Indolo[3,2-b]indole-Containing Donor–Acceptor Copolymers for High

Jan 3, 2017 - The organic solar cell (OSC) performance of a series of new donor–acceptor copolymers containing indolo[3,2-b]indole as a key donor bl...
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Indolo[3,2‑b]indole-Containing Donor−Acceptor Copolymers for High-Efficiency Organic Solar Cells Jaeyoung Hwang,†,# Jeonghun Park,∥,# Yu Jin Kim,‡,# Yeon Hee Ha,† Chan Eon Park,‡ Dae Sung Chung,*,§ Soon-Ki Kwon,*,∥ and Yun-Hi Kim*,† †

Department of Chemistry and RIGET, Gyeongsang National University, Jinju 660-701, Korea Department of Materials Engineering and Convergence Technology and ERI, Gyeongsang National University, Jinju 660-701, Korea ‡ POSTECH Organic Electronics Laboratory, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea § Department of Energy System Engineering, DIGIST, Daegu 711-873, Korea ∥

S Supporting Information *

ABSTRACT: The organic solar cell (OSC) performance of a series of new donor−acceptor copolymers containing indolo[3,2b]indole as a key donor block and benzothiadiazole (BT) units with various degrees of fluorination as acceptors is reported. Compared with the simple carbazole unit, the strategically developed indolo[3,2-b]indole unit is found to significantly extend π-conjugation and thus increase the intermolecular interactions of the resulting copolymer, as probed by density functional theory calculations, photophysical studies, and structural/morphological analyses. In addition, fluorination of BT can facilitate nanostructuring of the copolymers, mainly due to further planarization of the backbone, which leads to apparently higher hole/electron charge carrier mobilities. The OSC properties of this series of new copolymers blended with fullerene show a strong dependence on the fine and continuous fibrous nanostructure of the blend film. The indolo[3,2-b]indole-based copolymer with singly fluorinated BT units possesses optimal intermolecular interactions and achieves the highest power conversion efficiency of 8.84% under AM 1.5G illumination. This result shows the potential of π-extended carbazole moieties for achieving high-performance OSCs with many of the favorable properties induced by large heteroacene blocks.

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INTRODUCTION For the past decade, donor−acceptor (D−A)-based conjugated polymers (CPs) have been extensively studied for organic solar cell (OSC) applications, mainly because they are flexible, lightweight, and solution processable.1−6 Many research groups have tried to tune the absorption spectra, electronic band structures, structural/morphological characteristics, and charge carrier mobilities of CPs by introducing new building blocks to reach the threshold power conversion efficiencies (PCEs) required for commercialization.7,8 Among the large number of D−A CPs, carbazole-based copolymers such as PCDTBT have shown remarkably high photoconductive characteristics, with PCEs higher than 7% when applied as the donor material in bulk heterojunction OSCs.1,9 In addition to high photoconductivity, PCDTBT possesses outstanding thermal stability, with 7 years of device lifetime, which is the longest reported operating lifetime for polymer-based OSCs.10 In addition, PCDTBT has the important merit of low material cost owing to cheap, reliable, reproducible, and scalable synthetic procedures. Therefore, PCDTBT is now regarded as a second-generation benchmark for OSCs, next to P3HT.11 © 2017 American Chemical Society

Nonetheless, it is worth noting that the PCE values achieved by PCDTBT and its derivatives are still lower than those of state-of-the-art of OSCs, despite the use of various morphological and structural optimization strategies. Therefore, a synthetic approach that further enhances the photon absorption and charge transport abilities of a carbazole-based polymeric semiconductor without compromising its thermal stability would be a notable contribution to the field. Notably, recently reported other efficient polymeric semiconductors are usually based on large lactam units, such as diketopyrrolopyrrole (DPP), isoindigo, bidithienopyridinedione, and thienopyrroledione.2,10,12,13 These materials have various advantages, such as enhanced charge carrier mobility owing to enhanced intermolecular interactions and a narrow energy band gap owing to effective overlap of the expanded π-system.14−16 Moreover, it has been reported that the increased effective conjugation length decreases the reorganization energy, which Received: November 7, 2016 Revised: January 2, 2017 Published: January 3, 2017 2135

DOI: 10.1021/acs.chemmater.6b04745 Chem. Mater. 2017, 29, 2135−2140

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Chemistry of Materials Scheme 1. Synthetic Scheme of Monomer and Polymers

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can facilitate exciton separation to generate free charge carriers.17−19 Therefore, in this work, considering the many favorable properties that large heteroacene blocks may induce, we extended the degree of π-conjugation in the carbazole structure by introducing the indolo[3,2-b]indole building block to synthesize a series of new D−A CPs. Compared with carbazole, the indolo[3,2-b]indole block has many promising merits: (1) a symmetric conjugation structure that leads to a high degree of backbone rigidity and thus very efficient charge transport along the π-stacking direction,20−22 (2) two branched alkyl chains at the 5 and 10 positions that greatly enhance the solubility of the resulting copolymer,23,24 and (3) a lower band gap owing to extended charge delocalization.21,25 Despite such interesting skeletal characteristics for the indolo[3,2-b]indole unit, its solar cell application, particularly its deep exploration, has rarely been reported so far. As an acceptor unit, 2,1,3-benzothiadiazole (BT) was introduced, and moreover, to tune the energy levels, fluorinated BT units, obtained by substitution of the hydrogen atoms of the BT unit with one or two fluorine atoms, were introduced. The resulting copolymers were poly[4-(5-(5,10bis(2-decyltetradecyl)-5,10-dihydroindolo[3,2-b]indol-2-yl)thiophen-2-yl)-7-(thiophen-2-yl)benzo[c][1,2,5]thiadiazole] (PDHITHT), poly[4-(5-(5,10-bis(2-decyltetradecyl-5,10dihydroindolo[3,2-b]indol-2-yl)thiophen-2-yl)-5-fluoro-7-(thiophen-2-yl)benzo[c][1,2,5]thiadiazole] (PDHITFT), and poly[4-(5-(5,10-bis(2-decyltetradecyl)-5,10-dihydroindolo[3,2-]indol-2-yl)thiophen-2-yl)-5,6-difluoro-7-(thiophen-2-yl)benzo[c][1,2,5]thiadiazole] (PDHIT2FT).

RESULTS AND DISCUSSION

The monomer and polymer synthetic schemes are depicted in Scheme 1. The 5,10-dihydroindolo[3,2-b]indole moiety was easily obtained by cyclization of dibenzo[b,f ][1,5]diazocine6,12(5H,11H)-dione, which was synthesized by nucleophilic condensation of methyl-2-aminobenzoate following chlorination.26,27 To increase solubility, decyltetradecyl substituents were introduced into 5,10-dihydroindolo[3,2-b]indole by Nalkylation. The monomer 5,10-bis(2-decyltetradecyl)-2,7-bis( 4 , 4 , 5 , 5 - t e t ra m e t h y l - 1 ,3 ,2 - d i o x a b o r o l a n - 2 - y l ) - 5 , 1 0 dihydroindolo[3,2-b]indole was obtained by bromination (yield: 52%) following reaction with borate (yield: 42%) (detailed experimental procedures are provided in the Supporting Information). The other comonomers were prepared by literature methods.28 Further structural analyses including 1H NMR, 13C NMR, and mass spectrometry for intermediates and monomers are provided in the Supporting Information (Figures S1-1−S1-14). The polymers (PDHITHT, PDHITFT, and PDHIT2FT) were synthesized by Suzuki coupling reactions (see details in the Supporting Information). The polymers were purified by Soxhlet extraction using methanol, hexane, THF, and chloroform. The chloroform fraction was collected and reprecipitated. The number-average molecular weights of PDHITHT, PDHITFT, and PDHIT2FT were 31700, 25800, and 22500 g/mol, with PDIs of 3.06, 3.05, and 2.93, respectively (Table S1 and Figure S1-15−S1-17). The introduction of an indoloindole moiety instead of the 3,6-carbazole moiety can possibly increase the backbone planarity, leading to a narrow band gap energy. The electrochemical properties of the synthesized polymers 2136

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Figure 1. UV−vis absorption spectra of PDHITHT, PDHITFT, and PDHIT2FT in (a) solution state and (b) film states (b) at room temperature.

Figure 2. (a) Illuminated J−V curves under AM 1.5G illumination (100 mW cm−2) and (b) EQE spectra of devices with the general structure of ITO/MoO3/polymer:PC71BM/LiF/Al.

Table 1. Photovoltaic Parameters of Single-Junction PDHITHT, PDHITFT, and PDHIT2FT Devices blend PDHITHT:PC71BM PDHITFT:PC71BM PDHIT2FT:PC71BM a

champion mean ± SDa champion mean ± SDa champion mean ± SDa

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

0.89 0.88 ± 0.01 0.89 0.88 ± 0.01 0.89 0.88 ± 0.01

16.1 15.0 ± 0.8 18.4 17.2 ± 0.9 17.6 16.8 ± 0.6

52.3 50.6 ± 1.1 53.4 52.1 ± 1.1 52.9 51.1 ± 1.9

7.49 6.71 ± 0.6 8.84 8.51 ± 0.3 8.28 7.73 ± 0.5

The mean values are calculated from 12 cells.

and PDHIT2FT as active layers. As shown in Figure S4, topcontact (Au), bottom-gated (SiO2, 100 nm) FET with PDHITFT as an active layer yielded the highest hole mobility of 0.04 cm2/(V s), and two other derivatives also rendered high hole mobility values of ∼0.02 cm2/(V s). These charge carrier mobility values of indoloindole derivatives are apparently higher than that of a carbazole-based polymer such as PCDTBT,30 implying the effectiveness of extended piconjugation structure on charge transport. Thermogravimetric analyses showed that each polymer was stable up to 450 °C. In addition, differential scanning calorimetry measurements revealed that these polymers do not have distinctive transition temperatures, implying low overall crystallinity (Figure S5). The photoconductive ability of this series of new indolo[3,2b]indole-based CPs was investigated by measuring the performance of bulk heterojunction OSCs (Table S3). The solar cell performance of single-junction devices was evaluated in the conventional device architecture of ITO/MoO3/active layer/LiF/Al under AM 1.5G illumination (100 mW cm−2). For each polymer, the active layer blend (polymer:PC71BM) was systematically optimized in terms of blend composition ratio, type of solvent, and active thickness to maximize the PCE. The typical current density versus voltage (J−V) curves obtained for the optimized polymer blends are shown in Figure 2a, and corresponding detailed parameters are shown in Table

were studied using cyclic voltammetry (CV), and the results are summarized in Figure S2 and Table S2. Fluorination of the BT unit resulted in deeper HOMO and LUMO levels, in the order of PDHITHT > PDHITFT > PDHIT2FT, which revealed the possibility of obtaining OSCs with higher open-circuit voltage (VOC) values. The UV−vis absorption spectra of each polymer in chloroform solution and in film are depicted in Figure 1 and Figure S3. The λmax values of the polymers in solution were observed at 378 and 532 nm for PDHITHT, 374 and 532 nm for PDHITFT, and 365 and 512 nm for PDHIT2FT (Figure 1a and Table S2). The absorption band around 370 nm can be assigned to π−π* or n−π* transitions, and the absorption band around 530 nm can be attributed to the HOMO−LUMO transition. The absorption maximum showed a gradual blue shift as the number of F substituents increased, which is consistent with the behavior of other F-substituted polymers.29 The absorption spectra of the polymer films exhibited λmax values that were 60−70 nm red-shifted compared with those of the polymers in solution (Figure 1b). This large red shift may be explained by strong intermolecular interactions in the film state. The optical band gap of each polymer was calculated to be 1.71−1.76 eV. To study the charge transport nature of the indoloindole moiety compared to the 2,7-carbazole moiety, field effect transistors (FET) were fabricated and transfer characteristics were analyzed by using PDHITHT, PDHITFT, 2137

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the origin of the higher Jsc and FF values and thus the higher PCE values observed for PDHITFT and PDHIT2FT. Therefore, fluorination may result in better intermolecular interactions in the CPs, as discussed below. To understand the observed OSC characteristics and transport phenomena of the new indolo[3,2-b]indole-based CPs, the morphology of the blend films was probed by transmission electron microscopy (TEM), as shown in Figure 4a−c. The morphology of a BHJ OSC plays a critical role in achieving efficient exciton dissociation at the polymer/fullerene interfaces and charge transport in the polymer domains. As clearly shown in the TEM images, the active film of each polymer is composed of a quite homogeneous and uniform network. However, more fine and continuous fibrous structures are observed in the order of PDHITFT:PC 71 BM > PDHIT2FT:PC71BM > PDHITHT:PC71BM. Furthermore, particularly in the TEM image of PDHITHT:PC71BM, many nonideal aggregates are observed, which could be related to the ineffective charge transport and separation observed for this blend. Thus, the TEM nanomorphological images indicate the presence of a homogeneously percolating network, in which properly aggregated pathways lead to higher photocurrent and fill factor values and, thus, improved device performance. To further understand the detailed morphological properties of the blend films, we investigated the crystalline structure of the active layers using two-dimensional grazing-incidence wideangle X-ray scattering (2D-GIWAXS) (Figure 4d−f). Interestingly, all the CPs have more pronounced crystalline features than PCDTBT,31 which can be attributed to the more planar backbone of the indolo[3,2-b]indole unit compared with that of carbazole. In addition, more intense scattering reflections were observed in the following order: PDHITFT:PC71BM > PDHIT2FT:PC71BM > PDHITHT:PC71BM. The relatively well-developed long-range order of the fluorinated polymers can be attributed to strong intermolecular interactions driven by the inclusion of F atoms. It is known that F···S interactions can facilitate the planarization of polymer backbones,32 leading to more efficient intermolecular interactions. The optimal planarization of the backbone of PDHITFT owing to the singly fluorinated BT units maximized both hole and electron charge carrier mobilities. A halo arc peak was observed in the pattern of each polymer blend, which was attributed to the PCBM domains. Collectively, the morphological/structural analyses indicated that (1) the indolo[3,2-b]indole unit is a better donor building block than carbazole owing to its extended πconjugation and planar/symmetric nature and (2) fluorination of BT introduces an additional F···S interaction that can strengthen the intermolecular interactions of CPs. Overall, these factors lead to better OPV performance.

1. The best PDHITHT:PC71BM solar cell displayed the lowest Voc of 0.89 V, short-circuit current (Jsc) of 16.1 mA cm−2, and fill factor (FF) of 52.3%, and thus achieved a PCE of 7.49% (series resistance, Rs = 5.4 Ω cm2, and shunt resistance, Rsh = 2614.3 Ω cm2). In the case of PDHITFT, with a singly fluorinated BT unit incorporated in the polymer chains, the device parameters were significantly enhanced, especially the Jsc value: Voc = 0.89 V, Jsc = 18.4 mA cm−2, FF = 53.4%, and PCE = 8.84% (Rs = 6.2 Ω cm2 and Rsh =2071.5 Ω cm2). This PCE value of near 9% is quite higher than others recorded in the field of polymer solar cells. When the doubly fluorinated BT unit was introduced into the polymer chains in PDHIT2FT, the device performance was slightly reduced with a PCE value of 8.28% (Voc = 0.89 V, Jsc = 17.6 mA cm−2, FF = 52.9%, Rs = 9.1 Ω cm2, and Rsh =1498.7 Ω cm2). This result may be due to the photocurrent density and fill factor, which are strongly related to the active layer morphology, as discussed below. The EQE spectra are shown in Figure 2b, and the observed trend is well explained by the Jsc values of the devices. Throughout the entire wavelength range, the photon conversion efficiencies exhibit the following order: PDHITFT:PC71BM > PDHIT2FT:PC71BM > PDHITHT:PC71BM. To elucidate the observed trends in the Jsc and FF values, we used the SCLC method to investigate bulk charge transport in the active layer blends, which were prepared similarly to those for the OSC devices (except for the electrodes). To examine the hole carrier mobility, ITO/MoO3/active layer/Au structures were fabricated, whereas the electron mobility was measured in an Al/active layer/Al architecture. The dark current versus voltage characteristics are shown in Figure 3.

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SUMMARY Considering the positive effects of large heteroacene blocks in D−A copolymers, we extended the degree of π-conjugation in the carbazole structure by introducing the indolo[3,2-b]indole building block to synthesize a series of new D−A CPs. DFT calculations, photophysical analyses, charge transport studies, and structural/morphological analyses showed the many promising merits of the indolo[3,2-b]indole block in comparison with carbazole, such as a relatively high degree of backbone rigidity, higher charge carrier mobility, and extended charge delocalization. Furthermore, fluorination of the BT acceptor unit was found to strengthen intermolecular interactions in the CPs, resulting mainly from preferential F···

Figure 3. Dark J−V curves to extract bulk charge carrier mobilities of (a) hole and (b) electron for PDHITHT:PC71BM, PDHITFT:PC71BM, and PDHIT2FT:PC71BM devices.

The hole mobilities for PDHITFT:PC71BM, PDHIT2FT:PC71BM, and PDHITHT:PC71BM were determined as 3.14 × 10−3, 5.78 × 10−3, and 4.03 × 10−3 cm2 V−1 s−1, respectively. Similar trends were observed for the electron mobilities: 5.02 × 10−4, 9.75 × 10−4, and 6.11 × 10−4 cm2 V−1 s −1 for PDHITFT:PC 71 BM, PDHIT2FT:PC 71 BM, and PDHITHT:PC71BM, respectively. These results suggest that higher and hole/electron-balanced carrier mobilities could be 2138

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Figure 4. (Upper panel) high-resolution TEM images and (lower panel) 2D-GIWAXS patterns of, from left to right, (a and d) PDHITHT:PC71BM, (b and e) PDHITFT:PC71BM, and (c and f) PDHIT2FT:PC71BM.

S interactions. As a result, PDHITFT, with a single fluorine substituent in the BT unit, showed the highest OSC performance: Voc = 0.89 V, Jsc = 18.4 mA cm−2, FF = 53.4%, and PCE = 8.84%. This PCE value of near 9% for an indolo[3,2-b]indole-containing copolymer shows the bright future of extended carbazole units for OSC applications.

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In two diodes, all metal electrodes as well as the MoO3 layer are thermally deposited to the evaporation system in high vacuum. The active layer is spin-cast at the same conditions as the solar cell fabrication method. Charge carrier mobility (both hole and electron mobilities) was determined by the Mott−Gurney equation in the SCLC region (at voltage 2−3 V, slope = 2): J = 9εε0μh/eV2/8L3, in which is J is dark current density, ε is the dielectric constant (for polymer, assumed to be 3), ε0 is the permittivity of the vacuum, μh/e is the carrier mobility, V is internal voltage in the device, and L is film thickness of active layer.

EXPERIMENTAL SECTION

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Materials. See Supporting Information. Synthesis and Characterization. See Supporting Information. Device Fabrication and Characterization. Polymer solar cells were fabricated with the general sandwich structure through several steps. First, indium−tin−oxide (ITO) patterned coating glass substrates were cleaned by ultrasonication sequentially in detergent, diwater, acetone (CMOS grade), and 2-propanol (CMOS grade). To remove residues, the cleaned ITO glass substrates were treated with UV−ozone treatments for 20 min. After that, for the hole extraction layer, we chose an MoO3 layer (ca. 10 nm), which is deposited via thermal evaporation. An active layer is deposited onto MoO3/ITO glass substrates with 1500 rpm for 50 s, and the thickness is ca. 92 nm. Before spin-casting the active layer, the active solution is sufficiently stirred to 1:1 blend ratio with PC71BM in a nitrogen-filled glovebox for 12 h. Upon spin-casting the active layer, the substrates with two layers were baked onto a hot plate for 10 min at 80 °C to remove the remaining solvents. Finally, the LiF/Al double cathode is thermally deposited in high vacuum condition (2 × 10−6 Torr) to 0.8/100 nm with a 9 mm2 active area opening. The current−voltage (J−V) characteristics of the photovoltaic polymer solar cells were measured using a Keithley 2400 I−V measurement system (AM 1.5G/1 sun/100 mW cm−2). The external quantum efficiency (EQE) of the photovoltaic devices was recorded on a photomodulation spectroscopic system (Merlin model; Oriel Instruments, Irvine, CA, U.S.A.). A calibrated Si detector was used to measure the light intensity. Charge Carrier Mobility Measurement by the Space Charge Limited Current (SCLC) Method. Hole- and electron-only diodes were fabricated with the device structure of ITO glass/MoO3/active layer/ Au (100 nm) and Al (110 nm)/active layer/Al (110 nm), respectively.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b04745. Detailed experimental procedures and characterizations for materials (monomers and copolymers) and devices as well as additional figures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(D.S. Chung) E-mail: [email protected]. *(S.-K. Kwon) E-mail: [email protected]. *(Y.-H. Kim) E-mail: [email protected]. ORCID

Yun-Hi Kim: 0000-0001-8856-4414 Author Contributions #

Jaeyoung Hwang, Jeonghun Park, and Yu Jin Kim are equal contributors. Notes

The authors declare no competing financial interest. 2139

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ACKNOWLEDGMENTS This research was financially supported by the National Research Foundation of Korea (NRF) funded by Korea government (MSIP) (2015R1A2A1A10055620) and (NRF2016M1A2A2940911).

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DOI: 10.1021/acs.chemmater.6b04745 Chem. Mater. 2017, 29, 2135−2140