A new class of organic photovoltaic materials: Poly(rod-coil) polymers having alternative conjugated and non-conjugated segments

Article abstract of DOI:10.1039/c4cc03409k

A new class of organic photovoltaic materials, poly(rod-coil) polymers composed of alternatively definite conjugated and non-conjugated segments, have been proposed. The first five examples based on polyurethane chemistry showed photovoltaic performance s

Full text of DOI:10.1039/c4cc03409k

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A new class of organic photovoltaic materials:
poly(rod-coil) polymers having alternative
conjugated and non-conjugated segments†
Cite this: Chem. Commun., 2014,
50, 7720
Received 6th May 2014,
Accepted 28th May 2014
Hong-Jiao Li, Jin-Tu Wang, Chong-Yu Mei and Wei-Shi Li*
DOI: 10.1039/c4cc03409k
www.rsc.org/chemcomm
A new class of organic photovoltaic materials, poly(rod-coil) polymers composed of definite conjugated and non-conjugated segments in
composed of alternatively definite conjugated and non-conjugated an alternative fashion. Conjugated rigid segments are photo-active
segments, have been proposed. The first five examples based on poly- and basically determine optoelectronic properties of the material.
urethane chemistry showed photovoltaic performance surpassing the For photovoltaic application, these segments are suggested to be
reference compound, but less dependent on their molecular weight.
made of donor–acceptor (D–A) and related structures for efficient
light harvesting.1–3 Since they have definite chemical structures,
Over the past decade, remarkable achievement has been made like conventional photovoltaic compounds, the final material
in organic photovoltaic donor materials, which substantially would possess a performance less sensitive to its molecular weight
improved the power conversion efficiency (PCE) of organic solar and polydispersity. Furthermore, owing to their polymeric nature,
cells (OSCs).1 The so far reported donor materials can be categorized good film formation potential could be expected for these kinds
into two main classes: p-conjugated polymers1,2 and small mole- of materials.
cular compounds.1,3 Conjugated polymers have a one-dimensional
In this contribution, we report the first set of examples based on
p-conjugated backbone, which is favourable for light acquisition well-known polyurethane chemistry. Firstly, we synthesized com-
and the transportation of excitons and charge carriers. Besides, pound DPP(3TPOH)2 bearing one hydroxyl unit at both ends and a
polymer materials generally have a good film formation potential diketopyrrolopyrrole-centred D–A–D conjugated moiety5 (Scheme 1b
and are adaptable to various solution processing technologies. and ESI†). Secondly, DPP(3TPOH)2 was copolymerized with various
However, all polymers have issues of average molecular weight and diisocyanate monomers, including hexamethylene diisocyanate,
polydispersity, which always vary from batch to batch. Since their 1,4-phenylene diisocyanate, 1,3-phenylenediisocyanate, and
photovoltaic properties are sensitive to these parameters,4 conjugated methylene-diphenyl 4,40-diisocyanate, producing four kinds of
polymers usually suffer from poor batch-reproducibility, a severe polyurethane polymers named PU1, PU2, PU3, and two PU4
problem for their real applications. On the other hand, small (PU4-LW and PU4-HW) with different molecular weight, respectively.
molecular compounds do not have such problems since they have The average number molecular weight (Mn) and polydispersity index
a definite chemical structure and can be purified by means of many (PDI) were determined to be 4.12 kDa and 1.08 for PU1, while
well-developed techniques. However, for the purpose of promising 6.40 kDa and 1.15 for PU2, 6.66 kDa and 1.40 for PU3, 7.90 kDa and
light absorption and good charge transportation, these kinds of 1.39 for PU4-LW, and 16.7 kDa and 1.94 for PU4-HW. Obviously,
compounds usually have a large and rigid p-conjugated core. Conse- PU1, PU2, PU3 and PU4-LW have comparable molecular weight,
quently, they tend to aggregate or crystallize, and are hard to form a while Mn of PU4-HW is doubled. Meanwhile, compound DPP(3TP)2
well-qualified homogenous film, particularly in a large size. with saturated alkyl end chains was prepared as a small molecular
Herein, we propose a new class of polymeric photovoltaic reference compound for comparison.
materials with a structural feature in-between conventional con-
Compared with DPP(3TP)2, polymers PU1, PU2, PU3 and PU4
jugated polymers and small molecular compounds. As illustrated have different thermal properties and solid-state structures. Thermo-
in Scheme 1a, this class of materials are poly(rod-coil) polymers gravimetric analysis (TGA, Fig. S10, ESI†) revealed that these poly-
mers have a 5%-weight-loss decomposition temperature (Td) in
Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional
the range of 258–298 1C, lower than DPP(3TP)2 (383 1C). In the
differential scanning calorimetry (DSC, Fig. S11, ESI†), only a glass
transition around 60 1C and an weak endothermic peak at 160 1C
Molecules, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
345 Lingling road, Shanghai 200032, China. E-mail: liws@mail.sioc.ac.cn
† Electronic supplementary information (ESI) available: Materials and synthesis,
were observed for PU3 and PU4 in the second heating procedure,
while three small peaks at around 88, 101, and 187 1C for PU2.
TGA, DSC, XRD, UV, CV, TEM, and OSC device optimization. See DOI: 10.1039/
c4cc03409k
7720 | Chem. Commun., 2014, 50, 7720--7722
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Scheme 1 (a) Schematic representation of proposed poly(rod-coil) photovoltaic polymers. (b) Synthesis of photovoltaic polyurethanes.
In sharp contrast, a couple of intense phase transition peaks 10–19 nm-red-shifted ICT bands (Fig. 1b). This suggests the occur-
(51, 102, 138 and 176 1C) appeared in the second heating DSC rence of strong p–p interactions among the conjugated segments in
trace of DPP(3TP)2. Especially, the endothermic enthalpy for the film state. In comparison, DPP(3TP)2 only showed a relatively
the peak at 138 1C is extremely large, suggesting that DPP(3TP)2 weak shoulder at this region, indicating that its film structure is not
is a crystalline material. X-ray diffraction analysis (XRD, favourable for p–p interactions among the molecules. For PU1
Fig. S12, ESI†) further confirmed that DPP(3TP)2 is crystalline polymer film, an ICT band (606 nm) blue-shifted to that in solution
at room temperature while PU2–PU4 is amorphous. Although a (630 nm), together with a weak shoulder at 708 nm, was observed,
sharp phase transition was observed at around 115 1C together suggesting the formation of a different aggregation style, probably
with a weak one at around 155 1C for the PU1 polymer, the non- H-aggregates. Although the differences were observed in the absorp-
structured XRD profile indicates that it is also an amorphous tion peaks, the film state absorption spectra of either polymers or
material at room temperature.
DPP(3TP)2 displayed the same onset point at around 780 nm, giving
Fig. 1 displays UV-vis absorption spectra of the polymers and an energy band gap of 1.59 eV for all the materials. Cyclic voltam-
the reference compound in both solution and film states. metry further confirmed that all the materials have a similar highest
In chloroform solution, all the polymers exhibited a similar occupied molecular orbital (HOMO) energy level at ꢀ5.3 eV (Fig. S14,
electronic absorption spectrum to DPP(3TP)2, with two featured ESI†). These results indicate the change from small molecules to
bands in the regions of 300–500 nm and 500–750 nm for p–p* polymers and the different urethane linkers, as well as polymer
and intramolecular charge transfer (ICT) transition, respectively molecular weight, do not alter many basic optoelectronic properties
(Fig. 1a).6 It is valuable to point out that the two peaks at around of the materials, such as absorption bands in solution, energy band
622 and 654 nm observed for the ICT band of DPP(3TP)2 are due gap and molecular orbital energy levels, but does affect the
to the vibronic progression since they did not change upon aggregation-induced properties of the materials.
dilution (Fig. S13, ESI†). In the film state, PU2–PU4 displayed a
Bulk heterojunction OSCs with a conventional structure of
new intense peak in the range of 705–714 nm in addition to their ITO/PEDOT:PSS/active layer/Al using the synthesized polymers and
DPP(3TP)2 as donor components while [6,6]-phenyl-C61-butyric acid
methyl ester (PC61BM) as the acceptor component were fabricated. It
was found that the best weight ratio of donor/PC61BM varied with
the checked donor materials, in which 1 : 2 for DPP(3TP)2, 1:3 for
PU3, 1:5 for PU4-LW, while 1 : 4 for the rest of the polyurethanes
(Tables S1–S6, ESI†). Other fabrication conditions, including solvent,
concentration, the spin-coating rate, annealing temperature, and the
addition of 1,8-diiodooctane (DIO), were also optimized. Fig. 2 displays
the device performance of all the checked systems under their
respective optimized conditions, while their parameters are summar-
ized in Table 1. From these data, one can find that all the polymers
under optimized conditions displayed improved photovoltaic perfor-
mance compared to DPP(3TP)2 with an increasing factor of 29–73%.
Fig. 1 Normalized UV-vis absorption spectra of DPP(3TP)2, PU1, PU2,
Detailed comparison showed that the change from DPP(3TP)2 to
the polymers did not alter open-circuit voltage (VOC) and short-circuit
PU3, PU4-LW and PU4-HW in chloroform solutions with a concentration
of 1 ꢁ 10ꢀ5 M (a) and in film state (b).
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those of polymer blend films appeared to be homogeneous without
clear phase separation. Obviously, the latter is favourable to device
performance since it could provide much smaller phase domains
and a larger heterojunction interface. On the other hand, the
mobility measurements by the space-charge-limited method
indicate that the DPP(3TP)2/PC61BM blend film possesses hole
and electron mobilities larger than most of the polymer blend
films, but comparable to some cases (Table 1). These results
obviously suggest that the morphology change would be the
main reason for the above-mentioned performance improve-
Fig. 2 (a) J–V curves under AM 1.5 G illumination with a density of 100 ment observed for the polymers.
mW cmꢀ2 and (b) EQE spectra of the best OSCs based on DPP(3TP)2, PU1,
PU2, PU3, PU4-LW and PU4-HW as donor component while PC61BM
as acceptor components under the optimized conditions shown in
Tables S1–S6 (ESI†).
In summary, we have demonstrated a new class of polymeric
photovoltaic materials, which contain multiple conjugated rigid
segments but linked by non-conjugated soft chains. Compared
with their small molecular reference, the present five photovoltaic
polyurethane examples exhibited the performance improvement
by a factor of 29–73%. Moreover, the molecular weight seems no
longer to be one of the important factors that affect the material
properties. Although PCE in the present examples is low, this
work opens an avenue for the development of new photovoltaic
materials with the features of both small molecular compounds
and polymers. For improving the performance of these kinds of
materials, there are a lot of things that can be done, for example,
well design and optimization either of the opto-electronically
active segments, or the non-conjugated soft linking segments,
including the type of linking functionalities and pattern, the
nature and length of linking chains. Furthermore, this material
design strategy is not only limited to the OPV materials, but also
Table 1 Device parameters of the organic solar cells shown in Fig. 2
a
VOC
JSC
FF
PCEb
(%)
mh
me
Donor (V) (mA cmꢀ2) (%)
(cm2 Vꢀ1
s
ꢀ1) (cm2 Vꢀ1 sꢀ1
)
DPP(3TP)2 0.76 1.82 (1.98) 43.2 0.59 (0.55) 1.3 ꢁ 10ꢀ4
5.6 ꢁ 10ꢀ4
1.8 ꢁ 10ꢀ5
2.5 ꢁ 10ꢀ5
1.5 ꢁ 10ꢀ4
5.8 ꢁ 10ꢀ4
2.1 ꢁ 10ꢀ4
PU1
PU2
PU3
0.68 1.92 (1.62) 57.0 0.75 (0.71) 1.4 ꢁ 10ꢀ4
0.80 2.09 (1.87) 58.8 0.98 (0.95) 1.8 ꢁ 10ꢀ5
0.77 1.67 (1.49) 55.1 0.71 (0.68) 6.1 ꢁ 10ꢀ5
PU4-LW 0.75 1.75 (1.57) 58.4 0.77 (0.76) 4.7 ꢁ 10ꢀ5
PU4-HW 0.78 1.99 (1.78) 53.0 0.82 (0.77) 6.0 ꢁ 10ꢀ5
a
Data in parentheses are the JSC values calculated from EQE spectra
b
shown in Fig. 2b. Data in parentheses are the average values.
current ( JSC) a lot although in some cases a slight enhancement can be applied in the fields of organic field-effect transistors and
or reduction was observed. This can be well understood since organic light-emitting diodes.
the opto-electronically active segments for all the polymers are
We gratefully acknowledge the financial support from the
the same as that of DPP(3TP)2 and all polymers do possess a National Natural Science Foundation of China (No. 20974119,
similar HOMO energy level like DPP(3TP)2. As for JSC, external 90922019, and 21074147), Shanghai Science and Technology Com-
quantum efficiency (EQE) spectroscopy revealed the photocurrent mission (No. 13JC1407000), and Chinese Academy of Sciences.
of the polymer-based cells decreased in the range of 550–750 nm
but increased in the range of 300–550 nm (Fig. 2b). These two sides
compensated each other and thus resulted in comparable JSC
values. Therefore, the performance improvement of the polymer-
Notes and references
1 For selected recent reviews, see: (a) Y.-J. Cheng, S.-H. Yang and
C.-S. Hsu, Chem. Rev., 2009, 109, 5868; (b) P. M. Beaujuge and
´
based solar cells finally came from the enhancement in the FF
value, which increased from 43.2% for the DPP(3TP)2-based cell to
over 53% for the polymer-based ones. When the comparison was
carried out among the polymer blend films, one could easily find
that PU2 showed the best photovoltaic output with a PCE of nearly
1%. More importantly, the optimized solar cell based on PU4-LW or
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In order to investigate the origin of the performance difference,
the morphology and charge transportation of the blend films for the
best devices were studied. As revealed by transmission electron
microscopy (Fig. S15, ESI†), the DPP(3TP)2/PC61BM blend film
presented a large island-sea phase separation microstructure, while
7722 | Chem. Commun., 2014, 50, 7720--7722
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