An A-D-A Type Small-Molecule Electron Acceptor with End-Extended Conjugation for High Performance Organic Solar Cells

Article abstract of DOI:10.1021/acs.chemmater.7b02811

A new non-fullerene small molecule with an acceptor-donor-acceptor (A-D-A) structure, FDNCTF, incorporating fluorenedicyclopentathiophene as core and naphthyl-fused indanone as end groups, was designed and synthesized. Compared with the previous molecule FDICTF with the phenyl-fused indanone as the end groups, the extended π-conjugation at the end group has only little impact on its molecular orbital energy levels, and thus, the open-circuit voltage (V_oc) of its solar cell devices has been kept high. However, its light absorption and mobility, together with the short-current density (J_sc) and the fill factor (FF), of its devices have been all improved simultaneously. Through morphology, transient absorption, and theoretical studies, it is believed that these favorable changes are caused by (1) the appropriately enhanced molecular interaction between donor/acceptor which makes the charge separation at the interface more efficient, and (2) enhanced light absorption and more ordered packing at solid state, all due to the extended end-group conjugation of this molecule. With these, the solar cells with FDNCTF as the acceptor and a wide band gap polymer PBDB-T as the donor demonstrated a high power conversion efficiency (PCE) of 11.2% with an enhanced J_sc and a maintained high V_oc, and significantly improved FF of 72.7% compared with that of the devices of FDICTF with the phenyl-fused indanone as the end groups. These results indicate that the unexplored conjugation size of the end group plays a critical role for the performance of their solar cell devices.

Full text of DOI:10.1021/acs.chemmater.7b02811

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Article
An A-D-A type small molecule electron acceptor with end-
extended conjugation for high performance organic solar cells
Huanran Feng, Nailiang Qiu, Xian Wang, Yunchuang Wang, Bin Kan, Xiangjian Wan,
Mingtao Zhang, Andong Xia, Chenxi Li, Feng Liu, Hongtao Zhang, and Yongsheng Chen
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b02811 • Publication Date (Web): 29 Aug 2017
Downloaded from http://pubs.acs.org on August 30, 2017
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An A-D-A type small molecule electron acceptor with end-extended conjugation for
high performance organic solar cells
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⊥
⊥
Huanran Feng†, , Nailiang Qiu†, , Xian Wang‡, Yunchuang Wang†, Bin Kan†, Xiangjian
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Wan† Mingtao Zhang†, Andong Xia‡, Chenxi Li†, Feng Liu§,*, Hongtao Zhang†,*,
,
Yongsheng Chen†,*
†State Key Laboratory of ElementoꢀOrganic Chemistry, Centre of Nanoscale Science and
Technology and Key Laboratory of Functional Polymer Materials, College of Chemistry,
Nankai University, Tianjin, 300071, China.
‡Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of
Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190,
China
§Department of Physics and Astronomy, Shanghai Jiaotong University, Shanghai 200240,
China
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ABSTRACT:
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A new nonꢀfullerene small molecule with acceptorꢀdonorꢀacceptor (AꢀDꢀA)
structure, FDNCTF, incorporating fluorenedicyclopentathiophene as core and naphthylꢀ
fused indanone as endꢀgroups, was designed and synthesized. Compared with previous
molecule FDICTF with the phenylꢀfused indanone as the endꢀgroups, the extended πꢀ
conjugation at the endꢀgroup has only little impact on its molecular orbital energy levels,
and thus the open circuit voltage (Voc) of its solar cell devices has been kept high. But its
light absorption, mobility, together with the short current density (Jsc) and the fill factor
(FF) of its devices have been all improved simultaneously. Through morphology,
transient absorption and theoretical studies, it is believed that these favorable changes are
caused by 1) the appropriately enhanced molecular interaction between donor/acceptor
which makes the charge separation at the interface more efficient, 2) enhanced light
absorption and more ordered packing at solid state, all due to the extended end group
conjugation of this molecule. With these, the solar cells with FDNCTF as the acceptor
and a wide bandgap polymer PBDBꢀT as the donor demonstrated a high power
conversion efficiency (PCE) of 11.2% with an enhanced Jsc and a maintained high Voc,
significantly improved FF of 72.7% compared with that of the devices of FDICTF with
the phenylꢀfused indanone as the endꢀgroups. These results indicate that the unexplored
conjugation size of the end group plays a critical role for the performance of their solar
cell devices.
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Introduction
Bulk heteroꢀjunction (BHJ) organic solar cells (OSCs) have attracted extensive
attention due to their potentials in developing low cost, flexible, largeꢀscale and
semitransparent solar panels.1ꢀ5 Recently, benefiting from the rapid development of nonꢀ
fullerene (NF) smallꢀmolecule acceptors due to their better absorption and tunable band
structures, the overall performance of OSCs has improved significantly to 13%.6 Based
on the equation of PCE= Jsc×Voc×FF/Pin, it is best to improve open circuit voltage (Voc),
short circuit current (Jsc) and fill factor (FF) simultaneously when further higher power
conversion efficiency (PCE) is desired. Different with their inorganic counterparts, the
charge separation of exciton in organic solar cells is mainly limited by its shorter
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effective diffusion length.7ꢀ9 Both theoretical and experimental results have indicated that
the charge separation efficiency are expected to be potentially improved by appropriately
increasing the donor (D)/acceptor (A) intermolecular interaction.10ꢀ14 On the other hand,
Voc, Jsc and FF are often tangled together as both molecular properties (i.e. conjugation
length, band structure, etc.) and film morphology (the domain size and interpenetrating
network) in many cases may have opposite impact on these factors. For example, while it
is intrinsic to lower lowest unoccupied molecular orbital (LUMO) level of acceptor for
higher Jsc, the corresponding Voc could decrease accordingly.15,16 Also, while the
increased conjugation size of molecules may improve optical absorption, over extended
size of the backbone and high degree of coplanarity tend to make the molecules over
aggregated, thus leading to poor morphology with a large phase separation often for poor
FF.17 Therefore, dedicated balance on molecule design is highly required to achieve not
only high optical absorption and suitable energy level but also optimized film
morphology.
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In last few years, to maximize PCE, much work has been focused on the design of
various small molecules containing different electron deficient groups, such as perylene
diimide (PDI), naphthalene diimide (NDI), diketopyrrolopyrrole (DPP),
tetraazabenzodifluoroanthene diimide, rhodanine, 2ꢀ(3ꢀoxoꢀ2,3ꢀdihydroꢀ1Hꢀindenꢀ1ꢀ
ylidene)malononitrile (INCN) as NF acceptors.18ꢀ22 Interestingly, it should be noted that
currently, the most successful NF molecules all have the same acceptorꢀdonorꢀacceptor
(AꢀDꢀA) molecular architecture which has been proved to be very successful for many
donor molecules in last ten years.6,22,23 Some of these high performance AꢀDꢀA type NF
molecules include ITIC (3,9ꢀbis(2ꢀmethyleneꢀ(3ꢀ(1,1ꢀdicyanomethylene)ꢀindanone)ꢀ
5,5,11,11ꢀtetrakis(4ꢀhexylphenyl)ꢀdithieno[2,3ꢀd:2 ′ ,3 ′ ꢀd ′ ]ꢀsꢀindaceno[1,2ꢀb:5,6ꢀ
b ′ ]ꢀdithiophene),23 INIC (3,9ꢀbis(2ꢀmethyleneꢀ(3ꢀ(1,1ꢀdicyanomethylene)ꢀindanone)ꢀ
6,6,12,12ꢀtetrakis(4ꢀhexylphenyl)ꢀ indacenobis(dithieno[3,2ꢀb;2 ′ ,3 ′ ꢀd]thiophene),24
IDTBR (5Z,5′Z)ꢀ5,5′ꢀ{[5,5,11,11ꢀTETRAKIS(nꢀoctyl)ꢀdithieno[2,3ꢀd:2′,3′ꢀd′]ꢀ
sꢀindaceno[1,2ꢀb:5,6ꢀb’]dithiophene=2,8ꢀdiyl]bis[2,1,3ꢀbenzothiadiazoleꢀ7,4ꢀ
diyl(Z)methylꢀlidene]}bis(3ꢀethylꢀ2ꢀthioxoꢀ1,3ꢀthiazolidinꢀ4ꢀone))22 and PCEs over 13%
based on these AꢀDꢀA small molecule acceptors have been achieved.6 The reasons behind
the much success of AꢀDꢀA architecture might be very complicated and are yet to be
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fully understood. But some arguments we have are: 1) the spatial distribution of the
electron density of frontier molecular orbitals (HOMO and LUMO) of this type of
molecules might favor the important charge separation at the interface of the donor and
acceptor; 2) the easily tuned absorptions and energy levels of these AꢀDꢀA molecules; 3)
the optimal (not too high not too low) aggregation of the molecules due to its structure at
solid state. It should be noted that most of these successful NF acceptors comprise of
aromatic fused rings with extended conjugation as the donor core and an electronꢀ
deficient conjugation unit as the endꢀgroups, which would prompt intermolecular πꢀπ
overlaps and enhance intermolecular charge transport. The large central πꢀconjugated
unit would result in better absorption, thus higher Jsc could be achieved.24ꢀ27 But,
compared with many studies on the central core unit for these AꢀDꢀA small molecule
acceptors, 24,28ꢀ30 no larger conjugation end unit (almost all are based on INCN end unit)
has been studied to understand the influence of extended endꢀgroups on the properties of
these molecules.24,31 This is contrary to the intensive studies carried out for the small
molecule donors, where tuning the end groups would improve the material performance
significantly.32ꢀ34 On the other side, extending the πꢀconjugated length of the endꢀgroups
may provide a new solution for the tradeoff of Voc and Jsc by achieving a redꢀshifted
absorption while minimizing the adverse impact on the LUMO level and thus the Voc,
which has been used much for the design of many small molecule AꢀDꢀA type
donors.32,33 In addition, the introduction of large πꢀconjugated moiety onto the end of
small molecules would benefit the intermolecular endꢀtoꢀend πꢀπ interaction and
molecular packing, leading to enhanced charge separation and transport and thus
improving FF.34 With these, we believe that not only it is urgent and important to design
some new NF small molecule acceptors with extended end conjugation units and study
the impact of the end groups overall, but also such design might provide molecules with
better performance..
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Very recently, we reported an AꢀDꢀA type small molecule acceptor 2,9ꢀ
bis(2methylene(3(1,1dicyanomethylene)indanone))7,12ꢀdihydroꢀ4,4,7,7,12,12ꢀhexaoctylꢀ
4Hꢀcyclopenta[2″,1″:5,6;3″,4″:5′,6′] diindeno[1,2ꢀb:1′,2′ꢀb′] dithiophene (named as
FDICTF, Figure 1a) and a PCE of 10.06% with a high Voc of 0.94 V for its OSC
devices.35 While we thought the Voc was high, it seems that there is large potential for the
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improvement of the Jsc and FF. From the discussion above, we thought that extending the
end group conjugation might be able to manage the tradeoff of Jsc and Voc and achieve the
goal of improving the Jsc while maintaining its high Voc. This is based on the assumption
that different from the approaches of introducing strong electronegative group on the
end,24,31 the extended end conjugation length/size might not only minimize the impact on
the LUMO thus Voc, but also enhance the light absorption and molecular packing and thus
make the Jsc and FF improved simultaneously due to both the appropriately increased D/A
interfacial interaction for better exciton separation and more ordered packing.34 On the
basis of these thoughts, we extended the benzene unit of the previously widely used end
group INCN, and replace the benzene unit with a naphthalene unit and synthesized a new
active end group moiety, NINCN (2ꢀ3ꢀoxoꢀ2,3ꢀdihydroꢀ1Hꢀcyclopenta[b]naphthalenꢀ1ꢀ
ylidene)malononitrile. Using NINCN as the endꢀgroup, a new small molecule FDNCTF,
(2,9ꢀbis(2ꢀmethyleneꢀ(3ꢀ(1,1ꢀdicyanomethylene)ꢀbenz[f]indanone))ꢀ7,12ꢀdihydroꢀ4,4,7,7,
12,12ꢀhexaoctylꢀ4Hꢀcyclopenta[2'',1'':5,6;3'',4'':5',6']diindeno[1,2ꢀb:1',2'ꢀb']dithiophene
(Figure 1a), was synthesized. Indeed, the LUMO energy level was little changed
compared with the previous molecule FDICTF when INCN was at the end. Importantly,
as designed, at solid state a much redꢀshifted optical absorption than that of the previous
INIC based molecule FDICTF was achieved. This is due to the better molecular ordering
than that of FDICTF at solid arising from the stronger intermolecular interactions
between the end groups. In the OSC devices with the same donor material (PBDBꢀT),
due to the appropriately increased D/A interfacial interaction, the charge separation
efficiency improved significantly as evidenced by the transient absorption (TA) spectra
and other analysis. Thus, the OSCs based on PBDBꢀT:FDNCTF demonstrated a
significantly improved PCE of 11.2% with maintained Voc and improved Jsc and FF,
compared with the corresponding OSCs with FDICTF as the acceptor. These results
indicate using extended conjugation endꢀgroups might provide another important strategy
to design higher performance NF acceptors.
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Figure 1. Chemical structures of (a) FDICTF and FDNCTF and b) PBDBꢀT donor. (c)
Estimated energy levels of FDICTF, FDNCTF, and PBDBꢀT from electrochemical cyclic
voltammetry. (d) UVꢀvis absorption spectra of FDICTF and FDNCTF in chloroform
solution. (e) UVꢀvis absorption spectra of PBDBꢀT, FDICTF and FDNCTF in neat film.
Results and Discussion
Scheme 1. Synthetic Scheme of FDNCTF.
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Synthesis and Thermal Properties. The synthetic route of FDNCTF is shown in
Scheme 1, and the detailed procedures of them are provided in the Supporting
Information (SI). The central unit and its dialdehyde compound FDTCHO have been
prepared as before.36 The Compound 2 (NINCN) was obtained by Knoevenagel
condensation of Compound 1 with malononitile. The targeted compound FDNCTF was
prepared by the Knoevenagel condensation of NINCN and the dialdehyde compound
FDTCHO35 in a 62% yield. The solubility of FDNCTF is quite good in common organic
solvents, e.g., chloroform, chlorobenzene and orthoꢀdichlorobenzene. Thermogravimetric
analysis (TGA, Figure S3a) indicates its high thermally stability with the decomposition
temperature (Td) of 383 oC with 5% weight loss.
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Figure 2. (a) Theoretical density distribution for the frontier molecular orbitals of
FDNCTF and FDICTF. (b) The isosurface of charge density difference of LUMO
(HOMO) between FDNCTF and FDICTF.
DFT calculation. While the density functional theory (DFT) at the B3LYP/6ꢀ31G(d,p)
level is probably over simplified, as it is widely adopted in literatures and consumes less
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computing power, it was used to investigate the possible influence of the extended endꢀ
groups on the geometry properties of AꢀDꢀA small molecules. If we recall the major steps
of photovoltaic process, including the generation of exciton by photon absorption,
exciton migration/diffusion to the donor:acceptor interface, the charge separation of the
exciton at the interface, and the free charge transportation to the electrodes, clearly, the
interfacial interaction between the donor and acceptor plays a critical role for the efficient
charge separation of exciton generated by light absorption. Thus, such interaction, mainly
depending on the LUMO orbitals of the donor and acceptor after photoꢀcaused excitation
and generation of exciton (the donor’s electron is excited to its LUMO from its HOMO),
proper interaction between such two LUMO orbitals could favor an efficient charge
separation to generate free charges of electron and hole. Also, such interaction should be
not too weak or too strong. Generally, the factors affecting the molecular orbital
interaction include the orbital’s symmetry, energy difference, overlap and spatial distance.
For our case, with the new end group having a larger and planar conjugation system and
considering the general over bulky and rigid core structure of these NF molecules, it is
expected such an expanded end group structure would enhance the frontier orbital
interaction between the donorꢀacceptor at the interface, thus a more favorable charge
separation process would be achieved to improve the overall photovoltaic performance.
With these, we have analyzed the calculated HOMO electron density and LUMO spatial
distribution of FDNCTF and FDICTF to understand the effect of the larger endꢀgroup on
the HOMO and LUMO. While the quantitative results may not be accurate, the calculated
HOMO electron density and LUMO spatial distribution of FDNCTF and FDICTF in
Figure 2a indicate that the new molecule FDNCTF has more LUMO spatial distribution
extended to the end. The isosurface of charge density difference of LUMO/HOMO
between FDNCTF and FDICTF in Figure 2b offers a clear comparison between these two
molecules. As can be seen, the difference between the HOMOs of these two molecules is
much smaller than that of their LUMOs. These results indicate that the extended end
group significantly change the LUMO spatial distribution, and makes the spatial
distribution in LUMO move down to the end of the molecule. Such a LUMO spatial
density in the new molecule would favor an increased D/A interfacial interaction and if
appropriately, thus a more efficient charge separation can be expected.10,14 Regarding to
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the little change of HOMO distribution, it is intuitive to understand since the HOMO is
mainly decided by the central electronꢀrich group which remains the same for these two
molecules.30,34
The exciton binding energy in OSC is decided by Eq = e2/(4πε0εrr0), where e is the
elementary charge, ε0 is the permittivity of vacuum, εr is the dielectric constant of the
material, and r0 is the electron and hole separation distance.38,39 As the r0 for FDNCTF is
naturally larger than that of FDICTF, the Eq of FDNCTF should be smaller since the
dielectric constant of FDICTF and FDNCTF are about the same (~2.6) from the
measurement (Figure S6). This should also make the exciton of FDNCTF exhibits better
charge separation than that of FDICTF, favoring a higher FF and Jsc.
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Meanwhile, in order to further investigate the molecular interaction between
donor and acceptor in the active layer, the binding energies (ꢀG) of both systems with the
wellꢀknown donor PBDBꢀT were calculated (Figure S2).40ꢀ42 The binding energies of
PBDBꢀT:FDNCTF is ꢀ374.0 kcal/mol, more than that (ꢀ345.7 kcal/mol) of PBDBꢀ
T:FDICTF (see the SI for details). This indicates an appropriately stronger interaction
between PBDBꢀT with FDNCTF, thus a more optimized interaction between the donor
PBDBꢀT and acceptor FDNCTF and higher charge separation efficiency would be
expected.
Absorption Spectra and Electrochemical Properties. As shown in Figure 1d, in
chloroform solution, FDNCTF exhibites strong and broad absorption in the region of
550ꢀ760 nm with an maximum extinction coefficient of 3.2 × 105 Mꢀ1 cmꢀ1 at 697 nm,
which is higher than that of FDICTF (2.7× 105 Mꢀ1 cmꢀ1 at 665 nm). In thin film (Figure
1e), both FDNCTF and FDICTF exhibit red shifts and broader absorption than their
corresponding solution spectra with the maximum absorption peak of at 714 nm of
FDNCTF and 689 nm for FDICTF. The redꢀshifted absorption of FDNCTF is consistent
with the increased effective πꢀconjugation.43 The improved light absorption of FDNCTF
should contribute more current than FDICTF. The electronic energy levels of FDNCTF
was measured by electrochemical cyclic voltammetry (CV) in dichloromethane (Figure
S4). The experimental LUMO energy level of FDNCTF was estimated from the CV to be
ꢀ3.73 eV, 0.02 eV lower than that of FDICTF. And the HOMO energy level (ꢀ5.43 eV) of
FDNCTF is slightly upshifted compared with that (ꢀ5.41 eV) of FDICTF, consistent with
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the theoretical calculated HOMO/LUMO results of FDICTF (ꢀ5.56/ꢀ3.30eV) and
FDNCTF (ꢀ5.52/ꢀ3.32eV). As expected, FDNCTF as an acceptor exhibits only slightly
lower LUMO energy level thus minimizing the impact on Voc of the corresponding OSC
devices. The above absorption and energy level data are summarized in Table 1.
Table 1. Optical and electrochemical data of FDNCTF and FDICTF.
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opt
cv
sol
film
film
Eg
HOMO LUMO
Eg
λmax
εmax
(Mꢀ1 cmꢀ1)
λmax
λedge
Comp.
(eV)c
(eV)d
(eV)d
(eV)e
(nm)
(nm)a
(nm)b
FDICTF
665
697
2.7×105
3.2×105
689
714
760
774
1.63
1.60
ꢀ5.43
ꢀ3.71
ꢀ3.73
1.72
1.69
FDNCTF
ꢀ5.42
aMolecular films on quartz plate cast from chloroform solution. Absorption edge of the
b
opt
c
molecular films: Eg =1240/λedge. Optical bandgap was obtained from the onset
d
e
wavelength of the film. Energy levels evaluated by CV. Electrochemical bandgap
obtained from ELUMO − EHOMO
.
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Figure 3. (a) Current density−voltage (J−V) curves for the devices based on FDICTF and
FDNCTF at optimized conditions under the illumination of AM 1.5G (100 mW cm−2);
the inset shows the histogram of the PCE counts for 50 devices. (b) EQE Curves for for
the FDICTF and FDNCTFꢀbased OSC devices. (c) Jph versus Veff of the optimized
devices. (d) Light intensity (P) dependence of Jsc.
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Photovoltaic Properties. With better absorption and likely better charge separation, it is
expected that FDNCTF should have a better photovoltaic performance when the same
donor polymer material PBDBꢀT (Figure 1b) is used. Thus, the OPV devices of this new
electron acceptor were fabricated and evaluated with a conventional device configuration
of ITO/PEDOT:PSS/PBDBꢀT:FDNCTF/PDINO/Al, where PDINO is an efficient
cathode interlayer developed by Li et al.44 The detailed device optimization processes are
described in the Table S2ꢀ5, including different donor/acceptor weight ratios, different
processing solvents and solvent additives. The FDNCTF optimized performance were
obtained by utilizing chlorobenzene as the processing solvent with donor/acceptor weight
ratio 1:0.8 and the activeꢀlayer thickness of ~120 nm.
Table 2. The optimized photovoltaic parameters of the devices based on PBDBꢀ
T:FDICTF and PBDBꢀT:FDNCTF under the illumination of AM 1.5G (100 mW cmꢀ2).
Acceptor
FDICTFa
FDNCTF
Voc (V)
Jsc (mA cmꢀ2)
FF%
PCEmax
%
PCEb%
9.7±0.2
10.9±0.4
0.94± 0.01
0.93± 0.01
15.8 ± 0.2
66.0 ± 1.0
72.5 ± 0.8
10.0
16.3 ± 0.2
11.2
arefer to ref. 35. bThe average PCE value was calculated from 50 devices.
The optimized currentꢀdensityꢀvoltage (JꢀV) curve of the device based on PBDBꢀ
T:FDNCTF are shown in Figure 3a, and the corresponding photovoltaic parameters are
listed in Table 2. Meanwhile, the optimized JꢀV and photovoltaic parameters of the
PBDBꢀT:FDICTF device are also provided for comparison.35 As shown in Table 2, the
optimized OSC devices based on PBDBꢀT:FDNCTF exhibited a high PCE of 11.2% with
a Voc of 0.93 V, a Jsc of 16.5 mA cmꢀ2, and an FF of 72.7%. Compared with FDICTFꢀ
based device, the FDNCTFꢀbased device exhibited almost the same Voc but enhanced Jsc,
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consistent with the little changed LUMO level and redꢀshifted and stronger absorption of
FDNCTF. Note the FF was improved significantly too. Meanwhile, the FDNCTFꢀbased
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device exhibited decreased energy loss (Eloss = Eg ꢀ eVoc) of 0.67 eV comparing with
that (0.69 eV) of the FDICTFꢀbased device. The external quantum efficiency (EQE)
curves of FDNCTFꢀbased device (Figure 3b) shows stronger and wider photoresponse
than that of the FDICTFꢀbased device, which could be attributed to the stronger and redꢀ
shifted absorption of FDNCTF.
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To further understand the reasons behind of the improved FF of the FDNCTFꢀ
based devices, the exciton dissociation, charge collection and charge recombination were
investigated by measuring the photocurrent density (Jph) versus effective voltage (Veff)
and the dependence of the Jsc on light intensity.45 As can be seen from Figure 3c, Jph
values for both devices are saturated (Jsat) at ~2V, suggesting that charge recombination
in both devices are minimized at higher voltage due to the high internal electric field. The
Jph/Jsat ratio is up to 93% for the FDNCTFꢀbased devices under shortꢀcircuit conditions,
compared with 91% for the FDICTFꢀbased devices. Furthermore, these ratios are 80%
and 74% for FDNCTFꢀ and FDICTFꢀbased devices, respectively, at the maximal power
output conditions. These results indicate that FDNCTFꢀbased device possesses more
efficient exciton dissociation and charge collection relative to those of the FDICTFꢀbased
devices, reflecting our initial design. The lightꢀintensity (P) dependence of Jsc was also
measured to study the charge recombination kinetics.46 As shown in Figure 3d, the
powerꢀlaw exponents of the equation Jsc ∝ Pα for the FDNCTFꢀand FDICTFꢀbased
devices are 0.99 and 0.96, respectively, indicating less bimolecular recombination
occurred in the FDNCTFꢀbased device, supporting its improved FF and Jsc.45,46 In
addition, the enhanced fluorescence quenching efficiency (96%) of FDNCTF over that of
FDICTF (94%) obtained by the photoluminescence (PL) spectra (Figure S7) indicated a
higher exciton dissociation in the PBDBꢀT:FDNCTF blend. With all these together, thus
a better photovoltaic performance would be expected of FDNCTFꢀbased OSCs than that
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FDICTF.
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Figure 4. (a)The inꢀplane and outꢀofꢀplane line cuts of the GIXD patterns for the
FDNCTF and FDICTF; 2DꢀGIXD patterns for pure films of (b) FDNCTF and (c)
FDICTF; AFM height images of (d) FDNCTF and (e) FDICTF films.
Morphology Characterization. To understand why the endꢀgroup modulation causes
obvious difference in the photovoltaic performance. The molecular ordering in acceptor
thin films was studied using twoꢀdimensional grazingꢀincidence Xꢀray diffraction (2Dꢀ
GIXD). As shown in Figure 4a,c, FDICTF shows a quite low intensity diffraction peak at
around 0.38 Åꢀ1 and a strong diffraction halo at around 1.3 Åꢀ1, thus it is highly
amorphous in neat film.33 When larger size of endꢀgroups is introduced, the packing of
FDNCTF makes fundamental change. Quite strong (100) lamellar diffraction peak is seen
at 0.32Åꢀ1 with an interlamellar distance of 19.5 Å in inꢀplane direction; a sharp πꢀπ
stacking (010) diffraction peak is seen at 1.82 Åꢀ1 in outꢀofꢀplane direction with a dꢀ
spacing of 3.4 Å (Figure 4c). Thus, the minor structural change if proper could lead to a
completely new molecular ordering, turning an amorphous solid into highly ordered thin
film. The NINCN endꢀgroup is surely successful in inducing structure order, which can
be extended to other material systems. Highly ordered thin film leads to a rough surface.
As shown in Figure 4d,e, a larger surface roughness root mean square (RMS) of
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FDNCTF (0.90 nm) is seen over that of FDICTF (0.33 nm). In properties, the electron
mobility of FDNCTF (2.83×10ꢀ4 cm2 Vꢀ1 sꢀ1) is an order of magnitude larger than
FDICTF (3.79×10ꢀ5 cm2 Vꢀ1 sꢀ1) in single carrier spaceꢀchargeꢀlimited current (SCLC)
measurement, correlating well with structure details in thin film.
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Figure 5. (a) The outꢀofꢀplane (solid line) and inꢀplane (dotted line) lineꢀcut profiles of
2DꢀGIXD patterns. The 2DꢀGIXD patterns of PBDBꢀT:FDNCTF blend films (b) without
DIO and (c) with DIO; PBDBꢀT:FDICTF blend films (d) without DIO and (e) with DIO.
TEM images of (f) PBDBꢀT:FDNCTF with DIO and (h) PBDBꢀT:FDICTF blend films
with DIO.
The solidꢀstate packing of molecules in BHJ blends were characterized by 2Dꢀ
GIXD (Figure 5aꢀe), transmission electron microscopy (TEM, Figure 5f) and resonant
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soft Xꢀray scattering (RSoXS, Figure S10). As shown in Figure 5a, b, the GIXD
diffraction of PBDBꢀT:FDICTF blends without DIO was dominated by the polymer
diffraction signals with a (100) peak at 0.29 Åꢀ1 and a πꢀπ stacking (010) peak at 1.69 Åꢀ1
and there is no clear peak from the acceptor FDICTF. But PBDBꢀT:FDNCTF blends
processed without DIO showed some differences by comparing Figure 5a, b with Figure
5 a, d: 1) there is clearly a new peak at 0.32 Åꢀ1 from the new acceptor FDNCTF, close to
the original polymer (100) peak; 2) the original (010) peak at 1.69 Åꢀ1 has been
broadened due to the new acceptor FDNCTF. Thus, the introduced extended endꢀgroups
could significantly improve the morphology of BHJ blends, with results corresponding
well with the device characteristics, owing to the better crystallinity nature of FDNCTF
that breaking away from the intimate mixtures with polymer donor.
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When tiny amount of solvent additive was used, the low boiling nature of additive
molecules could effectively plasticize the blended thin films, making the glass transition
much lower than conventional mixtures, and the quicker kinetics of small molecules in
selfꢀassembling made pronounced molecular crystals in both cases, which directed better
morphologies and thus device performances. As seen from PBDBꢀT:FDNCTF blends
processed with DIO (Figure 5a,c), FDNCTF showed obvious crystallization peaks, which
by fitting, gave a CCL of 20.0 nm (polymer showed a CCL of 22.5 nm.) from inꢀplane
diffraction profiles. While PBDBꢀT:FDICTF processed with DIO exhibited rather similar
diffraction pattern as shown in Figure 5a and e with that (Figure 5a and c) of PBDBꢀ
T:FDNCTF with DIO, quantitatively it gave a CCL of 16.2 nm for FDICTF (polymer
showed a CCL of 24.5 nm.). The original fitting results is provided in the Figure S9 with
the fitting details and data accuracy. Thus, better crystallization of FDNCTF was
observed in its BHJ blends, which should help to improve device performances as shown
in previous discussions. Yet the global morphology is more complicated than just the
crystalline network, the phase separation and material aggregation also played an
important role. As seen from TEM characterizations (Figure 5f), PBDBꢀT:FDNCTF
blends processed with additive showed fibril network structure as well as dark area
material agglomerations; but PBDBꢀT:FDICTF thin film was smoother under the same
electron microscopy resolution, and though a larger inꢀplane (100) polymer CCL was
seen, the fibril network structure was less obvious (Figure 5h). These features were due to
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the better mixing of PBDBꢀT and FDICTF, where as in PBDBꢀT:FDNCTF blends, the
FDNCTF better crystallization led to stronger FDNCTF aggregated domains that defines
a multiꢀlength scaled morphology (fibril network in small length scale and material
aggregation in large length scale). Such a structure was quite well resolved with further
help of RSoXS where length scale of phase separation at around 160 nm was seen for
PBDBꢀT:FDNCTF and the scattering intensity was much stronger comparing to PBDBꢀ
T:FDICTF blends (Figure S10). These morphological features correspond well with their
SCLC mobility characterizations, which show that the higher hole and electron mobilities
of 1.77 ×10ꢀ4 cm2 Vꢀ1 sꢀ1 and 1.00 ×10ꢀ4 cm2 Vꢀ1 sꢀ1 of PBDBꢀT:FDNCTF than those of
PBDBꢀT:FDICTF ( 3.37 ×10ꢀ5 cm2 Vꢀ1 sꢀ1, 2.40 ×10ꢀ5 cm2 Vꢀ1 sꢀ1) measured by the SCLC
method (Figure S5) and is consistent with the microstructure analysis and accounted for
the improved FF in devices.
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Figure 6. (a, b) Differential absorption spectra as a function of probe wavelength with
pumping at 500 nm for neat PBDBꢀT film (black dashed line), at 720 nm for neat
FDICTF/FDNCTF (olive solid line) and blends of PBDBꢀT and FDICTF/FDNCTF films
(red solid line) at 5 ps time delay. For better comparison, all spectra shown are
normalized to their GSB peaks. (c, d) Target analysis of the normalized transient
absorption spectra of PBDBꢀT:FDICTF (c) and PBDBꢀT:FDNCTF (d) blend films at 720
nm excitation, which was obtained using the target model shown in the inset. (e, f)
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Concentrations of the individual species as a function of time.
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Charge separation dynamics. To understand the mechansim for higher performance
(meaning higher Jsc and FF) of this endꢀextended new acceptor, a series of studies were
carried out for the decay dynamics of the exciton in the devices. As it is well known that
different from fullerene acceptors, for NF electron acceptors, the current contribution due
to the acceptor absorption is also playing an important role.47,48 That is, there are two
channels for the exciton generation in the NF based OSCs, including the conventional
channel, exciton generated by the donor absoprtion of photons and another one by the
acceptor absorption, and both types of exciton migrate to the D/A interface to achieve
charge separation. Thus, both electron transfer (ET) from donor to acceptor and hole
transfer (HT) from acceptor to donor are important for the overall charge separation. To
investigate both the electron and hole generation processes at the donorꢀacceptor
interfaces, broadꢀband femtosecond transient absorption (TA) measurements were
performed at different pumping wavelengths for selectively exciting PBDBꢀT donor (at
500 nm) or acceptors (FDICTF and FDNCTF) (at 720 nm).47,49 The TA spectra of
PBDBꢀT:FDICTF and PBDBꢀT:FDNCTF blend films using 500 nm pump to just excite
donor PBDBꢀT for investigating the ET process was shown in Figure S11. Target
analysis results (speciesꢀassociated difference spectra, SADS and concentrations) were
exhibited in Figure S11 cꢀf. The normalized SADS shows a new feature for the chargeꢀ
separated (CS) state at 690 nm in PBDBꢀT:FDICTF blend (Figure S11c) and 720 nm in
PBDBꢀT:FDNCTF film (Figure S11d), respectively. The new feature was ascribed to the
groundꢀstate bleaching (GSB) of FDICTF or FDNCTF, indicating that efficient electron
transfer (ET) from donor to acceptor occurs in both two blend films. Corresponding fitted
exponential decay time constants and amplitudes by the target model were all listed in
Table S1 and PBDBꢀT:FDNCTF blend film shows slightly faster ET process, compared
with that for PBDBꢀT:FDICTF.
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We further investigated and compared the exciton dissociation and hole transfer
(HT) processes in the two blend films in detail, since the difference of the acceptors
should make the major difference. Figure 6aꢀb shows the differential absorption spectra
for five films: neat PBDBꢀT, neat FDICTF, neat FDNCTF, blends of PBDBꢀT:FDICTF
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and PBDBꢀT:FDNCTF. In order to simplify the discussion, using FDNCTF as an
example to demonstrate exciton dissociation and charge (hole) transfer from these TA
spectra. The negative features (Figure 6b) resulted from the GSB as excitons are created
in neat PBDBꢀT (with maxima at 630 and 580 nm) and neat FDNCTF (with maxima at
715 and 640 nm) films. The positive broad signals are the excitedꢀstate absorption (ESA)
of promptly photoꢀinduced exciton (EX) or chargeꢀtransfer (CT) in these neat films. The
normalized TA spectrum for blended PBDBꢀT:FDNCTF film at 5 ps with pumping at 720
nm shows, in addition to the GSB of neat FDNCTF, a new feature at 635 nm with
intensity much higher than that of neat FDNCTF at the same time delay. This evident
feature at 635 nm (though much overlap with the GSB of FDNCTF) is clearly ascribed to
the groundꢀstate bleaching of PBDBꢀT because of the similar band shape of ground state
absorption, suggesting hole transfer from FDNCTF to the donor. To extract the exciton
decay dynamics from overlapping TA spectra, target analysis was employed for global
fitting to obtain SADS (Figure 6d) and concentrations (Figure 6f). Chargeꢀseparated (CS)
state (generated via intermolecular exciton dissociation or charge transfer, see the inset in
Figure 6d) was observed in PBDBꢀT:FDNCTF blended films with weakening Columbic
attraction at interfaces. A new bleach band peaking at 580 nm in the spectral profile of CS
state relative to CT/EX occurs, which matches the GSB character of the donor,
suggesting efficient separation of holeꢀelectron pairs. Relative redshift bleach band peak
(722 nm) of CS to that (715 nm) of CT in Figure 6d is attributed to the Stark effect
induced exciton band shift,50 further verifying the generation of free carriers (holes). At
the same pumping wavelength, HT process is also seen in PBDBꢀT:FDICTF blend film
(Figure 6c) as the formation of CS state with clear GSB character of PBDBꢀT (maxima at
630 nm and 580 nm). As the fitting results shown in Table S1, the hole transfer from
FDNCTF to PBDBꢀT is much faster with a time constant of 4.8 ps than that from
FDICTF with a time constant of 9.5 ps, implying a more efficient hole transfer in PBDBꢀ
T:FDNCTF than that in PBDBꢀT:FDICTF. Comparing Figure 6e and 6f, there is higher
concentration of CS state for PBDBꢀT:FDNCTF at 1 ns. In addition, the yield of CS
(hole) in PBDBꢀT:FDNCTF blend is estimated about ~54%, much higher than that
(~43%) of PBDBꢀT:FDICTF, confirming the fact that better power conversion efficiency
obtained in PBDBꢀT:FDNCTF blend. With these results, it is clear that both the current
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generation channels are favored for the new molecule. These dynamic studies strongly
support our initial thought that the endꢀextended conjugation would improve the charge
separation and thus yield a better FF and overall performance under same situation.
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CONCLUSION
In conclusion, a new small molecule acceptor FDNCTF with an AꢀDꢀA structure
and incorporating NINCN as the new extended end group was designed and synthesized.
FDNCTF exhibited redꢀshifted optical absorption, little changed LUMO energy level,
higher charge carrier mobility and more ordered molecular packing. Nonꢀfullerene OSCs
using FDNCTF as the acceptor and a wide bandgap polymer PBDBꢀT as the donor,
demonstrated a high PCE of 11.2% with a Voc of 0.93 V, a Jsc of 16.5 mA cmꢀ2, and an FF
of 72.7%, which is much higher than the PCE of 10.06% for the corresponding FDICTF
based device. The higher Jsc and nearly maintained Voc, is benefitted from the better
complementary absorption and suitable LUMO energy level. The significantly improved
FF should be ascribed to the higher charge separation efficiency and efficient hole
transfer from FDNCTF to PBDBꢀT, confirmed by the transient absorption study. These
should be due to the appropriately enhanced intermolecular interaction between the donor
and acceptor because of the extended conjugation of the endꢀgroups. These results
indicate that employing larger conjugation endꢀgroups to the smallꢀmolecule acceptors is
an effective way to enhance both the absorption and molecular interaction of the NF
acceptor while keeping the LUMO level. This should offer a good strategy to manage the
difficult balance and tradeoff of Jsc and Voc in many cases and achieve higher Jsc and FF
while keeping the Voc relatively unchanged, simultaneously. Similar strategy might be
applied for the donor design.
ASSOCIATED CONTENT
Supporting Information
Experimental details, characterization data for the new compounds, thermogravimetric
analysis, differential scanning calorimetry, cyclic voltammogram, internal quantum
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efficiency, photoluminescence spectra, JꢀV characteristics of SCLC, NMR spectra, mass
spectra and other device data. This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: fengliu82@sjtu.edu.cn (F. Liu)
*Eꢀmail: htzhang@nankai.edu.cn (H. Zhang)
*Eꢀmail: yschen99@nankai.edu.cn (Y. Chen)
Author contributions
⊥H.R.F and N.L.Q contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the financial support from the Ministry of Science
and Technology of China (MoST) (2014CB643502 and 2016YFA0200200), NSFC
(91433101, 51422304, 51373078 and 91633301).
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