Polystyrene-block-Poly(ionic liquid) Copolymers as Work Function

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Polystyrene-block-poly(ionic liquid) copolymers as work function modifiers in inverted organic photovoltaic cells Jong Baek Park, Mehmet Isik, Hea Jung Park, In Hwan Jung, David Mecerreyes, and Do-Hoon Hwang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17635 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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ACS Applied Materials & Interfaces

Polystyrene-block-poly(ionic liquid) copolymers as work function modifiers in inverted organic photovoltaic cells Jong Baek Park†, Mehmet Isik‡, Hea Jung Park†, In Hwan Jung§, David Mecerreyes*,‡ and Do-Hoon Hwang*, † †

Department of Chemistry, and Chemistry Institute for Functional Materials, Pusan National

University, Busan 609-735, Republic of Korea ‡

POLYMAT, University of the Basque Country UPV/EHU Joxe Mari Korta Center, Avda.

Tolosa, 72, 20018 Donostia-San Sebastian, Spain §

Department of Chemistry, Kookmin University, 77 Jeongneung-ro, Seongbuk-gu, Seoul 02707,

Republic of Korea

ACS Paragon Plus Environment

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ABSTRACT

Interfacial layers play a critical role in building up the ohmic contact between the electrodes and functional layers in organic photovoltaic solar cells. These layers are based on either inorganic oxides (ZnO, TiO2) or water-soluble organic polymers such as poly[(9,9-dioctyl-2,7-fluorene)alt-(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)]

(PFN)

and

polyethylenimine

ethoxylated (PEIE). In this work, we have developed a series of novel poly(ionic liquid) nonconjugated block copolymers for improving the performance of inverted organic photovoltaic cells (OPVs) by acting as work function modifiers of indium tin oxide (ITO) cathode. Four nonconjugated block polyelectrolytes (n-CPE) based on polystyrene and imidazolium poly(ionic liquid) (PSImCl) were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. The ratio of hydrophobic/hydrophilic block copolymers was varied depending on the ratio of polystyrene to PSImCl block. The ionic density, which controls the work function of the electrode by forming an interfacial dipole between the electrode and n-CPE, was easily tuned by simply changing of the PSImCl molar ratio. The inverted OPV device with the ITO/PS29-b-PSImCl60 cathode achieved the best power conversion efficiency (PCE) of 7.55 % among the synthesized n-CPEs, exhibiting an even higher PCE than that of reference OPV device with PEIE (7.30%). Furthermore, the surface properties of n-CPE films were investigated by

contact

angle

measurements

to

explore

the

influence

of

the

controlled

hydrophobic/hydrophilic characters on the device performances.

KEYWORDS: poly(ionic liquids), block copolymers, interfacial layer, work function modifiers, non-conjugated block polyelectrolytes, organic photovoltaic cells


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INTRODUCTION Bulk heterojunction (BHJ) organic photovoltaic cells (OPVs) composed of a conjugated polymer donor and a fullerene-based acceptor have attracted significant attention as renewable energy sources owing to their low cost, flexibility, and the possibility of large-area module manufacturing.1–4 Recent developments have shown that the inverted OPV device configuration with indium tin oxide (ITO) as the bottom cathode and an air-stable high work function (WF) metal (Ag or Au) electrode as the top anode leads to better device stability by eliminating the degradation caused by the oxidation of PEDOT:PSS and Al in the normal structure.5–7 However, the large energy gap between the WF of the ITO cathode (~4.8 eV) and the lowest unoccupied molecular orbital (LUMO) level of the acceptor materials (3.7–4.2 eV) because of the high builtin field in the device results in Schottky barriers. Therefore, an additional interfacial layer, i.e., electron transport layer (ETL), is introduced for building up the ohmic contact between the cathode/BHJ layer interfaces.8 Metal oxide materials such as zinc oxide (ZnO) and titanium oxide (TiOx) have been previously applied as interfacial layers because of the feasibility of solution processing and outstanding electron selectivity. However, a high annealing temperature (over 200 ℃) is required to fabricate the metal oxide layer and achieve crystallinity of the layer.9–11 Therefore, water/alcohol-soluble organic molecules and polymers containing ionic functional groups have been developed as substitutes for metal oxide layers because they allow fabrication at low temperatures.12–19 The use of organic polymers is advantageous because of their ability to dissolve in environmentally friendly solvents such as alcohol and water, thus providing an orthogonal solvent system to prevent the solvent erosion problem during multilayer fabrication,


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and also enhance charge collection at the electrode because of the presence of polar functional groups. The thin film of water/alcohol-soluble organic polymers primarily works as an interfacial modifier that controls the WF of the electrode rather than as a hole or electron transport layer (HTL or ETL). When the thin film is fabricated on the metal electrode, the ionic functional group induces a permanent dipole moment on the electrode surface. This phenomenon leads to either an upward or a downward shift of the vacuum level of the metal electrode depending on the type of charges (positive or negative) and the number of ionic functional groups in the polymer.20,21 Conjugated polyelectrolytes (CPEs) such as poly[(9,9-dioctyl-2,7-fluerene)-alt-(9,9-bis(3'(N,N-dimethylamino)propyl)-2,7-fluorene)] (PFN) are comprised of conjugated main backbone, side chains, and ionic functional groups. The ionic functional groups of CPE help improve the solubility of the polymer in polar solvents such as alcohol and water, and also assist in controlling the WF of the electrode, which is proportional to the ion density.17 However, the introduction of a large number of ionic functional groups on the monomer to increase the ion density of the CPE requires several synthetic steps and the conjugated backbone of CPE can reduce the active-layer light absorption in the visible region.20–22,24,25 On the other hand, non-conjugated polyelectrolytes (n-CPEs) such as polyethylenimine ethoxylated (PEIE) are synthesized by radical polymerization and are comprised of an aliphatic backbone and either hydrophobic or hydrophilic side chains. They also have an identical operable mechanism as that of CPEs for WF tuning of the electrode in OPV devices and show good optical transparency in the visible region in addition to relatively simple synthetic procedures. Moreover, they are cost-effective and stable in aqueous solution.26-27


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In this study, we designed new non-conjugated block polyelectrolytes comprised of a hydrophobic polystyrene (PS) block and a hydrophilic poly(ionic liquid) block based on 1-(4vinylbenzyl)-3-methyl-imidazolium chloride (PSImCl). These block copolymers were then investigated as interfacial layers in OPVs. Block copolymerization allows control over the hydrophilic and hydrophobic character of the polymer via simple alteration of the monomer ratio, without disturbing the light absorption ability of the active layer when such a block copolymer is used as an interfacial layer in OPV. Therefore, we synthesized four kinds of block copolymers, designated as PS29-b-PSImClx (where x = 15, 29, 45, and 60), by changing the amount of PSImCl, via reversible addition-fragmentation chain transfer (RAFT) polymerization. The number of polystyrene units was fixed to 29 for retaining the hydrophobicity and for sake of comparability with an organic active layer. The number of PSImCl units was varied from 15 to 60 in order to identify an energy level that matched well with that of the acceptor in the active layer. The synthesized n-CPEs were investigated in inverted OPVs as interfacial layers for the modification of the work function of ITO.

EXPERIMENTAL Measurements 1

H NMR spectra were recorded in DMSO-d6 using a Varian Mercury Plus 300 MHz

spectrometer and the chemical shifts were reported in ppm relative to the residual signals of DMSO (2.50 ppm). Gel permeation chromatography (GPC) was performed to confirm the formation of the block copolymers and obtain the polydispersity indices (PDIs) with polystyrene (PS) standards. Ultraviolet photoelectron spectroscopy (UPS) was performed with a PHI 5000 Versaprobe spectrometer (Ulvac-PHI) equipped with a He(I) (hν = 21.2 eV) radiation source. For


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all measurements, a sample bias of –9 eV was applied and the photoemitted electrons were collected at normal emission with a pass energy of 0.585 eV and 4.0×10–8 torr base pressure. The performances of the OPV devices were determined by using a McScience K201 LAB50 Solar Simulator. The current density-voltage (J-V) characteristics of the OPV devices were measured by simulating solar light (AM 1.5G) condition and the illumination intensity was calibrated using a standard Si reference device (PV Measurements Inc., calibrated at the National Renewable Energy Laboratory). The external quantum efficiency (EQE) data were obtained by the McScience K3100 EQX system. The EQE values were characterized as a function of wavelength in the range from 300 to 1100 nm using a Xenon Short Arc Lamp as the light source and the calibration was performed by using a Si photodiode.

Materials All chemicals, styrene, 4-cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanol (CDP), 2,2'-azobisisobutyronitrile (AIBN), and PEIE were purchased from Aldrich, Alfa Aesar, or TCI Korea and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) were purchased from OSM. These materials used without further purification except for AIBN, which was purified by recrystallization from methanol. PSImCl and poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th) were synthesized as described in previous reports.28-30

Synthesis of the poly(styrene)-block-poly(styrene imidazolium chloride) copolymers


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Styrene and styrene imidazolium chloride were polymerized in the presence of the RAFT agent to give amphiphilic diblocks with a fixed hydrophobic block and various hydrophilic lengths (Scheme 1).

Synthesis of the macro-RAFT agent based on styrene Styrene (1.015 g, 9.744 mmol) and the chain transfer agent CPD (0.131 g, 0.336 mmol) were dissolved completely in toluene to obtain a 14% solid concentration in a round-bottom flask at room temperature. The flask was purged with N2 gas for 15 min before the addition of all of the AIBN initiator (0.005 g, 0.033 mmol) into the flask at 70 ℃. The reaction was conducted until full conversion of the monomer was achieved (Scheme 1 (a)). The reaction progress was followed by 1H NMR (Figure S1 (a)). After completion of the reaction, the flask was cooled to room temperature and the crude product was cleaned with methanol and analyzed by NMR and GPC to confirm the formation of the polymer. The molecular weight of the synthesized macroRAFT agent was 3,410 g mol–1, as calculated by 1H NMR.

General procedure for the synthesis of diblock copolymers The synthesis of PS29-b-PSImCl15 has been used as a standard example. The previously synthesized macro-RAFT agent-based styrene was polymerized with the PSImCl monomer by RAFT polymerization. The synthesized macro-RAFT agent (0.635 g, 0.444 mmol) and PSImCl monomer (1.562 g, 6.654 mmol)28 were dissolved in a methanol and chloroform mixture in a round-bottom flask at room temperature. As in the previous method, the solution was purged with N2 gas and the AIBN initiator (0.014 g, 0.087 mmol) was added at 70 ℃. Polymerization was conducted until full conversion of the styrene imidazolium chloride monomer could be


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confirmed (Scheme 1 (b)) by the disappearance of its characteristic peaks at 5.82 and 5.30 ppm in 1H NMR spectra. The crude products were purified by washing with different solvent combinations of diethyl ether, ethyl acetate, water etc. as the block copolymers exhibited different solubility patterns depending on their composition. All of the polymers were analyzed by NMR (Figure S1 (b–e)) and GPC to verify the full conversion of the monomer. Other diblock copolymers, PS29-b-PSImCl29, PS29-b-PSImCl45, and PS29-b-PSImCl60, were synthesized by following the similar synthesis procedure of PS29-b-PSImCl15 with different ratios of PSImCl.

Fabrication of OPVs The inverted OPV solar cells were fabricated as ITO/n-CPE/PTB7-Th:PC71BM/MoO3/Ag. The n-CPE materials were dissolved with various w/v ratios in a mixed solvent (chloroform:methanol = 1:1, v/v) and left overnight at room temperature (Table S2–S5). The active layer solution with PTB7-Th and PC71BM as the electron donor and acceptor materials, respectively, were dissolved in chlorobenzene with 3 vol% of 1,8-diiodooctane (1:1.5 w/w, total concentration = 25 mg mL–1) at ambient condition and stirred overnight at 70 ℃. The ITO-coated glass was cleaned using an ultrasound bath in sequence with a detergent solution, distilled water, acetone, and 2-propanol for 10 min each. After drying in an oven, the ITO substrate was treated with UV-ozone plasma for 20 min. The n-CPE was coated onto the ITO glass at 5000 rpm for 30 s and subsequently baked at 140 ℃ for 10 min on a hot plate under ambient conditions. The active layer was formed by a spin coater operating at 1300 rpm in the glove box filled with argon. The metal sources, namely, MoO3 (8 nm) and Ag (100 nm), were coated by thermal evaporation under a base pressure of ~10–7 torr. The measured active area of the device was 0.09 cm2.


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The electron-only devices (EOD) were fabricated with the following structure: ITO/nCPE/PTB7-Th:PC71BM/LiF/Al. Before the deposition of the LiF layer, all fabrication methods were the same as those used for the OPV devices. The LiF (0.5 nm) and Al (120 nm) layers were formed by thermal evaporation under a base pressure of ~10–7 torr. The devices were measured to have the voltage and the electron mobility was calculated by using the space charge limited current (SCLC) model according to the Mott-Gurney equation. 9 = 8 In this equation, J refers to the current density, ε0 is the free space permittivity (8.85×10–14 C V–1 cm–1), εr is the relative dielectric constant (3.5),31 and V is the difference in the WF of the anode and cathode and is expressed as Vappl – Vbi, where Vappl and Vbi are the applied potential and built-in voltage, respectively. Finally, L is the thickness of the active layer in the device.

RESULT AND DISCUSSION Synthesis and characterization of poly(styrene)-block-poly(styrene imidazolium chloride) The synthetic route of the amphiphilic polystyrene/poly(ionic liquid) block polymers is shown in Scheme 1. In the first step, a polystyrene macro-RAFT agent was synthesized. In a second, step, the prepared macro-RAFT agent was further utilized to synthesize four different block copolymers with varying mole ratios of the two blocks. GPC was performed to confirm the formation of the block copolymers, PS29-b-PSImClx, and determine their PDIs. A tetrahydrofuran (THF) solution of 10 mM of bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) was used as the eluent at 35 ℃ because of the presence of ionic groups in the polymers.


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Before the measurements, the samples were anion-exchanged with LiTFSI and then dissolved in the eluent for performing GPC measurements.32 The GPC results of the synthesized polymers are displayed and summarized in Figure 1 and Table 1. All block copolymers had reasonably low PDIs and the monomodal signals indicated that the macro-RAFT agent exerted good control over the polymerization. In addition, the GPC signal shifted to a lower retention volume than that of RAFT agent because of the increased molecular weight owing to the formation of the block copolymer.

Scheme 1. Synthetic route for (a) macro-RAFT agent and (b) the n-CPE, PS29-b-PSImClx series.


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Figure 1. Size exclusion chromatography (SEC) results of the synthesized polystyrene PS macroinitiator and PS29-b-PSImClx block copolymers. Table 1. Molecular weight properties of the synthesized polymers. Mna (g mol–1)

Mnb (g mol–1)

Mwb (g mol–1)

PDIb

Poly styrene macro-RAFT agent

3,410

6,200

7,300

1.17

PS29-b-PSImCl15

6,460

8,015

9,320

1.16

PS29-b-PSImCl29

9,310

10,400

12,000

1.16

PS29-b-PSImCl45

12,560

15,320

17,600

1.15

PS29-b-PSImCl60

15,620

21,080

24,500

1.16

a

Molecular weights calculated from 1H NMR. standards.

b

Determined by GPC based on polystyrene

The resultant PS29-b-PSImClx were soluble in polar solvents such as THF, methanol, and water, which facilitated their processing as thin films by spin-coating. Optical transmittance


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spectra of the thin films of PS29-b-PSImClx are shown in Figure 2 and compared to that of PEIE as a reference. The film samples for transmittance measurements were fabricated on quartz glasses by following the same procedure as that employed for the fabrication of OPV devices. The PS29-b-PSImClx block copolymers and the reference PEIE thin films (~10 nm) showed high optical transparency as they transmitted almost 100% of the incident light in the region from 250 nm to 600 nm, indicating that the n-CPE layer did not disturb the light absorption of the active layer in the OPV device. The UV-visible spectra of the active layers on PS29-b-PSImClx or PEIE are also shown in Figure S2 and there are no noticeable changes in the UV-visible spectra.

Figure 2. Transmittance spectra of PS29-b-PSImCl15, PS29-b-PSImCl29, PS29-b-PSImCl45, PS29b-PSImCl60, and PEIE thin films coated on quartz substrates.

Measurement of surface properties and WF of the PS29-b-PSImClx coatings


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The surface properties of ITO modified with PS29-b-PSImClx were investigated by contact angle measurements (Figure S3). Since the PS29-b-PSImClx films were damaged by water, diiodomethane (CH2I2) was used for measurement as the dropping liquid instead of water. All PS29-b-PSImClx films exhibited smaller CH2I2 contact angles (16.3–19.7°) than that of PEIE film (20.0°), indicating that the film surfaces coated with PS29-b-PSImClx were more hydrophobic than the PEIE film surface. As the content of the imidazolium salt side chain in the block copolymer increased, the CH2I2 contact angle also gradually increased because of the enhanced hydrophilicity of the film. In a comparison of the PS29-b-PSImClx and PEIE films, this result suggests that the hydrophilic imidazolium salt block in the former was mainly placed on the ITO side and the hydrophobic styrene block was located on the upper side.24 For this reason, the PS29-b-PSImClx coating leads to a slightly more hydrophobic surface than the PEIE layer with its irregular structure. The hydrophobic nature of the modified ITO (as confirmed by contact angle measurements) led to lower series resistances as a consequence of the improved interfacial contact between ITO and the organic active layer in the OPV devices.21,23,24 The AFM images of the PS29-b-PSImClx on ITO glass. There are no noticeable morphology changes on ITO surfaces. Since we diluted the concentration of n-CPEs solution to minimize their insulating properties onto ITO, it is expected that the thin n-CPE layer gave a minimal effect on the surface morphology of ITO. The AFM results are recorded in Figure S4. The WF (Φ) of the PS29-b-PSImClx-coated ITO layer was studied by UPS to confirm the effect of the quantity of imidazolium salt as a polar group on the binding-energy level of ITO/n-CPE. The UPS results of the ITO/PS29-b-PSImClx assembly are presented in Figure 3 (a), Figure S5 and Table S1. The WFs of the surface-modified ITO cathodes were 4.53, 4.42, 4.32, and 4.29 eV for ITO/PS29-b-PSImCl15, ITO/PS29-b-PSImCl29, ITO/PS29-b-PSImCl45, and ITO/PS29-b-


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PSImCl60, respectively, which were 0.25–0.49 eV smaller than that of the bare ITO (4.78 eV). The dramatic decrease in the WFs originated from the interfacial dipole caused by the imidazolium salt groups in the block copolymer. The ordered polar chains formed an aligned dipole, which consequently shifted up the vacuum level (VL) and resulted in the decrease of the WF (Figure 3 (b)).33–35 As the content of the imidazolium salt in the block copolymer was increased, the WF of the PS29-b-PSImClx-coated ITO decreased. The ITO/PS29-b-PSImCl60 assembly exhibited the lowest WF of 4.29 eV among the different PS29-b-PSImClx-coated ITOs, but it still had a slightly higher WF than that of ITO/PEIE (4.24 eV). Further details of the combination effect of the hydrophobic nature and the WFs of the PS29-b-PSImClx interfacial layers are described in the OPV performance section. The energy diagram of the devices that adopted the n-CPE interfacial layer on ITO is presented in Figure 3(c). In the general inverted OPV device, the great difference of energy levels between the cathode and PC71BM leads to the recombination of electrons because of the large Schottky barrier at the cathode/PC71BM interface.8,21 The decrease in the energy gap between the WF of the ITO cathode and the LUMO of PC71BM reduced the Schottky barrier and was expected to lead to an efficient electron collection in the OPV device.


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Figure 3. UPS spectra of the ITO substrate coated with PS29-b-PSImClx or PEIE interfacial layers obtained from (a) near the secondary electron cutoff (Ecutoff) range, (b) proposed WF modification scheme originating from net dipole alignment on the ITO and (c) Energy level diagram of the inverted OPVs with WF-modified cathodes.

Performances of the OPVs


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Inverted OPV devices were fabricated to investigate the influence of the WF modified ITO/PS29-b-PSImClx interfacial layer on the device performances with a device configuration of ITO/n-CPE/PTB7-Th:PC71BM/MoO3/Ag. PEIE was selected as a reference interfacial layer. The OPV results of the devices with the n-CPE interfacial layers are presented in Figures S6–S9 and Tables S2–S5. The EQE data of all devices were well-matched with the short circuit current (JSC) determined from the J-V curve within 5% error range. To control the thickness of the interfacial layer that acted as a WF modifier, the synthesized PS29-b-PSImClx coatings were dissolved in a co-solvent (methanol:chloroform = 1:1, v/v) system with various concentrations from 0.1 to 1.0 mg mL–1. For all of the PS29-b-PSImClx materials, the open circuit voltage (VOC) was improved as the concentration was increased, while the short circuit current (JSC) value was not as significantly affected by the changes in the concentration. However, the J-V curve of the fabricated devices with a high concentration of n-CPEs showed Sshape kinks because of the high resistance of n-CPEs acting as insulators.36 The best performances of all devices was shown at the PS29-b-PSImClx concentration of 0.5 mg mL–1. (Figure 4(a), (b) and Table 2). The device without the interfacial layer showed a low PCE of 0.06% because of the high Schottky barrier located at the ITO/active layer interface. The ITO modified devices with PS29-bPSImClx showed improved PCEs of 7.04, 7.24, 7.30, and 7.55% for ITO/PS29-b-PSImCl15, ITO/PS29-b-PSImCl29, ITO/PS29-b-PSImCl45, and ITO/PS29-b-PSImCl60. The PCE was increased as the content of imidazolium salt unit in the block copolymer was increased. The enhanced PCEs were related to the overall improved parameters such as VOC, JSC, and FF. The device with ITO/PS29-b-PSImCl60 showed the best results of VOC = 0.78 V, JSC = 15.97 mA cm–2, FF = 0.61,


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and PCE = 7.55%. The performance of this device was even better than that of the reference OPV device with ITO/PEIE. For a deeper understanding of the JSC and FF results of the ITO/PS29-b-PSImClx devices, shunt resistance (Rsh) and series resistance (Rs) were investigated by measuring the dark current of device. The electron mobility was also measured by fabricating an EOD by the SCLC method. The resistances and electron mobilities of the devices with n-CPE layers are shown in Figures 4(c), (d) and Table 3. The device without the interfacial layer had a low Rsh of 9.65 Ω cm2 which was caused by the loss of the generated photocarriers by the leakage current. Introduction of the n-CPE interfacial layer led to a significant increase in the Rsh and decrease in the Rs. As the number of polar units (imidazolium salt) in the block copolymer were increased, Rs decreased from 16.81 to 3.09 Ω cm2, while Rsh increased from 207,000 to 730,000 Ω cm2. In the device, the high Rsh and low Rs induced improvements in the device performances because of the reduction in current leakage and loss of photocarriers. Consequently, an increase in the imidazolium salt group ratio in the block copolymer led to high JSC and FF in the OPV device on account of the reduced WF by the permanent dipole preventing the current leakage. The ITO/PEIE devices showed lower Rsh and higher Rs than those of ITO/PS29-b-PSImCl60, even though the former exhibited a lower WF than that of the latter. This reveals that ITO/PS29-b-PSImCl60 has improved interfacial comparability with the upper organic active layer than PEIE, based on the decreased Rs result of the former. The electron mobilities of the devices modified with ITO/PS29-b-PSImClx were increased from 1.09×10–4 to 2.46×10–3 cm2 V–1 s–1 as the content of the imidazolium salt in the block copolymer increased. All ITO/n-CPEs showed faster electron mobilities than that of bare ITO because of the lowered Schottky barrier. The higher electron mobility of ITO/PS29-b-PSImCl60 (2.46×10–3 cm2


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V−1 s−1) than that of ITO/PEIE (1.58×10–3 cm2 V−1 s−1) revealed that the created photocarriers in the ITO/PS29-b-PSImCl60 device can be transferred quickly to the electrode as compare to the ITO/PEIE device, reducing leakage current.37,38 Compared to the reference device using the PEIE layer, the resistances and electron mobilities of the devices with the synthesized block copolymers were better than that of the reference PEIE device. As the ionic groups can support and redistribute the internal electric fields in OPV devices, the conductivity was enhanced by the self-doping effect.25 The stability of OPV devices is important for their practical applications and is affected by several factors such as irradiation, oxygen, moisture, temperature, mechanical stress, and morphology of active layer. 39-42 The stability of non-encapsulated OPV devices based on PTB7Th:PC71BM with the n-CPE layers was measured for 31 days. The device was stored in a glove box and withdrawn to ambient condition on measuring efficiencies. The time-dependent performances of the OPV devices are summarized in Figure S10 and Table S6. The PCEs of all devices were reduced as a result of the decrease in JSC and FF. After 31 days, the performances of the devices modified with PS29-b-PSImCl15, PS29-b-PSImCl29, and PS29-b-PSImCl45 had decreased to 12% of the initial performance, and that of the device modified with PS29-bPSImCl60 dropped to 18% of its initial performance. These are consistent with the reported results that ionic groups in interfacial layer could reduce the OPV device stability.43-46 All the fabricated OPV devices with PS29-b-PSImClx, however, showed better stability than the reference device with PEIE which showed PCE 26% drop for its initial performance.


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Figure 4. (a) J-V curves, (b) EQE spectra, (c) dark current condition adopted for n-CPE layers and (d) electron-only device with or without n-CPE. Only the best results are shown.


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Table 2. Performances of the OPV devices with various n-CPEs.

VOC (V) n-CPE

JSC (mA cm–2)

FF

PCE (%)

a

Average w/o interfacial layer

Best Average

Best

Average

Best Average

Best

5.51 0.05 ± 0.01 0.06 5.03 ± 0.61

[5.30]b

0.24± 0.02

0.23 0.06 ± 0.01 0.07

15.38 PS29-b-PSImCl15 0.77 ± 0.00 0.77 15.48 ± 0.12

[15.05]b

0.58 ± 0.01 0.59 6.92 ± 0. 10 7.04

15.76 PS29-b-PSImCl29 0.77 ± 0.00 0.77 15.58 ± 0.15

[15.20]b

0.59 ± 0.01 0.60 7.08 ± 0.11 7.24

15.90 PS29-b-PSImCl45 0.77 ± 0.00 0.77 15.88± 0.09

[15.27]b

0.59 ± 0.01 0.60 7.22 ± 0.06 7.30

15.97 PS29-b-PSImCl60 0.78 ± 0.00 0.78 15.95 ± 0.41

[15.72]b

0.59 ± 0.01 0.61 7.37 ± 0.16 7.55

15.90 PEIE a

0.77 ± 0.01 0.77 15.92 ±0.10

[15.17]b

0.58 ± 0.01 0.59 7.21 ± 0.07 7.30

ITO/n-CPE/PTB7-Th:PC71BM/MoO3/Ag configuration. b JSC of EQE.


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Table 3. Resistances of the OPV devices in dark conditions and electron mobilities of the devices with n-CPE.

n-CPE

RS (Ω cm2)

Rsh (Ω cm2)

Mobility µe (cm2 V−1 s−1)

w/o

9.60

9.65

3.67×10–5

PS29-b-PSImCl15

16.81

207,000

1.09×10–4

PS29-b-PSImCl29

6.70

254,000

1.64×10–4

PS29-b-PSImCl45

3.46

432,000

8.09×10–4

PS29-b-PSImCl60

3.09

730,000

2.46×10–3

PEIE

3.95

487,000

1.58×10–3

CONCLUSIONS In this work, we have applied new n-CPE type block copolymers as WF modifiers in an OPV device based on polystyrene with various amounts of the imidazolium chloride salt. Four kinds of block copolymers (PS29-b-PSImCl15, PS29-b-PSImCl29, PS29-b-PSImCl45, and PS29-bPSImCl60) were successfully synthesized by controlling the ratio of styrene imidazolium chloride using the poly styrene macro-RAFT agent. As the number of polar groups (imidazolium salt) was increased, the WF of the ITO electrode coated with n-CPE decreased because of the enhanced interfacial dipole. This phenomenon led to an improvement in the overall OPV parameters by reducing the Schottky barrier and strengthening the built-in potential of the device. As a result, the PCE of the device was more enhanced and proportional to the ratio of imidazolium salt in the block copolymer. The best performance was shown by the device


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modified with PS29-b-PSImCl60 (PCE = 7.55%). This performance was higher than that of the reference OPV device with PEIE (7.30%). Although ITO/PS29-b-PSImCl60 had a higher WF than ITO/PEIE, it exhibited a better OPV performance. Contact angle measurements proved that PS29b-PSImCl60 formed a more hydrophobic film surface than PEIE and consequently had better comparability with the organic active layer with decreased series resistance than PEIE. Thus, the approach of controlling the ratio of the simple polar pendant in the aliphatic block copolymer proved successful for tuning both the WF of the electrode and interfacial comparability.

ASSOCIATED CONTENT Supporting Information. 1H NMR spectra, surface properties of n-CPE thin films, photovoltaic properties of the devices having different n-CPE concentration, and stability properties of the OPV devices. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Prof. D.-H. Hwang ([email protected]) Prof. D. Mecerreyes ([email protected])

ACKNOWLEDGMENT This work was supported by the National Research Foundation (NRF) grants funded by the Korea Government (NRF-2017R1A2A2A05001345 and 2011-0030013 through GCRC SOP)


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and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20173010012960).

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Table of Contents (TOC)


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Polystyrene-block-Poly(ionic liquid) Copolymers as Work Function

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