Sehwan Kim

Electrochromic Diffraction from Nanopatterned Poly(3-hexylthiophene)

Poly(3-hexylthiophene) (P3HT) films were patterned by a soft lithography technique using a nanopatterned polydimethylsiloxane (PDMS) mold to generate one-dimensional (1D) grating and two-dimensional (2D) crossed line pillar patterns. The redox currents (i(p)) were significantly increased due to the facilitated diffusion of ClO4(-) counterions associated with redox processes at the P3HT electrode as analyzing cyclic voltammetry (CV) was performed at different scan rates (ν). It was found that the diffusion coefficient (D(f), cm(2) s(-1)) for ion diffusion in the patterned electrode was much larger than that of the pristine P3HT electrode. Furthermore, the value of D(f) in the 2D electrode was three times higher than that in a pristine film. As a result of such facilitated charge transport, the electrochromic (EC) properties of the patterned P3HT electrode were greatly enhanced and dependent on the dimension of the pattern. Thus, the electrochromic efficiency (E(e)), including the coloration (E(c)) and bleaching efficiencies (E(b)), was higher as the dimension of the pattern was increased; E(e) was maximized in the 2D patterned P3HT film. In a patterned cell, electrochromic diffraction was reversibly observed with a switching efficiency (R(DE)) of 2 and 2.5 for the 1D and 2D patterned cells, respectively.

Fabrication of silver nanowire transparent electrodes using electrohydrodynamic spray deposition for flexible organic solar cells

Fabrication of a flexible organic solar cell is demonstrated with an anode that is free of tin-doped indium oxide (ITO) by electrohydrodynamic (EHD) spraying of silver nanowires (Ag NWs). This methodology is applicable to fabricate patterned Ag NW thin-film electrodes for organic solar cells (OSCs). By optimizing the spray parameters and post-processing conditions, transparent electrodes with sheet resistances of ∼11 Ω sq−1 (on glass) and ∼20 Ω sq−1 (on polyethylene terephthalate, PET) and DC to optical conductivities of ∼70 (on PET) can be obtained at an optical transmittance of ∼80%. Bulk heterojunction OSCs are demonstrated with patterned Ag NW films serving as bottom transparent anodes on both glass substrates and flexible PET substrates. The photoactive layers are based on low band-gap polymers, poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4.5-b′]dithiophene)-2,6-diyl-alt-(4-(2-ehtylhexanoyl)-thieno[3,4-b]thiophene)]-2,6-diyl] and phenyl-C61-butyric acid methyl ester. Under AM 1.5 illumination, fabricated cells have high power conversion efficiencies of 5.27% (on glass) and 3.76% (on PET). This study indicates that a Ag NW electrode prepared by EHD spraying can serve as an alternative to the ITO electrode, which also enables its potential application in practical and flexible OSCs.

Strong Photocurrent Enhancements in Plasmonic Organic Photovoltaics by Biomimetic Nanoarchitectures with Efficient Light Harvesting

We propose the biomimetic moth-eye nanoarchitectures as a novel plasmonic light-harvesting structure for further enhancing the solar-generated photocurrents in organic photovoltaics (OPVs). The full moth-eye nanoarchitectures are composed of two-dimensional hexagonal periodic grating arrays on surfaces of both the front zinc oxide (ZnO) and rear active layers, which are prepared by a simple and cost-effective soft imprint nanopatterning technique. For the 380 nm period ZnO and 650 nm period active gratings (i.e., ZnO(P380)/Active(P650)), the poly(3-hexylthiophene-2,5-diyl):indene-C60 bis-adduct (P3HT:ICBA)-based plasmonic OPVs exhibit an improvement of the absorption spectrum compared to the pristine OPVs over a broad wavelength range of 350-750 nm, showing absorption enhancement peaks at wavelengths of ∼370, 450, and 670 nm, respectively. This leads to a considerable increase of short-circuit current density (Jsc) from 10.9 to 13.32 mA/cm(2), showing a large Jsc enhancement percentage of ∼22.2%. As a result, the strongly improved power conversion efficiency (PCE) of 6.28% is obtained compared to that (i.e., PCE = 5.12%) of the pristine OPVs. For the angle-dependent light-absorption characteristics, the plasmonic OPVs with ZnO(P380)/Active(P650) have a better absorption performance than that of the pristine OPVs at incident angles of 20-70°. For optical absorption characteristics and near-field intensity distributions of plasmonic OPVs, theoretical analyses are also performed by a rigorous coupled-wave analysis method, which gives a similar tendency with the experimentally measured data.

Fabrication of ordered bulk heterojunction organic photovoltaic cells using nanopatterning and electrohydrodynamic spray deposition methods

Organic photovoltaic cells with an ordered heterojunction (OHJ) active layer are expected to show increased performance. In the study described here, OHJ cells were fabricated using a combination of nanoimprinting and electrohydrodynamic (EHD) spray deposition methods. After an electron donor material was nanoimprinted with a PDMS stamp (valley width: 230 nm, period: 590 nm) duplicated from a Si nanomold, an electron acceptor material was deposited onto the nanoimprinted donor layer using an EHD spray deposition method. The donor-acceptor interface layer was observed by obtaining cross-sectional images with a focused ion beam (FIB) microscope. The photocurrent generation performance of the OHJ cells was evaluated with the current density-voltage curve under air mass (AM) 1.5 conditions. It was found that the surface morphology of the electron acceptor layer affected the current and voltage outputs of the photovoltaic cells. When an electron acceptor layer with a smooth thin (250 nm above the valley of the electron donor layer) surface morphology was obtained, power conversion efficiency was as high as 0.55%. The electrohydrodynamic spray deposition method used to produce OHJ photovoltaic cells provides a means for the adoption of large area, high throughput processes.

Synthesis and characterization of thiophene-based conjugated copolymer containing ferroceneunits for organic electronics

Effect of electroactive ferrocene on the optical and electrochemical properties of polythiophenes were examined. Thus the ferrocene unit were introduced into polythiophenes by Grignard metathesis (GRIM) method to produce ferrocene substituted polythiophenes (PThFc). These polymers were characterized by GPC, NMR, UV-Vis absorption spectrum, and cyclic voltammogram (CV). The PThFc showed lower redox potential and reversible electrochemical redox state due to the ferrocenyl functional group, as determined by the CV. The absorption maxima of PThFc was 556 nm in the film state with a broad absorption band covering from 350 nm to 650 nm. Fluorescence quenching efficiency of PL for the PThFc containing films was determined as > 90%. The short-circuit current densities (Jsc) for the cell containing PThFc was highly dependent on the thermal annealing process, indicating that the photovoltaic properties could be optimized by the control of polymer structure. (PThFc).