Won Suk Shin

Effects of functional groups at perylene diimide derivatives on organic photovoltaic device application

Four soluble perylene diimide derivatives (PDIs) have been prepared and their UV–visible and photoluminescence (PL) spectroscopy, cyclic voltammetry (CV) and thermal properties were studied. ITO/PEDOT∶PSS/poly(3-hexylthiophene) (P3HT)∶PDIs/LiF/Al photovoltaic devices were fabricated with PDIs as electron accepting and transporting materials. The highest incident photon-to-current conversion efficiency (IPCE) of 19% at 495 nm and the power conversion efficiency (PCE) of 0.18% under AM 1.5 (100 mW cm−2) with a short-circuit current density (JSC) of 1.32 mA cm−2, an open circuit voltage (VOC) of 0.36 V, and a fill factor (FF) of 0.38 have been achieved with 1 ∶ 4 ratio of P3HT ∶ N,N′-di(1-nonadecyl)perylene-3,4,9,10-bis(dicarboximide) (PDI-C9) after annealing at 80 °C for 1 h. 1,7-Bis(N-pyrrolidinyl)-N,N′-dicyclohexyl-3,4,9,10-perylenebis(dicarboximide) (5-PDI), which has the electron donating pyrrolidinyl group, absorbed the long wavelength region to give IPCE onset higher than 750 nm and the pyrrolidinyl group also raised the LUMO level of 5-PDI to render the high VOC (up to 0.71 V) in photovoltaic device.

Silver nanowire embedded in P3HT: PCBM for high efficiency hybrid photovoltaic device applications

A systematic approach has been followed in the development of a high-efficiency hybrid photovoltaic device that has a combination of poly(3-hexylthiophene) (P3HT), [6,6]-phenyl C61-butyric acid methyl ester (PCBM), and silver nanowires (Ag NWs) in the active layer using the bulk heterojunction concept. The active layer is modified by utilizing a binary solvent system for blending. In addition, the solvent evaporation process after spin-coating is changed and an Ag NWs is incorporated to improve the performance of the hybrid photovoltaic device. Hybrid photovoltaic devices were fabricated by using a 1:0.7 weight ratio of P3HT to PCBM in a 1:1 weight ratio of o-dichlorobenzene and chloroform solvent mixture, in the presence and absence of 20 wt % of Ag NWs. We also compared the photovoltaic performance of Ag NWs embedded in P3HT:PCBM to that of silver nanoparticles (Ag NPs). Atomic force microscopy, scanning electron microscopy, transmittance electron microscopy, UV-visible absorption, incident photon-to-current conversion efficiency, and time-of-flight measurements are performed in order to characterize the hybrid photovoltaic devices. The optimal hybrid photovoltaic device composed of Ag NWs generated in this effort exhibits a power conversion efficiency of 3.91%, measured by using an AM 1.5G solar simulator at 100 mW/cm(2) light illumination intensity.

Chemical compatibility between a hole conductor and organic dye enhances the photovoltaic performance of solid-state dye-sensitized solar cells

A series of organic dyes having an unsymmetrical geometry, 3-(5′-{4-[(4-tert-butyl-phenyl)-(4-fluoro-phenyl)-amino]-phenyl}-[2,2′]bithio-phenyl-5-yl)-2-cyano-acrylic acid (D-F), 3-(5′-{4-[(4-tert-butyl-phenyl)-p-tolyl-amino]-phenyl}-[2,2′] bithiophenyl-5-yl)-2-cyano-acrylic acid (D-CH33), and 3-(5′-{4-[(4-tert-butyl-phenyl)-(4-methoxy-phenyl)-amino]-phenyl}-[2,2′]bithiophenyl-5-yl)-2-cyano-acrylic acid (D-OCH3), were designed and synthesized for use in solid-state dye-sensitized solar cells (sDSCs). The dye regeneration energy levels and surface properties were characterized to determine the hole transfer yield from the oxidized dye to the hole conductor (spiro-OMeTAD) by measuring the degree of pore-filling by the spiro-OMeTAD and the transient absorption spectra (TAS). An electrode sensitized with D-OCH3 exhibited the highest spiro-OMeTAD filling fraction and hole transfer quantum yield (Φ) to spiro-OMeTAD, resulting in an enhanced photocurrent and a power conversion efficiency of 3.56% in the sDSC, despite a lower energy driving force for hole transfer compared to those of D-F, or D-CH33. This result illustrates the importance of the chemical compatibility between the hole conductor and the dye on the surface of TiO2.

Highly efficient uniform ZnO nanostructures for an electron transport layer of inverted organic solar cells

A highly uniform and predesigned ZnO nanostructure fabricated by single step direct nanoimprinting was used as the efficient electron transport layer (ETL) in inverted bulk heterojunction organic solar cells. Improved photovoltaic cell efficiency with long-term stability can be observed due to the large interface between the active layer and nanostructured ZnO ETL.

Surface-induced alignment of pentacene by photo-alignment technology for organic thin film transistors

A series of highly soluble maleimide-based polymers with photoreactive pendant group were synthesized and used as a gate dielectric insulator for organic thin film transistors. Photoalignment properties were characterized by UV-visible spectroscopy and the alignment of liquid crystals. Photopolymerization of polymer-coated organic thin film transistors with linearly polarized UV light induces anisotropy in field-effect mobility. The gate dielectric insulator based on the photoalignable maleimide shows higher field-effect mobility and on/off current ratio than those from a SiO2 gate dielectric insulator with the values of 0.3 cm2 V−1 s−1 and 104, respectively. Our results demonstrate that the photo-induced anisotropy of alignment films to control the molecular order of semiconducting pentacene is a promising technology for improving the performance of organic thin film transistors.

Effect of the alkyl chain length of C70-PCBX acceptors on the device performance of P3HT : C70-PCBX polymer solar cells

A series of soluble C70-derivatives (C70-PCBX) was synthesized by varying the alkyl chain length of the adduct (X = R1, R3, R5, R7, R9) attached to the C70-polyhedron. After blending with P3HT, bulk heterojunction polymer solar cells were fabricated, and their performances were evaluated. As the alkyl chain length increased, phase segregation between the P3HT and C70-PCBX domains in the P3HT : C70-PCBX blend active layer increased, which enhanced the device performance to a moderate degree. The performance enhancement may have been due to the increased solubility and self-aggregation of C70-PCBX as a result of the longer alkyl chains, as was predicted by molecular simulations. The nanomorphologies of the P3HT : C70-PCBX active layers offered an explanation for the trend in solar cell performance: as the series progressed from C70-PCBR1 to C70-PCBR7, the open circuit voltage and the short circuit current gradually increased, followed by a slight drop for C70-PCBR9. An optimal overall power conversion efficiency was observed for C70-PCBR7 with a P3HT : C70-PCBR7 blend ratio of 1 : 0.8 to 1 : 0.9.

High-efficiency photovoltaic cells with wide optical band gap polymers based on fluorinated phenylene-alkoxybenzothiadiazole

A series of semi-crystalline, wide band gap (WBG) photovoltaic polymers were synthesized with varying number and topology of fluorine substituents. To decrease intramolecular charge transfer and to modulate the resulting band gap of D–A type copolymers, electron-releasing alkoxy substituents were attached to electron-deficient benzothiadiazole (A) and electron-withdrawing fluorine atoms (0–4F) were substituted onto a 1,4-bis(thiophen-2-yl)benzene unit (D). Intra- and/or interchain noncovalent Coulombic interactions were also incorporated into the polymer backbone to promote planarity and crystalline intermolecular packing. The resulting optical band gap and the valence level were tuned to 1.93–2.15 eV and −5.37 to −5.67 eV, respectively, and strong interchain organization was observed by differential scanning calorimetry, high-resolution transmission electron microscopy and grazing incidence X-ray scattering measurements. The number of fluorine atoms and their position significantly influenced the photophysical, morphological and optoelectronic properties of bulk heterojunctions (BHJs) with these polymers. BHJ photovoltaic devices showed a high power conversion efficiency (PCE) of up to 9.8% with an open-circuit voltage of 0.94–1.03 V. To our knowledge, this PCE is one of the highest values for fullerene-based single BHJ devices with WBG polymers having a band gap of over 1.90 eV. A tandem solar cell was also demonstrated successfully to show a PCE of 10.3% by combining a diketopyrrolopyrrole-based low band gap polymer.

Impact of the Crystalline Packing Structures on Charge Transport and Recombination via Alkyl Chain Tunability of DPP-Based Small Molecules in Bulk Heterojunction Solar Cells.

A series of small compound materials based on benzodithiophene (BDT) and diketopyrrolopyrrole (DPP) with three different alkyl side chains were synthesized and used for organic photovoltaics. These small compounds had different alkyl branches (i.e., 2-ethylhexyl (EH), 2-butyloctyl (BO), and 2-hexyldecyl (HD)) attached to DPP units. Thin films made of these compounds were characterized and their solar cell parameters were measured in order to systematically analyze influences of the different side chains of compounds on the film microstructure, molecular packing, and hence, charge-transport and recombination properties. The relatively shorter side chains in the small molecules enabled more ordered packing structures with higher crystallinities, which resulted in higher carrier mobilities and less recombination factors; the small molecule with the EH branches exhibited the best semiconducting properties with a power conversion efficiency of up to 5.54% in solar cell devices. Our study suggested that tuning the alkyl chain length of semiconducting molecules is a powerful strategy for achieving high performance of organic photovoltaics.

Synthesis and Characterization of Regiosymmetric Poly(3,4-propylenedioxythiophene) Derivative

Chemical oxidative polymerization of 3,3-diheptyl-3,4-dihydro-2H-thieno[3,4-b] [1 4]dioxepine, (heptyl2-PDOT) with iron trichloride (FeCl3) gave the regiosymmetric poly(3,3-diheptyl-3,4-dihydro-2H-thieno[3,4-b] [1 4] dioxepine), poly(heptyl2-PDOT), which has a high glass transition temperature (195°C) and a good solubility behavior in most of common solvents. Electrochemically polymerized one showed higher conductivity at the positively applied potential (1.50 × 10−2 S/cm at +1.0 V) than the negatively applied one (2.99 × 10−5 S/cm at −0.3 V).

Carbazole linked phenylquinoline-based fullerene derivatives as acceptors for bulk heterojunction polymer solar cells: effect of interfacial contacts on device performance

To understand the effect of interfacial contact between the hole transporting layer (HTL) and fullerene derivatives in the active layer of bulk heterojunction polymer solar cells (BHJ PSCs), carbazole (Cz) linked phenylquinoline (PhQ)-based fullerene derivatives, PhQHCz-C61BM and PhQEOCz-C61BM, have been successfully synthesized. They are used as acceptors with a poly(3-hexylthiophene) (P3HT) donor in the active layer, and PEDOT:PSS and MoO3 were used as the HTL. Both the derivatives are highly soluble in common organic solvents and possess high thermal stability. BHJ PSCs are fabricated with configurations of ITO/PEDOT:PSS/P3HT:PhQHCz-C61BM/LiF/Al, ITO/PEDOT:PSS/P3HT:PhQEOCz-C61BM/LiF/Al, and ITO/MoO3/P3HT:PhQHCz-C61BM/LiF/Al, ITO/MoO3/P3HT:PhQEOCz-C61BM/LiF/Al, and the device characteristics were measured under AM1.5G (100 mW cm−2). Both derivatives exhibited much lower power conversion efficiencies (PCE) of ∼0.1% when PEDOT:PSS was employed as the HTL. In contrast, the PCE increases to ∼2.2% upon replacing PEDOT:PSS with MoO3 as the HTL. This is due to the fact that protonation of the pyridyl nitrogen of the acceptor in the active layer by the –SO3H group of PEDOT:PSS in the HTL, establishes a charge injection barrier at the interfacial contact and leads to restricted charge collection at the electrodes. This was indirectly confirmed by protonation of pyridyl nitrogen in PhQHCz-C61BM by the –SO3H group in p-toluenesulphonic acid.