Pyridination of hole transporting material in perovskite solar cells questions the long-term stability

Article abstract of DOI:10.1039/c8tc02242a

In this work, for the first time, reactive radical-cation species present in hole-transporting materials were shown to react with tert-butylpyridine additive, routinely used in hole transporting layer composition. As a result, new pyridinated products were isolated and characterized by NMR and MS analysis. Additionally, their optical and photophysical properties (i.e., solid-state ionization potentials (I_p), cyclic voltammetry (CV), UV/vis characteristics, and conductivities) were determined. Formation of the pyridinated products was confirmed in the aged perovskite solar cells by means of mass spectrometry, and shown to have negative influence on the overall device performance. We believe that these findings will help improve the stability of perovskite devices by either molecular engineering of hole-transporting materials or utilization of less-reactive or sterically hindered pyridine derivatives.

Full text of DOI:10.1039/c8tc02242a

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and V. Getautis, J. Mater. Chem. C, 2018, DOI: 10.1039/C8TC02242A.
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PAPER
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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ARTICLE
Pyridination of Hole Transporting Material in Perovskite Solar
Cells Questions the Long-term Stability
Artiom Magomedov,*a Ernestas Kasparavičius,a Kasparas Rakstys,b Sanghyun Paek,b Natalia
Gasilova,b Kristijonas Genevičius,c Gytis Juška,c Tadas Malinauskas,a Mohammad Khaja
Nazeeruddin,*b and Vytautas Getautis*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
In this work, for the first time, reactive radical-cation species present in hole-transporting materials were shown to react
with tert-butylpyridine additive, routinely used in hole transporting layer composition. As a result, new pyridinated
products were isolated and characterized by NMR and MS analysis. Additionally, their optical and photophysical properties
(i.e., solid-state ionization potentials (Ip), cyclic voltammetry (CV), UV/vis characteristics, and conductivities) were
determined. Formation of the pyridinated products was confirmed in the aged perovskite solar cells by means of mass
spectrometry, and shown to have negative influence on the overall device performance. We believe that these findings
will help improve the stability of perovskite devices by either molecular engineering of hole-transporting materials or
utilization
of
less-reactive
or
sterically
hindered
pyridine
derivatives.
Perovskite solar cells (PSCs) are efficient, yet achieving high demonstrated that hole transporting material (HTM) becomes
stability is still challenging1. The efficiencies of PSCs over the permeable to the gold migration at high temperatures13.
past decade have reached an impressive 22.7%2 for small However, gold-free architectures with carbon-based back
devices, 19.6%2 for 1 cm2 devices and 12.1%3 for >36 cm2 mini- electrode, which avoid fast degradation, still steadily
modules. Various scalable techniques (e.g., blade-coating4 and degrade14,15. Thus, attention must be paid to the increase in
roll-to-roll5 techniques) that have been applied for the stability of the HTM itself.
deposition of perovskites promise suitable market Molecules containing methoxydiphenylamine-substituted
applications. Further improvements in the stability of the fluorene16,17 and carbazole18 derivatives are among the best
devices are crucial for the successful commercialization of performing
PSCs. methoxyphenylamine)-9,9’-spirobifluorene
Although moisture and oxygen penetration can be efficiently remaining the most popular HTM. Other examples includes
suppressed by encapsulation6–8, the stability of the materials azomethine EDOT-OMeTPA19,
derivative fluorene–
and interfaces is essential. Significantly improved stability of dithiophene-based FDT16, and chemically more stable
the perovskite compositions have been achieved using multi- phthalocyanine-based HTMs20–22
Recently, our group
HTMs,
with
2,2’,7,7’-tetrakis-(N,N-di-p-
(spiro-OMeTAD)
.
cation (methylammonium, formamidinium, cesium9, and synthesized the promising HTM V88623, which is a cheaper
rubidium10). With this increase of the perovskite stability, the alternative to spiro-OMeTAD. However, above mentioned
hole-selective contact is typically considered as the main HTMs in their pristine form show low performance due to the
reason for efficiency loss, which is supported by the excellent low conductivity. To overcome this disadvantage, dopants,
long-term stability of hole-transporting material-free devices; such as lithium bis(trifluoromethane)sulfonamide24 (LiTFSI),
however, at the cost of significantly reduced efficiency of the FK20925 and tert-butylpyridine (tBP), are typically added
PSCs.11,12
before layer formation. While LiTFSI and FK209 act as oxidizing
The exact reasons for the degradation of the hole-selective agents26, improving the HTM conductivity; the role of tBP is
contact are still unclear. Recently, K. Domanski et al. still under investigation. Despite the positive effects on the
overall performance of solar cells, dopants typically result in
reduced long-term performance compared with dopant-free
HTM based devices 27–29
.
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Scheme 1. Oxidation and C-H pyridination of the dimethoxydiphenylamine-substituted carbazole fragment in V990.
Even though tBP has been used in PSCs to improve overall orange. After 20 min, the reaction was stopped, excess tBP
device performance, the exact role of tBP remains unclear, and was evaporated under reduced pressure and two products
several different mechanisms have been proposed. Since the were isolated after chromatographic purification.
first application in a dye-sensitized solar cell (DSSC)30 and The structures of the formed products were analyzed by 1H
further transition to a solid-state DSSC24, tBP has been used NMR and 13C NMR spectroscopy. One product was determined
1
without understanding this underlying mechanism. In 2014, N. to be unoxidized V990. In the H NMR spectrum of the other
Noel et al.31 suggested that pyridine base acts as a trap- product, down-field signals (8.98 (d, J = 6.5 Hz, 2H) and 7.86 (d,
passivation material, thus reducing recombination at the J = 6.9 Hz, 2H)), which are assigned to the pyridinium
HTM/perovskite interface. In 2016, S. Wang et al.32 suggested fragment, were observed. The singlet at 1.19 ppm (9H)
that tBP acts as a co-solvent, improving the uniformity of the confirmed the presence of the tert-butyl group in the product,
spin-coated layers. Later in the same year, the positive effect suggesting a structure with one pyridinium fragment attached
of tBP was proposed33 to result from a direct chemical to V990(tBP+)(TFSI-) (Scheme 1). The full spectrum is shown in
interaction between tBP and the perovskite material. Figure S5 in the ESI.
However, at approximately the same time,34 corrosion of the T. Morofuji et al. proposed that the pyridination site is
perovskite by
consistent with the lowest unoccupied molecular orbital
pyridine additives was demonstrated. All these mechanisms (LUMO) of the radical cation, which was determined by density
were supported by experimental evidence and may be functional theory (DFT) calculations37. As seen in Figure 1, the
complementary to one another. For the sake of further pyridination site is most likely located at the 4 position of the
1
developments in the field, a more detailed and complete carbazole moiety. This conclution is supported by the H NMR
mechanism is still greatly demanded.
spectra. In the range of 7.96 - 6.16 ppm, five signals with 1H
To date, no attention has been paid to the possible interaction integral can be observed with the spin-spin coupling pattern
between the HTM and tBP. Pyridine can act as a nucleophile (four doublets, one double doublet) well consistent with the
on the heteroaromatic radical cation species formed during substitution by pyridinium fragment at the 4 position of
doping process35. Since both the radical cation and pyridine carbazole. To further confirm proposed structure, 13C NMR
derivative are present in the HTM layer, the oxidized HTM and spectra was predicted by means of DFT calculations (more
tBP can potentially chemically interact with each other, which details are available in the ESI). Predicted spectra (Figure S8) is
would raise questions on the impact of tBP on the long-term in good agreement with the measured NMR spectra.
stability of the HTM material and devices themselves.
Characteristic signal of the carbon at the 4 position of
In this work, for the first time, we uncover the chemical t-butylpyridine ring was calculated to be at 174.34 ppm, and is
reaction between the oxidized HTM V886 and tBP. New in a good agreement with the experimentally measured values
pyridinated compounds were formed in high yields and were (172.46 ppm from dmso-d6, and 174.36 ppm from acetone-
isolated by standard chromatographic procedures for further d6).
characterization.
Initially, chemical interaction between tBP and model HTM
compound V990 was studied. Dimethoxydiphenylamine-
substituted carbazole fragments are present in several
efficient twin HTMs18,23, but materials with only one fragment
have equivalent chemical properties and are simpler to
analyse. V990 was prepared by a simple two-step synthesis in
high yield (Scheme S1). The cation-radical form (V990+(TFSI-)
)
was obtained via oxidation with AgTFSI, adapting the
procedure published by W. H. Nguyen et al.36
Next, we studied the chemical interaction between
V990+(TFSI-) and tBP. To reduce the number of possible by-
products and increase the speed of the reaction, V990+(TFSI-)
was dissolved in pure tBP and heated at 60°C. After 10 min
color of the solution changed from intensive green to dark
Figure 1. The lowest unoccupied molecular orbitals of V990+ obtained by DFT
calculations (B3LYP/def2-SVP). Isovalue = 0.04.
2 | J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
O
O
O
O
O
O
O
S
O
S
O
S
O
S
O
S
O
S
N
H+
O
S
O
F3C
N
CF3
F3C
N
O
CF3
F3C
N
CF3
S
O
O
O
O
O
O
N
F3
C
N
CF3
O
O
N+
+ H
N
N
N
N
N
N
N
N
O
O
O
O
O
V990+(TFSI-)
V990
N
N
N
V990+(TFSI-)
V990(tBP+)(TFSI-)
Scheme 2. Proposed mechanism of the C-H pyridination reaction.
Based on the obtained products and reaction conditions, a Additionally, as in the previous case, unoxidized V886 was
reaction mechanism was proposed (Scheme 2). isolated in a 20.5 % yield. More details, as well as UV/vis and
The proposed mechanism predicts formation of the unoxidized NMR spectra, are available in the ESI.
V990 and pyridinated V990(tBP+)(TFSI-) in a 1:1 molar ratio. Additionally, we studied the effect of this process on the
Indeed, the two products were isolated in a similar ratios electrical properties of the HTM. Presence of the oxidized
(38.5 % and 33 % yields based on the starting material).
species increase the conductivity of the HTM23, resulting in
better PSC performance. The conductivities of the formed
pyridinated products V886(tBP+)(TFSI-) and V886(tBP+)2(TFSI-)2
were on the order of 10-9 S cm-1 (Table S2), which is
significantly lower than the conductivity of oxidized V886
(4.2×10-5 S cm-1)23.
To estimate the energy levels of these materials, their
oxidation potentials were measured in solution using cyclic
voltammetry (CV) (Figure S18, Table S1). V886 showed two
quasi-reversible oxidation signals at 0.085 and 0.272 V vs
Fc/Fc+ (Fc = ferrocene). For V886(tBP+)(TFSI-), three redox
waves were observed: the first two signals were very close in
value to those of V886, and the third oxidation process
occurred at 0.527 V vs Fc/Fc+. V886(tBP+)2(TFSI-)2 showed only
two waves at 0.292 and 0.599 V vs Fc/Fc+. These results agree
with the structures of the materials, where two similar
fragments are interconnected by a bridging unit. CV data in
combination with the optical spectroscopy data was used to
determine HOMO/LUMO levels of the molecules and the
results are presented in Table S1. It can be seen, that
pyridination of the V886 results in more negative HOMO level
(-5.32, -5.34, and -5.53 eV for V886, V886(tBP+)(TFSI-), and
V886(tBP+)2(TFSI-)2 respectively). In addition, bandgap of the
V886 (2.72 eV) becomes narrower for both V886(tBP+)(TFSI-)
(1.70 eV) and V886(tBP+)2(TFSI-)2 (1.78 eV). As a consequence
lower LUMO levels were observed for V886(tBP+)(TFSI-), and
V886(tBP+)2(TFSI-)2 (-3.64, and -3.75 eV respectively)
compared with V886 (-2.60 eV) . Such change in LUMO energy
level could lead to reduced electron-blocking properties of the
HTM.
For better representation of the material energetics, the solid-
state ionization potential (Ip) values were measured by
photoelectron
V886(tBP+)(TFSI-)
spectroscopy
in
air
(PESA).
For
Figure 2. Structures of the V886, V886(tBP+)(TFSI-), and V886(tBP+)2(TFSI-)2.
,
the Ip value was 5.4 eV, and for
V886(tBP+)2(TFSI-)2, the Ip value was close to the measurement
limit of our equipment, estimated to be ~5.7 eV (Figure S17),
which is higher than that of the V886 (5.04 eV23). High Ip value
is unfavorable for the charge extraction from the perovskite
absorber material, due to the mismatch with the perovskite
valence band edge energy. Therefore it can be expected that
To determine whether the same process occurs in a well-
performing HTM, V88623 was tested under identical reaction
conditions. C-H pyridination of V8862+(TFSI-)2 occurred in the
same manner, two pyridinated products were isolated and
characterized by NMR. The mono- (V886(tBP+)(TFSI-)) and
dipyridinated (V886(tBP+)2(TFSI-)2) derivatives were obtained
in 37.9 % and 17.1 % yields, respectively (Figure 2).
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pyridinated species would serve as a trap-states for the In conclusion, for the first time we show that oxidized HTM
charges, reducing overall PSC performance.
species are prone to chemically interact with tBP additive,
forming new pyridinated products. In addition, it was shown
that the same process in situ occurs in the PSCs at elevated
temperatures during the relatively short aging time, and that
the formation of the pyridinated product V886(tBP+)(TFSI-) has
a negative impact on the device performance. These findings
raise new questions about the long-term stability of PSCs,
particularly the stability of organic HTMs upon doping. To
avoid negative impacts, we suggest using less-nucleophilic or
sterically hindered pyridine derivatives34 or substitute the
active hydrogen sites of the HTM.
25
20
15
10
V886
V886 + (10mol% V886(tBP+)(TFSI-))
5
Conflicts of interest
There are no conflicts to declare.
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Voltage (V)
Acknowledgements
Figure 3. J-V characteristics of PSC with V886, and V886+(10 mol% V886(tBP+)(TFSI-))
The authors acknowledge funding from the Research Council
of Lithuania via grant (No. LAT-07/2016). We thank E.
Kamarauskas for his help with ionization potential
measurements.
as a HTL.
To check, if pyridinatied products can be formed in the device,
PSCs with V886 as a HTM were prepared, following the
recently published procedure[34]
. Initially, device showed
17.5 % PCE, which after thermal stress (80 oC in air for 340 h)
dropped down to 13.81 % PCE (Figure S20). The aged PSC was
washed with the small amount of THF, followed by filtration of
the resulting extract. Concentrated sample was then directly
subjected to the MS analysis. As can be seen from the Figure
S22, weak signal at 1478.856 m/z can be observed, which is
attributed to the V886(tBP+) cation (1478.6694 Da). To further
confirm nature of this signal, high resolution MS/MS analysis
was performed (Figure S23) and the fragmentation pattern
confirmed that the observed signal arises from the
V886(tBP+)(TFSI-), the dipyridinated product V886(tBP+)2(TFSI-
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Journal of Materials Chemistry C
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DOI: 10.1039/C8TC02242A
Interaction of the hole transporting material with tert-butylpyridine as a possible degradation pathway of
perovskite solar cells.