Small bandgap naphthalene diimide copolymers for efficient inorganic-organic hybrid solar cells

Article abstract of DOI:10.1039/c4ra12188k

Two low band-gap naphthalene diimide (NDI)-based conjugated polymers have been designed and grafted with benzo[1,2-b:4,5-b′]dithiophene (BDT) or dithieno [3,2-b:2′,3′-d]pyrrole (DTP) by a Stille cross-coupling reaction. Inorganic-organic hybrid solar cells (HSCs) based on the copolymers deliver high performances by suitable molecular design and careful selection of chemical structures with different electronic nature. The well designed copolymers exhibit broader solar light absorption, which is attributed to their smaller band gaps. Density functional theory calculations and cyclic voltammetry characteristics reveal that the copolymers have small band gaps and deep HOMO and LUMO energy levels. Moreover, after introducing the copolymers, the energy level formation of the bulk-heterojunction become more matched, thus giving rise to excellent photovoltaic performances as HSCs. The results showed that an NDI-based copolymer with DTP donor segments exhibits a higher power conversion efficiency of 2.36%. This work highlights the development of bipolar host materials with a focus on molecular design strategies, which benefits the light harvest and enhances the efficiency by well aligned energy level formation. This journal is

Full text of DOI:10.1039/c4ra12188k

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The energy level control of Efficient Inorganic/Organic hybrid solar cells
induced by using copolymer was demonstrated

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Journal Name
RSCPublishing
ARTICLE
Small Bandgap Naphthalene diimide Copolymers for
Efficient Inorganic/Organic Hybrid Solar Cells
Cite this: DOI: 10.1039/x0xx00000x
Yuancheng Qin,*a Xing Li,a Weifu Sun,b Xubiao Luo,a Mingjun Li,a Xinghua
Tang,a Xiao Jin,a Yu Xie,a Xinhua Ouyanga and Qinghua Li*a
Received 00th January 2012,
Accepted 00th January 2012
Two low bandꢀgap naphthalene diimides (NDI)ꢀbased conjugated polymers have been designed
and grafted with benzo[1,2ꢀb:4,5ꢀb
′]dithiophene (BDT), Dithieno [3,2ꢀb:2′,3′ꢀd]pyrrole
DOI: 10.1039/x0xx00000x
(DTP) by Stille crossꢀcoupling reaction, respectively. The inorganic/organic hybrid solar cells
(HSCs) based on the copolymers deliver high performances by suitable molecular design and
careful selection of chemical structures with different electronic nature. The well designed
copolymers exhibit broader solar light absorption, which is attributed to their smaller band
gaps. The density functional theory calculations and cyclic voltammetry characteristics reveal
that the copolymers have small band gaps and deep HOMO and LUMO energy levels.
Moreover, after introducing the copolymers, the energy level formation of bulkꢀheterojunction
become more matchable, thus giving rise to excellent photovoltaic performances of HSCs. The
results showed that NDIꢀbased copolymer with DTP donor segments exhibits a higher power
conversion efficiency of 2.36%. This work highlights developed bipolar host materials with the
focus on molecular design strategies, which benefits the light harvest and enhances the
efficiency by well aligned energy level formation.
www.rsc.org/
research activities for developing new small band gap materials
with better aligned energy levels.
1. Introduction
Recently, organic/inorganic bulkꢀheterojunction (BHJ) hybrid
solar cells (HSCs) based on composites of pꢀtype conjugated
polymers and nꢀtype inorganic nanomaterials have attracted
much attention.1ꢀ3 HSCs can take advantages of the beneficial
properties of both types of materials, such as lowꢀcost solutionꢀ
processing of organic semiconductors at low temperatures and
high electron mobility of inorganic semiconductors.4ꢀ6 Despite
the advantages of organic/inorganic HSCs, further
improvement of power conversion efficiency (PCE) is still
required towards commercialization.7 Although the polymeric
solar cells that are made with regioregular poly(3ꢀ
hexylthiophene) (P3HT) as donor material have exhibited
excellent performances, nonetheless because of poor light
harvest of the solar spectrum and the mismatching of the energy
levels between the donor and acceptor, further efficiency
enhancement is hindered. In particular, most of the reported
PCE of organic/inorganic HSCs based on the conjugated
polymer P3HT is around 2%, ranging from 1.7~3.0%,
depending on the type of inorganic nanocrystals (except
quantum dots, which are more related to colloidal quantum dot
solar cells8), their morphologies and others.9ꢀ11 In order to
enhance efficiencies even further, this work is focusing on
In general, the influencing factors of PCE include the openꢀ
circuit voltage (Voc), shortꢀcircuit current ensity (Jsc), and fill
factor (FF).12 In HSCs, the degree of the matchment between
the absorption of the conjugated polymers and the solar
13
spectrum has an obvious effect on Jsc
.
The Voc is determined
by the difference between the energy levels of lowest
unoccupied molecular orbital (LUMO) of the inorganic
semiconductor acceptors and the highest occupied molecular
orbital (HOMO) of the conjugated polymer donors.14 Therefore,
further improvement in the PCE needs the development of
novel conjugated polymers with an appropriate energy level as
well as broader absorption of the solar spectrum. The energy
level band edges are as important as the band gap of the
material, both of which critically determine the photovoltaic
performance of HSCs. Narrowing the band gap of copolymers
will eventually result in a decreased PCE due to the decrease in
Voc.15 In practice, the optimal band gap will not only depend on
the suitable energy level formation between donor and acceptor
that enables efficient charge separation, but also the absorption
properties of the donor. An effective route to boost PCE is to
adjust energy level band edge positions of the donor by
modifying/designing the molecular structure of conjugated
polymers by copolymerizing with different segment units.
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As a structural analogue to perylene diimides, naphthalene otherwise. All solvents were distilled over appropriate drying
diimides (NDI) demonstrate comparable LUMOs and agent(s) prior to use and were purged with nitrogen. All
outstanding organic thinꢀfilm transistor (OTFT) performance, manipulations involving airꢀsensitive reagents were performed
which make NDI small molecules as one of the best nꢀchannel in a dry argon.
semiconductors.16 Therefore, NDI molecules has been
The nuclear magnetic resonance (NMR) spectra were
extensively employed and successfully incorporated into collected on a Bruker ARX 400 NMR spectrometer. The cyclic
diverse polymers.17ꢀ20 The small bandꢀgap, lowꢀlying LUMO, voltammetry (CV) results were obtained using a BAS 100B
and good electron mobility of NDI polymers herald their instrument at room temperature at a scan rate of 50mV/s,
potential as acceptor semiconductors in HSCs.21 It is accepted Tetrabutylammonium hexafluorophosphate (Bu4NClO4) in an
that the energy levels can be tuned effectively by the strong anhydrous and nitrogenꢀsaturated acetonitrile (CH3CN) solution
electronꢀwithdrawing imide groups, such as pꢀ or nꢀtype (0.1 mol Lꢀ1) was used as the supporting electrolyte, a
behaviors. However, imideꢀfunctionalized conjugated polymers platinized platinum (0.5 cm2) as the counter electrodes and
as pꢀsystems are rarely reported thus far. On one hand, Ag/Ag+ electrode as the reference electrode, the values are
copolymers containing naphthaleneꢀ1,4,5,8ꢀtetracarboxylic expressed in potentials versus Fc/Fc+. Polymer to be measured
bisimide (PFTBTN) have exhibited promising small band gap was coated on the platinum plate working electrodes from
characteristics with large absorption coefficient;22ꢀ24 on the dilute chloroform solutions. UVꢀvis spectra were recorded on a
other hand, the LUMO energy level can be elevated after PerkinElmer Lambda 900 UVꢀvis/NIR spectrometer.
introducing imide groups, by which the partial charge transfer Thermogravimetric analyses (TGA) were conducted on a TA
can be enhanced. Yet, the HSCs based on these materials such Instruments Model TGA Q500 thermogravimetric analyzer at a
as PFTBTN suffer from poor device performances. Inspired by heating rate of 10 °C minꢀ1 under N2 flow (100 mL minꢀ1). The
these incentives, we introduced NDI into conjugated polymer photocurrentꢀvoltage (
as an electron acceptor. With further construction in structural recorded on an Electrochemical Workstation (Xe Lamp Oriel
modification and device optimization, we anticipate that the Sol3A Class AAA Solar Simulators 94023A USA) under
JꢀV) curves of the assembled HSCs were
™
，
small band gap NDI polymers with better energy level irradiation of a simulated solar light from a 100 W xenon arc
alignment can be achieved, thus leading to a notable increase in lamp in ambient atmosphere.
PCE. For benzo[1,2ꢀb:4,5ꢀb′]dithiophene (BDT), however,
2.2 Synthesis of the Monomer.
steric hindrance between adjacent units is very small, as 4, 9ꢀ
bisꢀalkoxyꢀBDT has no substituent on 1, 3, 5, and 7 positions.25
Dithieno[3,2ꢀb:2′,3′ꢀd]pyrrole (DTP) has good planarity and
strong electronꢀdonating ability of nitrogen atoms, which might
lead to the development of a low band gap polymer.26 So, BDT
and DTP are ideal conjugated units for designing new
photovoltaic materials. Herein, copolymers containing the
combination of BDT and DTP as electronꢀdonating units, NDI
as strong electronꢀaccepting units were designed and
synthesized. We note that the suitable energy offset between the
donor/acceptor can be achieved by utilizing novel either BDT
or DTPꢀgrafted NDIꢀbased alternating conjugated copolymers
(namely P1 and P2, as seen in Chart 1), which enhances solar
spectrum harvest as well as heightens Voc in HSCs.
The synthesis and structure of the monomer are outlined in
Scheme 1.
Chart 1. Structures of the Low Band Gap Copolymers P1 and
P2
.
2. Experimental
Scheme 1. Synthetic route for monomers.
2.1 Materials and instruments.
2,
6-dibromonaphthalene-1,
4,
5,
8-
tetracarboxydianhydride, 1 18
All chemical reagents were purchased from Acros, TCI and
Aldrich Chemical Co. and were used as received except stated
2 | J. Name., 2012, 00, 1-3
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A mixture of 1, 4, 5, 8ꢀnaphthalenetetracarboxylic dianhydride 4H, NꢀCH2), 2.64 (t, 4H, αꢀCH2), 1.67ꢀ1.58 (m, 8H, CH2), 1.26ꢀ
(2.83 g) and oleum (20% SO3, 100 mL) was stirred at 55 ºC for 1.24 (m, 72H, CH2), 0.87 (t, 12H, CH3). 13C NMR (CDCl3,
2 hours. A solution of dibromoisocyanuric (3.12 g) in oleum 100MHz, δ/ppm): 162.0, 142.5, 140.1, 139.4, 136.4, 129.6,
(50 mL) was then added over a course of 40 mins. The resulting 127.4, 125.5, 122.9, 112.4, 41.2, 31.9, 29.7, 29.6, 29.5, 29.4,
mixture was then heated to 85 ºC and maintained at this 29.3, 28.0, 27.1, 22.7, 14.1.
temperature for 43 hours. Upon cooling to room temperature, 2, 6-Bis(tributyltin)-4, 8-didodecyloxybenzo[1, 2-b; 3, 4-
the reaction mixture was poured onto crushed ices. This b]dithiophene (M2)
mixture was diluted with water and then stirred at room M2 was synthesized following methods described in the
temperature for 1 hour. The precipitates were collected by literature.25 1H NMR (CDCl3, 400Hz, δ/ppm): 7.48 (s, 2H,
centrifuge, washed with distilled water and methanol, and dried thiopheneꢀH), 4.30 (t, 4H, OꢀCH2), 1.90ꢀ1.84(m, 4H, CH2),
under vacuum, finally leading to a greenish yellow solid (3.87g, 1.66ꢀ1.58 (m, 12H, CH2), 1.41ꢀ1.27 (m, 48H, CH2), 1.20ꢀ1.12
yield 81.1%).
N, N,- didodecyl-2, 6-dibromonaphthalene-1, 4, 5, 8- 100MHz, δ/ppm): 142.9, 139.9, 134.2, 132.9, 128.2, 73.6, 31.9,
tetracarboxylic acid bisimide, 2 27
30.6, 29.7, 29.7, 29.6, 29.4, 29.1, 29.0, 28.9, 27.3, 26.2, 22.7,
The mixture of compound (1 g), dodecylamine (2.2 mL), and 14.1, 13.7, 10.9.
(t, 12H, CH2), 0.93ꢀ0.88 (m, 24H, CH3). 13C NMR (CDCl3,
1
acetic acid (70 mL) was filtered and then refluxed for 4 h under 2,6-Di(tributyltin)-N-[1-(2’-ethylhexyl)-3-ethylheptanyl]-
argon. After having been cooled to room temperature, the dithieno[3,2-b:2’,3’-d]pyrrole (M3)
precipitate was filtered and washed with distilled water (300 M3 was synthesized following methods described in the
mL), the residue was purified by column chromatography literature.26 1H NMR (CDCl3, 400Hz, δ/ppm): 6.92(t, 2H,
(eluant: hexane:methylene chloride = 1:2) to give
orange crystal (1.2, 52.8%). H NMR (CDCl3, 400Hz, δ/ppm): 1.68(m, 10H, CH2), 1.31ꢀ1.39(m, 24H, CH2), 1.23ꢀ1.27(m, 16H,
8.85 (s, 2H, PhꢀH), 4.22ꢀ4.18 (m, 4H, NꢀCH2), 1.76ꢀ1.73 (m, CH2), 1.08ꢀ1.17(m, 10H, CH2), 0.84ꢀ0.92(m, 24H, CH3). 13C
2 as an thiopheneꢀH), 4.04(s, 2H, NꢀCH2), 1.99(s, 1H, CH), 1.49ꢀ
1
12H, CH2), 1.41ꢀ1.35 (m, 28H, CH2), 0.92ꢀ0.86 (m, 6H, CH3). NMR (CDCl3, 100MHz, δ/ppm): 148.3, 134.56, 120.10, 118.23,
13C NMR (CDCl3, 100MHz, δ/ppm): 162.5, 162.3, 161.7, 51.57,39.19, 31.97, 31.90, 31.66, 30.00, 29.66, 29.38, 29.15,
161.6, 135.9, 131.4, 128.0, 127.0, 126.0, 125.4, 124.0, 121.8, 29.04, 28.94, 27.60, 27.31, 27.03, 26.53, 26.47, 22.73, 22.70,
41.9, 31.9, 31.2, 29.6, 29.3, 27.9, 27.1, 22.6, 14.1.
Bis-(2, 2’-didodecylthienyl)-2, 6-naphthalene-N,
didodecyl-1, 4, 5, 8-tetracarboxylicdiimide, 3
A solution of (0.57g, 0.75 mmol), and 3ꢀdodecylꢀ5ꢀ
(tribytylstannyl) thiophene (1.01 g, 1.8 mmol) in freshly
distilled THF (15 mL) was degassed. The mixture was heated
14.16, 13.74, 10.92.
N’-
2.3 Polymerization.
2
The synthesis and structures of the copolymers are outlined in
Scheme 2.
A typical experimental procedure for the
polymerization of monomer M1 and monomer M2 was as
to reflux under
a
nitrogen atmosphere and then
follows:
dichlorobis(triphenylphosphine)palladium (II) [PdCl2(PPh3)2]
(14 mg, 0.02 mmol) was added. After 12 h, the THF was
removed off by vacuum distillation. The residues were purified
by column chromatography (eluant: hexane:methylene chloride
= 1 : 1) to give pure compound
3 as a red solid (0.5g, 67.5%).
1H NMR (CDCl3, 400Hz, δ/ppm): 8.70 (t, 2H, PhꢀH), 7.14 (s,
2H, thiopheneꢀH), 7.11 (s, 2H, thiopheneꢀH), 4.19ꢀ4.08 (m, 4H,
NꢀCH2), 2.68 (t, 4H, αꢀCH2), 1.72ꢀ1.65 (m, 6H, CH2), 1.42ꢀ1.24
(m, 74H, CH2), 0.86 (t, 12H, CH3). 13C NMR (CDCl3,
100MHz, δ/ppm): 162.6, 143.7, 141.1, 140.4, 136.4, 136.3,
131.3, 130.5, 129.6, 127.9, 126.8, 126.5, 126.1, 125.0, 123.3,
122.8, 41.1, 41.0, 31.9, 30.5, 30.4, 29.6, 29.5, 29.3, 28.0, 27.1,
25.6, 23.0, 22.7, 22.2, 14.1.
Bis-{2,
naphthalene-1,
didodecyldiimide (M1)
To a solution of (0.5 g, 0.45 mmol) in chloroform (25 mL),
Nbromosuccimide (NBS) (0.16 g, 0.9 mmol) was added. This
mixture was then stirred for 4 h at room temperature in
darkness. The solvent was removed off by vacuum distillation.
The residues were purified by column chromatography
(hexane:methylene chloride = 2 : 1) to give pure compound M1
(0.48 g. 85%) as a purple solid. 1H NMR (CDCl3, 400Hz,
δ/ppm): 8.70 (s, 2H, PhꢀH), 7.03 (s, 2H, thiopheneꢀH), 4.11 (t,
2’-(5,
5’-dibromo)-didodecylthienyl}-2,
4, 5, 8-tetracarboxylic-N,
6-
N’-
Scheme 2 Synthetic route for copolymers.
3
P1: Under argon atmosphere, monomer M1 (0.3 mmol,
0.3784 g) and monomer M2 (0.3 mmol, 0.3411 g) were
dissolved in 15 mL of toluene. The solution was flushed with
argon for 10 min, and then Pd2dba3 (5.40 mg) and P(oꢀtolyl3)
(7.20 mg) were added into the flask. The flask was purged three
times with successive vacuum and argon filling cycles. The
polymerization reaction was heated to 110 °C, and the mixture
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was stirred for 72 h under argon atmosphere. 2ꢀTributylstannyl 5% weight loss) at 343 and 340 °C for P1 and P2, respectively,
thiophene (23.50 ꢁL) was added to the reaction. After two as determined by thermogravimetric analysis (TGA) in Figure 1
hours, 2ꢀbromothiophene (7.90 ꢁL) was added. The mixture and shown in Table 1. The combinations of such good thermal
was stirred overnight to complete the endꢀcapping reaction. The properties are adequately suitable for photovoltaic and other
mixture was cooled to room temperature and poured slowly in optoelectronic devices.
400 mL of methanol. The precipitate was filtered and washed
with methanol and hexane in a soxhlet apparatus to remove the
oligomers and catalyst residue. Finally, the polymer was
extracted with chloroform. The solution was condensed by Table 1 Molecular weights and thermal properties of different
evaporation and precipitated into methanol. The polymer was copolymers.
collected as a dark purple solid with a yield of 76.8% (389 mg).
a
a
Copolymers
Mn
Mw
PDI
Td (℃)
1H NMR (CDCl3, 400Hz, δ/ppm): 8.28 (d, 2H, PhꢀH), 7.49ꢀ
7.33 (m, 4H, thiopheneꢀH), 4.22ꢀ4.08 (m, 8H, OꢀCH2, NꢀCH2),
1.96ꢀ1.50 (m, 124H, CH2, CH3), 0.80 (s, 18H, CH3).
P1
P2
15000 18000 1.2
9465 11519 1.2
343
P2: P2 was synthesized from monomers M1(0.3 mmol,
340
0.3784 g) and M3 (0.3 mmol, 0.2729 g) as a dark purple solid
with a yield of 89% according to the method of polymer P1
described above. 1H NMR(CDCl3, 400Hz, δ/ppm): 8.75(br, 4H,
thiopheneꢀH), 8.39(br, 1H, thiopheneꢀH), 7.26(br, 11H, PhꢀH),
6.97(br, 11H, PhꢀH), 5.12(br, 2H, NꢀCH2), 4.12(br, 28H, Nꢀ
CH2), 2.04(br, 16H, CH2),1.24(br,673H,CH2),0.86(br, 138H,
CH3).
aMn
, Mw and PDI of the different copolymers were obtained by
GPC technique using polystyrene in THF as standards.
1.0
0.9
0.8
0.7
0.6
2.4 Device fabrication.
TiO2 colloid was prepared by the procedures in a similar way
as done in the previous works.28,29 A layer of nꢀtype TiO2 film
with a thickness of ~300 nm was prepared by coating the TiO2
colloid on Fluorineꢀdoped tin oxide (FTO) glass using a spinꢀ
coated technique, followed by sintering in air at 450 ºC for 30
min. Then, the acceptor film was soaked in a 5 mg mLꢀ1 coꢀ
polymer in CHCl3 solution for 12 hr to uptake pꢀtype polymer
and dried under nitrogen flow. Next, PEDOT: PSS layer was
spinꢀcoated onto the BHJ. Finally, Pt electrodes were deposited
on the top of the PEDOT: PSS layer by thermal evaporation
under vacuum.
0.5
0.4
0.3
0.2
0.1
P1 polymer
P2 polymer
100
200
300
400
500
600
Temperature (oC)
Fig. 1. TGA thermograms of P1 and P2 polymers.
3.2 Absorption spectra of copolymers.
The absorption spectra of copolymers in CHCl3 solution and
copolymer films are shown in Fig. 2. The spectroscopic data are
listed in Table 2. As shown in Fig. 2, both P1 and P2 present
two absorption bands. For solution, the absorption peaks of the
P1 are located at about 394 and 644 nm, while those of P2 are
at about 388 and 673 nm. For the thinꢀfilm, the absorption band
edge is located at about 438 and 699 nm for P1, 433 and 759
nm for P2. In general, the thinꢀfilm absorption spectra are
similar in shape to those in dilute solutions. All absorption
spectra exhibit two characteristic bands centered at 350ꢀ500 and
500ꢀ1000 nm, respectively. As compared to their counterparts
in the solution, the broad peaks of the thinꢀfilm absorption
spectra show obvious redꢀshifts. This can be explained by the
formation of πꢀstacked structures in the solid state,34 which
could facilitate charge transport for photovoltaic applications.
The results also indicate that the absorption characteristics of
copolymer P1 and P2 could be readily tuned, which are very
useful for the design of the photovoltaic materials with
enhanced absorption. The energy band gap Eg calculated from
the absorption band edges of the optical absorption spectra
3. Results and discussion
3.1 Synthesis and characterization
Copolymers show good solubility at room temperature in
organic solvents such as chloroform, tetrahydrofuran and
toluene. The incorporation of alkyl side chains on NDI, BDT
and DTP segments enables them to have a good solubility. The
weightꢀaverage molecular weight (Mw) and polydispersity
index (PDI=Mw/Mn) were measured by gel permeation
chromatography (GPC) technique using THF as the eluent and
polystyrenes as the internal standards. As shown in Table 1, P1
gives a Mn of 15000 and a PDI of 1.2. P2 has a Mn of 9465 and
a PDI of 1.2. Stille coupling usually results in a broad
molecular weight distribution.30,31 However, it is worth
mentioning that the polymerization reaction was performed at
110 °C for 72 h under vigorous stirring. Such long time reaction
at relative high temperature may induce a uniform weight
distribution, which agrees well with the related systems.32,33
Moreover, the resulting copolymers exhibit good thermal
stability with the decomposition onset temperatures (Td) (about
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were about 1.62 and 1.57 eV for P1 and P2, respectively, as per absorption spectra as described above (i.e., 1.62 and 1.57 eV,
Eg=1240/λ eV, where λ is the absorption band edge. The respectively).
tunable optical bandgap and broad absorption bands across the
entire visible wavelength region of NDIꢀbased copolymer was
achieved by tailoring the donor structures, suggesting their
potentially useful applications in HSCs.
0.0025
0.0020
0.0015
0.0010
0.0005
0.0000
-0.0005
P1 polymer
P2 polymer
P1 solution
P1 film
1.0
P2 solution
P2 film
0.8
0.6
0.4
0.2
0.0
-3
-2
-1
0
1
Potential (V)
Fig. 3. Cyclic voltammogram measurements of different
copolymers.
400
500
600
700
800
900
3.4 Theoretical Calculations.
Wavelength(nm)
Fig. 2. Optical absorption spectra of copolymers in CHCl3
solution and thin film on quartz plates.
Table 2. Optical and Electrochemical Data of copolymers.
Absorptio
ox
peak
red
n peaks of Egopt
E
E
peak
Absorption
peaks in
CHCl3 (nm)
Eg
thin film
(eV)
/HOMO
/LUMO
(nm)
0.43/
ꢀ5.14
ꢀ1.39/
ꢀ3.32
1.82
1.79
P1
P2
394, 644
388, 673
438, 699
433, 759
1.62
1.57
0.51/
ꢀ5.22
ꢀ1.28/
ꢀ3.43
3.3 Electrochemical Properties.
The electrochemical properties of the copolymers are
investigated by CV technique. The CV characteristics are
carried out with Ag/Ag+ as reference electrode. The energy
level of the Ag/Ag+ reference electrode was calibrated against
FC/FC+ system and determined to be 4.71 eV below the vacuum
level.35 The cells are purged with pure argon prior to each scan.
The scans toward the anodic and cathodic directions were
performed at a scan rate of 50 mV sꢀ1 at room temperature. The
voltammetric measurements of the copolymer thin films are
shown in Fig. 3, and all the electrochemical redox data are
summarized in Table 2. Both oxidation and reduction waves are
clearly revealed in the CVs, which indicates the capability of
copolymers to transport both electrons and holes. Based on the
HOMO and LUMO values, the bandgap (Eg) of the P1 and P2
were found to be about 1.82 and 1.79 eV, respectively. These
values are comparable to those obtained from the optical
Fig. 4. Optimized geometry, molecular orbital surfaces of the
HOMO and LUMO of the model compounds, obtained at the
DFT/B3LYP/6ꢀ31G*.
To gain deeper insight into the effect of BDT, DTP and the
nature of the excited states of these NDIꢀbased copolymers, we
performed density functional theory (DFT) calculations at the
B3LYP/6ꢀ31G* level for NDIꢀBDT and NDIꢀDTP compounds
and optimized the geometrical structure, followed by
calculating the lowest vertical excitations using the Gaussian 09
program suite. DFT/B3LYP/6ꢀ31G* method was used here,
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DOI: 10.103J9o/Cu4rRnAa1l21N8a8Kme
because it has been found to be an accurate method for respectively, following excitation at 510 nm, are shown in Fig.
calculating the optimal geometry and electronic structures in 6. The emission of TiO2/copolymer BHJ was quenched by more
many molecular systems. The alkyl groups were replaced by than 80% as compared to copolymer emission, indicating that
methyl groups to simplify the calculations because the length of lots of the charges transport from donor to acceptor in BHJs has
alkyl chains does not significantly affect the equilibrium occurred.38 However, the PL for the BHJ has not yet been
geometry and the electronic properties.
completely quenched, which may be ascribed to the large pores
The optimized geometry molecular structures and frontier of acceptor films and hence copolymer is not in ideally direct
energy levels (HOMO and LUMO) of the model compound are but still intimate contact with the TiO2 acceptor.
100
shown in Fig. 4. As shown in Fig. 4, the results indicate that the
P1 polymer
90
80
70
60
50
40
30
20
10
0
P2 polymer
electron density of LUMO is mainly localized on the NDI unit.
The HOMO values were calculated to be ꢀ5.26 and ꢀ4.99 eV,
and the LUMO values to be ꢀ3.36 and ꢀ3.24 eV for NDIꢀBDT
and NDIꢀDTP, respectively. Thus, the bandgaps are determined
to be about 1.90 and 1.75 eV. The results are in good agreement
with those obtained by CV characteristics and UVꢀvis
absorption spectra.
TiO /P1 BHJ
2
TiO /P2 BHJ
2
Quenching
3.5. Photovoltaic Properties
It is generally accepted that the photogeneration in present HSC
can be boosted by simple consideration of the energetics at the
donor/acceptor interface (Fig. 5). First, copolymers act as a
donor by absorbing photons, after which delocalized charges
are created. Then the electrons diffuse towards an electron
acceptor (TiO2) interface while the holes are transferred to the
opposite phases by utilizing the hole extraction material
(PEDOT:PSS). However, not all of the holes and electrons can
reach the interface, owing to the fact that the electronꢀhole pairs
are strongly bound by Coulomb interaction. Note that, the
energy offset between the LUMO of copolymers and the CB of
TiO2 is 0.48 and 0.43 eV, respectively, which is close to the
Coulomb attraction (typically 0.1~0.5 eV), thus enabling the
initial electron transfer step to be energetically downhill.37
After the electrons and holes are dissociated into free electron
or hole charge carriers, they are transported through the two
respective phases and can be collected at the electrodes.
550
600
650
700
750
800
Wavelength (nm)
Fig. 6. Steady state PL of copolymer neat films and
TiO2/copolymers blend films.
The porous acceptor film was soaked in coꢀpolymer solution
to form BHJs. The crossꢀsectional SEM image in Fig. 7 shows
that the polymers were permeated well into the porous structure
of TiO2 film. The intimate contact between donor and acceptor
benefits the efficient charge transfer at the interface because of
the short diffusion length of the phtotoexcited carrier in
polymer, typically ~12 nm.39 The photocurrentꢀvoltage (
JꢀV)
characteristics of the TiO2/P1 BHJ and TiO2/P2 BHJ HSCs
under a simulated solar light irradiation of 100 mW cmꢀ2 are
shown in Fig. 8. Each type of the HSCs was tested for 5 times
under the identical experimental conditions, and the statistics of
the device parameters with standard derivations were listed in
Table 3. The corresponding Voc, Jsc, FF, and PCE parameters of
these devices under the illumination of AM1.5 (100 mW cmꢀ2)
are measured to be 0.64 V, 5.45 mA cmꢀ2, 0.53, and 1.94% for
copolymer P1, and 0.67 V, 6.19 mA cmꢀ2, 0.57, and 2.36% for
copolymer P2, respectively. The current density Jsc is governed
by the device quantum efficiency, which depends on the light
harvest capacity of the polymer.40 P2 exhibits better light
harvest ability than its counterpart P1 due to the smaller band
gap, thus delivering a notable current density. Additionally,
HOMO level of P2 is below than P1, while the energy levels of
acceptor (i.e. TiO2) remains unchanged, which is prone to give
rise to a higher Voc according to the following relationship:41,42
2


Neff
np
Fig. 5. Schematic of an HSC incorporating an active layer
TiO2/copolymers and the energy band diagram of the materials
used in HSC. The VB (ꢀ7.40 eV) and CB (ꢀ4.20 eV) of TiO2 is
obtained from the literature.36
To reveal the electron transport properties at the interface of
acceptor/donor BHJs, the steady state photoluminescence (PL)
spectra were measured and the steady state PL results of
copolymer donor neat films, TiO2/copolymer BHJ blend films,
kT
q
(1)
VOC = HOMOD − CBA
−
ln






where HOMOD is the highest occupied molecular orbital
(HOMO) of the donor, CBA is the conduction band of the
acceptor,
charge, T is temperature, Neff is the effective density of state,
and are the concentrations of electrons and holes,
k is the Boltzmann constant, q is the elementary
n, p
respectively. This relationship demonstrates that the Voc is
6 | J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
DOI: 10.1039/C4RA12188K
determined by the energy offset between HOMOD and CBA.43 2.36% than its counterpart (1.94%), i.e., enhanced by 21%,
Note that the CBꢀHOMO offset of TiO2/P2 is 1.02 eV, which is owing to its smaller band gap as well as more suitable energy
greater than TiO2/P1 (0.94 eV) system. The enhanced Jsc and levels formation. This study indicates that both the band gap
Voc finally result in a higher PCE for TiO2/P2 HSC than and HOMO level of the donor can be efficiently lowered by
TiO2/P1 HSC. The present results indicate that the NDIꢀbased grafting carefully chosen monomers. This work opens up a new
copolymers grafted with different donors are one of the route to achieve small band gap polymers with wellꢀmatched
promising efficient photovoltaic materials. Furthermore, energy level alignment for robust HSC applications.
suitable electronic, optical, and photovoltaic properties can be
easily achieved by simply tuning the molecular structures of
donors.
Acknowledgements
The work was financially supported by Aviation Science Fund
of China (No: 2013ZF56025), Natural Science Foundation of
Jiangxi Province (Grant no. 20142BAB213015), Key
Laboratory of Jiangxi Province for Persistant Pollutants Control
and Resources Recycle (No: ST201222004), and the Natural
Science Foundation of China (61366003).
Notes and references
aKey Laboratory of Jiangxi Province for Persistant Pollutants Control and
Resources Recycle, Nanchang Hangkong University, Nanchang, 330063,
P. R. China,
bSchool of Aerospace, Mechanical and Mechatronic Engineering, The
University of Sydney, Sydney,New South Wales 2006, Australia
Eꢀmail: qhli@hqu.edu.cn, qinyuancheng@hotmail.com.
Notes
Fig. 7. Crossꢀsectional SEM image of TiO2/coꢀpolymer BHJ.
The authors declare no competing financial interest.
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