Synthesis and search for design principles of new electron accepting polymers for all-polymer solar cells

Article abstract of DOI:10.1021/cm500832h

New electron withdrawing monomers, thieno[2′,3′:5′, 6′]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H)-dione (TPTI) and fluorenedicyclopentathiophene dimalononitrile (CN), have been developed and used to form 12 alternating polymers having different m

Full text of DOI:10.1021/cm500832h

Article
pubs.acs.org/cm
Synthesis and Search for Design Principles of New Electron
Accepting Polymers for All-Polymer Solar Cells
In Hwan Jung,† Wai-Yip Lo,† Jaeyoung Jang,† Wei Chen,‡ Donglin Zhao,† Erik S. Landry,‡ Luyao Lu,†
Dmitri V. Talapin,†,§ and Luping Yu*,†
†Department of Chemistry and The James Franck Institute, The University of Chicago, Chicago, Illinois 60637, United States
‡Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
§Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States
S
* Supporting Information
ABSTRACT: New electron withdrawing monomers, thieno-
[2′,3′:5′,6′]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H)-
dione (TPTI) and fluorenedicyclopentathiophene dimalononitrile
(CN), have been developed and used to form 12 alternating
polymers having different monomer combinations: (a) weak
donating monomer−strong accepting monomer, (b) weak
accepting monomer−strong accepting monomer, (c) weak
accepting monomer−weak accepting monomer, and (d) strong
donating monomer−strong accepting monomer. It was found that
lowest unoccupied molecular orbital (LUMO) energy levels of
polymers are significantly determined by stronger electron
accepting monomers and highest occupied molecular orbital
(HOMO) energy levels by the weak electron accepting
monomers. In addition, fluorescent quantum yields of the
TPTI-based polymers in chloroform solution are significantly decreased as the LUMO energy levels of the TPTI series of
polymers become deeper. The quantum yield was found to be closely related with the photovoltaic properties, which reflects the
effect of internal polarization on the photovoltaic properties. Only the electron accepting polymers showing SCLC mobility
higher than 10−4 cm2/(V s) exhibited photovoltaic performance in blend films with a donor polymer, and the PTB7:PNPDI
(1:1.8 w/w) device exhibited the highest power conversion efficiency of 1.03% (Voc = 0.69 V, Jsc = −4.13 mA/cm2, FF = 0.36)
under AM 1.5G condition, 100 W/cm2. We provide a large set of systematic structure−property relationships, which gives new
perspectives for the design of electron accepting materials.
electron accepting materials. For several decades, n-type
polymers are extensively studied for use in organic field-effect
transistors (FETs) and high electron mobility up to 1 cm2/(V·
s) has been achieved.16−22 However, only a few polymers were
successfully applied to solar cells.23−26 The most successful n-
type polymer, P(NDI2OD-T2),27−29 exhibited FET mobility
up to 0.85 cm2/(V·s)27 and space charge limited current
(SCLC) mobility of 5 × 10−4 cm2/(V·s).30 Recently,
diketopyrrolopyrrole-based PDPP2TzT polymer resulted in a
PCE of 2.9% with FET mobility up to 0.13 cm2/(V·s).31 The
Polyera Corp. disclosed a certified all-polymer photovoltaic cell
with a PCE over 6%.32
INTRODUCTION
■
Currently, most popular bulk heterojunction (BHJ) solar cells
require the use of fullerene derivatives as the electron acceptor
for facile photoinduced electron transfer from electron rich low
band gap polymer donors.1−4 The fullerene derivatives, phenyl-
C61(or71)-butyric acid methyl ester (PC61BM and PC71BM), are
almost exclusively used as acceptors due to their high electron
affinity and electron mobilities. However, these fullerenes are
rather expensive and exhibit limited absorption in the longer
wavelength region of the solar spectrum and thermal instability
in the morphology of blend films.5−9 There is a question
resonating in the community working on polymeric solar cells:
can we replace the fullerene with electron-deficient polymers?
Although the electron donating polymers have been extensively
studied and can generate power conversion efficiency (PCE)
over 7% routinely in small area devices in combination with
fullerenes,10−15 it is still challenging to find n-type polymers or
organic molecules that rival the electronic properties of
fullerenes. Both intellectual curiosity and economical reason
lead us to explore new, more cost-effective and efficient
Although the FET mobility of these polymers already
surpasses that of fullerene derivatives, there are many other
factors that influence the photovoltaic performances. A crude
design idea for the electron acceptors is that it must exhibit
Received: March 3, 2014
Revised: May 6, 2014
© XXXX American Chemical Society
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proper energy levels to match with the electron donor
Scheme 1. Synthetic Routes to the Synthesized Monomers
polymers, ideally similar to those of fullerene. Studies from
D
polymer/fullerene BHJ solar cells indicated that the ELUMO
−
A
ELUMO offset should be around 0.3−0.8 eV for efficient photo
induced charge transfer at the donor−acceptor interface.33,34 In
addition, the polymers must exhibit the proper absorption
bandgap to effectively harvest solar energy. It is also crucial to
achieve a nanoscale, bicontinuous, and interpenetrating net-
work of the donor and acceptor phases in order to facilitate
charge separation and transport.35
In this work, we attempt to further develop the general
design guidelines for synthesizing new electron accepting
polymers. A major focus is to figure out how to control and
fine-tune the energy levels and electron accepting properties.
First, we developed new electron accepting monomers, 4,10-
bis(2-butyloctyl)thieno[2′,3′:5,6]pyrido[3,4-g]thieno[3,2-c]-
isoquinoline-5,11-dione (TPTI) and fluorenedicyclopentathio-
phene dimalononitrile (CN).36 Based on these new monomers,
we have synthesized a series of alternating accepting polymers
with the following monomer−co-monomer combination: (a)
weak donating monomer−strong accepting monomer (WD-
SA), (b) weak accepting monomer−strong accepting monomer
(WA-SA), (c) weak accepting monomer−weak accepting
monomer (WA-WA), and(d) strong donating monomer−
strong accepting monomer (SD-SA). We investigated the
physical, optical, and electrochemical properties and the
electron transporting and photovoltaic characteristics of the
resultant polymers to extract a close relationship between each
characteristic and polymer structure. Several criteria for
designing electron accepting polymers for all-polymer solar
cells are suggested.
facilitating efficient π-electron delocalization and intermolecular
order. To enhance the solubility of the resulting polymers, a
fluorene moiety with the two alkyl side chains at the C-9
position is introduced.
The synthetic approaches to the two new monomers are
outlined in Scheme 1, and their detailed synthetic procedure is
described in the Supporting Information (SI). Starting from the
commercially available N-Boc-3-aminothiophene, compound 2
was synthesized by alkylation with 2-butyloctyl bromide using
sodium hydride. The Boc functional group in compound 2 was
easily deprotected by trifluoroacetic acid (TFA) to afford
compound 3. Without further purification, compound 3 was
reacted with acid chloride 4, generated in situ from the reaction
of 2,5-dibromoterephthalic acid with thionyl chloride, to give
the amide compound 5. Direct intramolecular arylation of
compound 5 was performed via C−H bond activation with
KOAc as the base and Pd(PPh3)4 as the catalyst at 120 °C in
dimethylacetamide (DMA). Under diluted condition (2 mM),
the arylation of compound 5 was completed within 6 h to
generate compound 6 with a yield higher than 80%. The final
monomer 7 was synthesized by stannanylation with trimethyl-
tin chloride after lithiation of compound 6 with lithium
diisopropylamide (LDA) at −78 °C. The 2,2′-(9,9-bis(2-
butyloctyl)-9H-fluorene-2,7-diyl)dithiophene-3-carboxylic acid
(11) was synthesized according to the literature.37 Bromination
of compound 11 with bromine under acid condition yielded
compound 12. The carboxylic acid of compound 12 was
converted into acyl chloride using thionyl chloride, which was
converted into compound 13 via intramolecular acylation.
The introduction of bromo functionality into the 2 and 2′
positions of compound 12 prevented intermolecular acylation
of compound 12, resulting in compound 13 (>80%).
Compound 14 (CN) was synthesized by Knoevenagel
condensation using malononitrile and TiCl4 under basic
condition. Monomer 13 (CO) containing a carbonyl group
exhibits weaker electron withdrawing property than monomer
14 (CN). EDOT is a stronger electron donating moiety than
BDT. Through the combination between two electron
donating monomers (EDOT and BDT) and two electron
accepting monomers (CO and CN), four kinds of D−A
alternating copolymers (PBCO, PBCN, PECO, and PECN)
were synthesized.
RESULTS
■
Synthesis and Structural Characterization. The initial
design strategy to n-type semiconducting polymers involves
considerations of the two issues. First of all, we need
compounds that can be converted into distannanyl monomers
for the Stille polycondensation, which will exclude many
electron-deficient compounds whose distannanyl derivatives
exhibit a limited reactivity for polymerization. Second, we need
to ensure that the energy levels of materials (highest occupied
molecular orbital/lowest unoccupied molecular orbital
(HOMO/LUMO)) are lowered enough to match with those
in donor polymers. The following molecular architectures of
accepting polymers were conceived with repeating units: (a)
WD-SA, (b) WA-SA, (c) WA-WA, and (d) SD-SA. Those with
SA-SA are not pursued here due to the limitation in
polymerization. The structures of the new monomers
(compounds 6 and 14) designed for these purposes are
shown in Scheme 1. Specifically, the new cyclic diamide
monomer, TPTI, shows a highly planar structure facilitating
intermolecular ordering. The end thiophene structures enable
further functionalization at the 2-position of the thiophenes to
introduce distannanyl groups (compound 7). The two electron
withdrawing carbonyl moieties enhance the electron affinity of
the conjugated system, resulting in a weak accepting monomer.
The alkyl groups allow tuning of the solubility of the rigid
conjugated backbone in organic solvents.
SW and TPD have similar molecular structure. The TPD
unit has an electron withdrawing imide group, while SW has a
stronger electron withdrawing sulfonimide unit. NDI and PDI
are well-known electron accepting units containing diimide
functionality. SF, BT, and NT have electron-deficient disulfone,
thiadiazole, and dinitro groups, respectively. By combining
these electron-accepting co-monomers (TPD, SW, SF, BT, NT,
A total of eight dibromo monomers, all electron-deficient,
were synthesized and co-polymerized with three distannanyl
monomers: monomer 7 (WA), BDT (WD), and EDOT (SD).
The CN (compound 14) monomer exhibits strong electron
accepting properties and has a rigid conjugated backbone,
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Scheme 2. Synthetic Scheme for the Electron Accepting Polymer
Table 1. Physical Properties of the Polymers
PNPDI
PNNDI
PNSF
PNSW
PNCN
PNBT
PNNT
PNTPD
PBCO
PBCN
PECO
PECN
a
Mr
1720
18 100
39 200
2.17
1147
21 800
49 400
2.27
1167
958
1474
28 800
55 100
1.91
793
831
922
1161
13 200
28 700
2.18
1257
857
953
b
c
Mn
16 300
28 800
1.77
11 500
18 500
1.61
15 700
38 700
2.46
13 600
25 200
1.86
10 900
18 000
1.65
14 400
40 600
2.81
18 700
43 100
2.31
21 700
52 900
2.43
Mw
PDI
d
Td , °C
387
331
353
420
358
423
339
409
382
368
381
363
a
b
c
d
Molecular weight of repeating unit. Number-average molecular weight. Weight-average molecular weight. Decomposition temperature
determined by TGA under N2 based on a 5% weight loss.
NDI, CN, and PDI) with the new cyclic amide monomer 7,
eight TPTI-based electron accepting polymers having varied
electron withdrawing strengths were synthesized.
functional theory (DFT) using the B3LYP functional and a 6-
31G* basis set.38−40 The calculated structures of the
compounds are shown in Figure S11 (SI), and the dihedral
angles at the junction of two monomers in the polymer
backbones are summarized in Table 2. The new electron
accepting monomers, TPTI and CN, exhibited completely
planar backbone structures, facilitating intermolecular ordering
in the film state. CO- and CN-based polymers showed a planar
polymer backbone with dihedral angle less than 10° due to the
small steric hindrance between two thiophene units at the
bonding sites of two monomers. However, PNSF and PNSW
containing sulfone functionality exhibited a twisted structure
with dihedral angle of ∼20° due to the bulky sulfone
functionality. PNNT containing bulky dinitro thiophene units
also showed distorted backbone structure similar to the sulfone-
based polymers. PNPDI exhibited a highly distorted backbone
structure with a dihedral angle of 48° due to the strong steric
hindrance between two hydrogens at the bonding site of TPTI
and PDI units. Although it was pointed out that all of the
polymers twisted up to about 40° should not have much
influence on the polymer conjugation,41 the twisted polymer
The detailed synthetic routes and structures of the final
polymers are shown in Scheme 2. The number-average
molecular weights (Mn) of the resulting polymers, determined
by gel permeation chromatography (GPC) with polystyrene as
standard, are shown in Table 1. It was found that the TPTI-
based polymers exhibited excellent solubility in common
organic solvents (i.e., THF, dichloromethane, chloroform,
chlorobenzene, and dichlorobenzene). On the other hand,
PBCO, PBCN, PECO, and PECN polymers only dissolve in a
limited number of organic solvents (i.e., chloroform, chlor-
obenzene, and dichlorobenzene). The thermal stabilities of the
polymers were evaluated by thermogravimetric analysis (TGA)
under N2 atmosphere. All polymers exhibited good stability,
showing less than 5% weight loss up to 331−420 °C (Figure
S10, SI).
Polymer Backbone Structure. The ground-state (S0)
geometric structure of the monomers and the four repeating
units of the synthesized polymers were optimized with density
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structures do affect the molecular ordering in the film states,
and thus, charge transport behaviors.
Optical Properties. The UV−vis absorption and photo-
luminescence (PL) emission spectra of the polymers in
chloroform (CF) and in films are shown in SI (Figures S14
and S15) and summarized in Table 2. The synthesized
polymers exhibited broad absorption covering the entire visible
region. The optical bandgap of polymers, estimated by
measuring the absorption edge in UV−vis absorption spectra
in films, was increased in the following order: PNPDI (1.56 eV)
< PNNDI (1.59 eV) < PNSF (1.61 eV) < PNBT (1.63 eV) <
PNNT (1.81 eV) < PNSW (1.83 eV) < PNTPD (1.88 eV).
The CN-based polymers (PBCN, PECN, and PNCN)
exhibited more red-shifted absorption maxima with a longer
wavelength absorption tail than CO-based polymers (PBCO
and PECO) since the dicyanovinyl group is a much stronger
electron withdrawing moiety than the carbonyl unit.
In a diluted chloroform solution of equal concentration
(concentration, 5 0.1 × 10−6 M, based on the repeating units
of the polymers), the molar absorption coefficients of polymers
were determined from the absorption maxima. Polymers
PNCN, PBCN, and PECN exhibited much higher molar
absorption coefficients due to the highly extended conjugated
backbone of the CN monomer. All polymers exhibited molar
absorption coefficients over 30,000 L/(mol·cm).
PL emissions of these polymers were measured in solution
with excitation at the wavelength of maximum absorption
except for the solutions of polymers PNTPD, PNSW, PNSF,
PNBT, PNPDI, and PNNDI, whose emission spectra are
largely overlapped with their absorption in solution. These
solutions were excited at the wavelengths of 463, 454, 545, 545,
622, and 658 nm, respectively. Most of the polymers exhibited
red-shifted emission in the film compared to those in solution
due to the enhanced intermolecular π−π stacking in the film
state. However, the emission of PNPDI and PNNDI is almost
similar both in solution and in the film state due to the highly
twisted backbone structures of the two polymers preventing
them from intermolecular interaction in the film states.
Electrochemical Properties. The electrochemical proper-
ties of the synthesized monomers and polymers were
investigated by using cyclic voltammetry (CV) and summarized
in Table 3. The cyclic voltammograms are shown in SI (Figures
S16 and S17). The HOMO and the LUMO energy levels of the
monomers and polymers were calculated from the oxidation
and reduction onset potentials relative to ferrocene (as an
internal standard).42,43 The general trend is that LUMO energy
levels of monomers was increased in the order of PDI, NDI
(functionality: diimide), and NT (nitro) < CN (dicyano) < SF
(disulfone) < SW (sulfonimide) < BT (benzothiadiazole) <
CO (dicarbonyl) < TPD (imide) < TPTI (diamide). The
diimide functionality is a much stronger electron withdrawing
unit than imide, resulting in lower LUMO energy levels of PDI
and NDI (−3.96 eV) than that of TPD (−3.11 eV). Since the
sulfone functionality is a stronger electron accepting unit than
the imide moiety, the LUMO energy level of SW bearing a
sulfonimide unit was lower than that of TPD having an imide
unit. SF having more sulfone units exhibits a lower LUMO
energy level than SW. The new diamide monomer, TPTI,
exhibited relatively higher LUMO energy levels of −2.95 eV
than TPD having an imide group, thus a weak electron
accepting monomer. The LUMO energy level of CN (−3.84
eV) was lower than those of CO and SF monomers.
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Table 3. Electrochemical Properties of the Polymers and Monomers
P-doping (V vs Ag/Ag+)
N-doping (V vs Ag/Ag+)
P-doping (V vs Ag/Ag+)
N-doping (V vs Ag/Ag+)
a
b
c
a
b
c
polymers
Eox/onset
EHOMO (eV)
Ered/onset
ELUMO (eV)
Eg
monomers
Eox/onset
EHOMO (eV)
Ered/onset
ELUMO (eV)
Eg
PBCN
PBCO
PECN
PECO
PNBT
PNNT
PNTPD
PNSW
PNSF
0.74
0.74
0.61
0.44
0.68
1.07
0.98
0.94
0.96
1.03
1.01
1.02
−5.45
−5.45
−5.32
−5.15
−5.39
−5.78
−5.69
−5.65
−5.67
−5.74
−5.72
−5.73
−0.84
−1.10
−0.86
−1.30
−1.29
−0.89
−1.47
−1.34
−1.07
−0.80
−0.80
−0.67
−3.87
−3.61
−3.85
−3.41
−3.42
−3.82
−3.24
−3.37
−3.64
−3.91
−3.91
−4.04
1.58
1.84
1.47
1.74
1.97
1.96
2.45
2.28
2.03
1.83
1.81
1.69
CN
CO
BT
1.23
1.12
1.84
2.20
1.95
1.76
1.18
1.58
1.63
1.01
−5.94
−5.83
−6.55
−6.91
−6.66
−6.47
−5.89
−6.29
−6.34
−5.72
−0.87
−1.29
−1.24
−0.74
−1.60
−1.16
−0.90
−0.75
−0.75
−1.76
−3.84
−3.42
−3.47
−3.97
−3.11
−3.55
−3.81
−3.96
−3.96
−2.95
1.99
2.52
3.08
2.94
3.55
2.92
2.08
2.33
2.38
2.77
NT
TPD
SW
SF
NDI
PDI
TPTI
PNCN
PNNDI
PNPDI
a
b
HOMO levels were determined from the Eonset of the first oxidation potential of ferrocene, −4.8 eV. LUMO levels were determined from the Eonset
of the first reduction potential of ferrocene, −4.8 eV. The electrochemical bandgap determined by cyclic voltammetry.
c
The difference in the LUMO energy levels of those
monomers brought a significant effect on the LUMO levels
of TPTI-based polymers. LUMO energy levels of these
polymers were increased in the following order: PNPDI
(−4.04 eV) < PNNDI, PNCN (−3.91 eV) < PNNT (−3.82
eV) < PNSF (−3.64 eV) < PNBT (−3.42 eV) < PNSW (−3.37
eV) < PNTPD (−3.24 eV). As shown in Figure 1, the LUMO
the case of EDOT-based polymers, the HOMO energy levels of
PECN and PECO were significantly decreased to −5.32 and
−5.15 eV, respectively, compared to that of EDOT monomer
(−4.60 eV). The frontier orbitals of the monomers and the
several repeating units of polymers were calculated using the
B3LYP function with basis set 6-31G*, and the results are
shown in SI (Figures S11 ans S12). The HOMOs of the TPTI-
based polymers were highly localized in the amide building
block, which means that the HOMOs of the TPTI-based
polymers are quite similar to that of the TPTI building block.
On the other hands, the LUMOs of the TPTI-based polymers
were largely localized in the electron accepting co-monomers. It
also indicates that the LUMOs of the TPTI-based polymers
resemble those of the stronger electron withdrawing co-
monomers. In addition, the LUMOs of polymers containing
strong electron accepting co-monomers (PDI, NDI, CN, and
SF) have electron density mainly localized on strong electron
acceptor units. As a result, those four polymers exhibited almost
identical LUMO energy levels with co-monomers. Therefore,
the HOMO energy levels of the TPTI-based polymers were
determined by a TPTI monomer and the LUMO energy levels
by the electron accepting co-monomers.
EDOT-based polymers exhibited similar electron distribution
with TPTI-based polymers in LUMOs; electron density is
highly withdrawn to the electron accepting moieties; however,
in HOMOs, electrons are well-delocalized along the backbone
rings. Therefore, the HOMO energy levels of SD-SA-type
polymers (PECO and PECN) are determined by both the
electron donating and accepting monomers, consistent with
experimental results.
FET Studies and Mobility. The carrier mobility of the
polymers was measured using two types of device structure:
FET devices using a top-contact geometry (Figure 2) and
sandwich-type devices for SCLC measurements as reported
elsewhere.44 The results are summarized in Table 2. The
microstructure of neat polymer films was characterized by using
grazing incidence wide-angle X-ray scattering (GIWAXS). Two
dimensional (2D) GIWAXS patterns of the neat polymers are
shown in Figure 3, and their out-of-plane and in-plane lincuts
are shown in the SI (Figure S18). In the FET devices, TPTI-
based polymers exhibited much higher electron mobilities than
BDT- and EDOT-based polymers. PECO polymer containing
strong electron donating EDOT and weak electron accepting
CO monomer exhibited only p-type characteristics due to the
Figure 1. The electrochemical bandgap diagram of the monomers and
the TPTI-based polymers. The colored lines in the middle column
represents the energy levels of the polymers result from copolymer-
izing TPTI with the corresponding monomers on the right.
energy levels of TPTI-based polymers were determined by
those of the electron accepting co-monomers. The strong
electron accepting monomers effectively decreased the LUMO
energy levels of polymers. All of the TPTI-based polymers
exhibited similar HOMO energy levels of −5.65 to −5.78 eV
regardless of the difference in electron accepting properties of
co-monomers, which are similar to that of the TPTI monomer
(−5.72 eV).
The BDT-based polymers also exhibited similar trends with
TPTI-based polymers. PBCO and PBCN exhibited identical
HOMO energy levels of −5.45 eV, which is also similar to the
HOMO energy levels of BDT (−5.51 eV). The LUMO energy
levels of PBCN and PBCO are −3.87 and −3.61 eV,
respectively, which are similar to the LUMO energy levels of
CN (−3.84 eV) and CO (−3.42 eV), respectively. However, in
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polymers exhibited electron mobilities in the range of (2−9) ×
10−4 cm2/(V·s).
However, the SCLC electron mobilities of the polymers
showed a significantly different trend from those in FET
devices. PNBT, PNCN, and PNTPD exhibited FET mobilities
of (6−8) × 10−4 cm2/(V·s), but SCLC mobilities of (2−5) ×
10−5 cm2/(V·s). On the other hand, PNPDI exhibited a higher
SCLC mobility of 2.7 × 10−4 cm2/(V·s), but no FET mobility.
PNNDI and PNSW showed similar FET and SCLC mobilities
of around 10−4 cm2/(V·s). GIWAXS results indicated that
PNBT and PNCN showing the ordering with a d-spacing of
∼3.5 Å in the edge-on mode exhibited a sizable FET mobility.
The polymers exhibiting the face-on orientation of conjugated
backbone (PNPDI, PNNDI, and PNSW) showed relatively
high SCLC mobilities compared to the other polymers. Since
the SCLC devices require a vertical electron transport pathway
from the bottom electrode, the ordering toward perpendicular
direction should promote electron transport in the SCLC
devices. CN-based polymers exhibited higher SCLC mobilities
than CO-based polymers due to the better ordering with a d-
spacing of ∼3.9 Å in the face-on mode, as shown in Figure 3.
Photovoltaic Effect. To examine the photovoltaic proper-
ties of the electron accepting polymers, several materials were
used as the electron donor in all-polymer solar cells, namely,
PTB7, P3HT, and benzoporphyrin (BP). The BHJ photo-
voltaic devices were fabricated with the following configuration:
ITO/PEDOT:PSS/electron donors:electron accepting poly-
mers/Ca/Al. The photovoltaic characteristics were measured in
a glovebox under nitrogen atmosphere. The J−V characteristics
of the devices (under AM 1.5G condition, 100 W/cm2) of the
PTB7:accepting polymer mixtures are shown in Figure 4a, and
Figure 2. Transfer characteristics of the polymers showing (a)
unipolar n-type and (b) ambipolar behavior in the saturation regime
for FETs fabricated by spin-coating electron accepting polymers on
octadecyltrichlorosilane (ODTS)-treated SiO2/Si substrates with
device structure shown in the top left corner, with the exception of
the PECO device utilizing Au as electrodes.
Figure 4. (a) J−V characteristics and (b) external quantum efficiency
(EQE) of BHJ photovoltaic devices with an active layer composed of
PTB7:electron accepting polymers. J−V characteristics of (c)
P3HT:accepting polymers BHJ devices and (d) BP:polymers bilayer
devices under simulated AM 1.5 G solar irradiation.
Figure 3. 2D GIWAXS patterns for the neat electron accepting
polymers.
the corresponding photovoltaic parameters are summarized in
Table 4. Detailed studies of the J−V characteristics as a function
of the weight ratio between donor and acceptor are shown in
Figure S19 (SI).
Among the synthesized 12 electron accepting polymers, only
four polymers exhibited photovoltaic properties. PNPDI
exhibited the highest PCE value of 1.03% (Voc = 0.69 V, Jsc =
−4.13 mA/cm2, and FF = 0.36) at a weight ratio of
dominant electron donating properties of EDOT. PNCN,
PNSF, PNBT, and PECN exhibited ambipolar field-effect
behaviors, and the other polymers showed n-type field-effect
characteristics. Among the TPTI-based polymers, PNSF
exhibited the highest FET electron mobility of 2.3 × 10−3
cm2/(V·s) while showing ambipolar behavior with a hole
mobility of 9.1 × 10−4 cm2/(V·s). The other TPTI-based
F
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Table 4. Summary of Photovoltaic Properties
a
active layer
device type
Jsc (mA/cm2)
Voc (V)
fill factor
PCE (PCEmax) (%)
PTB7:PNSW (1:1, w/w)
BP:PNSW
BHJ
−1.46 0.01
−1.10 0.02
−0.42 0.02
−0.34 0.06
−2.28 0.10
−1.19 0.07
−0.46 0.01
−4.13 0.09
−4.19 0.14
−1.63 0.08
0.93 0.02
0.86 0.01
0.76 0.01
0.46 0.01
0.71 0.01
0.45 0.01
0.45 0.01
0.69 0.01
0.42 0.01
0.43 0.01
0.26 0.01
0.32 0.003
0.32 0.004
0.29 0.004
0.27 0.01
0.28 0.005
0.20 0.003
0.36 0.004
0.43 0.005
0.57 0.004
0.35 0.02 (0.37)
0.30 0.01 (0.31)
0.10 0.01 (0.11)
0.04 0.01 (0.05)
0.43 0.03 (0.46)
0.15 0.01 (0.16)
0.04 0.01 (0.05)
1.03 0.03 (1.06)
0.75 0.04 (0.79)
0.40 0.01 (0.41)
bilayer
BHJ
PTB7:PNCN (1:1.5, w/w)
P3HT:PNCN (1:1.5, w/w)
PTB7:PNNDI (1:1, w/w)
P3HT:PNNDI (1:1.5, w/w)
BP:PNNDI
BHJ
BHJ
BHJ
bilayer
BHJ
PTB7:PNPDI (1:1.8, w.w)
P3HT:PNPDI (1:1.5, w.w)
BP:PNPDI
BHJ
bilayer
a
Average value from four to eight devices.
PTB7:PNPDI = 1:1.8. PNNDI and PNSW showed PCE of
0.43% (Voc = 0.71 V, Jsc = −2.28 mA/cm2, and FF = 0.27) and
0.35% (Voc = 0.93 V, Jsc = −1.46 mA/cm2, and FF = 0.26),
respectively, at a weight ratio of PTB7:PNNDI or PNSW = 1:1.
The electron accepting polymers exhibiting a SCLC mobility
less than 10−4 cm2/(V·s) did not exhibit any noticeable
photovoltaic performance when they were combined with
PTB7 that exhibits a hole mobility of ∼10−3 cm2/(V·s). The
polymers showing SCLC mobilities higher than 10−4 cm2/(V·s)
exhibited photovoltaic properties. There is no clear relationship
between the FET mobility and the photovoltaic performance. It
can be expected that if the electron mobilities of the accepting
polymers are increased to or above the level of ∼10−3 cm2/(V·
s), well-balanced with that of electron donor materials, much
higher FF and Jsc values in photovoltaic devices can be achieved
by reducing the charge recombination and space charge
formation as well as increasing exciton dissociation and carrier
transport.
Lastly, PNCN exhibited the lowest PCE value of 0.10% (Voc
=
0.76 V, Jsc = −0.42 mA/cm2, and FF = 0.32) at a weight ratio of
PTB7:PNCN = 1:1.5. The external quantum efficiency (EQE)
curves of PTB7:accepting polymer devices are shown in Figure
4b, which are in good agreement with the Jsc values. The open
circuit voltage is linearly dependent on the HOMO level of the
donor (p-type semiconductor quasi Fermi level) and the
LUMO level of the acceptor (n-type semiconductor quasi
Fermi level).45 As a result, the Voc was increased in the order of
PNPDI (−4.04 eV) < PNNDI (−3.91 eV) < PNCN (−3.91
eV) < PNSW (−3.37 eV). However, all of the solar cells
exhibited relatively small fill factors, less than 0.36. In general,
fill factor is determined by the series resistance (Rs), shunt
resistance (Rsh), and diode parameters (n and JS), all of which
are the function of charge charier mobility.46 The dominating
contribution to the Rs is the large resistivity of the organic
layers. Space charge formation and bimolecular recombination
from the carrier with low mobility induce charge carriers’ loss
and increase in Rsh.47 Therefore, both cases require the high
mobility of the electron accepting polymers to improve the FF
in the photovoltaic devices. When P3HT was used as an
electron donor, the Voc of the devices was significantly
decreased due to the higher HOMO energy level (∼5.0 eV)
of P3HT than that of PTB7 (−5.3 eV). Interestingly, thermal
annealing P3HT:PNPDI device resulted in higher FF and Jsc
values than PTB7:PNPDI devices. It is likely that the annealing
process enhanced the ordering of P3HT in the blended films,
which facilitated the formation of a nanoscale interpenetrating
network between donor and acceptor, enhancing charge
transport. We have also fabricated the bilayer devices using
BP as the electron donor. The donor layer was formed by
solution process from a soluble precursor, tetraethanotetra-
benzoporphyrin (CP), and then thermally converted in the
insoluble semiconducting BP film. BP:PNPDI bilayer device
exhibited a highest FF value of 0.57 compared to the other
devices. It is expected that the nongeminate (bimolecular)
recombination in the bilayer devices are minimized compared
to that in the BHJ devices, because the hole (or electron)
already transported to the electron donor (or acceptor) cannot
recombine with electron (or hole).48,49 BP:PNSW bilayer
devices also exhibited a higher FF than PTB7:PNSW BHJ
devices similar to PNPDI devices. However, the bilayer devices
exhibited lower Jsc values than BHJ devices due to the limited
interfacial area between electron donor and acceptor.
DISCUSSION
■
Determination of HOMO and LUMO Levels of
Electron Accepting Polymers. Among the four polymer
repeating structures, (a) WD-SA (PBCN and PBCO(, (b) WA-
SA (PNPDI, PNNDI, PNCN, PNSF, and PNNT), (c) WA-WA
(PNTPD, PNSW, and PNBT), (d) SD-SA (PECN and
PECO), the SA determines the LUMO energy levels of the
polymers. The HOMO energy levels of those polymers with
WA-SA (e.g., the TPTI-based polymers) and WD-SA (e.g., the
BDT-based polymers) are mainly determined by WA and WD,
respectively. Both the theoretical simulation of the frontier
orbitals of polymers and electrochemical studies support the
conclusion. The polymers with the WD-SA (PBCO and
PBCN) structure exhibit rather low-lying LUMO energy levels,
but their HOMO energy levels are too high due to the high-
lying HOMO energy level of the BDT monomer (WD), and
the electron mobilities are relatively poor.
An interesting observation of the consequence of low LUMO
energy levels is the relationship between LUMO energy levels
and fluorescence quantum yields (QYs) of the polymers
containing TPTI. The fluorescence QY of the polymers was
quantified in a diluted solution; the intensity at the absorption
maxima was kept at 0.03 to prevent the aggregation effect.
Fluorescein was used as a reference fluorescence dye, whose
QY in ethanol is known to be 0.79.50 The QYs of the polymers
as listed in Table 2 are significantly changed depending on the
electron accepting moiety. The QYs of the polymers are
nonlinearly reduced as the LUMO energy levels of polymers
are decreased (Figure 5). Since the LUMO energy levels of
TPTI-based polymers are highly related with the electron
withdrawing properties of the co-monomers, it is expected that
the stronger electron accepting co-monomers quenched the PL
emission of the polymers more effectively due to internal
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SCLC mobility plays a much more important role than FET
mobility for determining the photovoltaic performance. Overall,
this work offers a new outlook for developing better electron
accepting materials by providing an in-depth study of
structure−property relationships.
EXPERIMENTAL SECTION
■
Materials. 1,3-Dibromo-5-(2-ethylhexyl)-4H-thieno[3,4-c]pyrrole-
4,6(5H)-dione (TPD),51 1,3-dibromo-5-(2-ethylhexyl)thieno[3,4-d]-
isothiazol-3(2H)one-1,1-dioxide (SW),52 4,7-dibromobenzo[c][1,2,5]-
thiadiazole (BT),53 2,5-dibromo-3,4-dinitrothiophene (NT),54 2,6-
bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]-
dithiophene (BDT),55 5,7-bis(trimethylstannyl)-2,3-dihydrothieno-
[3,4-b][1,4]dioxine (EDOT),564,9-dibromo-2,7-bis(2-ethylhexyl)-
benzo[lmn][3,8]-phenanthroline-1,3,6,8(2H,7H)-tetraone (NDI),57
and N,N′-bis(2-decyltetradecyl)-1,7-dibromo-3,4,9,10-perylene dii-
mide (PDI),58 were synthesized according to literature procedures,
and their structural characterization through the MALDI-TOF mass
and NMR spectroscopy was described in the SI. All of the chemicals
were purchased from Aldrich except for tetrakis(triphenylphosphine)
palladium from Strem Chemicals. All reagents purchased commercially
were used without further purification except for toluene and
tetrahydrofuran (THF), which were dried over sodium−benzophe-
Figure 5. Fluorescent quantum yield (QY) vs LUMO levels of the
TPTI-based polymers.
polarization. Both LUMO and HOMO energy levels in
accepting polymers should be sufficiently low enough to
enhance the driving force for charge separation from a donor
polymer to an accepting polymer. In addition, the internal
polarization between the two monomers is also beneficial to
charge separation. Indeed, PNPDI and PNNDI having LUMO
energy levels of −4.04 and −3.91 eV, respectively, exhibited
higher Jsc values than the other polymers.
Interestingly, the polymers containing a strong electron
accepting CN moiety did not exhibit any emission in film
states. In solution, CN-based polymers also exhibited the worst
QYs in the series of polymers. However, PNCN having deep
LUMO levels of −3.91 eV showed a significantly poor Jsc value
in the photovoltaic devices. The difference from other
accepting monomers is that CN monomer has the two
dicyanovinylene moieties functionalized in the same direction
of the conjugated backbone, which can generate large
polarization between the ground and excited states. Therefore,
the dicyanovinylene moieties can act as electron trapping sites.
In addition, the dicyanovinylenyl groups orthogonally attached
to the conjugated backbone can lead to localized LUMOs and
low fluorescent QYs of the CN-based polymers.38 Indeed, CN-
based polymers exhibited poor SCLC electron mobilities in the
range of 10−5−10−6 cm2/(V·s) compared with those of PNPDI
and PNNDI even though CN-based polymers are more planar
and exhibited very small dihedral angles and strong preference
to the face-on orientation.
1
none. H NMR and 13C NMR spectra were recorded on a Bruker
DRX-400 spectrometer, with tetramethylsilane as an internal reference.
Matrix-assisted laser desorption ionization time-of-flight (MALDI-
TOF) mass spectra were recorded using a Bruker Ultraflextreme
MALDI-TOF/TOF mass spectrometer with dithranol as the matrix.
Elemental analysis was performed by Midwest MicroLab. The
number- and weight-average molecular weights of the polymers were
determined by gel-permeation chromatography (GPC) with a Waters
Associates liquid chromatography instrument equipped with a Waters
510 HPLC pump, a Waters 410 differential refractometer, and a
Waters 486 tunable absorbance detector. THF was used as the eluent
and polystyrene as the standard. TGA measurement of the polymers
was performed using a TA Q600 instrument. UV−vis absorption
spectra were measured on a Shimadzu UV-3600 device. Cyclic
voltammetry was performed on an AUTOLAB/PG-STAT12 model
system with a three-electrode cell in a 0.1 N Bu4NBF4 solution in
acetonitrile at a scan rate of 50 mV/s. A film of each polymer was
coated onto a Pt wire electrode by dipping the electrode into a
polymer solution in chloroform. All measurements were calibrated
against an internal standard of ferrocene (Fc), the ionization potential
(IP) value of which is −4.8 eV for the Fc/Fc+ redox system. AFM
images were obtained by using an Asylum MFP-3D microscope. TEM
measurements were performed by using a Tecnai F-30 with an
accelerating voltage at 300 kV.
PSC Device Fabrication. Polymeric solar cells with a device
configuration of glass/indium tin oxide (ITO)/poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)
/PTB7, P3HT or BP:accepting polymers/Ca/Al were prepared.
Prior to device fabrication, the ITO substrates were cleaned with
detergent and ultrasonicated in deionized water, acetone, and
isopropanol and then dried overnight in an oven. The substrate was
spin-coated by a PEDOT:PSS solution and dried at 100 °C in N2 for
30 min, and then transferred to a glovebox for spin-casting of the
polymer layer. The solution (chloroform:1,8-diiodooctane = 98:2 (v/
v)) containing a blended mixture of PTB7/accepting polymers was
spin-casted by 6000 rpm, onto the above substrate. PTB7/acceptor
films were used directly without annealing process. The solution
(chloroform:p-xylene = 3:2 (v/v)) of P3HT/accepting polymers was
spin-coated by 2800 rpm and annealed at 150 °C for 30 min. In case of
bilayer devices, a soluble precursor, tetraethanotetrabenzoporphyrin
(CP) in chloroform (1.0 wt %) was spin-coated by 1500 rpm and then
thermally converted in the insoluble semiconducting BP film at 180 °C
for 10 min. Onto the BP substrate, the electron accepting polymer in
chloroform (0.8 wt %) was spin-casted by 2800 rpm. Subsequently, the
device was pumped down under vacuum (<10−6 Torr) and the Ca (20
nm) and Al (80 nm) electrode was deposited by thermal evaporation
CONCLUSION
■
We have developed new electron-deficient TPTI and CN
monomers and synthesized a series of alternating electron
accepting polymers containing different monomer combina-
tions: (a) WD-SA, (b) WA-SA, (c) WA-WA, and (d) SD-SA. It
was found that LUMO energy levels of the electron accepting
polymers are significantly determined by those of stronger
electron accepting monomers and the HOMOs are largely
decided by the weak accepting (or weak donating) monomers.
Fluorescent measurements indicated that the QYs of the TPTI-
based polymers are nonlinearly reduced as the LUMO energy
levels of polymers are decreased, which has a close relationship
with PCE of photovoltaic cells, implying internal polarization is
important for photovoltaic application. To control the energy
levels of both HOMO and LUMO and at the same time
introduce internal polarization, polymers with WA-SA
structures were found to be desirable. It was found that the
H
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in the glovebox at a chamber pressure of ∼5.0 × 10−7 Torr. The active
area of the solar cell is 3.14 mm2, which is defined by the cathode area.
Current density−voltage (J−V) characteristics of the devices under
nitrogen were measured using a Keithley 238 Source Measure unit.
The photovoltaic properties were characterized under an Air Mass 1.5
Global (AM 1.5G) solar simulator with irradiation intensity of 100
mW/cm2.
FET Device Fabrication. Heavily doped Si substrates coated with
thermally grown 300 nm SiO2 were used as gate substrates (Silicon
Inc.). The substrate was cleaned by piranha treatment followed by
multiple rinsing with DI water, and then treated with octadecyltri-
chlorosilane (ODTS). 0.2−0.3 wt % solutions of the polymers (in
chloroform) were spin-coated on the ODTS-treated Si substrates. The
polymer films were annealed at 180 °C for 10 min to improve their
crystallinity. To complete top-contact polymer FET devices, Al was
thermally evaporated through a shadow mask, which forms 100 nm
thick source/drain electrodes (channel length/width (W/L) = 50 μm/
(1500 μm)). FET transfer characteristics were measured in the
saturation regime, and the FET mobility was calculated using the
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ASSOCIATED CONTENT
■
S
* Supporting Information
Scheme showing synthesis of monomers, text describing the
syntheses, general procedure for polymerization, and mobility
measurements, figures showing NMR spectra of compounds 6,
7, 13, 14, NDI, EDOT, SW, and TDP, MALDI-TOF mass
spectra of compounds 6, 7, 13, and 14, TGA thermograms of
polymers, HOMO and LUMO diagrams of the four polymer
repeating units and of the monomers, HOMO and LUMO
energy level diagrams, absorption, cyclic voltammograms of the
monomers, linecuts of neat electron accepting polymers, plots
of J−V characteristics, and AFM images, and a table listing
electrochemical properties of select polymers. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
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Corresponding Author
*E-mail: lupingyu@uchicago.edu.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by a U.S. National Science Foundation
Grant (NSF DMR-1263006), the Air Force Office of Scientific
Research and NSF MRSEC program at the University of
Chicago (Grant DMR-0213745), Department of Energy
(DOE) via the ANSER Center, an Energy Frontier Research
Center funded by the U.S. Department of Energy, Office of
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