Dye-Incorporated Polynaphthalenediimide Acceptor for Additive-Free High-Performance All-Polymer Solar Cells

Article abstract of DOI:10.1002/anie.201800035

All-polymer solar cells (all-PSCs) can offer unique advantages for applications in flexible devices, and naphthalene diimide (NDI)-based polymer acceptors are the widely used polymer acceptors. However, their power conversion efficiency (PCE) still lags behind that of state-of-the-art polymer solar cells, due to low light absorption, suboptimal energy levels and the strong aggregation of the NDI-based polymer acceptor. Herein, a rhodanine-based dye molecule was introduced into the NDI-based polymer acceptor by simple random copolymerization and showed an improved light absorption coefficient, an up-shifted lowest unoccupied molecular orbital level and reduced crystallization. Consequently, additive-free all-PSCs demonstrated a high PCE of 8.13 %, which is one of the highest performance characteristics reported for all-PSCs to date. These results indicate that incorporating a dye into the n-type polymer gives insight into the precise design of high-performance polymer acceptors for all-PSCs.

Full text of DOI:10.1002/anie.201800035

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Dye-incorporated Polynaphthalenediimide Acceptor for Additive-
free High-performance All-polymer Solar Cells
Authors: Dong Chen, Jia Yao, Jingping Yin, Ruizhi Lv, Bin Huang, Siqi
Liu, Chunhe Yang, Zhiguo Zhang, Lie Chen, Yiwang Chen,
and Yongfang Li
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201800035
Angew. Chem. 10.1002/ange.201800035
Link to VoR: http://dx.doi.org/10.1002/anie.201800035
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10.1002/anie.201800035
Angewandte Chemie International Edition
COMMUNICATION
Dye-incorporated Polynaphthalenediimide Acceptor for Additive-
free High-performance All-polymer Solar Cells
Dong Chen‡a,b, Jia Yao‡c,d, Lie Chen*a,b, Jingping Yina, Ruizhi Lva, Bin Huanga, Siqi Liua, Zhiguo
Zhang*c, Chunhe Yangd, Yiwang Chen*a,b, Yongfang Lic
Abstract: All-polymer solar cells (all-PSCs) can offer unique
advantages for applications in flexible devices, and naphthalene
diimide (NDI)-based polymer acceptors are the widely used polymer
acceptors. However, their power conversion efficiency (PCE) still
lags behind that of state-of-the-art polymer solar cells, due to low
light absorption, suboptimal energy levels and the strong
aggregation of the NDI-based polymer acceptor. In this work, by
means of simple random copolymerization, a rhodanine-based dye
molecule was introduced into the NDI-based polymer acceptor and
showed an improved light absorption coefficient, an up-shifted
polyN,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-
bis(dicarboximide)-2,6-diyl-alt-5,5′-(2,2′-bithiophene) (N2200),
are the most prominent one due to the advantages of suitable
energy levels, high electron mobility, and good solubility.[14-20]
However, N2200 suffered from a poor absorption coefficient in
films, which limited the photocurrent and efficiency improvement.
On the other hand, the high crystallinity of N2200 resulted in
high charge mobility.[21-23] However, such a close packing of a
polymer acceptor reduces the miscibility with a polymer donor,
resulting in a large phase separation between the donor and
lowest unoccupied molecular orbital level and reduced crystallization. acceptor. Furthermore, the large domain sizes are always
Consequently, additive-free all-PSCs demonstrated a high PCE of
8.13%, which is one of the highest performance characteristics
reported for all-PSCs to date. These results indicate that
incorporating dye into the n-type polymer yields insights into the
precise design of high-performance polymer acceptors for all-PSCs.
associated with unbalanced hole and electron transport.[24-28]
The high crystallinity of N2200 results in a problem that only
limited polymer donors can work well with N2200, which retards
its universal application as an acceptor for all-PSCs.[28-30] In
addition, the low-lying lowest unoccupied molecular orbital
(LUMO) of NDI-based polymers was also an important issue that
limited its open-circuit voltage (VOC), leading to a poor device
performance compared with that of small molecules.[31] To boost
device efficiency, most of the all-PSCs require additive
processing to optimize the morphology of the blend film.[9-13]
Additives, such as 1,8-diiodooctane (DIO) normally have a
deleterious effect on the thermal stability.[32-33]
Polymer solar cells (PSCs) have made considerable
progress during the past two decades.[1-3] The power conversion
efficiency (PCE) of single-junction fullerene-based PSCs has
been attained over 11%.[4] Recently, PCEs exceeding 13% have
been achieved with proper molecular designs of wide-band-gap
polymer donors and fused-ring electron acceptors.[5-7] Regarding
fullerene-based and fused-ring electron acceptor-based devices,
all-polymer solar cells (all-PSCs) composed of a conjugated
polymer donor and an n-type conjugated polymer offer unique
attractions due to their special advantages of morphological
stability and flexibility.[8-16] For practical applications, the
morphological stability of the all-PSCs against the mechanical
and thermal stresses is beneficial for applications in flexible
large-scale and stable devices.
In this work, we employed a small amount of rhodanine
derivative dye groups, 2-(1, 1′-dicyanomethylene)-4-(3-
thienylmethylene) rhodanine (TR), to partly replace the
naphthalene diimide (NDI) blocks in N2200 by random
polymerization to afford two new polymer acceptors (PNDI-2T-
TR(x), x=5 and 10). The molar percentage of the dye TR units to
NDI blocks were 5% and 10% (Scheme 1). It was observed that
incorporation of dye groups into the backbones dramatically
improve the absorption coefficient and LUMO level of the NDI-
based polymers. Simultaneously, simple random polymerization
enabled flexible molecular engineering on the backbones and
tunable crystallinity for the dye-modified copolymers. The
appropriate loading of dye moieties onto the NDI-based n-type
copolymer acceptor afforded a suitable miscibility and phase
separation with the polymer donor PBDB-T. Free of additives,
the PBDB-T:PNDI-2T-TR(5)-based device achieved a high PCE
of 8.13%, compared with that (5.15%) of the PBDB-T:N2200-
based device due to its overall improved short-circuit current
density (JSC), open-circuit voltage (VOC) and fill factor (FF). This
efficiency is one of the highest values reported for all-PSCs.
These results demonstrated that incorporating dye into the n-
type polymer by random polymerization yields insights into the
precise design of high-performance polymer acceptor for all-
PSCs, due to the improved absorption coefficient and miscibility
with donor polymers.
Although the PCE of all-PSCs has reached 10%,[15] the
performance of all-PSCs still lag behind that of state-of-the-art
small molecule-based devices, owing to the lack of suitable n-
type polymer acceptors. To date, the number of polymer
acceptors developed for all-PSCs are limited, and only five
studies have reported all-PSCs with a photovoltaic efficiency
over 8% for a binary system.[12-16] Among the polymer acceptors
reported, naphthalene diimide (NDI)-based acceptors, such as
aCollege of Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang
330031, China
bJiangxi Provincial Key Laboratory of New Energy Chemistry/Institute of
Polymers, Nanchang University, 999 Xuefu Avenue, Nanchang
330031, China
cCAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese
Academy of Sciences (ICCAS), Beijing 100190, China
dDepartment of Chemistry, School of Science, Beijing JiaoTong University,
Beijing 100044, China
The synthetic routes to PNDI-2T-TR(x) are depicted in
Scheme 1 and Scheme S1 in (supporting information SI). The
new dye group, 2-(1,1′-dicyanomethylene)-4-(2,5-dibromo-3-
thienylmethylene) rhodanine (TR), was simply prepared from
Corresponding author. Tel.: +86 791 83968703; Fax: +86 791 83969561;
E-mail: ywchen@ncu.edu.cn (Y. C.); chenlie@ncu.edu.cn (L. C.);
zgzhangwhu@iccas.ac.cn (Z. Z.)
‡ Author contributions. D. Chen and J. Yao contributed equally to this work.
2,5-dibromo-3-thiophenecarboxaldehyde
and
2-(1,1′-
dicyanomethylene) rhodanine with a high yield of 82%. The
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10.1002/anie.201800035
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random copolymers PNDI-2T-TR(x) were successfully obtained
by Stille coupling polymerization of three monomers, namely
2,6-dibromonaphthalene-1,4,5,8-tetracarboxylic-N,N′-bis(2-
octyldodecyl)diimide (NDI), 5,5′-bis(trimethylstannyl)-2, 2′-
bithiophene (2T) and TR. To simultaneously improve the light
absorption and reduce the crystallinity of the polymer but not to
significantly interrupt the molecular packing, we tried two
random copolymers PNDI-2T-TR(5) and PNDI-2T-TR(10) with
different feeding molar ratios of TR:NDI:2T (5:95:100 and
10:90:100, respectively). The resulting copolymers were easily
soluble in common organic solvents such as chlorobenzene, o-
dichlorobenzene and chloroform. Table S1 in SI summarized the
molecular weights of the polymers measured by gel permeation
chromatography (GPC) in chloroform. The number-average
molecular weights ( ) of N2200, PNDI-2T-TR(5) and PND I-2T-
TR(10) are 48.5, 58.9 and 45.6 kDa with corresponding
polydispersity index (PDI) of 2.1, 2.4, and 2.6, respectively. All
the intermediates and final materials were confirmed by element
analysis (EA) and nuclear magnetic resonance (NMR) (Figure
S1-6 in SI). The calculated ratios of the incorporated dye units
are consistent with the feeding ratios of the dye monomer.
for PNDI-2T-TR(5) and 1.50 eV for PNDI-2T-TR(10), which is
slightly larger than that of N2200 (1.44 eV). Figure 1b reveals
that both random copolymers had a complementary absorption
with the PBDB-T donor. Their blends films also displayed
improved absorption at 400-700 nm, where the dye-modified
acceptors absorb. The hyperchromicity of the copolymer
induced by the TR groups and the complementary absorption of
the polymer donor/acceptor blend film both contribute to the light
harvesting enhancement, which consequently increases the JSC
values of the all-PSCs.
Figure 1. (a) Absorption coefficient of the polymer PNDI-2T-TR(x) and N2200
in chloroform solutions; (b) UV-vis absorption spectra of polymer PBDB-T,
N2200, PNDI-2T-TR(x), PBDB-T:N2200 and PBDB-T:PNDI-2T-TR(x) blends.
From the cyclic voltammogram (CV) data shown in Figure
S8 and Table S2, the highest occupied molecular orbital (HOMO)
energy levels of the three polymers were almost the same, but
the lowest unoccupied molecular orbital (LUMO) energy levels of
the dye-modified copolymers up-shifted in comparison with that
of N2200. The up-lying LUMO levels are desirable for achieving
a higher VOC in a PSC device [31]. Figure 2a shows that the
energy levels of the random copolymers matched well with those
of the PBDB-T donor. The gradient energy levels could provide
a sufficient driving force for hole and electron dissociation.
The decomposition temperature (Td) of these random
copolymers at a 5 weight loss was approximately 370 °C in the
thermogravimetric analysis (TGA) (in Figure S9); thus, the
copolymers exhibited sufficient thermal stability for applications
in electronic devices. The differential scanning calorimetry (DSC)
curves was depicted in Figure 2b N2200 display a distinct
melting peak (Tm=320 °C) and a crystallization peak (Tc=290 °C)
during heating and cooling process, respectively, in agreement
with the values reported in the literature.[7] The copolymers with
different ratios of the dye unit only showed one melting peak,
indicating that random polymers had been obtained. In addition,
the incorporation of a small amount of dye groups into the
polymers greatly impacted the melting and crystallization
transitions. The Tm and Tc values decreased gradually as the
loading of dye groups increased (Table S1). In addition, the
melting enthalpy of the samples (ΔHm) were also sharply
reduced in the sequence of PNDI-2T-TR(10)<PNDI-2T-
TR(5)<N2200.[34] The results imply that the inclusion of disorder
by random polymerization could effectively reduce the
crystallinity of the NDI-based polymers, which may favour a
more miscible morphology of the donor and acceptor blend.
Introducing a 5 molar ratio of dye groups into the backbone
still maintained a good crystallinity in PNDI-2T-TR(5) for efficient
charge transport. However, for PNDI-2T-TR(10), which included
too many dye groups resulted in the melting and crystallization
peaks becoming unclear and hard to be distinguished. The
Scheme 1. (a) Chemical structure evolution of the acceptor polymers; (b)
Synthetic procedures for copolymers PNDI-2T-TR(5) and PNDI-2T-TR(10).
Figure 1 shows the UV-vis-NIR absorption spectra of the thin
film and solution of the copolymers PNDI-2T-TR(5), PNDI-2T-
TR(10), N2200, and their corresponding blended films with
PBDB-T donor. In solution and solid state, both PNDI-2T-TR(5)
and PNDI-2T-TR(10) exhibited similar absorption profiles
relative to N2200 with two distinct absorption bands centred at
300~380 nm and 600~700 nm, due to their similar conjugated
skeletons,[34] as shown in Figure 1a and Figure S7 in SI.
Intriguingly, by incorporation of a small amount of dye groups,
both random copolymers displayed much more pronounced
peaks in almost the entire absorption range from 300~900 nm
compared with those of N2200. The absorption coefficient at
short wavelength and long wavelength was gradually enhanced
with increased loadings of the TR units. As can be seen Figure
1a, the absorption coefficients for N2200, PNDI-2T-TR(5) and
PNDI-2T-TR(10) solution were 34.6, 39.8 and 50.7 Lg-1cm-1,
respectively. It was obvious that the maximum absorption
coefficient of PNDI-2T-TR(10) with only a 10% loading of TR
was approximately 45% higher than that of N2200. The films
show same trend in Figure S7. The optical band gap is 1.48 eV
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10.1002/anie.201800035
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disorder caused by the TR unit led to slightly blue-shifted
absorption bands associated with larger band gap of PNDI-2T-
TR(x), as revealed by the UV spectra (Figure 1). The reduced
crystallinity of PNDI-2T-TR(5) and PNDI-2T-TR(10) was verified
by X-ray diffraction (XRD) (Figure S10 in SI)
entire spectral response range. The PNDI-2T-TR(10)-based
device shows the lowest EQE values among the devices. The
calculated JSC values are 14.8 mAcm-2 for the PNDI-2T-TR(5)-
based device, 11.7 mAcm-2 for the N2200-based device and
11.3 mAcm-2 for the PNDI-2T-TR(10)-based device, consistent
with those values obtained from the J–V curves (Table 1). The
charge carrier mobilities in the blends were investigated using
space-charge-limited current (SCLC) measurements in hole-only
and electron-only device configurations (Figure S12 and Table
S5 in SI). The hole mobility (μh) of the blended films of PBDB-T
with N2200, PNDI-2T-TR(5) and PNDI-2T-TR(10) were
measured to be 9.61×10-4, 6.52×10-4 and 8.51×10-4 cm2V-1s-1,
respectively. Their electron mobilies (μe) were measured to be
8.32×10-5, 3.93×10-4 and 1.18×10-4 cm2V-1s-1, respectively. The
PBDB-T:PNDI-2T-TR(5) blend possessed
a
much more
Figure 2. (a) Energy levels of PBDB-T, PNDI-2TR(x) and N2200; (b) DSC
thermograms of neat PNDI-2T-TR(x) and N2200, measured with a scan rate of
10 °C/min.
balanced μh/μe ratio of 1.6 than those of PBDB-T:PN200 (11.5)
and PBDB-T:PNDI-2T-TR(10) blend (7.2), which can maximize
the photocurrent and suppress the bimolecular recombination in
the solar cells. As a result, higher JSC and FF values were
obtained in the PBDB-T:PNDI-2T-TR(5)-based device.
The all-PSCs were fabricated to characterize the
photovoltaic performance of the new polymer acceptors, using
poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate)
Photoluminescence (PL) curves of the blended films are
depicted in Figure 3c. The PL quenching efficiency (ΔPL) of
PBDB-T was 79% for N2200, 92% for PNDI-2T-TR(5) and 72%
for PNDI-2T-TR(10), as summarized in Table S6. The higher
ΔPL values reflected more efficient charge transfer at the D/A
interface and the improved miscibility between PBDB-T and the
random polymer blend. Thermal annealing further optimized the
nanoscale morphology, as verified by the more quenched PL of
the blends. Time-resolved photoluminescence (TRPL)
measurements of these blended films prepared under optimized
device conditions were conducted (Figure 3d). The lifetime ()
was calculated to be 0.76 ns for PBDB-T:N2200; 0.71 ns for
PBDB-T:PNDI-2T-TR(5) and 0.86 ns for PBDB-T:PNDI- T:PNDI-
2T-TR(10). The shortest lifetime of the PBDB-T:PNDI-2T-TR(5)
among these blends implied that the highest amount of
photoinduced charges has been transferred.
(PEDOT:PSS) as the hole transport layer and the perylene
diimide derivative (PDINO)[36] as the electron transport layer with
a conventional device structure of ITO/PEDOT:PSS/PBDB-
T:PNDI-2T-TR(x)/PDINO/Al (Table S3-4. and Figure S11 in SI).
The control device based on PBDB-T:N2200 was also fabricated
for comparison. Figure 3a shows the density-voltage
characteristics of the PSCs with and without thermal annealing
(TA) under an illumination of AM 1.5G simulated solar light of
100 mWcm-2. The control device based on PBDB-T:N2200
showed a PCE of 5.15% with a VOC of 0.80 V, JSC of 11.84
mAcm-2 and a fill factor (FF) of 54% (Table 1). After replacing
5% NDI blocks by TR units, the PCE of the device based on
PBDB-T: PNDI-2T-TR(5) dramatically improved to 7.23% with
overall improved device parameters, including VOC, JSC and FF.
The improved VOC by 0.06 V is derived from the up-shifted
LUMO levels of PNDI-2T-TR(5). The enhanced absorption
coefficient favoured by dye molecules boosted the JSC from
11.84 to 14.34 mAcm-2. In addition, a remarkably improved FF
was also observed for PNDI-2T-TR(5) with respective to N2200,
due to the more homogeneous morphology with improved
miscibility between the donor and acceptor (discussed later).
Generally, the additives were imperative to optimize the
performance of all-PSCs. However, the 1,8-diiodooctane (DIO)
additive exerted a negative impact on the device efficiency.
Delightfully, thermal annealing was found to further improve the
PCE of the PNDI-2T-TR(5)-based device to 8.13% together with
a higher JSC of 14.84 mAcm-2, a VOC of 0.85 V and a higher FF
of 64%. Moreover, the inclusion of more loading of TR units
diminished the device performance, as revealed by the sharply
decreased PCE of the PBDB-T:PNDI-2T-TR(10)-based devices
for both as-cast and thermal annealing films because of
disordered molecular packing and the resulting unfavourable
blend morphology.
Figure 3b shows the external quantum efficiency (EQE)
spectra of the devices with a broad spectral response in the
range from 300 to 850 nm, which agrees with their absorption
spectra. In comparison with the N2200-based device, the PNDI-
2T-TR(5)-based device showed an obvious enhancement in the
Figure 3. (a) J-V characteristics of the PBDB-T:PNDI-2T-TR(x) (2:1 w:w) and
PBDB-T:N2200 (2:1,w:w) solar cells; (b) Corresponding EQE spectra
measured under 100 mAcm-2 AM 1.5G solar illumination, (c) PL spectra
obtained from the films of PBDB-T, as well as blend films of PBDB-T: N2200
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10.1002/anie.201800035
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and PBDB-T:PNDI-2T-TR(x) (2:1 w:w) blends; and (d) Kinetic traces for the
blended films of the PBDB-T: acceptors at 750 nm.
Table 1. Photovoltaic parameters of the optimized PBDB-T:PNDI-2T-TR(x) and PBDB-T:N2200 solar cells
donor:accepter
PBDB-T:N2200
TA (°C)
As-cast
As-cast
130
VOC (V)
0.80
JSC (mA/cm2)
11.84
FF (%)
PCE (%)
5.15[5.04]a
7.23[7.12]
8.13[8.03]
4.89[4.82]
4.46[4.40]
54.13
58.67
64.32
53.53
47.31
0.86
14.34
PBDB-T:PNDI-2T-TR(5)
0.85
14.83
As-cast
130
0.80
11.36
PBDB-T:PNDI-2T-TR(10)
0.80
11.76
aThe values in the square brackets stand for the average PCEs from the 10 devices
The light-intensity dependence of the photovoltaic
performance of the all-PSCs was further investigated to
understand the charge recombination behaviour in the devices.
As shown in Figure S13a, the all-PSC based on PBDB-T:PNDI-
2T-TR(5) has a remarkable higher power law component ()
value than that based on PBDB-T:N2200 (0.965 vs 0.954). It
was inferred that bimolecular recombination was significantly
suppressed in the device with PNDI-2T-TR(5) as the acceptor,
which is also correlated with balanced hole and electron
transport. The VOC curves as a function of P were plotted in
Figure S13b. The slope (S) (1.134) of the PNDI-2T-TR(5) device
was lower than that (1.428) of the N2200-based all-PSC,
implying that the monomolecular recombination of the PBDB-
T:PNDI-2T-TR-(5) blends has been significantly reduced, partly
accounting for the improved device performance.
Figure 4. (a) (b) and (c) AFM topography (5×5 µm2) of the PBDB-T:PNDI-2T-
TR(x) (2:1 w:w) and PBDB-T:N2200 (2:1 w:w) blended films; (d) (e) and (f)
TEM images of PBDB-T:PNDI-2T-TR(x) (2:1 w:w) and PBDB-T:N2200 (2:1
w:w) blended films on the actual optimum devices prepared from CB.
In Figure 4a-c and Figure S14 the atomic force microscopy
(AFM) measurements of all the samples show smooth surfaces
with similar root-mean-square (RMS) roughness of values from
1.27~1.74 nm. Compared to PBDB-T: N2200, PBDB-T: PNDI-
2T-TR(5) has a more homogenous morphology. Especially after
thermal annealing at 130 °C, a nanofibrous surface with a fine
nanophase separation emerged in the PBDB-T: PNDI-2T-TR(5)
blend film, which is very desirable for the efficient charge
transfer and transport. Blending more disordered PNDI-2T-
TR(10) into PBDB-T destroyed the interpenetrating network and
formed a very miscible film. This finding explains why PNDI-2T-
TR(10) had the highest absorption but obtained relatively low JSC
and FF values. Such morphological evolution can be clearly
detected by transmission electron microscopy (TEM)
observation (Figure 4d-f). It inferred that appropriately
decreasing the crystallinity of N2200 could promote the donors
to penetrate the acceptor to form interpenetrating network
structures with more appropriate domain sizes and phase
separation.
PCE of 8.13% was achieved for the additive-free all-PSC based
on PBDB-T: PNDI-2T-TR(5) as the acceptor, which is much
higher than that of a device based on N2200. Substantially
suppressed bimolecular and monomolecular recombination were
observed in PNDI-2T-TR(5)-based device. The results
demonstrate that random polymerization of a small amount of
dye units into the NDI-based polymer is a facile and promising
strategy to develop high-performance polymer acceptors for
stable all-PSCs.
Acknowledgements
L. C. wishes to thank the National Natural Science Foundation of
China (NSFC) (51673092 and 21762029) for financial support. Y.
C. also thanks the National Science Fund for Distinguished
Young Scholars (51425304) for financial support.
In summary, two random polymer acceptors, namely, PNDI-
2T-TR(5) and PNDI-2T-TR(10), were synthesized by random
polymerization of a small amount of dye molecules into an NDI-
Keywords: all-polymer solar cells; n-type conjugated polymer
acceptor; miscibility; organic dyes; random polymerization.
based polymer.
A dramatic increase in the absorbance
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10.1002/anie.201800035
Angewandte Chemie International Edition
COMMUNICATION
Dong Chen, Jia Yao, Lie Chen*,
Jingping Yin, Ruizhi Lv, Bin Huang, Siqi
Liu, Zhiguo Zhang*, Chunhe Yang,
Yiwang Chen*, Yongfang Li
A rhodamine-based dye molecule was imbedded into a naphthalene diimide (NDI)-
based polymer acceptor to simultaneously improve the light absorption coefficient, up-
shift the lowest unoccupied molecular orbital (LUMO) level and reduce crystallization,
by means of simple random copolymerization. All-polymer solar cells based
on PBDB-T with a new dye-modified polymer acceptor achieved a high power
conversion efficiency of 8.13% without additives.
This article is protected by copyright. All rights reserved.