Effect on Electrode Work Function by Changing Molecular Geometry of

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Effect on Electrode Work Function by Changing Molecular Geometry of Conjugated Polymer Electrolytes and Application for Hole Transporting Layer of Organic Optoelectronic Devices Eui Jin Lee, Min Hee Choi, Yong Woon Han, and Doo Kyung Moon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11164 • Publication Date (Web): 21 Nov 2017 Downloaded from http://pubs.acs.org on November 23, 2017

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ACS Applied Materials & Interfaces

Effect on Electrode Work Function by Changing Molecular Geometry of Conjugated Polymer Electrolytes and Application for Hole Transporting 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, Gyeonggi-do 16910, Republic of Korea

*

To whom all correspondence should be addressed.

Tel.: +82-2-450-3498, Fax: +82-2-444-0765 E-mail: [email protected]

ACS Paragon Plus Environment


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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, 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 function 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 to electricity (organic solar cells, OSCs) or electricity to 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%, an 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 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 progressed13,14,16–18. To promote the transportation of charge carriers and enhance the performance of OOEDs, electron or hole-transport layers are introduced above or below the organic active layers.



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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)-alt2,7-(9,9–dioctylfluorene)] (PFN),21 3,3′,3″,3‴-((pyrene-1,3,6,8-tetrayltetrakis(benzene-4,1diyl))tetra-kis(oxy))tetrakis(N, N-dimethylpropan-1-amine) (PBPA),22 and 3,6-bis(5-(4-(3(dimethylamino)propoxy)-phenyl)thiophen-2-yl)-2,5-bis(2-hexyl-decyl)pyrrolo[3,4c]pyrrole-1,4(2H,5H)-dione (DPPA),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 intermixing with the organic active layers for multilayer-structure devices,

are

very

limited.

Poly(3,4-ethylenedioxythiophene):poly-styrenesulfonate

(PEDOT:PSS) is 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 In order to replace PEDOT:PSS, studies on transition metal 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 molecules37,38 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


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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-altthiophene] (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 (PFT-D, 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 function 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.

Experimental section Synthesis of CPEs Synthesis of monomers



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The purchased materials from Sigma Aldrich and Alfa Aesar were employed for all reaction and polymerization processes, without further purification. The reported methods

40

were

modified to synthesize the sodium 4-(2,7-dibromo-9-(4-sulfonatobutyl)-9H-fluoren-9yl)butyl sulfite (M1). Polymerization

Scheme 1. Polymerization process of CPEs.

Dimethylformamide (DMF, 54mL) was used as solvent to dissolve monomer M1 (2.0 mmol) and M2 (or M3 or M4) (2.0 mmol). The vessel containing result solution was purged with nitrogen

for

15

minutes

to

eliminate

oxygen

and

moisture.

Then,

tris(dibenzylideneacetone)dipalladium(0) (Pd2dba3) (0.1 mmol, 5 mol%) and tri-(otolyl)phosphine (0.4 mmol, 20 mol%) mixed with the above solution. The temperature of mixture was elevated to 100 °C and kept during 48 hours, 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


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cellulose membranes (12-kD molecular-weight-cutoff). After purification, the solvent was eliminated by the low-temperature drying method. 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 Preparation of PFT-D, PFtT-D, and PFbT-D Na2S2O8 (3 mol) was dissolved in 1 mL of H2O. 1 mL of 5 mg/mL aqueous solution of PFT, PFtT or PFbT was then added to the 1st solution. The solutions were mixed and reacted at room temperature over 2 hours. 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.



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Fabrication of OSCs Before fabrication process of the OSC devices, patterned ITO glass substrates were cleaned and surface treated through previously reported method.39,41 For the HTL solution, PFT-D, PFtT-D, and PFbT-D were each dissolved at a various concentrations in absolute methanol (HPLC grade) and filtered through a 0.45-µm polytetrafluoroethylene (PTFE) syringe filter. The filtered solutions and PEDOT:PSS (Heraeus, P VP AI 4083) were spin-coated with 3000 RPM for 30 seconds onto individual ITO surfaces as HTL. The thickness of CPE HTLs was calculated with 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-(2-ethylhexyl)thiophen-2-yl)benzo[1,2b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene)-2carboxylate-2-6-diyl] (PCE-10, 1-materials) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM, 1-materials) (1:1.5, w/w) were dissolved in mixed solvent of chlorobenzene and 1,8-diiodooctane additive (97:3 v/v), was employed on the HTLs by spin-coating in N2-filled glove box, 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

Effect on Electrode Work Function by Changing Molecular Geometry of

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