A ring-locking strategy to enhance the chemical and photochemical stability of A-D-A-type non-fullerene acceptors

Article abstract of DOI:10.1039/d0ta09924d

Recently, the power conversion efficiencies (PCEs) of bulk-heterojunction organic solar cells (BHJ-OSCs) based on non-fullerene acceptors (NFAs) have made a very impressive progress in the research field. However, less attention has been paid to the intrinsic chemical and photochemical stability of NFAs, although they are correlated greatly with the resulting device stability. Herein, we describe a new molecular design strategy to enhance the intrinsic chemical and photochemical stability of acceptor-donor-acceptor (A-D-A)-type NFAs by introducing ring-locked carbon-carbon double bonds between D-A conjugation, attributed to increased steric hindrance of nucleophilic attack and the formation of intramolecular C-H?O interactions. Based on this strategy, two types of NFAs were successfully prepared, 2-(1,1-dicyanomethylene)rhodanine-based IDT-CR and IDTT-CR and thiobarbituric acid-based IDT-CT and IDTT-CT. When blended with a wide-bandgap polymer donor (P3HT), the IDTT-CR-based solar cells can exhibit a PCE of 2.86%. Moreover, a much enhanced PCE of 6.13% was realized by adopting a low-bandgap polymer donor PTB7-Th to pair with IDTT-CT. The fabricated PTB7-Th:IDTT-CT-based OSCs showed very encouraging photostability, the PCE of which could retain >80% of the initial values after 200 h one sun irradiation in air without a UV filter. Such photostability performance has greatly outperformed those from conventional NFAs like ITIC, IT-4F, and IT-M, suggesting the effectiveness of our ring-locking design strategy. Moreover, PTB7-Th:IDTT-CT-based OSCs could retain ～70% of its initial PCE after heating at 85 °C for 100 h. Furthermore, we reported an inferior device stability for P3HT:IDTT-CR based OSCs, which is primarily attributed to the evolution of BHJ film morphology under light illumination.

Full text of DOI:10.1039/d0ta09924d

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DOI: 10.1039/D0TA09924D
ARTICLE
A ring-locking strategy to enhance the chemical and
photochemical stability of A-D-A-type non-fullerene acceptors†
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
b
a
Hongtao Liu,‡ Wen Wang,‡ Yinhua Zhou and Zhong’an Li*
DOI: 10.1039/x0xx00000x
Recently, the power conversion efficiencies (PCEs) of bulk-heterojunction organic solar cells (BHJ-OSCs) based on non-
fullerene acceptors (NFAs) have made a very impressive progress. However, few attentions have been paid to the
intrinsically chemical and photochemical stability of NFAs, although which are greatly correlated with the resulting device
stability. Herein, we describe a new molecular design strategy to enhance the intrinsically chemical and photochemical
stability of acceptor-donor-acceptor (A-D-A)-type NFAs by the introduction of ring-locked carbon-carbon double bonds
between D-A conjugation, attributed to the removal of hydrogen atoms attached onto the electrophilic double bonds and
the formation of intramolecular C–H⸱⸱⸱O interactions. Based on this strategy, two types of NFAs have been successfully
prepared, 2-(1,1-dicyanomethylene)rhodanine-based IDT-CR and IDTT-CR and thiobarbituric acid-based IDT-CT and IDTT-
CT. When blended with a wide-bandgap polymer donor (P3HT), the IDTT-CR-based solar cells can exhibit a PCE of 2.86%.
Moreover, a much enhanced PCE of 6.13 % was realized by adapting a low-bandgap polymer donor PTB7-Th to pair with
IDTT-CT. The fabricated PTB7-Th:IDTT-CT-based OSCs showed very encouraging photostability, the PCE of which can retain
over 80% of the initial values after 200 h one sun irradiation in air without UV filter. Such photostability performance has
greatly outperformed those from conventional NFAs like ITIC, IT-4F, and IT-M, suggesting the effectiveness of our ring-
locking design strategy. Moreover, PTB7-Th:IDTT-CT-based OSCs can also retain ~70% of its initial PCE after heating at 85
O
C for 100 h. Besides, we also found the inferior device stability for P3HT:IDTT-CR based OSCs is mianly due to the
evolution of BHJ film morphology under light illumination.
22
23
24
small molecule NFAs like ITIC, O-IDTBR , 6TBA , Y6 , etc. is
based on the acceptor-donor-acceptor (A-D-A) structure, in
which a strong intramolecular charge transfer (ICT) can be
generated to ensure a strong and broad light absorption for
effectively harvesting solar energy. Furthermore, such type of
NFAs possesses excellent synthetic flexibility to tune their
optical/electronic properties as needed by rationally
integrating different types of donor and acceptor units. In
general, most of A-D-A-type NFAs are synthesized by a typical
Knoevenagel condensation between the aldehyde groups in
the D units such as indacenodithieno[3,2-b]thiophene (IDTT)
derivatives and the active methylene groups in the A units
Introduction
Organic solar cells (OSCs) have attracted great research
interests in the past two decades due to their significant
advantages of low cost, short energy payback time, light
weight, flexible and semitransparent.
development of novel non-fullerene acceptors (NFAs) such as
1
-9
Recently, the
3
5
,9-bis(2-methylene-(3-(1,1-dicyanomethylene)- indanone))-
,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-
indaceno[1,2-b:5,6-b′]dithiophene (ITIC) has successfully
broken the long-term domination of fullerene-based acceptors
10
in OSCs , and has excitingly enabled the power conversion
11-17
such as 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile
efficiencies (PCEs) of OSCs to exceed 17%
PCEs of NFA-based OSCs have potentially met the
requirements of commercialization, the corresponding device
stability still confronts great challenges.
To data, the mainstream design strategy to develop efficient
. Although the
6
(
INCN), rhodanine (RD), and thiobarbituric acid (TBA). In this
manner, the resulting NFAs inevitably contain very
electrophilic carbon-carbon double bonds due to the strong
push-pull effect, which should be easily attacked by the
nucleophiles or photooxidized under light irradiation, as shown
in Fig. 1a. This thus would lead to unsatisfied chemical and
photochemical stability of NFAs to significantly deteriorate the
final device stability.
Typically, the fabrication of OSCs involves a variety of well-
known interface materials containing reactive species, such as
the acidic PEDOT and nucleophilic PEIE, and so on. For example,
Zhou et al. recently have demonstrated the ITIC derivatives can
readily react with the amine species in the PEIE via a typical
1
8-21
a.
Key Laboratory for Material Chemistry of Energy Conversion and Storage,
Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong
University of Science and Technology, Wuhan, 430074, PR China. E-mail:
lizha@hust.edu.cn
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science
and Technology Wuhan 430074, PR China
b.
†
Electronic
Supplementary
Information
(ESI)
available.
See
DOI: 10.1039/x0xx00000x
‡
These authors contributed to this work equally.
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3
4
on the end-groups and side-chains of the NFAs , and further
View Article Online
DOI: 10.1039/D0TA09924D
showed that the ITIC-2F and ITIC-Th based OSCs can
potentially have a device lifetime approaching 10 years under
irradiation by white light without UV light, despite affording
35-37
lower PCEs than those reported elsewhere
. Nonetheless,
new design strategies toward intrinsically photostable NFAs
are still lack.
Herein, based on conventional A-D-A type NFAs, IDT-R, IDTT-
38
39, 40
R, IDT-T and IDTT-T
, we have synthesized a series of
novel NFAs, IDT-CR, IDTT-CR, IDT-CT and IDTT-CT (Fig. 1b), by
using a cyclohexene π-conjugated bridge linked with electron-
withdrawing end-groups. Indacenodithiophene (IDT) and IDTT
are used as the electron-donating cores, while 2-(1,1-
dicyanomethylene)rhodanine (RCN) and TBA as the electron-
withdrawing end-groups. Accordingly, the resulting NFAs
exhibited significantly improved chemical stability against
nucleophilic reagent and photochemical stability in
comparison to the control NFAs without ring-locked bridge,
thereby making the fabricated OSCs with much enhanced
stability than those from other high-performance conventional
NFAs like ITIC.
Fig. 1 (a) The mechanism of A-D-A-type NFAs attacked by the
nucleophiles or photooxidized under light irradiation. (b) The
structures of IDT-R, IDTT-R, IDTT-T, IDT-CR, IDTT-CR, IDT-CT
and IDTT-CT.
Results and discussion
Synthesis and Structural Characterization
nucleophilic addition reaction (i. e. Michael addition), making
25
the resulting device performance extremely poor. Besides The synthetic route of ring-locked NFAs is shown in Scheme 1,
PEIE, inorganic charge transporting materials such as zinc oxide and the synthetic details are provides in the ESI.
†
First, 3-
(
ZnO) also contains a large amount of amine species such as bromocyclohex-2-enone was synthesized
(1)
from
41
ethanolamine (EA), which are adopted to stabilize the cyclohexene-1,3-dione as reported. Then, a typical base-
nanoparticles in solutions, however, EA has been also shown catalyzed Knoevenagel condensation was carried out, which,
2
6
to react with ITIC derivatives. Furthermore, they also however, did not afford the desired cyclohexene-locked
−
reported that OH anion as a nucleophile could destroy the acceptors, BrCR and ClCT (Scheme 1). After carefully
27
C=C bond in ITIC. As a result, it is essential to improve the optimizing the reaction conditions, we found a TiCl
chemical stability of NFAs in the presence of nucleophilic condensation can successfully afford the products, according
reagent from the molecular design strategy, which however to the literature report in which TiCl was shown to coordinate
has been received less attention.
the ketone species to promote the condensation. Strangely,
OSCs also face photostability challenges caused by the for the reaction between and TBA dye, the bromine
-promoted
4
4
4
2
1
photodegradation of active layers. Particularly, the active substitution on the resulting product was replaced by chlorine
electrophilic double bonds in A-D-A-type NFAs are more atom, the structure of which can be confirmed by high
chemically vulnerable than the stable aromatic units under resolution mass spectrum (HRMS, ESI
†
) and signal-crystal
28, 29
illumination.
Significant research efforts recently have structure (Fig. 3b). This thus suggests an unusual halogen
been devoted to studying the photochemical stability of NFAs. exchange reaction, to which we still have no rational
For example, ZnO is well-known as the efficient photocatalyst, explanation at this stage. Furthermore, it is worth noting that
which can lead to the degradation of organic dyes under light the attempt of synthesizing the INCN-based acceptors
30
illumination. When the surface defects of ZnO adsorbed unfortunately failed. Finally, the desired ring-locked NFAs, i.e.
oxygen and water molecules, ITIC and IT-4F will be IDT-CR, IDTT-CR, IDT-CT, and IDTT-CT, were successfully
decomposed at the active layer/ZnO interface under UV synthesized via a typical Still coupling in high yields over 80%,
28, 31
illumination.
Kim et al. recently investigated the by using the IDT-tin and IDTT-tin to react with the BrCR and
degradation process of structurally similar NFAs, planar O- ClCT acceptors, respectively. On the other hand, control NFAs
IDTBR and nonplanar O-IDFBR, and found that the planar without any ring-locked bridges were also prepared for
configuration
can
be
more
resistant
to
the comparison based on the piperidine-catalyzed Knoevenagel
All the new NFAs
3
2
photodegradation. Li et al. reported that the NFAs can condensation reaction as shown in the ESI.
†
possess better anti-photooxidation ability when having deeper have good solubility, and their structures were fully
lowest unoccupied molecular orbital levels (LUMOs) than the characterized by nuclear magnetic resonance (NMR) and mass
33
electron affinity of oxygen. Moreover, Brabec et al. revealed spectrometry, and the detailed data are provided in
that the photostability of ITIC derivatives is strongly dependent experimental section (ESI ).
†
2
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/D0TA09924D
Scheme 1. Synthetic routes of IDT-CR, IDTT-CR, IDT-CT and IDTT-CT.
1
Fig. 2 H NMR of BrCR, IDT-CR and IDTT-CR in Chloroform-d (*).
Interestingly, BrCR is found to have two different subsequent products, i.e. IDT-CR and IDTT-CR (Fig. 2).
configurations, E-BrCR and Z-BrCR, due to the rotation of RCN Furthermore, Pure E-BrCR and Z-BrCR isomers were
end-group that can make the formation of two types of successfully separated and their structures were confirmed by
noncovalent C–H⸱⸱⸱O interactions with the neighboring vinylic the NMR spectra in Fig. S24-S27. We then tried to use them to
groups (E-configuration) and methylene groups (Z- synthesize IDTT-CR acceptors with single conformation, which,
configuration) in the cyclohexene-locked bridge, respectively however, failed possibly due to the dynamic change of the
(Scheme 1). Such interactions enable the corresponding NMR noncovalent interactions under reaction conditions (Fig. S37-
signals with a clear downshift as shown in Fig. 2. For example, S38). On the contrary, it is worth noting that due to the
the signal of vinylic protons can downshift from 6.6 ppm in the symmetric structure of TBA end-group, its resulting products,
Z-configuration to 8.3 ppm in the E-configuration. Thus, based i.e. ClCT, IDT-CT and IDTT-CT, all do not have any isomers,
1
on the integration data, the molar ratio between E- and Z- while the H NMR spectra (ESI
†
) strongly indicate the existence
configurations is 0.64:0.36, which can be still remained in its of the noncovalent C–H⸱⸱⸱O interactions.
This journal is © The Royal Society of Chemistry 20xx
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(
εmaxs), and IDTT-CT thus exhibits the largest λ
of 641 nm
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x
ma
DOI: 10.1039/D0TA09924D
5
-1
-1
with a εmax of 1.34×10 M cm . Nevertheless, introduction of
ring-locked bridge into the NFAs also leads to decreased εmaxs,
possibly due to the decease of effective chromophore
concentration.
Relative to their solution ones, the film absorption spectra
of NFAs without ring-locked bridge all exhibit a red-shift of ~10
nm however with an increased short-wavelength shoulder
peak, somewhat indicating that the occurring of
intermolecular interactions in their films. On the contrary, the
ring-locked NFAs show a negligible red-shift by comparing the
film spectra to the solution ones. The optical bandgaps (Eg,opts)
of IDT-R, IDTT-R, and IDTT-T are estimated from the film
absorption edge as 1.96 eV, 1.84 eV, and 1.82 eV, respectively,
while those of IDT-CR, IDTT-CR, IDT-CT, and IDTT-CT films as
Fig. 3 The single crystal X-ray crystallography of (a) Z-BrCR and
(b) ClCT. For clarity, most of the hydrogen atoms are omitted,
and the hydrogen atoms are marked in white, carbon atoms in
blue, nitrogen atoms in magenta, oxygen atoms in red, sulfur
atoms in yellow, bromine atoms in green, and chlorine atoms
in purple.
Single Crystal X-Ray Crystallography
Single crystals of BrCR and ClCT were successfully obtained by
a slow evaporation of the co-solvent system to refine their
ground-state geometric structures based on the X-ray
crystallography. For BrCR, it is a pity that only Z-configuration
crystal was obtained and its structure (Orthorhombic, Pnma
1
.78 eV, 1.74 eV, 1.63 eV and 1.63 eV, respectively. The
decreased bandgap for the ring-locked NFAs could suggest a
potential of improving light-harvesting when used for OSCs.
The electrochemical (EC) properties of these NFAs were
investigated by cyclic voltammetry (CV) (Fig. S1†). As shown,
space group, Table S1†, CCDC2014380) is shown in Fig. 3a,
both IDT-R and IDTT-R exhibit irreversible oxidation and
reduction processes, while others all exhibit a quasi-reversible
oxidation process. Then, the highest occupied molecular
orbital (HOMO) and LUMO energy levels are calculated
according to the equation of EHOMO/LUMO = −(Eox/red,onest + 4.8) eV,
which are shown to be effectively affected by tuning the
central cores, the π-bridge, and the end-groups, as listed in
from which it is easily seen that there is a C–H⸱⸱⸱O bonding
existed between the carbonyl group in the RCN unit and the
methylene group (C6) in the cyclohexene bridge, with a
o
distance of 2.29 Å and a C6–H⸱⸱⸱O angle of 123 . Furthermore,
a weak C2–H⸱⸱⸱S bonding can be also observed with a distance
o
of 2.27 Å and an angle of 108 . The crystal structure (Triclinic,
P-1 space group, Table S2
Fig. 3b, in which the two carbonyl groups in the TBA unit form to -5.44 eV, and the LUMO energy levels range from -3.38 eV
two types of C–H⸱⸱⸱
bonding with the connecting to -3.67 eV. Due to enhanced electron-donating ability, the use
cyclohexene-based bridge, with a distance of 2.13 Å and 2.29 Å,
†, CCDC2014381) of ClCT is shown in Table 1. Overall, the HOMO energy levels range from -5.25 eV
O
of IDTT as the central core will lead to increased HOMO energy
levels compared to those from IDT, while the use of TBA as
end-groups will decrease the LUMO energy levels compared to
those from RCN, due to enhanced electron-withdrawing ability.
Their electrochemical energy bandgaps (Eg,ecs, Table 1) were
o
o
respectively, and an angle of 125 and 107 , respectively.
These noncovalent interactions could be able to act as the
conformational locks to planarize and rigidify the π-conjugated
molecular backbones of resulting NFAs, which has been well
also calculated according to the equation of Eg,ec = ELUMO
–
4
3
shown to improve charge carrier transport.
More
EHOMO, which are consistent with the above Eg,opts. The EC
characterization of NFAs was also carried out by differential
pulse voltammetry (DPV) to compare their CV data as shown in
importantly, based on these results and combined with the
1
above H NMR results, the chemical reactivity of resulting
NFAs are expected to be reduced to improve their
Fig. S1c
† and Table S3†. Based on these results, we can
chemical/photochemical stability, attributed to the removal of conclude that the introduction of ring-locked bridge is likely to
hydrogen atoms attached onto the electrophilic double bonds elevate the HOMO/LUMO energy levels.
in the D-A conjugation and the formation of intramolecular
interactions, which will be discussed later.
Photophysical and Electrochemical Properties
The UV−Vis absorption spectra of seven NFAs in chloroform
–
5
solutions (10 M) and as thin films are shown in Fig. 4, and the
corresponding data are summarized in Table 1. In solutions,
the absorption bands of the ring-locked NFAs are found to not
only show a red-shift about 20 nm but also an increased band-
broadening (~120 nm vs ~80 nm for full width at half maxima,
FWHM) when comparing with the corresponding control NFAs
without ring-locked bridge. This could be attributed to
enhanced intramolecular charge transfer (ICT) in the ring-
locked NFAs due to the enlarged conjugation length and
planarized molecular backbone. Moreover, by changing the
central core from IDT to IDTT or the end-groups from RCN to
TBA, the absorption maxima (λmaxs) of resulting NFAs all show Fig. 4 Normalized UV–Vis absorption spectra in chloroform
a clear red-shift with increased molar extinction coefficients solutions and as thin films.
4
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/D0TA09924D
ARTICLE
Table 1. Optical and electrochemical data of NFAs.
a
b
FWHMsolc
λfilb
FWHMfilc
Eg,optd
EHOMOe
ELUMOe
Eg,ecf
ε
λsol
max
NFAs
5
−1
−1
(
10 M cm ) (nm)
(nm)
(nm)
(nm)
97
(eV)
(eV)
(eV)
-3.53
-3.49
-3.64
-3.44
-3.38
-3.67
-3.65
(eV)
IDT-R
IDTT-R
IDTT-T
IDT-CR
IDTT-CR
IDT-CT
IDTT-CT
1.19
1.43
1.48
0.95
1.21
1.02
1.34
576
590
620
598
610
639
641
77
82
77
119
119
116
583
598
625
590
602
655
653
1.96
1.84
1.82
1.78
1.74
1.63
1.63
-5.44
-5.32
-5.51
-5.34
-5.25
-5.39
-5.28
1.91
1.83
1.87
1.90
1.87
1.78
1.68
116
103
151
154
144
126
155
a
-5
b
c
Molar extinction coefficients measured in 10 M chloroform solutions. The maximum absorption wavelength of NFAs. Full
d
width at half maxima of absorption bands. Calculated from the film absorption edge (λedge) according to the equation of Eg,opt
=
e
1
240/λedge. Measured by cyclic voltammograms of NFAs in dichloromethane/0.1 M Bu NPF solution. The HOMO and LUMO
4 6
f
energy levels are calculated according to the equation of EHOMO/LUMO = −(Eox/red + 4.80) eV. Calculated according to the equation
of Eg,ec = ELUMO – EHOMO
.
As discussed in the introduction, the A-D-A-type NFAs
synthesized by a typical Knoevenagel condensation generally
contain very electrophilic carbon-carbon double bonds that
can be easily attacked by the nucleophiles such as amine
derivatives in the many interface materials for OSCs. Fig. 5a
and S2a
between ITIC and EA, and the afforded products were well
confirmed by HRMS (Fig. S2b ). Initially, EA attacks the β-
carbon of ITIC and destroys the conjugated structure (Fig. 5a).
Then, the terminal acceptor moieties are lost (Fig. S2a ). Fig.
b shows the absorption spectra before and after adding EA
† illustrate a typical Michael addition underwent
†
†
5
-
3
(10 M) into the ITIC solution, in which a clear intensity
decrease can be found for the ICT-induced absorption band
due to the interruption of D-A conjugation. Similarly, the
addition of EA into IDTT-T solution also induces a substantial
absorption change (Fig. 5c). Nonetheless, the corresponding
ring-locked NFAs excitingly show a much enhanced chemical
stability against EA as we expected initially. As shown in Fig.
5
d, only a 12% decrease of absorption intensity was induced
after adding EA into IDTT-CT, and after keeping the mixed
solution for 40 min, it was still remained 45% of its initial
value. In addition, RCN-based NFAs show an enhanced
intrinsically chemical stability against EA, even without ring-
locked bridge. We attribute this to the decreased electron-
withdrawing ability of RCN-based end-groups that makes the
double bond less electrophilic than that in the INCN and TBA -
based NFAs. However, when increasing the EA concentration
from 10 M to 0.15 M and simultaneously extend the reaction
time, there is a significant difference still observed for RCN-
based NFAs; for IDT-R and IDTT-R (Fig. S3f† and 5e), the ICT-
induced absorption band showed a decrease of ~25% after 1.5
h, which almost disappeared after 19 h, while for IDT-CR and
Fig. 5 (a) The illustration of nucleophilic addition reaction of
ITIC with EA. Normalized UV–Vis absorption spectra of NFAs, (b)
ITIC, (c) IDTT-T, (d) IDTT-CT, (e) IDTT-R and f) IDTT-CR, before
-3
and after adding EA in THF:H O mixtures (96:4, V/V). The
2
-5
concentration of NFAs is controlled at 10 M, while that of EA
-3
is 10 M (b, c, d) and 0.15 M (e, f), respectively.
Chemical Stability against Nucleophilic Reagent
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IDTT-CR, over 85% was still remained after 19 h (Fig. S3g
f).
Photostability
†
and the photostability was observed as shown in Fig. 6b, indicating
View Article Online
DOI: 10.1039/D0TA09924D
5
UV-light irradiation is not the major reason for
photodegradation of ITIC-type NFAs. Encouragingly, both ring-
locked IDTT-CR and IDTT-CT (Fig. 6d and f) demonstrated
better photostability in comparison to their analogues without
ring-locked bridges (Fig. 6c and e) and ITIC. As shown, after 10
min irradiation, IDTT-T was photodegraded almost completely,
while the absorption intensity of IDTT-CT can still remain 90%
of its initial value. These results thus strongly illustrate that the
ring-locking strategy can significantly enhance the intrinsic
photostability of NFAs.
Photodegradation is one of long-standing challenges for
organic photovoltaic materials. We then tested the
photostability of NFAs in THF solutions under continuous one
2
sun irradiation (100 mW/cm ) by monitoring the change of
their absorption. As displayed in Fig. 6a, after 10 min
illumination, the UV−Vis absorption intensity of ITIC was
reduced by over 50%. To identify whether UV light has a more
pronounced effect, the absorption change of ITIC under light
with UV filter (transmission spectrum shown in Fig. S4
†) was
also monitored. However, a negligible impact on enhancing
Fig. 6 Normalized UV–Vis absorption spectra of (a) ITIC, (b) ITIC with UV filter, (c) IDTT-R, (d) IDTT-CR, (e) IDTT-T, and (f) IDTT-CT
2
-5
in THF solutions in air under different irradiation time (100 mW/cm ). The concentration of NFAs is controlled at 10 M.
Photovoltaic Performance and Film Morphology
Single-junction inverted OSCs with a device structure of
ITO/ZnO/polymer donor:NFAs/MoO /Ag were fabricated to
3
investigate the photovoltaic performance of these ring-locked
NFAs and the device fabrication details are provided in ESI†.
For comparison, the devices of IDT-R, IDTT-R and IDTT-T were
also fabricated at the same conditions. Two well-known
polymer donors, P3HT and PTB7-Th (Fig. 7a), were used in this
work. The energy level alignment of polymer donors and NFAs
is shown in Fig. 7b, in which it can be easily seen that all NFAs
seem to have compatible energy levels with P3HT donor.
Moreover, the LUMO levels of RCN-based NFAs are not
compatible to that of PTB7-Th, which are detrimental to the
exciton separation. We also employed another widely-used
polymer donor, i.e. PBDB-T, to blend with these ring-locked
acceptors, but the resulting performance was even worse than
that from PTB7-Th (Table S4†).
Fig. 7 (a) Chemical structures of polymer donors P3HT and
PTB7-Th. (b) Energy levels of polymer donors and seven
acceptors. J−V characteristics of (c) the P3HT:NFAs-based
devices and (d) the PTB7-Th:NFAs-based devices.
6
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a
Table 2 The optimal photovoltaic parameters of solar cells based on the six acceptors under the irradiation (AMVie
mW/cm ).
1
w
.
A
5
r
G
tic
,
le
1
Online
0
0
2
DOI: 10.1039/D0TA09924D
VOC
V)
0.73 ± 0.02
0.78 ± 0.02
0.66 ± 0.01
0.82 ± 0.03
JSC
FF
(%)
PCE
(%)
Donor:acceptor
2
(
(mA/cm )
3.73 ± 0.32
0.83 ± 0.28
2.62 ± 0.35
5.61 ± 0.38
P3HT:IDT-R
P3HT:IDTT-R
P3HT:IDT-CR
P3HT:IDTT-CR
51.25 ± 0.75 1.29 ± 0.28 (1.57)
48.18 ± 0.82 0.17 ± 0.26 (0.43)
49.26 ± 0.74 0.67 ± 0.32 (0.99)
55.06 ± 0.94 2.52 ± 0.34 (2.86 )
PTB7-Th:IDTT-T 0.98 ± 0.01 13.71 ± 0.31 62.39 ± 0.61 8.41 ± 0.31 (8.72)
PTB7-Th:IDT-CT 0.93 ± 0.01 11.19 ± 0.26 42.32 ± 0.68 4.40 ± 0.22 (4.62)
PTB7-Th:IDTT-CT 0.93 ± 0.01 12.58 ± 0.27 50.45 ± 0.55 5.89 ± 0.24 (6.13)
a
Data obtained from the average of 15 individual devices, and the best PCEs are shown in brackets.
Fig. 7c displays the current density-voltage (J-V) curves of
the champion devices of RCN-based NFAs with P3HT as the
polymer donor, and the corresponding device parameters are
summarized in Table 2. To date, rhodanine-based NFAs has
shown a particular compatibility with P3HT donor with an
6
impressive PCE of 7% realized for binary OSCs yet. However
here, all the device performance is not very satisfied compared
to the state-of-the-art P3HT-based OSCs reported so far,
possibly due to that their unfavorable absorption properties
(
Fig. S5†) that lead to very low short-circuit currents (J s).
SC
Overall, the P3HT:IDTT-CR based devices show the highest PCE
of 2.86% among them with an open-circuit voltage (V ) of
OC
2
0
.85 V, a JSC of 5.99 mA /cm and a fill factor (FF) of 56%.
Nonetheless, we could not claim that our proposed ring-locked
strategy could be able to improve the device efficiency given
the fact IDT-CR-based devices even exhibit a lower PCE (0.99%)
than that of IDT-R-based ones (1.57%). On the other hand,
TBA-based NFAs, IDT-CT and IDTT-CT were also used to blend
with the P3HT donor. However, the fabricated devices even
performed worse than RCN-based ones, although the TBA-
based NFAs exhibit a red-shifted absorption and decreased
LUMO levels. Fortunately, when using PTB7-Th as the polymer
donor for the TBA-based NFAs, the resulting device efficiency
can be significantly improved due to their enhanced
compatibility, and the IDT-CT and IDTT-CT -based devices can
produce PCEs of 4.62% and 6.13%, respectively, as shown in
Fig. 7d and Table 2. As shown, ring-locked IDTT-CT delivered a
decreased device performance compared to the IDTT-T
analogue with a best PCE of 8.72%. Nonetheless, IDTT-CT-
based devices exhibited much enhanced device stability as
discussed later. The external quantum efficiency (EQE) spectra
Fig. 8 The device performance variation under continuous
irradiation: (a) P3HT:IDTT-CR device, (b) PTB7-Th:IDTT-CT and
(
c) PTB7-Th:IDTT-T device in a glovebox with dry nitrogen
2
atmosphere under one sun irradiation (100 mW/cm ); PTB7-
Th:IDTT-CT devices in air under one sun irradiation (100
mW/cm ) (d) with UV filter and (e) without UV filter; (f) PTB7-
(Fig. S6†) of P3HT:IDTT-CR and PTB7-Th:IDTT-CT -based devices
were also measured under the AM 1.5G solar spectrum (100
2
2
mW/cm ). For the P3HT:IDTT-CR-based device, the J
SC
Th:IDTT-CT devices at 85
atmosphere.
ºC in a glovebox with dry nitrogen
calculated by the integration of the EQE spectrum is 4.80
2
mA/cm , which does not agree well with the experimental J .
SC
The device deterioration is the main reason and will be
discussed later. The calculated JSC (12.17 mA/cm ) for PTB7-
Th/IDTT-CT-based device was similar to that measured by J–V
measurements (5.3% mismatch).
2
To check whether the enhanced chemical/photochemical
stabilities of our designed ring-locked NFAs would help to
improve the device stability, we first studied the device
performance variations of P3HT:IDTT-CR, PTB7-Th:IDTT-T and
PTB7-Th:IDTT-CT blends under simulated AM 1.5G solar
spectrum irradiation in the glove box with dry nitrogen
atmosphere, and the corresponding degradation curves of
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ARTICLE
Journal Name
different photovoltaic parameters are shown in Fig. 8a-c, characterized the evolution of blend film morphVoielwogAriteicsle aOfntliener
respectively. The P3HT:IDTT-CR-based devices display dramatic light irradiation via AFM. As displayed inDFOigI:.190,.1t0h3e9/DP03THAT0:9ID92T4TD-
burn-in degradation in the first 5 h, although IDTT-CR has CR blend film show great change in terms of surface roughness
higher chemical/photochemical stabilities than IDTT-CT. After with the root mean square roughness (Rq) from 4.27 nm to
continuously irradiating for 200 h, the PCE of P3HT:IDTT-CR 8.10 nm after 24 h light irradiation, while a negligible change is
blend exhibited a serious decrease up to ~50%, while only 14% observed for the PTB7-Th:IDTT-CT blend film. Consequently,
was observed for that of PTB7-Th:IDTT-CT blend. the morphology change should be responsible for the
Encouragingly, the photostability of PTB7-Th:IDTT-CT device significantly decreased performance for the P3HT:IDTT-CR
was found to be much better than that of PTB7-Th:IDTT-T blend. The above results further indicate the light-induced
device which only remained 57% of initial PCE under similar performance degradation of OSCs is a complicated process,
test conditions. This result thus directly confirms the which is related to not only the intrinsic photochemical
advantages of our ring-locked strategy in enhancing the stability of materials but also the physical film morphology.
intrinsic stabilities of NFAs.
Beyond that, the photostability of PTB7-Th:IDTT-CT devices
in air under one sun irradiation with or without UV filter were
Conclusions
further evaluated (Fig. 8d and 8e), both of which can
encouragingly show a comparable stability as that treated in
dry nitrogen atmosphere, and retain over 80% of the original
PCEs. These results thus strongly suggest the negative effects
of oxygen and UV-light in degrading the device performance
can be alleviated effectively by taking advantage of our
proposed ring-locked strategy. By sharp contrast, the device
performance of traditional state-of-the-art NFAs such as ITIC,
ITIC-4F, and IT-M experienced a more severe decay under
similar test conditions, and less than 40% of their original PCEs
were retained only after 100 h under one sun exposure in dry
In summary, we designed and synthesized four ring-locked
NFAs, IDT-CR, IDTT-CR, IDT-CT and IDTT-CT by adapting
cyclohexene as π-conjugated bridge. Compared with the
control NFAs without ring-locked bridge, the ring-locked NFAs
have demonstrated significantly enhanced chemical stability
against nucleophilic reagents such as primary amine
derivatives as well as improved photochemical stability against
strong light irradiation, attributed the removal of β-hydrogen
atoms in the electrophilic double bonds and the formation of
intramolecular C–H⸱⸱⸱O interactions to stabilize the γ-hydrogen
atoms. Meanwhile, the introduction of cyclohexene-based
bridge can slightly enlarge the conjugation of acceptors,
leading to red-shifted UV-Vis absorption spectra with
decreased optical bandgaps. When blended with a wide-
bandgap polymer donor (P3HT), the IDTT-CR-based solar cells
exhibited a PCE of 2.86%. Adapting a low-bandgap polymer
PTB7-Th to blend with IDTT-CT, the optimized devices showed
a PCE of 6.13 %. Although the PCEs are not as high as those of
state-of-the-art NFA-based OSCs, these two blends
demonstrated a much enhanced device stability. In particular,
the PTB7-Th:IDTT-CT-based OSCs can retain over 80% of its
nitrogen atmosphere (Fig. S7
OSCs is also affected by heat, we further checked the
thermostability of PTB7-Th:IDTT based devices heated at 85
†). Since the device stability of
º
C
in dry nitrogen atmosphere. As shown in Fig. 8f, ~70% of initial
PCE can be retained after heating for 100 h.
initial PCE in air after one sun irradiation without UV filter for
o
2
00 h, while ~70% after heating at 85 C for 100 h. These
results thus strongly suggest the use of ring-locked π-
conjugated bridge is an effective strategy for improving NFAs
chemical/photochemical stability and provides
a good
molecular insight for designing stable NFAs. Moreover the
relatively inferior device stability for P3HT:IDTT-CR-based
devices further highlights the importance of maintaining stable
morphology on the device stability.
Fig. 9 AFM images (5.00 × 5.00 μm). P3HT:IDTT-CR blend films:
Conflicts of interest
There are no conflicts to declare.
(
a) as-cast and (b) under continuous light irradiation (100
2
mW/cm ) for 24 h; PTB7-Th:IDTT-CT blend films: (c) as-cast and
2
(d) under continuous light irradiation (100 mW/cm ) for 24 h.
Acknowledgements
As discussed above, IDTT-CR exhibits a better chemical and
photochemical stabilities than IDTT-CT, however the Z. Li thanks the financial supports from the Natural Science
P3HT:IDTT-CR devices reversely showed a worse device Foundation of China (No. 21704030; 21975085). Y. Zhou
stability. To reveal the reason for the distinct device stability acknowledges the same funding agency with a grant no.
between P3HT:IDTT-CR and PTB7-Th:IDTT-CT, We thus 51773072. The authors also would like to thank the Analytical
8
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