Polythiophenes comprising conjugated pendants toward long-term air-stable inverted polymer solar cells with high open circuit voltages

Article abstract of DOI:10.1039/c3ta11272a

A series of polythiophenes (PTs) functionalized with bulky conjugated side chains comprising tert-butyl substituted carbazole (tCz) as an electron donor pendant and bisbenzothiazolylvinyl (DBT) as an electron acceptor pendant were synthesized via Stille copolymerization for polymer solar cell (PSC) applications. We use the descriptors PTtCz, PT(tCz)_0.9(DBT) _0.1, PT(tCz)_0.64(DBT)_0.36, PT(tCz) _0.45(DBT)_0.55, and PTDBT to identify each of these conjugated polymers, with the names denoting the compositions of the bulky pendants. The tunable energy levels of the PTs were accomplished by incorporating both tCz as a donor pendant and DBT as an acceptor pendant, while retaining the low-lying HOMO levels (-5.26 to -5.39 eV). Furthermore, lower bandgaps were observed for the DBT-derived PTs because of stronger donor-π-acceptor characteristics and more efficient intramolecular charge transfer. Conventional PSCs were fabricated by spin-coating the blend of each PT and the fullerene derivative (PC₇₁BM). The conventional PSC devices exhibited high open circuit voltages (V_oc) of around 0.79-0.91 V. The power conversion efficiency (PCE) of the PSCs based on PTtCz : PC ₇₁BM (w/w = 1 : 2.5) reached 2.48% with a V_oc of 0.91 V, short circuit current (J_sc) of 6.58 (mA cm^-2) and fill factor (FF) of 41% under the illumination of AM1.5, 100 mW cm^-2. Furthermore, a PTtCz/PC₇₁BM-based inverted PSC with ZnO_x and MoO₃ as an electron extraction layer and a hole extraction layer respectively was capable of retaining ca. 80% of its original efficiency after storage under ambient conditions (without encapsulation) for 1032 h, according to the ISOS-D-1 shelf protocol. The highly durable inverted PSC accompanied by a large V_oc value was achieved for the PT-type polymers.

Full text of DOI:10.1039/c3ta11272a

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Materials Chemistry A
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PAPER
Polythiophenes comprising conjugated pendants
toward long-term air-stable inverted polymer solar cells
with high open circuit voltages
Cite this: J. Mater. Chem. A, 2013, 1,
8950
a
b
c
Hsing-Ju Wang, Chen-Wei Chou, Chih-Ping Chen, Ying-Hsiao Chen,b
*
b
a
*
*
Rong-Ho Lee and Ru-Jong Jeng
A series of polythiophenes (PTs) functionalized with bulky conjugated side chains comprising tert-butyl
substituted carbazole (tCz) as an electron donor pendant and bisbenzothiazolylvinyl (DBT) as an
electron acceptor pendant were synthesized via Stille copolymerization for polymer solar cell (PSC)
applications. We use the descriptors PTtCz, PT(tCz)0.9(DBT)0.1, PT(tCz)0.64(DBT)0.36, PT(tCz)0.45(DBT)0.55
,
and PTDBT to identify each of these conjugated polymers, with the names denoting the compositions
of the bulky pendants. The tunable energy levels of the PTs were accomplished by incorporating both
tCz as a donor pendant and DBT as an acceptor pendant, while retaining the low-lying HOMO levels
(ꢀ5.26 to ꢀ5.39 eV). Furthermore, lower bandgaps were observed for the DBT-derived PTs because of
stronger donor–p–acceptor characteristics and more efficient intramolecular charge transfer.
Conventional PSCs were fabricated by spin-coating the blend of each PT and the fullerene derivative
(PC71BM). The conventional PSC devices exhibited high open circuit voltages (Voc) of around 0.79–
0.91 V. The power conversion efficiency (PCE) of the PSCs based on PTtCz : PC71BM (w/w ¼ 1 : 2.5)
reached 2.48% with a Voc of 0.91 V, short circuit current (Jsc) of 6.58 (mA cmꢀ2) and fill factor (FF) of
41% under the illumination of AM1.5, 100 mW cmꢀ2. Furthermore, a PTtCz/PC71BM-based inverted PSC
with ZnOx and MoO3 as an electron extraction layer and a hole extraction layer respectively was capable
of retaining ca. 80% of its original efficiency after storage under ambient conditions (without
encapsulation) for 1032 h, according to the ISOS-D-1 shelf protocol. The highly durable inverted PSC
accompanied by a large Voc value was achieved for the PT-type polymers.
Received 29th March 2013
Accepted 16th May 2013
DOI: 10.1039/c3ta11272a
www.rsc.org/MaterialsA
(Voc) value of only about 0.6 V and narrow absorption spectra in
the UV-visible region which limited the photocurrent genera-
Introduction
tion of its related BHJ solar cell.9 In general, the Voc of a PSC is
proportional to the difference between the HOMO of the elec-
tron-sufficient polymer and the lowest unoccupied molecular
orbital (LUMO) of the electron-decient fullerene. Lower HOMO
levels of the polymers would provide a higher Voc according to
the theoretical prediction. Additionally, conjugated polymers
with higher ionization potentials are capable of minimizing the
p-doping level under ambient O2 and increasing the environ-
mental stability.10
Much of the improvement in PSCs can be attributed to the
design of novel conjugated polymers with (i) strong and broad
visible absorption with high charge carrier mobility to increase
the short-circuit current density ( Jsc) and (ii) low-lying HOMO
levels to achieve large Voc values. The absorption properties and
the energy levels can be modulated by introducing conjugated
side chains onto the conjugated polymers.11,12 Li and coworkers
rst studied a PT with conjugated pendants attached that
exhibited a broad absorption band in the UV and visible
regions, and was capable of harvesting a great amount of solar
light.13 Subsequently, the investigations on PTs comprising
Wet-processing is the main advantage of polymer solar cells
(PSC). In this process, the active layer can be manufactured
using fast roll-to-roll based methods at a cost that has the
potential to be competitive, pending further development.1–5 On
the basis of the concept of the bulk heterojunction (BHJ)
structure, PSCs made by blending poly(3-hexylthiophene)
(P3HT) as a donor and fullerene derivative [6,6]-phenyl-C61/71
-
butyric acid methyl ester (PC61BM/PC71BM) as an acceptor have
been intensively investigated, leading to power conversion
efficiencies (PCE) of up to 4–5%.6–8 Unfortunately, the highest
occupied molecular orbital (HOMO) (ꢀ4.9 to ꢀ5.1 eV) energy
level of P3HT resulted in an unsatisfactory open circuit voltage
aInstitute of Polymer Science and Engineering, National Taiwan University, Taipei 106,
Taiwan. E-mail: rujong@ntu.edu.tw; Fax: +886-2-33665237; Tel: +886-2-33665884
bDepartment of Chemical Engineering, National Chung Hsing University, Taichung
402, Taiwan. E-mail: rhl@nchu.edu.tw; Fax: +886-4-22854734; Tel: +886-4-22854308
cDepartment of Materials Engineering, Ming Chi University of Technology, New Taipei
City 243, Taiwan. E-mail: cpchen@mail.mcut.edu.tw; Fax: +886-2-29084091; Tel:
+886-2-29089899 extn. 4439
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conjugated pendants as side-chains have attracted a great deal copolymers/PC71BM blends, and then the dried lms were
of interest.14–19 Our previous studies on conjugated polymers sandwiched between a transparent anode ITO and cathodes
presenting bulky conjugated electron donor moieties as side- (Ca/Al). The PV performance of these PSCs was studied in terms
chains revealed that the sterically induced twisting of the of the inuence of the conjugated pendant groups. Additionally,
polymer backbone was responsible for achieving a low-lying in order to investigate the important issue of air stability, the
HOMO level and enhancing the oxidative stability of conjugated PTtCz based inverted devices without encapsulation were tested
polymers as compared to the parent P3HT.20,21 Larger Voc values under ambient conditions for 1032 h, under the ISOS-D-1 shelf
of the conjugated PTs functionalized with bulky pendants have measurement.30,31
been observed in conventional PSCs.13,22–24 Nevertheless, the
bulky substituents increase the degree of twisting from
planarity in the backbone, resulting in a decreased intra-
molecular charge transfer and larger optical bandgaps (Eg). To
Experimental
Chemicals
address this issue, the incorporation of electron acceptor
pendants on PTs was studied by Wei and coworkers.17,18 The
results showed that a lower Eg was obtained due to the presence
of the efficient intramolecular charge transfer from the PT
backbone to the conjugated acceptor side chain induced by the
electron-accepting imidazole moiety.
Trimethylstannyl
chloride,
n-bromosuccinimide,
2-for-
mylthiophene, phosphorous oxychloride, 2,20-methylene bisben-
zothiazole, n-butyllithium (2.5 M in hexane) and other reagents
and chemicals were purchased from Aldrich, Alfa and TCI
Chemical Co., and used as received. Dichloromethane (DCM),
tetrahydrofuran (THF), dimethylformamide (DMF), toluene, and
o-dichlorobenzene (o-DCB) were freshly distilled over appropriate
drying agents prior to use, while being purged with nitrogen. The
tert-butyl-substituted carbazole-containing moiety (tCz) and 2, 20-
methylenebisbenzothiazole substituted moiety (DBT) were
synthesized according to Scheme 2. Syntheses of a series of
PTs-based donor–acceptor random copolymers via Stille poly-
merization with various tCz/DBT feed ratios are illustrated in
Scheme 3. Compounds 2-(4-ethylhexyl-5-trimethylsilanyl-thio-
phen-2-yl)-4,4,5,5-tetramethyl-[1,3,2]dioxaborolane (1), diethyl(2,5-
dibromothiophen-3-yl)methylphosphonate (2), 5-(4-(3,6-di-tert-9-
carbazolyl)phenyl)thiophene-2-carbaldehyde (5) and 5,5000-
dibromo-3,3000-diethylhexyl-2,20; 50,200; 500,2000-quater-thiophene
were synthesized according to the literature.22,32
To systematically investigate the variation of photophysical
properties and energy levels via the strategy of developing PTs
with covalently attached both electron donor and acceptor
pendants, we rst synthesized PTs functionalized with various
contents of tert-butyl substituted carbazole (tCz) as an electron
donor pendant25,26 and bisbenzothiazolylvinyl (DBT) as an elec-
tron acceptor pendant.27,28 The synthesis, properties and PSC
performances of the novel PTs comprising 2-ethylhexyl-
substituted quaterthiophene (T) as the conjugated unit in the PT
backbone along with different composition ratios of tCz and DBT
as bipolar pendant groups were reported in this study (Scheme 1).
According to the literature,29 the optical and electronic properties
of the polymers can be easily manipulated through the attach-
ment of electron-withdrawing groups while taking the variation
of the acceptor strengths into account. Therefore, one would
anticipate further energy level adjustment when the acceptor
moiety DBT is incorporated. Depending on the copolymer
composition of PTs comprising conjugated pendants, samples
Synthesis of compound (3)
A mixture of compound (2) (0.58 g, 1.5 mmol), compound (1)
(1.21 g, 3.3 mmol), and Pd(PPh3)4 (0.037 g, 0.03 mmol) was
dissolved in a degassed mixture of toluene (12 mL) and 2 M
such as PTtCz, PT(tCz)0.9(DBT)0.1
,
PT(tCz)0.64(DBT)0.36,
PT(tCz)0.45(DBT)0.55, and PTDBT were synthesized and investi-
gated (Scheme 1). To validate the dual-adjustment of the lower-
lying HOMO levels and narrow Eg values, UV-vis absorption
spectroscopy and cyclic voltammetry (CV) were employed to study
the effect of bipolar pendants on the photophysical and electro-
chemical properties of the PT-based copolymers. We fabricated
conventional PSCs by spin-coating the solutions of PT-based
Scheme 2 Synthesis of the conjugated pendants tCz and DBT (i) Pd(PPh3)4, 2M
K2CO3 (aq.), toluene, reflux, 8 h; (ii) NBS, AcOH, DCM, 0 ꢁC, 1 h; (iii) CH3ONa, DMF,
0 ꢁC, 2 h; (iv) 2-carboxaldehyde, potassium tert-butoxide, THF,0 ꢁC, 1 h; (v) DMF,
phosphoryl chloride, dichloroethane, from 0 ꢁC to reflux, 8 h; (vi) 2,20-methyl-
enebisbenzothiazole, I2, K2CO3, DMF, 60 ꢁC, 6 h.
Scheme 1 PTs comprising bipolar pendant groups.
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7.38 (d, J ¼ 0.8 Hz, 2H), 7.44 (m, 2H), 7.54 (d, J ¼ 9.2 Hz, 2H),
7.77 (d, J ¼ 4 Hz, 2H), 8.12 (t, J ¼ 1.6 Hz, 2H). ESIMS (m/z): calcd
for C60H69Br2NS4: 1092.26. Found: 1092.4. Anal calcd for
C
50H50Br2N2S6: C, 65.98; H, 6.37; N, 1.28. Found: C, 65.72; H,
6.44; N, 1.14%.
Synthesis of compound (6)
A mixture of compound (4) (7.81 g, 10 mmol) and K2CO3 (2.76 g,
20 mmol) in 30 mL dry THF was stirred under an ice water bath
for 15 minutes. A solution of 2-carbadehydethiophene (1.68 g,
15 mmol) in dry THF (7 mL) was then added to the mixture.
Aer 2 h, the reaction was quenched with methanol. Further
purication was performed using silica gel chromatography
(hexane as eluent) to give a yellow oil (4.95 g, yield ¼ 67%). 1H-
NMR (d/ppm, 400 MHz, CDCl3): 0.87 (m, 12H), 1.27 (m, 16H),
1.52 (m, 2H), 2.45 (m, 4H), 6.81 (d, J ¼ 10 Hz, 1H), 6.98 (m, J ¼
8.4 Hz, 1H), 7.02 (s, 1H), 7.05 (s, 1H), 7.08 (d, J ¼ 3.2 Hz, 1H),
Scheme 3 Reaction scheme for preparing the PT-based random copolymers.
K2CO3(aq) (12 mL). The solution was stirred at reux tempera- 7.14 (s, 1H), 7.19 (d, J ¼ 5.2 Hz, 1H), 7.23 (d, J ¼ 3.6 Hz, 1H).
ture for 48 h, and then poured into methanol (100 mL). The
crude product was partitioned between EA and water, and the
organic phase was dried (MgSO4), ltered, and evaporated to
dryness. The residue was puried using silica gel chromatog-
Synthesis of compound (7)
POCl3 (7.45 mL, 80 mmol) was added dropwise to freshly
distilled DMF (3.09 mL, 40 mmol) at 0 ꢁC under nitrogen
raphy (EA : hexane ¼ 1 : 10) to give a yellow liquid (0.72 g, yield
1
atmosphere. Compound 6 (7.39 g, 10 mmol) dissolved in
dichloroethane (50 mL) was added dropwise to the above-
mentioned POCl3/DMF complex at room temperature. The
reaction mixture was stirred at 80 ꢁC for 3 h. Subsequently, the
reaction was cooled to 0 ꢁC and 2 M NaOH aqueous solution was
added to quench the reaction. The mixture was partitioned
¼ 63%). H NMR (d/ppm, CDCl3): 0.28 (s, 18H), 0.84 (m, 18H),
1.22 (m, 16H), 1.55 (m, 2H), 2.58 (m, 4H), 3.25 (d, J ¼ 6.4 Hz,
2H), 4.02 (m, 4H), 7.05 (s, 1H), 7.15 (m, 2H).
Synthesis of compound (4)
Compound (3) (6.22 g, 10 mmol) was dissolved in DCM (25 mL). between EA–water, and the organic phase was dried (MgSO4),
The solution was then stirred in an ice water bath for 15 ltered, and evaporated to dryness. The residue was puried
minutes. N-bromosuccinimide (3.55 g, 20 mmol) was dissolved using silica gel chromatography (EA : hexane ¼ 1 : 10) to give a
in a mixture of DCM (20 mL) and AcOH (10 mL). This mixture yellow oil (5.09 g, 77%). 1H-NMR (d/ppm, 400 MHz, CDCl3): 0.86
was then added dropwise to the compound (3) solution over 15 (m, 12H), 1.27 (m, 16H), 1.59 (m, 2H), 2.45 (m, 4H), 6.81 (d, J ¼
min at 0 ꢁC. The reaction mixture was warmed to room 10.4 Hz, 1H), 7.06 (s, 1H), 7.10 (s, 1H), 7.12 (d, J ¼ 2.8 Hz, 1H),
temperature and stirred for 2 h. Subsequently, Na2SO3 aqueous 7.23 (d, J ¼ 2.4 Hz, 1H), 7.25 (d, J ¼ 4.0 Hz, 1H), 7.63 (d, J ¼
solution was added to quench the reaction. The mixture was 5.6 Hz, 1H), 9.89 (s, 1H).
partitioned between EA–water, and the organic phase was dried
(MgSO4), ltered, and evaporated to dryness. The residue was
Synthesis of compound DBT
puried using silica gel chromatography (EA : hexane ¼ 1 : 10)
1
A mixture of dibenzo[d]thiazol-2-ylmethane (3.11 g, 11 mmol),
K2CO3 (0.04 g, 0.3 mmol), and I2 (0.07 g, 0.3 mmol) in 20 mL
DMF was stirred under room temperature for 15 minutes. A
solution of compound 7 (7.67 g, 10 mmol) in DMF (15 mL) was
added to the mixture, and then the reaction solution was heated
to give a yellow liquid (5.72 g, yield ¼ 81%). H-NMR (d/ppm,
400 MHz, CDCl3): 0.86 (m, 18H), 1.23 (m, 16H), 1.58 (m, 2H),
2.42 (m, 4H), 3.18 (d, J ¼ 5.2 Hz, 2H), 4.02 (m, 4H), 6.80 (s, 1H),
6.84 (s, 1H), 7.08 (s, 1H).
ꢁ
to 60 C. Aer 2 h, the reaction was quenched with methanol.
Synthesis of compound tCz
The mixture was partitioned between EA–water, and the organic
A mixture of compound (4) (7.81 g, 10 mmol) and CH3ONa phase was dried (MgSO4), ltered, and evaporated to dryness.
(1.78 g, 33 mmol) in 30 mL DMF was stirred under an ice water Further purication was performed using silica gel chroma-
bath for several minutes. Compound (5) (4.66 g, 10 mmol) was tography (EA : hexane ¼ 1 : 10) to give a red solid (7.43 g, yield ¼
then added to the solution. Aer 2 h, the reaction was quenched 72%). 1HNMR (d/ppm, CDCl3): 0.86 (m, 12H), 1.28 (m, 16H),
with methanol and a yellow powder was precipitated. Further 1.54 (m, 2H), 2.43 (m, 4H), 6.76 (d, J ¼ 8.0 Hz, 2H), 6.87 (s, 1H),
purication was performed using silica gel chromatography 6.91 (d, J ¼ 13.6 Hz, 1H), 6.98 (d, J ¼ 6.4 Hz, 2H), 7.15 (s, 1H),
(hexane as eluent) to give a yellow solid (7.19 g, yield ¼ 66%). 7.19 (t, J ¼ 4.4 Hz, 1H), 7.31 (t, J ¼ 16.4 Hz, 1H), 7.49 (t, J ¼ 12.8
1H-NMR (d/ppm, 400 MHz, CDCl3): 0.82 (m, 12H), 1.29 (m, 16H), Hz, 2H), 7.55 (t, J ¼ 16.8 Hz, 1H), 7.79 (d, J ¼ 7.6 Hz, 1H), 7.99
1.44 (s, 18H), 1.59 (m, 2H), 2.48 (m, 4H), 6.85 (d, J ¼ 1.2 Hz, 2H), (d, J ¼ 8 Hz, 2H), 8.16 (s, 1H), 8.22 (d, J ¼ 8.4 Hz, 1H). ESIMS
7.07 (m, 3H), 7.25 (d, J ¼ 8.8 Hz, 1H), 7.28 (d, J ¼ 7.6 Hz, 1H), (m/z):calcd for C50H50Br2N2S6: 1031.14 Found: 1031.0. Anal
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calcd for C50H50Br2N2S6: C, 58.24; H, 4.89; N, 2.72; S, 18.66 CDCl3): 0.87 (br, 84H), 1.26–1.30 (br, 64H), 1.45–1.56 (br, 8H),
Found: C, 58.24; H, 4.86; N, 2.77; S, 18.52%.
2.62–2.84 (br, 16H), 6.94–7.46 (br, 33H), 7.56 (br, 2H), 7.80 (br,
3H), 7.89 (br, 1H), 8.01 (br, 1H), 8.14 (br, 4H), 8.19 (br, 1H).
Synthesis of compound T
A solution of 5,5000-dibromo-3,3000-diethylhexyl-2,20; 50,200; 500,2000-
quaterthiophene (7.12 g, 10 mmol) in dry THF (150 mL) was
stirred at ꢀ78 ꢁC under a N2 atmosphere and then n-BuLi (2.5 M
in hexane, 8.40 mL, 21 mmol) was added dropwise to the
Synthesis of PT(tCz)0.45(DBT)0.55
Using the same procedure as that described for the synthesis of
PTtCz, the reaction of tCz (0.40 g, 0.37 mmol), DBT (1.16 g, 1.13
mmol), and T (1.32 g, 1.5 mmol) gave a dark-red solid (60%).
GPC: Mw, 28 875 g molꢀ1; PDI, 2.93. 1H-NMR (d/ppm, 400 MHz,
CDCl3): 0.88 (br, 84H), 1.27–1.30 (br, 64H), 1.46–1.70 (br, 8H),
2.72 (br, 16H), 6.95–7.46 (br, 33H), 7.57 (br, 2H), 7.80 (br, 3H),
7.91 (br, 1H), 8.01 (br, 1H), 8.14 (br, 4H), 8.20 (br, 1H).
ꢁ
solution. The solution mixture was maintained at ꢀ78 C with
stirring for 1 h, at which point trimethyltinchloride (4.98 g,
25 mmol) in dry THF (5 mL) was added dropwise. The mixture
was warmed to room temperature and stirred for 8 h. Subse-
quently, HCl (1 N) was added to quench the reaction. The
resulting mixture was partitioned between EA and water, and
the organic phase was dried (MgSO4), ltered, and evaporated to
dryness. The residue was puried using silica gel chromatog-
raphy (hexane, with 1% TEA) to give a greenish oil (7.13 g,
yield ¼ 81%). 1H-NMR (d/ppm, 400 MHz, CDCl3): 0.82 (m, 12H),
1.07 (m, 18H), 1.23 (m, 16H), 1.52 (m, 2H), 2.71 (m, 4H), 6.89 (s,
2H), 6.98 (d, J ¼ 3.6 Hz, 2H), 7.08 (d, J ¼ 4 Hz, 2H). ESIMS (m/z):
calcd for C38H58S4Sn2: 880.55 Found: 880.2. Anal calcd for
Synthesis of PTDBT
Using the same procedure as that described for the synthesis of
PTtCz, the reaction of DBT (1.54 g, 1.5 mmol) and T (1.32 g,
1.5 mmol) gave a dark-red solid (65%). GPC: Mw, 29 616 g
1
molꢀ1; PDI, 3.49. H-NMR (d/ppm, 400 MHz, CDCl3): 0.89 (br,
84H), 1.30–1.34 (br, 64H), 1.57–1.62 (br, 8H), 2.47–2.80 (br,
16H), 6.90–7.31 (br, 18H), 7.77 (br, 1H), 7.88 (br, 1H), 7.98
(br, 1H), 8.17 (br, 1H).
C
38H58S4Sn2: C, 51.83; H, 6.64 Found: C, 52.10; H, 6.74%.
Synthesis of PTtCz
Characterization of copolymers
A solution of tCz (1.64 g, 1.5 mmol) and T (1.32 g, 1.5 mmol) in 1H-NMR (400 MHz) spectra were recorded using a Varian Unity
dry toluene (20 mL) was purged with N2 and subjected to three Inova spectrometer. The average molecular weights of the
freeze–pump–thaw cycles to remove traces of O2. Pd(PPh3)4 polymers were measured by means of GPC on a Waters chro-
(30 mg, 1 mol%) was added to the mixture, which was then matography system (717 plus Autosampler) equipped with two
stirred and heated under reux for 48 h. Aer cooling to room Waters Styragel linear columns. Polystyrene standards were
temperature, the mixture was poured into MeOH (100 mL) and used, with THF as the eluent. The glass transition temperature
the precipitated material was ltered through a Soxhlet thimble. (Tg) and thermal decomposition temperature (Td, the tempera-
Soxhlet extractions were performed sequentially with MeOH, ture at which weight loss reaches 5%) of the copolymers were
hexane, acetone, and CHCl3. The polymer was recovered determined by means of differential scanning calorimetry
through rotary evaporation of the CHCl3 fraction. Drying the (TA Instruments, DSC-2010) and thermogravimetric analysis
sample under vacuum for 24 h provided an orange–red solid (TA Instruments, TGA-2050), respectively. Both analyses were
(70%). Gel permeation chromatography (GPC; THF): weight- performed under N2 atmosphere at a scanning (both heating
average molecular weight (Mw), 21 421 g molꢀ1; polydispersity and cooling) rate of 10 C minꢀ1. The absorption spectra were
ꢁ
1
index (PDI), 2.35. H-NMR (d/ppm, 400 MHz, CDCl3): 1.01 (br, measured using a Hitachi U3010 UV-Vis spectrometer. Fluo-
42H), 1.30–1.34 (br, 32H), 1.57–1.62 (br, 4H), 2.30–2.46 (br, 8H), rescence spectra were measured using a Varian Cary Eclipse
6.40–7.50 (br, 15H), 7.55 (br, 2H), 7.79 (br, 2H), 8.12 (br, 4H).
luminescence spectrometer. Dilute o-DCB solutions of the PTs
were ltered through a 0.45 mm lter to remove insoluble
materials before spectral measurements. Redox potentials of
the polymers were determined with a CHI 611D electrochemical
analyser (scanning rate: 50 mV sꢀ1) equipped with Pt electrodes
and an Ag/Ag+ (0.10 M AgNO3 in MeCN) reference electrode in
an anhydrous, N2-saturated solution of 0.1 M Bu4NClO4 in
MeCN. Bu4NClO4 (98%, TCI) was recrystallized three times from
ethyl acetate and then dried at 60 ꢁC under reduced pressure. A
Pt plate coated with a thin polymer lm was used as the working
electrode, a Pt wire and an Ag/Ag+ electrode were used as the
counter and reference electrodes, respectively. CV curves were
Synthesis of PT(tCz)0.9(DBT)0.1
Using the same procedure as that described for the synthesis of
PTtCz, the reaction of tCz (1.24 g, 1.13 mmol), DBT (0.38 g, 0.37
mmol), and T (1.32 g, 1.5 mmol) gave a dark-red solid (67%).
GPC: Mw, 41 531 g molꢀ1; PDI, 3.49. 1H-NMR (d/ppm, 400 MHz,
CDCl3): 1.01 (br, 84H), 1.30–1.34 (br, 64H), 1.57–1.62 (br, 8H),
2.60–2.80 (br, 16H), 6.40–7.50 (br, 33H), 7.55 (br, 2H), 7.79 (br,
3H), 7.88 (br, 1H), 7.99 (br, 1H), 8.16 (br, 4H), 8.22 (br, 1H).
Synthesis of PT(tCz)0.64(DBT)0.36
calibrated using ferrocene as the standard, whose HOMO is set
ox
onset
Using the same procedure as that described for the synthesis of at ꢀ4.8 eV with respect to zero vacuum level. The E
of
PTtCz, the reaction of tCz (0.82 g, 0.75 mmol), DBT (0.77 g, 0.75 ferrocene/ferrocene+ is 0.36 V in Bu4NClO4/MeCN solution
mmol), and T (1.32 g, 1.5 mmol) gave a dark-red solid (64%). (0.1 M). The HOMO energy levels were obtained from the
ox
onset
ox
GPC: Mw, 34 235 g molꢀ1; PDI, 3.23. 1H-NMR (d/ppm, 400 MHz, equation: HOMO ¼ ꢀe(E
ꢀ E
+ 4.8) (eV). The
onset,ferrocene
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LUMO levels of the polymer were obtained from the equation: (50 nm) was thermally-deposited through a shade mask with an
red
red
LUMO ¼ ꢀe(E
ꢀ E
+ 4.8) (eV).33
effective device area of 3 mm2. The use of metal oxides on both
sides of the PTtCz:PC71BM layer would prevent the diffusion of
moisture into the active layer. Under ambient atmosphere, the
measurements of long-term photovoltaic characteristics of the
onset
onset,ferrocene
Fabrication and characterization of PSCs34
All the conventional PSCs in this study were based on a structure PTtCz-derived inverted devices without encapsulation were per-
like the indium tin oxide (ITO)-coated glass/hole-transporting formed in the same manner as those of the conventional devices
material (HTM)/photoactive layer/Ca (30 nm)/Al (100 nm) struc- as described above.
ture, in which the photoactive layer consisted of an inter-
penetrating network of PTs/PC71BM.The ITO substrates
(obtained from Sanyo, Japan (8 U ,ꢀ1)) were rst patterned by
lithography, cleaned with detergent, and ultrasonicated in
acetone and isopropyl alcohol, dried on a hot plate at 120 ꢁC for 5
min, and nally treated with oxygen plasma for 5 min. PC71BM
was purchased from Nano-C and used as received. Poly(3,4-
Results and discussion
Characterization of conjugated polymers
The molar ratios (m/n; Fig. 1) of the PTs were manipulated by
controlling the feed ratios of the tCz/DBT conjugation units. In
addition, the actual values of m/n were determined from the
relative integral areas attributed to the peaks of tCz (7.48–
7.50 ppm) and DBT (7.77–7.98 ppm and 8.17 ppm) by 1H NMR
spectra.22,28 The characteristic signals of tCz and DBT moieties
are shown in Fig. 1 and indicated as blue and red arrows,
respectively. The copolymer compositions were determined and
named according to the actual ratios as indicated by PTtCz,
ethylene-dioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS,
Baytron P-VP AI4083) was ltered through a 0.45 mm lter before
being deposited on ITO, with a thickness around 30 nm, by spin
ꢁ
coating at 3000 rpm in the air and drying at 150 C for 30 min
inside a glove box. A solution mixture of PTs-based donor–
acceptor random copolymers and PC71BM (10 mg mLꢀ1 in
o-DCB) was stirred overnight, then ltered through a 0.2 mm
poly(tetrauoroethylene) (PTFE) lter and spin-coated (1200 rpm,
30 s) onto the PEDOT:PSS layer to prepare the PT/PC71BM
composite lm-based photo-active layer. The optimal thickness
of the active layers obtained under these conditions was ca.
60 nm. Subsequently, the device was completed by depositing ca.
(30 nm) and Al (100 nm) under <10ꢀ6 mm-Hg pressure, respec-
tively. The active area of the conventional device was 5 mm2.
Finally the cell was encapsulated using UV-curing glue (obtained
from Nagase, Japan). During the encapsulation process, the UV-
glue was dispensed onto the edge of a piece of glass in the air.
The UV-glue coated glass was transferred to the glove box for
covering the PSC device. The device was then sealed by pressing
the UV-glue coated glass on top of the device, and the device
underwent UV curing (254 nm) for 2 min. Upon device encap-
sulation, the I–V curves of the PSC devices were measured in an
ambient atmosphere at 25 ꢁC using a computer-controlled
Keithley 2400 source measurement unit (SMU) equipped with a
Peccell solar simulator under AM 1.5G illumination (100 mW
cmꢀ2). The illumination intensity was calibrated using a stan-
dard Si photodiode detector equipped with a KG-5 lter. The
output photocurrent was adjusted to match the photocurrent of
the Si reference cell to obtain a power density of 100 mW cmꢀ2
.
An inverted device of PTtCz/PC71BM-based lm with the struc-
ture of ITO/ZnOx/PTtCz:PC71BM/MoO3/Ag, in which ITO acted as
the cathode and Ag as the anode, was prepared using the
following procedure. A ZnOx precursor solution was spin-cast on
top of the pre-cleaned ITO-glass substrate. The lms were
ꢁ
annealed at 150 C for 1 h in air. The ZnOx lm thickness was
approximately 30 nm, as determined by a prolometer. The
ZnOx-coated substrates were then placed in a glove box. A pho-
toactive layer, PTtCz:PC71BM, was fabricated according to the
condition for preparing the conventional device. Subsequently, a
thin MoO3 layer with a thickness of approximately 7 nm was
Fig. 1 1H NMR spectra of the PTs-based donor–acceptor random copolymers (a)
PTtCz, (b) PT(tCz)0.9(DBT)0.1, (c) PT(tCz)0.64(DBT)0.36, (d) PT(tCz)0.45(DBT)0.55, and (e)
PTDBT. The blue and red arrows indicate the characteristic signals of the tCz and
vacuum-deposited on the top of the photoactive layer with an
evaporation rate of 0.1 A sꢀ1. In addition, the top electrode Ag DBT moieties, respectively.
˚
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PT(tCz)0.9(DBT)0.1
,
PT(tCz)0.64(DBT)0.36
,
PT(tCz)0.45(DBT)0.55,
and PTDBT. It is important to note that a higher actual m/n ratio
was obtained despite the equal tCz/DBT feed ratio during
polymerization, for example in the case of PT(tCz)0.64(DBT)0.36
.
This might be due to the fact that the relatively poor solubility of
the DBT bulky segment would inhibit the propagation of the
polymer chains. Furthermore, a higher content of DBT-con-
taining segments would also lead to precipitation at the initial
stages of polymerization, resulting in better propagation of the
tCz-containing segment. All of the conjugated copolymers were
soluble in common organic solvents, including chloroform,
THF, and o-DCB. In general, conjugated polymers exhibit poor
solubility due to strong p–p interaction between planar poly-
mer backbones. Moreover, the presence of the branched
2-ethylhexyl side-chains is effective in improving the solubility
of conjugated copolymers. Using GPC with THF as the eluent
and calibrating by a polystyrene standard, we determined the
weight-average molecular weights (Mws) and PDIs of the
conjugated PTs. The values of Mw and PDI of PTtCz,
PT(tCz)0.9(DBT)0.1
,
PT(tCz)0.64(DBT)0.36
,
PT(tCz)0.45(DBT)0.55,
and PTDBT were (21 421, 2.35), (41 531, 3.49), (34 235, 3.23),
(28 875, 2.93), and (29 616, 2.29), respectively.
The operational stability of the PSCs is directly related to the
thermal stability of the conjugated polymers. Thus, high Tgs
and Tds are desirable for the conjugated polymers. Tds (taken at
Fig. 2 UV spectra of PT-based random copolymers (a) in o-DCB and (b) as thin
films. The symbols of the PTs are indicated as the following: PTtCz (,),
PT(tCz)0.9(DBT)0.1 (B), PT(tCz)0.64(DBT)0.36 (O), PT(tCz)0.45(DBT)0.55 (>), and
PTDBT (*).
5
wt% weight loss) of PTtCz, PT(tCz)0.9(DBT)0.1
,
PT(tCz)0.64(DBT)0.36, PT(tCz)0.45(DBT)0.55, and PTDBT were 367,
348, 331, 355, and 288 ꢁC, respectively. Tgs were determined with increasing DBT content could be due to the stronger donor–
from the second round of DSC heating scans. The Tgs of PTtCz, p–acceptor characteristic and more efficient intramolecular
PT(tCz)0.9(DBT)0.1
,
PT(tCz)0.64(DBT)0.36
,
PT(tꢁCz)0.45(DBT)0.55
,
charge transfer. As shown in Fig. 2(b), the absorption bands of
and PTDBT were 122, 132, 120, 133, and 122 C, respectively, the PTs as thin lms were located between 350 and 650 nm.
which are considered to be good candidates for PSC applica- More red shi of the maximum absorption wavelengths and
tions in terms of thermal stability. A single endothermic glass absorption edges was observed for these polymers as solid lms
transition was observed for each conjugated copolymer, than those in o-DCB. Bandgap energies (Egs) of the conjugated
implying that the electron-donating and electron-withdrawing PTs were determined from the onset wavelengths of their
bulky pendants (tCz and DBT) were distributed homogeneously. absorption bands. As shown in Table 1, the values of Eg were in
the following order: PTtCz (1.97) > PT(tCz)0.9(DBT)0.1 (1.96) >
PT(tCz)0.64(DBT)0.36 (1.90) ¼ PT(tCz)0.45(DBT)0.55 (1.90) > PTDBT
Optical properties of the conjugated polymers
(1.87) (eV). The Eg decreased with an increasing molar ratio of
The normalized UV-vis absorption spectra of the PTs in o-DCB DBT in the PTs. In solution, these polymers exhibited absorption
solution and as solid lms are shown in Fig. 2. Table 1 edges ranging from 550 to 580 nm which underwent a 50–70 nm
summarizes the photophysical properties of the copolymers. As red shi in the solid state. Interestingly, the PTDBT lm revealed
shown in Fig. 2(a), the absorption bands of the PTs in o-DCB a broad and strong absorption band in the region from 350 to
ranged from 325 to 600 nm, and each polymer exhibited a single 650 nm, covering the UV and visible regions. This is because the
broad absorption band. In general, PTs functionalized with incorporation of the electron-decient DBT group results in a
conjugated pendants exhibit two maximum absorption peaks, smaller Eg of the polymer. With increasing the DBT contents of
one in the visible region attributed to the p–p* transition of the the PTs, the stronger donor–p–acceptor characteristic and more
conjugated polymer main chains, and the other in the UV region efficient intramolecular charge transfer between the polymer
attributed to the conjugated side chains.16,35 In this study, the backbones to conjugated pendants were manifested. Herein, the
PTs functionalized with conjugated tCz/DBT moieties showed absorbance edge was extended from 570 to 650 nm for the PTs
only a single broad absorption peak. This indicates that a high when the DBT content was increased to a certain extent. This
degree of intramolecular conjugation was present within the indicates that using geometric donor–p–acceptor conjugated
conjugated frameworks of the polymers when in solution. Due to moieties as pendants in PTs is a feasible strategy to adjust the Eg
the electron withdrawing characteristic of the DBT moiety, the of conjugated polymers. In other words, the photophysical
maximum absorption band located in the longer wavelength properties can be tailored for the pursuit of better photovoltaic
region would generally take place.27,28 Further red-shi of the performance via the variation of the molar ratios of conjugated
maximum absorption wavelengths (Table 1) for the copolymer pendants.
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Table 1 Optical properties and electronic energy levels of the PT-based random copolymers comprising donor and acceptor pendants
UV-visible absorption spectra
Cyclic voltammetry
ox
red
E
E
onset
onset
abs
max
abs
max
abs
onset
PTs
l
(nm)a
l
(nm)b
l
l (nm)b
Eogpt (eV)c
(V)
HOMO (eV)d
(V)
LUMO (eV)e
Eegc (eV)f
PTtCz
415
433
440
450
461
421
475
502
517
550
629
634
654
655
687
1.97
1.96
1.90
1.90
1.80
0.95
0.87
0.85
0.84
0.82
ꢀ5.39
ꢀ5.31
ꢀ5.29
ꢀ5.28
ꢀ5.26
ꢀ1.31
ꢀ1.30
ꢀ1.26
ꢀ1.23
ꢀ1.12
ꢀ3.13
ꢀ3.14
ꢀ3.18
ꢀ3.21
ꢀ3.32
2.26
2.17
2.11
2.07
1.94
PT(tCz)0.9(DBT)0.1
PT(tCz)0.64(DBT)0.36
PT(tCz)0.45(DBT)0.55
PDBT
a
The maximal absorption wavelength of the PTs in o-DCB. b The maximal absorption wavelength of the PTs as thin lms. c Calculated the from the
d
ox
onset
+ 4.8) (eV).
onset absorption (lonset) of polymer thin lms: ¼ 1240/lonset
.
The HOMO energy levels were obtained from the equation:HOMO ¼ ꢀe(E
ꢀ
ox
e
red
onset
red
E
+ 4.8) (eV). The LUMO levels of polymer were obtained from the equation: LUMO ¼ ꢀe(E
ꢀ E
onset,ferrocene
onset,ferrocene
f
Electrochemical bandgap energies E ¼ LUMO ꢀ HOMO (eV).
(ꢀ3.21) > PTDBT, (ꢀ3.32) (eV). The electrochemical bandgap
energies (Eegc) were calculated from the differences between the
LUMO and HOMO energy levels (Table 1). The variations of
electrochemical bandgap energies exhibited the same trend
with those of the optical Egs. The lower Eegc values for the PTs
with an increasing content of DBT are attributed to the stronger
donor–p–acceptor characteristic. The Eegc values of the studied
PTs were 0.1–0.3 eV larger than the Eogpt values, which is prob-
ably due to the presence of the exciton binding energy of
conjugated polymers.37 The HOMO levels of PTs were shied to
low-lying values by ca. 0.48–0.35 eV when compared to that of
P3HT. It is known that attachment of the tert-butyl groups on
the electrochemically active sites of aromatic amines leads to
better electrochemical stability.38 Additionally, the dihedral
angles formed between the 9-position substituted carbazole and
Electrochemical properties of the conjugated polymers
Since the PV performance of PSCs is closely related to the energy
levels of the conjugated polymer, we determined the onset
oxidation/reduction potentials of the PTs in air by employing
cyclic voltammetry to calculate the HOMO/LUMO energy levels.
This onset is dened by the intersection of the extrapolated
baseline with the tangent to the inection point. In Fig. 3, the
onset oxidation potentials (E) of PTtCz, PT(tCz)0.9(DBT)0.1
PT(tCz)0.64(DBT)0.36 PT(tCz)0.45(DBT)0.55 and PTDBT were
,
,
,
0.95, 0.87, 0.85, 0.84 and 0.82 V, respectively. The HOMO
energy levels were calculated and listed in the order as
following: PTtCz, (ꢀ5.39)
<
PT(tCz)0.9(DBT)0.1
,
(ꢀ5.31)
<
PT(tCz)0.64(DBT)0.36, (ꢀ5.29) < PT(tCz)0.45(DBT)0.55, (ꢀ5.28) <
PTDBT, (ꢀ5.26) (eV) (Table 1). These numbers are in an ideal
range to ensure better air-stability and larger attainable Voc in
the device.36 The onset reduction potentials (E) of PTtCz,
ꢁ
benzene plane were approximately 60 along with two bulky
tert-butyl group substituents, which inhibit the close p–p
aggregation effectively between the polymer main chains.39,40
On the other hand, two vicinal benzothiazolyl substituents of
donor–acceptor-substituted uorine uorophores are nearly
perpendicular to each other from X-ray structure analysis.28 The
rigid benzothiazolyl substituents prevent the uorine molecules
from p–p interaction. In this study, the most critical factor in
tailoring the oxidation potentials was the incorporation of bulky
conjugated pendants capable of twisting adjacent thiophene
rings through steric interactions, thereby reducing the p orbital
overlapping of PTs backbones and resulting in the deeper
HOMO energy levels. This phenomenon was more pronounced
in the tCz-derived polymers. Deeper HOMO energy levels were
observed while the tCz content was increased. In contrast, a
weak acceptor DBT group does not signicantly affect the
oxidation potentials.28 However, the attachment of the electron
withdrawing group (DBT) onto the polymer backbone also had a
positive effect in obtaining lower HOMO energy levels, as
compared to the parent P3HT.9 PTs with an increasing content
of DBT were still able to retain satisfactory anti-oxidation
stability as compared to the oxygen energy level (ꢀ5.2 eV with
respect to vacuum).10,41 As mentioned previously, the LUMO
energy level is strongly inuenced by the acceptor moiety.28
Apparently, the LUMO energy levels decreased with an
increasing DBT content of PTs in a signicant manner. Indeed,
PT(tCz)0.9(DBT)0.1
,
PT(tCz)0.64(DBT)0.36
,
PT(tCz)0.45(DBT)0.55,
and PTDBT were ꢀ1.31, ꢀ1.30, ꢀ1.26, ꢀ1.23, and ꢀ1.12 V,
respectively. The LUMO energy levels were calculated and listed
in the order as following: PTtCz, (ꢀ3.13) > PT(tCz)0.9(DBT)0.1
,
,
(ꢀ3.14) > PT(tCz)0.64(DBT)0.36, (ꢀ3.18) > PT(tCz)0.45(DBT)0.55
Fig. 3 Cyclic voltammograms of PT-based random copolymers thin films in an
MeCN solution of 0.1 M TBAP at a scan rate of 50 mV Sꢀ1
.
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Table 2 Photovoltaic performances of the PTs/PC71BM-based conventional PSCs
the tunable energy levels of PTs were accomplished by incor-
porating both tCz as donor pendants and DBT as acceptor
pendants. This approach revealed a molecular design avenue to
narrow the bandgap energies while retaining the low-lying
HOMO levels.
PTs
Voc (V)
Jsc (mA cmꢀ2
)
FF
PCE (%)
PTtCza
0.91
0.82
0.85
0.80
0.79
6.58
3.54
5.52
4.93
5.05
0.41
0.31
0.35
0.35
0.33
2.48
0.89
1.65
1.39
1.31
PT(tCz)0.9(DBT)0.1
PT(tCz)0.64(DBT)0.36
PT(tCz)0.45(DBT)0.55
PDBT
PV properties of PSCs based on PT/PC71BM lms
PSCs were fabricated based on the ITO/PEDOT:PSS (30 nm)/
PTs:PC71BM/Ca (30 nm)/Al (100 nm) structure.34 The optimum
PTs/PC71BM blend ratios were found to be 1 : 2.5 (w/w) and
1 : 3.5 (w/w) for PTtCz-based and other PT-based photoactive
lms, respectively. Fig. 4 displays the J–V curves of each PTs/
PC71BM-based PSC, whereas the essential photovoltaic param-
eters (Voc, Jsc, FF and PCE) are listed in Table 2. The devices
based on PTs/PC71BM all exhibited higher Voc values (0.91–
0.79 V), when compared with that of P3HT/PC71BM (0.65 V).42
These high values of Voc for the PT devices are consistent with
their deeper HOMO energy levels. The Voc value was decreased
with increasing the DBT content in PTs, which is also consistent
with the CV result. As expected, an excellent Voc (0.91 V) value
was observed for PTtCz due to its deep HOMO energy level
(ꢀ5.39 eV). Among these PT-based photoactive layers, the
PTtCz-based sample exhibited the best PCE (2.48%) because of
its higher Jsc and largest Voc, when compared with other
samples in this study. One would have expected to further
increase the PCE by introducing a DBT unit into the system.
However, the PCE of PTDBT device was signicantly limited by
its low Jsc and Voc, despite the fact that the UV-vis absorption
spectrum of PTDBT was more red-shied than that of PTtCz.
The results obtained in the PCE performances prompted us to
further investigate the relevance of the charge transport prop-
erties of the PTs, which will be addressed in a later section. The
lower photocurrent density might result from charge transport-
related recombination losses. Additionally, the PCE of
PT(tCz)0.9(DBT)0.1/PC71BM (0.89%) was the lowest among these
PT-based PSCs. The presence of low molecular-weight fractions
in the system (a rather broader PDI value (3.49) for
a
PTs/PC71BM ¼ 1 : 2.5.
performance in photovoltaic parameters. It has been demon-
strated that the broad molecular weight distributions of PTs are
responsible for this dramatic decrease in performance, partic-
ularly those with low molecular weight fractions.43 This
phenomenon is caused by distinctly reduced charge carrier
(hole) mobility in the donor phase of the devices built from the
lower molecular-weight fractions of the PT samples.44
Morphology of the thin lms of PT/PC71BM blends
The performance of a PSC is strongly dependent on the
morphology of the conjugated polymer/fullerene derivative
composite lm. To generate more effective free carriers from
bonding excitations and to avoid the recombination of excitons
and free carriers, the P/N heterojunction phase must be
controlled at the nanoscale level with an interpenetrating
network. This is because the diffusion range of the exciton is
approximately 10–20 nm.45,46 AFM was utilized to investigate the
compatibility and morphology of the PT/PC71BM composite
lms prepared in the same manner as those prepared in the
device fabrication (Fig. 5, topographic (a) and phase (b) images).
In each case, we observed a phase-separated interpenetrating
network with sizable PC71BM domains. The surface rough-
nesses of the PT/PC71BM lms were in the range of 0.21–
0.32 nm. The overall small root mean square (RMS) values are
probably due to the random conformation of the PTs and the
presence of the steric bulky pendants in these PTs. These two
factors prohibit the ordered packing of the polymer chains in
the solid state.47 The bulky pendants in the PTs provide suffi-
cient free volume for the PC71BM units to intercalate into the
polymer chains. This could permit better compatibility between
PC71BM and the polymer chains. In Fig. 5(b), a certain degree of
phase separation was observed, which is favorable for efficient
formation of free carriers to provide optimal PV properties of
PSCs. These results indicate that the compatibility and surface
morphology of the PTs/PC71BM lms were inuenced by the
geometric structure of the pendants, but not by the presence of
the electron donor or acceptor moieties.
PT(tCz)0.9(DBT)0.1
)
would normally lead to
a
reduced
Space charge limited current hole mobility
In addition to the morphology, we suspect that the variation of
hole mobility might play an important role in inuencing the
device performance. To evaluate the contribution of polymer
mobility, we employed hole-only devices using a high-work-
function material, palladium (Pd), as the cathode to block the
Fig. 4 Current density–potential curves of illuminated (AM 1.5 G, 100 mW cmꢀ2
)
PTs/PC71BM-based conventional PSCs.
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applied to the device, and L is the blend lm thickness. The
experimental dark-current densities of the PTs devices were
measured in the hole-only devices. The applied voltage was
corrected for the built-in voltage (VBI), which was estimated from
the difference between the work function and the HOMO energy
level of PTs. Based on the slopes from the plots of J0.5 with
respect to V for the corresponding devices, we calculated the
eld-independent mobilities of the PTtCz, PT(tCz)0.9(DBT)0.1
,
PT(tCz)0.64(DBT)0.36, and PT(tCz)0.45(DBT)0.55 derived devices to
be 1.35 ꢂ 10ꢀ5, 9.19 ꢂ 10ꢀ6, 9.23 ꢂ 10ꢀ6, and 4.11 ꢂ 10ꢀ6 (cm2
V
–1 sꢀ1), respectively. With increasing the tCz content in the PTs,
higher hole mobility values were observed, which is consistent
with the PCE performance mentioned earlier. This is attributed
to the electron-donating character and more efficient p–p*
charge transfer of the carbazole moiety, which has been widely
used as a hole transport material for organic electronics.26 The
increase in hole mobility exerts more inuence on the Jsc value of
the PTs-derived PSCs than the absorption characteristics. On the
contrary, the losing hole mobility and lowering Jsc values were
consistent for the PTs with increasing DBT content. The SCLC
measurements are in good agreement with the PCE perfor-
mances. With increasing the DBT content in the PTs, the pres-
ence of lower hole mobilities would lead to unbalanced charge
transport and hole accumulation. The hole accumulation
partially acted as the recombination centers of electron–hole
pairs. Subsequently the photocurrent collection efficiency of the
respective electrode was restricted.
Air stability of the PTtCz-derived inverted device
Our interests in understanding the air stability of our rationally
designed low HOMO materials led us to evaluate their PCE
degradation with time under ambient conditions. Due to the
better environmental stability of the inverted cells,49–52 we
further studied the durability of the PTtCz/PC71BM-based lm
based on an inverted structure of ITO/ZnOx/PTtCz:PC71BM/
MoO3/Ag, as shown in Fig. 6(a). The long-term current density-
potential characteristics of the PTtCz-derived inverted devices
without encapsulation under ambient condition is shown in
Table 3.The evaluated photovoltaic characteristics of the PTtCz/
PC71BM-based devices, as a function of elapsed time, are shown
in Fig. 6(b). For each set of points in the plot, the average
performance parameters of the devices were measured, and the
standard deviation was recorded over an extended period of
testing, further testifying the uniformity and robustness of
the devices. As expected, the low-lying HOMO level of PTtCz
(ꢀ5.39 eV), which is relatively stable against oxidation in
ambient conditions allowed the device to retain ca. 80% of its
original efficiency over a period of 1032 h (ISOS-D-1 shelf).30,31
Preliminary results indicate that this new synthetic material
exhibited good stability in ambient conditions (dened as
25 ꢁC/RH 50% in general) using an air-stable inverted archi-
tecture. Nevertheless, factors such as heat and humidity would
affect the OPV performance in long-term practical applica-
tions.53 Therefore, further long-term stability investigations are
Fig. 5 AFM tapping mode topographic (a) and phase (b) images of fabricated
photoactive PTs/PC71BM-based blend thin films (1-PTtCz, 2-PT(tCz)0.9(DBT)0.1
3-PT(tCz)0.64(DBT)0.36, 4-PT(tCz)0.45(DBT)0.55, and 5-PTDBT).
,
back injection of electrons.48 When a sufficient voltage is
applied to this hole-only device, the transport of holes through
the polymer lm is limited by the space charge that accumu-
lates. The space charge limited current (SCLC) is described by
eqn (1):
9
v2
hL3
J ¼ 3r30m
(1)
8
Where 3r is the dielectric constant of the polymer, 30 is the currently underway to understand the performance of inverted
permittivity of free space, mh is the hole mobility, V is the voltage devices under outdoor conditions (ISOS-O-1 shelf).31
8958 | J. Mater. Chem. A, 2013, 1, 8950–8960
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Journal of Materials Chemistry A
Acknowledgements
We thank the National Science Council of Taiwan for nancial
support (grant numbers NSC-101-2221-E-002-035-MY3 and
NSC101-2113-M-131-001-MY2).
Notes and references
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