The effect of side-chain length on regioregular poly[3-(4-n-alkyl) phenylthiophene]/PCBM and ICBA polymer solar cells

Article abstract of DOI:10.1039/c2jm31371e

The photovoltaic, charge transport, light absorption and morphology properties of a series of poly[3-(4-n-alkyl)phenylthiophene] (PAPT):acceptor (phenyl-C₆₁-butyric acid methyl ester (PCBM) or indene-C₆₀ bisadduct (ICBA)) blend films are reported as a function of alkyl side-chain length. Regioregular poly[3-(4-n-butyl)phenylthiophene] (PBPT), poly[3-(4-n-hexyl)phenylthiophene] (PHPT), poly[3-(4-n-octyl)phenylthiophene] (POPT), and poly[3-(4-n-decyl)phenylthiophene] (PDPT) were successfully synthesized by a modified Grignard metathesis (GRIM) polymerization method. The effects of the alkyl side-chain length on the optical, electrochemical and structural properties of the polymers were carefully investigated to establish a relationship between the molecular structure and device function. Bulk heterojunction solar cells made of PAPTs blended with either PCBM or ICBA showed a strong photovoltaic property dependence on the alkyl side-chain length. Among all PAPTs used in this study, the best performance was observed in PHPT-based devices. This is the first report of the use of PHPT in polymer solar cells. In addition, PAPT devices blended with ICBA generally showed higher open-circuit voltages (V_oc) and power conversion efficiencies than PCBM-based devices. For example, a PHPT:ICBA photovoltaic device showed a V_oc of 0.79 V and a power conversion efficiency of 3.7%, which is the highest performance reported thus far using PAPT polymers. While the optical properties of PAPT/electron acceptor films exhibit the strongest effect on the short-circuit current of the devices, a balance between the optical, electrical and morphological properties of blended films caused by the alkyl side-chain length determines the overall photovoltaic performance of these devices. Therefore, our work provides a fundamental understanding of the molecular structure-device function relationship, especially with respect to changes in the alkyl side-chain length in conjugated polymers.

Full text of DOI:10.1039/c2jm31371e

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PAPER
The effect of side-chain length on regioregular poly[3-(4-n-alkyl)
phenylthiophene]/PCBM and ICBA polymer solar cells†
a
Chul-Hee Cho,‡ Hyeong Jun Kim,‡ Hyunbum Kang, Tae Joo Shin and Bumjoon J. Kim
a
a
b
a
*
Received 5th March 2012, Accepted 5th May 2012
DOI: 10.1039/c2jm31371e
The photovoltaic, charge transport, light absorption and morphology properties of a series of
poly[3-(4-n-alkyl)phenylthiophene] (PAPT):acceptor (phenyl-C61-butyric acid methyl ester (PCBM) or
indene-C60 bisadduct (ICBA)) blend films are reported as a function of alkyl side-chain length.
Regioregular poly[3-(4-n-butyl)phenylthiophene] (PBPT), poly[3-(4-n-hexyl)phenylthiophene] (PHPT),
poly[3-(4-n-octyl)phenylthiophene] (POPT), and poly[3-(4-n-decyl)phenylthiophene] (PDPT) were
successfully synthesized by a modified Grignard metathesis (GRIM) polymerization method. The
effects of the alkyl side-chain length on the optical, electrochemical and structural properties of the
polymers were carefully investigated to establish a relationship between the molecular structure and
device function. Bulk heterojunction solar cells made of PAPTs blended with either PCBM or ICBA
showed a strong photovoltaic property dependence on the alkyl side-chain length. Among all PAPTs
used in this study, the best performance was observed in PHPT-based devices. This is the first report of
the use of PHPT in polymer solar cells. In addition, PAPT devices blended with ICBA generally showed
higher open-circuit voltages (Voc) and power conversion efficiencies than PCBM-based devices. For
example, a PHPT:ICBA photovoltaic device showed a Voc of 0.79 V and a power conversion efficiency
of 3.7%, which is the highest performance reported thus far using PAPT polymers. While the optical
properties of PAPT/electron acceptor films exhibit the strongest effect on the short-circuit current of
the devices, a balance between the optical, electrical and morphological properties of blended films
caused by the alkyl side-chain length determines the overall photovoltaic performance of these devices.
Therefore, our work provides a fundamental understanding of the molecular structure–device function
relationship, especially with respect to changes in the alkyl side-chain length in conjugated polymers.
small-molecule fullerene derivatives comprise the active layer in
so-called bulk-heterojunction (BHJ) photovoltaics. In such
devices, small modifications in conjugated polymer structure
can induce large differences in the function of BHJ PSC
devices.11–15 In particular, a change in the alkyl solubilizing
group in conjugated polymers is known to have a strong
influence on physical and electrical properties, such as solu-
bility, crystallinity, interchain packing, light absorption, elec-
tron affinity, and absorption coefficients.16–22 In addition, even
small modifications of the alkyl chain in conjugated polymers
can affect their interactions with electron acceptors, resulting in
dramatic changes in both the blend morphology and solar cell
performance.23–25 For example, blend of poly(3-hexylthiophene)
(P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)
is one of the most representative BHJ photovoltaics, with
a reproducible efficiency of 4%.26,27 However, BHJ solar cells
based on regioregular poly(3-alkylthiophenes) (P3ATs) with
different alkyl chain lengths have poor PCEs under the same
device fabrication conditions used for P3HT-based devices.28,29
The underlying reasons for this large dependence on P3AT
side-chain length, although not fully understood, appear to be
Introduction
Conjugated polymers, as active components in polymer solar
cells (PSCs), have received great attention because of their
potential for realizing large-scale, low-cost, solution fabricated
and lightweight flexible devices.1–4 Over the last decade, inten-
sive research on PSCs has provided significant improvements in
power conversion efficiency (PCE) and a better understanding
of fundamental device working principles, including light
absorption,5,6 charge transport,7,8 and blend morphology of
electron donors and acceptors.9,10 In current PSCs, blends of
electron-donating conjugated polymers and electron-accepting
aDepartment of Chemical and Biomolecular Engineering, Korea Advanced
Institute of Science and Technology (KAIST), Daejeon 305-701, Korea.
E-mail: bumjoonkim@kaist.ac.kr; Fax: +82-42-350-3910; Tel: +82-42-
350-3935
bPohang Accelerator Laboratory, POSTECH, Pohang 790-784, Republic
of Korea
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2jm31371e
‡ These authors contributed equally to this work.
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linked to changes in the light absorption and the charge
mobility of thiophene polymers caused by morphological
differences in solution-processed films.30–32 Several works
demonstrated that the grafting of different alkyl chains into
polymer backbones can dramatically change optoelectronic
properties and photovoltaic efficiency.28,33 For example, while
the use of longer and bulkier alkyl groups improves solution
processibility, it also hinders intermolecular ordering and
affects charge transport between polymer stacks.34–36 In this
respect, the synthesis of conjugated polymers with systemati-
cally controlled alkyl chain lengths offers a great opportunity
to further optimize their photovoltaic properties and provide
a fundamental understanding of the relationship between their
properties and molecular structure.
Experimental section
General procedure for the preparation of 3-(4-n-alkyl)
phenylthiophenes (1)
To
a solution of the appropriate 4-n-alkylbromobenzene
(23.5 mmol), 2.5 mol% Pd2(dba)3, 10 mol% X-Phos and K3PO4
(47.0 mmol) in n-BuOH (50 mL), 3-thienylboronic acid
(28.2 mmol) was added at room temperature. This mixture was
degassed by argon bubbling, and the reaction mixture was heated
at 110 ꢀC for 12 h with vigorous stirring. Upon cooling to room
temperature, the reaction mixture was extracted with hexanes
(250 mL). The organic layer was washed with water (3 ꢁ 150 mL)
and brine (50 mL), dried over MgSO4 and concentrated in vacuo.
The crude compound was purified by flash column chromatog-
raphy using hexane as the eluent to afford the corresponding
product 1.
Regioregular poly[3-(4-n-octyl)phenylthiophene] (POPT) has
been shown to exhibit very promising features: a lower-lying
highest occupied molecular orbital (HOMO) level, a smaller
optical band gap of 1.8 eV and air stability.37–41 Friend et al.37
demonstrated a high photocurrent using a laminated bilayer
3-(4-Butyl)phenylthiophene (1a)
POPT:poly[2-methoxy-5-(20-ethylhexyloxy)-1,4-(1-cyano-vinyl-
The product was obtained as a white solid (91% yield): 1H NMR
(500 MHz, CDCl3) d 7.52 (d, J ¼ 8.1 Hz, 2H), 7.41–7.43 (m, 1H),
7.36–7.40(m, 2H), 7.22(d, J ¼ 8.1Hz, 2H), 2.64(t, J ¼ 7.7Hz, 2H),
1.60–1.67 (m, 2H), 1.34–1.43 (m, 2H), 0.95 (t, J ¼ 7.3 Hz, 3H).
38
ꢀ
ene)phenylene] (CN-PPV) device. Moreover, Frechet et al.
reported a PCE of 2.0% in similar POPT:CN-PPV bilayer-type
solar cells, which represents one of the highest reported effi-
ciencies to date for solution-processed all-polymer solar cells.
Recently, our group reported a series of POPT analogs with
different side-chain densities.41 We found that controlling the
side-chain density of the POPT analogs allows tuning of their
packing structure and HOMO level, thus improving the Voc and
PCE values. The synthetic simplicity and tunability of POPT
make it very attractive for the exploration of molecular struc-
ture–device function correlations. While POPT has been used as
one of the best materials for bilayer all-polymer solar cells, poly
[3-(4-n-alkylphenyl)thiophene]s (PAPTs) with other alkyl chain
lengths have not been studied to date.
3-(4-Hexyl)phenylthiophene (1b)
The product was obtained as a white solid (67% yield): 1H
NMR (500 MHz, CDCl3) d 7.51 (d, J ¼ 8.1 Hz, 2H), 7.40–7.42
(m, 1H), 7.36–7.39 (m, 2H), 7.21 (d, J ¼ 8.1 Hz, 2H), 2.62 (t,
J ¼ 7.7 Hz, 2H), 1.59–1.67 (m, 2H), 1.27–1.38 (m, 6H), 0.89 (t,
J ¼ 6.9 Hz, 3H).
3-(4-Octyl)phenylthiophene (1c)
The product was obtained as a white solid (82% yield): 1H NMR
(500 MHz, CDCl3) d 7.56 (d, J ¼ 8.2 Hz, 2H), 7.38–7.46 (m, 3H),
7.25 (d, J ¼ 8.2 Hz, 2H), 2.67 (t, J ¼ 7.7 Hz, 2H), 1.66–1.73 (m,
2H), 1.27–1.45 (m, 10H), 0.95 (t, J ¼ 6.8 Hz, 3H).
In this study, we synthesized a series of PAPTs with different
side-chain lengths (number of carbons in alkyl side-chain; n ¼
4, 6, 8, and 10) to systematically investigate the effect of the
side-chain length on photovoltaic performance. We found
a strong device performance dependence on the alkyl side-chain
length, observing that poly[3-(4-n-hexylphenyl)thiophene]
(PHPT) exhibited the best photovoltaic properties among
PAPTs with different alkyl chain lengths. In addition, indene-
C60-bisadduct fullerene (ICBA) with a higher lowest unoccu-
pied molecular orbital (LUMO) energy level than that of singly
functionalized PCBM42–46 was applied as an alternative elec-
tron-acceptor in blends with PAPTs. The PAPT:ICBA blend
devices showed higher efficiency than PCBM-based devices
because of a higher Voc; PAPT:ICBA devices also exhibited
a device performance trend as a function of alkyl chain length
similar to that in PAPT:PCBM devices. For example, the
PHPT:ICBA system showed the highest PCE (3.7%) among the
PAPT:ICBA devices. It could be possible to establish a balance
between the competing effects of solution processibility,
miscibility with PCBM/ICBA, and solid-state polymer packing
by tuning PAPT side-chain length. Variation in device perfor-
mance as a function of side-chain length was investigated by
careful characterization of the electrical, optical and morpho-
logical properties of films of four different PAPTs (n ¼ 4, 6, 8,
10) blended with either PCBM or ICBA.
3-(4-Decyl)phenylthiophene (1d)
The product was obtained as a white solid (80% yield): 1H NMR
(500 MHz, CDCl3) d 7.51 (d, J ¼ 8.2 Hz, 2H), 7.40–7.42 (m, 1H),
7.36–7.38(m, 2H), 7.21(d, J ¼ 8.1Hz, 2H), 2.62(t, J ¼ 7.7Hz, 2H),
1.60–1.66 (m, 2H), 1.23–1.38 (m, 14H), 0.89 (t, J ¼ 6.8 Hz, 3H).
General procedure for the preparation of 2-bromo-3-(4-n-alkyl)
phenylthiophenes (2)
To a stirred solution of the appropriate 3-(4-n-alkyl)phenyl-
thiophene 1 (15.0 mmol) in chloroform and acetic acid (200 mL,
3 : 1 v/v), NBS (15.75 mmol) was added at room temperature for
2 h. The reaction was monitored by TLC to establish completion.
The organic layer was extracted with CHCl3 (150 mL). The
remaining excess NBS was removed by washing with satd aq
Na2S2O3 (50 mL), 1.0 N aq NaOH (2 ꢁ 50 mL), and water (2 ꢁ
100 mL), dried over MgSO4 and concentrated in vacuo. The
resulting crude oil product was purified by gravity column
chromatography on silica gel using hexanes as the eluent to
afford the corresponding product 2.
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2-Bromo-3-(4-butyl)phenylthiophene (2a)
2-Bromo-5-iodo-3-(4-octyl)phenylthiophene (3c)
1
The product was obtained as a colorless oil (89% yield): 1H NMR
(500 MHz, CDCl3) d 7.47 (d, J ¼ 8.1 Hz, 2H), 7.29 (d, J ¼ 5.7 Hz,
1H), 7.25 (d, J ¼ 8.4 Hz, 2H), 7.03 (d, J ¼ 5.7 Hz, 1H), 2.66 (t,
J ¼ 7.8 Hz, 2H), 1.61–1.68 (m, 2H), 1.36–1.44 (m, 2H), 0.96 (t,
J ¼ 7.4 Hz, 3H).
The product was obtained as a pale yellow oil (84% yield): H
NMR (500 MHz, CDCl3) d 7.44 (d, J ¼ 8.0 Hz, 2H), 7.26 (d, J ¼
8.0 Hz, 2H), 7.22 (s, 1H), 2.68 (t, J ¼ 8.0 Hz, 2H), 1.64–1.72 (m,
2H), 1.25–1.45 (m, 10H), 0.94 (t, J ¼ 7.0 Hz, 3H).
2-Bromo-5-iodo-3-(4-decyl)phenylthiophene (3d)
2-Bromo-3-(4-hexyl)phenylthiophene (2b)
1
The product was obtained as a pale yellow oil (83% yield): H
The product was obtained as a colorless oil (83% yield): 1H NMR
(500 MHz, CDCl3) d 7.46 (d, J ¼ 8.1 Hz, 2H), 7.28 (d, J ¼ 5.7 Hz,
1H), 7.24 (d, J ¼ 8.1 Hz, 2H), 7.02 (d, J ¼ 5.7 Hz, 1H), 2.64 (t,
J ¼ 7.7 Hz, 2H), 1.61–1.68 (m, 2H), 1.28–1.41 (m, 6H), 0.90 (t,
J ¼ 7.4 Hz, 3H).
NMR (500 MHz, CDCl3) d 7.40 (d, J ¼ 8.1 Hz, 2H), 7.23 (d, J ¼
8.1 Hz, 2H), 7.19 (s, 1H), 2.63 (t, J ¼ 7.8 Hz, 2H), 1.61–1.67 (m,
2H), 1.22–1.40 (m, 14H), 0.89 (t, J ¼ 7.0 Hz, 3H).
General procedure for the synthesis of poly[3-(4-n-alkyl)
phenylthiophene] (PAPT)
2-Bromo-3-(4-octyl)phenylthiophene (2c)
A dry 250 mL two-neck flask was flashed with argon and was
charged with 2-bromo-5-iodo-3-(4-n-alkyl)phenylthiophene 3
and anhydrous THF (0.05 M conc). The resulting solution was
cooled to ꢂ78 ꢀC and stirred for 0.5 h. Then, i-PrMgCl (2.0 M in
THF, 0.95 equiv.) was added to the reaction mixture dropwise.
The product was obtained as a colorless oil (87% yield): 1H NMR
(500 MHz, CDCl3) d 7.52 (d, J ¼ 8.0 Hz, 2H), 7.33 (d, J ¼ 5.5 Hz,
1H), 7.29 (d, J ¼ 8.5 Hz, 2H), 7.07 (d, J ¼ 5.5 Hz, 1H), 2.69 (t,
J ¼ 7.5 Hz, 2H), 1.66–1.73 (m, 2H), 1.24–1.45 (m, 10H), 0.95 (t,
J ¼ 6.8 Hz, 3H).
ꢀ
Then, after stirring at ꢂ78 C for 1 h, the reaction mixture was
allowed to cool to room temperature, and then 0.3 mol%
Ni(dppp)Cl2 was added to the reaction mixture. After 12 h at
65 ꢀC, the polymer was precipitated into methanol from the THF
solution and filtered through a Soxhlet thimble. The polymer was
isolated from the solvent fraction, followed by precipitation into
methanol. The polymer was dried under vacuum to give the
desired PAPT polymer.
2-Bromo-3-(4-decyl)phenylthiophene (2d)
The product was obtained as a colorless oil (83% yield): 1H NMR
(500 MHz, CDCl3) d 7.47 (d, J ¼ 8.2 Hz, 2H), 7.29 (d, J ¼ 5.7 Hz,
1H), 7.24 (d, J ¼ 8.1 Hz, 2H), 7.03 (d, J ¼ 5.7 Hz, 1H), 2.64 (t,
J ¼ 7.7 Hz, 2H), 1.61–1.69 (m, 2H), 1.23–1.40 (m, 14H), 0.88 (t,
J ¼ 6.8 Hz, 3H).
Poly[3-(4-n-butyl)phenylthiophene] (PBPT) was prepared by
the reaction of 2-bromo-5-iodo-3-(4-n-butyl)phenylthiophene 3a
(1.0 g, 2.38 mmol) and THF (100 mL) with i-PrMgCl (2.0 M in
THF, 1.13 mL, 2.26 mmol) in the presence of Ni(dppp)Cl2
(0.3 mol%, 3.9 mg) at the reflux temperature instead of 65 ꢀC
with more dilute condition (z 0.025 M conc.). This compound
was prepared according to a modified general procedure. The
desired polymer was isolated by precipitation into methanol and
purified by Soxhlet extraction using methanol, acetone, hexane,
chloroform and chlorobenzene as solvents. The polymer was
dried under vacuum to give a black solid with metallic luster
General procedure for the preparation of 2-bromo-5-iodo-3-(4-n-
alkyl)phenylthiophenes (3)
To a stirred solution of the appropriate 2-bromo-3-(4-n-alkyl)
phenylthiophene 2 (12.0 mmol) in chloroform and acetic acid
(160 mL, 3 : 1 v/v), N-iodosuccinimide (NIS) (14.4 mmol) was
added, and the mixture was warmed and stirred at 60 ꢀC for 10 h.
The reaction was monitored by TLC to establish completion.
The excess NIS was removed by washing with saturated aq
Na2S2O3 (100 mL). The organic layer was extracted with CHCl3
(120 mL), washed with 1.0 N aq NaOH (2 ꢁ 40 mL) and water
(2 ꢁ 100 mL), dried over MgSO4 and concentrated in vacuo. The
resulting crude product was purified by gravity column chro-
matography on silica gel using hexanes as the eluent to afford the
corresponding product 3.
1
(110 mg, 21.4%): H NMR (500 MHz, CDCl3) d 7.21 (d, J ¼
7.8 Hz, 2H), 7.15 (d, J ¼ 7.8 Hz, 2H), 6.77 (s, 1H), 2.62 (t, J ¼ 7.6
Hz, 2H), 1.55–1.65 (m, 2H), 1.27–1.43 (m, 2H), 0.86 (t, J ¼ 7.2
Hz, 3H); Mn ¼ 10.3 kg molꢂ1, PDI ¼ 1.94.
Poly[3-(4-n-hexyl)phenylthiophene] (PHPT) was prepared by
the reaction of 2-bromo-5-iodo-3-(4-n-hexyl)phenylthiophene 3b
(838 mg, 1.87 mmol) and THF (38 mL) with i-PrMgCl (2.0 M in
THF, 0.89 mL, 1.78 mmol) in the presence of Ni(dppp)Cl2
(0.3 mol%, 3.1 mg). The desired polymer was isolated by
precipitation into methanol and purified by Soxhlet extraction
using methanol, acetone, hexane, chloroform and chlorobenzene
as solvents. The polymer was dried under vacuum to give a black
2-Bromo-5-iodo-3-(4-butyl)phenylthiophene (3a)
1
The product was obtained as a pale yellow oil (81% yield): H
NMR (500 MHz, CDCl3) d 7.40 (d, J ¼ 8.1 Hz, 2H), 7.23 (d, J ¼
8.1 Hz, 2H), 7.18 (s, 1H), 2.64 (t, J ¼ 7.8 Hz, 2H), 1.58–1.67 (m,
2H), 1.33–1.43 (m, 2H), 0.95 (t, J ¼ 7.4 Hz, 3H).
1
solid with metallic luster (171 mg, 37.4%): H NMR (500 MHz,
CDCl3) d 7.22 (d, J ¼ 8.0 Hz, 2H), 7.15 (d, J ¼ 8.0 Hz, 2H), 6.78
(s, 1H), 2.61 (t, J ¼ 7.6 Hz, 2H), 1.55–1.65 (m, 2H), 1.20–1.38 (m,
6H), 0.86 (t, J ¼ 6.8 Hz, 3H); Mn ¼ 22.4 kg molꢂ1, PDI ¼ 1.98.
Poly[3-(4-n-octyl)phenylthiophene] (POPT) was prepared by
the reaction of 2-bromo-5-iodo-3-(4-n-octyl)phenylthiophene 3c
(1.0 g, 2.10 mmol) and THF (42 mL) with i-PrMgCl (2.0 M in
2-Bromo-5-iodo-3-(4-hexyl)phenylthiophene (3b)
1
The product was obtained as a pale yellow oil (78% yield): H
NMR (500 MHz, CDCl3) d 7.41 (d, J ¼ 8.2 Hz, 2H), 7.24 (d, J ¼
8.2 Hz, 2H), 7.19 (s, 1H), 2.65 (t, J ¼ 7.7 Hz, 2H), 1.62–1.69 (m,
2H), 1.28–1.42 (m, 6H), 0.91 (t, J ¼ 7.2 Hz, 3H).
14238 | J. Mater. Chem., 2012, 22, 14236–14245
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THF, 1.0 mL, 1.99 mmol) in the presence of Ni(dppp)Cl2
(0.3 mol%, 3.4 mg). The desired polymer was isolated by
precipitation into methanol and purified by Soxhlet extraction
using methanol, acetone, hexane, chloroform and chlorobenzene
as solvents. The polymer was dried under vacuum to give a black
filtered through a 0.45 mm PTFE syringe filter. Blend solutions
composed of PBPT, PHPT, POPT and PDPT mixed with PCBM
or ICBA were prepared to a final PAPT concentration of 14 mg
mLꢂ1. Each solution was spin-cast onto an ITO/PEDOT:PSS
substrate at 900 rpm for 40 s. The substrates were then placed in
an evaporation chamber and held under high vacuum (below
10ꢂ1 Torr) for at least 1 h before deposition of approximately
0.7 nm of LiF and 100 nm of Al with a shadow mask that
produced four independent devices on each substrate. Then, all
PAPT:PCBM and ICBA devices were thermally annealed at
1
solid with metallic luster (244 mg, 42.7%): H NMR (500 MHz,
CDCl3) d 7.23 (d, J ¼ 8.0 Hz, 2H), 7.15 (d, J ¼ 8.0 Hz, 2H), 6.79
(s, 1H), 2.61 (t, J ¼ 7.6 Hz, 2H), 1.52–1.65 (m, 2H), 1.15–1.40 (m,
10H), 0.87 (t, J ¼ 6.7 Hz, 3H); Mn ¼ 22.0 kg molꢂ1, PDI ¼ 1.74.
Poly[3-(4-n-decyl)phenylthiophene] (PDPT) was prepared by
the reaction of 2-bromo-5-iodo-3-(4-n-decyl)phenylthiophene 3d
(1.12 g, 2.22 mmol) and THF (45 mL) with i-PrMgCl (2.0 M in
THF, 1.06 mL, 2.11 mmol) in the presence of Ni(dppp)Cl2
(0.3 mol%, 3.6 mg). The desired polymer was isolated by
precipitation into methanol and purified by Soxhlet extraction
using methanol, acetone, hexane, and chloroform as solvents.
The polymer was dried under vacuum to give a black solid with
ꢀ
180 C for 2 min. The photovoltaic performance of the devices
was characterized with an air-mass (AM) 1.5 G filter. The
intensity of the solar simulator was carefully calibrated using an
AIST-certified silicon photodiode. Current–voltage behavior
was measured using a Keithley 2400 SMU. The active area of the
fabricated devices was 0.102 cm2. The hole mobilities of the
PAPT:PCBM and PAPT:ICBA blended systems were measured
by the SCLC method using an ITO/PEDOT:PSS/blend/Au
device structure. The SCLC is described by:
1
metallic luster (76 mg, 11.4%): H NMR (500 MHz, CDCl3)
d 7.22 (d, J ¼ 8.2 Hz, 2H), 7.15 (d, J ¼ 8.2 Hz, 2H), 6.78 (s, 1H),
2.61 (t, J ¼ 7.6 Hz, 2H), 1.58–1.68 (m, 2H), 1.15–1.40 (m, 14H),
0.87 (t, J ¼ 7.0 Hz, 3H); Mn ¼ 7.9 kg molꢂ1, PDI ¼ 1.82.
9
8
V2
L3
JSCLC
¼
330m
where 30 is the permittivity of free space, 3 is the dielectric
constant of the polymer, m is the charge carrier mobility, V is the
potential across the device (V ¼ Vapplied ꢂ Vbi ꢂ Vr), and L is the
polymer layer thickness. The series and contact resistances of
the device (ꢃ25 U) were measured using a blank device (ITO/
PEDOT:PSS/Au), and the voltage drop caused by this resistance
(Vr) was subtracted from the applied voltage.
Characterization methods
UV-visible absorption spectra were obtained with a JASCO V-
570 spectrophotometer. The blend morphology of the active
layer PAPT:PCBM and PAPT:ICBA blend films was examined
by atomic force microscopy (AFM) (Veeco Dimension 3100) in
tapping mode. Different samples (PBPT:PCBM, PHPT:PCBM,
POPT:PCBM, PDPT:PCBM, PBPT:ICBA, PHPT:ICBA, POP-
T:ICBA and PDPT:ICBA) were prepared on a PEDOT:PSS/Si
substrate for device fabrication (1 : 0.7, w/w). Grazing incidence
X-ray diffraction (GIXRD) measurements were performed at
beamline 9A in the Pohang Accelerator Laboratory (South
Results and discussion
The chemical structure of the PAPTs and fullerene acceptors is
shown in Fig. 1. PAPT side-chain length increases in the order
PBPT, PHPT, POPT, and PDPT according to the number of
carbon atoms in the alkyl chain, n ¼ 4, 6, 8 and 10, respectively.
Scheme 1 presents the synthesis of four types of PAPTs con-
taining different alkyl chains at the para position on the phenyl
ring. The appropriate precursors, 2-bromo-5-iodo-3-(4-n-alkyl)
phenylthiophenes 3a–d, were obtained by a three-step synthetic
path via successive Suzuki–Miyaura coupling, regioselective
bromination, and iodination. The key steps in the monomer
synthesis are bromination and iodination at the 2- and 5-posi-
tions, respectively, of the 3-(4-n-alkyl)phenylthiophenes 1a–d.
Compounds 1–3 were successfully prepared and identified by 1H
NMR spectroscopy (Schemes S1–S4†). Then, the desired PAPT
polymers with different alkyl chain lengths (i.e., PBPT, PHPT,
POPT, and PDPT) were obtained by a modified Grignard
metathesis (GRIM) polymerization at 65 ꢀC.37,47 The monomers
used here were 2-bromo-5-iodo-3-(4-butyl)phenylthiophene 3a,
2-bromo-5-iodo-3-(4-hexyl)phenylthiophene 3b, 2-bromo-5-
iodo-3-(4-octyl)phenylthiophene 3c, and 2-bromo-5-iodo-3-(4-
decyl)phenylthiophene 3d, which have good selectivity for
activation by isopropylmagnesium chloride and subsequent
polymerization via nickel catalyst.
ꢁ
Korea). X-rays with a wavelength of 1.1010 A were used. The
incidence angle (ꢃ0.15ꢀ) was chosen to allow for complete
penetration of X-rays into the polymer film. The scattering
spectra were collected as a 2D image map that can be divided into
a component in the plane of the substrate (qxy) and a component
perpendicular to the substrate (qz).
Devices fabrication and measurement
BHJ PSC cells were fabricated using an ITO/PEDOT:PSS/
PAPT:acceptor/LiF/Al structure. PAPTs (PBPT, PHPT, POPT
and PDPT) were used as electron donors and either PCBM or
ICBA was used as the electron acceptor. ITO-coated glass
substrates were ultrasonicated in acetone and 2% Helmanex soap
in water, extensively rinsed with deionized water, then ultra-
sonicated in deionized water and isopropyl alcohol. Finally, the
substrates were dried for several hours in an oven at 80 ꢀC. The
ITO substrates were treated with UV-ozone prior to
PEDOT:PSS deposition. A filtered dispersion of PEDOT:PSS in
water (PH 500) was applied by spin-coating at 3000 rpm for 40 s
ꢀ
and baking for 30 min at 140 C in air. After application of the
PEDOT:PSS layer, all subsequent procedures were performed in
a glove box under a N2 atmosphere. Separate solutions of PBPT,
The number-average molecular weight (Mn) and poly-
dispersity index (PDI) of the obtained polymers were analyzed by
high-temperature size-exclusion chromatography (SEC) cali-
brated with polystyrene standards using UV and RI detectors
PHPT, POPT, PDPT, PCBM and ICBA in o-DCB (30 mg mLꢂ1
)
were prepared and stirred at 120 ꢀC for at least 24 h before being
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Fig. 1 Molecular structure of regioregular PAPTs and fullerene acceptors (PCBM and ICBA) used in this study.
Scheme 1 Synthetic route for different alkyl side-chain-containing regioregular PAPTs. Reaction conditions: (i) 2.5 mol% Pd2(dba)3, 10 mol% X-Phos,
K3PO4 (2.0 equiv.), n-BuOH (0.5 M conc.), 3-thienylboronic acid (1.2 equiv.), 80 ꢀC, 12 h. (ii) NBS (1.05 equiv.), AcOH : CHCl3 (1 : 3, v/v), rt, 2 h. (iii)
NIS (1.2 equiv.), AcOH : CHCl3 (1 : 3, v/v), 60 ꢀC, 10 h. (iv) i-PrMgCl (0.95 equiv.), THF, ꢂ78 ꢀC to rt, 1.5 h. (v) 0.3 mol% Ni(dppp)Cl2, 65 ꢀC, 20 h.
(Table S1†). The polymerization conditions, including reaction
time and concentration, were carefully controlled to produce
similar Mn and PDI values for different PAPTs. The Mn of all
polymers ranged from 10 to 20 kg molꢂ1, with a PDI of 1.7–2.0,
and all polymers were found to have high regioregularity (>99%),
1
as determined by H NMR (Scheme S5†).
Grazing incidence X-ray diffraction (GIXRD) patterns of the
four regioregular PAPTs are shown in Fig. 2. A thin layer (20–
30 nm) of PEDOT:PSS was spin-coated onto silicon substrates
followed by the polymer layer on top; the substrates were then
thermally annealed at 180 ꢀC for 1 h. Fig. 2(a)–(d) show the
GIXRD patterns of PBPT, PHPT, POPT and PDPT, respec-
tively; each of the 2D GIXRD pattern images can be divided into
a component in the plane of the substrate (qxy) and a component
perpendicular to the substrate (qz). In all four samples, the
diffraction peaks were strongest in the out-of-plane direction,
indicating that the polymer films had a well-organized structure
with lamellar conjugated polymer stacks oriented along the
perpendicular axis of the substrate. The interlayer domain spac-
ings of PBPT, PHPT, POPT and PDPT calculated from the qxy
values of GIXRD patterns were 2.15, 2.51, 2.99 and 3.31 nm,
respectively, indicating that the distance between adjacent PAPT
Fig. 2 GIXRD images of (a) PBPT, (b) PHPT, (c) POPT, and (d) PDPT
thin films after annealing for 1 h at 180 ꢀC.
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lamellae increased by approximately 0.4 nm with the addition of
two carbons to the alkyl chains. Longer side-chains resulted in
larger interlayer spacings, clearly indicating that the planar thio-
phene backbone stacks were uniformly spaced by the phenyl-alkyl
side-chains. A similar trend was observed in poly(3-alkylth-
iophene) (P3AT) systems; regioregular P3ATs (P3BT, P3HT,
P3OT, and P3DT) showed that the interlayer domain spacing
increased by approximately 0.4 nm with the addition of two
carbons to the alkyl side-chain (i.e., from P3HT to POPT, from
P3OT to P3DT).16 PHPT (n ¼ 6) shows a significantly higher
domain spacing than P3HT (2.51 vs. 1.65 nm), which has the same
number of carbons in the alkyl side-chain.48–50 The increased
domain spacing by 0.86 nm is due to the presence of bulky phenyl
rings between the alkyl side-chain and thiophene ring in PHPT.
Moreover, for a given number of carbons in the alkyl chain, the
difference in interlayer domain spacing between PAPTs and
P3ATs is found to be constant at approximately 0.9 nm.29
indicates that a longer and thus more flexible alkyl chain can
better accommodate the twisted phenyl conformation. In
general, p–p stacking is critical for both charge transport and
light absorption in conjugated polymers.51,52 The phenyl ring
torsion can induce different intermolecular interactions that lead
to different optoelectronic properties.
Fig. 4(a) and (b) show current density versus voltage (J–V)
curves of the ITO/PEDOT:PSS/PAPT:electron acceptors ((a)
PCBM or (b) ICBA)/LiF/Al devices under AM 1.5 illumination
at 100 mW cmꢂ2. The PAPT:PCBM blend solutions in o-
dichlorobenzene (o-DCB) were filtered and spin-coated on ITO/
glass substrates covered by a PEDOT:PSS layer. PBPT, because
of its short alkyl chain, has very limited solubility even in hot o-
DCB; for this reason, PBPT-based solar cell devices were fabri-
cated without filtration. Optimized devices with a 1 : 0.7 w/w
PAPT:acceptor weight ratio had active layer thicknesses between
ꢀ
80 and 100 nm and were annealed for 2 min at 180 C. Photo-
To provide some general insight into the polymer packing
structure, high resolution bulk X-ray diffraction (XRD) patterns
were also measured for the four different regioregular PAPTs.
XRD patterns were normalized with respect to the peak at q ¼
voltaic performances are summarized in Table 1. The PCE of the
POPT:PCBM device was 2.13% (Voc ¼ 0.51 V, Jsc ¼ 8.25 mA
cmꢂ2, FF ¼ 0.50), which agrees well with previous findings.38,41
Among the PAPT:PCBM-based BHJ PSCs, the PHPT:PCBM
ꢂ1
ꢁ
1.25 A and plotted in Fig. 3. The PAPT diffraction features
device exhibited the highest PCE of 2.51% (Voc ¼ 0.48 V, Jsc ¼
centered at q ¼ 1.25 and 1.70 Aꢂ1 correspond to spacings of 0.38
9.73 mA cmꢂ2, and FF ¼ 0.53). We observed a nonlinear device
performance dependence on the alkyl chain length, increasing in
the order of PDPT, PBPT, POPT and PHPT. To gain insight into
the effect of different alkyl chain lengths on photovoltaic prop-
erties, we also fabricated solar cells using ICBA as an electron
acceptor, which has a higher LUMO energy level than PCBM
and thus provides a higher Voc. A similar photovoltaic perfor-
mance trend with respect to alkyl side-chain length was observed
in PAPT:ICBA devices, increasing in the order of PDPT, PBPT,
POPT and PHPT. Note that the PHPT:ICBA device exhibited
the highest PCE value among all PAPT-based BHJ devices,
3.73% (Voc: 0.78 V, Jsc: 9.60 mA cmꢂ2, and FF: 0.50). The
essential device parameters (Voc, Jsc, FF and PCE) of the
PAPT:PCBM and PAPT:ICBA systems are compared in Fig. 5.
PAPT devices with ICBA as an electron acceptor exhibited
higher performances than devices based on PAPT:PCBM blends.
The increase in PCE was largely caused by an increase in Voc;
increases of more than 0.2–0.25 V over those of the PAPT:PCBM
devices were observed (PBPT: 0.45 to 0.73 V, PHPT: 0.48 to 0.78
V, POPT: 0.51 to 0.78 V, and PDPT: 0.57 to 0.79 V). Interest-
ingly, the increase in Voc is more pronounced for PAPTs with
a shorter alkyl chain length. For example, while the increase in
Voc for PDPT:ICBA over that of PDPT:PCBM was 0.22 V, the
Voc increase was 0.3 V for the PHPT-based devices. The
enhancement in the Voc and device performance observed in this
study agrees well with previous reports regarding P3HT:ICBA
systems.42,43 Due to higher-lying LUMO levels than PCBM,
bisadduct-type fullerenes including ICBA show potential for
replacing PCBM. However, the increase in PCE has been limited
to P3HT-based systems.53,54 Therefore, our results suggest that
bisadduct-type fullerenes can be applied as electron acceptors to
achieve enhanced Voc and PCE values in non-P3HT-based
systems.
ꢁ
and 0.51 nm, respectively. The numbers, which are independent
of side-chain length, can be attributed to the p–p stacking
distance between PAPT chains. The presence of two peaks at
0.38 and 0.51 nm suggests the presence of two different p–p
stacking distances in PAPTs, which is consistent with a report by
39
ꢀ
Frechet et al. on POPT polymers.
These two different p–p stacking distances arise from the two
major phenyl ring conformations relative to the PAPT backbone:
the p–p stacking distance of 0.51 nm corresponds to an orien-
tation of the phenyl ring perpendicular to the backbone, while
that at 0.38 nm corresponds to a parallel phenyl ring. The twisted
phenyl ring causes an increase in distance between adjacent
polymer chains. Interestingly, whereas the 0.38 and 0.51 nm peak
intensities were constant regardless of the alkyl chain length, the
intensity of the shoulder intermediate between the two peaks
increased as the alkyl chain length increased. This finding
Interestingly, the Voc of PAPT-based BHJ devices increased
gradually with increasing PAPT alkyl chain length. In particular,
the PAPT:PCBM systems showed Voc values ranging from
0.45 V (PBPT) to 0.57 V (PDPT). Despite the linear trend in Voc,
Fig. 3 Normalized XRD patterns of regioregular PAPT films. Two
ꢂ1
ꢁ
different (010) peaks at q ¼ 1.25 and 1.7 A indicate two major
conformations of POPT polymers, where phenyl rings are either flat or
twisted perpendicular relative to the backbone.
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Fig. 4 Current density–voltage (J–V) characteristics of BHJ PSCs based on blends of PAPTs (PBPT, PHPT, POPT and PDPT) as donors with two
types of acceptors ((a) PCBM or (b) ICBA) under AM 1.5 G simulated solar illumination (100 mW cmꢂ2). The weight ratio of electron donor to acceptor
was kept constant (1 : 0.7 w/w).
Table 1 Device characteristics of BHJ PSCs composed of PAPT:elec-
tron acceptor (PCBM or ICBA) blends under AM 1.5 G simulated solar
The measured mobility value for the POPT:PCBM device
(2.98 ꢁ 10ꢂ4 cm2 Vꢂ1 sꢂ1) agrees well with that reported previ-
ously in the literature.38,40
illumination (100 mW cmꢂ2
)
The light absorption of the active layer is one of the major
factors affecting the Jsc values of PSC devices. Fig. 6 shows the
thin film optical absorption spectra of (a) PAPT:PCBM and (b)
PAPT:ICBA blend films, each of which was constructed under
conditions affording the most efficient devices. The optical
properties of the blend films are summarized in Table 2. The
optical absorption spectra of the PAPT:acceptor films display
three maxima in the 500–700 nm range but show significant
differences in peak location and relative peak intensities with
changing alkyl chain length. The spectra of PHPT:acceptor films
showed the strongest red-shift of the PAPT devices, with the
maximum absorption (lmax) found at 603 nm for PHPT:PCBM
and 604 nm for PHPT:ICBA (Table 2). The lmax values of POPT
blend films are slightly blue-shifted relative to those of PHPT
blends; POPT:PCBM and POPT:ICBA showed maximum
absorption at 601 and 600 nm, respectively. PBPT showed
further blue-shifted absorption spectra, showing maximum
absorption at approximately 596 nm for PBPT:PCBM devices
and 598 nm for PBPT:ICBA devices. PDPT showed a more
dramatic blue-shift in absorption spectra (lmax ¼ 588 nm for
PDPT:PCBM and lmax ¼ 587 nm for PDPT:ICBA). This blue-
shift indicates weaker intermolecular ordering between conju-
gated polymers; conversely, the strong red-shift of PHPT
absorption features indicates a significantly higher degree of
ordering in PHPT polymers compared with PBPT and POPT
polymers, which is among the reasons for improved solar-cell
performance. In addition to differences in the location of the
maximum absorption peak, the optical spectra of the
PAPT:PCBM and PAPT:ICBA films exhibited different inten-
sities for the lmax and vibronic peaks at 600 and 650 nm,
respectively. The vibronic peak at ꢃ650 nm is a unique charac-
teristic of highly ordered PAPT chains.38,40,41 The absorption
spectra of PAPT:PCBM and PAPT:ICBA blends were normal-
ized with respect to the first absorption peak at approximately
550 nm; the relative intensities of the lmax peak at ꢃ600 nm
(Imax/I1st) and the vibronic peak at ꢃ650 nm (Ivib/I1st), compared
Active layer
Voc (V)
Jsc (mA cmꢂ2
)
FF
PCE (%)
PBPT:PCBM
PHPT:PCBM
POPT:PCBM
PDPT:PCBM
PBPT:ICBA
PHPT:ICBA
POPT:ICBA
PDPT:ICBA
0.45
0.48
0.51
0.57
0.73
0.78
0.78
0.79
5.88
9.73
8.25
3.38
8.18
9.60
8.67
2.24
0.45
0.53
0.50
0.44
0.45
0.50
0.47
0.38
1.19
2.51
2.13
0.85
2.67
3.73
3.16
0.67
PHPT-based devices showed the highest efficiencies because of
their much higher Jsc and FF values (Fig. 5(b) and (d)). It was
observed that the dramatic changes in PCE of the PAPT:ac-
ceptor devices upon altering the alkyl side-chain length were due
to the large differences in Jsc and FF values. For example, the
PDPT:ICBA device, which has the highest Voc value (Voc
¼
0.79 V) of the four PAPT:ICBA devices, suffered from low
current density (Jsc ¼ 2.24 mA cmꢂ2) and low PCE (0.67%).
To better understand the effect of alkyl side-chain length on
the electrical properties of PAPT solar cells, the space-charge-
limited current (SCLC) of hole-only devices was measured and
analyzed (Fig. S2†). The SCLC mobility measures hole mobility
in the direction perpendicular to the electrodes and is thus the
most reasonable measurement of the electrical properties of
PAPTs in blend films. Hole-only devices were constructed under
optimized device conditions, with the structure of ITO/
PEDOT:PSS/PAPT:acceptor/Au (using either PCBM or ICBA
as the acceptor material) and hole mobilities were calculated
using the Mott–Gurney equation. Whereas PAPT:PCBM BHJ
photovoltaic devices showed very different Jsc values, ranging
from 3.38 to 9.73 mA cmꢂ2 (PDPT and PHPT, respectively), no
significant differences between the devices were found in the hole
mobilities. Hole mobilities were in the range of 1.86 ꢁ 10ꢂ4 to
4.57 ꢁ 10ꢂ4 cm2 Vꢂ1 sꢂ1 for PAPT:PCBM devices and 1.27 ꢁ 10ꢂ4
to 1.90 ꢁ 10ꢂ4 cm2 Vꢂ1 sꢂ1 for PAPT:ICBA devices (Table S2†).
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Fig. 5 Variation of key solar cell parameters ((a) Voc, (b) Jsc, (c) FF and (d) PCE) with alkyl chain length.
Fig. 6 UV-vis absorption spectra of (a) PAPT:PCBM and (b) PAPT:ICBA blend films under optimized device conditions.
with the first absorption peak at ꢃ550 nm, were then calculated
(Table 2). Regardless of the acceptor type, PHPT-based blend
films show the most prominent vibronic features, indicating
a higher order of crystallization; conversely, PDPT-based films
exhibited vibronic peaks with reduced intensities and thus lower
crystallization order. The addition of longer alkyl chains may
disrupt the two-dimensional polymer packing and subsequent
crystallization. The observed trend in the results is similar to that
reported for P3AT systems.19,29 We speculate that the reason for
the weak vibronic features of PBPT is due to the extremely poor
solubility of PBPT that may disturb the efficient packing of
PBPT chains during the solution processing of films. Both
intensity ratios (Imax/I1st and Ivib/I1st) increased in the same order
as the device performance (in the decreasing order PHPT, POPT,
Table 2 Optical properties of PAPT:PCBM and PAPT:ICBA blend
films
PAPT:PCBM
PAPT:ICBA
lmax
(nm)
lmax
(nm)
a
b
a
b
Imax/I1st
Ivib/I1st
Imax/I1st
Ivib/I1st
PBPT
PHPT
POPT
PDPT
596
603
601
588
1.04
1.10
1.07
1.01
0.59
0.68
0.65
0.52
598
604
600
587
1.05
1.09
1.06
1.00
0.64
0.67
0.65
0.57
a
The relative intensity of maximum absorption peak (Imax) to first
absorption peak (I1st). The relative intensity of vibronic peak (Ivib) to
first absorption peak (I1st).
b
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Fig. 7 Tapping-mode AFM topography images of PAPT:PCBM and PAPT:ICBA blend films: (a) PBPT, (b) PHPT, (c) POPT, and (d) PDPT blends
with PCBM, (e) PBPT, (f) PHPT, (g) POPT, and (h) PDPT blends with ICBA. All samples were prepared under identical conditions to that for
optimized devices. The scale bar is 1 mm. RMS roughness: (a) 4.3 ꢄ 0.4 nm, (b) 3.2 ꢄ 0.3 nm, (c) 2.8 ꢄ 0.2 nm, (d) 1.4 ꢄ 0.2 nm, (e) 3.4 ꢄ 0.3 nm, (f) 1.2 ꢄ
0.2 nm, (g) 1.8 ꢄ 0.1 nm, and (h) 1.0 ꢄ 0.4 nm.
PBPT and PDPT). Furthermore, the trend in lmax for different
PAPTs (in the decreasing order PHPT, POPT, PBPT and PDPT)
is consistent with that of PCE. For this reason, the optical
properties of PAPTs in the blend film could be one of the major
contributions to the difference of the device performance as
a function of side-chain length. In addition, the thin film optical
absorption spectra of pristine PAPT films were measured for
comparison (Fig. S4†). The change in the peak positions of
PAPT films as a function of alkyl chain length is much less
pronounced compared to that in the PAPT:acceptor blend films,
indicating that the addition of PCBM or ICBA molecules in the
PAPT blend films affects the packing structure of PAPT poly-
mers. And, the interaction between the acceptor molecule and
PAPT polymer is dependent on its alkyl chain length.
spin-coating process while electron acceptors with relatively
good solubility are precipitated at a much later stage. This
unbalanced precipitation causes extensive phase separation and
produces rough films. The inhomogeneous morphology of
polymers with short alkyl chains provides a reasonable expla-
nation for the lower PCE in PBPT-based systems: the inho-
mogeneous blend morphology reduces the interfacial area
between electron donor and acceptor domains available for
exciton dissociation and causes poor electrical contact between
the active layer and cathode.
Conclusions
In this study, we demonstrated the correlation between PAPT
alkyl chain length and BHJ photovoltaic device functions.
Careful investigation of BHJ blend film properties was per-
formed using four PAPTs with different alkyl chain lengths
blended with PCBM or ICBA. While the optical properties of
PAPT/electron acceptor films exhibit the strongest effect on the
short-circuit current of the devices, a balance between the
optical, electrical and morphological properties of PAPT:elec-
tron acceptor blend films caused by changes in the alkyl side-
chain length was found to determine the overall photovoltaic
performance of these devices. Among PAPTs, the hexyl group
provides a good balance between crystallinity and miscibility
with acceptors to achieve optimal optical properties and photo-
voltaic device performance. In addition, a bisadduct-type
fullerene acceptor successfully enhanced the Voc values and PCE
of solar-cell devices. For example, PHPT:ICBA devices showed
a PCE of 3.73%, which is the highest PCE for PAPT-based BHJ
solar-cell devices reported thus far. GIXRD measurements
revealed that the interlayer domain spacing of the PHPT is
2.51 nm, 0.86 nm longer than that of P3HT, which shows the best
photovoltaic performance among P3ATs.
To gain deeper insight into the effect of alkyl chain length on
the interaction between PAPT and electron acceptors, the
nanoscale morphology of BHJ blend films was investigated
using tapping-mode atomic force microscopy (AFM). Fig. 7
shows a series of AFM topography images of PAPT:PCBM (a–
d) and PAPT:ICBA (e–h) blends, which were prepared under
conditions identical to those used for optimized BHJ devices.
The PAPT:ICBA blend films generally show lower root-mean-
square (RMS) surface roughness than PAPT:PCBM blend films
because of the intrinsically lower crystallinity of ICBA.55
Regardless of the acceptor type, the RMS roughness was
reduced with increasing alkyl chain length. PDPT:ICBA device
shows the lowest RMS roughness among the different
PAPT:PCBM and PAPT:ICBA devices. The trend of
decreasing roughness with increasing alkyl side-chain length
appears to be related to the increasing solubility of PAPTs with
increasing side-chain length. The polymer with the longest alkyl
chain (PDPT) retains better solubility as well as miscibility with
fullerene acceptors, which results in smooth BHJ films. In
contrast, poorly soluble PBPT begins to precipitate early in the
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25 H. Zhou, L. Yang, S. Xiao, S. Liu and W. You, Macromolecules,
2010, 43, 811–820.
Acknowledgements
26 W. L. Ma, C. Y. Yang, X. Gong, K. Lee and A. J. Heeger, Adv. Funct.
Mater., 2005, 15, 1617–1622.
27 G. Li, V. Shrotriya, J. S. Huang, Y. Yao, T. Moriarty, K. Emery and
Y. Yang, Nat. Mater., 2005, 4, 864–868.
28 G. Q. Ren, P. T. Wu and S. A. Jenekhe, Chem. Mater., 2010, 22,
2020–2026.
29 L. H. Nguyen, H. Hoppe, T. Erb, S. Gunes, G. Gobsch and
N. S. Sariciftci, Adv. Funct. Mater., 2007, 17, 1071–1078.
30 W. D. Oosterbaan, J. C. Bolsee, A. Gadisa, V. Vrindts, S. Bertho,
J. D’Haen, T. J. Cleij, L. Lutsen, C. R. McNeill, L. Thomsen,
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This research was supported by the Korea Research Foundation
Grant funded by the Korean Government (2011-0030387, 2011-
0010412), the New & Renewable Energy KETEP grant (2010-
T100100460) and the Fundamental R&D Program Grant for
Core Technology of Materials funded by the Ministry of
Knowledge Economy Republic of Korea. Experiments at PLS
were supported in part by MEST and POSTECH.
31 H. Xin, F. S. Kim and S. A. Jenekhe, J. Am. Chem. Soc., 2008, 130,
5424–5425.
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