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Kang R.,Heeger Center for Advanced Materials | Oh S.-H.,Korea Atomic Energy Research Institute | Kim D.-Y.,Heeger Center for Advanced Materials | Kim D.-Y.,Gwangju Institute of Science and Technology
ACS Applied Materials and Interfaces | Year: 2014

In this work, we synthesized water-soluble polyfluorene derivatives (WPFs) with anionic and/or cationic side chains, which were used as an indium tin oxide (ITO) cathode interfacial layer in inverted polymer solar cells. Three WPFs (WPFN+, WPFZW, and WPFS-) were obtained via Suzuki coupling reactions. Their solubility in polar solvents allowed the WPFs to be used as interfacial layers in inverted polymer solar cells (I-PSCs). Among the WPF-modified ITO electrodes, WPFN+ (with ammonium side chains)-modified ITO can be used as a cathode for electron extraction, while WPFS- (with sulfonate side chains)-modified ITO cannot extract electrons in I-PSCs based on poly(3-hexylthiophene): [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC61BM). The electron extraction of WPF-modified ITO can mainly be attributed to the different dipole formations at the WPF/ITO interfaces, based on the types of ionic groups on the side chains of the polyfluorene. In addition, we observed that the extent of ITO work-function modification was not always exactly correlated with the device performance based on the results obtained using a WPFZW (with ammonium and sulfonate side chains)-modified ITO electrode. © 2014 American Chemical Society. Source


Kim J.,Heeger Center for Advanced Materials | Khim D.,Heeger Center for Advanced Materials | Kang R.,Heeger Center for Advanced Materials | Lee S.-H.,Heeger Center for Advanced Materials | And 4 more authors.
ACS Applied Materials and Interfaces | Year: 2014

Here, we report the simultaneous attainment of efficient electron injection and enhanced stability under ambient conditions for top-gate/bottom-contact (TG/BC), n-type, organic field-effect transistors (OFETs) using water-soluble polyfluorene derivatives (WPFs). When inserting the WPF interlayers between a semiconductor and the BC Au electrodes, initially the ambipolar (6,6)-phenyl-C61butyric acid methyl ester (PCBM) OFETs were fully converted to unipolar charge transport characteristics that were exclusively n-type with significantly increased electron mobilities as high as 0.12 cm 2/(V s) and a decreased threshold voltage. These improvements were mostly attributed to the interfacial dipoles of WPF layers that aligned to form a favorable energy band structure for efficient electron injection and to effectively block counter charge carriers. These were confirmed when values for the reduced work function of metal electrodes with WPFs and their correlated contact resistance were measured via the ultraviolet photoemission spectroscopy and the transmission-line method, respectively. Moreover, the WPF interlayers played an important role in air stability of PCBM OFETs that exhibited higher and appreciably enhanced by increasing the ethylene-oxide side chain lengths of WPFs, which presumably was due to the water/oxygen/ion capturing effects in the hydrophilic interlayers. © 2014 American Chemical Society. Source


Kang M.,Heeger Center for Advanced Materials | Baeg K.-J.,Korea Electrotechnology Research Institute | Khim D.,Heeger Center for Advanced Materials | Noh Y.-Y.,Dongguk University | Kim D.-Y.,Heeger Center for Advanced Materials
Advanced Functional Materials | Year: 2013

The effects of using a blocking dielectric layer and metal nanoparticles (NPs) as charge-trapping sites on the characteristics of organic nano-floating-gate memory (NFGM) devices are investigated. High-performance NFGM devices are fabricated using the n-type polymer semiconductor, poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2, 6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P(NDI2OD-T2)), and various metal NPs. These NPs are embedded within bilayers of various polymer dielectrics (polystyrene (PS)/poly(4-vinyl phenol) (PVP) and PS/poly(methyl methacrylate) (PMMA)). The P(NDI2OD-T2) organic field-effect transistor (OFET)-based NFGM devices exhibit high electron mobilities (0.4-0.5 cm2 V-1 s-1) and reliable non-volatile memory characteristics, which include a wide memory window (≈52 V), a high on/off-current ratio (I on/Ioff ≈ 105), and a long extrapolated retention time (>107 s), depending on the choice of the blocking dielectric (PVP or PMMA) and the metal (Au, Ag, Cu, or Al) NPs. The best memory characteristics are achieved in the ones fabricated using PMMA and Au or Ag NPs. The NFGM devices with PMMA and spatially well-distributed Cu NPs show quasi-permanent retention characteristics. An inkjet-printed flexible P(NDI2OD-T2) 256-bit transistor memory array (16 × 16 transistors) with Au-NPs on a polyethylene naphthalate substrate is also fabricated. These memory devices in array exhibit a high Ion/Ioff (≈10 4 ± 0.85), wide memory window (≈43.5 V ± 8.3 V), and a high degree of reliability. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source


Na S.-I.,Korea Institute of Science and Technology | Na S.-I.,Heeger Center for Advanced Materials | Yu B.-K.,Heeger Center for Advanced Materials | Kim S.-S.,Kunsan National University | And 4 more authors.
Solar Energy Materials and Solar Cells | Year: 2010

We report on cost-effective ITO-free organic solar cells (OSCs) fabricated by a spray deposition method. All solution-processable layers of solar cells-a highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer and a photoactive layer based on poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM)-were spray-coated. PEDOT:PSS anode films with various thicknesses were prepared by controlling the spray deposition time. The transmittance and sheet resistance of PEDOT:PSS anodes were varied from 89.0% to 67.4% and from 358 to 63.3 Ω/squares, respectively, corresponding to an increase in film thickness. The best device exhibited a high power conversion efficiency of 2.17% under 100 mW cm-2 illumination with air mass (AM) 1. 5 global (G) condition. More importantly, the efficiency of the fully spray-coated OSC with the PEDOT:PSS anode was comparable to that of conventional ITO-based devices, demonstrating the feasibility of fabricating all-spray-deposited OSCs without a conventional spin-coating method and the possibility of replacing the costly vacuum-deposited indium tin oxide (ITO) with highly conductive polymer films fabricated by inexpensive spray deposition techniques. © 2010 Elsevier B.V. Source


Yu K.,Heeger Center for Advanced Materials | Lee J.M.,Korea Advanced Institute of Science and Technology | Kim J.,Heeger Center for Advanced Materials | Kim G.,Heeger Center for Advanced Materials | And 9 more authors.
Nano Letters | Year: 2014

Organic semiconductors are key building blocks for future electronic devices that require unprecedented properties of low-weight, flexibility, and portability. However, the low charge-carrier mobility and undesirable processing conditions limit their compatibility with low-cost, flexible, and printable electronics. Here, we present significantly enhanced field-effect mobility (μFET) in semiconducting polymers mixed with boron-doped carbon nanotubes (B-CNTs). In contrast to undoped CNTs, which tend to form undesired aggregates, the B-CNTs exhibit an excellent dispersion in conjugated polymer matrices and improve the charge transport between polymer chains. Consequently, the B-CNT-mixed semiconducting polymers enable the fabrication of high-performance FETs on plastic substrates via a solution process; the μFET of the resulting FETs reaches 7.2 cm2 V-1 s-1, which is the highest value reported for a flexible FET based on a semiconducting polymer. Our approach is applicable to various semiconducting polymers without any additional undesirable processing treatments, indicating its versatility, universality, and potential for high-performance printable electronics. © 2014 American Chemical Society. Source

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