Systematic investigation of benzodithiophene- and diketopyrrolopyrrole- based low-bandgap polymers designed for single junction and tandem polymer solar cells

Article abstract of DOI:10.1021/ja301460s

The tandem solar cell architecture is an effective way to harvest a broader part of the solar spectrum and make better use of the photonic energy than the single junction cell. Here, we present the design, synthesis, and characterization of a series of new low bandgap polymers specifically for tandem polymer solar cells. These polymers have a backbone based on the benzodithiophene (BDT) and diketopyrrolopyrrole (DPP) units. Alkylthienyl and alkylphenyl moieties were incorporated onto the BDT unit to form BDTT and BDTP units, respectively; a furan moiety was incorporated onto the DPP unit in place of thiophene to form the FDPP unit. Low bandgap polymers (bandgap = 1.4-1.5 eV) were prepared using BDTT, BDTP, FDPP, and DPP units via Stille-coupling polymerization. These structural modifications lead to polymers with different optical, electrochemical, and electronic properties. Single junction solar cells were fabricated, and the polymer:PC₇₁BM active layer morphology was optimized by adding 1,8-diiodooctane (DIO) as an additive. In the single-layer photovoltaic device, they showed power conversion efficiencies (PCEs) of 3-6%. When the polymers were applied in tandem solar cells, PCEs over 8% were reached, demonstrating their great potential for high efficiency tandem polymer solar cells.

Full text of DOI:10.1021/ja301460s

Article
pubs.acs.org/JACS
Systematic Investigation of Benzodithiophene- and
Diketopyrrolopyrrole-Based Low-Bandgap Polymers Designed for
Single Junction and Tandem Polymer Solar Cells
Letian Dou,† Jing Gao,† Eric Richard,† Jingbi You,† Chun-Chao Chen,† Kitty C. Cha,† Youjun He,†
Gang Li,† and Yang Yang*,†,‡
‡
†Department of Materials Science and Engineering, and California NanoSystems Institute, University of California, Los Angeles,
California 90095, United States
S
* Supporting Information
ABSTRACT: The tandem solar cell architecture is an
effective way to harvest a broader part of the solar spectrum
and make better use of the photonic energy than the single
junction cell. Here, we present the design, synthesis, and
characterization of a series of new low bandgap polymers
specifically for tandem polymer solar cells. These polymers
have a backbone based on the benzodithiophene (BDT) and
diketopyrrolopyrrole (DPP) units. Alkylthienyl and alkylphen-
yl moieties were incorporated onto the BDT unit to form
BDTT and BDTP units, respectively; a furan moiety was incorporated onto the DPP unit in place of thiophene to form the
FDPP unit. Low bandgap polymers (bandgap = 1.4−1.5 eV) were prepared using BDTT, BDTP, FDPP, and DPP units via Stille-
coupling polymerization. These structural modifications lead to polymers with different optical, electrochemical, and electronic
properties. Single junction solar cells were fabricated, and the polymer:PC71BM active layer morphology was optimized by adding
1,8-diiodooctane (DIO) as an additive. In the single-layer photovoltaic device, they showed power conversion efficiencies (PCEs)
of 3−6%. When the polymers were applied in tandem solar cells, PCEs over 8% were reached, demonstrating their great potential
for high efficiency tandem polymer solar cells.
spectrum.14−17 For example, the Shockley−Quiesser limitation
1. INTRODUCTION
of single junction solar cell is ∼34% (this requires a
semiconducting material with Eg ∼1.4 eV).18 Via enhancing
photon utilization efficiency, inorganic multijunction tandem
solar cells have reached up to 43.5% efficiency.8,9 Using
conservative assumptions, Brabec et al. predicted that the
theoretical maximum PCE for a tandem polymer solar cell is
15%, whereas it is 10% for a single junction cell.19,20 Realizing
solution processed tandem solar cell is very promising for
achieving high efficiency and low cost photovoltaic devices.
Most of the work on tandem PSCs has focused on the
double junction structure, which consists of a front cell with a
high-bandgap material, an interconnecting layer (ICL), and a
rear cell with a low-bandgap (LBG) material.21−24 Most of
those studies have focused on improving the ICL, and only a
few have demonstrated high efficiencies of 4−7%.25−30 In
addition to the ICL, the photoactive materials also play a
critical role in determining the PCE. One of the problems
holding back the progress of tandem solar cell is the lack of a
high performance low bandgap polymer (Eg < 1.5 eV),
designed for the tandem structure. As a result, the efficiency
for previous tandem solar cells has been limited to below 7%
Organic photovoltaics (OPV) technology has great potential
for low-cost solar electrical energy generation and has drawn a
great deal of attention.1−4 Tremendous efforts on processing,
material, and device have been devoted to this field in recent
years to improve the efficiency, with record efficiencies being
reported both in polymer solar cells (PSCs) and in small-
molecule OPV, both in the academia field and industry (PSCs)
and small-molecule OPV both in the academia field and in
industry (e.g., Solarmer Energy Inc., Kornaka Inc., Mitsubishi
Chemical, and Heliatek).5−9 Polymer-based OPV attracts more
attention due to its easy synthesis and purification as well as
better processability. So far in the literature, single junction
PSCs based on conjugated polymers as electron-donor
materials blended with [6,6]-phenyl-C71-butyric acid methyl
ester (PC71BM) as an electron-acceptor material have achieved
around 8% power conversion efficiency (PCE) using a bulk
heterojunction (BHJ) device structure.10−12 However, further
increasing PCE is challenging for single junction devices,
because photons with energy smaller than the bandgap cannot
be absorbed (loss of photocurrent) and photons with larger
energy will lose their excess energy via a thermalization process
of the hot carriers (loss of photovoltage).13 Alternatively,
tandem solar cells can reduce these losses by using separate
subcells, each converting a different part of the solar
Received: February 13, 2012
Published: May 29, 2012
© 2012 American Chemical Society
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for awhile.21−30 Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-
cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothia-
diazole)] (PCPDTBT) was the first polymer demonstrated to
be highly effective in tandem PSCs, and a PCE of 6.5% was
reported by Heeger et al. in 2007.6,25 Later, our group replaced
a carbon atom of PCPDTBT with a silicon atom and achieved a
LBG polymer poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-
d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl]
(PSBTBT).31 Tandem devices based on this material, together
with a newly developed robust ICL, showed 7% PCE.26
Another important tandem PSC with 4.9% PCE was reported
by Janssen et al. with the LBG polymer poly[3,6-bis(4′-dodecyl-
[2,2′]bithiophenyl-5-yl)-2,5-bis(2-ethylhexyl)-2,5-
dihydropyrrolo[3,4-]pyrrole-1,4-dione] (pBBTDPP2).23
Scheme 1. Synthesis of Monomers, PBDTT-DPP, PBDTP-
DPP, PBDTT-FDPP, and PBDTP-FDPP
a
It is worth mentioning that the diketopyrrolopyrrole (DPP)
unit, which was developed over four decades ago for high-
performance pigments, appeared to be a promising building
block for LBG polymers.32 It is highly absorbing in the visible
region and strongly electron withdrawing. When polymerized
with electron-donating monomers, the resulting polymers show
energy bandgaps smaller than 1.5 eV. In recent years, there has
been an intensive effort to develop LBG polymers based on the
DPP unit for photovoltaic applications, and single junction
́
devices with PCEs of 4−5% were reported by Janssen, Frechet,
and Yang.33−36 Because of poor solubility, relatively low
molecular weights, and high highest occupied molecular orbital
(HOMO) levels, the devices showed either low open circuit
voltage (VOC), low short circuit current (JSC), or low fill factor
(FF). They are far from ideal for a tandem structure because
when two subcells are connected in series, the overall tandem
cell performance will be limited by the poorest-performing
subcell.
a
(I) 2-Butyloctyl bromide, K2CO3, DMF, 140 °C. (II) NBS, CHCl3,
room temperature. (III) 4-(2-Ethylhexyl)bromobenzene, Mg, then
SnCl2·2H2O, THF, room temperature. (IV) n-Butyl lithium, then
SnMe3Cl, in THF, room temperature. (V) Pd(PPh3)4, toluene/DMF,
110 °C.
Recently, we have demonstrated a tandem PSC with
National Renewable Energy Laboratory (NREL) certified
efficiency of 8.62%, incorporating a new LBG polymer,
PBDTT-DPP, based on alternating DPP and alkylthienylben-
zodithiophene (BDTT) units (Figure 1).37 However, the
furan group was incorporated into the DPP unit to form the
FDPP unit using a reported method.35 Both BDTT (M4) and
BDTP (M3) were polymerized with DPP (M1) and FDPP
(M2) via Stille-coupling polymerization. Four polymers,
PBDTT-DPP, PBDTP-DPP, PBDTT-FDPP, and PBDTP-
FDPP, were obtained in good yield. As compared to early
efforts on DPP based polymers, the newly designed polymers
show deeper HOMO levels and better charge transport
properties, which lead to higher VOC and JSC in PSC devices.
The low energy bandgap (1.4−1.5 eV) and high photovoltaic
performance enable them to be applied in tandem PSCs, and
PCEs close to 9% were achieved.
2. EXPERIMENTAL SECTION
Materials. Benzo[1,2-b:4,5-b′]dithiophene-4,8-dione (3), 2,5-dieth-
ylhexyl-3,6-bis(5-bromofuran-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione
(M2), 2,6-bis(trimethyltin)-4,8-bis(5-ethylhexyl-2-thienyl)-benzo[1,2-
b:4,5-b′]dithiophene (M4), 4-ethylhexylbromobezene, 2-ethylhexylth-
iophene, and Indene-C60 bisadduct (ICBA) were synthesized
according to the procedures reported in the literature.7,35,38 Poly(3-
hexylthiophene) (P3HT) was purchased from Rieke Metals. [6,6]-
Phenyl-C71-butyric acid methyl ester (PC71BM) was purchased from
Nano-C. Unless otherwise stated, all of the chemicals were purchased
from Aldrich and used as received. The monomers and polymers were
synthesized according to Scheme 1.
3,6-Dithiophen-2-yl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-
dione (1). Potassium tert-butoxide (20 g, 180 mmol) was added to a
round-bottom flask with argon protection. A solution of t-amyl alcohol
(125 mL) and 2-thiophenecarbonitrile (16.4 g, 150 mmol) was
injected in one portion. The mixture was warmed to 100−110 °C, and
a solution of dimethyl succinate (7.30 g, 50 mmol) in t-amyl alcohol
(40 mL) was dropped in slowly in 1 h. The reaction was kept at the
Figure 1. Chemical structures of PBDTT-DPP, PBDTP-DPP,
PBDTT-FDPP, and PBDTP-FDPP.
detailed synthesis and characterization of this new family of
LBG polymers have not been reported. Here, we report the
design, synthesis, and characterization of a series of polymers
for tandem PSCs derived from modification of PBDTT-DPP
(Scheme 1). To tune the structural and electronic properties, a
new monomer (BDTP) with an alkylphenyl group attached to
the BDT unit instead of the alkylthienyl group was developed; a
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same temperature for about 1 h, and then the methanol byproduct was
distilled off and the reaction was kept at 100−110 °C for another 2 h.
The mixture was then cooled to 65 °C, diluted with 50 mL of
methanol, neutralized with acetic acid, and refluxed for another 10
min. Next the suspension was filtered, and the black filter cake was
washed with hot methanol and water twice and dried in a vacuum to
get the crude product, which could be used directly in the next step
without further purification (10.6 g, yield 72%).
7.64 (d, 4H), 7.39 (d, 4H), 7.38 (s, 2H), 2.69 (t, 4H), 1.79 (m, 2H),
1.44−1.30 (br, 16H), 0.90 (t, 12H), 0.35 (t, 18H). 13C NMR (CDCl3,
100 MHz): d = 142.45, 141.63, 137.10, 130.84, 129.52, 129.19, 128.84,
41.08, 40.19, 32.48, 28.94, 25.78, 23.04, 14.16, 10.93, −8.22 ppm. MS:
792.5 (calculated: 792.2).
Polymerization for PBDTP-DPP. M1 (0.1952 g, 0.2456 mmol)
and compound M4 (0.2360 g, 0.2456 mmol) were dissolved into 10
mL of toluene and 1 mL of DMF in a flask protected by argon. The
solution was flushed with argon for 10 min, and then 10 mg of
Pd(PPh3)4 was added into the flask. The solution was flushed with
argon again for 20 min. The oil bath was heated to 110 °C gradually,
and the reaction mixture was stirred for 10 h at 110 °C under argon
atmosphere. Next, the mixture was cooled to room temperature, and
the polymer was precipitated by addition of 100 mL of methanol and
the precipitated solid was collected and purified by Soxhlet extraction.
The title polymer was obtained as a dark green-purple solid, yield 30%.
The polymer can be readily dissolved into THF, chloroform,
chlorobenzene, or dichlorobenzene, etc. The polymer was thermally
2,5-Dibutyloctyl-3,6-dithiophen-2-ylpyrrolo[3,4-c]pyrrole-
1,4-dione (2). Compound 1 (13.0 g, 43.3 mmol) and anhydrous
potassium carbonate (24 g, 173 mmol) were dissolved into N,N-
dimethylformamide (250 mL) in a two-neck round flask and heated to
145 °C under argon protection. 2-Butyloctyl bromide (49.8 g, 200
mmol) was injected in one portion by syringe. After the reaction was
stirred for 12 h at 145 °C, the solution was cooled to room
temperature, poured into 500 mL of icewater, and then filtered. The
filter cake was washed with water and methanol several times. After
being dried in a vacuum, the crude product was purified by silica gel
chromatography using dichloromethane as eluent to obtain a purple-
black solid powder (17.3 g, yield 76%). 1H NMR (CDCl3, 400 MHz):
8.93 (d, 2H), 7.63 (d, 2H), 7.28 (d, 2H), 4.06 (m, 4H), 1.76 (m, 2H),
1.43−1.25 (m, 32H), 0.87 (m, 12H). MS: 636.2 (calculated: 636.4).
2,5-Dibutyloctyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-
c]-pyrrole-1,4-dione (M1). Compound 2 (5.47 g, 8.62 mmol) and
N-bromosuccinimide (3.14 g, 17.6 mmol) were dissolved into
chloroform (100 mL) in a two-neck round-bottom flask under
argon protection, and then the solution was protected from light and
stirred at room temperature. After 40 h, the mixture was poured into
400 mL of methanol and then filtered. The filter cake was washed with
hot methanol twice. After drying in a vacuum, the pure product was
obtained as a purple-black solid (5.21 g, yield 76%). 1H NMR (CDCl3,
400 MHz): d = 8.61 (d, 2H), 7.22 (d, 2H), 3.92 (d, 4H), 1.88 (m,
2H), 1.49−1.21 (br, 32H), 0.85 (t, 12H). 13C NMR (CDCl3, 100
MHz): d = 161.38, 139.38, 135.29, 131.42, 131.16, 118.98, 108.00,
46.31, 37.72, 31.76, 31.15, 30.86, 29.65, 28.38, 26.14, 23.03, 22.64,
14.09, 14.02 ppm. MS: 792.5 (calculated: 792.2).
1
stable up to 290 °C (3% weight loss by TGA). H NMR (400 MHz,
CDCl3): d = 6.7−8.6 (br, 10H), 1.8−4.9 (br, 14H), 0.6−1.5 (br, 78H).
Mn = 40.7 k; polydispersity = 2.2.
Polymerization for PBDTP-DPP. PBDTP-DPP was prepared
using the same procedure as PBDTT-DPP. The polymer was
1
thermally stable up to 300 °C (3% weight loss by TGA). H NMR
(400 MHz, CDCl3): d = 6.5−8.6 (br, 14H), 1.8−4.5 (br, 14H), 0.6−
1.5 (br, 78H). Mn = 36.3 k; polydispersity = 1.9.
Polymerization for PBDTT-FDPP. PBDTT-FDPP was prepared
using the same procedure as PBDTT-DPP. The polymer was
1
thermally stable up to 275 °C (3% weight loss by TGA). H NMR
(400 MHz, CDCl3): d = 6.4−8.6 (br, 10H), 1.8−4.2 (br, 14H), 0.6−
1.5 (br, 62H). Mn = 28.8 k; polydispersity = 2.2.
Polymerization for PBDTP-FDPP. PBDTP-FDPP was prepared
using the same procedure as PBDTT-DPP. The polymer was
1
thermally stable up to 285 °C (3% weight loss by TGA). H NMR
(400 MHz, CDCl3): d = 6.7−8.8 (br, 14H), 1.7−4.8 (br, 14H), 0.6−
1.6 (br, 62H). Mn = 20.9 k; polydispersity = 2.0.
1
Characterization. H and 13C NMR spectra were measured on a
4,8-Bis(4-ethylhexyl-1-phenyl)-benzo[1,2-b:4,5-b′]-
dithiophene (4). Under protection of argon, 1-bromo-4-(2-
ethylhexyl)benzene (4.96 g, 18.4 mmol) was added slowly to
magnesium turnings (0.538 g, 22.1 mmol) in anhydrous THF (22
mL) with a catalytic amount of I2 (∼10 mg). The mixture was
maintained at reflux until the color of the solution became dark and
the magnesium began to be consumed (1−5 h). Reflux was continued
one more hour, and then the solution was cooled to room
temperature. The solution was added slowly to 4,8-dehydrobenzo-
[l,2-b:4,5-b′]dithiophene-4,8-dione (1.35 g, 6.15 mmol) suspended in
10 mL of THF at room temperature. The temperature was maintained
at 50 °C for 1 h. The diketone dissolved to form a brown solution.
SnCl2 (8.9 g) dissolved in 10% aqueous HCl (12.4 mL) was added
dropwise, and the mixture was stirred one more hour at 50 °C. The
reaction mixture was poured into water (100 mL) and extracted with
ether (100 mL), washed three times with water (100 mL), and then 50
mL of saturated NaHCO3. The residue of the organic phase after
evaporation was purified by chromatography on silica gel with pure
hexane to give the title compound as a yellow-green oil (1.63 g, 47%
yield), which crystallized upon standing. 1H NMR (CDCl3, 400
MHz): d = 7.81 (d, 4H), 7.64 (d, 4H), 7.36 (m, 4H), 2.76 (t, 4H),
1.76 (m, 2H), 1.49−1.21 (br, 16H), 0.88 (t, 12H). MS: 567.0
(calculated: 566.3).
Bruker ARX-400 spectrometer. Absorption spectra were taken on a
Varian Cary 50 ultraviolet−visible spectrometer. The molecular weight
of the polymers was measured by the GPC method, and polystyrene
was used as a standard and chloroform was used as eluent. TGA
measurement was performed on a Perkin-Elmer TGA-7. The
electrochemical cyclic voltammetry (CV) was conducted with Pt
disk, Pt plate, and Ag/AgCl electrode as working electrode, counter
electrode, and reference electrode, respectively, in a 0.1 mol/L
tetrabutylammonium hexafluorophosphate (Bu4NPF6) acetonitrile
solution. The polymer films for electrochemical measurements were
coated from a polymer chloroform solution, ca. 5 mg/mL. For
calibration, the redox potential of ferrocene/ferrocenium (Fc/Fc+) was
measured under the same conditions, and it is located at 0.39 V vs the
Ag/AgCl electrode. It is assumed that the redox potential of Fc/Fc+
has an absolute energy level of −4.80 eV to vacuum. The energy levels
of the highest (HOMO) and lowest unoccupied molecular orbital
(LUMO) were then calculated according to the following equations:
EHOMO = −(φ + 4.41)(eV), ELUMO = −(φ + 4.41)(eV)
ox
re
where φox is the onset oxidation potential vs Ag/AgCl and φre is the
onset reduction potential vs Ag/AgCl.
Device Fabrication. Regular Structure Single Cell. PBDTT-
DPP, PBDTT-FDPP, or PBDTP-FDPP was codissolved with PC71BM
in 1,2-dichlorobenzene (DCB) with a weight ratio of 1:2 with a
concentration of 8 mg/mL. PBDTP-DPP was codissolved with
PC71BM in chloroform with a weight ratio of 1:2 and a concentration
of 5 mg/mL. Mixed solvents with about 1−4% (volume) 1,8-
diiodooctance were used to further improve the device performances.
ITO-coated glass substrates (15 Ω/cm2) were cleaned stepwise in
detergent, water, acetone, and isopropyl alcohol under ultrasonication
for 15 min each and subsequently dried in an oven for 5 h. A thin layer
(∼30 nm) of PEDOT:PSS (Baytron P VP A1 4083) was spin-coated
onto the ITO surface, which was pretreated by ultraviolet ozone for 15
2,6-Bis(trimethyltin)-4,8-bis(4-ethylhexyl-1-phenyl)-benzo-
[1,2-b:4,5-b′]dithiophene (M3). Under protection of argon at room
temperature, 1.6 M n-butyllithium solution in hexanes (4.23 mL, 6.77
mmol) was added dropwise to 4,8-bis-(4-(2-ethylhexyl)phenyl)benzo-
[1,2-b:4,5-b′]dithiophene (1.62 g, 2.82 mmol) dissolved in 30 mL of
THF. The solution was warmed to 40 °C for 1 h, and then 1.0 M
trimethyltin chloride solution in THF (7.34 mL, 7.34 mmol) was
added. The reaction mixture was poured into water (100 mL) and
extracted with ether (50 mL). The ether phase was evaporated, and
the residue was recrystallized from 20 mL of acetone to afford the title
1
compound in 65% yield (1.54 g). H NMR (CDCl3, 400 MHz): d =
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min. Low-conductivity PEDOT:PSS was chosen to minimize measure-
ment error from device area due to lateral conductivity of
PEDOT:PSS. After being baked at 120 °C for ∼20 min, the substrates
were transferred into a nitrogen-filled glovebox (<0.1 ppm O2 and
H2O). A polymer/PC71BM composite layer (ca. 100 nm thick) was
then spin-cast from the blend solutions at 2500 rpm on the ITO/
PEDOT:PSS substrate without further special treatments. The film
was then transferred into a thermal evaporator, which is located in the
same glovebox. A Ca layer (20 nm) and an Al layer (100 nm) were
deposited in sequence under a vacuum of 2 × 10−6 Torr. The effective
area of the device was measured to be 0.10 cm2.
Inverted Tandem Cells. The device architecture of the tandem
solar cell is shown in Figure S5. The precleaned ITO substrates were
treated with UV-ozone. The P3HT:ICBA38 at a 1:1 weight ratio in
1.8% DCB solution was spin-casted at 800 rpm for 30 s on top of a
layer of ZnO (the synthesis of ZnO nanoparticles can be found in ref
39). The films were annealed at 150 °C for 10 min. PEDOT:PSS was
spin-coated on the first active layer and annealed at 150 °C for 10 min.
After that, a thin layer of ZnO film was spin-cast, followed by thermal
annealing at 150 °C for 10 min. Polymer:PC71BM (1:2) from 8 mg/
mL DCB solution was then spin-coated without any subsequent
processing. The device fabrication was completed by thermal
evaporation of 15 nm MoO3 and 100 nm Ag as the anode under
vacuum at a base pressure of 2 × 10−6 Torr. The effective area of the
device was measured to be 0.10 cm2.
cells, (1) lowering the bandgap for spectral matching with the
front cell, (2) controlling HOMO/LUMO levels to enhance
V
OC, and (3) increasing the molecular weight to enhance JSC
and FF, high photovoltaic performance can be achieved.37 The
encouraging results of PBDTT-DPP led us to choose its
polymer backbone as the structural platform to investigate
structure/property relationships of new polymers. Following
the three guidelines, a series of LBG polymers were designed
and synthesized. The synthesis of BDTT, DPP, and FDPP was
performed according to previously reported methods.35,36,41
The new monomer BDTP was synthesized via a Grignard
reaction as shown in Scheme 1. 4-Alkyl bromobenzene was
treated with magnesium in tetrahydrofuran (THF) under argon
protection to form the Grignard reagent, and then the reagent
was transferred to another flask containing benzodithiophene-
dione dispersed in THF under argon protection. The
nucleophilic reaction completed upon heating to 50 °C for
several hours. After reduction by SnCl2, 4,8-bis(4-ethylhexyl-1-
phenyl)-benzo[1,2-b:4,5-b′]dithiophene was obtained as a
yellow-green oil. It is noteworthy that the butyl-lithium
method41 used for the synthesis of BDTT does not work for
BDTP, probably due to the poor stability of the alkylphenyl
anion upon heating. We believe that the successful synthetic
route of the BDTP unit reported here is of great importance for
the OPV community because it provides a promising new
family of weak electron-donating units with large π-conjugated
systems and good planarity.
Hole Mobility. Hole mobility was measured using the space charge
limited current model (SCLC),40 using a diode configuration of ITO/
PEDOT:PSS/polymer:PC71BM/Au and taking current−voltage meas-
urements in the range of 0−6 V and fitting the results to a space
charge limited form, where the SCLC is described by:
It is important to note that the alkyl side chain of the
polymers plays a critical role in determining the solar cell
performance.42 In this work, PBDTT-DPP with different alkyl
chains was synthesized to find the optimum side chain length.
Both n-octyl and 2-ethylhexyl chains were used on the BDTT
unit, and 2-ethylhexyl, 2-butyloctyl, and 2-hexyldecyl chains
were grafted on the DPP unit. Among the six combinations, it is
interesting that only the one shown in Figure 1 performs well.
Shorter alkyl chains led to very poor solubility (cannot be used
for solution processing), and longer alkyl chains led to very
good solubility but low JSC in single junction devices (4−6 mA/
cm2). The preliminary photovoltaic performance data of two
representative polymers with 2-hexyldecyl chains on DPP unit
are shown in the Supporting Information, Figure S1. Thus, the
combination of 2-ethylhexyl on BDTT and 2-butyloctyl on
DPP was chosen for all other polymers reported here except for
FDPP, which showed better performance using shorter alkyl
chains as reported.35 On the basis of the optimized side chain,
monomers M1, M2, M3, and M4 were synthesized as shown in
Scheme 1. PBDTT-DPP, PBDTP-DPP, PBDTT-FDPP, and
PBDTP-FDPP were synthesized via a Stille-coupling reaction.
J = (8/9)εrε0μ(V2/L3)
e
where ε0 is the permittivity of free space, εr is the dielectric constant of
the polymer, μ is the hole mobility, V is the voltage drop across the
device (V = Vappl − Vr − Vbi, where Vappl is the applied voltage to the
device, Vr is the voltage drop due to contact resistance and series
resistance across the electrodes, and Vbi is the built-in voltage due to
the difference in work function of the two electrodes), and L is the
polymer thickness. The dielectric constant εr is assumed to be 3, which
is a typical value for conjugated polymers. The thickness of the
polymer films is measured by using a Dektak profilometer.
Current−Voltage Measurement. The fabricated device was
encapsulated in a nitrogen-filled glovebox by UV cured epoxy and a
cover glass. The current density−voltage (J−V) curves were measured
using a Keithley 2400 source-measurement unit. The photocurrent
was measured under AM 1.5 G illumination at 100 mW/cm2 under a
Newport Thermal Oriel 91192 1000W solar simulator. The light
intensity was determined by a KG-5 filter diode (traceable to NREL
calibration) as a reference cell, followed by the calculation of spectral
mismatch factor and then short circuit current (JSC) correction.
External quantum efficiencies were measured using a lock-in amplifier
(SR830, Stanford Research Systems) with current preamplifier
(SR570, Stanford Research Systems) under short-circuit conditions.
The devices were illuminated by monochromatic light from a xenon
lamp passing through a monochromator (SpectraPro-2150i, Acton
Research Corp.) with a typical intensity of 10 μW. The photocurrent
signal is then amplified by an SR570 and detected with an SR830. A
calibrated monosilicon diode with known spectral response is used as a
reference.
1
The structures of polymers were characterized with HNMR
spectroscopy; all were consistent with the proposed ones. Gel
permeation chromatography (GPC) studies showed that these
polymers have similar molecular weights (Mn) between 20.9k
and 40.7k with a relatively narrow polydispersity index (PDI)
around 2 as listed in Table 1. These polymers have good
solubility in chlorinated solvents such as chloroform (CF),
chlorobenzene (CB), and dichlorobenzene (DCB) except that
PBDTP-DPP can only be dissolved in CF beyond 5 mg/mL.
Thermogravimetric analysis (TGA) indicates that the polymers
are stable up to about 270 °C.
AFM Measurement. AFM samples are prepared as follows: Films
of polymer/PC71BM blend are cast from solutions with or without 1,8-
diiodooctane treatment (ratio following best device conditions),
respectively, on clean glass slides. Both pristine and treated films are
examined by an Automated Nanite A atomic force microscope. Figure
5 shows the tapping mode phase images of these blend films.
Optical and Electrochemical Properties. The absorption
spectra of the polymer films are showed in Figure 2a, and
characteristics of the polymers absorption are summarized in
Table 1. The polymers have similar optical bandgap around
3. RESULTS AND DISCUSSION
Material Design and Synthesis. As we demonstrated, by
using three design principles for LBG polymers for tandem
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mobilities of 3.1 1.0 × 10−4, 2.2 1.0 × 10−4, 8.8 1.0 ×
10−4, and 4.5 1.0 × 10−4 cm2/V·s were found for PBDTT-
DPP, PBDTP-DPP, PBDTT-FDPP, and PBDTP-FDPP,
respectively. It is interesting to note that polymers containing
the FDPP unit showed higher hole mobility (8.8 1.0 × 10−4
and 4.5 1.0 × 10−4 cm2/V·s) than polymers with the DPP
unit (3.1 1.0 × 10−4 and 2.2 1.0 × 10−4 cm2/V·s). This
may be attributed to weaker steric hindrance of the shorter side
chain on the FDPP unit (2-ethylhexyl) than that on the DPP
unit (2-butyloctyl), which led to better π−π interaction
between polymer backbones. Also, it was found that polymers
containing the BDTT unit showed higher hole mobility than
polymers with BDTP unit ((8.8 1.0) × 10−4 vs (4.5 1.0) ×
10−4 cm2/V·s for FDPP-based polymers and (3.1 1.0) × 10−4
Table 1. Molecular Weights and Absorption Properties of
the Polymers
polymers
Mn (kDa) PDI λpeak (nm) λonset (nm) Egopt (eV)
PBDTT-DPP
PBDTP-DPP
PBDTT-FDPP
PBDTP-FDPP
40.7
36.3
28.8
20.9
2.2
1.9
2.2
2.0
710/769
687/753
681/757
676/752
858
849
800
816
1.44
1.46
1.55
1.52
1.4−1.5 eV. Because of the weaker electron-donating property
of the phenyl group as compared to thienyl group on BDT,
PBDTP-DPP and PBDTP-FDPP show slightly higher bandgap
than BDTT-based polymers. All of these polymers show
absorption ranges from 600 to 800 nm. Among them, PBDTP-
DPP and PBDTP-FDPP showed stronger absorption from 600
to 700 nm. Because they all showed very similar absorption in
solution (see the Supporting Information, Figure S2), the
broader absorption of PBDTP-DPP and PBDTP-FDPP in the
solid state was possibly caused by stronger interchain
aggregation. As compared to the absorption spectrum of
poly(3-hexylthiophene) (P3HT, Eg ∼1.9 eV, Figure 2a), which
is the most commonly used front-cell material, the overlap of
the spectra of the LBG polymers and P3HT is small, and these
materials cover the solar spectrum from 350 to 850 nm
complementarily, indicating a good match for the tandem
structure. The HOMO and LUMO energy levels of the
polymers were determined by cyclic voltammetry (CV), and
the results are summarized in Figure 2b (the original data can
be found in the Supporting Information, Figure S3). Because of
the higher ionization potential of benzene and furan, the
electron densities are expected to be lower for these moieties
than thiophene.33 Thus, by replacing the thiophene moiety with
benzene or furan moieties, the HOMO values of the resulting
polymers are expected to be lower, which will lead to higher
VOC in photovoltaic devices. The CV results reveal that both
PBDTP-DPP and PBDTP-FDPP have lower HOMO levels
and larger electrochemical bandgaps than PBDTT-DPP, but
PBDTT-FDPP has a slightly higher HOMO level and smaller
electrochemical bandgap.
vs (2.2
1.0) × 10−4 cm2/V·s for DPP-based polymers),
indicating the thienyl-based BDTT unit is possibly a better hole
transporting moiety than the phenyl-based BDTP unit.
Regular Single Layer BHJ Solar Cell Performance.
Photovoltaic properties of the polymers were investigated first
in single junction solar cells with the regular structure of ITO/
PEDOT:PSS/polymer:PC71BM/Ca/Al. The polymer active
layers were spin-coated from a DCB solution except for
PBDTP-DPP, which was coated from CF. The ratio of polymer
to PC71BM was adjusted from 1:1 to 1:3 (by weight), and the
optimized condition was 1:2 for all of them. To make sure the
optimized morphology was being achieved for the blends, we
studied the effect of adding 1,8-diiodooctane (DIO) as an
additive in detail.6 Solutions with DIO concentrations varying
from 0% to 4% (by volume) were prepared and used to coat
the active layer. In most of the high performance LBG polymer
systems reported so far, adding a certain amount of DIO can
facilitate the formation of nanoscale phase separation of
polymer:PCBM blends during spin-coating and thus enhance
the PCE greatly. For example, PCPDTBT6 showed 5.5% PCE
versus 2.8% upon adding 2% DIO, PTB47 showed 6.1% PCE
versus 3.1% upon adding 3% DIO, and PDPPTPT34 showed
5.5% PCE versus 2.0% upon adding 3% DIO. Interestingly, we
found that the four polymers in this work, which are similar in
structure, showed very different responses to the addition of
DIO in the solution, and one of them gave the best
performance without adding any DIO.
Hole Mobility. Hole mobility of the polymer:PC71BM
blends was measured using the space charge limited current
(SCLC) model, and the J−V characteristics are plotted in
Figure S4 (see the Supporting Information). The hole
Figure 3 shows the VOC, JSC, FF, and PCE results as a
function of DIO concentration for the single junction devices
Figure 2. (a) UV−vis absorption spectra of the polymer films. (b) HOMO and LUMO energy levels of the polymers. Energy levels of PC71BM are
listed for comparison.
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Figure 3. Solvent additive (DIO) concentration dependence of regular single layer solar cell performance of PBDTT-DPP, PBDTP-DPP, PBDTT-
FDPP, and PBDTP-FDPP: (a) VOC, (b) JSC, (c) FF, and (d) PCE.
fabricated from the new polymers. For VOC (Figure 3a),
PBDTP-DPP, PBDTT-FDPP, and PBDTP-FDPP showed 0.05
V higher values than PBDTT-DPP at 0% DIO condition,
indicating our design was successful in lowering the HOMO
level to enhance the VOC of the devices by introducing benzene
and furan moieties into the polymers. As the DIO
Table 2. Characteristic Properties of Regular Single Layer
BHJ Solar Cells
VOC
(V)
JSC
FF
PCEmax
/
ave
polymers
solvent D:A
(mA/cm2) (%)
(%)
PBDTT-
DPP
DCB
1:2
1:2
1:2
1:2
0.73
0.76
0.77
0.77
14.0
13.6
13.8
65
60
55
59
6.6/6.5
6.2/6.0
5.8/5.5
3.3/3.0
a
concentration increases, all of them show slightly lower VOC
.
PBDTP-
DPP
CF
VOC’s (0.7−0.8 V) for these polymers were notably higher than
b
c
PBDTT-
FDPP
DCB
DCB
those for previously reported LBG materials due to their deeper
HOMO levels (a detailed comparison correlating HOMO
levels and VOC’s can be found in the Supporting Information,
Table S1). For JSC (Figure 3b), without adding DIO, PBDTP-
DPP and PBDTP-FDPP showed very low JSC’s, probably due to
the poor morphology of the blends; upon adding DIO, all of
them except PBDTT-DPP showed enhanced JSC. Devices made
from PBDTP-DPP and PBDTT-FDPP can reach around 14
mA/cm2 at 1% and 3% DIO, respectively. The low JSC obtained
for PBDTP-FDPP may be due to the morphology not being
fully optimized through adding DIO. The fill factors of different
materials (Figure 3c) showed different responses to DIO. As
the DIO concentration increased, the FF of PBDTT-DPP
decreased slightly (from 65% to 62%) and PBDTP-DPP
increased slightly (from 57% to 60%); FF of PBDTT-FDPP
decreased to 48% at 1% DIO and then went to a peak value of
55% at 3% DIO; the FF of PBDTP-FDPP increased
dramatically from 28% to 59% upon adding 1% DIO and
decreased slightly at higher DIO concentration. Thus, as shown
in Figure 3d, the PCE of PBDTT-DPP showed its highest value
of 6.6% at 0% DIO and decreased slightly at higher DIO
concentrations; both PBDTP-DPP and PBDTP-FDPP showed
their maximum values of 6.2% and 3.3% at 1% DIO, while
PBDTT-FDPP showed the highest value of 5.8% at 3% DIO. It
is interesting to note that the highest PCE of 6.6% for a single
junction device was achieved without using DIO during the
processing, which is a very unique property of this polymer.
The characteristic properties of regular single layer BHJ solar
cells are summarized in Table 2, and the J−V characteristics
and EQEs for the best devices are shown in Figure 4. Among
the four polymers, three of them gave PCEs around 6% under
the optimized conditions. From EQE results (Figure 4b), broad
response ranges covering 350−850 nm were obtained with
average EQEs of 48% within this region for PBDTT-DPP,
PBDTP-DPP, and PBDTT-FDPP. Besides the photovoltaic
performance, the effects of adding DIO on the optical
properties and the hole mobility of the polymers in the blend
film have been investigated. As shown in Figure S5 (see the
Supporting Information), because the UV−vis absorption
spectra of the films only showed a very slight red-shift, and
the SCLC mobility of the polymers was not increased
dramatically after adding DIO as additive (Supporting
PBDTP-
FDPP
7.42
a
b
1% DIO was added to the solution. 3% DIO was added to the
c
solution. 1% DIO was added to the solution.
Information, Table S2), the significant enhancement of the
photovoltaic performance is most likely due to better film
morphology.
Morphology Studies. Polymer:PC71BM blend film
morphology was investigated using atomic force microscopy
(AFM). AFM phase images of films cast without DIO and with
optimized DIO concentration are shown in Figure 5. For
PBDTT-DPP (Figure 5a and b) and PBDTT-FDPP (Figure 5e
and f), film morphology was almost identical, and the
photovoltaic performance of the devices fabricated with and
without DIO was similar. This is consistent with the measured
J−V characteristics as shown in Figure 3. However, for PBDTP-
DPP (Figure 5c and d) and PBDTP-FDPP (Figure 5g and h),
dramatic changes in the film morphology were observed.
Without DIO, high surface roughness and very weak phase
separation were obtained for PBDTP-DPP:PC71BM (Figure
5c) and PBDTP-FDPP:PC71BM (Figure 5g) blend films. The
nonoptimized morphology was probably due to fast drying of
the solvent (CF) for PBDTP-DPP:PC71BM and extraordinarily
good miscibility between polymers and PC71BM. Nanoscale
phase separation was achieved by adding 1% DIO as cosolvent
for PBDTP-DPP:PC71BM (Figure 5d) and PBDTP-
FDPP:PC71BM (Figure 5h) blends, respectively. Because of
the more optimized morphology, both of the polymers showed
significant enhancement of JSC and FF, which lead to an
increase in PCE from 1.5% to 6.2% for PBDTP-DPP and 0.4%
to 3.3% for PBDTP-FDPP. Although DIO had a great effect on
film morphology for PBDTP-FDPP-based devices, the
maximum PCE was still not as high as other polymers. This
can be attributed to the nonoptimum morphology of PBDTP-
FDPP-based devices. As compared to Figure 5b, d, f, larger
phase-separated features throughout the PBDTP-
FDPP:PC71BM film were observed in Figure 5h.
Inverted Tandem Solar Cell Performance. Tandem
polymer solar cells with inverted configuration24,44 were
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Figure 4. (a) Current−voltage characteristics of polymer/PC71BM solar cells under AM 1.5 condition (100 mW/cm2). (b) EQEs of the
corresponding devices.
Figure 5. AFM phase images of polymer:PC71BM blend films: (a) PBDTT-DPP:PC71BM without DIO, (b) PBDTT-DPP:PC71BM with 2% DIO,
(c) PBDTP-DPP:PC71BM without DIO, (d) PBDTP-DPP:PC71BM with 1% DIO, (e) PBDTT-FDPP:PC71BM without DIO, (f) PBDTT-
FDPP:PC71BM with 3% DIO, (g) PBDTP-FDPP:PC71BM without DIO, and (h) PBDTP-FDPP:PC71BM with 1% DIO.
fabricated using the newly designed LBG polymers as the rear
cell donor materials (PBDTP-FDPP was not tested in tandem
devices due to the poor performance in single junction
devices). The device structure is ITO/ZnO(30 nm)/
P3HT:IC60BA(150 nm)/PEDOT:PSS(30 nm)/ZnO (30
nm)/LBG polymer:PC71BM (100 nm)/MoO3 (5 nm)/Ag as
shown in Figure S6 (see the Supporting Information).37 It is
worthy of noting that the HOMO levels of around −5.3 eV for
these LBG polymers not only help to increase Voc, but also
could help to improve charge injection at interfaces between
MoO3 and low bandgap polymers.45 The J−V characteristics of
the tandem cells and the performance parameters are shown in
Figure 6. The best devices based on PBDTT-DPP, PBDTP-
DPP, and PBDTT-FDPP showed PCEs as high as 8.8%, 8.5%,
and 8.3%, respectively. All of the devices showed VOC of ∼1.6
V, which is equal to the sum of the single front and rear cells;
high JSC of ∼8.5 mA/cm2 and FF over 60% were achieved,
Figure 6. J−V characterstics of the tandem solar cells under AM1.5G
illumination condition (100 mW/cm2).
leading to high PCEs for the tandem devices. As compared to
PBDTT-DPP-, PBDTP-DPP- and PBDTT-FDPP-based devi-
ces showed slightly higher VOC but lower FF, which are in
be the average of the two subcells.43 Here, we have a P3HT-
accordance with the results of single junction devices. It should
based front cell with JSC around 9 mA/cm2 and FF around 68%
be noted that the difference in FF was not as big as single
junction devices (65%, 60%, and 55% for single cells and 66%,
64%, and 61% for tandem cells) because the FF of a tandem
device is determined by the two subcells. When the
photocurrents from both cells are almost equal, the FF will
(see the Supporting Information, Figure S7), and thus the
tandem devices would be expected to show FF around 66%,
64%, and 61% for PBDTT-DPP-, PBDTP-DPP-, and PBDTT-
FDPP-based devices.
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The higher PCE achieved by using these three polymers as
compared to other polymer systems22−26 in tandem cells can be
attributed to the simultaneous enhancement of VOC, JSC, and
FF. Although pBBTDPP2-based tandem cells showed VOC as
high as 1.58 V, their JSC and FF were much lower; whereas
PCPDTBT-based tandem cells showed a high FF of 67% but a
low VOC of 1.2 V and JSC of 7.6 mA/cm2. Here, the VOC of
around 1.6 V and FF around 65% we obtained are among the
highest values. More importantly, the high JSC values of around
8.5 mA/cm2 obtained here are more than 10% higher than the
best reported data for tandem polymer solar cells (7.6 mA/
cm2) using other LBG polymers.25,26 This can be attributed to
the ability of PBDTT-DPP, PBDTP-DPP, and PBDTT-FDPP
to utilize the low-energy portion (600−800 nm) of the solar
irradiation very efficiently and enable balanced photocurrent
from front and rear cells (external quantum efficiency of the
tandem devices can be found in the Supporting Information,
Figure S8, and the measurement details can be found in ref 37).
It should be noted that during the peer review process of this
Article, another high performance DPP derivative (PDPP5T)
and its application in tandem PSCs with 7% PCE was reported
by Janssen et al.30 The drawback of the lower VOC of the
PDPP5T-based rear cell was compensated by using a
PCDTBT-based front cell with higher VOC than P3HT:ICBA.
The overall PCE of the tandem was mainly limited by lower FF,
probably due to the lower performance of the PCDTBT-based
front cell or the ICL between the two subcells.
Jun Yang, Mr. Xuanrong Guo, and Ms. Rene Green from
UCLA for helpful discussion, material synthesis, and testing.
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In summary, we have designed and synthesized a series of LBG
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AUTHOR INFORMATION
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Corresponding Author
yangy@ucla.edu
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Science
Foundation (NSF), Air Force Office of Scientific Research
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