Polymers Containing Conjugated Pyridyl H-Acceptor
Pendants and Various Low-Band-Gap H-Donor
Dyes Bearing Cyanoacrylic Acid Groups for
Organic Solar Cell Applications
TZUNG-CHI LIANG,1I-HUNG CHIANG,1PO-JEN YANG,1DHANANJAY KEKUDA,2 CHIH-WEI CHU,2,3HONG-CHEU LIN1
1
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan, Republic of China
2Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan, Republic of China 3Department of Photonics, National Chiao Tung University, Hsinchu, Taiwan, Republic of China
Received 8 June 2009; accepted 14 July 2009 DOI: 10.1002/pola.23643
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Novel supramolecular side-chain polymers were constructed by complexa-tion of proton acceptor (H-acceptor) polymers, i.e., side-chain conjugated polymers P1–P2 containing pyridyl pendants, with low-band-gap proton donor (H-donor) dyes S1–S4 (bearing terminal cyanoacrylic acids) in a proper molar ratio. Besides unique mesomorphic properties confirmed by DSC and XRD results, the H-bonds of supra-molecular side-chain structures formed by pyridyl H-acceptors and cyanoacrylic acid H-donors were also confirmed by FTIR measurements. H-donor dyes S1–S4 in solid films exhibited broad absorption peaks located in the range of 471–490 nm with opti-cal band-gaps of 1.99–2.14 eV. Furthermore, H-bonded polymer complexes P1/S1–P1/ S4 and P2/S1–P2/S4 exhibited broad absorption peaks in the range of 440–462 nm with optical band-gaps of 2.11–2.25 eV. Under 100 mW/cm2of AM 1.5 white-light illu-mination, the bulk heterojunction polymer solar cell (PSC) devices containing an active layer of H-bonded polymer complexes P1/S1–P1/S4 and P2/S1–P2/S4 (as elec-tron donors) mixed with [6,6]-phenyl C61butyric acid methyl ester (i.e., PCBM, as an electron acceptor) in the weight ratio of 1:1 were investigated. The PSC device con-taining H-bonded polymer complex P1/S3 mixed with PCBM (1:1 w/w) gave the best preliminary result with an overall power conversion efficiency (PCE) of 0.50%, a short-circuit current of 3.17 mA/cm2, an open-circuit voltage of 0.47 V, and a fill factor of 34%.VVC2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5998–6013, 2009
Keywords: dyes/pigments; H-bonded polymer complex; liquid-crystalline polymers (LCP); low-band-gap dye; polymer solar cell; supramolecular polymer; supramolecular structures
INTRODUCTION
Self-assembled phenomena through molecular recognition between complementary constituents have been explored in various areas, such as the
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 47, 5998–6013 (2009)
V
VC2009 Wiley Periodicals, Inc.
Additional Supporting Information may be found in the online version of this article.
Correspondence to: H.-C. Lin (E-mail: linhc@cc.nctu.edu. tw)
applications of biomaterials, liquid crystalline (LC) materials, and electro-optical materials.1–8 Not only innovative LC properties of novel supra-molecules consisting of two counterparts can be generated through intermolecular hetero-hydro-gen-bonding interactions, but also particular self-assembly of nanoscaled building blocks using non-covalent interactions (e.g., hydrogen bonding, acid/base proton transfer, and electrostatic forces) may be amplified into macroscopically observable phenomena.9 More recently, direct energy har-vesting from sunlight by using photovoltaic cells (PVCs) has increasingly attracted intensified attention to utilize renewable energy of the na-ture, especially for the development of organic so-lar cells.10–12Compared with inorganics (such as Si), organic materials (especially polymers) have the benefits to be easily made into devices with light weight, large area, and flexible panels, so different concepts of solar cell architectures have been developed by organics, including blends of polymers13–20 and block copolymers21 with [6,6]-phenyl C61 butyric acid methyl ester (i.e., PCBM,
as an electron acceptor). Among the organic solar cell materials investigated so far, semiconducting conjugated polymers with electron donor–acceptor architectures are one of the most effective ways to build intramolecular charge transfer (ICT) interac-tion between the electron donor (D) and electron acceptor (A) segments.22–30ConjugatedD-A copoly-mers with strong ICT effects are promising materi-als for the development of high performance poly-mer-based PVCs due to the merits of narrow band-gaps,25–27 broad absorption bands extend-ing into the near-infrared spectral range, effi-cient photoinduced charge transfer and separa-tion, pronounced charge photogeneration and col-lection, and high mobility of ambipolar charge transport.28–30Furthermore, different concepts of solar cell architectures, including the dye blends containing inorganics (PCBM, TiO2, and ZnO as
electron acceptors)31 and/or polymers,32–34 have been successfully progressed the efficiencies of the bulk heterojunction polymer solar cell (PSC) devices. However, due to the aggregations of the dyes originated from their strong p–p inter-actions, the power conversion efficiency (PCE) values of PSC devices are limited by the dye con-tents in the polymer blends. Therefore, the H-bonded interactions of supramolecular polymers in this work can be introduced to reduce the aggregations of the low-band-gap organic dyes (as H-donors), and thus, to improve the PCE values for the organic solar cell applications.
It is noticeable that the well-known electron-withdrawing unit would be an aryl-substituted cyano or nitro group, which has been widely uti-lized in organic solar cell materials, including metal-free dye sensitized solar cell (DSSC) mate-rials.35On account of the electron-rich sulfur and nitrogen atoms, especially in heterocyclic struc-tures, polymers, and organic molecules36,37 con-taining carbazole, triphenylamine, and thiophene units as the electron-donating moieties have lately attracted considerable interests in the applications of light-emitting diodes,38,39 photo-voltaic devices,40,41 and organic field effect tran-sistors (OFETs).42 In the past years, various attempts have been made to increase the delocali-zation of p-electrons by constructing more copla-nar conjugated systems to generate low-band-gap dyes. Another approach is to incorporate electron-accepting (A) units (e.g., cyano or nitro groups) with electron-donating (D) units (e.g., carbazole or triphenylamine groups) to produce low-band-gap dyes with resonance structures (i.e., D-A $ DþA).43,44 Recently, Lin and coworkers have introduced some novel conjugated spacers by inserting benzothiadiazole, benzoselenadiazole, and 1H-phenanthro[9,10-d]imidazole segments into low-band-gap dyes (bearing D-A structures) for the applications of photovoltaic devices.45,46
To incorporate low-band-gap organic dyes (as H-donors) into supramolecular polymers for or-ganic solar cell applications, conjugated pyridyl H-acceptors were integrated into the side-chain poly-meric structures as the pendent groups rather than as small molecules (acting as luminescent chromophores) in our previous studies.47–50 As shown in the schematic illustration of Figure 1, supramolecular side-chain polymers (i.e., H-bonded polymer complexes) were constructed by complexation of pyridyl H-acceptor polymers, i.e., side-chain polymers P1–P2 containing conjugated pyridyl pendants, with low-band-gap H-donor dyes S1–S4 (bearing terminal cyanoacrylic acids) in a molar ratio of 1:1 for pyridyl and acid units, which would have much more uptaken loads of photovoltaic dyes in the supramolecular polymeric structures (without phase separation) compared with the normal polymer blends. Our detailed investigations will prove that larger aggregations of the acid protected dyes occurred in the polymer blends due to the lack of supramolecular interac-tions, and a polymer blend of the conjugated H-acceptor polymer P1 and the acid-protected dye S1P (refer Fig. S1 of the Supporting Information) illustrated an obvious reduction in the PCE value
in contrast to the supramolecular analogue P1/S1. Different molar ratios of conjugated H-acceptor monomer PBB (containing a pyridyl terminus) and hole-transporting monomer CAZ (bearing a carbazole unit) were copolymerized through free radical polymerization to obtain H-acceptor poly-mers (P1 and P2). Both terminal carbazole or tri-phenylamine groups as electron-donating (D) units were in conjunction with cyanoacrylic acid groups as electron-accepting (A) units to yield low-band-gap H-donor dyes S1–S4, which were bridged through various numbers of fluorene, bithiazole, and thiophene units (refer Fig. 2). By incorporating side-chain conjugated H-acceptor polymers with low-band-gap H-donor dyes, the LC and PVC properties of the supramolecular poly-mer complexes can be easily adjusted. The present investigation is mainly to explore the supramolec-ular structures of H-bonded side-chain polymers containing low-band-gap H-donor dyes for the PSC applications. Therefore, the bulk heterojunc-tion PSC devices containing an active layer of H-bonded polymer complexes P1/S1–P1/S4 and P2/ S1–P2/S4 (as electron donors) mixed with PCBM (as an electron acceptor) were evaluated.
EXPERIMENTAL
Materials
Chemicals and solvents were reagent grades and purchased from Aldrich, ACROS, TCI, and
Lan-caster Chemical. Dichloromathane and THF were distilled to keep anhydrous before use. The other chemicals were used without further purification. The synthetic routes of side-chain conjugated H-acceptor polymers P1 and P2 (as shown in Fig. 2) were reported in our previous publication.51 Synthesis and characterization of H-donor dyes S1–S4 and their intermediates (refer Fig. 2) are described in the Supporting Information. The chemical structures for all products were con-firmed by 1H NMR spectroscopy and elemental analyses.
Preparation of Supramolecular Polymer Complexes
In all cases, all H-donor dyes and H-acceptor pol-ymers (as shown in Fig. 2) were dissolved in THF to make a clear solution. After then, most of the solvents were evaporated under ambient condi-tions, which were followed by drying in a vacuum oven at 60C for several hours. The complexation of H-donor acids and H-acceptor polymers through hydrogen bonding was proceeded during the solvent evaporation. The H-bonded side-chain polymers of all H-acceptor polymers complexed with H-donor dyes S1–S4 had the equal molar amount of pyridyl H-acceptor and carboxylic acid H-donor groups (in 1:1 M ratio) to form supramolecular polymer complexes (i.e., H-bonded side-chain polymers).
Figure 1. Schematic illustration of complexation processes for H-bonded side-chain polymers. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
Measurements and Characterization
1
H NMR spectra were recorded on a Varian unity 300 MHz spectrometer using d-DMSO as sol-vents. Elemental analyses were proceeded on a HERAEUS CHN-OS RAPID elemental analyzer. Fourier transform infrared (FT-IR) spectra were performed on a Nicolet 360 FT-IR spectrometer. The textures of mesophases were characterized by a polarizing optical microscope (POM, model: Leica DMLP) equipped with a hot stage. Tempera-tures and enthalpies of phase transitions were determined by differential scanning calorimetry (DSC, model: Perkin–Elmer Pyris 7) at a heating and cooling rate of 10 C/min under nitrogen. Thermogravimetric analyses (TGA) were con-ducted on a Du Pont Thermal Analyst 2100 sys-tem with a TGA 2950 thermogravimetric analyzer at a heating rate of 20C/min under nitrogen. Gel permeation chromatography (GPC) analyses were executed with a Water 1515 separations module using polystyrene as a standard and THF as an
eluant. UV-visible absorption spectra were recorded in dilute THF solutions (106 M) on a HP G1103A spectrophotometer, and photolumi-nescence (PL) spectra were obtained on a Hitachi F-4500 spectrophotometer. Thin films of UV-vis and PL measurements were spin-coated on quartz substrates from THF solutions with a concentra-tion of 1 wt %. Cyclic voltammetry (CV) measure-ments were carried out using a BAS 100 electro-chemical analyzer with a standard three-electrode electrochemical cell in a 0.1 M tetrabutylammo-nium hexafluorophosphate [(TBA)PF6] solution
(in acetonitrile) at room temperature with a scan-ning rate of 50 mV/s. During the CV measure-ments, the solutions were purged with nitrogen for 30 s. In each case, a carbon working electrode coated with a thin layer of copolymers, a platinum wire as the counter electrode, and a silver wire as the quasi-reference electrode were used, and Ag/ AgCl (3 M KCl) electrode was served as a
Figure 2. H-acceptor polymers (P1 and P2) and H-donor dyes (S1–S4) used in the H-bonded polymer complexes (P1/S1–P1/S4 and P2/S1–P2/S4).
reference electrode for all potentials quoted herein. The redox couple of ferrocene/ferrocenium ion (Fc/Fcþ) was used as an external standard. The corresponding highest occupied molecular or-bital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels were calculated using Eox/
onset and Ered/onset for experiments in solid films
of H-acceptor polymers (P1 and P2), H-donor dyes (S1–S4), and H-bonded polymer complexes (P1/S1–P1/S4 and P2/S1–P2/S4), which were performed by drop-casting films with a similar thickness from THF solutions (5 mg/mL). The LUMO level of PCBM used was in accordance with the literature datum.52 The onset potentials were determined from the intersections of two tangents drawn at the rising currents and back-ground currents of the cyclic voltammetry (CV) measurements. Synchrotron powder X-ray diffrac-tion (XRD) measurements were performed at beamline BL17A of the National Synchrotron Radiation Research Center (NSRRC) in Taiwan, where the X-ray wavelength used was 1.33,366 A˚ . XRD data were collected using imaging plates (IP, of an area¼ 20 40 cm2and a pixel resolution of 100) curved with a radius equivalent to the sam-ple-to-image plate distance of 280 mm, and the diffraction signals were accumulated for 3 min. The powder samples were packed into a capillary tube and heated by a heat gun, where the temper-ature controller was programmable by a PC with a PID feed back system. The scattering angle theta values were calibrated by a mixture of silver behenate and silicon.
Device Fabrication and Characterization of PSCs The PSC devices in this study were composed of an active layer of blended H-bonded side-chain polymers (P1/S1–P1/S4 and P2/S1–P2/S4) mixed with [6,6]-phenyl C61 butyric acid methyl ester
(i.e., PCBM) in solid films, which was sandwiched between a transparent indium tin oxide (ITO) an-ode and a metal cathan-ode. Before device fabrica-tion, ITO-coated glass substrates (1.5 1.5 cm2) were ultrasonically cleaned in detergent, deion-ized water, acetone, and isopropyl alcohol. After-ward, the substrates were treated with UV ozone for 15 min, and a layer of poly(ethylene dioxythio-phene): polystyrenesulfonate (PEDOT:PSS, 30 nm) was subsequently spin-coated onto the substrates. After baking at 130C for 1 h, the sub-strates were transferred to a nitrogen-filled glovebox. The PSC devices were fabricated by spin-coating solutions of blended H-bonded
poly-mer complexes:PCBM (with various weight ratios) onto the PEDOT:PSS modified substrates at 600 rpm for 60 s (200 nm), and placed in a covered glass Petri dish. Initially, the blended sol-utions were prepared by dissolving both H-bonded polymer complexes (P1/S1–P1/S4 and P2/S1–P2/ S4) and PCBM (with a 1:1 weight ratio initially and then with various weight ratios for the optimum H-bonded polymer complex) in chloro-benzene (20 mg/1 mL), followed by continuous stirring for 12 h at 50 C. In the slow-growth approach, blended H-bonded polymer complexes in solid films were kept in the liquid phase after spin-coating by using the solvent (chlorobenzene) with a high boiling point. Finally, a calcium layer (30 nm) and a subsequent aluminum layer (100 nm) were thermally evaporated through a shadow mask at a pressure below 6 106 Torr, and the active area of the device was 0.12 cm2. All PSC devices were prepared and measured under ambient conditions.
RESULTS AND DISCUSSION
FT-IR Spectroscopy of H-Bonded Polymer Complexes
All H-bonded side-chain polymers consisting of the appropriate molar ratio (fully H-bonded com-plexes in a molar ratio of 1:1 for pyridyl and acid units) of acceptor polymers (P1 and P2) and H-donor dyes (S1–S4) were prepared by slow evapo-ration of THF solutions and followed by drying in vacuo. The formation of hydrogen bonding in supramolecular side-chain polymers containing H-donor dyes (S1–S4) was confirmed by FT-IR spectroscopy. As shown in Figure 3, IR spectra of acceptor polymer P2, donor dye S1, and H-bonded complex P2/S1 are compared to analyze the hydrogen bonds in the supramolecular struc-ture of H-bonded polymer complex P2/S1. In con-trast to the OAH band of pure S1 at 2640 and 2510 cm1, the weaker OAH band observed at 2497 and 1902 cm1 in H-bonded polymer com-plex P2/S1 is indicative of stronger hydrogen bonding between the pyridyl group of P2 and the carboxylic acid of S1 in the H-bonded complex. On the other hand, a C¼¼O stretching vibration appeared at 1722 cm1in H-bonded polymer com-plex P2/S1, which shows that the carbonyl group was in a less associated state than that in pure S1 with a weaker C¼¼O stretching vibration appeared at 1690 cm1. Both results suggest that hydrogen bonds were formed between H-acceptor
polymer P2 and H-donor dye S1 in the solid state of H-bonded polymer complex P2/S1. The other H-bonded polymer complexes also have the simi-lar consequences of H-bonding formation as the H-bonded complex demonstrated here.53However, in comparison with H-bonded polymer complex P1/S1, physical blend P1/S1P (without H-bonds) in Figure S2 (refer the Supporting Information) shows a weaker C¼¼O stretching vibration appeared at 1706 cm1 for lack of H-bonding interactions.
Phase Behavior
The phase transition temperatures of H-acceptor polymers (P1 and P2), H-donor dyes (S1–S4), and H-bonded side-chain polymers (i.e., H-bonded polymer complexes P1/S1–P1/S4 and P2/S1–P2/ S4) are summarized in Table 1, which were deter-mined by DSC (under nitrogen) and POM. The weight-average molecular weights (Mw) of
H-acceptor polymers P1 and P2 (determined by GPC) are 14,400 g/mol (PDI¼ 1.72) and 38,100 g/ mol (PDI ¼ 3.24), respectively. The glass transi-tion temperatures (Tg) of H-acceptor polymers P1
and P2 are 63 and 88C, respectively.51 To eluci-date the H-bonding effect of H-bonded pendants on the thermal properties of supramolecular side-chain polymers, H-donor dyes S1–S4 were introduced to be incorporated with H-acceptor side-chain polymers P1 and P2. As shown in Table 1, both series of H-bonded complexes con-taining H-acceptor polymers P1 and P2 showed only a single glass transition, which suggests good miscibilities between H-donor dyes (i.e., S1– S4) and H-acceptor polymers (i.e., P1 and P2).
Since no melting and crystallization transitions were observed in the DSC measurements, it sug-gests that these H-bonded complexes possess amorphous characteristics. However, the Tg
val-ues of the H-bonded complexes are notably higher than those of their corresponding H-acceptor poly-mers P1 and P2. The increases of Tgvalues in
H-bonded complexes are probably due to the larger p–p interactions originated from the increased rigid-rod lengths of the integrated H-bonded pendants (containing both pyridyl H-acceptor units and H-donor dyes). In contrast to H-acceptor homopolymer P1 and its bonded complexes, H-acceptor copolymer P2 and its H-bonded com-plexes possessed higher Tgvalues due to the
inte-gration of more bulky and rigid CAZ components in copolymer P2. Comparing the H-bonded com-plexes containing fluorene-linked dyes (S1 and
S2) and bithiazole-linked dyes (S3 and S4), owing to the higher rigidity of bithiazole units in H-donor dyes S3 and S4, the latter H-bonded com-plexes (P1–P2/S3 and P1–P2/S4) have higher Tg
values than the former H-bonded complexes (P1– P2/S1 and P1–P2/S2), respectively. This obvi-ously indicates that the rigid bithiazole linkers will enhance the aggregation of the pendants in the H-bonded complexes effectively. In contrast to the H-bonded complexes (P1–P2/S1 and P1–P2/ S3) containing end-capping triphenylamine dyes (S1 and S3), owing to the higher rigidity and coplanarity of end-capping cabazole units in H-donor dyes (S2 and S4), the analogous H-bonded complexes (P1–P2/S2 and P1–P2/S4) containing end-capping cabazole dyes (S2 and S4) have higher Tgvalues.
The isotropization temperatures (Ti) have the
similar trends as the glass transition tempera-tures (Tg) in H-bonded polymer complexes (P1/
S1–P1/S4 and P2/S1–P2/S4). Moreover, compar-ing analogous H-bonded complexes consistcompar-ing of the same H-donor dyes, H-bonded complexes con-taining H-acceptor homopolymer P1 possess the higher isotropization temperatures (Ti) and the
broader mesophasic ranges than those containing H-acceptor copolymer P2. In addition, compared with H-bonded polymer complexes P1/S1 and P1/ S2 bearing fluorene-linked dyes (S1 and S2), H-bonded polymer complexes P1/S3 and P1/S4 bearing bithiazole-linked dyes (S3 and S4) have higher Tivalues and broader mesophasic ranges.
In general, the isotropization temperatures (Ti)
and mesophasic ranges of H-bonded side-chain polymers would be enhanced while the H-bonded central cores are longer and more rigid.
As shown in Figure 4(a), the mesomorphic behavior of H-bonded polymer complex P2/S4 (cooling at 130C) was confirmed as the nematic phase by the schlieren texture of POM, which was further elucidated by X-ray diffraction (XRD) measurements in Figure 4(b) that no sharp d-spacing values, i.e., no layered structures of the smectic phase, were observed in the XRD inten-sity against angle profiles of H-bonded polymer complexes P1/S1 and P2/S4 at 130 C (in the mesophasic range). According to the POM and XRD measurements, H-acceptor homopolymer P1 and all H-bonded polymer complexes (P1/S1–P1/ S4 and P2/S1–P2/S4) in Table 1 were verified to possess the nematic phase, but H-acceptor copoly-mer P2 bearing 50% molar ratio of CAZ units did not possess any mesophase. Hence, the integra-tion of CAZ units in copolymer P2 is detrimental
to the formation of the mesophase, which can be explained by that the CAZ units with nonmeso-morphic property may dilute and hinder the molecular packing of the LC arrangements in copolymer P2. However, the nematic phase was introduced to the corresponding H-bonded poly-mer complexes (P2/S1–P2/S4) of copolypoly-mer P2 due to the extended H-bonded mesogens by com-bination of H-acceptor pedants with H-donor
dyes. Moreover, the mesophasic ranges and Ti
val-ues of the H-bonded polymer complexes (P2/S1– P2/S4) containing copolymer P2 were apparently reduced by the CAZ units of H-acceptor copoly-mer P2, which diluted and interfered the LC arrangements of the H-bonded mesogens in their subsequent H-bonded polymer complexes. How-ever, acid-protected dye S1P and physical blend P1/S1P (without H-bonds) have lower phase transition temperatures (including the isotropiza-tion temperature Ti) than H-bonded polymer
com-plex P1/S1 due to the dilution effect of the acid-protected dye S1P moieties in the physical blend P1/S1P (refer Table S1 of the Supporting Information).
Figure 4. (a) Optical texture of the nematic phase in H-bonded polymer complex P2/S4 observed by POM at 130 C (cooling) and (b) XRD intensity against angle profiles obtained from H-bonded poly-mer complexes P1/S1 and P2/S4 at 130C (in the ne-matic phase).
Figure 3. FTIR spectra of H-acceptor polymer P2, H-donor dye S1, and H-bonded polymer complex P2/ S1.
Table 1. Thermal Properties of H-Acceptor Polymers (P1–P2), H-Donor Dyes (S1–S4), and H-Bonded Polymer Complexes (P1/S1–P1/S4 and P2/S1–P2/S4)
Compound Phase Transitions (C)a,b
P1 G 63 N 125cI P2 G 88 K 110cI S1 K 156 (10.8) I S2 K 163 (13.4) I S3 K 173 (25.9) I S4 K 180 (26.3) I P1/S1 G 87 N 151cI P1/S2 G 89 N 155cI P1/S3 G 96 N 167cI P1/S4 G 99 N 172cI P2/S1 G 96 N 141cI P2/S2 G 98 N 147cI P2/S3 G 104 N 156cI P2/S4 G 105 N 162cI
aPhase transition temperatures (C) and enthalpies (in parentheses, kJ/mol) were determined by DSC at a heating rate of 10C/min.
bG, glassy state; K, crystalline; N, nematic; I, isotropic. cPhase transition temperatures were obtained by POM
Optical Properties
The UV-visible absorption spectra of H-acceptor polymers P1–P2 and H-donor dyes S1–S4 (in both THF solutions and solid films), and H-bonded polymer complexes P1/S1–P1/S4 and P2/S1–P2/ S4 (in solid films) are displayed in Figures 5 and 6, and their photophysical properties are demon-strated in Table 2. The absorption energy band-gaps of H-bonded polymer complexes (P1/S1–P1/ S4 and P2/S1–P2/S4) could be easily tuned by the introduction of H-donor dyes (S1–S4), and their absorption spectra covered broad wave-length ranges for both solutions and solid films. As shown in Figure 5, the maximum absorption wavelength (kabs) of H-acceptor polymers P1–P2
in THF solutions and solid films were 385 and 393 nm, respectively, which were mainly contrib-uted from the PBB units. The maximum absorp-tion wavelength (kabs) of H-donor dyes S1–S4 in
THF solutions were in the range of 458–462 nm (in THF solutions) and 471–490 nm (in solid films). Because of the interchain association and p–p stacking of these polymers and dyes in solids, the absorption spectra of all H-acceptor polymers and H-donor dyes in solid films were generally larger than those in dilute solutions (i.e., 8 nm red shifts in polymers and 11–30 nm red shifts in dyes). After complexation (in solid films as shown in Fig. 6), H-bonded polymer complexes P1/S1–
P1/S4 and P2/S1–P2/S4 displayed blue-shifted absorption peaks (at 440–462 nm) in contrast to H-donor dyes S1–S4. The blue shifted absorption (blue shifted wavelengthDkabs ¼ 19–39 nm) was
due to the dilution effect of H-acceptor polymers as solid solvents for dyes (as solutes) in solid H-bonded polymer complexes. Compared with the H-bonded complexes containing fluorene-linked dyes (S1 and S2), the corresponding H-bonded complexes containing bithiazole-linked dyes (S3 and S4) have longer absorption wavelengths and thus to have lower optical band-gaps, which were originated from the smaller optical band-gaps of bithiazole-linked dyes (S3 and S4) in solid films. Therefore, the H-bonded complexes containing bithiazole-linked dyes (S3 and S4) might have the gifts of lower optical band-gaps for further good performance in photovoltaic properties. However,
Table 2. Absorption and Photoluminescence Spectral Data of H-Acceptor Polymers (P1–P2), H-Donor Dyes (S1–S4), and H-Bonded Polymer Complexes (P1/S1–P1/S4 and P2/S1–P2/S4) Compound kabs,sola (nm) kabs,filma (nm) kPL,film (nm) P1 385 393 496 P2 385 393 487 S1 460 471 631 S2 462 478 638 S3 458 481 668 S4 460 490 677 P1/S1 – 440 614 P1/S2 – 445 621 P1/S3 – 462 640 P1/S4 – 451 645 P2/S1 – 440 611 P2/S2 – 448 618 P2/S3 – 457 638 P2/S4 – 452 638
aAbsorption and PL emission spectra were recorded in
dilute THF solutions.
Figure 5. UV-visible absorption spectra of H-acceptor polymers P1–P2 and H-donor dyes S1–S4 (a) in THF solutions and (b) in solid films.
due to the lack of supramolecular interactions in polymer blend P1/S1P and a larger aggregation of the acid-protected dye S1P, a red-shifted (33 nm) absorption in the solid film of polymer blend P1/S1P than that of H-bonded polymer complex P1/S1 was observed (refer Fig. S3 and Table S2 of the Supporting Information).
The photoluminescence (PL) spectra of H-acceptor polymers P1–P2, H-donor dyes S1–S4, and H-bonded polymer complexes P1/S1–P1/S4 and P2/S1–P2/S4 (in solid films) are summarized in Table 1. Similar to the UV-visible absorption spectra, the PL emission wavelengths of H-bonded polymer complexes P1/S1–P1/S4 and P2/ S1–P2/S4 (at 611–645 nm) were all blue-shifted in contrast to those of H-donor dyes S1–S4 (at 631–677 nm). The PL emission spectra (in solid
films) of the H-acceptor polymers P1 and P2 were dramatically quenched by adding H-donor dyes S1–S4 in the H-bonded polymer complexes P1/S1– P1/S4 and P2/S1–P2/S4. The corresponding opti-cal quenching properties of these H-bonded com-plexes in solid films, including the broad optical absorptions and low optical band-gaps, proposed the potential applications in photovoltaic cells. Electrochemical Properties
Narrow-band-gap H-bonded side-chain polymers were designed as donor–acceptor type materials by using H-donor dyes containing electron-donat-ing carbazole and triphenylamine moieties and electron-withdrawing cyano moieties. To under-stand the energy band structures of these new narrow-band-gap H-bonded polymer complexes for the PSC device application, the electronic states, i.e., highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels, of the H-bonded side-chain poly-mers were investigated by the cyclic voltammetry (CV) measurements. The oxidation and reduction cyclic voltammograms of H-bonded polymer com-plexes P1/S1–P1/S4 and P2/S1–P2/S4 in solid films are displayed in Figure 7. H-bonded polymer complexes P1/S1–P1/S4 and P2/S1–P2/S4 exhib-ited quasi-reversible (or reversible) oxidation and reduction peaks as evident from the areas and close proximity of the anodic and cathodic scans. The onset oxidation and reduction potentials (in solid films) of acceptor polymers P1–P2, H-donor dyes S1–S4, and H-bonded polymer com-plexes P1/S1–P1/S4 and P2/S1–P2/S4 are dem-onstrated Table 3. Ag/AgCl was served as a refer-ence electrode, and it was calibrated by ferrocene (E1/2ferrocene ¼ 0.45 mV vs. Ag/AgCl). The HOMO
and LUMO energy levels were estimated by the oxidation and reduction potentials from the refer-ence energy level of ferrocene (4.8 eV below the vacuum level) according to the following equation: EHOMO/LUMO¼ [(Eonset 0.45) 4.8] eV.
Accord-ing to the previous estimation, the HOMO and LUMO energy levels as well as the band-gap values directly measured from CV (Eg,cv) of all
compounds are also summarized in Table 3. As can be seen, all band-gap values of Eg,cvhad
the analogous sequences as confirmed by the opti-cal band-gap values observed from UV-vis spectra (Eg,opt). By using the H-donor dyes with electron
donor-acceptor effects, H-bonded polymer com-plexes P1/S1–P1/S4 and P2/S1–P2/S4 displayed narrower band-gaps (0.68 eV smaller in Eg,optand
Figure 6. UV-visible absorption spectra of (a) bonded polymer complexes P1/S1–P1/S4 and (b) H-bonded polymer complexes P2/S1–P2/S4 in solid films.
0.51 eV smaller in Eg,cv) compared to H-acceptor
polymers P1–P2 (refer Table 3). However, due to the dilution results of H-acceptor polymers P1– P2, H-bonded polymer complexes P1/S1–P1/S4 and P2/S1–P2/S4 presented wider band-gaps in contrast to H-donor dyes S1–S4. It is worthwhile to note that the H-bonded complexes containing bithiazole-linked dyes (S3 and S4) have lower op-tical band-gaps (Eg,cv and Eg,opt) than the
corre-sponding H-bonded complexes containing fluo-rene-linked dyes (S1 and S2), which was origi-nated from the smaller optical band-gaps of bithiazole-linked dyes (S3 and S4) in solid films. Compared with H-acceptor polymers P1–P2 and H-donor dyes S1–S4, the medium HOMO and LUMO energy levels of H-bonded polymer com-plexes (P1/S1–P1/S4 and P2/S1–P2/S4) could be
adjusted. Therefore, the electrochemical reduc-tions of H-bonded complexes showed similar LUMO energy levels at about (2.92)(3.04) eV, which represented to possess high electron affin-ities and also make these H-bonded complexes suitable donors for electron injection and trans-porting to PCBM acceptors (with 0.71–0.83 eV off-sets in LUMO levels regarding PCBM with a LUMO level of 3.75 eV as shown in Fig. 8)23(b) for the polymeric bulk heterojunction solar cell devices. On the basis of the oxidation potential data, the introduction of electron-withdrawing cyano groups in H-donor dyes to the H-bonded complexes can induce the decreases of HOMO energy levels at about (5.36)(5.52) eV, which represented to possess high hole transporting properties and also make these H-bonded com-plexes suitable donors for hole injection and transporting to PEDOT:PSS layer and then to ITO electrode (with 0.06–0.22 eV offsets in HOMO levels regarding PEDOT:PSS layer with a HOMO level of 5.3 eV as illustrated in Fig. 8)23(b)for the polymeric bulk heterojunction solar cell devices.54 Thus, the electrochemical proper-ties of H-bonded complexes could be adjusted by introducing electron-withdrawing cyano groups and electron-donating amine groups of H-donor dyes to the H-bonded complexes, which can reduce the HOMO energy levels and increase the LUMO energy levels of the H-bonded side-chain polymers, and thus, to have narrower band-gaps. Photovoltaic Cell Properties
The design and syntheses of the H-bonded side-chain polymers P1/S1–P1/S4 and P2/S1–P2/S4 is to utilize new narrow band-gap H-donor dyes self-assembled with side-chain conjugated H-acceptor polymers into supramolecular polymeric struc-tures for the PSC applications. To investigate the potential use of H-bonded complexes in PSCs, bulk heterojunction PSC devices with a configura-tion of ITO/PEDOT:PSS/H-bonded polymer com-plexes:PCBM (1:1 w/w)/Ca/Al were fabricated from an active layer where H-bonded complexes were blended with a complementary fullerene-based electron acceptor PCBM in a weight ratio of 1:1 (w/w) initially (and later followed with various weight ratios for the optimum H-bonded polymer complex). The PSC devices were measured under AM 1.5 illumination for a calibrated solar simula-tor with an intensity of 100 mW/cm2. The prelimi-narily photovoltaic properties are summarized in Table 4, and the typical I–V characteristics of all
Figure 7. Cyclic voltammograms of H-bonded poly-mer complexes (a) P1/S1–P1/S4 and (b) P2/S1–P2/S4.
PSC devices are shown in Figure 9. Under the white-light illumination, the short circuit current density (Isc), open circuit voltage (Voc), fill factor
(FF), and PCE values of the PSC devices com-posed of H-bonded polymer complexes were in the range of 0.42–3.17 mA/cm2, 0.38–0.59 V, and 24–
34%, 0.06–0.50%, respectively.
The photovoltaic properties of the PSC devices containing H-bonded polymer complexes P1/S1– P1/S4 and P2/S1–P2/S4 were dependent on the solubility and film-forming quality of the bonded complexes. However, the PCE values of
bonded complexes P2/S1–P2/S4 containing H-acceptor copolymer P2 were apparently smaller than those of P1/S1–P1/S4, respectively, because the 1:1 M ratio of pyridyl and acid units of fully H-bonded P2/S1–P2/S4 would reduce the content of low-band-gap dyes complexed with H-acceptor copolymer P2 bearing 50 mol % of pyridyl units. As shown in Table 4, both series of H-bonded com-plexes (P1/S1–P1/S4 and P2/S1–P2/S4) contain-ing electron donors of end-cappcontain-ing triphenyl-amine dyes (S1 and S3) had better PCE values than those containing end-capping carbazole dyes
Table 3. Electrochemical Potentials and Energy Levels of H-Acceptor Polymers (P1–P2), H-Donor Dyes (S1–S4), and H-Bonded Polymer Complexes (P1/S1–P1/S4 and P2/S1–P2/S4)
Compound konset,abs(nm)a Eg,opt(eV)a Eox(eV)b HOMO (eV)c Ere(eV)b LUMO (eV)c Eg,cv(eV)
P1 445 2.79 1.18 5.53 1.67 2.68 2.85 P2 445 2.79 1.17 5.52 1.65 2.70 2.82 S1 580 2.14 0.96 5.31 1.47 2.88 2.43 S2 587 2.11 1.04 5.39 1.40 2.95 2.44 S3 622 1.99 1.00 5.35 1.33 3.02 2.33 S4 620 2.00 1.03 5.38 1.30 3.05 2.33 P1/S1 552 2.25 1.01 5.36 1.43 2.92 2.44 P1/S2 558 2.22 1.09 5.44 1.40 2.95 2.49 P1/S3 584 2.12 1.01 5.36 1.33 3.02 2.34 P1/S4 586 2.11 1.04 5.39 1.32 3.03 2.36 P2/S1 554 2.24 1.09 5.44 1.42 2.93 2.51 P2/S2 557 2.23 1.17 5.52 1.37 2.98 2.54 P2/S3 581 2.13 1.03 5.38 1.31 3.04 2.34 P2/S4 589 2.11 1.08 5.43 1.35 3.00 2.42
aAbsorption wavelengths obtained in solid films and optical band-gaps calculated from the equation of E
g,opt¼ 1240/kedge. bOnset oxidation and reduction potentials.
cE
HOMO/ELUMO ¼ [(Eonset0.45)4.8] eV where 0.45 V is the value of ferrocene versus Ag/Agþand 4.8 eV is the energy
level of ferrocene below the vacuum.
Figure 8. Energy band diagram with HOMO/LUMO levels of H-bonded polymer complexes P1/S1–P1/S4 (as electron donors) and PCBM (as an electron acceptor) in relation to the work functions of ITO, PEDOT:PSS, and Al (HOMO value of PCBM was obtained from ref. 23). [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]
(S2 and S4), respectively. It might be due to the larger aggregations of end-capping carbazole dyes (S2 and S4) to reduce the PCE values, which were confirmed by the redder shifted maximum absorption wavelengths (Dkabs
¼ kabs,solid kabs,solution) in solid films of
end-cap-ping carbazole dyes (Dkabs ¼ 16 nm for S2 and
Dkabs¼ 30 nm for S4) than those of end-capping
triphenylamine dyes (Dkabs ¼ 11 nm for S1 and
Dkabs ¼ 23 nm for S3), respectively. Among the
PSC devices containing H-bonded polymer com-plexes, those composed of H-donor dye S3, i.e., P1/S3 and P2/S3, had the best photovoltaic per-formance with enhanced Isc values in the
corre-sponding H-bonded complexes P1/S1–P1/S4 and P2/S1–P2/S4, respectively, which might be due to the promoted solubility and better film-forming capability of S3. Ideally, the Iscvalues were
deter-mined by the product of the photoinduced charge carrier densities and the charge carrier mobilities within the organic semiconductors.11Thus, it can be recognized that the better results of Iscand FF
in the PSC device containing P1/S3 and P2/S3 were obtained likely due to the well-balanced charge flow and less significant recombination loss24,55 originated from the highly order struc-tural packing of alkyl side chains. However, the relatively low Isc and FF values in the PSC
de-vices containing P1/S4 and P2/S4 were poorly understood at this time, but it might be related to the largest aggregation of S4 (with the reddest
shifted maximum absorption wavelengths in solid,Dkabs¼ 30 nm) and geminate charge
recom-bination at the interface due to stable charge-transfer states, which limited the values of the photocurrents.56 Though Voc values were related
to the differences between the HOMO energy lev-els of the polymers and the LUMO energy levlev-els of the acceptors,57but it was not noticeably varied among the PSC devices containing H-bonded com-plexes. In addition, the I–V curves and photovol-taic properties of dyes S1–S4 without complexa-tion with the polymers are illustrated in Figure S4 and Table S3 (refer the Supporting Informa-tion), which can be compared with the PCE values
Table 4. Photovoltaic Properties of PSC Devices Containing an Active Layer of H-Bonded Polymer Complexes:PCBM¼ 1:1 (w/w) with a Device Configuration of ITO/PEDOT:PSS/H-Bonded Polymer Complexes:PCBM/Ca/Ala Active Layerb H-Bonded Complexes: PCBM Voc (V) Isc (mA/cm2) FF (%) PCE (%) P1/S1c 0.59 1.67 27 0.28 P1/S2 0.54 1.72 28 0.26 P1/S3 0.47 3.17 34 0.50 P1/S4 0.43 1.70 26 0.19 P2/S1 0.58 0.95 24 0.13 P2/S2 0.53 0.42 27 0.06 P2/S3 0.51 2.29 28 0.32 P2/S4 0.38 0.73 24 0.07
aMeasured under AM 1.5 irradiation, 100 mW/cm2. bH-Bonded Polymer Complexes:PCBM¼ with the fixed
weight ratio of 1:1 (w/w).
cPCE of P1/S1P: 0.16%.
Figure 9. I–V curves (under simulated AM 1.5 solar irradiation) dependencies of PSC devices with an active layer of blended (a) H-bonded polymer com-plexes P1/S1–P1/S4:PCBM (1:1 w/w) and (b) H-bonded polymer complexes P2/S1–P2/S4:PCBM (1:1 w/w).
shown in Table 4. In general, the H-bonded poly-mer complexes containing H-acceptor polypoly-mers (P1 and P2) have higher PCE values for the or-ganic solar cells, even though the corresponding dye contents of S1–S4:PCBM¼ 1:1 (w/w) without complexation with polymers in the active layer were almost doubled than those of H-bonded poly-mer complexes (H-bonded polymer complex-es:PCBM¼ 1:1 w/w). Hence, H-acceptor polymers (P1 and P2) do really improve and facilitate the fabrication of solar cells. To demonstrate the con-tribution of supramolecular structures in H-bonded polymer complexes, one more PSC device containing an active layer of physical blend P1/ S1P (without H-bonds) has been fabricated to compare their photovoltaic properties with those of H-bonded polymer complex P1/S1. Compared with H-bonded polymer complex P1/S1 (PCE ¼ 0.28%), physical blend P1/S1P (without H-bonds) has a smaller PCE value (0.16%) in Figure S4 and Table S5 (refer the Supporting Information). The larger aggregations of the acid-protected dye S1P occurred in the polymer blend P1/S1P due to the lack of H-bonding interactions, which also can be confirmed by the red-shifted (33 nm) absorption of polymer blend P1/S1P in contrast to that of H-bonded polymer complex P1/S1 in solid films.
Since the best performance of PSC device (with the highest PCE value in Table 4) was fabricated by the blend of H-bonded polymer complex P1/ S3:PCBM (1:1 w/w), the current–voltage charac-teristics of PSC devices as a function of the weight ratio in H-bonded complex and PCBM were sur-veyed, and their photovoltaic properties are
shown in Figure 10 and Table 5. The optimum photovoltaic performance with the maximum PCE value of 0.50% (Isc¼ 3.17 mA/cm2, Voc¼ 0.47
V, FF¼ 34%) was obtained in the PSC device hav-ing a weight ratio of P1/S3:PCBM ¼ 1:1. Using lower weight ratios of PCBM in blended H-bonded polymer complex P1/S3:PCBM (2:1 w/w) led to the reduction in Isc values due to the inefficient
charge separation and electron transporting prop-erties by the possibly increased aggregation of H-bonded complex P1/S3, resulting in the lower PCE results.58 However, loading larger weight ratios of PCBM in blended H-bonded polymer complex P1/S3:PCBM (1:2 and 1:4 w/w) also reduced the Isc and PCE values, which could be
probably attributed to the increased aggregation of PCBM so as to affect the separation of charges. Moreover, an unbalanced charge transporting property would be introduced due to the large PCBM ratio. Hence, both Isc and PCE values
decreased with larger PCBM molar ratios of 1:2 and 1:4 (w/w) because of the two reasons described here.59 Overall, the PSC device fabri-cated by H-bonded polymer complexes P1/ S3:PCBM (1:1 w/w) reached the highest PCE of 0.50%, with a short circuit current density (Isc) of
3.17 mA/cm2, an open circuit voltage (Voc) of 0.47
V, and a fill factor (FF) of 0.34.
CONCLUSIONS
In conclusion, novel supramolecular side-chain polymers (i.e., H-bonded polymer complexes) were
Figure 10. I–V curves of PSC devices containing an active layer of H-bonded polymer complex P1/ S3:PCBM (w/w) with different weight ratios under simulated AM 1.5 solar irradiation.
Table 5. Photovoltaic Parameters for Bulk-Heterojunction PSC Devices Containing Different Weight Ratios of Blended H-Bonded Polymer Complex P1/S3:PCBMa Weight Ratios of Blended H-Bonded Complex P1/S3:PCBM Voc (V) Isc (mA/cm2) FF (%) PCE (%) 2:1 0.46 2.90 20 0.40 1:1 0.47 3.17 34 0.50 1:2 0.45 2.23 30 0.28 1:4 0.44 1.70 33 0.25
aPSC devices with the configuration of ITO/PEDOT:PSS/
H-Bonded Polymer Complex P1/S3:PCBM/Ca/Al containing an active blended layer composed of various weight ratios of H-bonded polymer complex P1/S3 and PCBM were meas-ured under AM 1.5 irradiation, 100 mW/cm2.
constructed by complexation of pyridyl H-acceptor polymers with low-band-gap H-donor dyes in a molar ratio of 1:1 for pyridyl and acid units, which would have much more uptaken loads of photovol-taic dyes in the supramolecular polymeric struc-tures compared with the normal polymer blends. Because of the lack of supramolecular interac-tions, the larger aggregations of the acid-pro-tected dyes occurred in the polymer blends, and thus a polymer blend (without H-bonds) contain-ing conjugated H-acceptor polymer P1 and acid-protected dye S1P illustrated an obvious reduc-tion in the PCE value in contrast to the supramo-lecular analogue P1/S1. H-donor dyes (S1–S4) and H-acceptor polymers (P1 and P2) were uti-lized to control the mesomorphic, photophysical, and photovoltaic properties effectively by the con-cept of supramolecular architecture. The supra-molecular architectures of H-bonded side-chain polymers were also confirmed by FTIR and XRD measurements. The nematic phase was observed by the introduction of various H-donor dyes and H-acceptor polymers with corresponding supra-molecular side-chain structures. In addition, com-pared with H-donor dyes, the optical properties demonstrated that blue-shifted absorptions occurred in these bonded complexes as the H-donor dyes were complexed with H-acceptor polymers. Thus, the electrochemical properties of H-bonded complexes were adjusted by introducing withdrawing cyano groups and electron-donating amine groups of donor dyes to the H-bonded complexes, which could reduce the HOMO energy levels and increase the LUMO energy lev-els of the H-bonded side-chain polymers, and thus, to have narrower band-gaps than H-acceptor polymers. Because of the reduced content of low-band-gap dyes complexed with H-acceptor copolymer P2, the PCE values of H-bonded com-plexes P2/S1–P2/S4 containing H-acceptor copol-ymer P2 were apparently smaller than those of P1/S1–P1/S4, respectively. Preliminary PSC de-vices based on these H-bonded polymer complex P1/S3 blended with PCBM acceptors (1:1 w/w) had the PCE up to 0.50%, which gave the best performance with the values of Isc¼ 3.17 mA/cm2,
Voc¼ 0.47 V, and FF ¼ 34%.
The authors are grateful to the National Center for High-performance Computing for computer time and facilities. The powder XRD measurements are sup-ported by beamline BL17A (charged by J.-J. Lee) of the National Synchrotron Radiation Research Center (NSRRC), in Taiwan. The financial supports of this pro-ject provided by the National Science Council of Taiwan
(ROC) through NSC 97-2113M-009-006-MY2, National Chiao Tung University through 97W807, and Energy and Environmental Laboratories (charged by C.-C. Yang) in Industrial Technology Research Institute (ITRI) are acknowledged.
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