14.7% Efficiency Organic Photovoltaic Cells Enabled by Active Materials with a Large Electrostatic Potential Difference

Article abstract of DOI:10.1021/jacs.8b12937

Although significant improvements have been achieved for organic photovoltaic cells (OPVs), the top-performing devices still show power conversion efficiencies far behind those of commercialized solar cells. One of the main reasons is the large driving force required for separating electron-hole pairs. Here, we demonstrate an efficiency of 14.7% in the single-junction OPV by using a new polymer donor PTO2 and a nonfullerene acceptor IT-4F. The device possesses an efficient charge generation at a low driving force. Ultrafast transient absorption measurements probe the formation of loosely bound charge pairs with extended lifetime that impedes the recombination of charge carriers in the blend. The theoretical studies reveal that the molecular electrostatic potential (ESP) between PTO2 and IT-4F is large, and the induced intermolecular electric field may assist the charge generation. The results suggest OPVs have the potential for further improvement by judicious modulation of ESP.

Full text of DOI:10.1021/jacs.8b12937

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Article
14.7% Efficiency Organic Photovoltaic Cells Enabled by
Active Materials with a Large Electrostatic Potential Difference
Huifeng Yao, Yong Cui, Deping Qian, Carlito S. Ponseca, Alireza Honarfar, Ye Xu,
Jingming Xin, Zhenyu Chen, Ling Hong, Bowei Gao, Runnan Yu, Yunfei Zu, Wei
Ma, Pavel Chabera, Tõnu Pullerits, Arkady Yartsev, Feng Gao, and Jianhui Hou
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b12937 • Publication Date (Web): 22 Apr 2019
Downloaded from http://pubs.acs.org on April 22, 2019
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14.7% Efficiency Organic Photovoltaic Cells Enabled by Active
Materials with a Large Electrostatic Potential Difference
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Huifeng Yao,† Yong Cui,†, ‡ Deping Qian,§ Carlito S. Ponseca, Jr.,§ Alireza Honarfar,‖ Ye Xu,†, ‡
Jingming Xin,⊥ Zhenyu Chen,⊥ Ling Hong,†, ‡ Bowei Gao,†, ‡ Runnan Yu,†, ‡ Yunfei Zu,†, ‡ Wei Ma,⊥
Pavel Chabera,‖ Tönu Pullerits‖, Arkady Yartsev,‖ Feng Gao,§ and Jianhui Hou†, ‡*
† Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Polymer Physics and Chemistry, CAS
Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, China
§ Department of Physics, Chemistry and Biology (IFM), Linköping University, Linköping 58183, Sweden
⊥ State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, P. R. China
‖ Division of Chemical Physics, Kemicentrum, Lund University, Lund, SE-22100, Sweden
‡ University of Chinese Academy of Sciences, Beijing 100049, China
Keywords: organic solar cells, molecular electrostatic potential, non-fullerene acceptor, charge generation
ABSTRACT: Although significant improvements have been achieved for organic photovoltaic cells (OPVs), the top-performing devices
still show far behind power conversion efficiencies than the commercialized solar cells. One of the main reasons is the large driving force
required for separating electron-hole pairs. Here, we demonstrate an efficiency of 14.7% in the single-junction OPV by using a new polymer
donor PTO2 and a non-fullerene acceptor IT-4F. The device possesses an efficient charge generation at a low driving force. Ultrafast transient
absorption measurements probe the formation of loosely bound charge pairs with extended lifetime that impedes the recombination of charge
carriers in the blend. The theoretical studies reveal that the molecular electrostatic potential (ESP) between PTO2 and IT-4F is large and the
induced intermolecular electric field may assist the charge generation. The results suggest OPVs have the potential for further improvement
by judicious modulation of ESP.
■ INTRODUCTION
new electron accepting materials have significantly contributed to
the increase of PCEs9,13-20
.
Organic photovoltaic cells (OPVs), comprising of a bulk
heterojunction active layer with an electron donor and acceptor,
possess many attractive advantages like flexibility, lightweight and
low-cost 1-3. Over the past two decades, significant improvements
have been made and over 14% power conversion efficiencies
(PCEs) have been realized, demonstrating its potential for
commercialization 4-7. However, state-of-the-art PCEs are still
much lower than the other photovoltaic devices like c-Si and
At the early stage development of OPVs, many significant
results were achieved by employing fullerene derivatives as the
electron acceptor 21. To provide sufficient driving force for
generating free charge, one of the primary molecular design rules
was that the electron affinity (EA) and ionization potential (IP) of
the donors should be higher than that of the fullerene acceptors by
at least 0.3 eV 22,23. Under such a condition, the CT state below the
singlet excited state (Eg) forms and leads to the additional energy
loss, which has been demonstrated in the state-of-art fullerene-
8
perovskite cells . Organic semiconductors have inherently low
dielectric properties, such that the incident light generates highly
bounded electron-hole pairs, also known as excitons, with strong
Coulombic binding energy. Therefore, excess energy is needed to
based OPVs 24
.
Recently, non-fullerene acceptors (NFAs) have attracted
extensive attention and achieved rapid progress 25-28. Interestingly,
some NFAs-based OPVs showed good external quantum
efficiencies (EQEs) when the energy offsets of EAs or IPs between
the donors and acceptors were very small 22,24. In these devices, the
ECTs are close or even identical to the Egs, suggesting that CT states
are generated efficiently even with lower energy offset 29. This
feature promises high VOC for non-fullerene devices but meanwhile
triggers a critical question on how charge generation occurs in these
systems. The existing theoretical studies such as the energetic
drive its separation to form free charges, which involves the
9,10
formation and dissociation of the charge transfer (CT) state
.
This results in energy loss, that is, the energy offset from the
bandgap to open-circuit voltage (VOC) is large in OPVs and hence
limits the PCEs 11. The increase of CT energy (ECT) may improve
the VOC but will lead to a lower driving force for generating CT
states from excitons 12. In recent years, the studies of photovoltaic
materials and devices with comparatively low energy loss using
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disorder, donor:acceptor intermolecular orientation/morphology,
We conducted the square-wave voltammetry (SWV)
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and entropic gain can partly explain the exciton dissociation but at
some specific conditions only. However, they were established
based on fullerene-OPV systems and have weak relationships with
the chemical structures 30-33. The spectroscopic studies such as
time-resolved photoluminescence and transient absorption were
measurement (Figure S2) and illustrated the energy levels of the
three materials (Figure 1D). The calculated EA/IP of PTO2 is
3.67/5.59 eV, which is higher than that of PC71BM over 0.3 eV. For
PTO2 and IT-4F, the IPs are very close (about 0.07 eV offset). We
also performed the ultraviolet photoelectron spectroscopy (UPS)
measurement to estimate the energy levels of the materials (Figure
S3), which show consistent results with the SWV.
used as useful tools to investigate the dynamics of CT process 23, 25
,
but the obtained results provide limited information of the driving
force and cannot be directly related to the chemical structure.
Therefore, it is of great importance to study the underlying working
mechanism from the perspective of chemical structures of the new
emerging acceptors, which will benefit the design of efficient OPV
materials. On the other hand, the lower energy loss leads to a higher
VOC and thus lifts the upper-limit of PCE. At present, 13%
efficiencies of OPVs were demonstrated with comparatively larger
energy losses 28; it is feasible to improve its state-of-the-art PCE by
adopting a combination of donor and acceptor with aligned Eg and
Furthermore, we performed highly sensitive EQE (s-EQE) and
electroluminescence (EL) measurements of the PTO2:IT4F and
PTO2:PC71BM blends. In accordance with the previous work36,
ECT of PTO2:IT-4F is estimated as 1.58 eV (Figure 1E). As
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displayed in Figure 1F, by fitting the s-EQE and EL spectra, the
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ECT of PTO2:PC71BM blend is 1.65 eV. From the EQE curves
,
the Egs are calculated as 1.58 and 2.02 eV for PTO2:IT-4F and
PTO2:PC71BM, respectively (Figure S4), and clear CT emission is
observed in the EL spectra of PTO2:PC71BM blend. Clearly, the
energetic offset between Eg and ECT is negligible for the PTO2:IT-
4F system while in PTO2:PC71BM it is 0.37 eV. The gaps between
Egs and ECTs are the major differences in energy losses for the two
systems (Figure S5). As shown in Figure S6, the
photoluminescence (PL) spectra to was used to estimate the exciton
dissociation in the blends. Interestingly, even though there is no
energy offset between the Eg and ECT of the PTO2:IT-4F blend, the
PL quenching is quite efficient, but for the POT2:PC71BM blend
films, the PL quenching is less efficient.
ECT
.
Here, we report a polymer donor PTO2, which delivers a
certified PCE of 14.6% by incorporating a non-fullerene acceptor
IT-4F 34. Even though the driving force is negligible as evaluated
by the commonly used techniques, the device still shows a
maximum EQE of 87%, indicating efficient charge generation.
Ultrafast transient absorption measurements probe extended
lifetime of charge pairs that impedes its recombination. The
theoretical studies demonstrate that PTO2 and IT-4F have very
different electrostatic potential (ESP) and the intermolecular
electric field (IEF) formed between them may assist the charge
generation. In contrast, an additional electric field is needed to
drive the charge transfer between PTO2 and PC71BM because the
IEF is relatively weaker than that in PTO2:IT-4F. These results
suggest that a larger difference in ESP between the donor and
acceptor is beneficial for charge extraction. We anticipate that
understanding the working mechanism and designing photoactive
materials with large ESP difference will open a new possibility in
boosting the PCEs of OPVs.
Photovoltaic Performance. We obtained the photovoltaic
performance of PTO2:IT-4F and PTO2:PC71BM, where
a
conventional device structure of indium tin oxide (ITO)/ poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS)/active layer/PFN-Br 38/Al were used. We optimized
the device processing conditions including donor:acceptor ratio,
solvent/additive and thermal annealing treatment to obtain the
optimal PCE. The current density–voltage (J–V) curves of the best
devices are shown in Figure 2A and the detailed photovoltaic
parameters are summarized in Table 1. By using CB as the host
processing solvent and 1,8-diiodooctane (DIO) as the additive, the
best-performing PTO2:IT-4F device gives a PCE of 14.7% with a
VOC of 0.91 V, a JSC of 21.5 mA cm–2 and a fill factor (FF) of 0.75.
For the PTO2:PC71BM-based device, a modest PCE of 5.0% was
recorded with a VOC of 1.0 V, a JSC of 8.1 mA cm–2 and a FF of
0.62. The energy losses are 0.67 eV and 1.02 eV for the PTO2:IT-
4F and PTO2:PC71BM devices, respectively. The PCE histogram
in Figure 2B shows the PCE distribution of sixty PTO2:IT-4F cells
made from six batches. We then sent the best cell to National
Institute of Metrology, China (NIM) for certification and a PCE of
14.6% was recorded (Figure 2C and Figure S7).
■ RESULTS AND DISCUSSION
Optical Absorption and Molecular Energy Levels of the
Materials. We designed and synthesized the polymer named PTO2
(Figure 1A) following the synthesis route illustrated in Scheme S1,
where the trimethyltin-containing benzodithiophene (BDT) and
ester-substituted dibromide thiophene compounds are used as
monomers. The BDT unit has been widely used to construct highly
efficient donors. The dibromide monomer can be synthesized easily
via two steps from a simple starting compound, and the alkyl side
chains can be tuned easily to modify the solubility of the resulting
polymers in the last step, lowing the material cost when comparing
the similar ester-substituted materials 35. The electron-withdrawing
substituents, fluorine and ester groups, can downshift the molecular
energy levels to obtain high VOC. PTO2 has a number-average
molecular weight (Mn) of 27 kDa with a polydispersity index of 2.6,
and good solubility in the common solvents such as tetrahydrofuran
(THF), chloroform, toluene and chlorobenzene (CB).
Figure S8 shows the internal quantum efficiency (IQE) spectra
of the devices. The PTO2:IT-4F device has much higher IQE
values than the PTO2:PC71BM device. From the EQE curves in
Figure 2D, the calculated current densities are 21.2 and 8.0 mA
cm–2 for the PTO2:IT-4F and PTO2:PC71BM devices, respectively,
which are consistent with the JSC obtained from the J–V
measurements. The PTO2:IT-4F device displays a broad and high
EQE spectrum with a peak of 87% at 547 nm while the maximum
EQE of the PTO2:PC71BM device is only 58%. These results
indicate that the charge generation in the former is more efficient
than that in the latter.
As plotted in Figure 1B and Figure S1, we measured the UV-
vis absorption spectra of PTO2, IT-4F, and PC71BM. The PTO2
film has strong absorption from 300 to 600 nm with a maximum
absorption coefficient of 7.69 × 104 cm–1 at 533 nm. The absorption
spectrum of PTO2 complements very well with that of IT-4F in the
whole region from 300 to 800 nm, and hence a high short-circuit
current (JSC) can be expected. The temperature-dependent UV-vis
absorption spectra (Figure 1C) indicate PTO2 has strong
aggregation effect in the diluted solution and the color of the
solution changes gradually from red to yellow by increasing
temperature.
To evaluate the charge transport properties in these two devices,
we performed the single-carrier (hole or electron) mobility
measurements via the space-charge-limited current (SCLC)
method. By fitting the J–V curves (Figure S9), we estimated the
hole/electron mobilities of PTO2:IT-4F and PTO2:PC71BM blends
as 1.6 × 10–3/6.4 × 10–4 and 9.4 × 10–4/5.8 × 10–4 cm2 V–1 S–1,
respectively. Then we conducted photo-induced charger-carrier
extraction in
a linearly increasing voltage (photo-CELIV)
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measurement to get the mobilities of faster carrier component in the
calculated to be 1.1 × 10–4 and 8.5 × 10–5 cm2 V–1 S–1, respectively.
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real working devices. As shown in Figure 2E, the photo-CELIV
mobilities of the PTO2:IT-4F and PTO2:PC71BM devices are
The transient photovoltage (TPV) results imply that the carrier
lifetime of the
PTO2:IT-4F and PTO2:PC71BM devices are 2.8 and 1.1 µs under the 1-sun illumination (Figure 2F). The measurements of photocurrent
dependence on the light intensity indicate that both devices have weak bimolecular recombination (Figure S10) 39. These results suggest that
the two blends have comparable charge transport properties at these time scales.
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Figure 1. Molecular structures, absorption spectra and molecular energy levels of the materials. A) Chemical structures of the polymer
donor, fullerene and non-fullerene acceptors. B) Normalized absorption spectra of the neat donor and acceptors in thin films. C) Absorption
of PTO2 in CB solutions at different temperatures; the insets show the colors of the solutions. D) Schematic energy alignment of materials.
The determination of ECTs by EL and s-EQE spectra, E) PTO2:IT-4F and F) PTO2:PC71BM.
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Figure 2. Photovoltaic performance of the OSCs. A) J–V curve of the best devices. B) Histogram of the PCEs of eighty devices in eight
batches. C) Certified J–V curve in NIM. D) EQE spectra of the optimal IT-4F and PC71BM devices. E) Photo-CELIV plots of the devices
used to calculate the carrier mobilities. F) Plots of the carrier lifetime under varied light intensities acquired from TPV measurements.
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Table 1. Detailed photovoltaic parameters of the devices.
around 1 nm. The TEM patterns (Figure 3C and 3D) demonstrate
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that the nano-scale networks form in the bulk of the two blends. As
shown in the GIWAXS patterns (Figure 3E and 3F), the (010)
peaks appear in the out-of-plane direction, suggesting that both
PTO2:IT-4F and PTO2:PC71BM films exhibit preferable face-on
orientation. The (010) peaks locate at around qz = 1.78 Å–1,
contributing to a π-π stacking distance of 3.53 Å. The (010)
coherence length was calculated using Scherrer analysis as 2.0 and
1.9 nm for the PTO2:IT-4F and PTO2:PC71BM films, respectively.
The morphologies of the two blend films may be mainly controlled
by the polymer donor as it has a strong self-aggregation effect
(Figure S11) 26,28. The morphology features of the two blends are
similar to those of many highly efficient donor:acceptor systems
and should not be detrimental factors affecting the charge dynamics
JSC
VOC
(V)
Devices
FF
PCE (%)a
(mA cm–2)
PTO2:IT-4F
0.91
1.00
21.5
8.1
0.75
0.62
14.7 (14.4 ± 0.2)
5.0 (4.9 ± 0.1)
PTO2:PC71BM
a average PCE values are obtained from 10 different devices.
Morphology Characterizations. We performed the atomic
force microscopy (AFM), transmission electron microscopy (TEM)
and grazing-incidence wide-angle X-ray scattering (GIWAXS)
measurements to investigate the morphological properties of the
two blends. As displayed in Figure 3A and 3B (phase images
shown in Figure S11), both PTO2:IT-4F and PTO2:PC71BM films
show a smooth surface with mean-square surface roughness (Rq) of
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and device performance 40
.
Figure 3. Morphology characterizations of the PTO2:IT-4F and PTO2:PC71BM blend films. (A) and (B) AFM height images. (C) and (D)
TEM patterns. (E) and (F) 2D GIWAXS images. (G) In-plane and out-of-plane cuts.
Ultrafast transient absorption spectroscopy. Figure 4 is the
transient absorption (TA) kinetic trace of the films measured at
near-infrared region (1040 nm) where charge transfer dynamics in
organic solar cell materials has been reported earlier 41,42. The
decrease in the TA kinetic traces of the neat films are fitted by an
exponential decay to determine their lifetimes, which are estimated
to be 350 and 70 ps, for PTO2 and IT-4F, respectively. These decay
rates represent the lifetime of excitons generated in neat films. In
blend films, the initial ultrafast decay (t < 1 ps) is associated with
electron transfer from excited donor to acceptor 42. This decay (t <
1 ps) does not have any discernable difference, which means that
charge transfer rates are quite similar. This is not surprising as the
LUMO offsets of the donors and acceptors in the two blends are
high, i.e. 0.52 eV for PTO2 and IT-4F and 0.32 eV for PTO2 and
PC71BM. More importantly, from about 10 ps, a rise of TA
absorption is observed for PTO2:IT-4F whereas in PTO2:PC71BM
this rise in nearly absent. A similar rise in the TA kinetic trace was
observed in the works of De, et al.41 and Pal, et al.42, which they
related to the separation of initially Coulombically bound charge
pairs to form nearly independent charge carriers at a greater
separation distance. In our case, the analogous phenomenon may
be happening. On a longer time scale, the decay of the TA in
PTO2:IT-4F appears clearly slower than in PTO2:PC71BM. These
results mean that at this time scale, and if electrodes are present as
in devices, charges may be extracted efficiently due to the long
lifetime of loosely bound charge pairs. For PTO2:PC71BM, this
window is a lot shorter, i.e. less than 100 ps, when they start to
recombine. Similar kinetic behavior was obtained at a probe
wavelength of 1100 nm (Figure S12).
Figure 4. Transient absorption kinetic traces of neat PTO2, neat
IT-4F and blend films PTO2:IT-4F and PTO2:PC71BM measured
at probe wavelength of 1040 nm and up to pump-probe delay of 10
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ns. All samples were excited at 530 nm (Iexc = 2.5  1012 ph/cm2)
except for neat film of IT-4F (exc = 720 nm, Iexc = 9.0  1012
ph/cm2).
the acceptors approach the donor from the vertical direction of the
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conjugated plane (Figure 5E), the intermolecular binding energies
in PTO2:IT-4F (ca –160 kcal mol–1) are also much lower than that
in PTO2:PC71BM (ca. –80 kcal mol–1). The results suggest IT-4F
has stronger non-covalent interaction with POT2 than PC71BM.
Calculations. The experimental results discussed above
demonstrate that the PTO2:IT-4F-based device can achieve very
good photovoltaic performance. However, a critical question arises
on the correlation of charge generation with a small driving force.
On one hand, ECT of the PTO2:IT-4F blend is aligned with its Eg
where a very high PCE is realized. On the other hand, although the
PTO2:PC71BM blend possesses a larger energetic difference to
drive charge generation in accordance with established rule43, the
device still shows relatively low EQE. From the perspective of
chemical structures, we studied the contribution of ESP of the
molecules and the charge transfer states to provide insight on this
issue. We studied the static charge distribution around the donor
and acceptor molecules.
We then calculated the charge density distributions of the lowest
excited state for PTO2:IT-4F and PTO2:PC71BM with a face-to-
face distance of 3.5 Å. As IT-4F has higher ESP than PC71BM, the
intermolecular electric field (IEF) between PTO2 and IT-4F is
clearly larger than that in PTO2:PC71BM system. Figure 5F and
5G exhibit the charge density distributions of the lowest excited
states for the two systems. For PTO2:PC71BM, the distributions of
electron and hole are localized on PC71BM and highly overlapping.
In contrast, the hole density is observed in the donor PTO2 for the
PTO2:IT-4T system, suggesting there is charge transfer following
the excitation of the acceptor. Enhanced charge density localization
(the electron density is delocalized on the adjacent IT-4F molecule)
is observed at the heterojunction of bimolecular donor and acceptor
(Figure S17), which may be related to the larger ESP of the
acceptor.
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Shown in equation (1) is the ESP at any point r in the
surrounding space, which is a simple sum of the Coulomb
potentials and reflects the contribution of each nucleus and every
electron in the molecule 44,45. The value of ESP at any point r is
V(r), where ZA is the charge on nucleus A located at RA, ρ(r) is the
electronic density, ε0 is the vacuum permittivity, e is the charge of
an electron.
We also studied more donor:acceptor combinations, including
P3HT:PC71BM, PTB7-Th:PC71BM, PTB7-Th:IEICO, and
PDCBT-2F:IT-M (the molecular structures are shown in Figure
S18) 23. Among them the polymer donor PDCBT-2F has larger IP
than the IT-M acceptor. The experimental results have
demonstrated that the non-fullerene-based combinations, PTB7-
Th:IEICO and PDCBT-2F:IT-M, have low energy offsets and good
charge generation efficiencies. The calculation results are shown in
Figure S19, where we can find the charge transfer is formed in the
non-fullerene systems. This means that when the ESP difference
between donor and acceptor is large enough, highly efficient OPV
cells are possible at aligned molecular energy levels, opening a new
molecular design strategy. The complex made of tetrathiafulvalene
(TTF) as donor and tetracyanoquinodimethane (TCNQ) as an
acceptor was also investigated. As displayed in Figure S20A, we
found that electron-rich TTF has large and negative ESPs
distributed on most of its surface while the TCNQ has the exact
opposite. The CT state result suggests there is efficient charge
transfer in this system (Figure S20B). To get more connections
between the ESP and device performance, we then selected the
other three non-fullerene acceptors with varied end groups
including IT-2F, ITIC, and IT-M to fabricate the OPV cells (Figure
S21). As the electron-withdrawing properties of the end groups are
weakened gradually from IT-4F, IT-2F, ITIC, to IT-M, their ESP
values are decreased correspondingly. For the lowest excited states,
the hole density distribution on PTO2 in the combinations of
PTO2:IT-2F, PTO2:ITIC, and PTO2:IT-M decrease, resulting in
increased Coulomb attractive energies (ECAs). In the devices, IQE
values have corresponding decrease trend (Figure S22), which may
have a connection with the decreased photovoltaic performance
(Table S1). Therefore, we think the large ESP difference between
donor and acceptor may be important for charge generation at the
low energy offset condition, which helps to obtain high VOC and JSC
in the devices.
′
′
( )
푧퐴푒
휌 퐫 푑퐫 )
1
푉(퐫) =
[
― 푒
]
∑
∫
|
|
|퐫′ ― 퐫|
4휋휀0
푅퐴 ― 퐫
퐴
We established the donor and acceptor molecular models by the
theoretical methods (Figure S13). The non-conjugated side chains
are simplified to methyl groups for PTO2 and IT-4F. To get a better
study of the impact of ESP on the intermolecular interactions and
excited states. We make the molecular geometries very planar to
avoid unexpected atomic interactions and this approximation
allows the field direction perpendicular to the plane. The ESP
distributions of the three molecular models are mapped in Figure
5A on their van der Waals surfaces (electron density isosurfaces of
0.001 au) 46. The averaged ESP values of the atoms in the
conjugated backbones and the surface area distribution with
different ESP are shown in Figure 5B and 5C, respectively. PTO2
shows negative ESP values on the most part of its conjugated
backbone, especially for the BDT units, and a minimal ESP value
of -1120 meV is calculated. For IT-4F, the chemical groups with
strong electronegative atoms like oxygen, nitrogen, and fluorine
have negative ESP while most of its conjugated surface has positive
ESP. As the acceptor has a symmetric structure, the overall
molecular dipole is small in spite of the strong electron-
withdrawing substituents. At this condition, the ESP distribution
reflects an uneven feature of charge density distribution that could
be used as a tool to study intermolecular electrostatic interactions.
For PC71BM, the buckyball has positive ESP on its surface, but the
values are much lower than that of IT-4F. We also studied the ESP
distributions of the aggregates by taking bimolecular examples
whose results are shown in Figure S14. We can find that the effects
of adjacent molecules on the ESP distributions are relatively small.
Taking molecular ESP into consideration, the electrostatic
attraction between PTO2:IT-4F will be stronger than that between
PTO2:PC71BM 44. We fix the face-to-face distance between the end
group of IT-4F and the backbone of PTO2 to 3.5 Å, a typical value
for the adjacent planar conjugated surfaces and evaluate the
intermolecular binding energy change by moving IT-4F along with
the PTO2 backbone (Figure S15). As shown in Figure 5D, the
intermolecular binding energies of PTO2:IT-4F are in the range of
ca. –100 to –150 kJ mol–1, which are obviously lower than that of
PTO2:PC71BM system (ca –50 to –75 kJ mol–1). The changes in
binding energies are closely related to the ESP distributions of
PTO2 along the moving direction (Figure S16). Similarly, when
The charge separation in the two systems can be enhanced by
applying additional electric fields. In the PTO2:PC71BM system,
complete hole/electron separation can be observed by adding an
electric field of 0.0025 au. In contrast, a much smaller electric field
of 0.0005 au can drive complete charge separation in PTO2:IT-4F
system. We studied the impact of the additional electric field on the
lowest excited states of the two systems (Figure S23), the distances
and ECAs of the electron-hole pairs under different electric fields.
Without an additional electric field, PTO2:IT-4F shows an
electron-hole distance of 1.2 Å and a ECA of 0.65 eV. For
PTO2:PC71BM, the electron-hole distance enlarges significantly
from 0.3 to 5.4 Å when the electric field increases from 0 to 0.0035
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au, which consequently decreased the ECA from 1.1 to 0.72 eV.
acceptor. For example, if we put IT-4F around the thiophene units
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Clearly, in comparison with PTO2:IT-4F, the electron-hole pair in
PTO2:PC71BM is tightly bound due to the stronger Coulomb
attraction and an additional electric field is needed to drive the
charge transfer in PTO2:PC71BM. According to the above analysis,
efficient charge generation in PTO-2:IT-4F may be related to the
stronger IEF. In addition, the impact of ESP on charge transfer is
highly dependent on the interaction position between donor and
in PTO2 with relatively high ESP (Figure S24), the electron/hole
separation is not observed without additional electric field because
the IEF is weakened. Finally, this strong ESP between PTO2 and
IT-4F may be favorable in preventing the long-lived loosely-bound
excitons, as probed by transient absorption, to meet and recombine.
Its absence in PTO2:PC71BM may have promoted faster
recombination lowering the current that can be extracted.
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Figure 5. ESP of the donor and acceptors and their impact on the intermolecular interactions. A) ESP distributions of the donor and acceptors.
B) Averaged ESP values of the atoms in the conjugated backbones. C) ESP area distributions of the molecules. D) The intermolecular binding
energies between PTO2 and the acceptors at different points parallel to the PTO2 backbone (ESP of PTO2 in the moving direction is shown
as red) at an intermolecular distance of 3.5 Å and E) vertical with backbone. The electron/hole density distributions of the lowest excited
states without/with the additional electric field between F) PTO2 and PC71BM G) PTO2 and IT-4F; red and blue represent hole and electron
densities, receptively.
Detailed synthesis, material characterizations, device fabrication
and measurement, calculation method and results.
■ CONCLUSION
In conclusion, we report a polymer donor PTO2 and study its
applications in OPVs. By using a non-fullerene acceptor IT-4F, we
obtain a high PCE of 14.7% in a single-junction solar cell. At a very
AUTHOR INFORMATION
small energetic offset from the singlet exciton to CT state, the
PTO2:IT-4F-based device exhibits very high charge generation
efficiency. Presence of loosely-bound excitons are detected in
transient absorption of PTO2:IT-4F while it is absent in
Corresponding Author
* hjhzlz@iccas.ac.cn;
Notes
The authors declare no competing financial interest.
PTO2:PC71BM blend. Our results indicate that PTO2 and IT-4F
have a very larger difference in molecular ESP, and the induced
IEF may assist separation of the excitons. In contrast, the charge
ACKNOWLEDGMENT
transfer in the PTO2:PC71BM system requires the assistance of an
J. Hou would like to acknowledge financial support from National
additional electric field in the calculation, which may connect to
Natural Science Foundation of China (91633301 and 51673201),
the energy loss of the device. Most importantly, these findings
and the Chinese Academy of Science (XDB12030200, KJZD-EW-
imply that fine-tuning of ESP can be used as a molecular design
J01); H. Yao thanks the National Natural Science Foundation of
strategy for organic photovoltaic materials. This approach will
China (21805287) and the Youth Innovation Promotion
improve the output voltage and hence boost the PCEs of OPVs to a
Association CAS (No. 2018043); W. M acknowledge the National
new stage.
Natural Science Foundation of China (21504066 and 21534003)
and the Ministry of Science and Technology (No.
2016YFA0200700). T. Pullerits acknowledge the support from the
ASSOCIATED CONTENT
Swedish Energy Agency.
Supporting Information
Supporting Information is available free of charge via the Internet
at http://pubs.acs.org.
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