Full Papers
doi.org/10.1002/cssc.202100080
ChemSusChem
contrast, the P(QxCN-T3) blend film showed a very low PL Conclusions
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quenching efficiency of only 33%, suggesting poor charge-
transfer. These trends are consistent with those observed in
P(E,T) and corresponding JSC values in the all-PSCs.
In this work, we report the development of a series of Qx-
based PAs by introducing CN groups on the 6- and 7-position
of Qx. In the series of QxCN-based PAs, their structural and
electrical properties were investigated by modifying the
electron-donating counterparts. The OFET measurements re-
vealed that all the QxCN-based PAs had electron transport
abilities with unipolar n-type characteristics. Among them,
P(QxCN-T2) and P(QxCN-TVT) showed well-ordered semi-
crystalline structures, forming good vertical conducting path-
ways. As a result, P(QxCN-T2) and P(QxCN-TVT) demonstrated
considerably high μe,SCLC of over 1.0×10À 4 cm2 VÀ 1 sÀ 1. In the all-
PSCs, the P(QxCN-T2)-based device achieved the highest PCE
of 5.32% among the QxCN-based PAs, attributed to the higher
electron mobility and exciton-dissociation probability with
reduced charge recombination. Thus, we demonstrated that
QxCN-based PAs can be applied as promising n-type compo-
nents for organic electronics.
It is known that the PL quenching behaviors are also related
to their blend morphologies, such as the PD–PA interfacial area,
at which exciton-dissociation occurs.[24] Accordingly, the blend
morphologies were investigated by atomic force microscopy
(AFM). Figure S6 shows that the surface roughness of the
P(QxCN-T3)-based blend film increased compared to those of
P(QxCN-TVT)- and P(QxCN-T2)-based blend films as supported
by its higher root-mean-square roughness (Rq) value. The
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P(QxCN-T2) blend film showed
a relatively uniform and
intermixed blend morphology, which can increase the PD–PA
interfacial area and promote efficient exciton-dissociation in all-
PSCs during operation. Consequently, when considering the
combined results of the electrical and morphological analyses,
the efficient charge generation and transportation in the
P(QxCN-T2) blend resulted in the enhanced PCE of all-PSCs with
the improved JSC and FF values.
Overall, we developed a new building block, QxCN, for n-
type conjugated polymers and successfully demonstrated the Experimental Section
potential of QxCN-based PAs for application in all-PSCs. To
evaluate the current level of photovoltaic performance for
QxCN-PA-based all-PSCs, standard all-PSC devices using a well-
known PA (N2200, poly[[N,N’-bis(2-octyldodecyl)-naphthalene-
1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5’-(2,2’-bithiophene)])
with the same PBDB-T PD were fabricated and compared
(Table S2).[1f,4h,12d] The all-PSCs showed an average PCE of 6.56%
with a VOC of 0.89 V, a JSC of 12.82 mAcmÀ 2, and a FF of 0.58.
Thus, when compared to the standard device, the P(QxCN-T2)-
based device produced a higher VOC value but lower JSC and FF
values. These results are mainly due to relatively low P(E,T)
value (72%) and high S value (1.36 kBTqÀ 1), which indicate that
the photo-induced excitons do not effectively dissociate to free
charges in this system. In addition, significant amount of the
generated free charges become recombined before reaching
the electrodes.[12b,c,18a] The highest FF (0.49) of the P(QxCN-T2)-
based devices in this study is also still much lower than those of
well-known high-performance all-PSCs. This encourages further
optimization of the morphological properties of the QxCN-PA
based blend films in terms of domain size, purity, and polymer
orientation. At the same time, insufficient electron affinity of
QxCN-based PA may hinder efficient exciton dissociation at the
interfaces between the QxCN-based PA and PD. From the
material-design perspective, we suspect that the presence of
the electron-donating alkoxy side chains in the QxCN unit
might reduce the n-type characteristics of the resulting PAs.[25]
To address this issue, we are developing another series of
QxCN-based PAs by (i) replacing the alkoxy-based side chains of
QxCN with other appropriate side chains and (ii) copolymerizing
QxCN unit with different counterparts that are more electron-
deficient than T2 (e.g., 3,3’-difluoro-2,2’-bithiophene). Based on
these design rules, we anticipate that the new QxCN-based PAs
will realize all-PSCs with enhanced photovoltaic performances.
Synthesis procedures for monomer and polymers
1,2-Bis(3,4-bis(octyloxy)phenyl)ethane-1,2-dione (1): 1,2-Bis
(octyloxy)benzene[9a] (7.0 g, 20.9 mmol) and AlCl3 (1.4 g,
12.3 mmol) in 1,2-dichloroethane (DCE) were added to a solution
°
of oxalyl chloride (1.6 g, 12.7 mmol) in DCE at 0 C under nitrogen
condition. After stirring for 30 min, the reaction solution was
allowed to warm up to RT and further stirred overnight. The
reaction mixture was poured to 1 m aqueous HCl solution and
the separated organic layer was dried over MgSO4. The crude
product was purified by column chromatography (hexane/
dichloromethane= 1:1) to yield yellow powder. Yield: 57%, 1H
NMR (400 MHz, CDCl3): δ= 7.55 (d, J= 4 Hz, 2H), 7.44 (dd, J=
4 Hz, J= 8 Hz, 2H), 6.85 (d, J=8 Hz, 2H), 4.04 (m, 8H), 1.84 (m,
8H), 1.46 (m, 8H), 1.28 (m, 32H), 0.88 ppm (m, 12H). MALDI-TOF
MS: calculated for C46H74O6 722.55; found: 722.97 (M+).
4,5-Diamino-3,6-dibromophthalonitrile (2): 4,5-Diaminophthaloni-
trile (3.6 g, 22.8 mmol) was dried under vacuum and dissolved in
degassed acetic acid under nitrogen condition. After cooling the
°
solution to 0 C, Br2 (8.1 g, 50.5 mmol) was added dropwise and the
reaction mixture was stirred overnight at RT. Aqueous Na2S2O3
solution was poured into the mixture and precipitate was filtered
to yield the product, which was used for the next reaction without
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further purification. Yield: 92%, H NMR (400 MHz, DMSO-d6): δ=
6.43 ppm (s, 4H). MALDI-TOF MS: calculated for C8H4Br2N4 313.88;
found: 314.48 (M+).
QxCN monomer (3): Compound 1 (3.7 g, 5.4 mmol) and 2 (1.7 g,
5.4 mmol) were dried under vacuum and dissolved in acetic acid
under nitrogen condition. The reaction mixture was refluxed
overnight and the solvent was condensed. The crude product was
diluted with dichloromethane and washed with water. The organic
phase was dried and purified by column chromatography (hexane/
toluene=2:8) to provide orange powder. Yield: 61%, 1H NMR
(400 MHz, CDCl3): δ=7.33 (s, 4H), 6.86 (d, J=12 Hz, 2H), 4.03 (m,
4H), 3.88 (m, 4H), 1.84 (m, 4H), 1.75 (m, 4H), 1.28 (m, 40H), 0.89 ppm
(m, 12H). MALDI-TOF MS: calculated for C54H74Br2N4O4 1000.41;
found: 1004.02 (M+).
ChemSusChem 2021, 14, 1–9
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