General design of self-doped small molecules as efficient hole extraction materials for polymer solar cells

Article abstract of DOI:10.1039/c6ta09925d

The development of high performance hole transport materials (HTMs) without a chemical dopant is critical to achieve long-term device durability. The general design of self-doping materials based on a phenolamine structure with strong electronic spin conc

Full text of DOI:10.1039/c6ta09925d

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ISSN 2050-7488
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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
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General Design of Self-Doped Small Molecules as Efficient Hole
Extraction Materials for Polymer Solar Cells
Yuyuan Xue,a Peipei Guo,b Hin-Lap Yip,b, * Yuan Lia, * and Yong Caob
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The development of high performance hole transport materials term device durability and practical application.
(HTMs) without chemical dopant is critical to achieve long-term
device durability. The general design of self-doped materials based
on phenolamine structure with strong electronic spin concentration
is reported for the first time. The phenol-enhanced self-doped
mechanism is also proposed. Comparing with their precursors,
demethyl phenolamine derivatives, TBP-OH4, TPD-OH4 and Spiro-
OH8, displayed much higher spin concentration in their neutral
states. Phenol acts as hole trap in traditional concept, however, the
films of TBP-OH4, TPD-OH4 and Spiro-OH8 exhibited higher
conductivities than those of methoxyl precursors. Meanwhile,
phenolamine derivatives are good soluble in polar organic solvents
and show good solvent resistance in chlorobenzene. Considering
the relatively good band alignment, film-formation and solvent
resistance against chlorobenzene, Spiro-OH8 and TPD-OH4
exhibited comparable performance with that of PEDOT:PSS-4083.
Most importantly, a new generation of self-doped system based on
phenolamine structure might provide new insight to develop
efficient HTMs for organic electronics.
Figure 1. Chemical structure of diadical cation and self-doped triarylamine
derivatives.
Herein, a series of polar solvent soluble phenolamine
derivatives, TPA-OH2, TBP-OH4, TPD-OH4 and Spiro-OH8,
were obtained by substitution of the -OMe groups on
triphenylamine derivatives with -OH groups. The detailed
synthetic scheme and characterization of the products were
provided in the supporting information (SI) (Scheme S1, Figure
S2-S8).[3] Interestingly, a neutral radical system was found in
triphenylamine derivatives, namely as self-doped behavior,
which was completely different with the traditional radical
cation of triarylamines by Wang (Figure 1).[4b, 4e] The radical
species and physical property of triphenylamine derivatives
were thoroughly studied by cyclic voltammograms (CV), UV-
vis-NIR spectra, variable temperature 1H NMR (VT NMR),
electron spin resonance spectra (ESR) and hole-only electronic
device. It is worth noting that phenolamine derivatives show a
much higher spin concentration than those of methoxyl
Solution-processed polymer solar cells (PSCs) based on bulk
heterojunction (BHJ) active layer structure, have gained
particular attention because of their potential for low-cost
fabrication and high power conversion efficiency (PCE).[1-2] p-
Doped
and
water
soluble
poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS)
is the most widely used hole extraction material (HEM) in PSCs
due to the high conductivity, anti-erosion and solution
processability.[2g-2i] However, indispensable chemical dopant of
PSS gives rise to a series of negative effects, like device
fabrication and durability.[2a] The manufacturing of high hole-
mobility of HEMs without additional chemical dopants, is
becoming one of the most critical issues for achieving long-
Table 1. Optical, Electrochemical Properties and Energy Level of TPA-OH2, TBP-OH4,
TPD-OH4 and Spiro-OH8
Opt
Eg
Eoxonset (V vs
Ag/AgCl)
0.59
HOMO
(eV)
-5.29
-4.91
-5.16
-5.08
LUMO
(eV)
-2.07
-1.89
-2.14
-2.15
Samples
λabs (nm)
(eV)
3.22
3.02
3.02
2.93
TPA-OH2
TBP-OH4
TPD-OH4
Spiro-OH8
294
310
296, 350
301, 386
0.21
0.46
0.38
precursors. Comparing with the methoxyl precursors, the
strong self-doped behavior of phenolamine derivatives
resulted in a higher conductivity. Finally, for the relatively good
anti-erosion and film-formation properties, Spiro-OH8 and
a.School of Chemistry and Chemical Engineering, State Key Laboratory of Pulp and
Paper Engineering. South China University of Technology, Guangzhou, China.
b.Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory
of Luminescent Materials and Devices. South China University of Technology,
Guangzhou, China.
† E-mail: celiy@scut.edu.cn, msangusyip@scut.edu.cn.
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TPD-OH4 were applied as HEM in PSCs and their hole temperature (Figure S12). It revealed that the radical species
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DOI: 10.1039/C6TA09925D
extraction property were also studied.
The typical peaks of UV-vis absorption appeared at 294 nm,
of TBP-OH4 was not from its diradical cation.
Moreover, the 1H NMR peaks of TBP-OH4 became obviously
310 nm, 350nm and 380 nm for TPA-OH2
and Spiro-OH8, respectively (Table 1). Before and after particularly for the protons b and c+d (figure 3). Meanwhile,
, TBP-OH4,
TPD-OH4 sharper, when the temperature decreased to -80 oC,
demethylation
，
the compounds showed relatively similar
the decrease of temperature led to downfield shift of the
phenolic proton, which can be explained by enhanced
intermolecular interaction. This phenomenon of VT NMR
would not happened in the molecule of monoradical cation,
owing to the S=1/2 state. It indicated that the radical species
of TBP-OH4 was also not from its monoradical cation.
Secondly, TPA-OH2, TBP-OH4, TPD-OH4 and Spiro-OH8 can
be readily purified with the mixed solution of petroleum ether
and ethyl acetate on the silica gel. It indicated that the whole
molecules of TPA-OH2, TBP-OH4, TPD-OH4 and Spiro-OH8
were in the neutral state instead of forming radical cations due
to its low polarity (Figure S14). Furthermore, the XPS and
element analyses of Spiro-OH8 also confirmed the neutral
state because there was no Br- anion, which might come from
the by-product of the raw material of BBr3, in its powder form
(Table S1 and Figure S13).
Finally, the products with deep color could also be eluted in
the thin layer chromatography (TLC) analysis, especially for
TBP-OH4, owing to its obviously visible deep green color
(Figure S14). It was demonstrated that the low-energy
absorbance come from the corresponding radical state. This
Figure 2. Electron spin resonance (ESR) spectra of solid powders of (a) TPA-OMe
and TPA-OH2, (b) TBP-OMe and TBP-OH4, (c) TPD-OMe and TPD-OH4, (d) Spiro-
OMeTAD and Spiro-OH8 at room temperature.
UV-vis absorption bands and photoluminescence (PL) spectra
(Figure S10). It was noteworthy that the color of the products
obviously deepened in powder form after demethylation,
which was in good agreement with the results of the enlarged
UV-vis spectra in the low-energy absorbance of the four
products (Figure S11). Considering the molecular structure of
the four phenolamine derivatives, their low-energy
absorbance might come from the radical state.[4]
Therewith, ESR experiments were illustrated in Figure 2 to
investigate the radical state of the products. The g values of
TPA-OH2, TBP-OH4, TPD-OH4 and Spiro-OH8 were 2.040,
2.0031, 2.0036 and 2.0033, respectively, which are very similar
with each other. Meanwhile, the spin concentration of the
four products significantly increased after the demethylation.
Especially, the spin concentration of TBP-OH4 increased to 104
fold than that of TBP-OMe. Meanwhile, the spin concentration
result further confirmed that the radical species of TPA-OH2
,
TBP-OH4, TPD-OH4 and Spiro-OH8 were not from their radical
cations.
All the three experiments above provided solid evidences
for the neutral state of the radical species of the four products.
Meanwhile, methoxyl precursors also showed a lower spin
concentration comparing with those of phenolamine
derivatives (Figure 2 and S15). Furthermore, we also provided
another low band-gap triphenylamine derivative, TPA-OMe-
DPP, based on 2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP)
block and the synthesis, 1H NMR and mass spectrum were
provided in SI (Scheme S2, Figure S1, S9 and S15). TPA-OMe-
DPP also showed ESR signal in the solid form and 1H NMR
signal broadening in aromatic protons in CDCl3 (Figure S15a).
It revealed that the mechanism of radical species above not
only existed in phenolamine derivatives but also in methoxyl
derivatives. Considering the general radical signal and neutral
state of triphenylamine derivatives, we conjectured a radical
state existed in pristine triphenylamine derivatives instead of
the radical cation (Figure 3a).[4] The proposed mechanism is
described as follows. For example, two electrons were excited
from the symmetrical phenolamine groups into the LUMO of
TBP-OH4, respectively, thus leaving two holes in the HOMO of
of TPA-OH2, TPD-OH4 and Spiro-OH8 were also estimated to
be 8, 24 and 3352 fold, respectively, with the corresponding
methoxyl precursors as reference. It is interesting that TBP-
OH4 showed relatively higher spin concentration and no clear
hyperfine spectrum could be detected in its 1H NMR result
(Figure S5). It is likely that the reason stems from the excited
triplet state at room temperature.[4a, 4b, 4d]
Firstly, in order to reveal the origin of radical signal of these
neutral compounds, the VT NMR of TBP-OH4 and its radical
cation species was investigated. TBP-OH4 showed obviously
1
TBP-OH4 [5]
The symmetrical phenolamine groups still contain
.
delocalized radical with a relatively broadening H NMR signal
at room temperature and further broadened at elevated
temperatures (Figure 3b). It indicated that the more thermally
excited triplet diradical species was generated with the
increasing temperature. However, it has been demonstrated
that the diradical cation of TBP-OH4, which was obtained by
one lone electron, respectively, and therefore the holes in the
TBP-OH4 are backfilled by the electron transferred from the
symmetrical phenolamine groups. A pair of free electrons
remains in the LUMO of TBP-OH4, and is stabilized by the
quinonoid resonance structure and intermolecular interaction.
As a result, the TBP-OH4 molecule partially exists as excited
triplet state at room temperature with an S=1 state.[5] Thus,
1
adding excess [NO]+[PF6]-, was no H NMR signal of aromatic
protons in CD3OCD3 solution with a total triplet state at room
2 | J. Name., 2012, 00, 1-3
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the whole molecule of TBP-OH4 is neutral state. The low- NMR (Figure S12). Similarly, TPD-OH4 could be easily oxidized
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DOI: 10.1039/C6TA09925D
energy absorbance of TBP-OH4 was from the resonance into monoradical cation (λmax: 493, 719 and 1295 nm), and
structure of the connection of two nitrogen radical centers. then diaradical cation (λmax: 498 nm) with excess [NO]+[PF6]-.
The other triphenylamine derivatives have the similar According to the CV data, Spiro-OH8 undergoes two-electron
mechanism with TBP-OH4
.
oxidation to diradical dication with trace [NO]+[PF6]- (a broad
absorption between 435 nm and 838 nm; λmax: 1338 nm), and
then two-electron oxidation to the tretracation with excess
[NO]+[PF6]- (λmax: 522 nm).
Figure 3. (a) The proposed resonance structure. (b) Variable-temperature 1H
NMR spectra of TBP-OH4 in CD3COCD3 and assignment of all aromatic and
phenolic protons.
It is worth noting that phenolamine derivatives show a
much higher spin concentration than those of their
corresponding methoxyl precursors. When the -OMe
substituent was replaced by -OH group, oxidation potential of
triphenylamine derivatives showed slight decrease (Figure S16).
In addition, comparing with the corresponding methoxyl
derivatives, the band gap of phenolamine derivatives was
obviously narrower in the solid state, resulting in the low-
energy absorbance, which was related with their hydrogen
bond of phenolamine derivatives in solid state. Thus, with the
increased HOMO level and decreased energy band gap, the
electron might be excited from the ground state more easily.
We conclude that -OH group is beneficial for the generation
and stabilization of radical species.
Figure 4. Electrochemical stability experiment of (a) TPA-OH2, (b) TBP-OH4, (c)
TPD-OH4 and (d) Spiro-OH8 was performed in CH2Cl2/CH3CN
(VCH2Cl2/VCH3CN=250/1) solution at a scan rate of 50 mV/s; the films were spin-
coating on ITO.
The electrochemical stability of phenolamine derivatives
were also conducted (Figure 4). The CV curves of TPD-OH4 and
Spiro-OH8 were measured both in solution and film states.
Owning to the poor film-formation, the CV results of TPA-OH2
and TBP-OH4 were only measured in solution. All of them had
the reversible peaks under the oxidative process and showed
negative change after 20 repetitive cycles, which was
comparable with their corresponding methoxyl precursors
(Figure S17). It indicated that phenolamine derivatives showed
a good electrochemical stability.
As the discussion above, the easily excited electron from
ground state and intermolecular interaction play very Table 2. The conductivities of Spiro-OMeTAD, TBP-OH4, TPD-OH4 and Spiro-OH8 filmsa)
important roles in the self-doped system.
In order to further study the radical state of TPA-OH2
Spiro-
OMeTAD
90
Spiro-
OH8
30
, TBP-
Samples
TBP-OH4
TPD-OH4
OH4 TPD-OH4 and Spiro-OH8 in their neutral states, the
,
Thickness (nm)
60
30
oxidized and charged state of these materials were probed by
chemical oxidation (Figure S18). Therefore, considering the CV
measurements of the phenolamine derivatives (Figure S16),
Conductivity (S cm-1)
6.7×10-7
1.3×10-5
1.0×10-5
3×10-6
a)The conductivities of TBP-OMe and TPD-OMe were lower than 10-8 S cm-1. TPA-
OMe and TPA-OH2 cannot be detected as their poor film formation property.
the nitrosonium hexafluorophosphate [NO]+[PF6]- with
a
formal potential of 1.4 V, should be an effective oxidant to
The self-doped behavior could greatly enhance the charge
transport property of the material.[6] Therefore, the
conductivities of TBP-OMe, TPD-OMe, Spiro-OMeTAD, TBP-
OH4, TPD-OH4 and Spiro-OH8 films with no treatment, were
measured (Table 2). Due to the poor film formation, the
obtain their radical cations.[4d] TPA-OH2 could be easily
oxidized into
a monocation, with two relatively sharp
absorptions with λmax at 500 and 747 nm. TBP-OH4 was
oxidized to the monoradical cation by moderate [NO]+[PF6]-,
with three absorption band over the visible and NIR regions
(λmax: 418, 589 and 1014 nm). After adding excess [NO]+[PF6]-,
the radical cation was further oxidized into diaradical cation
(λmax: 515 nm). The -OH group in TBP-OH4 could not be
conductivities of TPA-OMe and TPA-OH2 were not measured.
The conductivity of TBP-OH4, TPD-OH4 and Spiro-OH8 films
were 1.3 × 10-5 S cm-2, 1.0 × 10-5 S cm-2 and 3.0 × 10-6 S cm-2,
respectively. The conductivity of Spiro-OMeTAD film was 6.7 ×
1
oxidated with excess [NO]+[PF6]-, which was confirmed by H
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10-7 S cm-2. However, the sheet resistances of TBP-OMe and the slight difference of HOMO level between SpViireow-AOrtiHcle8Oanlninde
TPD-OMe films were too large and cannot be measured in our TPD-OH4 (Table 1). Moreover, the perfoDrmOIa: n10c.e10s3o9f/CS6pTiAr0o9-9O2H5D8
test. The obvious increase of conductivity was also one of the and TPD-OH4 are much higher than that of bare ITO-based
solid evidence to prove the self-doped mechanism in devices. The PEDOT: PSS control device showed a relatively
phenolamine materials. Moreover, the conductivity of Spiro- better performance, yielding the highest average PCE of
OH8 also had an obvious increase with the increasing 8.40 % (Voc=0.780 V, Jsc=15.52 mA cm-2, FF=69.5 %).[2d] PEDOT:
annealing temperature in the hole only device, which indicated PSS based devices showed a slightly higher Voc than Spiro-OH8
that the radical state of Spiro-OH8 could be stimulated by and TPD-OH4 due to the relatively appropriate HOMO level
temperature (Figure 5b).
between that of ITO and PTB7-Th. Considering the relatively
As expected, after demethylation, the four products were poor film property in terms of electrical conductivity, Spiro-
highly soluble in DMSO, DMF (> 300 mg/mL), methanol, OH8 showed a lower FF than PEDOT: PSS (Figure S23).[9]
acetone, and ethyl acetate but insoluble in chloroform, Interestingly, comparing the Jsc of Spiro-OH8 with that of
toluene and chlorobenzene, which was contrary to methoxyl- PEDOT: PSS, Spiro-OH8 showed an obvious advantage.
based materials and provided a good orthogonal solvent
processibility to contract multilayer devices process from
solutions (Figure S22a). Meanwhile, considering the good anti-
erosion, film formation and band alignment, Spiro-OH8 was
applied in the PSC devices.
The water contact angle, thermal property and energy level
of Spiro-OH8 were studied before the devices fabrication
(Figure S19, S20 and S22). The surface of Spiro-OH8 (68.7o) is
more hydrophobic than that of PEDOT: PSS (57.3o). In addition,
the water contact angle of other two materials, TBP-OH4
(71.4o) and TPD-OH4 (68.3o), also showed higher
hydrophobicity than that of PEDOT: PSS. It revealed that the
film of phenolamine derivatives would help to improve the
coverage of the active layer as it is typical hydrophobic in
nature.[7] Meanwhile, phenolamine derivatives might have a
high performance to expel moisture away from the devices.
Spiro-OH8 exhibited a thermal decomposition temperature (Td)
of 371 °C and glass transition temperature (Tg) of 98 °C. The
Figure 5. (a) Energy level diagram for each layer in PVS device. (b) I-V curves of
hole-only devices with different annealing temperature. (c) J-V curves of the
devices fabricated with none HEM, Spiro-OH8 (5 mg/mL) and PEDOT: PSS as
HEM, respectively; and (d) the corresponding EQE spectra.
HOMO of Spiro-OH8 was calculated to be -5.08 eV from CV
Table 3. Photovoltaic parameters for polymer solar cell using with different HEMs
measurement, which is almost the same with that of PEDOT:
PSS (Table 1).[2]
Jsc
(mA/cm2)
15.52
16.97
16.51
PCE
(%)
HEM
Voc (V)
FF
As we know, proper doping is benefit for the performance
of Spiro-OMeTAD in PVSC.[8] For the light-induced doping
effect on doped Spiro-OMeTAD film, the doping level is not
controllable and easily lead to degradation of solar cell device
performance.[8b] However, UV-irradiation ESR experiment
showed that Spiro-OH8 powder was very stable under strong
UV-irradiation of 100 W (Figure S21).
PEDOT: PSS 40nm
TPD-OH4 5 mg/mL
Spiro-OH8 5 mg/mL
Spiro-OH8 10 mg/mL
Spiro-OH8 16 mg/mL
0.780
0.700
0.740
0.701
0.707
69.5
68.5
67.1
64.8
64.5
8.40
8.16
8.20
7.60
7.47
16.70
16.37
Meanwhile, the dependence of device performance on HEM
The performance of Spiro-OH8 was evaluated by fabricating thickness was investigated by spin-coating different
BHJ polymer solar cells (Table 3, Figure 5 and S25), with a concentration Spiro-OH8 solutions from 5-16 mg/mL. The
device structure of indium tin oxide (ITO)/HEM/PTB7- roughness decreased with the increasing spin-coating
Th:PC71BM/PFN-Br/Al (Figure 5a and S24), and the detailed concentration of Spiro-OH8 (Figure S23), however, the
PSC preparation is described in Supporting Information (SI). performance of the devices declined gradually with increasing
The best Spiro-OH8 based device exhibited a PCE of 8.20 %, HEM thickness. The degradation of the PCEs is mainly due to
with an open circuit voltage (Voc) of 0.740 V, a short current the decline of Voc and FF, whereas Jsc did not change. To
density (Jsc) of 16.51 mA cm-2 and a fill factor (FF) of 67.1 % by understand the high performance of Spiro-OH8, the external
spin-coating Spiro-OH8 at 5 mg/mL. Meanwhile, in order to quantum efficiency (EQE) of the devices was also investigated
further understand the performance of phenolamine (Figure 5d and S25b). Spiro-OH8 showed higher EQE in the
derivative in PSCs as HEM, TPD-OH4 was also employed in 370-450 nm, despite the EQE of Spiro-OH8 in the 300-370 nm
devices. The device with TPD-OH4 as HEM also showed a good range was lower than that of PEDOT: PSS. In contrast, with the
performance, yielding the highest PCE of 8.16 % (Voc=0.700 V, increasing thickness of Spiro-OH8, the EQE decreased in the
Jsc=16.97 mA cm-2, FF=68.5 %). The PCE of Spiro-OH8 based UV absorption range of Spiro-OH8, which was in good
device is better than that of TPD-OH4 based device, which is agreement with J-V results and showed similar trends (Figure
mainly related to the improved Voc. It is good agreement with
4 | J. Name., 2012, 00, 1-3
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1
(a) H. Bin, L. Gao, Z. G. Zhang, Y. Yang, Y. Zhang,VCie.wZAhraticnlegO, nSl.ine
S25b). As the whole, Spiro-OH8 and TPD-OH4 exhibited
comparable performance with that of PEDOT:PSS-4083.
Chen, L. Xue, C. Yang, M. Xiao and Y. LiD, ONIa: t1.0C.1o0m39m/Cu6nTA.,029091256D,
7
, 13651; (b) Z. Li, K. Jiang, G. Yang, J. Y. L. Lai, T. Ma, J. Zhao,
Finally, the shelf-life stability of PSC devices with self-doped
materials as HTMs, was also studied. The devices based on
Spiro-OH8 and TPD-OH4 presented enhanced stability than
that based on PEDOT:PSS-4083 after 216 h of storage. The
PCEs based on Spiro-OH8 and TPD-OH4 droped 16.5% (Figure
S26) and 17.3% after 216 h, respectively, comparing with
24.2% of PEDOT:PSS (Figure 6). The decrease of the PSCs
performance was attributed to the parameters of FF and Jsc.
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Conclusions
In summary, a novel phenolamine based neutral radical
system with high spin concentration, was found for the first
time and its self-doped mechanism was proposed.
Phenolamine structure is beneficial for the generation and
stabilization of radical species, owning to the strong
intermolecular interaction and electron-donating effect. Self-
doped Spiro-OH8 was successfully applied as HEM in polymer
BHJ solar cells, showing PCE values up to 8.20%. This is an
encouraging result as the performance is comparable to that
based on commercial PEDOT: PSS material. Phenolamine
structure is an efficient building block to achieve high-efficient
HEM. Meanwhile, the activation of phenolamine structure
induced the radical state with an enhanced conductivity. This
new design strategy might provide a new access to develop
novel and efficient semiconductors for organic electronics.
5
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Acknowledgements
The authors would like to acknowledge the financial support of
the National Natural Science Foundation of China (21402054,
21436004), the Ministry of Science and Technology
(2014CB643501, 2014CB643505) and the Natural Science
Foundation of China (51323003, 51273069, 51573057).
Energ. Environ. Sci., 2016, 9, 656.
9
L. Xia, Y. Xue, K. Xiong, C. Cai, Z. Peng, Y. Wu, Y. Li, J. Miao, D.
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Notes and references
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Journal of Materials Chemistry A
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DOI: 10.1039/C6TA09925D
The design of self-doped materials as efficient hole extraction materials for polymer solar cells is
reported for the first time.