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Effects of Graphene in Dye-Sensitized Solar Cells Based on Nitrogen-Doped TiO Composite 2
Seong-Bum Kim, Jun-Yong Park, Chan Soo Kim, Kikuo Okuyama, Sung-Eun Lee, Hee Dong Jang, and Tae Oh Kim J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 01 Jul 2015 Downloaded from http://pubs.acs.org on July 2, 2015
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The Journal of Physical Chemistry
Effects of Graphene in Dye-Sensitized Solar Cells Based on Nitrogen-Doped TiO2 Composite Seong-Bum Kimaǂ, Jun-Yong Parkaǂ, Chan-Soo Kimb Kikuo Okuyamac, Sung-Eun Leed*, Hee-Dong Jange*, Tae-Oh Kima* a Department of Environmental Engineering, Kumoh National Institute of Technology, Daehakro 61, Gumi, Gyeongbuk 730-701, Republic of Korea b Jeju Global Research Center, Korea Institute of Energy Research, 200, Haemajihaean-ro, Gujwa-eup, Jeju 695-974 c Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi Hiroshima 739-8527, Japan d School of Applied Biosciences, Kyungpook National University, Daegu 702-701, Republic of Korea e Department of Rare Metals Research Center, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Republic of Korea
ABSTRACT : Graphene (GR) exhibits impressive photoelectric properties, including a large specific surface area, high charge-carrier mobility, high conductance, and fast electron transfer. In this study, the effect of GR on the performance of dye-sensitized solar cells (DSSCs) was
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investigated by mixing GR into N-doped TiO2 photoelectrodes. GR/N-doped TiO2 (GNT) nanoparticles were prepared using the sol-gel method. After preparation, the presence of GR in the photoelectrodes was confirmed using transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy analyses. After the addition of GR, the photoelectrodes displayed enhanced dye adsorption properties with lower internal resistances and faster transport times. Accordingly, DSSCs with these photoelectrodes generated high current density with a low electron-recombination rate. The maximum power conversion efficiency of DSSCs with GR/N-doped photoelectrodes was 9.32% with optimized DSSC parameters; this represents an enhancement of approximately 22% over that of DSSCs with N-doped photoelectrodes. The addition of excess GR weakened the crystallization of particles on the surface of photoelectrodes, which resulted in low dye adsorption and decreased efficiency of the DSSCs. In summary, the addition of GR promoted increased dye loading and enhanced DSSC efficiency. The optimal amount of GR for high- efficiency DSSCs was successfully determined in this study.
KEYWORDS : Charge-carrier mobility, High efficiency, Electron-recombination, Graphene oxide, Impedance, Chemical capacitance
INTRODUCTION Dye-sensitized solar cells (DSSCs), which were first introduced in 1991, produce electrons through the excitation of dye molecules attached to semi-conductor photoelectrodes after exposure to solar rays. The generated electrons move to the counter-electrodes, are recycled in
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the cells through redox reactions with electrolytes, and finally produce electricity.1 DSSCs have potential applications in a variety of industrial areas.2 However, DSSCs are less efficient than silicon solar cells because of their low dye-adsorption and high electron-recombination rates. Accordingly, many studies have focused on enhancing the dye-adsorption rates and reducing the electron-recombination rates via structural modification of the photoelectrodes and the addition of transition elements.3-6 Therefore, materials for such photoelectrodes absorb within the visible light region, possess a low conduction band for active electron transfer, and have appropriate band-gap energy.7,8 TiO2 is frequently used in DSSCs as a photoelectrode material because it is cheap and relatively thermally stable; however, it only absorbs in the ultraviolet (UV) light region. To overcome this disadvantage, TiO2 photoelectrodes have been doped with nitrogen to increase the optical performance by expanding absorption to the visible light region.9-11 It has been reported that the addition of transition elements such as Zr, Cu, and W to the photoelectrode increases the maximum power conversion efficiency.11-13 Graphene (GR) has been used to prepare photoelectrodes because of it fast electron transfer.14,15 GR is a monolayer of secondary graphite, which is 0.35 nm thick, i.e., as thick as an atom,16,17 and possesses high flexibility and stiffness, impressive thermal stability, and a large specific surface area.18,19 GR exhibits high chargecarrier mobility (200,000 cm2V−1s−1) and high conductance because of its low band-gap, which enables fast electron transfer.20-22 Graphene is one of strong compounds and has unique properties as high thermal conduction and high optical transmittance.23 These characteristics have been used to memories, biosensors, transistors and DSSCs. In this regard, DSSCs containing graphene have showed enhancement on overall conversion efficiency in the previous studies.24 Graphenes are also used as electrolytes and counter-electrodes, and they are also
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considered as photoelectrodes of DSSCs by formation of new compounds with NiO and MoS2.2528
However, no study has reported the addition of both nitrogen and graphene into
photoelectrodes to enhance the efficiency of DSSCs. In this study, GR/N-doped TiO2 (GNT) composite was prepared to both expand absorption into the visible light region and to enhance the electron-transfer rate; the material was used as a photoelectrode in our DSSC system, which had a higher maximum power conversion efficiency than that of plain TiO2 photoelectrodes. EXPERIMENTAL SECTION Preparation of graphene oxide. Graphene oxide (GO) was prepared using a previously reported method.29 For the pretreatment, P2O5 (3.75 g, phosphorous(V) oxide, Alfa Aesar, MA) and K2S2O8 (3.75 g, potassium peroxydisulfate, Alfa Aesar) were dissolved in 10 mL of H2SO4 and left in an oil bath at 80 °C for 20 min. Graphite (5 g, SP-1, Bay Carbon Inc., MI) and H2SO4 (9 mL) were added to the solution, which was then covered with a glass cap and mixed for 4.5 h. After the reaction, the graphite solution was diluted to 1 L with water and filtered using filter paper (GF/F, 110 mm i.d.). The filtrate was adjusted to a pH of 5 with distilled water and dried for 24 h until the graphite powder formed. Pretreated graphite powder (5 g) was dissolved in H2SO4 (187.5 ml) in an ice bath (
