 3H6, Canada and
Vector Institute, Toronto, ON M5G 1M1, Canada.
Department of Chemical and Biological Engineering, The University
of British Columbia, 2360 East Mall, Vancouver, British Columbia,
V6Y 1Z3, Canada.
Stewart Blusson Quantum Matter Institute, The University of British
Columbia, 2355 East Mall, Vancouver, British Columbia, V6T 1Z4,
Canada.
synthesized using
a simple one-pot condensation reaction
between phenols and a fluorenone moiety. This condensation
yields juxtaposed fluorene and xanthene units at a tetrahedral
carbon centre. This asymmetric fluorene-9,9′-xanthene core can
be functionalized with TPA groups, analogous to that of Spiro-
OMeTAD, to generate hole-transport properties that produce high
PCEs in the PSCs.[20,22,23]
These authors contributed equally to this work.
Supporting information and the ORCID identification numbers for the
authors of this article can be found under: a link at the end of the
document.
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10.1002/anie.201810809
Angewandte Chemie International Edition
COMMUNICATION
Table 1. Electrochemical and Hole-Transport Properties of HTMs.
[a]
[b]
[c]
Compound
EHOMO
εHOMO
Tg
Hole Mobility [d]
(× 10-4 cm2 V-1s-1)
(V vs NHE)
(V vs NHE)
(°C)
Spiro-
OMeTAD
0.63
-5.13
125
6.9
HTM-FX
HTM-F
0.67
0.67
0.76
0.97
0.66
-5.17
-5.17
-5.26
-5.47
-5.16
107
104
94
1.4
0.4
0.5
0.3
4.8
HTM-X
HTM-X’
HTM-FX’
130
137
[a] The half-wave potential corresponding to the first oxidation process in the
cyclic voltammogram. Measured in DCM and referenced to NHE by addition
of ferrocene. [b] εHOMO = − (EHOMO + 4.50 eV). [c] Determined by differential
scanning calorimetry (DSC). All Tm values determined to be >300 °C and
above the detection limits of our instrument. [d] Determined by space-charge-
limit current (SCLC) method from doped HTMs.
Replacing one of the two fluorene units in Spiro-OMeTAD with
a xanthene unit to form HTM-FX[22,23] enables us to test which
specific TPA groups affect the redox properties and thermal
temperatures of HTMs. We used computational methods to
screen the five HTMs in Figure 1 containing this asymmetric core
for appropriate frontier orbital energy levels, low reorganization
energies and photophysical properties. The frontier orbitals were
modeled (Figure S1) and the computed reduction potentials
corresponding to the first oxidation event (εHOMO) were predicted
to all exhibit suitable reduction potentials for hole extraction from
the perovskite layer. The reorganization energies (Ereorg) for the
entire HTM series (Table S1) were evaluated using Nelsen’s four
point method.[22,24,25] HTMs containing TPA moieties at the Rx or
Rx′ positions of the HTM scaffold were predicted to exhibit smaller
Ereorg values that are favorable for efficient hole transport
according to Marcus theory.[26] The vertical electron affinity of the
cation at the adiabatic cation minimum appears to be the more
dominant factor that affects Ereorg, and HTM-F was found to
contain the largest relative contribution from this term. Modeling
the spin-density differences of the adiabatic minima for the singly
Figure 2. Differential pulse voltammograms (DPVs) for HTM series normalized
to their first oxidation event. The number of electrons that contribute to each
redox process was determined by integrating the area beneath the peak fitting.
(a) The first deconvoluted oxidation events are shown in shaded colors to
highlight the progressively anodic shift of EHOMO for HTM-F, HTM-X and HTM-
X′, respectively. Traces for (b) HTM-FX and (c) HTM-FX′ are deconvoluted to
highlight that the EHOMO is linked to the TPA unit positioned on the fluorene unit.
The simple reaction chemistry associated with spiro-based
HTM-FX inspired us to elucidate how each of the TPA units about
the asymmetric spiro-core affects the HTM properties and the
corresponding performance in a PSC. We discovered that the
TPA groups positioned about the highly conjugated fluorene
fragment are the relevant electrochemically active units (i.e.,
associated with the redox couple that represents the HOMO
energy level, EHOMO) while those positioned on the xanthene
moiety affect the structural and thermal properties. We were able
to resolve this structure-property relationship by placing TPA
groups exclusively on the fluorene (HTM-F) or the xanthene
(HTM-X) units. These results pointed to the xanthene substituents
governing the thermal properties, prompting us to test how para-
and meta-substitution of TPAs at the xanthene moiety (i.e., HTM-
X and HTM-X′) affects the thermal properties. This decoupling of
the EHOMO levels and bulk thermal properties (e.g., Tg) yielded
HTM-FX′ characterized by a strikingly high Tg of 137 °C. This HTM
was found to be capable of generating a PCE of 20.8% in a PSC,
a value that compares favorably with those measured for Spiro-
OMeTAD and HTM-FX under the same experimental conditions.
Moreover, devices containing HTM-FX′ could be prepared with a
notably high level of reproducibility. This identification of how the
TPAs impact the redox activity, thermal properties and device
characteristics will guide the design of new high-performance
asymmetric HTMs with orthogonal halves.
oxidized molecules of the series (Figure S2) provides
a
visualization of the hole on an oxidized HTM and reveals
differences in hole delocalization within the series. Fluorene was
confirmed to be a highly conjugated unit and the positions of TPA
groups on the xanthene moieties affect the electronic structure of
the oxygen bridge. All computed Ereorg were between 0.14-0.20
eV and close to that of Spiro-OMeTAD (0.14 eV).
The preparation of HTM-FX followed a modified literature
procedure where 2,7-dibromofluorenone is reacted with 4-
bromophenol to yield a tetra-brominated product that, in turn,
undergoes a Pd-catalyzed Buchwald-Hartwig coupling with 4,4′-
dimethoxyphenylamine to yield the product.[22] This procedure
was adapted to access the rest of the HTM series (Scheme S1)
using either non-brominated fluorenone for HTM-X and HTM-X′
and/or 3-bromophenol for HTM-FX′ and HTM-X′. All compounds
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in the HTM series were prepared in merely two reaction steps
under relatively benign reaction conditions and a single column
purification step. By contrast, the Spiro-OMeTAD core requires a
multi-step synthesis involving highly air and moisture-sensitive
reagents, hazardous lithiation reactions and several time-
consuming column chromatographic purification steps.[15]
The melting point (Tm) and Tg of each HTM measured by
differential scanning calorimetry (DSC) is tabulated in Table 1.
The Tm and Tg values for every member of the series were
measured to be >300 °C and >93 °C, respectively. (A minimum
Tg value of 90°C is required to avoid morphology changes during
solar cell operation[6].) There is no apparent trend between
number of TPA units and glass transition temperature. However,
substitution at the meta-position of the xanthene moiety was
found to increase Tg by over 30 °C (e.g., Tg (HTM-X′) = 130 °C c.f.
Tg (HTM-X) = 94 °C). This striking increase in Tg through meta-
substitution of the xanthene was also observed in the case of
HTM-FX′ (Tg = 137 °C c.f. 107 °C for HTM-FX).
Cyclic voltammograms (CVs) for each HTM was recorded in
0.1 M n-NBu4PF6 DCM solutions to determine the EHOMO for the
series (Table 1 and Figure S15). The EHOMO values for HTMs
bearing TPAs on the fluorene (HTM-F, HTM-FX, and HTM-FX′)
were all found to be ~0.66 V vs NHE, which is slightly more
positive than Spiro-OMeTAD (0.63 V vs NHE) and therefore
appropriate for energy alignment with the perovskite layer.
Differential pulse voltammetry (DPV) was used to deconvolute
and quantify each of the successive oxidation events for the
series (Figure 2 and Table S3). The number of oxidation events
tracked the number of TPA units on the HTM, and we were able
to assign the redox activity to each TPA unit by taking a number
of different observations into account. For example, the EHOMO
levels for the HTMs with only para- or meta-substituted xanthene
TPAs (HTM-X and HTM-X′, respectively) are higher than that of
the HTM with only fluorene TPAs (HTM-F) and reflect a lower
degree of electronic coupling based on successive oxidation of
their two TPA groups (Figure 2a). This conjecture is fully
supported by the trend in computed εHOMO values (Table S1). The
first reduction potentials (EHOMO) for HTM-FX and HTM-FX′ can
therefore be linked to the first oxidations of the fluorene TPAs
(EF1), in that the first oxidation event of HTM-FX and HTM-FX′
matches that measured for HTM-F (0.67 V vs NHE). The second
oxidation event for HTM-FX at 0.87 V vs NHE is attributed to the
first oxidation of the xanthene TPAs (EX1) by benchmarking
against HTM-X (0.78 V vs NHE). We can assign the third and
fourth oxidations events of HTM-FX to the second faradaic events
for HTM-F (EF2) and HTM-X (EX2) according to their electronic
coupling (ΔE1/2 in Figure 2) with the first two oxidations. Similarly,
all four oxidation events for HTM-FX′ have been resolved even
though the second to fourth oxidation features overlapped
significantly and appeared to be a three-electron process (0.8 to
1.1 V vs NHE). Our assignments of the redox activity for each TPA
unit, which was corroborated by modeling the degree of hole
delocalization of the cationic HTM species (Figure S2) provides a
visualization of the hole on an oxidized HTM, clearly shows that
the relevant redox activity is confined to the TPA groups
appended to the fluorene units. This finding indicates that the
xanthene units can be modified without compromising the EHOMO
levels corresponding to the fluorene TPA units. The similar UV-
visible absorption and emission maxima (Table S2) of HTM-F,
HTM-FX and HTM-FX′ confirm the similar electronic structures of
the compounds’ frontier orbitals. All HTMs under study have
limited light absorption in the visible region which may be
beneficial in maximizing light absorption by the perovskite layer
(Figure S14).
The hole mobilities were determined using the space-charge-
limited current (SCLC) method (Table 1). The J-V characteristics
of hole-only devices with various HTMs are shown in Figure 3a.
The HTMs are doped with 4-tert-butylpyridine (tBP) and
bis(trifluoromethane)sulfonimide lithium salt (LiTFSI). The
mobilities of Spiro-OMeTAD, HTM-FX and HTM-FX′ obtained
here are comparable to the previous reported values.[21,22,27] The
mobility of HTM-FX′ is close to that of Spiro-OMeTAD and is much
higher than that of all other HTM compounds in the series under
study. While the EHOMO level of HTM-FX′ is similar to HTM-FX, the
higher mobility is ascribed to the meta-substitution affecting the
chemical structure or electronic delocalization. The low hole
mobilities of HTM-X and HTM-X′ may be due to the EHOMO levels
being too high in energy for efficient doping by LiTFSI. Given that
the extent of hole doping for HTM-F and HTM-FX by LiTFSI is
expected to be similar based on the similarities in EHOMO, the low
mobility of HTM-F is attributed to the higher Ereorg value (0.21 eV)
relative to the other HTMs (<0.16 eV). Each of the HTMs were
then tested in planar PSCs with the device architecture
glass/ITO/TiO2-Cl/perovskite/HTM/Au.[17] Table 2 summarizes the
statistical performance of devices fabricated with otherwise-
identical device processing for each HTM, and the J-V curves of
champion devices for each HTM are shown in Figure 3b.
Figure 3. Performance of perovskite solar cells with HTMs: (a) J-V curves of
hole-only devices with various HTMs; (b) J-V curves of solar cells with various
HTMs measured under solar simulator (100 mW cm-2); (c) Histogram of PCEs
for 44 devices with HTM-FX′; (d) J-V curves of champion device with HTM-FX′
scanned in reverse and forward directions (scanning rate of 20 mV/s).
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The glass transition temperatures, however, are governed by the
TPAs about the xanthene unit: substitution at the meta-position
raises the Tg by 30 °C over those substituted at the para-position.
The synergistic effects of a positively shifted EHOMO and high glass
transition temperature in HTM-FX′ yielded a device power
conversion efficiency of >20.8%, which is commensurate with
current state-of-the-art PSCs. Electrochemical, photophysical and
structural findings were fully supported by computational analysis
of the frontier orbital and reorganization energies of the neutral
HTMs and spin density difference maps of cationic species,
thereby highlighting the role predictive tools can play in screening
future state-of-the-art HTMs. This set of design principles that
enables selective control of redox and mesoscopic properties of
asymmetric HTMs will be elaborated on in future studies in pursuit
of robust higher performance PSCs.
Table 2. Electrochemical and Hole-Transport Properties of HTMs.[a]
Voc
(V)
Jsc
(mA·cm-2)
PCE
(%)
Champion
PCE (%)
Compound
FF
Spiro-
OMeTAD
1.16(2)
21.9(3)
0.78(2)
19.7(5)
20.4
HTM-FX
HTM-F
HTM-X
1.16(1)
1.08(2)
0.88(11)
0.77(6)
1.17(1)
21.2(3)
14.7(9)
3.8(6)
0.74(2)
0.41(2)
0.32(2)
0.26(1)
0.78(2)
18.2(7)
6.5(5)
1.1(2)
0.2(1)
19.7(6)
19.5
7.0
1.3
HTM-X
′
1.4(1)
0.3
HTM-FX
′
21.7(3)
20.8
Keywords: hole-transport materials; perovskite solar cells; solar
energy; organic semiconductors; glass transition temperature
Statistical performance of 41 Spiro-OMeTAD devices, 24 HTM-FX devices,
44 HTM-FX′ devices, 8 HTM-F devices, and 4 devices for each HTM-X and
HTM-X′, is shown here.
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The HTMs exhibit similar charge extraction kinetics and
effective photoluminescence quenching for all HTMs as revealed
by time-resolved and steady-state photoluminescence (PL)
measurements (Figure S17). Hole extraction from the perovskites
to each of the HTMs in this study was not found to be a limiting
factor, except for HTM-X′ which showed incomplete PL quenching
(Figure S17a). The HTMs exhibit similar charge extraction
kinetics between Spiro-OMeTAD, HTM-FX, and HTM-FX′ (Figure
S17b) though the initial PL decay rates in HTMs with only two TPA
moieties follow the trend of HTM-F > HTM-X > HTM-X′ (Figure
S17c).
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a
previous report.[22] HTM-FX′
outperformed HTM-FX (20.8% vs 19.5%, respectively) and the
performance enhancement can be attributed to the 3-fold higher
hole mobility imparted by the change of TPA substitution pattern
on the xanthene group (4.8 vs 1.4 × 10-4 cm2V-1s-1, respectively).
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OMeTAD. PSCs containing HTM-FX′ exhibit excellent
reproducibility, as indicated by the narrow PCE distribution over
44 devices (Figure 3c). Moreover, PSCs containing HTM-FX′ also
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10.1002/anie.201810809
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Entry for the Table of Contents
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Valerie A. Chiykowski, Yang Cao,
Hairen Tan, Daniel P. Tabor, Edward H.
Sargent,* Alán Aspuru-Guzik,* Curtis P.
Berlinguette*
Page No. – Page No.
Precise Control of Thermal and
Redox Properties of Organic Hole-
Transport Materials
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