New iridium-containing conjugated polymers for polymer solar cell applications

Article abstract of DOI:10.1039/C8NJ03410A

A series of novel donor-acceptor (D-A) copolymers P1-P5 with iridium-complexed moieties in their side chains have been synthesized on the basis of a new iridium-containing monomer. The results obtained show that P1-P5 have good thermal stability (317-347 °C) for photovoltaic applications. These copolymers absorb visible light in a broad spectral range up to 680 nm. The optical bandgaps of P1-P5 are in the range of 1.96-2.08 eV, respectively. The HOMO and LUMO energy levels of the polymers P1-P5 estimated from cyclic voltammetry measurements indicate that these copolymers are suitable as electron donors along with PC₇₁BM as an electron acceptor for bulk heterojunction polymer solar cells. BHJ polymer solar cells were developed based on blend compositions (P1-P5):PC₇₁BM. The values of J_sc, V_oc, and FF are in the range of 0.95-4.44 mA cm^?2, 0.67-0.69 V and 34.6-56.8%, and the power conversion efficiencies (PCE) are in the range of 0.22-1.74%, respectively, the highest value of 1.74% being for P3. Increase of the photovoltaic parameters was achieved with increasing iridium complex percentage in the polymers due to involvement of triplet effects. The improvement in the efficiency of the triplet-forming polymers P2 and P3 in comparison with P1 appears to be due to the formation of triplet excitons in comparison with singlet excitons in the polymer P1 which does not contain heavy metals. With further increase in the content of iridium complex fragments in the polymers, for example, up to 3 mol% for polymer P4, the efficiency falls to 1.23% and further decreased to 0.22% for P5.

Full text of DOI:10.1039/C8NJ03410A

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New iridium-containing conjugated polymers
for polymer solar cell applications
Cite this: New J. Chem., 2018,
42, 17296
a
a
a
b
M. L. Keshtov,
S. A. Kuklin,
I. O. Konstantinov,
Fang-Chung Chen,
c
d
Zhi-yuan Xie and Ganesh D. Sharma
*
A series of novel donor–acceptor (D–A) copolymers P1–P5 with iridium-complexed moieties in their
side chains have been synthesized on the basis of a new iridium-containing monomer. The results
obtained show that P1–P5 have good thermal stability (317–347 1C) for photovoltaic applications. These
copolymers absorb visible light in a broad spectral range up to 680 nm. The optical bandgaps of P1–P5
are in the range of 1.96–2.08 eV, respectively. The HOMO and LUMO energy levels of the polymers
P1–P5 estimated from cyclic voltammetry measurements indicate that these copolymers are suitable as
electron donors along with PC71BM as an electron acceptor for bulk heterojunction polymer solar cells.
BHJ polymer solar cells were developed based on blend compositions (P1–P5):PC71BM. The values of
ꢀ2
J
sc, Voc, and FF are in the range of 0.95–4.44 mA cm , 0.67–0.69 V and 34.6–56.8%, and the power
conversion efficiencies (PCE) are in the range of 0.22–1.74%, respectively, the highest value of 1.74%
being for P3. Increase of the photovoltaic parameters was achieved with increasing iridium complex
percentage in the polymers due to involvement of triplet effects. The improvement in the efficiency of
the triplet-forming polymers P2 and P3 in comparison with P1 appears to be due to the formation of tri-
plet excitons in comparison with singlet excitons in the polymer P1 which does not contain heavy
metals. With further increase in the content of iridium complex fragments in the polymers, for example,
up to 3 mol% for polymer P4, the efficiency falls to 1.23% and further decreased to 0.22% for P5.
Received 9th July 2018,
Accepted 20th September 2018
DOI: 10.1039/c8nj03410a
rsc.li/njc
organic photovoltaics are largely based on narrow-bandgap
conjugated polymers, it is known that it is difficult to achieve
Introduction
Over the past few years, encouraging progress has been made in a high efficiency value for practical use on the basis of pure
the field of polymer solar cells (PSCs) based on a bulk hetero- polymers only. One of the reasons limiting the efficiency
1
junction (BHJ) active layer as an inexpensive alternative to of PSCs is the short diffusion length of the singlet exciton,
silicon solar cells due to their unique advantages, such as which is determined by its mobility and lifetime. One possible
low cost, light-weight, manufacturability and flexibility of the approach to increasing the diffusion length of excitons and,
2
devices. In connection with this, great efforts have been made as a consequence, the effectiveness of PSCs is the use of triplet-
to optimize the spectral characteristics of the conjugated polymers forming organo-metallic polymers. The formation of a large
by reducing the optical band gap and optimizing their energy number of charge-transfer triplet states, which have a micro-
ꢀ
6
levels. Although many narrow-bandgap polymers have been second lifetime (B10 s) in comparison with the relatively
ꢀ9
proposed for photovoltaic applications, the power conversion short nanosecond lifetime (B10 s) of the singlet state, is
efficiency (PCE) of single BHJ layer PSCs has now reached about attractive for improving the photovoltaic characteristics of
3
5
1
0–12%, which limits their introduction into industrial produc- PSCs. A difference of three orders of magnitude can be used
tion (it is believed that the efficiency of PSCs should be about 15% to significantly increase the diffusion length of excitons, which
4
6
for commercialization). Although recent advances in the field of is about 5–10 nm in most photovoltaic materials. In connec-
tion with this, interest in the conjugated triplet organo-metallic
a
polymers as donor materials for PSCs has recently grown.
In particular, Holdcroft reported a photovoltaic study in which
Ir complexes are coordinated with conjugated poly(fluorene-co-
Institute of Organoelement Compounds of the Russian Academy of Sciences,
Vavilova St., 28, 119991, Moscow, Russian Federation.
E-mail: keshtov@ineos.ac.ru
b
c
Department of Photonics, National Chiao Tung University, Hsinchu 30010, Taiwan
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, China
7
phenylpyridine), where for the PSCs based on poly(9,9-dioctyl-
fluorene-co-tris(2-phenylpyridine)-iridium)(III) the PCE values
increased 35-fold compared to the device based on poly(9,9-
dihexylfluorene-co-2-phenylpyridine), which was attributed to
d
Department of Physics, The LNM Institute for Information Technology, Jamdoli,
Jaipur, India
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the formation of triplet states in the iridium-containing polymers.
Yang et al. studied mixtures of poly(3-hexylthiophene) (P3HT) with
8
molecular Ir(mppy)3 using CdSe nanoparticles as an acceptor
and demonstrated an increase in the generation of triplet excitons
due to an increase in the degree of singlet–triplet conversion,
while the magnitude of the short circuit current ( Jsc) increased by
a factor of two and the value of open circuit voltage (Voc) by 50%,
after addition of a triplet metal complex in comparison with
a metal-free reference device. In addition, Chen et al. recently
reported PSCs based on conjugated polymers containing indace-
nodithiophene and cyclometallated platinum complexes with a
9
maximum power conversion efficiency (PCE) of 2.9%. Wong et al.
also demonstrated effective platinopolyine/PC BM based solar
71
cells, which achieved the highest efficiency among metal-
1
0
containing conjugated polymers (4–5%). Although platino-
polyines of the type trans-[Pt(PR –CRC–R–CRC–] attracted
considerable attention for use in optoelectronic devices,
Scheme 1 Synthetic route of monomer M1.
3
)
2
n
1
1–14
in
most cases their efficiency was low due to the large bandgap
width (Eg 4 2.5 eV), the possible unfavorable arrangement
of energy levels and low charge-transport properties. In poly-
platinines, where platinum atoms are located in the main
chain, the energy overlap of the 5d Pt and 2p C orbitals leads
to a significant decrease of the conjugation length compared
Reaction of the acid 3 with carbonyldiimidaxole (CDI)
followed by treatment with ammonia solution gave amide 4
with the yield of 95%. The amide 4 was treated with Lowesson
reagent (5) with formation of thioamide 6, which was purified
by means of column chromatography and obtained with a
moderate yield of 57%. Interaction of 6 with an equimolar
amount of chloro-ketone 7 led to thiazole derivative 8, which
was obtained with the yield of 64%. Oxidative cyclization of 8
1
5
to similar polyphenylene ethylene polymers.
Therefore,
organometallic conjugated polymer architectures, other than
polyplatinines, deserve attention as donor materials for a better
understanding of the role of heavy metal atoms in the for-
mation of triplets in conjugated polymers for possible applica-
tion in photovoltaics. In the present article, in contrast to the
reviewed studies, iridium atoms connected directly to the side
chain of macromolecules and do not interfere with delocaliza-
tion of the excitons along the polymer chain, providing an
increase in the diffusion length of excitons, and simultaneously
decreasing the width of the bandgap of the polymer. In this
regard, the design of metal polymers will be aimed at developing
not only phosphorescent triplet polymer materials, but also
phosphorescent triplet polymers with a narrow band gap for
high-performance PSCs. It is expected that this work will stimulate
the further development of new organometallic polymers for
photovoltaic applications and help the achievement of high
efficiency PSCs with bulk heterojunctions.
3
occurs upon treatment with anhydrous FeCl in a dichloro-
methane–nitromethane solvent mixture and gave targeted
monomer M1 with yield of 79% (as shown in Scheme 1).
0
Synthesis of bis(2-(4-fluorophenyl)-4-phenylquinolinato-C2,N )-
[1,3-bis(5-bromothiophen-2-yl)propan-1,3-dionato-O,O]iridium(III)
(M2) was conducted according to Scheme 2; at first, the new
ligand, 1,3-bis(5-bromothiophen-2-yl)propan-1,3-dione (12) and
an intermediate dimer complex (14) were prepared. The dione
ligand 12 was obtained in a three stage synthesis starting
from the oxidation of 2-bromo-5-acetylthiophene (9) by sodium
hypobromide to yield the acid 10. In the second stage the acid
10 was converted to its ethyl ester 11 under the action of
Results and discussion
New fused aromatic monomers, (5,8-dibromo-2-[5-(3-octyltridecyl)-
0
0
thiophene-2-yl]bisthiophen-[2-yl]bisthieno[3,2-e:2 ,3 -g][1.3]-
benzothiazole) (M1) and triplet-forming iridium containing
0
bis((2-4-fluorophenyl)-4-phenylquinolinato-C2,N )[1,3-bis(5-bromo-
thiophen-2-yl)propan-1,3-dionato-O,O]iridium(III) (M2) have been
synthesized. Monomer M1 was synthesized by a three stage
method including metallalization of 5-methylthiophen-
2
carboxylic acid (1) with lithium diisopropyl amide with sub-
sequent alkylation of carbanione with 2-octyldodecylbromide
2) to give 5-(3-octydodecyl)thiophene-2-carboxylic acid (3) with
a yield of 97%.
(
Scheme 2 Synthetic routes of monomer M2.
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dicyclohexylcarbodiimide (DCC) in the presence of dimethyl-
aminopyridine (DMAP). Equimolar amounts of ester 11 and
starting compound 9 react in the condensation reaction in
the presence of an excess of sodium hydride with formation of
1,3-bis(5-bromothiophen-2-yl)propan-1,3-dione (12), which was
purified by column chromatography. Reaction of two-molar
1
6
excess of 2-(4-fluorophenyl)-4-phenylquinoxaline (13)
in
2
-ethoxyethanol with an aqueous solution of IrCl xH O gave
3
2
red-orange intermediate dimer 14 with excellent yield (99%),
which was used in the reaction with the dione ligand 12 for
preparation of target monomer M2 with 87% yield.
The compositions and structures of all the intermediate
compounds 2–14 and the target monomers M1 and M2 were
1
13
confirmed by elemental analysis data and H and C NMR
spectroscopy. In particular, in the low-field region of the
1
H NMR spectrum of the target monomer M1 two singlet
and two doublet signals were observed at d 7.87, 7.40, 7.25
and 6.80 ppm with the intensity ratio of 1 : 1 : 1 : 1, which related
to the protons of thiophene fragments (Fig. 1). In the up-field
region at d 2.87 ppm a triplet resonance appears typical of a
methylene group directly connected to a thiophene ring, and in
the range of d 1.76–0.90 ppm there are signals related to other
protons of the aliphatic chains.
1
1
13
The target iridium complex M2 was characterized by H and Fig. 2 H (a) and C (b) NMR spectra of monomer M2 in CDCl
3
.
1
3
C NMR spectroscopy and elemental analysis, which confirmed
its structure and composition. For example, two doublet and one
singlet resonances d = 7.04, 6.88 and 5.66 ppm with the intensity
ratio of 2 : 2 : 1 related to the diketone ligand can be observed
non-fluorinated fragments of phenylquinoline ligands. The number
13
of signals in the C spectrum and the integral intensity values in
1
1
the H spectrum confirm the structure of complex M2 (Fig. 2).
in the low-field region of the H NMR spectrum. Another nine
Based on the monomers M1, M2 and M3, we synthesized
five new D–A conjugated polymers P1–P5 with different contents
of iridium triplet complexes (0, 0.5, 1.0, 1.5, 3.0 and 100 mol%)
in the chain (Scheme 3). In order to maintain the linear
structure of the polymer backbone and minimize the charge
loss and transport properties of the polymers, the iridium-
containing metal complex was inserted into the polymer back-
bone statistically through the b-diketonate auxiliary ligand.
signals at d = 8.49 (d, 2H), 7.98 (s, 2H), 7.90 (dd, 2H), 7.82 (d, 2H),
3
7.60 (m, 10H), 7.35 (t, 2H), 7.28 (m, 2H + CDCl ), 6.77 (td, 2H),
and 6.32 (dd, 2H) are related to protons of 2-(4-fluoro-
1
3
phenyl)quinoline ligands. The C NMR spectrum contains six
doublet resonances at d = 162.39 (d, J = 253.1 Hz), 151.82 (d, J =
6.04 Hz), 130.73 (d, J = 21.34 Hz), 127.41 (d, J = 9.46 Hz), 122.10
(d, J = 17.32 Hz), and 108.99 (d, J = 23.47 Hz), and their multi-
plicity was caused by spin–spin interaction with a fluorine
atom. Another singlet signal is related to carbon atoms of
1
Fig. 1 H NMR spectrum of 5,8-dibromo-2-[5-(3-octyltridecyl) thiophen-
0
0
2
-yl]bisthieno[3,2-e:2 ,3 -g][1,3]benzothiazole (M1).
Scheme 3 Synthetic routes of copolymers P1–P5.
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Polycondensation was carried out under Stille cross-coupling Table 1 Molecular weights and thermal properties of the polymers P1–P5
conditions in boiling toluene using tetrakis(triphenylphosphine)-
Polymer (Ir, mol%) Yield (%)
M
n
M
w
M
w
/M
n
T5% (1C)
palladium as the catalyst. The resulting polymers were purified
from catalyst residues, and low molecular weight impurities by
reprecipitation from solution twice to methanol and subsequent
extraction with methanol, hexane, acetone and chloroform. The P4
process proceeded under homogeneous conditions and allowed
P1
P2
P3
79
77
72
71
69
50 100 156 200 3.12
48 700 109 400 2.25
54 700 187 500 3.43
53 500 160 500 3.00
72 600 232 300 3.20
317
317
330
336
347
P5
polymers with high molecular weights to be obtained in the form
of a dark purple powder with yields in the range of 69–79%,
respectively.
The temperature values of five percent weight loss (T5%) of
polymers P1–P5 in air, found using TGA, range from 317 to
29 1C, respectively (Table 1). These results show that the
polymers have a sufficiently high thermal stability for photo-
voltaic applications and other optoelectronic devices.
The composition and structure of polymers P1–P5 are
3
1
confirmed by the elemental analysis data and H NMR spectro-
scopy. In particular, there are multiplets in the weak-field
region in the range of d 8.10–6.50 ppm belonging to the
1
aromatic protons of the polymer unit in the H NMR spectrum
Optical properties
of the polymer P1; in the strong-field region in the range of d
4
.30–2.70 ppm there are two multiplets characteristic of two
The optical properties of polymers P1–P5 were investigated
by UV and visible spectroscopy, and the results are given in
Table 2, and the corresponding absorption spectra in chloro-
form and films are shown in Fig. 5(a and b), respectively. As one
can see from the figures, all polymers exhibit two absorption
peaks in solution and films, which are characteristic for donor–
acceptor copolymers. Absorption peaks at short wavelengths of
different CH groups which are directly connected to the
2
thiophene ring and alkoxy group. Signals that belong to the
remaining hydrogen atoms of the alkyl substituents are detected
within the range of d 1.47–0.60 ppm (Fig. 3).
Although the proton spectrum of P2 is complex, the ratio
of the integral intensity of the aromatic part to the aliphatic
part corresponds to the proposed structure. These results,
combined with elemental analysis, show that the reaction
process was successful and complete. The number-average
360–407 nm in films refer to p–p*-transitions of the conjugated
chains, while long-wavelength absorption peaks of 552–570 nm
are attributes of strong intramolecular charge transfer in the
transition state in the donor–acceptor segments, which are the
most important for covering the spectrum of solar radiation.
Polymers P1–P4 show almost identical absorption profiles in
the range of 350–600 nm, since they have very similar composi-
tion with a very low content of iridium containing blocks.
On the other hand, polymer P5 showed a completely different
absorption profile due its high percentage of iridium blocks.
The absorption maxima in solid films were slightly shifted to
the red region in comparison with the solutions and show
bathochromic shifts within 10–19 nm due to aggregation,
strong interchain interactions and the formation of p-stack
molecular weights (M ) and polydispersity (M /M ) of polymers
n
w
n
P1–P5, determined by the GPC method, range from 48 700–
2 600 and 2.10–3.43, respectively. The polymers are soluble
7
in common organic solvents such as chloroform, THF, and
o-dichlorobenzene, which made it possible to obtain films
based on them for electrochemical and photovoltaic studies.
The introduction of low concentrations of Ir into the polymers
showed a small effect on their solubility.
Thermal properties
The thermal properties of the polymers P1–P5 were examined
by means of thermogravimetric analysis (TGA); the results are
listed in Table 1, and the corresponding TGA curves are shown
in Fig. 4. All the polymers exhibit high thermal stability.
structures and better packing of the macromolecules in thin
opt
films. The optical band gap (E
g
) of polymers P1–P5, found
from the absorption edges of the polymer films by the formula
240/ledge vary within 1.96–2.08 eV (Table 2).
1
Fig. 4 TGA plots of the polymers P1–P5 in air at a scanning rate of
1
ꢀ1
Fig. 3 H NMR spectrum of polymer P1 in CDCI
3
.
501 min
.
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Table 2 Optical and electrochemical properties of the polymers P1–P5
opt
Polymer
l
max(solution)
l
max(film)
HOMO LUMO
E
g
(Ir, mol%) (nm)
(nm)
(eV)
(eV)
(eV)
P1
P2
P3
P4
P5
377, 490, 539 390, 500, 552 ꢀ5.21
377, 490, 539 390, 500, 552 ꢀ5.21
384, 500, 532 407, 513, 558 ꢀ5.18
390, 508, 543 401, 501, 555 ꢀ5.17
ꢀ2.93
ꢀ3.00
ꢀ2.99
ꢀ2.92
ꢀ3.08
2.08
2.07
2.04
2.04
1.96
350, 560
360, 570
ꢀ5.42
Fig. 6 Cyclic voltammograms of copolymers P1–P5.
ons
LUMO = ꢀ(Ered + 4.84) (eV),
ec
g
ons
ons
E
= (Eox ꢀ Ered) (eV).
The HOMO and LUMO energies of the polymers P1–P5
found onset oxidation and reduction potentials, respectively
observed in the voltammograms are ꢀ5.21, ꢀ5.21, ꢀ5.18,
5.17, 5.42 eV, and ꢀ2.93, ꢀ3.00, ꢀ2.99, ꢀ2.92 and ꢀ3.08 eV,
respectively. Polymers P1–P4 show fairly close HOMO and
LUMO levels and a small difference in the energy levels of the
HOMO and LUMO suggests a slight effect of the inclusion of
iridium complexes. All the polymers exhibit low-lying HOMO
levels within (ꢀ5.42)–(ꢀ5.18 eV), which favors a high open
circuit voltage, since usually Voc is proportional to the difference
between the HOMO level of the polymer (donor) and the LUMO
level of fullerene (acceptor). The difference between the LUMO of
the polymers and the LUMO of PC BM (ꢀ4.2 eV) varies between
71
1.12 and 1.27 eV. These energy offset values are greater than 0.3 eV,
which is the generally accepted minimum driving force for
Fig. 5 Normalized absorption spectra of the polymers P1–P5 in (a) efficient separation of the excitons into free charge carriers.
chloroform solution and (b) thin films.
Photovoltaic properties
Electrochemical properties
In order to evaluate the influence of triple-forming iridium
complexes on the photovoltaic characteristics of PSCs, bulk
heterojunction polymer solar cells with a structure of ITO/
PEDOT-PSS/copolymer:PC BM/Ca/Al were created on the basis
of the polymers P1–P5. The main parameters of the PSCs are
given in Table 3, and the corresponding J–V characteristics are
The electrochemical properties of the polymers P1–P5 were
investigated by cyclic voltammetry; the results are given in
Table 2, and the corresponding cyclic voltammograms are
presented in Fig. 6. Cyclic voltammetry of the polymer films
7
1
4 4
was carried out in acetonitrile with 0.1 M Bu NBF at a scan rate
ꢀ
1
shown in Fig. 7. The short circuit current density ( J ), the open
sc
of 50 mV s . The platinum electrode was used as a counter
+
circuit voltage (V ) and the filling factor (FF) of the PSCs based
oc
electrode, Ag/Ag in a 0.1 M solution of silver nitrate was used
ꢀ2
+
on P1–P5 vary within the ranges of 0.95–4.44 mA cm
;
as a reference electrode, and Fc/Fc was used as an external
standard. All the polymers exhibit reversible oxidation–
0.67–0.69 V and 34.6–56.8%, and the efficiency (PCE) in the
onset
ox
range 0.22–1.74%, respectively (Table 3). The value of Voc for all
the devices is almost the same due to the similar values of
HOMO energy levels of the copolymers. However, the PCEs
of the PSCs are different, mainly due to the different values of
Jsc and FF. With an increase of the triplet iridium complex
concentration from 0 to 1.5%, the photovoltaic performance
increases due to the participation of the triplet effect. The device
reduction properties. From the onset oxidation (E
) and
onset
reduction (Ered ) potentials of the polymers, the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) energies and bandgap values of the polymers
ec
g
(E
) were calculated according to equations
ons
HOMO = ꢀ(Eox + 4.84) (eV),
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Table 3 Photovoltaic performance of PSCs with the polymers P1–P5
ꢀ2
Polymer (Ir, mol%) (Voc) (V) ( Jsc) (mA cm
) (FF) (%) PCE (%)
P1 (0.0)
P2 (1.0)
P3 (1.5)
P4 (3.0)
P5 (100)
0.67
0.69
0.69
0.67
0.67
3.86
4.00
4.44
3.60
0.95
51.8
52.9
56.8
51.0
34.6
1.34 (ꢁ0.02)
1.46 (ꢁ0.02)
1.74 (ꢁ0.03)
1.23 (ꢁ0.03)
0.22 (ꢁ0.01)
Fig. 8 EQE spectra of the PSCs under illumination based on polymers
P1–P5:PC71BM thin films.
quantum efficiency (EQE) spectra of the devices are shown in
Fig. 8. It can be seen from this figure that the EQE spectra
consist of two bands i.e. a shorter wavelength region peak
around 384 nm, which corresponds to the absorption of
PC71BM, and another band in the wavelength region (480–
7
00 nm), which corresponds to the absorption band of the
Fig. 7 J–V plots of the PSCs under illumination based on polymers
P1–P5:PC71BM thin films.
polymers. This indicates that both PC71BM and polymers
contribute to the exciton generation and thereby photocurrent
generation. Moreover, the EQE values are well consistent with
based on the polymer P3 shows the greatest efficiency equal to the Jsc values of the corresponding PSCs. Further improvement
.74%. Apparently, this is due to higher values of Jsc and FF of this in efficiency is expected when optimizing the content of iridium
1
device. The improvement in the efficiency of the triplet-forming in these systems and selecting D–A conjugated polymers with a
polymers P2 and P3 in comparison with the polymer P1 is wider absorption spectrum covering the entire spectrum of solar
apparently due to the formation of triplet excitons in comparison radiation.
with singlet excitons in the polymer P1 containing no heavy
metals.
Device fabrication and testing
Triplet-forming polymers prevent the recombination of PSCs were fabricated on patterned indium tin oxide (ITO)-coated
the germinal pairs, have a longer exciton lifetime and, as glass substrates. The ITO-coated glass was treated with UV ozone
a consequence, a longer exciton diffusion length, thereby prior to use. Poly(3,4-ethylenedioxythiophene)–poly(styrene-
increasing the percentage of excitons reaching the donor/ sulfonate) (PEDOT:PSS) was spin coated onto the glass sub-
acceptor interface, that is, the efficiency of charge generation strates and then baked at 120 1C for 1 h. The photoactive layer,
and photocurrent generation increases. Indeed, the triple- prepared from a blend of different polymers and P70CBM
forming polymers P2 and P3 containing heavy metals exhibit (1 : 2, w/w) in 1,2-dichlorobenzene, was spin-coated onto the
larger values of Jsc at an almost identical value of Voc and FF PEDOT:PSS layers. Note that 3% (v/v) 1,8-diiodooctane (DIO)
compared to P1 (Table 3). With further increase in the content was added in the solution to improve the morphology of the
of triplet iridium complexes in the polymers, for example, up to active films. The wet film underwent solvent annealing in a
3
mol% for polymer P4, the efficiency drops to 1.23%. Photo- glass Petri dish for at least 10 min before transfer to the
voltaic characteristics of the device (PCE, Jsc, FF) based on P5 thermal evaporator. Then, the device cathode, consisting of Ca
with 100 mol% Ir complex content decreased to a minimum (30 nm) and Al (100 nm), was thermally deposited to complete
ꢀ2
value of 0.22%, 0.95 mA cm and 34.6%, respectively. This the device fabrication. The J–V (current density–voltage)
may be due to the bad charge transfer of the static iridium characteristics of the PSCs were measured using a Keithley
complex, which destroys the polymer conjugated chain and 2400 source-measure unit. The photocurrent was measured
further reduces the mobility of charges so reducing the short- under illumination from a 150 W Thermal Oriel solar simu-
circuit current in the device. The efficiency of P1–P5 is lower lator (AM 1.5G). The intensity of the light source was corrected
than that for most known polymer:fullerene systems based on using a standard Si photodiode. The IPCE measurement
the generation of singlet exciton charges, but this is probably system (Enli Technology) comprised
a quartz tungsten
due to the unfavorable absorbing power of P1–P5. The external halogen (QTH) lamp as the light source, a monochromator,
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an optical chopper, a lock-in amplifier, and a calibrated
silicon photodetector.
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We have synthesized five D–A copolymers based on various
moles of iridium-complexed moieties in their side chain and
their optical and electrochemical properties were investigated.
These copolymers exhibit medium optical bandgap in the range
of 1.96–2.08 eV and suitable HOMO and LUMO energy level
matching with the PC71BM. These copolymers were used as an
electron donor along with PC71BM as an electron acceptor for
their potential application in solution processed PSCs. The
highest value of PCE (1.74%) was obtained for the P3 based
polymer solar cell. They are primarily investigated without
any treatment of the active layer. We expect that the value of
PCE can be improved further after adding a solvent additive,
thermal annealing, solvent annealing and a combination of
these to improve the active layer morphology. Work in this
direction is under progress and will be communicated later on.
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Conflicts of interest
2
There are no conflicts to declare.
5
8
C. M. Yang, C. H. Wu, H. H. Liao, K. Y. Lai, H. P. Cheng,
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Acknowledgements
This work was supported by the Russian foundation for basic
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