Article pubs.acs.org/Macromolecules
TPD-Based Copolymers with Strong Interchain Aggregation and High Hole Mobility for Efficient Bulk Heterojunction Solar Cells Ye Rim Cheon,† Yu Jin Kim,§ Jong-jin Ha,‡ Myeong-Jong Kim,‡ Chan Eon Park,*,§ and Yun-Hi Kim*,† †
Department of Chemistry & ERI and ‡School of Materials Science & ERI, Gyeongsang National University, Jin-ju 660-701, Republic of Korea § POSTECH Organic Electronics Laboratory, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea S Supporting Information *
ABSTRACT: A−D−A conjugated polymer, PTPD-TVT, containing thienopyrroledione and thiophene−vinylene−thiophene (TVT) units was synthesized as an electron donor for organic photovoltaic devices. It possesses a small bandgap and has excellent coplanarity and high hole mobility. To further enhance the interchain interactions between the polymer chains, a selenophene−vinylene−selenophene (SVS) unit was also introduced and copolymerized to form the PTPD-SVS polymer. Devices made from PTPD-TVT and PTPD-SVS have rather promising power conversion efficiencies (PCEs) of 4.87 and 5.74%, respectively. The higher PCE value for solar cells based on PTPD-SVS was attributed to an enhanced carrier mobility resulting from stronger interchain aggregation in the BHJ active layer. These results show that the incorporation of a vinylene unit in TPD-based polymers is an effective way to reduce the bandgap and thereby improve charge transport for efficient photovoltaic devices.
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INTRODUCTION The need to develop high-performance polymer solar cells (PSCs) as renewable and eco-friendly energy resources has fostered significant progress in the power conversion efficiency (PCE) of PSCs over the past decade.1,2 The most promising architecture of PSCs is the donor−acceptor bulk heterojunction (BHJ) structure, typically consisting of a photoactive conjugated polymer and fullerene derivative.3,4 To date, single-junction BHJ PSCs with PCEs up to 9.48% have been realized.5 Although the molecular design of new semiconducting materials has played a key role in enhancing of the efficiency of PSC devices, it is still important to further improve their light-harvesting and charge-transporting properties to use them in practical applications. In particular, there is a need to develop polymers with more rigidified structures to reduce reorganization energy to improve charge mobility.6,7 In theory, high charge mobility of a polymer donor can accelerate carrier transport in the active layer of a PSC and decrease the recombination of holes and electrons, allowing realization of higher PCE.8,9 Among various donor materials developed for BHJ devices, conjugated polymers incorporating a vinylene bridge unit are quite attractive because of their superior optical and electrical properties.10,11 Results from Jo, Kim, and Marks have shown that coplanarity of the vinylene group facilitates strong intermolecular interactions with a large overlapping area, © XXXX American Chemical Society
providing an effective way to reduce their band gap. Additionally, the more rigid structures of these polymers can also prevent rotational disorder, thereby reducing reorganization energy, which may contribute to enhancing charge separation and carrier mobility.12−15 The incorporation of a thieno[3,4-c]pyrrole-4,6-dione (TPD) unit into the polymer backbone is known to induce strong intra- and interchain interactions along and between chain backbones, affording good electron delocalization in the solid state due to its relatively compact, symmetrical, and planar structure.16−18 Thus, conjugated polymers containing TPD units are expected to have enhanced charge transport properties compared with polymers without a TPD moiety. Here, we report the synthesis and photovoltaic properties of a highly planar conjugated polymer composed of TPD and TVT, poly[(N-decylheptadecylthieno[3,4-c]pyrrole-4,6-dione)co-thiophene−vinylene−thiophene (PTPD-TVT), and compare the properties of PTPD-TVT with those of poly[(Ndecylheptadecylthieno[3,4-c]pyrrole-4,6-dione)-co-selenophene−vinylene−selenophene (PTPD-SVS), a TPD-based polymer with SVS, to examine the effect of the selenophene on the photovoltaic properties of conjugated polymers. When a solar cell device was fabricated from the PTPD-TVT:PC71BM Received: September 12, 2014 Revised: November 16, 2014
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dx.doi.org/10.1021/ma501888z | Macromolecules XXXX, XXX, XXX−XXX
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Scheme 1. Synthesis and Molecular Structures of the Monomers and Two Conjugated Polymers
and PTPD-SVS:PC 71BM blend, the PTPD-SVS device exhibited a higher PCE value of 5.74%. The higher value of PCEs for solar cells based on PTPD-SVS was attributed to the relatively higher value of carrier mobility due to stronger interchain aggregation in the BHJ active layer.
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RESULTS AND DISCUSSION Synthesis and Thermal Properties. Synthetic routes for all monomers and copolymers are shown in Scheme 1. NDecylheptadecylthienopyrrole-4,6-dione was obtained by condensation of thieno[3,4-c]furan-1,3-dione and the decylheptadecylamine synthesized by a Gabriel amine synthesis reaction. The polymers were synthesized by a Stille coupling reaction using Pd2(dba)3 and P(o-tol)3. The structures of PTPD-TVT and PTPD-SVS were confirmed by H NMR and FT-IR. Both polymers had good solubility in common organic solvents such as o-dichlorobenzene, chlorobenzene, chloroform, toluene, and tetrahydrofuran due to their long alkyl side chains. The weightaverage molecular weights (Mw) of the polymers PTPD-TVT and PTPD-SVS were determined to be 24.7 and 31.4 kDa, with corresponding polydispersity indices (PDIs) of 1.52 and 1.45, respectively, as estimated by gel permeation chromatography (GPC) using polystyrene as a standard. In the fabrication of solution-processed solar cells, the thermal stability of the donor material is of great importance. Thus, thermogravimetric analysis (TGA) was used to evaluate the thermal properties of the resulting copolymers. As shown in Figure 1, the onset decomposition temperatures with 5% weight loss (Td) of copolymers PTPD-TVT and PTPD-SVS were 435 and 417 °C, respectively, indicating sufficient thermal stability of the copolymers.19 Optical Properties. One of the major properties of copolymers assessed in terms of their application as photoactive materials for solar cells is their absorption coefficient in the visible and near-IR ranges. The electronic absorption properties of the copolymers synthesized were measured both in chloroform solution and as thin films spin-coated onto quartz
Figure 1. Thermogravimetric analysis plots of the TPD-based polymers.
substrates, as shown in Figures 2a (PTPD-TVT) and 2b (PTPD-SVS). The results of the spectral data are presented in Table 1. As shown in Figure 2a,b, the PTPD-TVT solution exhibited a maximum absorption peak (λmax) at 480 nm, whereas the λmax for PTPD-SVS was significantly red-shifted, to 619 and 701 nm due to the stronger interchain aggregates resulting from the selenophene unit.20 A red-shift in the absorption spectra of polymer films from those of the corresponding solutions is commonly observed in conjugated polymers due to aggregation of the polymer main chains and enhanced interchain interactions in the solid state.21,22 The optical band gaps (Eopt g ) of the PTPD-TVT and PTPD-SVS films were determined to be 1.72 and 1.58 eV, respectively, by measuring their UV−vis absorption onsets. This appeared to demonstrate that, consistent with earlier studies and experimental results, the replacement of thiophene with selenophene resulted in a reduction in the optical band gap due to an increase in the quinoid character of the polymer backbone.23,24 PL spectra of the pristine polymers and polymers blended with PC71BM were examined for the charge transfer process B
dx.doi.org/10.1021/ma501888z | Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. UV−vis absorption spectra of copolymers (a) PTPD-TVT and (b) PTPD-SVS. (c) Photoluminescence spectra of PTPD-TVT, PTPDSVS, PTPD-TVT:PC71BM, and PTPD-SVS:PC71BM films.
Table 1. Photophysical and Electronchemical Properties of the Conjugated Polymers UV−vis absorption spectrum
cyclic voltammetry
polymer
λmax (nm) solution
λmax (nm) film
λonset (nm) film
a Eopt g (eV)
Eox onset (eV)
EHOMO (eV)
ELUMO (eV)b
PTPD-TVT PTPD-SVS
480 619, 701
486 631, 705
723 787
1.72 1.58
0.92 0.86
−5.28 −5.22
−3.56 −3.64
Estimated from the onset of the absorption in the thin films (Eg = 1240/λonset eV). bCalculated using the optical band gap and HOMO energy level of the polymers. a
Figure 3. (a) Cyclic voltammogram of PTPD-TVT and PTPD-SVS thin films. (b) Energy-level diagrams of PTPD-TVT and PTPD-SVS.
highest occupied molecular orbital (HOMO) energy level of the copolymers was calculated based on the equations HOMO = −(Eox onset − ferroceneonset) − 4.8 eV, and the resulting HOMO energy levels of PTPD-TVT and PTPD-SVS were −5.28 and −5.22 eV, respectively (ferroceneonset is the onset oxidation potential of ferrocene, 0.44 eV, used as a reference26). The lowest unoccupied molecular orbital (LUMO) levels were determined to be −3.56 and −3.64 eV for PTPD-TVT and PTPD-SVS, respectively, based on the calculated HOMO values and the optical band gap. Detailed electrochemical properties are summarized in Table 1. By incorporating a selenium moiety into the polymer chain, the LUMO energy level of the resulting copolymer PTPD-SVS decreased by 0.08 eV with respect to its counterpart, PTPDTVT. This observation was consistent with the properties of other reported conjugated polymers systems with a selenophene unit.27,28 To help understand the geometric and electronic properties of these conjugated polymers, density functional theory (DFT) calculations were examined. Becke’s three-parameter gradientcorrected functional (“B3LYP”) with a polarized 6-31G** basis
from polymers to PC71BM. Figure 2c compares the PL spectra of PTPD-TVT, PTPD-SVS, PTPD-TVT:PC71BM, and PTPDSVS:PC71BM composites in solid thin-film states with a 1:4 weight ratio. The PL measurements were made by excitation at a wavelength based on their optical absorption peaks. PTPDTVT showed a strong PL emission band with an emission maximum at 572 nm, whereas PTPD-SVS exhibited a redshifted emission band at 742 nm. Upon addition of PC71BM, the emission bands for both PTPD-TVT and PTPD-SVS were quenched almost completely. The highly efficient photoluminescence quenching in both copolymers suggests efficient photoinduced charge transfer from the polymer to PCBM.25 Electrochemical Properties. The electrochemical properties of the copolymers were evaluated by cyclic voltammetry (CV) measurements, and the corresponding characteristics are shown in Figure 3a. As shown, the CV waves revealed that the two copolymers showed a strong oxidation peak (p doping) as a result of the electron-donating thiophene and selenophene segments. For PTPD-TVT and PTPD-SVS, the oxidation onset + potentials (Eox onset) referred to ferrocene/ferrocenium (Fc/Fc ) were determined to be 0.92 and 0.86 V, respectively. The C
dx.doi.org/10.1021/ma501888z | Macromolecules XXXX, XXX, XXX−XXX
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Figure 4. (a, c) Current density−voltage (J−V) curves of the PSCs based on polymer/PC71BM (1:4, w/w) without or with 2% DIO (v/v) as additive under the illumination of AM 1.5G (100 mW/cm2). (b, d) External quantum efficiency (EQE) curve of the corresponding devices.
Table 2. Device Performance Parameters for BHJ PSCs Based on PTPD-TVT and PTPD-SVS
a
polymer:PCBM
condition
Voc (V)
Jsc (mA/cm2)
FF (%)
best PCE (%)
av PCE (%)a
PTPD-TVT:PC71BM PTPD-TVT:PC71BM PTPD-SVS:PC71BM PTPD-SVS:PC71BM
as-cast DIO 2% v/v as-cast DIO 2% v/v
0.87 0.87 0.84 0.85
9.8 11.4 10.5 12.8
46.2 49.2 49.8 52.8
3.94 4.87 4.39 5.74
3.89 4.68 4.32 5.61
Average values from eight devices.
blend ratio influenced device performance significantly. Thus, we carefully changed the polymer:PC71BM blend ratio from 1:1 to 1:4 (w/w) using a concentration of 40 mg/mL in chlorobenzene (CB). Active layers PTPD-TVT:PC71BM and PTPD-SVS:PC71BM, both with a weight ratio of 1:4 (w/w), showed the best performance, with open-circuit voltages (Voc) of 0.86 and 0.83 V, short-circuit currents (Jsc) of 6.7 and 7.9 mA cm−2, fill factors of 44.4 and 46.6%, and PCEs of 2.56 and 3.20%, respectively. Moreover, the material concentration was adjusted to provide a higher PCE of 3.94% for PTPDTVT:PC71BM and a PCE of 4.39% for PTPD-SVS:PC71BM. Based on these results, the key current density−voltage (J−V) curves are shown in Figure 4a, and the corresponding performance parameters are summarized in Table 2. Because PTPD-TVT has a lower HOMO energy level than PTPD-SVS and PTPD-TVT do, it produced slightly higher VOC values in the solar cell devices.30 However, the higher PCE obtained in the PTPD-SVS:PC71BM device was due mainly to the improved JSC and FF values. JSC and FF depend strongly on film morphology, nanostructural order, and charge carrier mobility, as discussed below. The external quantum efficiency (EQE) spectra of these devices are shown in Figure 4b. It is apparent that the photovoltaic devices exhibited efficient photon conversion properties, with a broad response from 300 to 900 nm. The higher and broader coverage of EQE for
was used for a full geometric optimization. The alkyl chains were replaced by methyl groups as a reasonable simplification to save computational time. As shown in Figure S5, the πconjugated orbital densities of the HOMO and LUMO in the PTPD-TVT polymer were found to extend along the entire length of the backbone. In PTPD-SVS, however, the orbital density of the HOMO was located predominantly on the selenophene−vinylene−selenophene unit, whereas the orbital density of the LUMO has TPD character. These calculations demonstrate that effective intramolecular charge transfer can occur in the PTPD-SVS when excited by light energy. The optimized geometries of both oligomers were also investigated. Both PTPD-TVT and PTPD-SVS exhibited highly planar conformations with very small torsion angles of
