Effects of conjugated bridges on the photovoltaic properties of: Ortho -functionalized perylene diimides for non-fullerene polymer solar cells

Article abstract of DOI:10.1039/c8tc03303j

Perylene diimide (PDI) derivatives have great potential to replace the commonly used fullerene derivatives in bulk heterojunction (BHJ) polymer solar cells (PSCs). However, PDI-based molecules have a strong tendency to aggregate in thin films, which prevents the formation of appropriate phase separation. To alleviate this problem, much effort has been devoted to developing twisted PDI dimers with functional bridges at bay positions. Unfortunately, the bay-functionalized PDI dimers exhibit relatively low electron mobilities due to the excessive twist geometry. To solve this dilemma, an efficient method is to functionalize the PDI at less sterically-encumbered ortho-positions. In this article, we synthesized two novel ortho-functionalized PDI dimers, namely PDI-2Th-PDI and PDI-ThFTh-PDI, and investigated the influence of the conjugated bridge length on the physical and photovoltaic properties of PDIs. After introducing a fluorene bridge between the thiophene units, PDI-ThFTh-PDI with the extended conjugate length possessed a relatively low-crystalline nature and therefore exhibited one order of magnitude lower electron mobility compared to PDI-2Th-PDI. As a result, the power conversion efficiency (PCE) of solar cells fabricated with PDI-2Th-PDI as the electron acceptor and PTB7-Th as the electron donor is almost twice that of the solar cell with PDI-ThFTh-PDI as the acceptor. Our findings provide important guidelines for the design of more efficient ortho-functionalized PDI acceptors.

Full text of DOI:10.1039/c8tc03303j

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Effects of conjugated bridges on the photovoltaic
properties of ortho-functionalized perylene
diimides for non-fullerene polymer solar cells†
Cite this:DOI: 10.1039/c8tc03303j
Jiazun Wu,‡ab Xiangchun Li,‡c Xiaodong Liu,*ab Shuanghong Wu,*a
ab
Wen-Yong Lai *c and Yonghao Zheng
*
Perylene diimide (PDI) derivatives have great potential to replace the commonly used fullerene derivatives
in bulk heterojunction (BHJ) polymer solar cells (PSCs). However, PDI-based molecules have a strong
tendency to aggregate in thin films, which prevents the formation of appropriate phase separation. To
alleviate this problem, much effort has been devoted to developing twisted PDI dimers with functional
bridges at bay positions. Unfortunately, the bay-functionalized PDI dimers exhibit relatively low electron
mobilities due to the excessive twist geometry. To solve this dilemma, an efficient method is to
functionalize the PDI at less sterically-encumbered ortho-positions. In this article, we synthesized two
novel ortho-functionalized PDI dimers, namely PDI-2Th-PDI and PDI-ThFTh-PDI, and investigated the
influence of the conjugated bridge length on the physical and photovoltaic properties of PDIs. After
introducing a fluorene bridge between the thiophene units, PDI-ThFTh-PDI with the extended conjugate
length possessed a relatively low-crystalline nature and therefore exhibited one order of magnitude lower
electron mobility compared to PDI-2Th-PDI. As a result, the power conversion efficiency (PCE) of solar
cells fabricated with PDI-2Th-PDI as the electron acceptor and PTB7-Th as the electron donor is almost
twice that of the solar cell with PDI-ThFTh-PDI as the acceptor. Our findings provide important guidelines
for the design of more efficient ortho-functionalized PDI acceptors.
Received 5th July 2018,
Accepted 30th July 2018
DOI: 10.1039/c8tc03303j
rsc.li/materials-c
donor materials have led to a power conversion efficiency (PCE)
of over 11% for single-junction polymer/fullerene-based
1. Introduction
Over the past few decades, bulk heterojunction (BHJ) polymer solar PSCs.11–13 Despite this remarkable success, further improve-
cells (PSCs) have attracted extensive interest owing to their distin- ment was limited by the weak absorption at long wavelength,
guished advantages, such as low cost, light weight, solution poor chemical and electronic tunability of fullerenes, and the
processability and mechanical flexibility.1–5 Fullerene derivatives, metastable morphology in polymer:fullerene blends.14–17 To
such as [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) and resolve these drawbacks, a number of non-fullerene acceptors,
[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), are the most including small molecules and polymers, have been explored to
effective and widely used acceptor materials because of their strong replace fullerene derivatives in BHJ PSCs.11,18–36
electronegativity, high electron mobility and universal compatibility
Among the various classes of non-fullerene acceptors, per-
with multiple donor polymers.1,6–10 To date, advances in efficient ylene diimides (PDIs) are one of the most promising alternatives
due to their high electron mobility, strong absorption in the
visible region, low synthesis cost and excellent stability.19,33,37–46
Despite these favorable properties, PDI-based PSCs usually show
a School of Optoelectronic Science and Engineering, University of Electronic Science
and Technology of China (UESTC), Chengdu 610054, P. R. China.
low performance owing to the formation of large crystalline
E-mail: xdliu@uestc.edu.cn, shwu@uestc.edu.cn, zhengyonghao@uestc.edu.cn
b Center for Applied Chemistry, University of Electronic Science and Technology of
domains in the active layer, which is derived from the planar
perylene core and their aggregation tendency.37,39,47 To restrain
the aggregation effects of PDIs, several approaches have been
employed to reduce the strong p-stacking.33,48 An effective
method is the introduction of alkyl chains or aromatic groups
at bay positions (1-, 6-, 7-, and 12-positions) or imide positions of
the PDI molecules.39,49–51 Another frequently used method is to
develop twisted PDI dimers, in which two PDI units are linked at
China (UESTC), Chengdu, 611731, P. R. China
c Key Laboratory for Organic Electronics and Information Displays (KLOEID) &
Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation
Center for Advanced Materials (SICAM), Nanjing University of Posts &
Telecommunications, 9 Wenyuan Road, Nanjing 210023, P. R. China.
E-mail: iamwylai@njupt.edu.cn
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8tc03303j
‡ These authors contributed equally to this work.
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glass substrates (CSG Holding Co., Ltd, 10 ohm sqꢀ1) were
cleaned stepwise with detergent, deionized water, acetone, and
ethanol under sonication for 15 min each. The substrates were
dried in a nitrogen stream and subjected to ultraviolet-ozone
(UVO) treatment for 30 min to generate the hydrophilic surface.
A sol–gel-derived ZnO electron transport layer was prepared
according to literature procedures.58,59 In brief, the ZnO pre-
cursor was prepared by dissolving zinc acetate dihydrate
(Zn(CH3COO)2ꢁ2H2O, Sigma-Aldrich, 499.0%, 0.109 g) and
ethanolamine (NH2CH2CH2OH, Sigma-Aldrich, 499.5%, 32 mL)
in 2-methoxyethanol (CH3OCH2CH2OH, Sigma-Aldrich, 99.8%,
anhydrous, 1 mL) under vigorous stirring for at least 12 h for
the hydrolysis reaction in air. The ZnO precursor solution was
filtered through a 0.45 mm nylon syringe filter and spin-cast onto
the cleaned ITO substrate at 4000 rpm for 60 s. After being baked
on a hot plate at 200 1C for 1 h in air, the samples were transferred
into a nitrogen-filled glove box (o0.1 ppm O2 and H2O) for the
rest of the device fabrication.
The active layer was spin-coated on the ZnO surface at
different spin rates; PTB7-Th : PDI-2Th-PDI (1 : 2 w/w) and
PTB7-Th : PDI-ThFTh-PDI (1 : 2 w/w) were dissolved in chloro-
benzene with a total concentration ranging from 30 to
45 mg mLꢀ1 and kept under stirring for at least 12 h before
spin-coating the active layer. After spin-coating, the blend film
was annealed on a hot plate at 120 1C for 5 min. To complete
the device fabrication, 10 nm MoO3 and 100 nm Ag were
sequentially deposited by thermal evaporation at a base
pressure of 5.0 ꢂ 10ꢀ5 Pa. The deposition rate and film
thickness were monitored using a quartz crystal sensor.
A shadow mask was put on the sample to define an active area
of 4 mm2 before the metal deposition.
SCLC mobility measurements. Space charge-limited currents
(SCLCs) were tested in hole-only devices with a structure of ITO/
PEDOT:PSS/active layer/MoO3/Ag and electron-only devices with
a structure of ITO/ZnO/active layer/Ca/Al. The devices were
prepared following the identical procedure as described above
for OPV devices, except that PEDOT:PSS was spin-coated on the
ITO substrate instead of ZnO for hole-only devices and Ca/Al was
deposited on the active layer instead of MoO3/Ag for electron-
only devices. The mobilities were determined by fitting the dark
current to the modified Mott–Gurney equation60,61
Scheme 1 Synthesis routes for the ortho-functionalized PDIs: PDI-2Th-
PDI and PDI-ThFTh-PDI.
imide or bay positions either directly or through different linkage
units.51–54 Unfortunately, the bay-position functionalized PDI shows
a lowered electron mobility due to its highly twisted molecular
conformation.38,48,55 Therefore, there is a trade-off between the
suppression of molecular p–p stacking and the crystallinity of the
blend layer.56 To balance the trade-off, an attractive approach is to
functionalize the less sterically encumbered ortho-positions (2-, 5-, 8-,
and 11-positions). So far, only a few ortho-functionalized PDIs have
been reported due to their complicated synthesis.48,57 Recently, we
reported a simple and low-cost method to synthesize desirable ortho-
substituted PDIs, and we realized high-performance non-fullerene
PSCs with a single PDI core as the acceptor.
In this work, we first investigated the effect of the conju-
gated bridge length on the physical and photovoltaic properties
of ortho-functionalized PDIs. We designed two novel PDI
dimers, PDI-2Th-PDI and PDI-ThFTh-PDI, with different bridge
lengths (Scheme 1). Compared to PDI-ThFTh-PDI, PDI-2Th-PDI
showed better crystallinity and thus higher electron mobility,
which indicates that introducing a fluorene bridge between the
thiophene units severely inhibited the crystallization of PDI
molecules. When blending the PDI-2Th-PDI acceptor with a
low-bandgap polymer donor PTB7-Th, the non-fullerene PSCs
exhibited an impressive PCE of 4.00% without any solvent
additives, which is significantly higher than that of the PSCs
with PTB7-Th as the donor and PDI-ThFTh-PDI as the acceptor.
!
rffiffiffiffiffiffiffiffi
2
9
Veff
L3
Veff
J ¼ e0erm0
exp 0:89b
8
L
where e0 is the vacuum permittivity, er is the relative dielectric
constant of the active layer, m0 is the zero-field mobility of
the charge carriers, and Veff is the effective applied voltage,
which is equal to the applied voltage minus the built-in
potential originating from the work function difference between
the electrodes, L is the thickness of the active layer, and b is the
field-activation factor.
2. Experimental section
2.1. Fabrication of PSCs
2.2. Characterization
OPV device fabrication. BHJ PSCs were fabricated with an The current density–voltage ( J–V) characteristics were measured
inverted structure of ITO/ZnO/active layer/MoO3/Ag. ITO-coated using a Keithley 2400 Source Meter unit under simulated
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Air Mass 1.5 Global (AM 1.5 G) solar illumination at an intensity
of 100 mW cmꢀ2, which was calibrated by a reference silicon
solar cell. The measurements were carried out with the PSCs
inside the glove box (o0.1 ppm O2 and H2O). The external
quantum efficiency (EQE) spectra were measured using a
QTEST HIFINITY 5 (Crowntech Inc., USA) at room temperature
in air. The light intensity was calibrated using a single-crystal Si
photovoltaic cell as a standard. Atomic force microscopy (AFM)
measurements were conducted on an Asylum MFP-3D-BIO
system (Oxford instruments) operating in tapping mode.
Absorption spectra were acquired using a Shimadzu UV-2600
Fig. 1 UV-vis absorption spectra of PDI-2Th-PDI and PDI-ThFTh-PDI (a)
in DCM and (b) in films cast without blending and with blended PTB7-Th.
UV-vis spectrophotometer. Steady-state photoluminescence PTB7-Th thin film exhibits a low-bandgap absorption in the
(PL) spectra were recorded on a Hitachi F-4600 fluorescence region of 550–750 nm, which complements well with the absorp-
spectrophotometer. Cyclic voltammetry (CV) was performed tion of our PDI electron acceptors.66 The PTB7-Th:acceptor blend
in an electrolyte solution of 0.1 M tetrabutylammonium hexa- films show wide absorption bands spanning almost the whole
fluorophosphate in dichloromethane (DCM) at a scanning rate visible spectrum, which probably results in high short-circuit
of 50 mV sꢀ1. The counter electrode was a platinum wire and current density ( Jsc) values.
the working electrode was a glassy carbon electrode. A silver
To study the exciton dissociation efficiency at the donor/
wire coated with 0.1 M silver nitrate (Ag/AgNO3) was used acceptor hetero-interface, the PL spectra of the PTB7-Th:acceptor
as a reference electrode in combination with the ferrocene– blend films were measured and compared with the neat PTB7-Th
ferrocenium (Fc/Fc+) redox couple as an internal standard. The and two PDI acceptor films, as shown in Fig. S2 (ESI†). The
vacuum level of Fc/Fc+ was assumed to be ꢀ4.80 eV.
PL of the PTB7-Th:acceptor blends is significantly quenched
relative to the PL intensity of the pure PTB7-Th film when
excited at 620 nm where only PTB7-Th is excited (Fig. S2a, ESI†),
which indicates the efficient electron transfer process from
PTB7-Th to the PDI acceptors. Upon excitation at 490 nm,
where only the PDI acceptor is excited, the PTB7-Th:PDI-2Th-PDI
blend film shows a higher PL quenching efficiency compared
to the PTB7-Th:PDI-ThFTh-PDI blend film (Fig. S2b, ESI†), thus
implying a more efficient hole transfer from PDI-2Th-PDI to
PTB7-Th than that from PDI-ThFTh-PDI to PTB7-Th.67
3. Results and discussion
3.1. Molecular design and synthesis
The synthetic procedures of PDI-2Th-PDI and PDI-ThFTh-PDI
are depicted in Scheme 1. The synthesis method of PDI-ThBr
was modified from our previous work,38 which afforded the key
intermediate in 41% yield. PDI-2Th-PDI was synthesized by a
simple self-coupling reaction.62–64 Suzuki coupling of PDI-ThBr
and 9,9-dioctylfluorene-2-bis(boronic acid pinacol ester) occurred
smoothly to give PDI-ThFTh-PDI in reasonable yields. The
structures of all the compounds were characterized by mass
spectroscopy, 1H and 13C NMR spectroscopy, and elemental
analysis (ESI†). Thermogravimetric analysis (TGA) demonstrated
the good thermal stability of PDI-2Th-PDI and PDI-ThFTh-PDI
with temperatures of 5% weight loss at 352 1C and 379 1C,
respectively, as shown in Fig. S1 (ESI†).
3.3. Photovoltaic performance of PTB7-Th:PDI acceptor-based
PSCs
To demonstrate the potential of the two PDI derivatives as
electron acceptors, we fabricated BHJ PSCs in an inverted
device architecture of ITO/ZnO/PTB7-Th:acceptor/MoO3/Ag,
where ZnO and MoO3 were applied individually as the cathode
and anode interlayers. Fig. 2a and b show the inverted device
structure and the energy levels of the component materials,
respectively. The highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) energy levels
3.2. Physical properties of PDI-2Th-PDI and PDI-ThFTh-PDI
Fig. 1a shows the normalized UV-vis absorption spectra of PDI- of the two PDI acceptors were determined by CV, and
2Th-PDI and PDI-ThFTh-PDI in DCM solution. Both of the the results are shown in Fig. S3 and Table S1 (ESI†). The
molecules show broad absorption bands in the wavelength HOMO/LUMO levels of PDI-2Th-PDI and PDI-ThFTh-PDI
range from 300 to 550 nm, with a complete spectral overlap are almost the same, located at ꢀ5.73/ꢀ3.79 eV and ꢀ5.71/
in the range of 445–550 nm that is attributed to the PDI segment ꢀ3.81 eV, respectively, which indicates that introducing a
absorption.10,65 In the range of 300–445 nm, PDI-ThFTh-PDI fluorene unit does not affect the energy levels of our PDI
exhibits a stronger absorption than PDI-2Th-PDI due to the system. Computational studies by density functional theory
extended p-conjugation length as a result of the incorporation (DFT) were additionally performed. The dihedral angles between
of the fluorene unit. Fig. 1b depicts the UV-vis spectra of the two constituent PDI units were calculated to be 71 for
the donor polymer PTB7-Th, two PDI acceptors and PTB7-Th/ PDI-2Th-PDI and 01 for PDI-ThFTh-PDI, which demonstrates
acceptor mixtures in the film state. The thin-film absorption that the structures of the two PDI molecules are almost planar
spectra of the two PDI acceptors are broadened and red-shifted (Fig. S4b, ESI†). As shown in Fig. S4c (ESI†), the HOMO/LUMO
relative to their solution spectra, covering a broad absorption levels obtained from DFT calculations are in good agreement
range from 300 to 650 nm. The absorption spectrum of the with the experimental results estimated from CV. As expected,
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Fig. 2 (a) Schematic illustration of the PSCs studied in this work. (b) Energy-level diagram for each component of the devices. (c and d) J–V
characteristics for PTB7-Th:PDI-2Th-PDI and PTB7-Th:PDI-ThFTh-PDI-based solar cells (c) under illumination of AM 1.5 G, 100 mW cmꢀ2 and (d) in the
dark. (e) EQE curves of the corresponding devices. (f) Light intensity dependence of the short-circuit current density of the devices.
the HOMO–LUMO map of both molecules suggests that the such as the donor/acceptor (D/A) blend ratio, photoactive layer
HOMO and LUMO orbitals locate at the conjugated bridge and thickness and thermal annealing temperature, are presented
the PDI cores, respectively. In addition, the bridge length in Fig. S5–S7 and Tables S2–S4 (ESI†). The PSCs based on
of PDI-2Th-PDI is 9 Å, which is shorter than that (17 Å) of PTB7-Th:PDI-2Th-PDI achieved a high PCE of 4.00%, with a
PDI-ThFTh-PDI.
Jsc of 10.72 mA cmꢀ2, an open-circuit voltage (Voc) of 0.81 V,
Fig. 2c shows the J–V characteristics of the optimized devices and a fill factor (FF) of 46.0%. In contrast, the PSCs based on
based on two PDI acceptors (and PTB7-Th as the donor) under PTB7-Th:PDI-ThFTh-PDI possessed a relatively low PCE of 2.22%,
illumination of AM 1.5 G, 100 mW cmꢀ2, and Table 1 lists the with a Jsc, Voc and FF of 6.81 mA cmꢀ2, 0.84 V and 39.0%,
photovoltaic parameters of the corresponding PSCs for a clear respectively. For both of these two PDI acceptors, the optimized
comparison. The details of the systematic device optimization, D/A blend weight ratio is 1 : 2, and only a simple thermal
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Table 1 Photovoltaic performance of the PSCs based on PTB7-Th:PDI-2Th-PDI and PTB7-Th:PDI-ThFTh-PDI under illumination of AM 1.5 G,
100 mW cmꢀ2
Jsc (mA cmꢀ2
a
)
FFa (%)
PCEa (%)
Jsc (mA cmꢀ2
b
)
a
Active layer
Voc (V)
PTB7-Th:PDI-2Th-PDI
PTB7-Th:PDI-ThFTh-PDI
0.81 (0.81 ꢃ 0.01)
10.72 (10.08 ꢃ 0.38)
6.81 (6.47 ꢃ 0.24)
46.0 (46.7 ꢃ 0.6)
39.0 (39.3 ꢃ 0.5)
4.00 (3.82 ꢃ 0.10)
2.22 (2.14 ꢃ 0.06)
10.50
6.79
0.84
a
b
The best values are given, followed by the averages and standard deviations in parentheses, calculated from at least eight devices. Jsc calculated
from the EQE spectrum.
annealing procedure is employed to optimize the active layer 3.4. Charge transport properties of PTB7-Th:acceptor blend
morphology, without using any additives. Fig. 2d presents the films
dark J–V characteristics of the optimized devices based on two
PDI acceptors. The PTB7-Th:PDI-ThFTh-PDI device exhibits a
higher turn-on voltage than that of the PTB7-Th:PDI-2Th-PDI
PTB7-Th:PDI-ThFTh-PDI device, can be attributed to the higher
device, indicating a larger built-in potential across the device,
and thus a higher Voc of the PTB7-Th:PDI-ThFTh-PDI PSC.
To confirm the accuracy of the photon J–V measurements of
the devices, their corresponding EQE spectra are presented in
According to the above study, the much better performance
of the PTB7-Th:PDI-2Th-PDI device, in comparison with the
Jsc and FF originating from the reduced bimolecular recombi-
nation. To gain more insight into the remarkably high Jsc and
FF of the PTB7-Th:PDI-2Th-PDI device, the charge transport
properties of the optimized PTB7-Th:acceptor blend films were
Fig. 2e. An enhanced efficiency is observed throughout the full
investigated using the SCLC method with the device structures
visible spectral range from 300 to 800 nm for the PTB7-Th:PDI-
of ITO/ZnO/active layer/Ca/Al for electron and ITO/PEDOT:PSS/
active layer/MoO3/Ag for hole transport measurements.57,66
2Th-PDI device when compared to the PTB7-Th:PDI-ThFTh-PDI
device, which is consistent with the variation trend of Jsc values
Fig. 3a and b, respectively, show the dark J–V curves, plotted
on a log–log scale, of the electron-only and hole-only devices
obtained from the J–V characteristics. By integrating the
product of the EQE data with the AM 1.5 G solar photon flux,
based on two PDI acceptors. The charge carrier mobilities of the
the calculated Jsc values for the PTB7-Th:PDI-2Th-PDI and
blend films were extracted by fitting the J–V curves in the near
PTB7-Th:PDI-ThFTh-PDI devices are 10.50 and 6.79 mA cmꢀ2
,
quadratic region according to the modified Mott–Gurney
equation.60,61 Table 2 gives the estimated electron and hole
mobilities. The electron mobility (me) of the PTB7-Th:PDI-2Th-
respectively, which are very close to the measured values from
the J–V curves shown in Table 1.
The dependence of J–V characteristics on light intensity
(Plight) can give information on the recombination loss of
PDI blend film (1.10 ꢂ 10ꢀ4 cm2 Vꢀ1
s
ꢀ1) is one order of
magnitude higher than that of the PTB7-Th:PDI-ThFTh-PDI
devices.52 In principle, Jsc has a power-law dependence on Plight
,
blend film (1.34 ꢂ 10ꢀ5 cm2 Vꢀ1
s
ꢀ1), whereas the hole
mobility (mh) of the PTB7-Th:PDI-2Th-PDI blend film (4.33 ꢂ
À
Á
which can be expressed as Jsc / Plight a, where the power-law
component (a) should be unity when the bimolecular recombi-
nation in the device is negligible.67,68 Fig. 2f illustrates Jsc as a
function of Plight for the optimized devices based on two PDI
acceptors. The extracted a values are 0.988 and 0.965 for the
PTB7-Th:PDI-2Th-PDI and PTB7-Th:PDI-ThFTh-PDI devices,
respectively. This result indicates that the bimolecular recom-
bination was significantly suppressed in the PSCs with PDI-
2Th-PDI as the acceptor.
10ꢀ4 cm2 Vꢀ1 sꢀ1) is almost one order of magnitude lower than
that (3.40 ꢂ 10ꢀ3 cm2 Vꢀ1
s
ꢀ1) of the PTB7-Th:PDI-ThFTh-PDI
blend film. As a result, the PTB7-Th:PDI-2Th-PDI blend possessed
a much more balanced mh/me (3.9) compared to that (253.7) of the
PTB7-Th:PDI-ThFTh-PDI blend, which can explain the reduced
bimolecular recombination, and therefore the enhanced Jsc and
FF of the PTB7-Th:PDI-2Th-PDI device.69
3.5. Morphological characterization of the active layers
The morphology of the active layer is an important factor in
determining the charge separation and transport, and thus it
affects the carrier mobilities in the blend films.69,70 The ideal
BHJ morphology contains a bi-continuous interpenetrating
network of pure donor and acceptor domains that are smaller
than the typical exciton diffusion length of less than 10 nm.71,72
The surface morphology of our PTB7-Th:acceptor blend films
was investigated by AFM imaging. The AFM height and phase
images (2 ꢂ 2 mm2) are shown in Fig. 4. The PTB7-Th:PDI-2Th-
PDI blend film exhibits a network of fibrils with diameters
of B40 nm, which should favor a high exciton dissociation
efficiency (Fig. 4c).73 The fine surface morphology is consistent
with the efficient PL quenching of the blend film. In contrast, a
less defined phase separation was found for the PTB7-Th:PDI-
ThFTh-PDI blend (Fig. 4d). The root-mean-square (RMS) for
Fig. 3 J–V characteristics in the dark of (a) electron-only devices in the
configuration of ITO/ZnO/active layer/Ca/Al and (b) hole-only devices in
the configuration of ITO/PEDOT:PSS/active layer/MoO3/Ag. The symbols
represent experimental data, and the solid lines are the fit to the experi-
mental data by the modified Mott–Gurney equation. The applied voltage is
corrected for the built-in potential due to the work function difference of
the two electrodes.
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Table 2 Calculated hole mobilities, electron mobilities, and hole-to-electron-mobility ratios for PTB7-Th:PDI-2Th-PDI and PTB7-Th:PDI-ThFTh-PDI-
based devices. The thicknesses and root-mean-square (RMS) surface roughnesses of the two blend films are also listed
Active layer
Thickness (nm)
mh (cm2 Vꢀ1 sꢀ1
)
me (cm2 Vꢀ1 sꢀ1
)
mh/me
RMS (nm)
PTB7-Th:PDI-2Th-PDI
PTB7-Th:PDI-ThFTh-PDI
120
106
4.33 ꢂ 10ꢀ4
1.10 ꢂ 10ꢀ4
3.9
253.7
0.95
1.37
3.40 ꢂ 10ꢀ3
1.34 ꢂ 10ꢀ5
synthesized and characterized. When increasing the bridge
length of the molecule, the PDI dimers showed a relatively
low electron mobility and therefore poor performance. As a
result, PDI-2Th-PDI with a short bridge length of 9 Å exhibited a
best PCE of 4.00%, whereas PDI-ThFTh-PDI with a long bridge
length of 17 Å gave a PCE of only 2.22% when they were each
blended with the donor polymer PTB7-Th. Further efforts are
ongoing to modify the molecular structures of PDI dimers,
especially the conjugated bridge groups, for improving the
photovoltaic properties of ortho-functionalized PDIs.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Fig. 4 AFM height (a and b) and phase (c and d) images (2 ꢂ 2 mm2) of
(a and c) PTB7-Th:PDI-2Th-PDI and (b and d) PTB7-Th:PDI-ThFTh-PDI
blend films under identical fabrication conditions with photovoltaic devices.
This work was supported by the National Natural Science
Foundation of China (61604101) and the Scientific Research
Foundation of UESTC for Young Teacher (ZYGX2016KYQD134).
PTB7-Th:PDI-2Th-PDI is 0.95 nm and the RMS for PTB7-Th:PDI-
ThFTh-PDI is 1.37 nm (Table 2). The smoother surface is
believed to be beneficial for electric contact between the active
layer and the MoO3/Ag electrode, which would help to decrease
both contact resistance and leakage current.74
Notes and references
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