Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 44060−44069
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Effect on Electrode Work Function by Changing Molecular Geometry of Conjugated Polymer Electrolytes and Application for HoleTransporting Layer of Organic Optoelectronic Devices Eui Jin Lee,‡ Min Hee Choi,§ Yong Woon Han,† and Doo Kyung Moon*,† †
Department of Materials Chemistry and Engineering, Konkuk University, 120, Neungdong-ro, Gwangjin-gu, Seoul 05029, Republic of Korea ‡ Convergence Research Center for Solar Energy, Daegu Gyeongbuk Institute of Science and Technology, 333, Techno Jungang daero, Dalseong-gun, Daegu 42988, Republic of Korea § Progressive Technology Research Group, KOLON Central Research Park, 30, Mabuk-ro 154beon-gil, Giheung-gu, Yongin-si 16910, Gyeonggi-do, Republic of Korea S Supporting Information *
ABSTRACT: In this study, we synthesized three conjugated polymer electrolytes (CPEs) with different conjugation lengths to control their dipole moments by varying spacers. P-type CPEs (PFT-D, PFtT-D, and PFbT-D) were generated by the facile oxidation of n-type CPEs (PFT, PFtT, and PFbT) and introduced as the hole-transporting layers (HTLs) of organic solar cells (OSCs) and polymer light-emitting diodes (PLEDs). To identify the effect on electrode work function tunability by changing the molecular conformation and arrangement, we simulated density functional theory calculations of these molecules and performed ultraviolet photoelectron spectroscopy analysis for films of indium tin oxide/CPEs. Additionally, we fabricated OSCs and PLEDs using the CPEs as the HTLs. The stability and performance were enhanced in the optimized devices with PFtT-D CPE HTLs compared to those of PEDOT:PSS HTL-based devices. KEYWORDS: organic optoelectronic devices, stability, hole-transporting layer, conjugated polymer electrolytes, molecular dipole
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INTRODUCTION Organic optoelectronic devices (OOEDs), which can convert light into electricity (organic solar cells, OSCs) or electricity into light (organic light-emitting diodes, OLEDs), have attracted tremendous academic and industrial attention for sustainable energy conversion applications.1−4 Recently, with the research and development of various organic semiconducting materials and device structures, the efficiency and stability of OOEDs have improved dramatically.5−11 The power conversion efficiency (PCE) of OSCs is more than 11%, a level acceptable for commercialization.5,9,12 Prototypes and commercial applications of OLEDs in lighting and displays have already been developed. The device structures, including the materials and the functions of each layer in OSCs and OLEDs, which typically consist of substrates, transparent electrodes, buffer layers, and organic active layers, are almost same. Because of this structural, material, and functional similarity, studies on OSCs and OLEDs are complementary in nature. For example, conjugated polymer electrolytes (CPEs)13,14 and transition-metal oxides15 have been investigated as buffer materials in OOEDs to facilitate the transportation of charge carriers (electrons or holes). In terms of the efficiency and stability of OOEDs, charge transport, which means the extraction of charge carriers for OSCs and the injection of carriers for OLEDs, is a critical © 2017 American Chemical Society
factor. Therefore, controlling the interfacial properties of each layer is necessary to obtain highly efficient and stable devices. By understanding and improving charge transport via interfacial engineering, the performance and long-term stability of OOEDs have progressed.13,14,16−18 To promote the transportation of charge carriers and enhance the performance of OOEDs, electron- or holetransport layers are introduced above or below the organic active layers. Zirconium acetylacetonate (ZrAcac),19 cesium carbonate (Cs2CO3),20 and conjugated organic compounds, for example, poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN),21 3,3′,3″,3‴((pyrene-1,3,6,8-tetrayltetrakis(benzene-4,1-diyl))tetrakis(oxy))tetrakis(N,N-dimethylpropan-1-amine),22 and 3,6-bis(5(4-(3-(dimethylamino)propoxy)-phenyl)thiophen-2-yl)-2,5-bis(2-hexyl-decyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione,23 have been introduced as electron-transport layer (ETL) of OOEDs. Specifically, PFN and derivatives thereof are commonly sold by suppliers like 1-materials, Lumtec, and Ossila. However, materials for hole-transport layers (HTLs), which should be soluble in both water and alcohols to avoid Received: July 28, 2017 Accepted: November 21, 2017 Published: November 21, 2017 44060
DOI: 10.1021/acsami.7b11164 ACS Appl. Mater. Interfaces 2017, 9, 44060−44069
Research Article
ACS Applied Materials & Interfaces Scheme 1. Polymerization Process of CPEs
tional theory (DFT) calculations of these molecules and ultraviolet photoelectron spectroscopy (UPS) analysis of indium tin oxide (ITO)/CPE films. Additionally, we fabricated OSCs and polymer light-emitting diodes (PLEDs), by introducing the CPEs as the HTLs. In comparison to the reference device in which the PEDOT:PSS was used as HTL, the stability and performance of the device with the PFtT-D CPE HTL were enhanced.
intermixing with the organic active layers for multilayerstructure devices, are very limited. Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is an extensively used material for HTL and transparent electrode because of its remarkable electrical conductivity, optical properties, and solution processability.24−28 However, PEDOT:PSS accelerates the degradation of devices because it is both acidic and hygroscopic.24 To replace PEDOT:PSS, studies on transitionmetal oxide precursors for molybdenum oxide (MoOx),29 nickel oxide (NiOx),30 tungsten oxide (WO3),31 and organic materials, such as conjugated polymer electrolytes32−36 and small molecules,37,38 have been performed. The results of these reported works exhibit comparable performance to PEDOT:PSS-based devices. However, transition-metal oxides for solution processing have some disadvantages, including high annealing temperatures, surface defects, and hydrophilicity.29−31,39 CPEs, which consist of conjugated polymeric main chain and branched ionic pendant groups, can form molecular dipole moments at the interface of organic active layer/anode or cathode for charge transport and energy-level control. Moreover, CPEs can be dissolved in alcohols and neutral-pH water, and high-temperature thermal annealing is unnecessary during HTL formation. For this reason, they are regarded as suitable materials for printing processes to fabricate flexible OOEDs as replacements for PEDOT:PSS HTLs. In this study, we synthesized three CPEs of poly[9,9-bis(4′sulfonatobutyl)fluorene-alt-thiophene] (PFT), poly[9,9-bis(4′sulfonatobutyl)fluorene-alt-thieno[3,2-b]thiophene] (PFtT), and poly[9,9-bis(4′-sulfonatobutyl)fluorene-alt-2,2′bithiophene] (PFbT) with different conjugation lengths to control their dipole moments by varying spacers. The n-type CPEs were oxidized by Na2S2O8 oxidant to form p-type CPEs (PFTD, PFtT-D, and PFbT-D) containing radical cations (i.e., polarons) in their main chain.40 The self-doped p-type CPEs, comprising π-conjugated polymeric chains with ionic pendant groups, can reduce the energy barrier, which can be generated by different energy levels between anode and organic semiconducting materials in organic electronic devices. To achieve high performance, matching energy levels at interfaces is essential. To identify the effect on the dipole moment and electrode work function tunability by changing the molecular conformation and arrangement, we performed density func-
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EXPERIMENTAL SECTION
Synthesis of CPEs. Synthesis of Monomers. The materials purchased from Sigma-Aldrich and Alfa Aesar were employed for all reaction and polymerization processes, without further purification. The reported methods40 were modified to synthesize sodium 4-(2,7dibromo-9-(4-sulfonatobutyl)-9H-fluoren-9-yl)butyl sulfite (M1). Polymerization. Dimethylformamide (DMF, 54 mL) was used as solvent to dissolve monomer M1 (2.0 mmol) and M2 (or M3 or M4) (2.0 mmol). The vessel containing the result solution was purged with nitrogen for 15 min to eliminate oxygen and moisture. Then, tris(dibenzylideneacetone)dipalladium(0) (Pd2dba3) (0.1 mmol, 5 mol %) and tri-(o-tolyl)phosphine (0.4 mmol, 20 mol %) were mixed with the above solution. The temperature of the mixture was elevated to 100 °C and kept for 48 h before being poured into 300 mL of acetone. The yellow products were collected by vacuum filtration and rinsed with acetone (300 mL, twice). The collected products were then dissolved in water to be purified through cellulose membranes (12 kD molecular-weight cutoff). After purification, the solvent was eliminated by the low-temperature drying method (Scheme 1). Poly[9,9-bis(4′-sulfonatobutyl)fluorene-alt-thiophene] (PFT). Yellow solid, 0.65 g (yield = 58%). 1H NMR (400 MHz, DMSO-d, d): δ 7.85 (d, 2H), 7.68 (s, 2H), 2.35 (m, 2H), 2.14 (s, 3H), 2.08 (s, 2H), 1.56 (m, 2H), 1.42 (m, 4H), 0.56 (m, 2H). GPC in buffer pH 9 + 30% MeOH: MW 10 980 Da Poly[9,9-bis(4′-sulfonatobutyl)fluorene-alt-thieno[3,2-b]thiophene] (PFtT). Yellow solid, 0.50 g (yield = 40%). 1H NMR (400 MHz, DMSO-d, d): δ 7.84 (d, 2H), 7.65 (d, 2H), ∼7.43−7.31 (m, 2H), 2.33 (s, 2H), 2.12 (d, 2H), 2.08 (s, 2H), 1.56 (m, 2H), 1.38 (s, 4H), 0.64−0.52 (m, 2H). GPC in buffer pH 9 + 30% MeOH: MW 20 961 Da Poly[9,9-bis(4′-sulfonatobutyl)fluorene-alt-2,2′bithiophene] (PFbT). Yellow solid, 0.62 g (yield = 48%). 1H NMR (400 MHz, DMSO-d, d): δ 8.02 (d, 2H), 7.87 (m, 2H), 7.66 (d, 2H), ∼7.48−7.26 (m, 1H), 2.41−2.32 (m, 2H), 2.14 (m, 2H), 2.08 (s, 2H), 1.52 (m, 2H), 1.42 (m, 4H), 0.65−0.50 (m, 4H). GPC in buffer pH 9 + 30% MeOH: MW 16 925 Da 44061
DOI: 10.1021/acsami.7b11164 ACS Appl. Mater. Interfaces 2017, 9, 44060−44069
Research Article
ACS Applied Materials & Interfaces
Figure 1. Preparation process of CPEs. Preparation of PFT-D, PFtT-D, and PFbT-D. Na2S2O8 (3 mol) was dissolved in 1 mL of H2O, and 1 mL of 5 mg/mL aqueous solution of PFT, PFtT, or PFbT was then added to the first solution. The solutions were mixed and reacted at room temperature for over 2 h. The result solutions were filtered by vacuum filtration. The collected powders were rinsed with acetone and cold H2O several times. The products were dried in a vacuum oven, and pale yellow solids were obtained. Fabrication of OSCs. Before fabrication process of the OSC devices, patterned ITO glass substrates were cleaned and surfacetreated through a previously reported method.39,41 For the HTL solution, PFT-D, PFtT-D, and PFbT-D were each dissolved at various concentrations in absolute methanol (HPLC grade) and filtered through a 0.45 μm poly(tetrafluoroethylene) (PTFE) syringe filter. The filtered solutions and PEDOT:PSS (Heraeus, P VP AI 4083) were spin-coated at 3000 rpm for 30 s onto individual ITO surfaces as HTL. The thickness of CPE HTLs was calculated by a previously reported method (see Figure S1 of Supporting Information).42,43 The optimal annealing temperatures for CPEs and PEDOT:PSS HTL were 70 and 140 °C, respectively. The solution in which poly[4,8-bis(5-(2ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene)-2-carboxylate-2-6diyl] (PCE-10, 1-materials) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM, 1-materials) (1:1.5, w/w) were dissolved in a mixed solvent of chlorobenzene and 1,8-diiodooctane additive (97:3 v/v) was employed on the HTLs by spin-coating in a N2-filled glovebox, to form photoactive layers with thicknesses of 80 nm. To deposit electron extraction layers, PFN solution (2 mg/mL in methanol with 2 μL/mL of acetic acid) was used for spin-coating onto the photoactive layers. Finally, Al cathodes (100 nm) were thermally deposited under
