Ordered orientation and compact molecule packing due to coplanar backbone structure of interlayer: Improvement in fill factor for photovoltaic device

European Polymer Journal 116 (2019) 330–335

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Ordered orientation and compact molecule packing due to coplanar backbone structure of interlayer: Improvement in fill factor for photovoltaic device

T

Doo Hun Kimb,1, Woong Cheol Seoka,1, Jong Tae Leema,1, Yong Woon Hanc, Ju Hui Kanga, ⁎ Ho Jun Songa, a Research Institute of Sustainable Manufacturing Systems, Intelligent Sustainable Materials R&D Group, Korea Institute of Industrial Technology, 89 Yangdaegiro-gil, Ipjang-myeon, Seobuk-gu, Cheonan-si, Chungcheongnam-do 331-822, Republic of Korea b Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST), Wanju, Jeonbuk 55324, Republic of Korea c Department of Chemical Engineering, Konkuk University, 120 Neungdong-ro, Gwangjin-gu, Seoul 05029, Republic of Korea

A R T I C LE I N FO

A B S T R A C T

Keywords: Small molecule Interlayer OPVs Orientation

A novel alcohol/water-soluble small molecule was obtained using a p-type planar backbone. The synthesized molecule was dissolved in organic solvents and highly polar solvents. The 3,3′-(((3,3″′-dimethyl[2,2′:5′,2″:5″,2″′-quarterthiophene]-5,5″′-diyl)bis(4,1-phenylene))bis(oxy))bis(N,N-dimethylpropan-1-amine) (QTA) film exhibited a red-shifted spectrum compared with the solution spectrum owing to its many more planar molecular conformations in the solid state. According to X-ray diffraction (XRD) measurements, the QTA film showed sharp diffraction peaks near 3.6–11.0°, which indicates the formation of an interdigitated and ordered structure as an out-of-plane peak (1 0 0) due to the alkyl side chain of the quarter-thiophene backbone. A comprehensive analysis of the out-of-plane and in-plane XRD data suggests that a large fraction of the QTA derivatives was oriented edge-on relative to the substrate. A photovoltaic device containing QTA exhibited an open-circuit voltage of 0.85 V, current density of 15.5 mA/cm2, fill factor of 62.9%, and power-conversion efficiency of 8.4%. The photovoltaic device containing the QTA derivative exhibited improved power conversion efficiency compared with those containing PFN (8.0%) due to the ordered orientation and compact molecule packing of QTA.

1. Introduction Conjugated polymers have been used widely in organic light-emitting diodes (OLEDs) [1–4], organic photovoltaic cells (OPVs) [5–11], and organic thin-film transistors (OTFTs) [12,13] for several decades. On several occasions, OPVs have drawn considerable attention for these applications due to the universal technological tendency toward economic potential and continual growth coupled with efforts to preserve the environment. However, the poor power-conversion efficiency (PCE) of these materials has been the greatest barrier in organic photovoltaic development [6]. To improve the poor PCEs, various efforts are ongoing to control the materials of the active layer and device structure, using various donor–acceptor (D-A)-type polymers, additives, thermal treatments, and morphology control. However, there is still inevitable loss at the

interfaces because of the charge-transport barriers between the active layer and metal cathode [14]. To achieve effective charge transport between the interfaces, several investigations to study the effects of introducing an interlayer have been reported recently. In particular, most research efforts have focused on alcohol/water-soluble conjugated polymer electrolytes (CPEs) for the interlayer. The Cao group reported a PCE of 8.3% with a device structure that introduced poly[(9,9-bis(30-(N,N-dimethylamino) propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) between the active layer and cathode. An inverted device configuration with PFN improved the PCE to 9.2% with increases in the short-circuit current density (JSC), open-circuit voltage (VOC), and fill factor (FF) [15,16]. It has been suggested that these interlayers introduce an enhanced builtin potential across the device because of the presence of an interface dipole, which is conducive to refinement in the charge transport,

⁎

Corresponding author. E-mail address: [email protected] (H.J. Song). 1 Doo Hun Kim, Woong Cheol Seok and Jong Tae Leem contributed equally to this work. https://doi.org/10.1016/j.eurpolymj.2019.04.025 Received 13 February 2019; Received in revised form 10 April 2019; Accepted 10 April 2019 Available online 11 April 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.

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Japan (10 Ω/γ)] were sequentially lithographically patterned, cleaned with detergent, and ultrasonicated in deionized water, acetone, and isopropyl alcohol. The substrates were then dried on a hot plate at 120 °C for 10 min and treated with oxygen plasma for 10 min to improve the contact angle immediately before the film-coating process. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Baytron P 4083 Bayer AG) was passed through a 0.45-μm filter before being deposited onto the ITO substrates at a thickness of approximately 32 nm by spin coating at 4000 rpm in air and then were dried at 120 °C for 20 min inside a glove box. Composite solutions with polymers and PCBM were prepared using chlorobenzene (CB) with 1,8-diiodooctane (DIO). The concentration was adequately controlled in the 0.3–0.5 wt% range. The solutions were then filtered through a 0.45-μm PTFE filter and spin coated (500–2000 rpm, 30 s) on top of the PEDOT:PSS layer. The PFN solution in methanol and acetic acid was spin coated on the top of the obtained active layer at 4000 rpm for 30 s to form a thin interlayer of 8–10 nm. The device fabrication was completed by depositing thin layers of Al (200 nm) at pressures of less than 10−6 torr. The active area of the device was 4.0 mm2. Finally, the cell was encapsulated with UV-curing glue (Nagase, Japan). In this study, all of the devices were fabricated with the following structure: ITO glass/ PEDOT:PSS/polymer:PCBM/with or without interlayer/Al/encapsulation glass. The illumination intensity used to test the OPVs was calibrated using a standard Si photodiode detector equipped with a KG-5 filter. The output photocurrent was adjusted to match the photocurrent of the Si reference cell to obtain a power density of 100 mW/cm2. After encapsulation, all of the devices were operated under ambient atmosphere at 25 °C. The current–voltage (I–V) curves of the photovoltaic devices were measured using a computer-controlled Keithley 2400 source measurement unit (SMU) that was equipped with a Peccell solar simulator under an illumination of AM 1.5G (100 mW/cm2).

elimination of the built-up space charge, and decrease in the recombination losses of the charge carriers [14,15]. Small molecules have lots of advantages compared with polymers, such as straightforward synthesis and purification, monodispersity, precise structures, residual end functionality, and good reproducibility [17,18]. Numerous research studies on active materials using small molecules have been reported [19–21]; however, only a few investigations of small molecule interlayers produced by solution processing have been reported in the fields of OLEDs and OPVs [4,22]. The Fang group reported a PCE of 9.2% with a small molecule interlayer using an n-type benzothiadiazole derivative in an inverted cell [23]. Recently, we reported small molecule interlayers using p- and n-type derivatives with a PCE of 7.9% in a conventional device structure [24]. In this study, we synthesized alcohol/water-soluble small molecules with electron-rich mesogen backbones. We introduced the mesogen molecule into the backbone to improve the charge-transfer properties between the interfaces and form a well-ordered orientation. Because of the effective charge transfer and ordered orientation, the FF, VOC, and PCE values of the device were improved in these evaluations. 2. Experimental section 2.1. Instruments and characterization Unless otherwise specified, all reactions were performed under a nitrogen atmosphere. The solvents were dried using standard procedures. All column chromatography was performed with silica gel (230–400 mesh, Merck) as the stationary phase. 1H NMR spectra were collected using a Bruker ARX 400 spectrometer on solutions in CDCl3 with chemical concentrations recorded in ppm using TMS as the internal standard. Elemental analyses were performed using an EA1112 apparatus with a CE Instrument. The electronic absorption spectra were measured in chloroform using an HP Agilent 8453 UV–Vis spectrophotometer. The cyclic voltammetric curves were obtained using a Zahner IM6eX electrochemical workstation with a 0.1 M acetonitrile solution (purged with nitrogen for 20 min) containing tetrabutylammonium hexafluorophosphate (Bu4NPF6) as the electrolyte at a constant scan rate of 50 mV/s. Indium tin oxide (ITO), Pt wire, and silver/silver chloride [Ag in 0.1 M KCl] were used as the working, counter, and reference electrodes, respectively. The electrochemical potential was calibrated against Fc/Fc+. The highest occupied molecular orbital (HOMO) levels of the polymers were determined using the oxidation onset value. The onset potentials are defined as the values obtained from the intersection of the two tangents drawn at the rising current and the baseline changing current of the capacitance–voltage curves. Thermographic analysis was performed on a NETZSCH TG 209 F3 thermogravimetric analyzer. Differential scanning calorimetry (DSC) was used to determine the phase-transition temperatures on a Netzsch DSC 200 F3 Maia differential scanning calorimeter with a constant heating/cooling rate of 10 °C/min. All gel-permeation chromatography analyses were performed using tetrahydrofuran as an eluent and a polystyrene standard as a reference. Grazing-incidence X-ray diffraction (GIXD) patterns were obtained using a SmartLab 3 kW (40 kV, 30 mA, Cu target, wavelength: 1.541871 Å) instrument from Rigaku, Japan. Topographic images of the active layers were obtained through atomic force microscopy (AFM) in the tapping mode under ambient conditions using an XE-100 instrument. Scanning Kelvin probe microscopy (SKPM) measurements were carried out on AFM equipment using the standard SKPM mode. Theoretical analyses were performed using density functional theory (DFT) as approximated by the B3 LYP functional and employing the 6-31G* basis set in Gaussian09.

2.3. Materials All reagents were purchased from Aldrich, Acros, or TCI. All chemicals were used without further purification. PTB7 was purchased from Nano Clean Tech (Product No.: OS0737). The following compounds were synthesized following modified literature procedures: 5,5′bis(3-dodecylthiophen-2-yl)-2,2′-bithiophene (M3) [8] and N,N-dimethyl-3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenoxy) propan-1-amine (M2) [24]. 3,3′-(((3,3″′-dimethyl-[2,2′:5′,2″:5″,2″′-quarterthiophene]5,5″′-diyl)bis(4,1-phenylene))bis(oxy))bis(N,N-dimethylpropan-1amine) (QTA) M2 (0.28 g, 0.93 mmol), M3 (0.25 g, 0.31 mmol), and Pd(PPh3)4(0) (0.07 g, 0.06 mmol), were placed in a Schlenk tube and purged with three nitrogen/vacuum cycles. Under nitrogen atmosphere, 2 M degassed aqueous K2CO3 (10 mL) and dry 1,4-dioxane (20 mL) were added. The mixture was heated to 110 °C and stirred in the dark for 48 h. After reaction quenching, the mixture was poured into 50 mL water and extracted with CHCl3 (100 mL). The combined organic layers were washed with brine and dried over anhydrous MgSO4. The solvent was removed by rotary evaporation, and the final product was obtained after vacuum drying. A yellowish-green solid was obtained (0.21 g, yield: 67%). 1H NMR (400 MHz; CDCl3; Me4Si): δ = 7.53 (d, 4H), 7.15 (d, 2H), 7.06 (d, 4H), 6.93 (d, 4H), 4.06 (t, 4H), 2.79 (t, 4H), 2.48 (t, 4H), 2.02 (t, 4H), 1.68 (m, 14H), 1.27 (m, 40H), 0.89 (t, 6H), 0.093 (t, 6H). 13C NMR (100 MHz; CDCl3; Me4Si): 158.75; 142.06; 140.69; 136.48; 135.46; 128.85; 126.80; 126.05; 125.10; 123.81; 114.88; 66.34; 56.39; 45.54; 31.95; 30.58; 29.72; 29.63; 29.51; 29.39; 27.55; 22.72; 14.15; 1.03. Anal. Calcd for: C62H68N2O2S4: C, 74.36; H, 6.84; N, 2.80; O, 3.20; S, 12.81. Found: C, 70.4; H, 8.6; N, 2.6; O, 4.5; S, 12.3.

2.2. Fabrication and characterization of polymer solar cells All of the bulk-heterojunction PV cells were prepared using the following device-fabrication procedure. Glass/ITO substrates [Sanyo, 331

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Scheme 1. Synthesis route of QTA.

3. Results and discussion 3.1. Synthesis and thermal properties Fig. 1. Absorption spectra of (a) QTA molecule in solution & thin film and (b) without or with spin-coated QTA interlayer on the top of the active layer (PBDB-T:ITIC).

Scheme 1 shows the chemical structures of the materials and the synthesis process. As shown in Scheme 1, QTA was synthesized by a Suzuki coupling reaction with monomers M2 and M3. The reaction mixtures were heated for 48 h at 110 °C with palladium(0) catalysts and a 2 M potassium carbonate solution in 1,4-dioxane as a solvent. The synthesized QTA was purified by several rounds of re-crystallization with methanol/H2O. The yield of QTA was 67%. The obtained QTA was soluble in organic solvents (chlorobenzene, ortho-dichlorobenzene, and chloroform) and highly polar solvents (methanol and ethanol) (see Table 1). The solubility of QTA in methanol was investigated. Acetic acid was added to methanol to improve the solubility relative to the protonation of the terminal dimethylamino groups. For the same weight ratio (1 mg of material/cosolvent, 4 μL acetic acid + 1 mL MeOH), QTA completely dissolved with only a bit of heat treatment. Thus, QTA exhibited higher solubility in highly polar solvents than other molecules, which was due to the long alkyl chain of its backbone. By using QTA solutions, uniform

and semi-transparent films were formed by spin coating. Fig. S3 shows the thermal properties of the QTA molecule. QTA experienced 5% weight loss at temperatures of 373 °C. This result was similar to the thermal stability of a conjugated polymer, which indicates that a QTA derivative is applicable to OLED and OPVs demanding high thermal stability above 300 °C [6]. We investigated the thermal phenomena using DSC (Fig. S4). The DSC thermograms of QTA revealed no obvious liquid-crystal phase transitions and exhibited only melting to an isotropic phase upon heating at 97.1 °C. 3.2. Optical and electrochemical properties Fig. 1 exhibits the UV–visible (UV–vis) spectra of QTA derivative.

Table 1 Optical & electrochemical properties of the all molecules. Molecules

QTA

Absorption, λmax (nm) Solutiona

Filmb

261, 305, 406

268, 307, 416

Eoxonset (V)

EHOMO (eV)c

ELUMO (eV)d

Eopt (eV)e

Absorption coefficient

0.83

−5.15

−2.80d

2.35

3.7 × 10−4

Absorption spectrum in MeOH solution (10−6 M). Spin-coated thin film (50 nm). c Calculated from the oxidation onset potentials under the assumption that the absolute energy level of Fc/Fc+ was −4.8 eV below a vacuum. d HOMO – Eopt. e Calculated from the redox onset potentials under the assumption that the absolute energy level of Fc/Fc+ was −4.8 eV below a vacuum. Estimated from the onset of UV–vis absorption data of the thin film. a

b

332

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Fig. 3. (a) X-ray diffraction pattern in thin films of QTA molecule (out-of-plane & in-plane) and (b) torsion angels of QTA molecule through the 6-31G* basis set in Gaussian09.

acid and MeOH. In the out-of-plane diffraction pattern of QTA, as shown in Fig. 3, sharp diffraction peaks occurred at 3.6°, 7.3°, and 11.0°, which indicates the formation of an interdigitated and ordered structure of an out-ofplane peak (1 0 0) due to the alkyl side chain of quarter-thiophene backbone. No prominent diffraction peak occurred between 20° and 30°, whereas in the in-plane diffraction pattern of QTA, prominent diffraction peaks were exhibited at 21.2° and 26.7°. This result indicates an in-plane peak (0 1 0) of the molecular packing by π–π stacking, and the shortest π–π stacking distance of QTA was 0.33 nm (λ = 2dsinθ) [6,25]. Because of this short π–π stacking distance, it is expected that the shunt resistance and FF of the OPV device would increase. In addition, based on comprehensive analysis of the out-of-plane and inplane XRD data, it is suggested that a large fraction of the QTA derivatives were oriented edge-on relative to the substrate. In other words, the π–π stacking direction was vertical to that of the substrate [26]. The torsion angles were comparatively investigated by calculating QTA derivative using a DFT calculation. For the QTA molecule, the largest torsion angles between thiophene derivatives (θ1, θ2, θ4) were 16–35°. However, the center torsion angle between thiophenes derivatives (θ3) was 3–5°, which was much lower than the others. This results of DFT calculation suggest that QTA molecules have a planar backbone, which leads them to have strong π–π stacking interactions and a short π–π stacking distance [25]. This result coincided with the aforementioned XRD results. As shown in the XRD results, the QTA molecule showed a considerably short π–π stacking distance of 0.33 nm.

Fig. 2. (a) cyclic voltammogram of QTA and (b) band diagram of QTA, ITO, PC71BM, Al.

The maximum absorption peaks of QTA (λmax) were observed at 261, 305, and 408 nm in solution (10−5 M in a co-solvent of acetic acid and methanol) and at 268, 307, and 416 nm in the thin film. The QTA film spectrum was red-shifted compared with the solution spectrum, which was explained by its many more planar molecular conformations in the solid state [25]. The calculated optical band gap of QTA from the measured value of UV onset for the films were 2.35 eV. Fig. 1(b) shows the UV–vis spectra of ITO/PEDOT:PSS/PBDBT:ITIC/without or with QTA interlayer. The absorption spectrum of the structure with QTA showed increased in intensity from 350 nm to 500 nm compared to that without an interlayer. These results may be attributed to the energy transfer between the active layer and the QTA interlayer, which is expected to improve the photocurrent in OPV devices [23]. Fig. 2 shows the cyclic voltammograms of all of the molecules. The cyclic voltammograms were recorded in 0.1 M tetrabutylammoniumhexafluorophosphate acetonitrile solution. As shown in Fig. 2, QTA showed typical p-type oxidation peaks. The oxidation (Eoxonset) of QTA exhibited at +0.83 V, and the HOMO energy levels of QTA determined through calculation were −5.15 eV. The lowest unoccupied molecular orbital (LUMO) energy levels were obtained from the gap between the HOMO energy levels and the optical band-gap energies. As a result, the LUMO levels of QTA were −2.80 eV. As shown in the band diagram in Fig. 2(b), the QTA HOMO level (−5.15 eV) was similar to that of PTB7 (−5.15 eV) or PEDOT:PSS (5.20 eV), which can behave as a holetransport layer in OLED or perovskite cells.

3.4. Characterization of OPV devices Fig. 4 and Table 2 show the data for device performance. Each device was fabricated as follows: ITO (170 nm)/PEDOT:PSS (40 nm)/ active layer (50–80 nm)/interlayer (∼8 nm)/Al (100 nm). In the active layer, we introduced two donor-type polymers (poly[thieno[3,4-b]thiophene-benzodithiophen], PTB7, and poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl) benzo[1,2-b:4,5-b′]dithiophene)-co-(1,3-di(5-

3.3. XRD analysis and DFT calculation Fig. 3 exhibits the XRD investigation of the QTA-derivative film to analyze the ordering structures. The film sample was fabricated as spincasted films on the surface of Si wafers using solutions containing acetic 333

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Table 2 Photovoltaic performance of the BHJ solar cells. Device structure

VOC (V)

JSC (mA/ cm2)

FF (%)

PCEave (%)

PCEmax (%)

Rs (Ω/ cm2)

Rsh (Ω/ cm2)

Ala QTA/Ala Alb MeOH/ Alb PFN/Alb QTA/Alb ZnOc ZnO/ QTAc

0.53 0.69 0.61 0.61

13.3 13.4 15.1 15.1

60.8 71.8 52.3 55.1

4.7 7.2 4.8 5.0

4.7 7.2 4.9 5.1

4.0 2.5 10.6 8.4

702 1307 594 421

0.85 0.85 0.89 0.87

15.6 15.5 16.4 15.1

59.7 62.9 55.5 67.5

8.0 8.3 8.1 9.0

8.0 8.4 8.2 9.0

8.3 7.2 9.9 6.4

620 1291 532 1210

a Conventional device structure: ITO/PEDOT:PSS/PTB7:PC71BM(1:1.5)/ without or with interlayer/Al. b Conventional device structure: ITO/PEDOT:PSS/PBDB-T:ITIC(1:1)/ without or with interlayer/Al. c Inverted device structure: ITO/ZnO/without or with interlayer /PBDBT:ITIC(1:1)/MoO3/Ag.

ratio for PBDB-T/ITIC was 1:1 by weight, and the active layer was spin coated from a mixed solvent (chlorobenzene 97 vol%, 1,8-diiodoctane 3 vol%). A film of the QTA solution was formed on top of the active layer (thickness of 8 nm) using spin coating. An Al electrode (100 nm) was deposited on top by evaporation through a shadow mask. More than 100 devices were fabricated to confirm device reproducibility. The OPV device without an interlayer exhibited a VOC of 0.53 V, JSC of 13.3 mA/cm2, FF of 60.8%, and PCE of 4.7%, which are similar to the results of a PTB7 device reported in the literature [15,27]. In the case of PTB7:PC71BM/QTA/Al device structure, the VOC, JSC, FF, and PCE were 0.71 V, 13.4 mA/cm2, 71.8%, and 7.2%, respectively. Upon introduction of the QTA interlayer, the FF dramatically increased from 60.8% to 71.8%, which suggests that the short π–π stacking distance of the QTA derivative leads to an improved FF. This result was confirmed again for the other device structure. In the case of PBDB-T:ITIC/Al device structure, the VOC, JSC, FF, and PCE were 0.61 V, 15.1 mA/cm2, 52.3%, and 4.9%, respectively. When the active layer was rinsed with methanol solution, the FF was somewhat improved compared with that of the basic structure (52.3% → 55.1%). However, upon introducing the QTA interlayer, the FF dramatically increased from 52.3% to 62.9%, which is due to the short π–π stacking distance of the QTA derivative. Finally, the PBDB-T:ITIC/QTA/ Al device structure showed VOC 0.85 V, JSC 15.5 mA/cm2, FF 62.9%, and PCE 8.4%. To compare with another interlayer effect, we investigated Poly [(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-ioctylfluorene)] (PFN) of the representative interlayer. In the case of the PBDB-T:ITIC/PFN/Al device structure, the VOC, JSC, FF, and PCE were 0.85 V, 15.6 mA/cm2, 59.7%, and 8.0%, respectively. The device with PFN showed similar VOC and JSC values, but exhibited a lower FF value (59.7%) compared with that of QTA (62.9%). This indicates that a wellordered orientation and the dense π–π stacking of the QTA interlayer influence the improvement in FF more than the PFN interlayer [24]. We also fabricated an inverted device as follows: ITO (170 nm)/ZnO (10 nm)/without or with QTA (∼5 nm)/active layer (50–80 nm)/MoO3 (5 nm)/Ag (100 nm). For a device without QTA, VOC, JSC, FF, and PCE were 0.89 V, 16.4 mA/cm2, 55.5%, and 8.2%, respectively. The inverted OPV device with QTA showed VOC of 0.87 V, JSC of 15.1 mA/ cm2, FF of 67.5%, and PCE of 9.0%. To investigate the oxidative stability of photovoltaic devices based on molecules (PFN, QTA), the current density–voltage (J–V) curves of the devices were measured under ambient conditions for 20 days. All of the devices were encapsulated to prevent oxidation of the metal electrode and maintained under ambient conditions. As shown in Fig. 4(c),

Fig. 4. (a) J-V characteristics of active layer (PTB7:PC71BM), (b) J-V characteristics of active layer (PBDB-T:ITIC) for the BHJ solar cells with the device (Conventional device structure: ITO/PEDOT:PSS/active layer/without or with interlayer/Al. Inverted device structure: ITO/ZnO/without or with interlayer /active layer /MoO3/Ag.) and (c) Durability test data of photovoltaic devices without ETL and with PFN, QTA for convention device structures.

thiophene-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c′]dithiophene4,8-dione)], PBDB-T) and two acceptor-type derivatives (phenyl-C71butyric acid methyl ester, PC71BM, and indacenodithieno[3,2-b]thiophene-based nonfullerene, ITIC). The blend ratio for PTB7/PC71BM was 1:1.5 by weight, and the active layer was spin coated from a mixed solvent (chlorobenzene 97 vol%, 1,8-diiodoctane 3 vol%). The blend 334

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the PCE values of the devices without ETL and with PFN showed sharp decrease after 200 h. However, the PCE values of the device with QTA had changed slightly after 200 h. Moreover, the PCE values of the device with QTA decreased by only approximately 91–92% after 450 h, which showed superior durability compared with that with PFN (68–69% after 450 h) because of the electrochemical stability of QTA.

[7]

[8]

4. Conclusions

[9]

In this study, we synthesized novel alcohol/water-soluble small molecules with p-type and mesogen derivatives (QTA) using a simple synthesis process. The synthesized molecule exhibited good thermal stability and high solubility for organic and polar solvents. The QTA film showed sharp diffraction peaks near 3.6–11.0°, which indicates the formation of an interdigitated and ordered structure as an out-of-plane peak (1 0 0) due to the alkyl side chain of quarter-thiophene backbone. Specially, QTA film showed short π–π stacking distance as 0.33 nm compared with other interlayer molecules. The OPV device using QTA exhibited improved characteristics compared with the devices without an interlayer (FF 52.3% vs. 62.9%, PCE 4.9% vs. 8.4%) due to the reduction of the interfacial resistance and ordered orientation. QTA was assumed to form flat and ordered packing due to its planar structure, which may lead to electron transport compared with the packing of a PFN film. In the case of an inverted device structure, the OPV device using QTA exhibited more improved PCE (9.0%) than the conventional device structure.

[10]

[11]

[12]

[13]

[14]

[15]

[16]

Acknowledgements

[17]

This research was financially supported by the Ministry of Trade, Industry and Energy (MOTIE) and Korea Institute for Advancement of Technology (KIAT) through the Research and Development for Regional Industry.

[18]

[19]

Appendix A. Supplementary material

[20]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.eurpolymj.2019.04.025.

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