Control of vertical distribution of thiophene-based copolymers containing 4,7-Dithien-2-yl-benzo[C][1,2,5]thiadiazole and 3,6-Dithien-2-yl-pyrrolo[3,4-C]pyrrole-1,4(2H,5H)-dione as Side Groups for Photovoltaics

Article abstract of DOI:10.1002/pola.28157

Four new D—A type copolymers with 2D-conjugated side-chain identified PfToBT, PbToBT, PfTDPP and PbTDPP, containing two acceptors 4,7-dithien-2-yl-benzo[c][1,2,5]thiadiazole (DTBT), and diketopyrrolopyrrole (DPP) linked by thiophene donors, are obtained using Pd-catalyzed Stille-coupling reaction. These polymers show a broad visible-near-infrared absorption band (E_g?=?1.79–1.66?eV) and possess a relatively low-lying HOMO level at ?5.34 to ?5.12?eV. All the polymer:PC₇₀BM blend films showed edge-on structure and have similar d_π-spacing values. According to the structure of conjugated side-chain, the vertical distributions of polymer chains and PC₇₀BM within the BHJ (bulk heterojunction) were different. When DPP used as an acceptor, conjugated side chains of the polymer coexisted with PC₇₀BM in same position. The BHJ film prepared from PfToBT, PbToBT had a discontinuous network between polymer and PC₇₀BM, whereas films from PfTDPP and PbTDPP formed continuous and evenly distributed network between them. This optimized vertical morphology promotes hole transport along respective pathways of polymers and fullerenes in the vertical direction, leading to high J_SC. PbTDPP shows PCE up to 2.9% (Jsc of 9.4?mA/cm², Voc of 0.68 V, and FF of 0.44).

Full text of DOI:10.1002/pola.28157

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Control of Vertical Distribution of Thiophene-Based Copolymers Containing
4,7-Dithien-2-yl-benzo[C][1,2,5]thiadiazole and 3,6-Dithien-2-yl-pyrrolo[3,4-C]
pyrrole-1,4(2H,5H)-dione as Side Groups for Photovoltaics
Min-Hee Choi, Tae Ho Lee, Yong Woon Han, Doo Kyung Moon
Department of Materials Chemistry and Engineering, Konkuk University, 1 Hwayang-Dong, Gwangjin-Gu, Seoul 143-701, Korea
Correspondence to: D. K. Moon (E-mail: dkmoon@konkuk.ac.kr)
Received 21 March 2016; accepted 27 April 2016; published online 00 Month 2016
DOI: 10.1002/pola.28157
ABSTRACT: Four new D—A type copolymers with 2D-
conjugated side-chain identified PfToBT, PbToBT, PfTDPP and
PbTDPP, containing two acceptors 4,7-dithien-2-yl-ben-
zo[c][1,2,5]thiadiazole (DTBT), and diketopyrrolopyrrole (DPP)
linked by thiophene donors, are obtained using Pd-catalyzed
Stille-coupling reaction. These polymers show a broad visible-
near-infrared absorption band (Eg 5 1.79–1.66 eV) and possess
a relatively low-lying HOMO level at 25.34 to 25.12 eV. All the
polymer:PC70BM blend films showed edge-on structure and
have similar dp-spacing values. According to the structure of
conjugated side-chain, the vertical distributions of polymer
chains and PC70BM within the BHJ (bulk heterojunction) were
different. When DPP used as an acceptor, conjugated side
chains of the polymer coexisted with PC70BM in same position.
The BHJ film prepared from PfToBT, PbToBT had a discontinu-
ous network between polymer and PC70BM, whereas films
from PfTDPP and PbTDPP formed continuous and evenly dis-
tributed network between them. This optimized vertical mor-
phology promotes hole transport along respective pathways of
polymers and fullerenes in the vertical direction, leading to
high JSC. PbTDPP shows PCE up to 2.9% (Jsc of 9.4 mA/cm2,
C
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Voc of 0.68 V, and FF of 0.44). 2016 Wiley Periodicals, Inc. J.
Polym. Sci., Part A: Polym. Chem. 2016, 00, 000–000
KEYWORDS: bulk heterojunctions; conjugated polymers; D-A
type polymer; organic solar cells; orientation; phase separa-
tion; two-dimensional conjugation; vertical distribution
region by identifying the relationship between the structure
of the conjugated backbone and photovoltaic property.4–7
Although the PCE of conventional PSCs is already around
7%, further optimization of various characteristics is neces-
sary to gain better performance.8 The inverted PSCs have
significantly improved long-range air stability, giving rise to
PCE values greater than 10% in comparison with conven-
tional PSCs.9,10 Certainly, both a sensible moleculer design
and the optimization of device physics are crucial in the
development of high-efficiency PSCs.11,12
INTRODUCTION Recently, polymer solar cells(PSCs) have been
to the fore at academic and industrial world as promising alter-
natives to inorganic photovoltaics because of their lightweight,
low manufacturing costs, flexibility, and the possibility of large
scale production via solution processing.1 Bulk heterojunction
(BHJ) system, which is a blend structure of an electron donor
(i.e., conjugated polymer) and an electron-acceptor (i.e., the full-
erene), has been widely researched to that end.2,3 Conditions of
a best polymer as the donor in PSCs should be possessed are
acknowledged well: low bandgap, high absorption in the visible
and near-infrared regions, appropriate molecular orbital levels
that line up with the electron-acceptor, high charge mobility, and
excellent miscibility with the electron-acceptor that forms a max-
imizing the interface between the electron-donor and electron-
acceptor network. Judiciously coordinating and managing the
parameters of the polymer donor can achieve high-efficiency of
the PSCs due to high values of short-circuit current density (Jsc),
open-circuit voltage (Voc), and fill factor (FF).
Two-dimensional (2D)-conjugation and D–A concept have
been confirmed a successful theories in development of
high-performance electron-donor polymers. 2D-conjugated
polymers, such as polythiophenes containing conjugated side
chains reported by Li and Tang’s group, show low bandgap,
higher charge mobility and relatively low-lying HOMO levels,
which are suitable properties for high performance conju-
gated polymer as an electron-donor for PSCs.13–15 A new
series of polymer donors containing conjugated side chains
has been successfully achieved in the past few years through
The fundamental design of narrow band gap (NBG) polymer
is donor–acceptor (D–A) architecture with broad absorption
Additional Supporting Information may be found in the online version of this article.
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FIGURE 1 Molecular structures of PfToBT, PbToBT, PfTDPP, and PbTDPP.
the efforts of several different research groups. In a previous
publication, Tan et al. reported a 2D-conjugated D–A copoly-
mer, PTG1, using 4,7-dithien-2-yl-2,1,3-benzodiathiazo-
le(DTBT) as electron-acceptor and conjugated side chains.
PTG1 showed great hole mobility and a best PCE up to
ꢀ5%.16,17 However, its comparatively narrow absorption
region and weak absorption coefficient restrict any more
enhancement of its photovoltaic performance. Li et al.
attached fluorine as a substituent group on the acceptor to
expand the absorption region, but the electron withdrawing
effect of the fluorinated acceptor unit was very small
(Eg ꢀ 1.90 eV). The fluorinated acceptor also limited solubil-
ity and molecular weight of the polymer.13 Majority of these
D–A polymers containing conjugated side-chain do not show
equivalent photovoltaic properties to their counterpart poly-
mers which have same main-chain components. In order to
improve performance of D–A polymers containing conjugated
side-chain, it is needed to develop new, stronger conjugated
side chains for decreasing the band gap of such polymers.
HOMO energy level and increased hole mobility. Based on
the aforementioned points, we designed and synthesized
four copolymers of new 2D D–A type copolymers with conju-
gated side-chain labeled PfToBT, PbToBT, PfTDPP, and
PbTDPP (as shown in Fig. 1), with thiophene derivatives
such as thieno[3,2-b]thiophene and 2,2’-bithiophene as a
donor and thiophene containing different conjugated side-
groups as an acceptor. To enhance the absorption region of
these polymers, DTBT and DPP as electron-accepting groups
are adopted into the side-groups. We attached alkoxy and
branched 2-ethylhexyl groups on the acceptor to promote
the solubility of the resulting polymers. The conjugated side-
chain groups were connected to the main-chain using the 2-
vinylene group. The vinylene connector eliminates torsional
strains between the main-chain and electron-accepting side-
chain. It helps to extend the p-conjugation and improve the
coplanarity of the polymer backbone, leading to a reduced
optical bandgap and adjustable energy levels of frontier
molecular orbital. The effects of the conjugated side chains
on the thermal, photophysical, electrochemical, and photovol-
taic properties of the copolymers were also researched spe-
cifically. Lastly, the distribution rate and contact between the
2D D–A copolymers with conjugated side-chain and PC70BM
in an active layer were investigated to determine the mecha-
nism of their interaction.
DTBT and pyrrolo[3,4-c]pyrrole-1,4-dione(DPP) are well-
known strong electron-accepting units, and are mostly used
to make high-performance D–A copolymers.18,19 Especially,
DPP has several other beneficial attributes including its pla-
nar structure, strong electron withdrawing nature and highly
conjugated lactam structure (which induces large intermolec-
ular interactions because of p–p stacking between the chains
of these polymers), favorable supramolecular order, and
increased charge carrier mobility.20 By varying the donor
segments, it has afforded a series of donor–acceptor poly-
mers showing extraordinary hole mobility with the highest
value being ꢀ2 [cm2/(V s)] and the best photovoltaic device
performance being over 5.5%.21–23 DPP achieves a high
degree of coplanarity in the polymer backbone when com-
bined with other aromatic blocks such as thiophene, bithio-
phene, thienothiophene, naphthalene and benzothidiazole.
Such high coplanarity is favorable for efficient charge trans-
port properties and stronger D–A interactions along the con-
jugated backbone.24 However, most of theses DPP-based
copolymers have low miscibility with fullerene derivatives
and need to be processed with additives to achieve desired
film morphologies.25 So DPP-based copolymers need a new
conjugation motif that retains their unique advantages.
EXPERIMENTAL
Materials
2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene(D1) and all
starting materials were purchased the TCI, Sigma Aldrich,
and Alfar Aesar companies. And the materials were used
without further purification. THF, ether, toluene were dis-
tilled before use. The 2,5-bis(trimethylstannyl)22,20-bithio-
phene(D2),26 5,6-bis(octyloxy)24,7-dithien-2-yl-benzo[c][1,2,
5]thiadiazole(1),27 2,5-bis(2-ethylhexyl)23,6-dithien-2-yl-pyr-
rolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2),28 diethyl-2-(2,5-
dibromothiophen-3-yl)ethyl phosphonate(3)29 were synthe-
sized modified procedures according to the reported
procedures.
5-(5,6-Bis(octyloxy)-7-(thien-2-yl)benzo[c][1,2,5]
thiadiazol-4-yl)thien-2-carbaldehyde (4)
Anhydrous DMF (0.8 mL, 10.2 mmol) was added dropwise
to a solution of 1 (5.68 g, 10.2 mmol) in C2H4Cl2 (50 mL)
under the nitrogen, then POCl3 (0.96 mL, 10.2 mmol) was
added slowly at 0 8C. The mixture was heated and stirred at
Among 2D-conjugated D–A polymers, those based on thio-
phene derivatives containing conjugated acceptor side-chain
indicate advantages of enhanced absorption, decreased
2
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85 8C for 12 h. After cooling to room temperature (RT),
60 mL water was added to end the reaction. The mixture
was extracted with CH2Cl2. After the solvent was distilled,
the residue was purified by silica gel chromatography using
petroleum ether:dichloromethane (1:1) as eluent. 4 was
obtained an orange solid (Yield: 4.24 g, 71%). 1H-NMR
(400 MHz, CDCl3, d): 9.84 (s, 1H), 8.47 (d, 1H, J 5 4.0 Hz),
8.37 (d, 1H, J 5 1.2 Hz), 7.65 (d, 1H, J 5 4.0 Hz), 7.38 (d,
1H, J 5 4.0 Hz), 7.08 (d, 1H, J 5 3.6 Hz), 4.00 (t, 4H,
J 5 7.2 Hz), 1.86–1.79 (m, 4H), 1.31 (m, 5H), 1.22–1.18 (m,
17H), 0.80–0.76 (m, 6H). 13C-NMR (400 MHz, CDCl3, d):
183.4, 153.6, 151.3, 150.6, 143.5, 135.7, 133.7, 131.2, 128.1,
126.7, 119.5, 116.0, 74.6, 31.87, 30.38, 29.5, 29.33, 26.0,
22.7, 14.17. Anal. calcd. for C31H40N2O3S3: C, 63.66; H, 6.89;
N, 4.79; O, 8.21; S, 16.45. Found: C, 63.78; H, 6.84; N, 4.75;
O, 8.19; S, 16.44. GC/MS (C31H40N2O3S3) m/z: calcd. for
584.22; found 584.24.
2H), 7.50 (d, J 5 4.8 Hz, 1H), 7.23 (t, J 5 5.6 Hz, 2H), 7.19
(s, 1H), 7.06 (d, J 5 16 Hz, 1H), 6.90 (d, J 5 16 Hz, 1H),
4.145 (m, 4H), 1.98-1.93 (m, 4H), 1.50–1.45 (m, 4H), 1.32–
1.28 (m, 18H), 0.89–0.85 (m, 6H). 13C-NMR (400 MHz,
CDCl3, d): 150.8, 149.7, 141.9, 137.9, 133.0, 130.4, 129.6,
126.2, 125.8, 123.4, 118.6, 116.7, 116.2, 110.9, 108.8, 73.4,
30.8, 29.5, 28.7, 28.5, 28.3, 25.0, 21.7, 13.1. Anal. calcd. for
C36H42Br2N2O2S4: C, 52.55; H, 5.15; Br, 19.42; N, 3.40; O,
3.89; S, 15.59. Found: C, 52.53; H, 5.08; Br, 19.46; N, 3.39; O,
3.85; S, 15.69. GC/MS (C36H42Br2N2O2S4) m/z: calcd. for
822.05; found 821.95.
(E)-3-(5-(2-(2,5-dibromothiophen-3-yl)vinyl)thien-2-yl)-
2,5-bis(2-ethylhexyl)-6-(thien-2-yl)pyrrolo[3,4-c]pyrrole-
1,4(2H,5H)-dione (A2)
A2 was synthesized with the same procedure of A1. Under a
nitrogen atmosphere, a solution of 3 (1.42 g, 3.62 mmol)
and 5 (2.0 g, 3.62 mmol) in THF (36 mL) was stirred and
degassed for 0.5 h at RT. Then mixture of potassium tertbut-
oxide (0.812 g, 7.24 mmol) in THF (24 mL) was dropwised
to the solution and stirred at RT for 8 h. The mixture
refluxed at 60 8C for 12 h. After cooling to RT, the mixture
was extracted with CH2Cl2 and washed with dilute HCl aque-
ous solution, water. The organic layer was dehydrated using
anhydrous MgSO4 and the solvent was distilled by rotary
evaporation. By silica gel chromatography, the crude product
was purified using hexane: CH2Cl2 (3:1) as eluent. A purple
solid was obtained. Yield: 2.5 g (70%). 1H-NMR (400 MHz,
CDCl3, d): 8.94 (t, J 5 4.6 Hz, 2H), 7.62 (t, J 5 2.4 Hz, 1H),
7.27 (d, J 5 2.4 Hz, 1H), 7.22 (t, J 5 9.0 Hz, 2H), 7.04 (d,
J 5 16 Hz, 1H), 6.92 (d, J 5 16 Hz, 1H), 4.03 (m, 4H), 1.59
(m, 2H), 1.37–1.26 (m, 17H), 0.92–0.86 (m, 12H). 13C-NMR
(400 MHz, CDCl3, d): 161.7, 147.1, 140.1, 139.8, 138.3, 136.6,
135.4, 130.6, 129.9, 128.4, 128.0, 127.1, 123.3, 121.9, 112.4,
108.2, 45.93, 39.2, 30.23, 28.4, 23.6, 23.1, 14.1, 10.6. Anal.
calcd. for C20H8Br2N2O2S3: C, 42.57; H, 1.43; Br, 28.32; N,
4.96; O, 5.67; S, 17.05. Found: C, 42.52; H, 1.48; Br, 28.38; N,
4.92; O, 5.63; S, 17.07. GC/MS (C20H8Br2N2O2S3) m/z: calcd.
for 563.81; found 563.87.
5-(2,5-Bis(2-ethylhexyl)-3,6-dioxo-4-(thien-2-yl)-2,3,5,6-
tetrahydropyrrolo[3,4-c]pyrrol-1-yl)thien-2-carbaldehyde
(5)
Material 5 was synthesized with the same procedure of 4. To
solution of 2 (1.04, 10.20 mmol) in C2H4Cl2, anhydrous DMF
(0.8 mL, 10.2 mmol) was added dropwise. Then, POCl3
(0.96 mL, 10.2 mmol) was added slowly. The mixture was
heated and stirred at 85 8C for 12 h. After cooling to RT,
water was poured into the mixture and extracted with chlo-
roform. The organic layer was removed the solvent, and the
crude product was purified using silica gel chromatography
(eluent: DCM:PE 5 1:1 to 1:0). 5 was obtained a purple solid
(0.27 g, 49%). 1H-NMR (400 MHz, CDCl3, d): 10.01 (s, 1H),
9.06 (t, J 5 2.0 Hz, 1H), 8.94 (d, J 5 4.0 Hz, 1H), 7.86 (d,
J 5 4.0 Hz, 1H), 7.71(t, J 5 2.4 Hz, 1H), 7.31 (d, J 5 1.2 Hz,
1H), 4.05 (t, J 5 3.8 Hz, 4H), 1.88–1.83 (m, 2H), 1.36–1.24
(m, 17H), 0.88–0.85 (m, 13H). 13C-NMR (400 MHz, CDCl3, d):
182.8, 161.9, 161.3, 145.5, 142.9, 137.7, 136.5, 134.9, 132.0,
129.5, 128.8, 110.9, 108.1, 46.0, 39.3, 39.1, 30.2, 30.1, 28.3,
23.5, 23.1, 14.0, 10.5. Anal. calcd. for C15H6N2O3S2: C, 55.20;
H, 1.85; N, 8.58; O, 14.71; S, 19.65. Found: C, 55.16; H, 1.83;
N, 8.59; O, 14.69; S, 19.73. GC/MS (C15H6N2O3S2) m/z: calcd.
for 325.98; found 326.08.
General Polymerizations
To a mixture of tris(dibenzylideneacetone)dipalladium(0)
(Pd2dba3) (7.3 mg, 0.008 mmol, 2.7 mol %), tri-(o-tolyl)-
phosphine (9.7 mg, 0.032 mmol, 10.7 mol %), stannane com-
pound (0.2 mmol, e.g., D1 and D2) and equal halogen
compound (e.g., A1 and A2) were dissolved in 8 mL of anhy-
drous toluene. Under the nitrogen, the mixture was power-
fully stirred and refluxed at 90–95 8C for 48 h. 2-
bromotihophene was added to the reaction after 48 h. And
after 3 h, 2-trimethylstannyl thiophene was added to end-
capping reaction. After cooling the mixture to RT, it was put
into methanol and filtered. The filtered precipitate was rep-
recipitated after dissolving in CHCl3, dropping into methanol
and then filtered. The polymer was further purified by Soxh-
let extraction using methanol, acetone, CHCl3, and o-dichloro-
benzene, respectively for 24 h. The chloroform part was
reprecipitated with methanol, then filtrated, and desiccated
under reduced pressure at 50 8C.
(E)-4-(5-(2-(2,5-dibromothiophen-3-yl)vinyl)thien-2-yl)-
5,6-bis(octyloxy)-7-(thien-2-yl)benzo[c][1,2,5]thiadiazole
(A1)
Under a nitrogen atmosphere, a solution of 3 (1.21 g,
3.2 mmol) and 4 (1.87 g, 3.2 mmol) in THF (30 mL) was
stirred and degassed for 0.5 h at RT. Then mixture of potas-
sium tertbutoxide (0.672 g, 6.0 mmol) in THF (20 mL) was
added dropwise into it. The reaction mixture was stirred at
RT for 8 h, and then refluxed at 60 8C for 12 h. After cooling
to RT, the reaction mixture was extracted with CH2Cl2 and
washed with dilute aqueous HCl solution, water. The organic
layer was dehydrated using anhydrous MgSO4, and the sol-
vent was distilled by rotary evaporation. By silica gel chro-
matography, the residue was purified using a hexane/CH2Cl2
mixture (1:1 by volume) as eluent. A red solid was obtained.
Yield: 2.564 g (97%). 1H-NMR (400 MHz, CDCl3, d): 8.48 (s,
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Poly[2,5-thieno[3,2-b]thiophene-alt-(E)-4-(5-(2-thien-
3-yl)vinyl)thien-2-yl)-5,6-bis(octyloxy)-7-(thien-
2-yl)benzo[c][1,2,5]thiadiazole] (PfToBT)
electrode 5 the polymer film on a ITO glass, counter
electrode 5 a ITO glass, and reference electrode 5 silver/sil-
ver chloride [Ag in 0.1M KCl]. The constant scan rate was
50 mV/s. The electrochemical potential was calibrated
against Fc/Fc1. X-ray diffraction (XRD) patterns was per-
formed using a New D8-Advance instrument of Bruker-AXS
and SmartLab 3kW (40 kV 30 mA, Cu target) of Rigaku.
Thicknesses of the active layers were recorded by a KLA Ten-
cor Alpha-step 500 surface profilometer. The depth profile of
the blend film was detected by X-ray photoelectron spectros-
copy (XPS) measurement which was performed from PHI
5000 VersaProbe ll system with a microfocused (100 mm,
25 W) Al X-ray beam during in situ sputtering. In brief, the
Ar1 ion beam was created from Ar1 ion source (Fig-5CE
ꢁ5.0 lA at 5 kV). Using turbomolecular and ion-getter
pumps, the measurement was accomplished at the pressure
of <1 3 1027 Pa by evacuating the main chamber. The cur-
rent density–voltage (J–V) curves of the PSCs were per-
formed by Keithley 2400 source measurement unit (SMU)
furnished with a Class A Oriel solar simulator under an AM
1.5G illumination, 100 mW/cm2. To obtain topographic
images of the active layers, atomic force microscopy (AFM)
in tapping mode was performed by an XE-100 instrument
under ambient conditions.
Dark black purple solid, 0.150 g (yield 5 62%). 1H-NMR
(400 MHz, CDCl3, d): d 8.47 (m, 3H), 7.40 (m, 4H), 7.15 (m,
3H) 4.11 (m, 4H), 1.88–1.83 (m, 6H), 1.25 (2, 19H), 0.84–
0.81 (m, 9H). Anal. calcd. for C42H44N2O2S6: C, 63.57; H,
6.06; N, 3.37; O, 3.85; S, 23.14. Found: C, 64.04; H, 7.47; N,
2.45; O, 2.84; S, 23.2
Poly[2,2’-bithiophene-alt-(E)-4-(5-(2-thien-3-
yl)vinyl)thien-2-yl)-5,6-bis(octyloxy)-7-(thien-2-
yl)benzo[c][1,2,5]thiadiazole] (PbToBT)
Dark black purple solid, 0.165 g (yield 5 66%). 1H-NMR
(400 MHz, CDCl3, d): d 8.44 (m, 4H), 7.52–7.40 (m, 4H), 7.15
(m, 3H) 4.09 (m, 4H), 1.92 (m, 7H), 1.25 (s, 21H), 0.86–0.85
(m, 9H). Anal. calcd. for C44H46N2O2S6: C, 64.45; H, 6.11; N,
3.27; O, 3.73; S, 22.44. Found: C, 66.2; H, 7.27; N, 3.04; O,
3.44; S, 23.49
Poly[2,5-thieno[3,2-b]thiophene-alt-(E)-3-(5-(2-(thien-3-
yl)vinyl)thien-2-yl)-2,5-bis(2-ethylhexyl)-6-(thien-2-
yl)pyrrolo[3,4-c]pyrole-1,4(2H,5H) -dione] (PfTDPP)
Dark black purple solid, 0.200 g (yield 5 86%). 1H-NMR
(400 MHz, CDCl3, d): d 8.89 (m, 3H), 7.73–7.44 (m, 4H), 7.01
(m, 3H) 4.03 (m, 4H), 1.85 (m, 3H), 1.25 (m, 16H), 0.87 (m,
8H). Anal. calcd. for C41H44N2O2S5: C, 66.12; H, 6.31; N, 3.51;
O, 4.00; S, 20.06. Found: C, 67.3; H, 6.59; N, 3.25; O, 3.86; S,
19.0
Photovoltaic Cell Fabrication and Treatment
The photovoltaic cells were constructed in the following con-
ventional structure of ITO (170 nm)/PEDOT:PSS (30 nm)/
polymer:PC70BM/BaF2 (1 nm)/Ba (2 nm)/Al (200 nm). The
patterned indium tin oxide (ITO) glass [Sanyo, Japan(10 X/
c)] was cleaned with detergent, deionized water, acetone,
and isopropyl alcohol in ultrasonicator. It dried at 120 8C for
10 min on a hot plate and treated with ultraviolet–ozone
chamber for 10 min.
Poly[2,2’-bithiophene-alt-(E)-3-(5-(2-(thien-3-
yl)vinyl)thien-2-yl)-2,5-bis(2-ethylhexyl)-6-(thien-2-
yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione] (PbTDPP)
Dark black purple solid, 0.180 g (yield 5 75%). 1H-NMR
(400 MHz, CDCl3, d): d 8.90 (m, 4H), 7.52–7.31 (m, 4H), 7.02
(m, 4H) 4.00 (m, 4H), 2.04ꢀ1.85 (m, 5H), 1.25 (m, 17H),
0.87 (m, 11H). Anal. calcd. for C43H46N2O2S5: C, 66.95; H,
6.35; N, 3.39; O, 3.88; S, 19.43. Found: C, 66.94; H, 6.57; N,
3.29; O, 3.84; S, 19.36
Poly(3,4-ethylene-dioxythiophene):poly(styrene
(PEDOT:PSS, Baytron P 4083 Bayer AG) was filtered through
0.45-mm filter and spin-coated on cleaned-ITO at
sulfonate)
a
4000 rpm in air. Then, substrate was dried at 120 8C for
20 min and transferred to a N2-filled glove box. An o-dichlor-
obenzene solutions of the polymers and 1-(3-methoxycarbo-
nyl)propyl-1-phenyl-[6,6]-C71(PC70BM) blended in 1:2 (w/
w) and 1:3 (w/w) were spin-coated (500–3000 rpm, 30 s)
on to the PEDOT:PSS layer. The substrates with the spin-
casted active layer films were dried for 10 min at 120 8C
inside a glove box. BaF2 (1 nm) and Ba (2 nm) were then
deposited as an electron injection cathode on to the active
layer. The device was completed by depositing of 200-nm
aluminum layer using thermal evaporator at pressures lower
than 1026 Torr. The yielded devices had an active area of
4 mm2. Finally, the devices were encapsulated using a UV-
curable epoxy (Nagase, Japan) and glass slide.
Measurements
13
1H- and C-NMR data were performed on a Bruker AMX400
€
spectrometer in CDCl3. And TMS as the internal standard in
CDCl3 calibrated the chemical shifts indicated in measures of
ppm. The Ultraviolet–visible (UV–vis) absorption spectra
were carried out on an Agilent 8453 UV–visible spectroscopy
system. The solutions used for experiments of the UV–visible
absorption spectra were dissolved in CHCl3 at various con-
centrations. The films were drop-coated from the 1 mg/mL
solution on a quartz substrate. The molecular weight and
polydispersity index (PDI) of the polymers were determined
by gel permeation chromatography (GPC) analyses using
THF as the eluent and a polystyrene standard as the refer-
ence. The thermal analyses were recorded using a TG 209
F3 thermogravimetric analyzer (TGA) under a N2 atmos-
phere. The cyclic voltammetry (CV) was recorded in solid-
state using a Zahner IM6eX electrochemical workstation with
three electrodes system in a 0.1M tetrabutylammonium hex-
afluorophosphate (Bu4NPF6) acetonitrile solution: working
The single-carrier diodes were fabricated based on a configu-
ration of ITO (170 nm)/PEDOT:PSS (40 nm)/Poly-
mer:PC70BM/MoO3 (30 nm)/Al (100 nm) for calculating hole
mobility. The hole mobilities of the active layers were
extracted from the J–V graphs of the hole-only devices
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SCHEME 1 Synthetic route to monomers and polymers.
recorded in the dark conditions. The Mott–Gurney equation
is as follow:
age (Vbi – Vr 1 Vappl), where Vbi is the built-in voltage; Vr is
the voltage drop because of series resistance across the elec-
trodes; Vappl is the applied voltage to the device.
rffiffiffi
9
hðeÞ V2
V
L
J5 ee0l
exp ð0:89Þc
L3
8
RESULTS AND DISCUSSION
where e0 (8.85 3 10214 F/cm) is the permittivity of vacuum;
e is the permittivity of the polymer:PC70BM blend; l is the
mobility; L is the thickness of the thin film; and V is the volt-
Synthesis and Characterization of Polymers
Scheme 1 and Figure 1 showed the copolymer structures
and common methods of synthesis. Compound
3 was
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TABLE 1 Polymerization Results and Degradation Temperatures of Polymers
Yield
(%)
Mna
Mwa
Degree of
Td
Polymer
(kDa)
(kDa)
PDIa
Polymerization
( 8C)
PfToBT
PbToBT
PfTDPP
PbTDPP
62
66
86
75
60
85
66
82
281
348
220
272
4.6
4.1
3.3
3.3
72
390
394
382
344
99
83
100
a
Indicated by solution in CHCl3 using GPC. Calibrated by polystyrene standards.
synthesized by starting with the bromination of 3-
methylthiophene using NBS at 80 8C, which afforded 2,5-
dibromo-3-methylthiophene. Then bromination of 2,5-
dibromo-3-methylthiophene in ethylacetate yielded 2,5-
dibromo-3-bromomethylthiophene. Compound 3 was synthe-
sized from 2,5-dibromo-3-bromomethylthiophene by the Wit-
tig–Horner reaction. Compounds 4 and 5 were prepared
from compounds 1 and 2 using the Vilsmeier–Haack reac-
tion. A1 and A2 were then prepared from compound 3 and
aldehyde-containing materials (Compounds 4 and 5) in aver-
age yields according to the Wittig–Horner reaction30 for the
resulting copolymers PfToBT, PbToBT, PfTDPP, and PbTDPP
using a Stille coupling reaction with D1 and D2. The struc-
tures of the synthesized monomers and polymers were con-
firmed using 1H, 13C-NMR (as shown in Supporting
Information Figs. S1–S9) and elemental analysis (EA).
PfTDPP and PbTDPP exhibit good solubility in common
organic solvents such as tetrahydrofuran (THF), chloroform
(CHCl3), o-dichlorobenzene (o-DCB), and chlorobenzene (CB).
Conversely, PfToBT and PbToBT are not soluble in these sol-
vents at RT and are soluble only at elevated temperature.
sisting of only thiophene derivatives, which in turn may have
an impact on the photovoltaic performances.
The thermal weight loss in nitrogen atmosphere of the
copolymers was performed by TGA at 20 8C min21 heating
rate. All of the synthesized polymers (PfToBT, PbToBT,
PfTDPP, and PbTDPP) exhibited superior thermal stability
with onset degradation temperatures (Td) of a 5% weight-
loss from 344 to 394 8C, as shown in Supporting Information
Figure S10. The obtained copolymers had adequate thermal
stability, which proved to be suitable for application in opto-
electronics and device fabrication.
Optical Properties
The UV–visible absorption of all copolymers in solution and
thin film was investigated. The correlative data of the copoly-
mers are summarized in Figure 2 and Table 2. Figures 2(a)
and Supporting Information Figure S11 indicate the UV–vis
absorption spectra of PfToBT, PbToBT, PfTDPP, and PbTDPP
in diluted CHCl3 solutions at various concentrations and cal-
culation of molar absorption coefficients (e). The copolymers
show three distinct absorption peaks. The first peak around
311–343 nm is caused by the n–p* absorption of the DPP or
the 5,6-bis(octyloxy)24,7-di(thien-2-yl)benzo[c][1,2,5]thiadia-
zole (DToBT) group.36 The second peak with the absorption
maxima(ks, max) at 388 nm for PfToBT, 388 nm for PbToBT,
408 nm for PfTDPP, and 408 nm for PbTDPP is caused by
the p–p* transition of the delocalized excitons. Whereas the
third peak at 501, 502, 560, and 564 nm, respectively, can
be identified to a localized intramolecular transition between
donor and acceptor.37 PfTDPP and PbTDPP exhibit a little
wider absorption region which is resulted from the intramo-
lecular charge transfer (ICT) interaction between the poly-
mer backbone and the conjugated side chain.38 Evidently,
the ICT interactions of PfTDPP and PbTDPP are gradually
increased because of the stronger electron-withdrawing abil-
ity of the DPP of A2 than DToBT of A1. The calculated
absorption coefficients of PfToBT, PbToBT, PfTDPP, and PbTDPP
The measured polymer molecular weights and thermal prop-
erties are summarized in Table 1. PfToBT, PbToBT, PfTDPP,
and PbTDPP have molecular weights (Mw) of 281, 348, 220,
and 272 kDa, respectively, and broad PDI of 4.6, 4.1, 3.3, and
3.3, respectively. These data were indicated using GPC, cali-
brated by polystyrene standards with CHCl3 base. As listed
in Table 1, the molecular weights of PbToBT and PbTDPP are
higher than PfToBT and PfTDPP, presumably due to bithio-
phene donor unit that release the diheral angle. This
decrease of steric hindrance directly impacts the polymer
molecular weight, a dominant factor in determining the
chain packing and mechanical properties of polymer thin
films.31 To compare 2D-conjugated polymers with CT-type
polymers which have similar chemical structures, the molec-
ular weight of the four copolymers is relatively higher, and
the PDI is broader.32–35 The vinylene linkage eliminates tor-
sional strains between the main-chain and electron-accepting
side-chain. So it can help to extend the p-conjugation and
improve the coplanarity of the polymer backbone. The Mw
and PDI are affected by the polymerization conditions (i.e.,
the purity of the monomers and the solubility of the mono-
mers and polymers).16 Moreover, the degree of polymeriza-
tion may also be influenced the high degree of planarity of
PfToBT, PbToBT, PfTDPP, and PbTDPP’s main backbone con-
are 3.64, 4.00, 5.03, and 6.28 3 104 (g mL21 21
tively at ks,max
)
cm21, respec-
.
The absorption coefficients of PfTDPP and PbTDPP are
higher than those of PfToBT and PbToBT, because of the
introduction of brilliant red dye (in DPP unit) to the side
group. These results convincingly demonstrate that DPP-
containing polymers (PfTDPP and PbTDPP) have better light-
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likely associated with the increased polarizability and extent
of p–p stacking of the polymers.39 The optical bandgaps
(Eogpt) of PfToBT, PbToBT, PfTDPP, and PbTDPP are 1.79,
1.78, 1.68, and 1.66 eV, respectively, inferred from the edge
of the absorptions in film. Compared to the parent polymers
(PPDToBT, PABToBT, DPPT-TT, and pBBTDPP2) which have
linear CT-type structure with similar donor and acceptor
units, the narrow absorptions of the graft structure copoly-
mers PfToBT, PbToBT, PfTDPP, and PbTDPP are observed
from 300 to 800 nm. The observed blue-shifted absorption
phenomenon are related to weaker D-A interaction in the
graft-type copolymer compared to linear-type copolymer.40
These results clearly indicate that introducing different con-
jugated side-groups can control the optical properties of
these 2D polymers.16
Electrochemical Measurements and Computational Study
of Polymers
Cyclic voltammetry (CV) was produced to investigate electro-
chemical behavior of the copolymers.
The results data of corresponding with CV curves are pre-
sented in Figure 3 and summarized in Table 2. Redox poten-
tials were calibrated by using the ferrocene/ferrocenium(Fc/
Fc1) redox couple as the internal standard. The redox poten-
tial of Fc/Fc1 is assumed 0.43 V versus Ag/AgCl, which have
a definite vacuum energy level of 4.8 eV.
The oxidation onset potentials (Eox) for PfToBT, PbToBT,
PfTDPP, and PbTDPP are 0.97, 0.95, 0.83, and 0.75 V, respec-
tively. The calculated HOMO levels are at 25.34, 25.32,
25.23, and 25.12 eV for PfToBT, PbToBT, PfTDPP, and
PbTDPP, respectively. The copolymers have lower HOMO lev-
els than air oxidation threshold (25.20 eV). PfToBT, PbToBT,
and PfTDPP are expected to have good oxidative stability.16
Introducing benzothiadiazole(BT) unit to the backbone con-
tributes to the relatively lower HOMO energy levels of
PfToBT and PbToBT compared with PfTDPP and PbTDPP due
to the BT unit has a lower HOMO level than the DPP unit.
This result is in agreement with that obtained by You et al.
regarding the HOMO level of a polymer containing BT, which
has a side-chain in its core.41 The HOMO energy level is pri-
marily determined by the donor unit in the type of a D–A
FIGURE 2 Normalized UV–vis absorption spectra of copolymers:
(a) in solution and (b) film. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
harvesting ability than DToBT-containing polymers (PfToBT
and PbToBT) indicating that the former can give a higher Jsc
than the latter.13 As shown in Figure 2(b), the absorption
peaks of all copolymers in the films were red-shifted com-
pared to those in solution. This characteristics may be
accounted for the improved aggregation and interchain inter-
action of the polymer backbones in solid state, which is
TABLE 2 Photophysical and Electrochemical Measurements of Polymers
Polymer
UV–Vis Absorption
Cyclic Voltammetry
HOMOb
(eV)
LUMOb
(eV)
onset
ox
E
Egop;a
CHCl3 Solution
Film
(V)
(eV)
ks,max (nm)
kf,max (nm)
konset (nm)
PfToBT
PbToBT
PfTDPP
PbTDPP
340, 388, 501
346, 390, 524
693
1.79
1.78
1.68
1.66
0.97
0.95
0.83
0.75
25.34
25.32
25.23
25.12
23.55
23.54
23.52
23.46
343, 388, 502
311, 408, 560
312, 408, 564
334, 411, 525
312, 412, 569
312, 411, 571
694
738
747
a
b
From the absorption spectrum of film state, Egop calculated from the
point of contact with the baseline and the tangent on the low energetic
edge.
EHOMO
5 2e[Eonset(vs. Ag/AgCl) – E1/2(Fc/Fc1 vs. Ag/AgCl)] –
(or LUMO)
4.8 eV.
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FIGURE 3 Cyclic voltammograms of polymers. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
copolymer. Also the HOMO levels of PfToBT and PfTDPP are
slightly deeper than those of PbToBT and PbTDPP because
the HOMO energy level of the thieno[3,2-b]thiophene
electron-donating unit of PfToBT and PfTDPP is relatively
lower than the bithiophene electron-donating unit of PbToBT
and PbTDPP.42 The PfToBT and PbToBT with same conju-
gated side chains have a similar HOMO level. So we expected
that they would show a similar Voc value to the resulting
PSCs. As PfToBT and PbToBT have deep-lying HOMO levels,
these PSCs would have high VOC values. The fact, value of
Voc in BHJ PSCs is directly proportional to the difference
between the HOMO level of the p-type and the LUMO level
of the n-type, has been demonstrated. In addition, the LUMO
levels are 23.55, 23.54, 23.52, and 23.46 eV for PfToBT,
PbToBT, PfTDPP, and PbTDPP, respectively. The energy differ-
ences of the LUMO levels are higher than 0.3 eV to yield suf-
ficient exciton dissociation and efficient charge transfer
when comparing LUMO levels of the copolymers with those
of the acceptor (PC70BM 5 24.3 eV).43
FIGURE 4 The frontier molecular orbitals for polymer dimer
models. [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
PbToBT, the electrons are distributed primarily on the oBT
side group with a small distribution on the thiophene donor
groups (HOMO). The LUMO is spread out on the oBT side-
group only, which allows the external excitations are able to
electron transfer from the left to right side in molecules. The
distribution of the HOMO electron density of PfTDPP and
PbTDPP is weighted more towards the accepting side group
than in PfToBT and PbToBT because DPP is better electron
acceptor than oBT.
To gain a understanding of the electron density of states dis-
tribution and electronic properties, as well as the molecular
geometries of the polymers with different side-groups, simu-
lations were performed using density functional theory
(DFT). Computation was performed at the B3LYP/6-31G(d)
with Gaussian 09, which is suitable program for using theo-
retical calculations. For the purpose of computational simpli-
fication, two repeating units were selected to the polymer
backbones. Figure 4 shows the molecular orbitals of calcu-
lated HOMO and LUMO. The data of these calculations are
summarized in Table 3. DPP has greater electron-
withdrawing capability as an acceptor than oBT, which is
reflected in the shapes of electron density of these HOMO
and LUMO orbitals. The HOMOs and LUMOs are localized on
oBT and DPP unit as the acceptor. This is because of the fea-
tures of the quinoid structure built between the nonbonding
electron pairs of nitrogen and sulfur, which have electron-
accepting characteristics.44 In the case of PfToBT and
As shown in Table 3, the HOMO levels determined from the
DFT calculations tend to increase using the bithiophene
TABLE 3 Theoretical Data of Polymers
Dihedral Angle (8)
HOMOcal.
(eV)
LUMOcal.
(eV)
Polymer
1
2
3
4
PfToBT
PbToBT
PfTDPP
PbTDPP
0.76 46.65 1.59 36.26 27.119
0.41 50.71 0.31 35.49 27.050
1.63 46.01 1.57 45.86 26.958
1.48 49.50 0.28 42.81 26.934
0.438
0.446
0.356
0.368
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FIGURE 5 X-ray measurements data with thermally annealed thin films of polymers and polymer:PC70BM blend on silicon wafer:
(a), (b), out-of-plane mode and (c) inplane mode. [Color figure can be viewed in the online issue, which is available at wileyonline-
library.com.]
donor or DPP derivative on the polymer backbone. This
agrees with the CV measurements in which strengthen of the
electron-donating ability of the donor unit makes the HOMO
level increases. Moreover, the dihedral angles 1, 2, 3, and 4
between the donor unit, vinyl thiophene and side groups
(summarized in Table 3) demonstrate similar values for each
of the four polymers. The dihedral angles 1, 3, and 4 (0.418/
0.318/35.498, and 1.488/0.288/42.818) of the polymers with
bithiophene donor (PbToBT and PbTDPP, respectively) are
observed to be smaller than those (0.768/1.598/36.268 and
1.638/1.578/45.868) of the polymers with the fused thio-
phene donor (PfToBT and PfTDPP, respectively). Therefore,
the structure and planarity of the donor unit plays a decisive
role in determining the HOMO energy level. In the case of
PbToBT and PbTDPP, the introduction of the bithiophene
extends the conjugation in the polymer main chains. This
increases the electron-donating activity of PbToBT and
PbTDPP and results in the increased HOMO levels.
of the polymer pristine films acquired in the out-of-plane
mode were shown in Figure 5(a) to analyze the arrangement
structure. PfToBT, PbToBT, PfTDPP, and PbTDPP thin films,
obtained in the out-of-plane mode, showed small diffraction
peaks for the (100) planes and broad diffraction peaks for
the (010) crystal planes. This suggests that the polymers
simultaneously form (h00) and (0h0) lamellar structures. In
particular, (010) peaks involved with p–p stacking are exhib-
ited at 23.638, 24.428, 24.128, and 25.028, respectively. Based
on of a calculation using Bragg’s law (i.e., k 5 2dsinh), p–p
stacking distances (dp) are 3.77, 3.65, 3.69, and 3.56 Å,
respectively. The dp-spacing (i.e., the length between the
inter-chains of the PfToBT, PbToBT, PfTDPP, and PbTDPP
polymers) decreases when going from using fusedthiophene
donor to bithiophene donor and when changing the acceptor
unit from oBT to DPP. This is in good agreement with the
research published by Ong et al., where bithiophene release
space for the side-chain to interpenetrate with the
polymers.45
XRD Measurements
To gain a better understanding of the ordering and orienta-
tion in thin films of the graft-structure polymers, X-ray dif-
fraction (XRD) measurements were performed with their
associated annealing temperatures. XRD diffraction patterns
Figure 5(b,c) presents the XRD profiles of the poly-
mer:PC70BM blended thin films performed in the out-of-
plane and inplane mode, respectively. In the out-of-plane
mode, strong (010) diffraction peaks for the polymer:
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PC70BM blend films are measured at 18.878, 18.878, 18.818,
and 18.818 due to the crystallinity of fullerene. 2h value of
the PfToBT, PbToBT, PfTDPP, and PbTDPP:PC70BM thin films,
(010) peaks, observed at 26.778, 26.898, 26.718, and 26.958,
respectively. The calculated interlamellar (dp) spacings of the
blend thin films are 3.33, 3.32, 3.34, and 3.31 Å, respectively.
This suggests that the polymer: PC70BM blended films form
(0h0) lamellar structure. When a p–p stacking peak is shown
for the (010) plane in the out-of-plane mode, it has highly
possibility that the thin films adopts a crystalline form with
face-on or edge-on orientation. This can be verified by
acquiring XRD data using an inplane mode. Figure 5(c)
presents the XRD diffraction patterns of the polymer:PC70BM
blend thin films detected in the in-plane mode.
centrations at their top-most surfaces. The decrease of N
ratio in the active layer region indicates that the N-rich side
chains diffuse into the surface of active layer. In particular,
this implies that the side chains of the polymers are oriented
look upward in the active layer. On the other hand, the
PfToBT:PC70BM blend film shows a lower concentration of N
at its surface. The N ratio increases in the bottom of the
active layer region, indicating that the N-rich side chains dif-
fuse into the bottom of active layer and side-chain of PfToBT
are oriented look down. The polymer side chains have both
O and N in the same ratio. In the PbToBT, PfTDPP and
PbTDPP:PC70BM blend films, the concentration of O is higher
than N in the active layer due to the presence of PC70BM. In
the bottom of the active layer, PfTDPP and PbTDPPs:PC70BM
blend films show decreased amounts of O, but the
PbToBT:PC70BM blend film has an unchanged amount of O.
This indicates that PfTDPP and PbTDPPs:PC70BM blend films
have PC70BM that diffuses into the top of active layer, while
in the PbToBT:PC70BM blend film that PC70BM diffuses into
the bottom of active layer. The PfToBT:PC70BM blend film
has a lower concentration of O at the boundary between the
HTL and active layer, indicated by the fact that the
PfToBT:PC70BM blend film has PC70BM in the top of the
layer in the absence of polymer side chains.
The XRD profiles for all of the polymer:PCBM blend films in
the in-plane mode show diffraction peaks at high angles. The
aforementioned XRD data imply that polymer:PC70BM blend
films have edge-on structures in the thin film. In the in-plane
mode, the dp-spacings of the PfToBT, PbToBT, PfTDPP, and
PbTDPP: PC70BM thin films are similar with the calculated
values in the out-of-plane mode (3.32 3.29, 3.31, and
3.28 Å). For the blended films, the polymers using bithio-
phene donor also show closer intermolecular distances than
the polymers using fusedthiophene donor.
As shown in Figure 4, we confirmed that the electron clouds
of PfToBT, PbToBT, PfTDPP and PbTDPP are localized on the
acceptor oBT and DPP grafted side-chain units. The grafted
side chains of PbToBT (or PfToBT) and PC70BM exist on dif-
ferent sides of the active layer and do not come into contact,
and as such. These films have difficulty in obtaining efficient
charge transfer from polymer to PC70BM.
Cross-Sectional SEM and XPS Depth Profile
Measurements
In order to examine the composition gradient in the poly-
mer:PC70BM BHJ layer, cross-sectional SEM images were col-
lected and atomic concentration of thiophene (characteristic
of polymers and PEDOT:PSS), oxygen (characteristic of poly-
mers side-chain and PC70BM), and nitrogen (characteristic of
polymers side-chain only) were investigated using XPS depth
profile, as shown in Figure 6. The component layers of the
device are clearly seen in SEM images. Samples were pre-
pared in simplified structure as ITO glass/PEDOT:PSS/Poly-
mer:PC70BM/Al. The thicknesses of the optimum devices
measured from SEM images are in good agreement with
Conversely, PfTDPP (or PbTDPP) and PC70BM exist on the
same side of the active layer, and these films have clear
intermolecular charge transfer from polymer to PC70BM.
Increased concentrations of S and O are indicative of the
HTL layer, which consists of PEDOT:PSS. Below the HTL,
decreasing S and increasing O are observed in the ITO and
glass substrate areas. It is noteworthy that although the total
thickness of the organic layer is thinner than thickness of
the inorganic layer, the time required to remove the organic
layer is longer. The apparent slower sputtering rate of the
organic layer can be attributed to ion-induced cross-linkage
between organic molecules.46
those measured by a-step (ITO
5
170–180 nm,
PEDOT:PSS 5 30 nm, active layer 5 100–110 nm and
Al 5 150 nm). The depth profile correctly reflects the multi-
layer structure of the polymer solar cell device. It can be
concluded that such a profiling technique is suitable for
profiling the cross sectional construction of these devices.
We therefore performed XPS profiling of these cross-sections
with regards to sulfur (S), oxygen (O), and nitrogen (N) to
investigate the vertical distribution of polymer chains and
PC70BM within the BHJ films. In XPS images, the BHJ films
have a different network between polymer chains and
PC70BM from PEDOT:PSS layer (the bottom) to Al electrode
layer (the top).
This optimized vertical morphology promotes hole and elec-
tron transport along the respective pathways of the polymers
and fullerenes in the vertical direction, leading to high Jsc
and FF.
Photovoltaic Device Characteristics
BHJ PSCs with ITO/PEDOT:PSS/polymer:PC70BM/BaF2/Ba/Al
structures were fabricated to determine the photovoltaic
(PV) properties of graft-type polymers. The J–V curves
acquired with different PC70BM ratio are shown in Figure
7(a,b). And incident photon to charge carrier efficiency
(IPCE) profiles are presented in Figure 7(c). The
At the top-most surface, all of the polymer:PC70BM blend
films show high concentrations of O. This implies that poly-
mer side-chain or PC70BM which contain the O sources are
present at the top of the active layer. PbToBT, PfTDPP, and
PbTDPP polymer:PC70BM blend films also show high N con-
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FIGURE 6 Cross-sectional SEM images and XPS depth profile of conventional device with recorded elements of S (resolved to sul-
fonate and thiophene), O (polymer side-chain and PC70BM) and N (polymer side-chain). [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
manufactured devices were encapsulated in a glove box. The
current density–voltage (J–V) plots were determined under
an ambient atmosphere with a 4 mm2 active area and were
estimated under standard AM 1.5G, 100 mW/cm2 illumina-
tion. The polymer:PC70BM weight ratios used in the mea-
surement were 1:2 and 1:3, and the optimized conditions
were 1:2 (PfToBT, PbToBT) and 1:3 (PfTDPP, PbTDPP),
respectively. r-dichlorobenzene(ODCB) was used as a solvent
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exhibits a PCE of 2.9% under a Voc of 0.677 V, Jsc of 9.4
mA/cm2, and FF of 44.4%. PCE values were recorded that
Jsc of PfToBT and PbToBT is low because of the exciton dis-
sociation rate is low. In particular, the low Jsc are measured
for PfToBT and PbToBT because these polymers have diffi-
culty in contacting the polymer side-chain and PC70BM in
blend films. When DPP is used as a conjugated side-chain,
the bandgap decreases and the extinction coefficient
increases because of strong electron affinity. Moreover, the
side chains of PfTDPP and PbTDPP easily make contact with
PC70BM, causing an increase in Jsc and overall device per-
formance. However, VOC is determined according to the gap
between the LUMO levels of PC70BM and the HOMO level of
the polymer. The oxidation stability decreases when using
DPP as the acceptor unit, leading to upshift the HOMO level
and reduce the VOC values. The Voc of PfTDPP and PbTDPP
decrease compared to those of PfToBT and PbToBT.
The shapes of IPCE spectra of the devices are similar to each
other. These devices exhibit sufficiently broad IPCE spectra
in the visible region (300–800 nm). And higher external
quantum efficiency (EQE) values in the UV–vis absorbance
range of polymers but lower values above 800 nm. The
PbTDPP:PC70BM blend-used cell showed the highest EQE
that is agree with the highest photo current, in which the
values reached 48.41% at ꢀ460 nm. PfToBT, PbToBT, and
PfTDPP recorded 38.04, 43.61, and 46.16%, respectively.
Hole Carrier Mobility and Morphological Properties
In BHJ solar cells, Jsc and FF are directly limited by the hole
mobility of the polymer, making it a crucial parameter for
determining photovoltaic performance. We determined the
hole mobility of the polymer and PC70BM blend films from
the logJ versus logV graphs presented in the Supporting
Information, Figures S12–S15 and Table S1 via a space charge
limited current (SCLC) method. A structure of hole-only
devices in this work is ITO/PEDOT:PSS/Polymer:PC70BM/
MoO3/Al.
Based on the equation and the logJ versus logV graph, the
hole mobility values of PfToBT, PbToBT, PfTDPP, and PbTDPP
blended with PC70BM are 3.85 3 1024, 2.13 3 1023
,
2.52 3 1023, and 2.66 3 1023 [cm2/(V s)], respectively. The
TABLE 4 Device Performances of Polymers with Different
FIGURE 7 The device characteristics of PSC based on polymer:
PC70BM with different weight ratio in 1:2 (w/w) (a) and 1:3 (w/
w) (b). The IPCE spectra (c). [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
PC70BM Ratio
PC70BM
Ratios
Jsc
Voc
(V)
FF
PCE
(%)
Polymer
PfToBT
(mA/cm2)
(%)
1:2
1:3
1:2
1;3
1:2
1:3
1:2
1:3
6.8
5.6
7.4
7.2
8.7
8.8
8.1
9.4
0.717
0.717
0.737
0.737
0.697
0.677
0.677
0.677
41.0
37.6
43.1
42.1
41.1
45.5
44.6
44.4
2.0
1.6
2.5
2.2
2.5
2.7
2.5
2.9
to spin-coat the active layers. Table 4 summarizes the photo-
voltaic response data including VOC, JSC, FF, and PCE.
PbToBT
PfTDPP
PbTDPP
PfToBT has a PCE of 2.0%, with an open circuit voltage
(Voc) of 0.717 V, short-circuit current density (Jsc) of
6.8 mAcm2, and the FF of 41.0%. In PbToBT, a PCE of 2.5%
is observed under a Voc of 0.737 V, Jsc of 7.4 mA/cm2, and
FF of 43.1%. PfTDPP exhibits a PCE of 2.7% under a Voc of
0.677 V, Jsc of 8.8 mA/cm2, and FF of 45.5% and PbTDPP
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respectively. The polymer:PC70BM blended films have a smooth
surface due to its nanoscale features and had a low root-mean-
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In this study, four graft-type polymers prepared with differ-
ent types of donor and acceptor, PfToBT, PbToBT, PfTDPP,
and PbTDPP, were successfully synthesized via the Stille cou-
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strong electron affinity of the DPP group. All the poly-
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dissociation and electron transfer. On the other hand, the
PfTDPP and PbTDPP:PC70BM blend films indicated that
PC70BM coexisted in the top of the layer with the conjugated
side chains of the polymer. These had efficient charge trans-
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showed values of 9.4 mA/cm2, 0.677 V, 44.4, and 2.9%,
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This research was supported by the New and Renewable
Energy Core Technology Program of the Korea Institute of
Energy Technology Evaluation and Planning (KETEP) grant
funded by the Ministry of Trade, Industry and Energy (MI,
Korea) (No. 20133030000180) and New and Renewable
Energy Core Technology Program of the Korea Institute of
Energy Technology Evaluation and Planning (KETEP), grant
financial resource from the Ministry of Trade, Industry and
Energy, Republic of Korea (No. 20153010140030).
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