Synthesis, Characterization, and Photovoltaic Properties of 4,8

Apr 7, 2015 - Synthesis, Characterization, and Photovoltaic Properties of 4,8-Dithienylbenzo[1,2-b:4,5-b′]dithiophene-Based Donor–Acceptor Polymer...
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Synthesis, Characterization, and Photovoltaic Properties of 4,8Dithienylbenzo[1,2‑b:4,5‑b′]dithiophene-Based Donor−Acceptor Polymers with New Polymerization and 2D Conjugation Extension Pathways: A Potential Donor Building Block for High Performance and Stable Inverted Organic Solar Cells Nallan Chakravarthi,† Kumarasamy Gunasekar,† Chang Su Kim,‡ Dong-Ho Kim,‡ Myungkwan Song,*,‡ Young Geun Park,§ Jin Yong Lee,§ Yurim Shin,∥ In-Nam Kang,∥ and Sung-Ho Jin*,† †

Department of Chemistry Education, Graduate Department of Chemical Materials, BK 21 PLUS Team for Advanced Chemical Materials, and Institute for Plastic Information and Energy Materials, Pusan National University, Busan 609-735, Republic of Korea ‡ Surface Technology Division, Korea Institute of Materials Science, Changwon 641-831, Republic of Korea § Department of Chemistry, Sungkyunkwan University, Suwon 440-746, Republic of Korea ∥ Department of Chemistry, The Catholic University of Korea, Bucheon, Republic of Korea S Supporting Information *

ABSTRACT: In all the previously reported 4,8dithienylbenzo[1,2-b:4,5-b′]dithiophene (DTBDT)-based πconjugated polymers, the polymerization and two-dimensional (2D) conjugation extension pathways were through the thiophenes fused to the phenyl core of DTBDT and through the thiophenes linked to the benzene core of DTBDT, respectively (BDT-directed DTBDT). Herein, with the aim of discovering another potential way to introduce the DTBDT motif in the donor−acceptor alternating polymer structure, we first report the synthesis of three new π-conjugated polymers, P1, P2, and P3, with a modified DTBDT building block as a donor unit. This modification results in new polymerization and 2D conjugation extension pathways for the polymers through the thiophenes linked to the benzene core of DTBDT and through the thiophenes fused to the phenyl core of the DTBDT, respectively (dithienylbenzene-directed DTBDT). Although these modified polymerization pathways of DTBDT result in less delocalized conjugation along the dithienylbenzene direction, the optical and electrochemical properties reveal that the electron-donating property of dithienylbenzene-directed DTBDT was strong enough to generate strong intramolecular charge transfer (ICT) and maintain low-lying highest occupied molecular orbital (HOMO) energy levels (−5.21 to −5.28 eV) for high air stability. Inverted organic solar cells (IOSCs) were fabricated with the configuration of ITO/ZnO/polymer:PC71BM/ PEDOT:PSS/Ag. By systematic optimization of the performance of the IOSCs using polar solvent treatment, the IOSCs based on P1, P2, and P3 displayed promising power conversion efficiencies (PCE) of 6.31, 5.65, and 7.10%, respectively, which compare well with the PCE of already reported BDT-directed DTBDT-based polymers. More importantly, the stability of the IOSCs was demonstrated by their retention of 83% PCE after ambient storage for 30 days. These study results revealed the promising potential of the proposed molecular design strategy for introducing new 2D conjugation extension and polymerization pathways for a DTBDT unit for high performance and stable IOSCs. This strategy can be applied to the judicious molecular design of new polymeric materials for achieving high PCE.

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INTRODUCTION

and interfacial properties are important factors for achieving high performance polymer-based OSCs.2−7 Among the various molecular building blocks reported so far, 4,8-dithienylbenzo[1,2-b:4,5-b′]dithiophene (DTBDT) has

Organic solar cells (OSCs), prepared from a blend of electrondonating π-conjugated polymers and electron-accepting fullerene derivatives (like PCBM), have been intensively explored as important features for realizing inexpensive, lightweight, flexible, and large area devices.1 Significant progress has been made in the performance of OSC devices. The design of new πconjugated polymer donor materials, morphological control, © 2015 American Chemical Society

Received: January 19, 2015 Revised: March 27, 2015 Published: April 7, 2015 2454

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Figure 1. Chemical structures of BDT-directed DTBDT-based polymers in the literature (top) and dithienylbenzene-directed DTBDT-based polymers in this work (bottom).

direction than in the dithienylbenzene direction in the crossconjugated DTBDT system by synthesizing two isomeric push−pull small molecules for the dye-sensitized solar cell application.16 This study clearly demonstrated the possibility of incorporating the basic DTBDT unit in the push−pull system in two different directions: (1) via the BDT direction and (2) via the dithienylbenzene direction. Although the modulations of band gaps and energy levels of the polymers based on BDTdirected DTBDT have been well reported,11,12,17 no study of the donor moiety has examined the application of the basic DTBDT unit in a donor−acceptor alternating polymer structure by swapping the polymerization and 2D conjugation extension pathways, which are entirely opposite to the pathways of BDT-directed DTBDT. Swapping the 2D conjugation extension and polymerization pathways of DTBDT backbone is a promising strategy to increase the diversity of the variation of the molecular structures, with the aim of developing efficient π-conjugated polymers. In this scenario, we first report the synthesis of a new dithienylbenzene-directed DTBDT building block possessing polymerization and conjugation extension pathways through the thiophenes linked to the benzene core of DTBDT and through those fused to the phenyl core of the DTBDT unit, respectively. The dithienylbenzene-directed DTBDT was judiciously designed to maintain the 2D conjugation and the

been considered the most efficient donor building block for developing high performance donor−acceptor type π-conjugated polymers.8−10 The DTBDT unit contains four thiophene rings with a benzene core, and it can be functionalized in two vertical directions to fabricate semiconducting materials. Currently, the DTBDT unit is typically introduced into the polymer backbone by the following BDTdirected DTBDT approach: (1) establishing the polymerization pathway through the two thiophenes fused to the phenyl core of DTBDT and (2) establishing the two-dimensional (2D) conjugation extension pathway through the two thiophenes linked to the benzene core of DTBDT. On the basis of this directional approach, Huo et al. performed extensive investigations of the photovoltaic performance of BDT-directed DTBDT-containing polymers and showed a maximum power conversion efficiency (PCE) of 9%.11−15 Even though further expansion of the materials based on BDT-directed DTBDTs is expected to produce exceptional π-conjugated polymers, the structural modification of the overall BDT-directed DTBDTbased polymers was merely limited to the variation of acceptor units used for polymerization. This was because the solubilizing side chains are attached at the α-positions of the linked thiophenes of the DTBDT unit, which hinders any further chemical modification. In addition, Jiang et al. demonstrated that the charge transfer was much stronger in the BDT 2455

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Scheme 1. Synthetic Routes of Dithienylbenzene-Directed DTBDT and DTBDT-Containing Polymers (P1, P2, and P3)

solubility for the polymers by introducing 2-ethylhexylthiophene as a side chain substituent at the available α-positions of the fused thiophenes of DTBDT. In order to tune the absorption and molecular energy levels to meet the requirements of an ideal donor with highly efficient photovoltaic performance, we investigated three acceptor units: 1-(4,6dibromothieno[3,4-b]thiophen-2-yl)-2-ethylhexan-1-one (TTC), 2-ethylhexyl-4,6-dibromothieno[3,2-c]thiophene-2-carboxylate (TT-E), and 2,5-ethylhexyl-3,6-bis(5-bromothiophen-2yl)pyrrolo[3,4-c]-pyrrole-1,4-dione (DPP) to polymerize with dithienylbenzene-directed DTBDT to get three new polymers: poly[(5,5′-(2,6-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2b:4,5-b′]dithiophene-4,8-diyl)bis(thiophene-5,2-diyl))-alt-[1(thieno[3,4-b]thiophen-2-yl)-2-ethylhexan-1-one] (P1), poly[(5,5′-(2,6-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5b′]dithiophene-4,8-diyl)bis(thiophene-5,2-diyl))-alt-[ethylhex-

yl-thieno[3,4-b]thiophene-2-carboxylate] (P2), and poly[(5,5′(2,6-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-4,8-diyl)bis(thiophene-5,2-diyl))-alt-2,5-dihexyldecyl-3,6-bis(5-thiophen-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione (P3), respectively (Figure 1). Herein, by cautious optimization of the bulk heterojunction (BHJ) interface morphology (via solvent treatment), dithienylbenzene-directed DTBDT is used as an excellent electrondonor building block. Despite generating weak charge transfer interaction in the dithienylbenzene direction, this new polymerization pathway of DTBDT afforded high PCE values of 6.31% for P1, 5.65% for P2, and 7.10% for P3 in inverted OSCs (IOSCs). To the best of our knowledge, this is the first report of using dithienylbenzene-directed DTBDT as a building block and of synthesizing its derivatives for the fabrication of IOSC devices. Moreover, the high PCE value of 7.10% for P3 2456

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Figure 2. (a) TGA and UV−vis absorption spectra in (b) CF solution, (c) thin film, and (d) CV of P1, P2, and P3.

was attained via a simple fabrication process without any additional modified layers, which is a desirable feature for costeffective OSCs.

polymers exhibited good solubility in common organic solvents, such as tetrahydrofuran (THF), chloroform (CF), and chlorobenzene (CB). The chemical structures of the synthesized monomers and polymers were confirmed by 1H, 13C NMR spectroscopy (Figures S1−S9 in the Supporting Information), and elemental analysis (EA). The weight-average molecular weight (Mw) of P1, P2, and P3 was 21 900, 24 180, and 45 170 Da with polydispersity index (PDI) of 1.51, 1.52, and 1.74, respectively. The disparity in the Mw of the three dithienylbenzene-directed DTBDT-based polymers may either due to the reactivity of acceptor units or the different solubility of the resulting polymers. The thermal properties of the three polymers were measured by thermal gravimetric analysis (TGA) at a heating rate of 10 °C/min (Figure 2a). The onset of decomposition temperature (Td, corresponding to 5% weight loss) of P1, P2, and P3 was 295, 292, and 412 °C, respectively, indicating their very high thermal stability. Optical and Electrochemical Properties of Newly Designed Dithienylbenzene-Directed DTBDT-Based Polymers. Figures 2b and 2c show the UV−vis absorption spectra of P1, P2, and P3 in dilute CF solution and in thin films, respectively, with the absorption maxima values listed in Table 1. The three polymers showed two distinct features: a π−π* transition at higher energy region and an ICT transition at lower energy region. P1 and P2 exhibited an absorption maximum at a relatively short wavelength compared to P3, which was ascribed to the relatively weak accepting nature of TT-C and TT-E, respectively. The thin film spectra of P1 and P2 exhibited a similar trend to that observed for the solutions; however, the absorption bands were slightly shifted toward the relative longer wavelength region because of the interactions between the polymer chains in the solid state. As anticipated, the absorption maximum and the band gap of P1, P2, and P3 varied with increasing ICT, as the acceptor unit was varied. The optical band gap (Egopt) of P1, P2, and P3 was estimated from

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RESULTS AND DISCUSSION Synthesis and Characterization. The synthetic routes for dithienylbenzene-directed DTBDT and the DTBDT-based polymers (P1, P2, and P3) are outlined in Scheme 1. (2Thienyl)trimethylsilane, TT-C, TT-E, and DPP were synthesized according to the reported procedures.18a−e In our synthetic strategy, compound 2 was synthesized by generating the anion of (2-thienyl)trimethylsilane at the α-position, followed by treatment with dihydrobenzo[1,2-b:4,5-b′]dithiophene-4,8-dione. Then, compound 2 was distannylated and Stille coupled with 2-bromo-5-(2-ethylhexyl)thiophene to form compound 3. Finally, compound 3 was desilylated and then distannylated to afford the target new dithienylbenzenedirected DTBDT (D). Our synthetic strategy for modifying the DTBDT unit allowed us to functionalize at the α-positions of the fused thiophenes and linked thiophenes for 2D conjugation extension and polymerization pathways, respectively. The arrangement of an electron-rich (donor) and an electrondeficient unit (acceptor) in a polymer main chain is a familiar synthetic approach to yield a low band gap (LBG) with rather high charge carrier mobilities, most likely due to the intramolecular charge transfer (ICT). These attractive features focused our attention on further work toward the synthesis of new polymers based on dithienylbenzene-directed DTBDT. Hence, three polymers, P1, P2, and P3, were prepared by polymerizing a new soluble dithienylbenzene-directed DTBDT donor moiety (D) with the acceptor moieties, TT-C, TT-E, and DPP, respectively, under microwave-assisted Stille reaction conditions. The resulting polymers were carefully purified through Soxhlet extraction using methanol, acetone, and hexane, to remove the low molecular weight fractions. All 2457

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eV). This is due to the weak withdrawing strength of TT-C and TT-E, which cannot interact strongly with dithienylbenzenedirected DTBDT, whereas DPP, due to its high withdrawing tendency, interacts strongly with ICT and hence extends the absorption to longer wavelengths for P3 than for P1 and P2. This interesting result suggests that the electron-donating strength of dithienylbenzene-directed DTBDT is comparable to that of BDT-directed DTBDT, which affords it a strong ability to generate efficient ICT between the donor and acceptor units. It is also worth saying that the commonly explored approach to lower the band gap in TT containing polymers is via the quinoid resonance structure stabilization.18f The quinoid resonance form is lower in energy than the aromatic form; thus, the band gap of corresponding polymers is effectively reduced by stabilizing the quinoid form. It has been reported that the quinoid approach is much explored in a series of polymers of alternating BDT-directed DTBDT and TT units and have set excellent PCE in OSCs.8 This noteworthy achievement is attributed to the capability of the quinoid approach in reducing the band gap of the related polymers, whereas the quinoid stabilization is slightly less effective in the case of polymers of alternating dithienylbenzene-directed DTBDT and TT, thus increasing the band gap compared to BDT-directed DTBDT-co-TT polymers. This is due to the less delocalized conjugation along the dithienylbenzene direction that will perturb the whole conjugated backbone in forming

Table 1. Optical and Electrochemical Properties of P1, P2, and P3 polymer

solution λmaxa (nm)

film λmaxb (nm)

Egopt c (eV)

HOMO (eV)

LUMO (eV)

P1 P2 P3

533 523 634

552 550 636

1.74 1.74 1.40

−5.24 −5.21 −5.28

−3.50 −3.47 −3.88

a

Absorption maxima measured from UV−vis absorption spectrum in CF solution. bAbsorption maxima measured from UV−vis absorption spectrum in thin film state. cEstimated from the onset of the absorption in thin films (Egopt = 1240/λonset).

the onset absorption edge of the thin film absorption spectra and found to be 1.74, 1.74, and 1.40 eV, respectively. Some interesting facts were revealed when the Egopt of BDT-directed DTBDT-based polymers (PBDTTT-C-T, PBDTTT-E-T, and PBDTT-DPP)8,9 were compared to that of the dithienylbenzene-directed DTBDT-based polymers (P1, P2, and P3). This comparison was valid since the difference in the connectivity of the donor (DTBDT) to the same acceptor units (TT-C, TT-E, and DPP) will result in different absorption properties for the resultant polymers. The Egopt value of 1.74 eV for both P1 and P2, with TT-C and TT-E as acceptor units, respectively, is larger than that of PBDTTT-C-T (1.58 eV) and PBDTTT-E-T (1.58 eV), whereas the Egopt of 1.40 eV for P3, with DPP as an acceptor unit, is almost the same as that of PBDTT-DPP (1.44

Figure 3. DFT calculated HOMO and LUMO wave functions of the geometry optimized repeating unit structure of P1, P2, and P3. 2458

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of dithienylbenzene-directed DTBDT-based polymers could be effectively fine-tuned by using suitable electron-withdrawing TT-C, TT-E, and DPP groups. Theoretical Calculations. Density functional theory (DFT) calculations with a suite of Gaussian 09 programs21 were performed to further examine the electrochemical properties of P1, P2, and P3. Becke’s three parametrized Lee−Yang−Parr exchange functional (B3LYP) and 6-31G* basis sets were used for geometry optimization (Figure 3). For the corresponding repeating units, the electron density of the LUMO was mainly located on the electron-accepting unit, whereas the electron density of the HOMO was well distributed along their backbones with the exception of P3. The HOMO energies were calculated to be −4.77, −4.76, and −4.76 eV for P1, P2, and P3, respectively, which is in agreement with the experimental results (−5.24, −5.21, and −5.28 eV, respectively). The calculated LUMO energies were −2.06, −1.89, and −2.53 eV for P1, P2, and P3, respectively, which is also in agreement with the experimental results (−3.50, −3.47, and −3.88 eV, respectively) in that the LUMO energies of P1 and P2 are similar, while that of P3 is lower than those of P1 and P2 by about 0.5 eV. As a result, the HOMO− LUMO gap of P3 was smaller than those of P1 and P2. Figure 3 shows that in P3 two oxygen and two nitrogen atoms, which are part of the acceptor unit, have greatly reduced electron density in both LUMO and as well as in HOMO. Replacing the acceptor group’s ketone in P1 with ester in P2 noticeably affects neither the electrochemical property nor the ground state geometry. Theoretical calculations also specify that the HOMO−LUMO gaps decrease with the increasing withdrawing ability of the acceptors groups, which supports the improved electron-accepting ability of DPP relative to that of TT-C and TT-E. This conjecture is further supported by the optical and electrochemical results (Table 1). Considering the superior features such as enhanced ICT, we propose P3 as the best candidate to maximize the performance in organic thin film transistors (OTFTs) and OSCs. OTFT Characteristics of Newly Designed Dithienylbenzene-Directed DTBDT-Based Polymers. To investigate how the acceptor units polymerized with dithienylbenzenedirected DTBDT affect charge transport, the field effect hole mobility (μhole) of thin films of P1, P2, and P3 was measured by fabricating OTFTs. The OTFTs were fabricated on a silicon wafer using bottom contact geometry with a channel length of 12 μm and a width of 120 μm under a N2 atmosphere. Figure S10 shows the OTFT transfer characteristics of the devices fabricated using P1, P2, and P3, which were annealed at 140 °C. Related data are presented in Table S1. The OTFTs made from P1, P2, and P3 exhibited notable (μhole) of 1.6 × 10−5, 6.7 × 10−6, and 2.2 × 10−4 cm2 V−1 s−1 and on/off ratios of 5 × 103, 1 × 103, and 1 × 103, respectively. From these OTFTs, it could be inferred that the dithienylbenzene-directed DTBDTbased polymers possess hole transporting (p-type) characteristics. The μhole values of these polymers indicate a reduced photocurrent loss and hence suggest high performance for the OSCs.22a As expected, DPP-containing polymer P3 exhibited high charge carrier (hole) mobility, which enhances the transportation and collection of free charges in OSCs and is expected to provide high photocurrents and fill factor (FF).22b Since the LUMO level of P3 is −3.88 eV, we also investigated the n-channel transport behavior of P3 by fabricating organic field effect transistor (OFET), and the related data are listed in Table S2. Interestingly, we observed that, as the temperature is

effective quinoid-type resonance structures. Whereas, in the case of P3 with DPP as acceptor unit, the band gap is modulated through the formation of a quinoid resonace via ICT between DPP and the electron-rich donor unit. Hence, the band gap obtained for P3 (1.40 eV) is almost similar to or slightly lower than that of PBDT-DPP, which indicates the better electron donating ability of dithienylbenzene-directed DTBDT. The LBG of 1.40 eV for P3 was achieved even though the conjugation is less delocalized in the dithienylbenzene direction, which is a unique property of this polymer. The DPPcontaining polymer P3 exhibited broader absorption spectra in the long wavelength region than did P1 and P2, probably denoting that higher photocurrent densities may be obtained in the OSCs based on P3 than on the other two polymers. Since the DPP moiety is a stronger acceptor than TT-E and TT-C, this led to a smaller Egopt for P3 than for P1 and P2. Additionally, P3 displayed a shoulder peak adjacent to the absorption maxima in the thin film state, probably due to the formation of intermolecular π−π stacking and the planarization of the backbone, as observed in other DPP-based polymers.19a Overall, the band gaps and absorption profiles of P1, P2, and P3 should be beneficial to their application as photoactive materials in OSCs. The electrochemical properties of dithienylbenzene-directed DTBDT-based polymer films were investigated by cyclic voltammetry (CV), and the results are summarized in Figure 2d and Table 1. All polymers exhibited quasi-reversible processes in the positive potential range and the HOMO energy levels of P1, P2, and P3 were −5.24, −5.21, and −5.28 eV, respectively, which are significantly deeper than that of P3HT (−4.76 eV)19b and close to the ideal HOMO energy level (−5.2 eV), which suggests their high air stability.19c The small difference in HOMO values (∼−5.21 to ∼−5.28 eV) for the polymers can be attributed to the presence of an identical donor unit, since the HOMO for a π-conjugated polymer with alternating donor−acceptor structure is more confined on the donor. In general, the HOMO levels of dithienylbenzenedirected DTBDT-based polymers are higher than those of BDT-directed DTBDT unit because of the less conjugated system in the former type of DTBDT unit, which increases the electron density and results in high-lying HOMO levels. Accordingly, it is expected that the high-lying HOMO levels of the three polymers will result in inferior open-circuit voltage (Voc) of the resultant OSCs. However, in addition to the difference between the HOMO of the donor unit and the lowest unoccupied molecular orbital (LUMO) of the PC71BM derivative, other features such as morphology and recombination dynamics can also affect Voc.19d As it is very difficult to observe the peaks in the negative potential range, the LUMO levels are calculated from Egopt and the HOMO energy levels. The LUMO levels of P1, P2, and P3 were calculated to be −3.50, −3.47, and −3.88 eV, respectively, as estimated from Egopt and the HOMO energy levels. The much lower LUMO value of P3 (∼−3.88 eV) compared to that of P1 (∼−3.50 eV) and P2 (∼−3.47 eV) can be attributed to the much stronger electron-withdrawing ability of the DPP acceptor unit with respect to the TT-E and TT-C moieties. In view of the HOMO and LUMO energy levels, all polymers have sufficient driving force for efficient exciton dissociation with fullerene derivatives and they are all stable in air, which demonstrates the promising potential of the dithienylbenzene-directed DTBDT unit as a donor unit for OSCs with high Voc.20 The aforementioned results indicated that the band gap and molecular energy levels 2459

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Figure 4. Current density−voltage (J−V) curves of the IOSCs with (a) P1, (b) P2, (c) P3 and IPCE spectra of the IOSCs with (d) P1, (e) P2, and (f) P3.

Table 2. Photovoltaic Properties of Polymer:PC71BM BHJ IOSCs at a Blend Ratio of 1:2 polymer P1 P1 P2 P2 P3 P3 a

4% CN

Jsc (mA/cm2)

Voc (V)

FF (%)

Rs (Ω·cm2)

Rsh (Ω·cm2)

no yes no yes no yes

± ± ± ± ± ±

± ± ± ± ± ±

± ± ± ± ± ±

± ± ± ± ± ±

× × × × × ×

10.66 13.66 7.77 11.36 10.35 13.02

0.28 0.20 0.18 0.49 0.25 0.43

0.82 0.82 0.81 0.81 0.84 0.83

0.01 0.01 0.01 0.01 0.01 0.01

42.11 50.81 40.32 49.19 46.42 54.05

1.59 1.09 2.07 0.46 0.13 1.15

18.20 6.26 28.20 7.06 12.02 4.95

0.89 0.21 0.62 0.18 0.93 1.02

4.12 7.65 2.95 3.83 2.71 9.87

4

10 105 104 105 105 105

PCEa (%) 3.68 5.65 2.53 4.53 4.06 5.88

± ± ± ± ± ±

0.01 0.13 0.18 0.13 0.06 0.14

integrated Jsc (mA/cm2) 10.12 13.21 7.70 10.96 10.27 13.18

± ± ± ± ± ±

0.42 0.31 0.10 0.52 0.36 0.28

PCE is average of 10 OSCs.

the importance of dithienylbenzene-directed DTBDT for the OSC application and investigated the photovoltaic properties of P1, P2, and P3 under illumination of a simulated A.M 1.5G (100 mW/cm2). The photoactive layers were spin-coated from the CB solution of the corresponding polymer and PC71BM blend. To optimize the device performance, several device and processing conditions such as polymer to PC71BM ratio, photoactive layer thickness, and processing solvent were systematically optimized. The optimum photovoltaic properties were obtained with the blend ratio of 1:2 w/w + 4% 1chloronaphthalene (CN) for the polymer:PC71BM. The photoactive layers were spin-coated from solutions in CB and CB/CN mixed solvent systems. The current density−voltage (J−V) curves of polymer:PC71BM (1:2), polymer:PC71BM (1:2) + 4% CN, and polymer:PC71BM (1:2) + 4% CN (1butanol treatment) are presented in Figure 4a−c, and the detailed data are collected in Table 2. The photovoltaic data and J−V curves of polymer:PC71BM at different ratios and different solvent treatments are shown in Tables S3, S4 and Figures S13, S14, respectively. In the preliminary investigation, the pristine device based on P3:PC71BM exhibited a PCE of 4.06% with Voc = 0.84 V, short-circuit current density (Jsc) =

increased during the device fabrication based on P3, it showed both electron and hole mobility, which indicates that P3 possess ambipolar character (Figures S11 and S12). As compared with other BDT-directed DTBDT polymers, we postulate an amorphous structure for the P1, P2, and P3, which is well corroborated by the comparable or relatively lower hole mobilities obtained from the OTFT measurements. Photovoltaic Properties and Optimization of Dithienylbenzene-Directed DTBDT-Based IOSCs. Fabrication of IOSCs is an effective approach to overcome the reduced performance of conventional devices due to continual exposure to air, etching of ITO by the acidic PEDOT:PSS, quick oxidation of low work function metal cathode, and sustaining long-term stability.22c If the polarity of charge collection is reversed in a conventional cell, the combination of highly airstable Ag metal with PEDOT:PSS can replace the air-sensitive Al as the anodic contact for effective hole collection. This reversed configuration necessitates the insertion of an inorganic metal oxide (ZnO, TiO2, or TiOx) between ITO and the photoactive layer to work as an electron-receiving contact. In this scenario, we implemented the inverted device structure of ITO/ZnO/polymer:PC71BM/PEDOT:PSS/Ag to demonstrate 2460

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Table 3. Photovoltaic Properties of the BHJ IOSCs Based on Polymer:PC71BM at a Blend Ratio of 1:2 + 4% CN with 1-Butanol Treatment

a

polymer

Jsc (mA/cm2)

Voc (V)

FF (%)

Rs (Ω·cm2)

Rsh (Ω·cm2)

PCEa (%)

integrated Jsc (mA/cm2)

P1 P2 P3

13.86 ± 0.74 13.46 ± 0.40 14.63 ± 0.12

0.84 ± 0.01 0.83 ± 0.03 0.81 ± 0.03

54.31 ± 1.91 50.75 ± 1.15 60.51 ± 0.12

5.23 ± 0.12 6.31 ± 0.19 3.67 ± 0.23

1.03 × 10 5.85 × 105 5.78 × 106

6.31 ± 0.22 5.65 ± 0.13 7.10 ± 0.07

13.36 ± 0.52 13.10 ± 0.39 14.29 ± 0.42

6

PCE is average of 10 OSCs.

10.35 mA/cm2, and FF = 46.42%. The PCEs of the pristine devices of P1 and P2 also showed moderate PCEs of 3.68 and 2.53%, respectively. The higher performance of P3 results from its higher Jsc and FF, which is probably due to its better absorption at the longer wavelength region and higher charge mobility than those of P1 and P2 (Figure 2b and Table S1). The higher Mw of P3 than those of P1 and P2 is also accountable for the higher PCE of P3, which is consistent with the other reports.22d The impressive efficiency for P1 stems from the high FF and Jsc when compared to P2. In light of the favorable effect of a processing additive, such as CN, on the photovoltaic performance of the polymers, we also examined the effect of CN and found a positive effect on the photovoltaic properties of the three polymers.23 As shown in Table 2, the addition of a small amount of CN (4%, v/v) sharply increased the performances of the devices based on P1, P2, and P3, with a high PCE of 5.65, 4.53, and 5.88%, respectively. The enhanced PCE resulted from the gradual increase of the devices’ Jsc and FF, which improved the miscibility with PC71BM and the morphology of the blend films based on dithienylbenzene-directed DTBDT-based polymers. Various research groups have recently demonstrated that the device performance could be significantly enhanced by postdeposition polar solvent treatment to obtain nanoscale morphology for photoactive layer by controlling the phase separation between donor and acceptor.24,25 In this regard, we tried to use three polar solvents (ethanol, 1-butanol, and acetone) to optimize the performance of the IOSCs based on P1, P2, and P3. Surprisingly, after treating the photoactive layer with these three solvents, the blends based on the devices of the three polymers efficiently responded to the 1-butanol treatment, and the PCE of all IOSCs was dramatically enhanced. For example, for P3, the most appropriate solvent was also 1butanol, and the PCE achieved ranged from 4.06 to 7.10% with Jsc ranging from 10.35 to 14.63 mA/cm2 and a FF from 46.42 to 60.51%. Tables 2 and 3 clearly show that the PCE improvement after 1-butanol treatment is due to the higher Jsc and FF because Voc remains almost the same. In earlier research, Jiang et al. reported that the charge transfer interaction is much stronger in the BDT direction than in the dithienylbenzene direction in the isomeric crossconjugated DTBDT-based small molecules.16 On the basis of the charge transfer interaction phenomenon, they showed that the BDT-directed DTBDT isomer possesses LBG and higher PCE than the dithienylbenzene-directed DTBDT isomer. Although a similar phenomenon can be applied to the BHJ OSC systems, the device’s PCE depends upon various other factors such as well-matched energy levels (HOMO/LUMO) of donor materials with fullerene derivatives, BHJ photoactive layer morphology, and charge transfer phenomena. Even though the charge transfer interaction was strong in the case of BDT-directed DTBDT-based polymers, the design strategy of the polymers was restricted to the variation of acceptor units due to the impossibility of DTBDT structural modification. In

contrast, our design strategy clearly differs from the aforementioned strategy, as it has the polymerization pathway through the dithienylbenzene direction and 2D conjugation extension pathway through the BDT direction. Taking into account the weak charge transfer interaction in the dithienylbenzne direction of DTBDT, the polymers might possess inferior properties such as larger band gaps compared to the BDT direction, since the conjugation is less delocalized in the dithienylbenzene direction. Nevertheless, we firmly believe that the band gap issue will not hamper the improvement in PCE, as the literature reports various medium band gap (MBG) polymers with Egopt of ∼1.9 eV delivering excellent PCE with the fine-tuning of other features such as rational molecular design (introduction of F atoms), morphology control, and device engineering approach.26 Based on this premise, even though the charge transfer interaction of DTBDT is weaker in the dithienylbenzene direction than in the BDT direction, by cautious device optimization of the performance of OSCs, we could achieve remarkable PCE (P1: 6.31%; P2: 5.65%; P3: 7.10%) based on the dithienylbenzenedirected DTBDT unit. This supports our molecular design idea of introducing the new polymerization and 2D conjugation pathways of the DTBDT unit to maximize the PCE of the resulting polymers with cautious device optimization. The enhanced PCE originates from improved charge transport, or enhanced efficiency of charge carrier collection, which can all result from the creation of an ideal morphology. This will be discussed in detail in the next sections. As shown in Figure 4d− f, the polymer:PC71BM (4% CN) blend of P1, P2, and P3 covers the external quantum efficiency (EQE) from 300 to 800 nm, which are well matched with their absorption patterns. Interestingly, after treating the photoactive layer with 1-butanol, EQE was considerably increased for all the devices based on P1, P2, and P3. The Jsc values calculated from integration of the EQE spectra were well matched with Jsc obtained from the J−V measurements, considering the common error of 5−10% (Tables 2 and 3). The enhanced performance, especially the increase in FF after solvent treatment, can be attributed to the rise in shunt resistance (Rsh) and lowering of series resistance (Rs) of the device (Table 3). As shown in the dark current J−V curves (Figure S15), all IOSCs with 1-butanol solvent exposure exhibited a higher Rsh and a reduced Rs, compared to without 1butanol treatment. For example, the Rsh values were calculated to be 9.87 × 105 and 5.78 × 106 Ω·cm2 for the pristine and 1butanol-treated IOSCs, respectively. These results demonstrate the excellent diode characteristics that were attained after 1butanol treatment. Hole Mobility. To further understand the origin of enhanced Jsc and PCE after 1-butanol treatment, the holeonly devices with a configuration of ITO/PEDOT:PSS (40 nm)/polymer:PC71BM (80 nm) + 4% CN/PEDOT:PSS (30 nm)/Ag (150 nm) were fabricated in order to estimate the hole mobilities of these polymers by space charge limited current (SCLC) model (Figure 5a).27 The hole mobility curves of 2461

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Figure 5. (a) Schematic diagram of hole-only device structure and (b) current density−voltage (J−V) curve using P1, P2, and P3 with/without 1butanol treatment and Nyquist plots of impedance spectra of (c) P1, (d) P2, and (e) P3 with/without 1-butanol treatment.

the surface morphology and its correlation with enhanced photovoltaic performance after 1-butanol treatment. The AFM images of polymer:PC71BM + 4% CN and polymer:PC71BM + 4% CN (1-butanol treatment) are shown in Figure 6. The AFM images of polymer:PC71BM + 4% CN (ethanol treatment) and polymer:PC71BM + 4% CN (acetone treatment) are shown in Figure S17. The AFM images revealed that after 1-butanol treatment the blend films of polymer:PC71BM + 4% CN exhibited different morphologies compared to the untreated blend films. The root-mean-square (RMS) roughness values (Figure 6a,c,e) of P1, P2, and P3 blend films were found to be 1.5, 2.2, and 0.9 nm, respectively, whereas after 1-butanol treatment the RMS values (Figure 6b,d,f) of P1, P2, and P3 blend films were decreased to 0.21, 0.18, and 0.20 nm, respectively. Upon 1-butanol treatment, the size of polymer aggregates reduced, and each component appears to be uniformly mixed throughout the BHJ photoactive layer possibly due to the penetration of PC71BM molecules between polymer chains which enhanced Jsc. As FF is determined by charge carriers reaching the electrodes in a blend film, which is related to not only charge transport but also nanomorphology, so the high FF in all polymer:PC71BM devices after 1-butanol treatment can also be attributed to smooth surface and small phase separation.28 Importantly, the desirable morphological change should be related with the much improved photovoltaic performance, hole mobility, and low bulk resistance in the 1butanol-treated polymer:PC71BM devices. To summarize the study results, the molecular structure of dithienylbenzenedirected DTBDT unit is supportive of establishing the ideal film morphology with bicontinuous percolation pathways in the BHJ network, as shown in various BDT-directed DTBDT polymers.8,11 Stability Study. Stability is also a very important parameter for practical OSC usage. Figure S18 shows the PCE of the

polymer:PC71BM + 4% CN blends with and without 1-butanol treatment are shown in Figure 5b, and the data are summarized in Table S5. The J−V characteristics of the hole-only devices of polymer:PC71BM + 4% CN at the blend ratio of 1:2 with ethanol or acetone treatment are shown in Figure S16. The hole mobilities of all polymer:PC71BM + 4% CN blends were enhanced after 1-butanol treatment. The hole mobilities of P1-, P2-, and P3-based devices without 1-butanol treatment were 9.50 × 10−6, 4.92 × 10−6, and 5.91 × 10−5 cm2 V−1 s−1, respectively, rising 2.8-, 2.1-, and 1.7-fold to 2.74 × 10−5, 1.01 × 10−5, and 9.89 × 10−5 cm2 V−1 s−1, respectively, after 1-butanol treatment, which is consistent with the enhanced Jsc and FF resulting in the increased PCE. The larger Jsc for the 1-butanoltreated devices compared to the untreated devices can be attributed to their higher hole mobility. The hole mobility order that was observed in the OTFT method was the same as that for the SCLC method: P3 exhibited higher hole mobilities than P1 and P2. The higher Jsc values of the IOSCs based on P3 can be attributed to its higher hole mobilities. Impedance Spectroscopy Study. The improvement of PCE in (P1, P2, and P3):PC71BM-based IOSCs after 1-butanol treatment was further examined by electrical impedance spectroscopy (EIS). Figure 5c−e shows Nyquist plots of impedance measurements of polymer:PC71BM + 4% CN devices with and without 1-butanol treatment with a frequency range from 1 Hz to 1 MHz. The Nyquist plots clearly show that there is a significant reduction in bulk resistance of all the 1butanol-treated devices, which can be inferred from the diameter of semicircle on the real axis (Re Z) of the impedance curve. Thus, it is confirmed from the EIS study that 1-butanol treatment has considerably reduced interfacial resistance for dithienylbenzene-directed DTBDT-based polymeric devices. Morphological Studies. Atomic force microscope (AFM) images of the polymer:PC71BM films were taken to compare 2462

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Figure 6. AFM images of (a) P1:PC71BM (1:2) + 4% CN, (b) P1:PC71BM (1:2) + 4% CN (1-butanol), (c) P2:PC71BM (1:2) + 4% CN, (d) P2:PC71BM (1:2) + 4% CN (1-butanol), (e) P3:PC71BM (1:2) + 4% CN, and (f) P3:PC71BM (1:2) + 4% CN (1-butanol).

IOSCs based on polymer (P1, P2, P3):PC71BM (4% CN) over a month period. As shown in Figure S18, the IOSCs initially exhibited good performance with a PCE of 5.60, 4.52, and 5.88% for P1, P2, and P3, respectively. After 30 days of air

storage, the devices still exhibited a high PCE of 4.80, 3.90, and 5.10% for P1, P2, and P3, respectively, which indicated a retention of 83% of their highest efficiency. The excellent stability observed for the IOSCs based on P1, P2, and P3 2463

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ACKNOWLEDGMENTS This work was supported by grant fund from the National Research Foundation (NRF) (2011-0028320) and the Pioneer Research Center Program through the NRF (2013M3C1A3065522) by the Ministry of Science, ICT & Future Planning (MSIP) of Korea.

suggests a good potential of dithienylbenzene-directed DTBDT-based semiconducting materials for photovoltaic application.

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CONCLUSIONS In summary, we synthesized the dithienylbenzene-directed DTBDT unit as a new electron-rich building block via functionalization of the available α-positions on linked and fused thiophenes of DTBDT. The dithienylbenzene-directed DTBDT was introduced in donor−acceptor alternating polymer structure to afford three polymers P1, P2, and P3, which possessed polymerization and 2D conjugation extension pathways through the thiophenes linked to the benzene core of DTBDT and through the thiophenes fused to the phenyl core of DTBDT, respectively. The optical properties revealed that the dithienylbenzene-directed DTBDT unit possessed an excellent electron-donating ability capable of producing strong ICT to attain MBG for P1 and P2 and LBG for P3, even though the conjugation was less delocalized in the dithienylbenzene direction than in the BDT direction. Irrespective of the increased electron density in the dithienylbenzene direction, the low-lying HOMO energy levels (−5.21 to −5.28 eV) of P1, P2, and P3 revealed their reasonable air stability. In the initial investigation, pristine polymer:PC71BM devices delivered PCE values of 3.68, 2.53, and 4.06% for P1, P2, and P3, compared to 5.65, 4.53, and 5.88% for polymer:PC71BM + 4% CN devices, respectively. Further morphology optimization of the IOSCs by 1-butanol treatment increased the PCE values to 6.31% for P1, 5.65% for P2, and 7.10% for P3. The positive effects of 1butanol treatment on the enhancement in the Jsc and FF of IOSCs were shown to originate from the increased hole mobilities and the formation of good morphology. The high stability of the IOSCs was demonstrated by their retention of 83% of their highest efficiency after 30 days of air storage. Above all, P1, P2, and P3 with polymerization pathways in the dithienylbenzene direction of DTBDT achieved high PCEs which is not inferior to the already reported polymers that have the polymerization pathways in the BDT direction of DTBDT. These study findings will benefit the future development of dithienylbenzene-directed DTBDT polymers as donor materials for highly efficient IOSCs. Furthermore, their future extension is promising in the fields of rational chemical modification and engineering the interfacial layers and device architectures in order to maximize PCE.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures, 1H and 13C NMR of monomers and polymers, OTFT, SCLC, AFM images, and J−V characterization data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Tel +82 55 280 3686, Fax +82 55 280 3570, e-mail [email protected] (M.S.). *Tel +82 51 510 2727, Fax +82 51 581 2348, e-mail shjin@ pusan.ac.kr (S.-H.J.). Notes

The authors declare no competing financial interest. 2464

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