High-Efficiency Organic Photovoltaics with Two-Dimensional

Apr 28, 2017 - We synthesized and characterized two kinds of regioregular polymers that were based on thieno[3,4-b]thiophene as an electron-accepting ...
1 downloads 9 Views 5MB Size
Download PDF
Article pubs.acs.org/cm

High-Efficiency Organic Photovoltaics with Two-Dimensional Conjugated Benzodithiophene-Based Regioregular Polymers Honggi Kim,† Bogyu Lim,*,‡ Hyojung Heo,† Geonik Nam,† Hyungjin Lee,† Ji Young Lee,‡ Jaechol Lee,‡ and Youngu Lee*,† †

Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science and Technology (DGIST), 333, Techno jungang-daero, Hyeonpung-Myeon, Dalseong-Gun, Daegu 42988, Republic of Korea ‡ Corporate R&D, Future Technology Research Center, LG Chem Research Park, 188, Moonji-ro, Yuseong-gu, Daejeon 34122, Republic of Korea S Supporting Information *

ABSTRACT: We synthesized and characterized two kinds of regioregular polymers that were based on thieno[3,4-b]thiophene as an electron-accepting unit and benzo[1,2-b:4,5b′]dithiophene as the electron-donating unit with different side chain, alkylthio and alkyl thiophenes, named rr-PTBS and rrPTB7-Th, respectively. Because of the partial introduction of the alkylthio thiophene side chain, rr-PTBS showed red-shifted absorption and a deeper HOMO level compared to those of rrPTB7-Th. In addition, both rr-PTBS:PC71BM and rr-PTB7Th:PC71BM blended films showed face-on orientations stronger than those of regiorandom PTB7-Th. However, the rr-PTB7-Th:PC71BM blended film showed a peak in the outof-plane direction much weaker than those of rrPTBS:PC71BM and PTB7-Th:PC71BM blended films. Moreover, the rr-PTBS:PC71BM blended film exhibited charge carrier mobility (μe/μh ∼ 1.01) much more balanced than that of the rr-PTB7-Th:PC71BM blended film (μe/μh ∼ 1.23). The bulkheterojunction organic photovoltaic (OPV) device based on rr-PTBS and the 1,8-diiodooctane additive showed a high power conversion efficiency (PCE) of 8.68%, while the OPV device based on rr-PTB7-Th and the 1,8-diiodooctane additive showed a PCE of 7.04%. Finally, an OPV device using rr-PTBS, the diphenyl ether additive, and Micro Lens Film exhibited a short-circuit current (Jsc) of 19.72 mA/cm2, an open-circuit voltage (Voc) of 0.82 V, and a fill factor (FF) of 63.82%, thus resulting in a PCE of 10.31%.

1. INTRODUCTION Bulk-heterojunction (BHJ) organic photovoltaic (OPV) devices have been extensively studied over the past decade because of their unique advantages, some of which are their low cost of manufacturing, the fact that they are lightweight, their mechanical flexibility, their visible transparency, and the fact that they are aesthetically pleasing.1−7 Great efforts have been devoted to improving the photovoltaic performance of BHJ OPV devices, including development of novel materials, device architecture, and the printing process, over the past two decades. The most representative approach to high-performance BHJ OPV devices was to develop new low-bandgap materials with a deep highest occupied molecular orbital (HOMO) level and high mobility.8−13 Among many reported low-bandgap donor polymers, alternating copolymers based on thieno[3,4-b]thiophene (TT) as an electron-accepting unit (A) and benzo[1,2-b:4,5-b′]dithiophene (BDT) as an electrondonating unit (D) (called PTB polymer series) have attracted considerable attention because of their low bandgap properties, relatively high hole mobility, and promising photovoltaic properties.14−18 These polymers exhibited low-bandgap char© 2017 American Chemical Society

acteristics through quinoid resonance structure stabilization, which could potentially enhance hole mobility because of the rigid backbone.19 A PTB7 polymer with a fluorine atom in a TT unit reached a power conversion efficiency (PCE) of 7.4% because of the improved open-circuit voltage (Voc) caused by the HOMO level being deeper than those of previous PTB series.20 Recently, Liu et al. achieved a PCE of ≥10% using a two-dimensional conjugated polymer by replacing alkoxy side chains with alkyl thiophene side chains in a BDT unit of PTB7, namely PTB7-Th, which exhibited an extended absorption range, improved hole mobility, and a deeper HOMO level compared to those of PTB7.21 Even though this polymer has achieved high efficiency, however, the fluorine atom from the TT unit has undefined regioregularity in the polymer backbone, which can limit the electrical and optical properties of polymer films. Received: February 12, 2017 Revised: April 11, 2017 Published: April 28, 2017 4301

DOI: 10.1021/acs.chemmater.7b00595 Chem. Mater. 2017, 29, 4301−4310

Article

Chemistry of Materials Conjugated polymers with regioregularity demonstrated well-ordered structure in film because their π-stacking orientation was better than that of regiorandom polymers.22−24 Therefore, these polymers exhibited better device performance. For example, Ying et al. reported a dramatic increase in field-effect hole mobility and highly improved photovoltaic performance using regioregular pyridal[2,1,3]thiadiazole-based polymers.25 Recently, regioregular low-bandgap polymers using monomers with an A−D−A structure showed greatly enhanced OPV device performance.26−32 Kim et al. synthesized a regioregular BDT- and TT-based polymer, which exhibited an OPV device performance that was better than that of a regiorandom counterpart because of an enhanced short-circuit current (Jsc). The improved device performance is caused by an increase in charge carrier mobility due to highly effective molecular ordering between polymer backbones.27 Very recently, to improve the photovoltaic performance of PTB7Th, we synthesized a regioregular PTB7-Th (Scheme S1, coded rr-PTB7-Th) through a previously reported synthetic route.28 Contrary to expectations, however, the PCE of the regioregular PTB7-Th-based OPV device was lower than that of the conventional PTB7-Th-based OPV device. In this study, we report the synthesis and characterization of regioregular PTB7-Th-based copolymers with A−D1−A−D2 and A−D1−A−D1 configurations containing BDT as D and TT as A, named rr-PTBS and rr-PTB7-Th, respectively. In rrPTBS, an ethylhexyl thio-thiophene side chain-substituted BDT as the D2 unit has been successfully introduced to control its physical properties and improve processability by breaking the molecular symmetry of the polymer backbone. Both polymers exhibit improved hole mobility values because they are regioregular. In addition, replacing alkyl side chains with an alkylthio side chain on rr-PTBS can enhance Voc because of the deeper HOMO level.33 BHJ OPV devices based on rr-PTBS demonstrated a PCE of 8.68% that was higher than that of BHJ OPV devices based on rr-PTB7-Th (7.04%). It is also found that rr-PTBS promotes the formation of highly ordered crystals with a π−π stacking interval much shorter than that of rrPTB7-Th. Furthermore, a rr-PTBS:PC71BM blended film has a much more ideal morphology with favorable phase separation and a bicontinuous network of internal donor−acceptor heterojunctions. In addition, the rr-PTBS:PC71BM blended film shows values of hole and electron mobility that are higher than those of the rr-PTB7-Th:PC71BM blended film. As a result, a rr-PTBS-based BHJ OPV device exhibits a Voc of 0.82 V, a fill factor (FF) of 63.82% and a Jsc of 19.72 mA/cm2, resulting in an excellent PCE of 10.31% using a diphenyl ether additive with Micro Lens Film.

Figure 1. (a) Chemical structures of PTB7-Th, PBDTT-S-TT, rrPTB7-Th, and rr-PTBS. (b) Normalized ultraviolet−visible absorption spectra of rr-PTBS and rr-PTB7-Th as thin films and in a chlorobenzene solution.

rr-PTB7-Th were 91 and 42%, respectively. Unlike rr-PTBS, rrPTB7-Th dissolved well in chloroform, and the yield of rrPTB7-Th in the chloroform fraction was 52%. The numberaverage molecular weights (Mn) and polydispersity indices (PDI, Mw/Mn) of rr-PTBS and rr-PTB7-Th were estimated by high-temperature gel-permeation chromatography (HT-GPC) analysis at 150 °C in 1,2,4-trichlorobenzene as an eluent relative to polystyrene standards to be 40.7 and 35.8 kDa, respectively, and 2.53 and 2.49, respectively. The GPC profiles of rr-PTBS and rr-PTB7-Th show that both polymers are mainly composed of high-molar mass polymers (Figures S7 and S8). It is wellknown that the molecular mass distribution of the polymers has a strong effect on their optoelectronic properties and photovoltaic performances.34,35 Figure 1b shows the normalized ultraviolet−visible (UV−vis) absorption spectra of rr-PTBS and rr-PTB7-Th in a chlorobenzene solution and thin solid films. The corresponding optoelectronic properties, including absorption peak wavelength and optical bandgap, are summarized in Table 1. Both polymers exhibit a broad absorption band from 300 to 800 nm. In addition, they show distinct vibrational features next to their absorption maxima in solution as well as film, indicating the existence of ordered aggregation and strong intermolecular π−π stacking. Because rr-PTBS possesses an electron-donating ethylhexyl thio-thiophene side chain in the BDT segment, it shows an absorption in solution and film that is red-shifted by 10−15 nm compared to that of rr-PTB7-Th.33 Both polymers exhibit strong vibronic features in the UV−vis spectra of the diluted solution, which are similar to the UV−vis spectra of their films. This result indicates that their molecular rigidity can form intermolecular interactions in solution.36 When both

2. RESULTS AND DISCUSSION The molecular structures of rr-PTBS and rr-PTB7-Th are shown in Figure 1a. To synthesize regioregular rr-PTBS and rrPTB7-Th, the bis-brominated TT-BDT-TT monomer was synthesized according to the literature procedures.28 Then, both polymers were synthesized by conventional Stille crosscoupling reactions between bis-brominated TT-BDT-TT monomers and bis-trimethylstannylated BDT monomers as shown in Scheme S1. The detailed synthesis of the monomers and polymers is described in Scheme S1. After polymerization, both polymers were purified by Soxhlet extraction with methanol, acetone, hexane, dichloromethane, chloroform, and chlorobenzene. The chlorobenzene fraction of each polymer was obtained as a dark black solid. The yields of rr-PTBS and 4302

DOI: 10.1021/acs.chemmater.7b00595 Chem. Mater. 2017, 29, 4301−4310

Article

Chemistry of Materials Table 1. Optical and Electrochemical Properties of rr-PTBS and rr-PTB7-Th polymer

λmax sola

λmax filmb

λonset filmb

HOMOc(eV)

LUMOd(eV)

Eg UVe(eV)

Eg CVf(eV)

rr-PTBS rr-PTB7-Th

705 695

710 695

785 770

−5.22 −5.20

−3.84 −3.68

1.58 1.61

1.38 1.52

a Measurements in a chlorobenzene solution. bMeasurements in films spin-casted on a glass. cHOMO level estimated from cyclic voltammetry measurement. dLUMO level estimated from cyclic voltammetry measurement. eOptical bandgap determined from the onset of the absorption in the film. fElectrochemical bandgap estimated by cyclic voltammetry measurement.

solutions are heated, their absorption maxima are blue-shifted because of a decrease in the level of interaction between the polymer backbones as shown in Figure S10.37 The maximal absorption peak of rr-PTBS in film exhibits a slight red-shift compared to that of rr-PTBS in solution because of the electron-donating ethylhexyl thio-thiophene side chain.33 The optical bandgaps (Egopt) of rr-PTBS and rr-PTB7-Th are 1.58 and 1.61 eV, respectively. These results suggest that both polymers can be used as promising electron donor materials for BHJ OPV devices. The highest occupied molecular orbital (HOMO, EHOMO) and lowest unoccupied molecular orbital (LUMO, ELUMO) energy levels of both polymers were evaluated by electrochemical cyclic voltammetry (CV) as shown in Figure S11 and Table 1. The HOMO levels of rr-PTBS and rr-PTB7Th are −5.22 and −5.20 eV, respectively. The LUMO levels of rr-PTBS and rr-PTB7-Th are −3.84 and −3.68 eV, respectively. These results clearly demonstrate that both HOMO and LUMO energy levels of rr-PTBS are down-shifted because of its ethylhexyl thio-thiophene side chain compared to those of rrPTB7-Th.33 To investigate and compare the photovoltaic properties of rrPTBS and rr-PTB7-Th, BHJ OPV devices with an indium tin oxide (ITO)/ZnO/polymer:[6,6]-phenyl C71 butyric acid methyl ester (PC71BM)/MoO3/Ag inverted device configuration were fabricated. The OPV devices were fabricated using rr-PTBS or rr-PTB7-Th as an electron donor and PC71BM as an electron acceptor material in the photoactive layer. The optimization of the BHJ OPV device was systemically performed by adjusting major factors such as the blend ratio of the donor and acceptor, the polymer:PC71BM concentration, and the ratio of the processing additive (Figures S12, S13, S16, and S17). The OPV device performance was first optimized by processing polymer:PC 71 BM (1:1.5 weight ratio) at a concentration of 15 mg/mL in a chlorobenzene solvent with 3% (v/v) 1,8-diiodooctane (DIO) as an additive, which were typically optimized conditions for PTB7-based polymers. Figure 2a shows the current density−voltage (J−V) characteristics of the BHJ OPV devices under AM 1.5G illumination (100 mW/cm2). The photovoltaic characteristics of the OPV devices, including the corresponding open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF), and power conversion efficiency (PCE), are listed in Table 2. Contrary to our expectations, rr-PTB7-Th-based OPV devices show a power conversion efficiency (PCE) of 7.04% with an open-circuit voltage (Voc) of 0.78 V, a short-circuit current (Jsc) of 15.36 mA/cm2, and a fill factor (FF) of 59.08%. The Jsc value is increased because of the regioregularity of rr-PTB7-Th, while the FF value is decreased compared to those of conventional PTB7-Th-based OPV devices.33 In contrast, OPV devices based on rr-PTBS exhibit a highly improved Jsc of 16.97 mA/cm2, a FF of 64.16%, and a Voc of 0.80 V, resulting in a PCE of 8.68%. These results verify that the significant improvement in the photovoltaic performance of the rr-PTBS is mainly due to the incorporation of alkyl thio-thiophene-substituted BDT with

Figure 2. rr-PTBS or rr-PTB7-Th:PC71BM (1:1.5 weight ratio) in a chlorobenzene solvent with 3% (v/v) 1,8-diiodooctane (DIO) as an additive: (a) current density−voltage (J−V) characteristics and (b) EQE spectra of BHJ OPV devices based on both polymers.

regioregularity in the polymer backbone. Figure 2b shows the external quantum efficiencies (EQEs) of the OPV devices based on the rr-PTBS:PC71BM and rr-PTB7-Th:PC71BM blended films with 3% (v/v) DIO under AM 1.5G illumination (100 mW cm−2). The rr-PTBS-based OPV device shows a photoresponse in the range of 350−800 nm and has the highest EQE value of >70% at 710 nm, which is consistent with the UV−vis absorption spectrum of the rr-PTBS film. However, the rr-PTB7-Th-based OPV device exhibits a relatively lower EQE, ∼60% in the range of 480−700 nm, with a maximum of 63.1% at 690 nm. This result supports the possibility that the rr-PTBS-based OPV device shows a Jsc and a PCE that are higher than those of the rr-PTB7-Th-based OPV device. To understand why the rr-PTBS-based OPV devices show better photovoltaic performance than rr-PTB7-Th-based OPV devices, we examined the dependence of Jsc and Voc on light intensity. Figure 3a presents the double-logarithmic diagram of the Jsc on the light intensity. It is linearly fitted with the power law Jsc ∝ Iα, where I is the light intensity and α is the recombination parameter. It is well-known that the α value can be related to bimolecular recombination in the photoactive layer. In addition, an α value closer to 1 leads to negligible 4303

DOI: 10.1021/acs.chemmater.7b00595 Chem. Mater. 2017, 29, 4301−4310

Article

Chemistry of Materials

Table 2. Comparison of Photovoltaic Characteristics with Those of Previously Reported Highly Efficient PTB-Based Polymersa PTB7-Th PBDTT-S-TT rr-PTB7-Thb rr-PTBSb rr-PTBSc rr-PTBSd

Jsc (mA/cm2)

Voc (V)

polymer 0.77 0.84 0.78 0.80 0.83 0.82

(0.78 (0.80 (0.82 (0.82

± ± ± ±

0.01) 0.01) 0.01) 0.01)

14.99 15.32 15.36 16.97 18.53 19.72

(15.35 (17.02 (18.06 (19.63

± ± ± ±

FF (%) 63.92 65.49 59.08 64.16 63.50 63.82

0.01) 0.05) 0.47) 0.09)

(58.27 (62.06 (64.13 (63.33

± ± ± ±

PCE (%)

0.81) 2.10) 0.63) 0.49)

7.42 8.42 7.04 (6.96 ± 0.08) 8.68 (8.46 ± 0.22) 9.71 (9.48 ± 0.23) 10.31 (10.19 ± 0.12)

ref 33 33 this this this this

work work work work

a Average values in parentheses were obtained from >16 devices. bPolymer:PC71BM (1:1.5 weight ratio) in a chlorobenzene solvent with 3% (v/v) DIO. cPolymer:PC71BM (1:1.5 weight ratio) in a chlorobenzene solvent with 2% (v/v) DPE. dPolymer:PC71BM (1:1.5 weight ratio) in a chlorobenzene solvent with 2% (v/v) diphenyl ether and Micro Lens Film (MLF).

To understand this enhancement of photovoltaic performance based on rr-PTBS compared to rr-PTB7-Th, it is necessary to investigate the various physical properties of the polymers such as crystallinity, BHJ film morphology, and charge carrier mobility. First, to investigate the molecular ordering structures of polymers, grazing incidence wide-angle X-ray scattering (GIWAXS) analysis was performed on pristine and blended polymer films. Figure 4 shows the GIWAXS images of pristine polymers and polymer:PC71BM [1:1.5 (w/ w)] blended films containing 3% (v/v) DIO. As shown in Figure 4a−c, GIWAXS images on the pristine films of rr-PTBS, rr-PTB7-Th, and PTB7-Th show a perfect face-on orientation. However, the rr-PTBS and rr-PTB7-Th films exhibit a face-on orientation pattern with the pronounced π−π stacking peak (010) intensities in the out-of-plane direction (qz ≈ 1.59−1.63 Å−1) that is stronger than that of the PTB7-Th film. This result indicates that the structural regioregularity for polymer backbones of rr-PTBS and rr-PTB7-Th improves the face-on orientation in their films compared to that of PTB7-Th with a regiorandomly oriented packing structure. Furthermore, the rrPTBS film exhibits π−π stacking spacing [d(010) ≈ 3.83 Å] much shorter than those of rr-PTB7-Th and PTB7-Th films, which were 3.93 and 3.90 Å, respectively (Figure S14b and Table S3). This dense π−π stacking ordering of rr-PTBS can be favorable for effective charge transport and consequently improved Jsc and FF. On the other hand, rr-PTBS shows a lamellar spacing [d(100) ≈ 24.18 Å] slightly longer than those of rr-PTB7-Th [d(100) ≈ 23.97 Å] and PTB7-Th [d(100) ≈ 23.48 Å]. This result can be attributed to the elongated side chains by introducing a sulfur atom on the BDT segment of rr-PTBS compared to the side chain without the sulfur atom of rr-PTB7Th or PTB7-Th. The crystallite correlation length (Lc) for the pristine polymers is calculated with the Scherrer equation.44,45 The corresponding lamellar correlation length [Lc(100) from the in-plane direction] and π−π stacking correlation length [Lc(010) from the out-of-plane direction] of rr-PTBS, rr-PTB7-Th, and PTB7-Th are summarized in Table S4. The lamellar correlation lengths of rr-PTBS, rr-PTB7-Th, and PTB7-Th pristine films are 67.16, 57.62, and 56.53 Å, respectively. The π−π stacking correlation lengths of rr-PTBS, rr-PTB7-Th, and PTB7-Th pristine films are 18.84, 16.71, and 16.91 Å, respectively. The rrPTBS exhibits crystallite correlation lengths in both lamellar and π−π stacking for the face-on orientation more increased than those of rr-PTB7-Th and PTB7-Th, whereas rr-PTB7-Th and PTB7-Th show similar lamellar and π−π correlation lengths. As a result, rr-PTBS shows improved crystallinity as well as face-on orientation packing in a film compared to those of rr-PTB7-Th and PTB7-Th films. It is well-known that the face-on orientation with the high crystallinity of polymers is advantageous for photovoltaic applications and produces

Figure 3. Dependence of (a) Jsc and (b) Voc on light intensity for the rr-PTBS and rr-PTB7-Th:PC71BM blended film with DIO.

bimolecular recombination. The extracted α values of the rrPTBS- and rr-PTB7-Th-based OPV devices are 0.992 and 0.949, respectively. This result clearly indicates that the bimolecular recombination in the rr-PTBS-based OPV devices is suppressed.38−42 Figure 3b shows the plots of Voc as a logarithmic function of light intensity with a slope of kT/q, where k is Boltzmann’s constant, q is the elementary charge, and T is the temperature. If the slope is larger than kT/q, trapassisted recombination takes place in the photoactive layer. This can be caused by a decreased bimolecular recombination rate. The rr-PTBS-based OPV devices show less Voc dependence on light intensity (1.18kT/q) than the rr-PTB7-Th-based OPV devices (1.38kT/q). Therefore, trap-assisted recombination is also suppressed in the photoactive layer of the rr-PTBSbased OPV devices. As a result, the rr-PTBS-based OPV devices show less bimolecular and trap-assisted recombination, resulting in highly enhanced Jsc, FF, and PCE values.43 4304

DOI: 10.1021/acs.chemmater.7b00595 Chem. Mater. 2017, 29, 4301−4310

Article

Chemistry of Materials

Figure 4. Grazing incidence wide-angle X-ray scattering (GIWAXS) of pristine polymer films for (a) rr-PTBS, (b) rr-PTB7-Th, and (c) PTB7-Th and polymer:PC71BM blended films with 3% (v/v) DIO for (d) rr-PTBS, (e) rr-PTB7-Th, and (f) PTB7-Th.

Figure 5. Tapping mode AFM topographies and TEM images of blended films of (a and d) the rr-PTBS:PC71BM blended film with DIO, (b and e) the rr-PTB7-Th:PC71BM blended film with DIO, and (c and f) the PTB7-Th:PC71BM blended film with DIO.

a face-on orientation that is better than that of the rr-PTB7Th:PC71BM blended film. However, π−π stacking spacings, d(010), for the blended films of rr-PTBS, rr-PTB7-Th, and PTB7-Th are almost similar (3.93, 3.94, and 3.97 Å, respectively). The lamellar spacing values, d(100), in the inplane direction of the rr-PTBS:PC71BM, rr-PTB7-Th:PC71BM, and PTB7-Th:PC71BM blended films are 22.21, 21.51, and 21.65 Å, respectively. The out-of-plane lamellar spacing values, d(100), of the rr-PTBS:PC71BM, rr-PTB7-Th:PC71BM, and PTB7-Th:PC71BM blended films are 21.33, 20.38, and 20.85 Å, respectively. The rr-PTBS:PC71BM blended film shows a lamellar spacing in both directions due to the introduction of a sulfur atom on its side chains that is slightly longer than those

efficient charge carrier transport of separated holes and electrons.46−48 Panels d−f of Figure 4 and panels c and d of Figure S14 display the GIWAXS images and profiles of rrPTBS, rr-PTB7-Th, and PTB7-Th blended films with PC71BM containing 3% (v/v) DIO. In the blended films, polymer ordering structures have dramatically changed to show bimodal structures with edge-on and face-on orientations. As shown in Figure 4d−f and Figure S14d, the rr-PTBS:PC71BM and PTB7Th:PC71BM blended films show π−π stacking peaks in the outof-plane direction (qz ≈ 1.58−1.59 Å−1), whereas the rr-PTB7Th:PC71BM blended film exhibits a much weaker peak (qz ≈ 1.59 Å−1) in the same direction. As a result, the rrPTBS:PC71BM and PTB7-Th:PC71BM blended films possess 4305

DOI: 10.1021/acs.chemmater.7b00595 Chem. Mater. 2017, 29, 4301−4310

Article

Chemistry of Materials of rr-PTB7-Th:PC71BM and PTB7-Th:PC71BM blended films. The calculated lamellar stacking correlation lengths, Lc(100), in the in-plane direction are 125.72, 94.25, and 115.22 Å for the rr-PTBS:PC 71 BM, rr-PTB7-Th:PC 7 1 BM, and PTB7Th:PC71BM blended films, respectively (Table S5). The calculated lamellar stacking correlation lengths, Lc(100), in the out-of-plane direction are 97.45, 83.12, and 93.56 Å for rrPTBS:PC71BM, rr-PTB7-Th:PC71BM, and PTB7-Th:PC71BM blended films, respectively (Table S5). These results confirm that rr-PTBS has molecular ordering structure and crystallinity that are favorable for photovoltaic applications not only in the pristine film but also in the blended film compared to those of rr-PTB7-Th and PTB7-Th. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to evaluate the nanoscale morphology of rr-PTBS:PC71BM, rr-PTB7-Th:PC71BM, and PTB7-Th:PC71BM blended films prepared in chlorobenzene without and with DIO as a processing additive. Without the DIO processing additive, large aggregations in all blended films are observed in AFM and TEM images as shown in Figure S15. They can reduce the level of exciton dissociation and charge recombination, which lowers Jsc and FF. On the other hand, after all the blended films are processed with the DIO additive, their morphology changes obviously. As shown in Figure 5, the rr-PTBS:PC71BM and PTB7-Th:PC71BM blended films show favorable phase separation and a homogeneous interpenetrating network with a moderate root-mean-square (RMS) surface roughness of 2.41 and 2.97 nm, respectively.49 However, the rrPTB7-Th:PC71BM blended film exhibits a very smooth surface with a RMS roughness of around 1 nm, leading to a reduced level of charge collection because of the decrease in the contact area between the photoactive layer and interfacial layer.50,51 In addition, it shows the fibril-like morphology with small domains (around 30 nm in diameter), which cause an unfavorable exciton dissociation and charge transport.40,52 Therefore, it results in Jsc, FF, and PCE values that are lower than the values of the others. The balanced charge carrier mobility between the hole and electron is also an important factor for high-efficiency BHJ OPV devices. To examine the charge carrier mobility of the rr-PTBS:PC71BM and rr-PTB7-Th:PC71BM blended films, hole- and electron-only devices were fabricated with ITO/ PEODT:PSS/polymer:PC 71 BM/Au and ITO/ZnO/polymer:PC71BM/LiF/Al configurations, respectively.53−55 Hole and electron mobility were calculated from the space-chargelimited current (SCLC) measurements as shown in panels a and b of Figure 6. The hole mobilities of rr-PTBS and rr-PTB7Th are 9.25 × 10−5 and 4.74 × 10−5 cm2 V−1 s−1, respectively. The electron mobilities of rr-PTBS and rr-PTB7-Th are 9.33 × 10−5 and 5.85 × 10−5 cm2 V−1 s−1, respectively.56 These results verify that the hole and electron mobility of rr-PTBS are ∼2.0 and ∼1.6 times higher than those of rr-PTB7-Th, respectively. In addition, rr-PTBS exhibits a charge carrier mobility (μe/μh ∼ 1.01) much more balanced than that of rr-PTB7-Th (μe/μh ∼ 1.23). This result clearly confirms that rr-PTBS has bicontinuous interpenetrating pathways for hole and electron transport more desirable than those of rr-PTB7-Th, leading to enhanced FF and Jsc values. To further improve the performance of the rr-PTBS polymer for BHJ OPV devices, we utilized different additives to fabricate OPV devices (see Figure S16).57,58 In particular, the diphenyl ether (DPE) additive showed the best photovoltaic performance. As shown in Figure 7b, the BHJ OPV device with rr-PTBS using 2% (v/v) DPE shows a Jsc of

Figure 6. (a) Hole carrier mobility (μh) and (b) electron carrier mobility (μe) of rr-PTBS:PC71BM and rr-PTB7-Th:PC71BM blended films with DIO.

Figure 7. (a) Schematic representation of the inverted structure BHJ OPV with Micro Lens Film. (b) Current density−voltage (J−V) characteristics of inverted BHJ OPVs based on rr-PTBS and PC71BM blended films using a CB solvent containing 2% (v/v) DPE without and with Micro Lens Film.

4306

DOI: 10.1021/acs.chemmater.7b00595 Chem. Mater. 2017, 29, 4301−4310

Article

Chemistry of Materials 18.53 mA/cm2, a Voc of 0.83 V, and a FF of 63.50%, thus resulting in a PCE of 9.71%. The improvement in the photovoltaic performance is mainly due to the enhanced charge extraction (as shown in Figure S17). Finally, to achieve the highest PCE of ≥10% using rr-PTBS, Micro Lens Film (MLF) is integrated into the bottom side of the BHJ OPV device as shown in Figure 7a.59,60 After MLF is integrated, the BHJ OPV device exhibits a PCE of 10.31% with a Jsc of 19.72 mA/cm2, a Voc of 0.82 V, and a FF of 63.82%, indicating that the Jsc value can be increased by ∼6% because of additional light absorption by the photoactive layer.

mmol] was added to the solution. The mixed solution was stirred at 100 °C for 72 h. After polymerization, 4-bromobenzotrifluoride (0.8 mL) was added to the solution. Then, the resulting solution was stirred for 12 h and cooled to room temperature.61 Then, the reaction mixture was poured into methanol (200 mL). The precipitated polymer was purified by Soxhlet extraction using methanol, acetone, hexane, dichloromethane, chloroform, and chlorobenzene. The chlorobenzene extract was concentrated and then poured into methanol to obtain the purified polymer. The purified polymer was collected and dried under vacuum overnight. Therefore, purified rrPTBS with a purple-black luster (915 mg, 91%) was obtained. Mn = 40.7 kDa, and PDI = 2.53. Anal. Calcd (%) for (C99H117F2O4S14)n: C, 64.00; H, 6.31; S, 24.16. Found: C, 63.48; H, 6.35; S, 24.15. 4.3. Characterization and Measurements. Materials were characterized by 1H nuclear magnetic resonance (NMR) and 13C NMR spectroscopy (AVANTEC III, 400 MHz). The number- and weight-average molecular weights of polymers were measured by hightemperature gel-permeation chromatography (HT-GPC) using 1,2,4trichlorobenzene as an eluent at 150 °C relative to polystyrene standards. UV−vis spectra were obtained with Mecasys Optizen Pop spectrophotometer. All cyclic voltammetry (CV) measurements were performed using an AutoLab analyzer with platinum as a counter electrode, indium tin oxide (ITO) coated with a thin film as a working electrode, and a Ag/Ag+ electrode as a reference electrode at a scan rate of 100 mV/s. The supporting electrolyte was 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) in an anhydrous acetonitrile solution. A ferrocene/ferrocenium (Fc/Fc+) redox couple was used as a calibration reference to estimate the energy levels of polymers from the vacuum energy level. The half-wave potential (E1/2) for oxidation of the Fc/Fc+ redox couple is assumed to be 4.8 eV below the vacuum level. The surface morphology of polymers was determined using atomic force microscopy (AFM) (NX10, Park systems) and transmission electron microscopy (TEM) (Hitachi, HF3300). 4.4. GIWAXS Measurement and Characterization. GIWAXS measurements were performed at the Pohang Accelerator Laboratory’s PLS-II 9A U-SAXS beamline. X-rays originating from an in-vacuum undulator (IVU) were monochromated (Ek = 11.070 keV, and λ = 1.119 Å) using a Si (111) double-crystal monochromator and focused horizontally and vertically at the sample position [450 (H) μm × 60 (V) μm in full width at half-maximum (fwhm)] with a K−B-type focusing mirror system. A GIWAXS sample stage was equipped with a seven-axis motorized stage for fine alignment of thin samples, and the incidence angle of the X-rays was adjusted to 0.12°. GIWAXS patterns were recorded using a two-dimensional CCD detector (Rayonix SX165). X-ray irradiation required 3 s, depending on the saturation level of the detector. The diffraction angle was calibrated to a sucrose standard (monoclinic, P21, a = 10.8631 Å, b = 8.7044 Å, c = 7.7624 Å, and β = 102.938°). The sample−detector distance was ∼225.42 mm. Samples for GIWAXS measurements were prepared by spin-coating polymer or polymer:PC71BM (1:1.5 w/w) blended solutions (chlorobenzene, 15 mg/mL) without or with 3% (v/v) DIO onto Si substrates. The GIWAXS images were evaluated on the basis of the relationship between the scattering vector (q) and d spacing (q = 2π/ d). The crystallite correlation lengths (Lc in angstroms) for the pristine polymers and polymer:PC71BM blended films with 3% (v/v) DIO were calculated with the equation Lc = (2πK)/fwhm (Å−1). K is a shape factor (∼0.9). The fwhm values are estimated by a Gaussian function fitted to the (100) and (010) diffractions. 4.5. Hole and Electron Mobility Measurements. Hole-only and electron-only devices were fabricated with an ITO/PEDOT:PSS/ polymer:PC71BM/Au and ITO/ZnO/polymer:PC71BM/LiF/Al structure.54,55 The hole and electron carrier mobility were calculated by Mott−Gurney’s law (eq 1)

3. CONCLUSION In conclusion, we demonstrate a systematic study of molecular construction in the PTB7-Th-based regioregular polymers with an A−D1−A−D2 or A−D1−A−D1 configuration. These two regioregular polymers employ the same conjugated backbone but possess different side chains, ethylhexyl thiophene and ethylhexyl thio-thiophene. A different molecular construction has led to different intermolecular interactions, optoelectronic properties, and device performance. The rr-PTB7-Th-based OPV devices exhibited a PCE of only 7.04%, but rr-PTBS with alternating ethylhexylthio side chains showed an improved PCE of 8.68%. The GIWAXS results indicated that the rrPTBS:PC71BM blended film exhibited a face-on orientation that was stronger than that of the rr-PTB7-Th:PC71BM blended film. In the morphology analysis, the rr-PTBS:PC71BM blended film exhibited favorable phase separation and moderate surface roughness, while the rr-PTB7-Th:PC71BM blended film showed a very smooth surface and fibril-like morphology with small domains. The rr-PTBS:PC71BM blended film exhibited hole and electron mobility more balanced than that of the rrPTB7-Th:PC71BM blended film, which indicates that rr-PTBS has bicontinuous interpenetrating pathways for hole and electron transport that are more desirable than those of rrPTB7-Th. The best-optimized rr-PTBS-based OPV devices showed a PCE of 10.31%. These results can serve as a guideline for future material design. 4. EXPERIMENTAL SECTION All chemical reagents were purchased from commercial supplies and used without further purification. 4.1. rr-PTB7-Th. Monomeric TB7 (0.305 g, 0.224 mmol) and a DiSn-BDT (0.202 g, 0.224 mmol), toluene (5 mL), and dimethylformamide (DMF) (3 mL) were charged into a flask under a nitrogen atmosphere to obtain a clear solution. The solution was bubbled with nitrogen for 30 min. Then, tetrakis(triphenylphosphine)palladium [Pd(PPh3)4, 5.5 mg, 4.8 μmol] was added to the solution. The mixed solution was stirred at 100 °C for 72 h. After polymerization, 4-bromobenzotrifluoride (0.5 mL) was added to the solution. The resulting solution was stirred for 12 h and then cooled to room temperature.61 Then, the reaction mixture was poured into methanol (100 mL). The precipitated polymer was purified by Soxhlet extraction using methanol, acetone, hexane, dichloromethane, chloroform, and chlorobenzene. The chlorobenzene extract was concentrated and then poured into methanol. The purified polymer was collected and dried under vacuum overnight. Therefore, purified rr-PTB7-Th with a purple-black luster (164 mg, 42%) was obtained. Mn = 35.8 kDa, and PDI = 2.49. Anal. Calcd (%) for (C99H117F2O4S12)n: C, 66.29; H, 6.57; S, 21.45. Found: C, 66.29; H, 6.44; S, 21.32. 4.2. rr-PTBS. Monomeric TB7 (0.739 g, 0.543 mmol) and DiSnBDTS (0.526 g, 0.543 mmol), toluene (12 mL), and DMF (5 mL) were charged into a flask under a nitrogen atmosphere to obtain a clear solution. The solution was bubbled with nitrogen for 30 min. Then, tetrakis(triphenylphosphine)palladium [Pd(PPh3)4, 11.6 mg, 0.01

J=

9 V2 εrε0μ 3 8 L

(1)

where ε0 is the free-space permittivity, εr is the dielectric constant of the polymer:PC71BM blend film, μ is the charge carrier mobility, V is 4307

DOI: 10.1021/acs.chemmater.7b00595 Chem. Mater. 2017, 29, 4301−4310

Chemistry of Materials

■

the applied voltage (V = Vap − Vbi − Vr, where Vap is the applied bias, Vbi is the built-in potential due to the difference in the electrical contact work function, and Vr is the voltage drop due to contact resistance and series resistance across the electrodes), and L is the thickness of a photoactive layer.53 4.6. Fabrication and Measurements of BHJ OPV Devices. BHJ OPV devices were prepared as follows. An indium tin oxide (ITO)-coated glass was cleaned using ultrasonic treatment in acetone, deionized (DI) water, and isopropyl alcohol. The cleaned ITO-coated glass was treated in a UV−ozone chamber for 20 min and spin-coated with a ZnO solution. The ZnO solution was prepared by dissolving zinc acetate dihydrate [Zn(CH3COO)2·2H2O, 1.0 g] and ethanolamine (NH 2 CH 2 CH 2 OH, 0.28 g) in 2-methoxyethanol (CH3OCH2CH2OH, 10 mL). The ZnO-coated ITO glass was annealed for 1 h at 200 °C in the atmosphere. The thickness of the ZnO layer was ∼40 nm. The blended polymer and [6,6]-phenyl-C71butyric acid methyl ester (PC71BM) in different weight ratios were dissolved in chlorobenzene with DIO or DPE. The polymer:PC71BM blended solution was spin-coated onto the ZnO layer and dried at room temperature. Then, MoO3 (10 nm) and Ag (100 nm) were deposited using thermal evaporation in a high-vacuum thermal evaporator (