Enhanced efficiency of dye-sensitized solar cells doped with green phosphors LaPO4:Ce, Tb or (Mg, Zn)Al11O19:Eu
© Hong et al.; licensee Springer. 2013
Received: 18 March 2013
Accepted: 26 April 2013
Published: 8 May 2013
We have successfully introduced green phosphors LaPO4:Ce, Tb (G4) or (Mg, Zn)Al11O19:Eu (G2) into TiO2 photoelectrode of dye-sensitized solar cells. The conversion efficiency of the G4-doped device was enhanced by 30% compared with the pristine TiO2 photoelectrode. The green phosphor doped at 5-wt.% ratio contributed to the reduction of resistances of the surface and interface of the photoelectrode and to the great enhancement of the absorption spectrum in UV-visible and near-infrared regions. The internal resistances and absorbance of the photoelectrode directly affect the power conversion efficiency. Green phosphor plays an important role towards the realization of high-efficiency dye-sensitized solar cells.
KeywordsGreen phosphor Dye-sensitized solar cell Photoluminescence Conversion efficiency
The uses of different types of nanostructured materials in dye-sensitized solar cells (DSSC) have attracted worldwide attention as a low-cost alternative to traditional photovoltaic device [1–5]. This is because nanostructures of materials enhance the surface area to allow a higher amount of dye molecules to be adsorbed, and the nature of electron transport in oxide nanoparticle films is fairly well understood. The scientific community is still struggling to find optimum nanostructures and materials for the best solution to overcome issues associated with stability, efficiency, and cost-effective mass production [6, 7].
Normally, in DSSCs, photons interact with dye molecules to create excitons. These excitons come into contact with nanoparticles/nanostructures at the surface of the photoelectrode and are rapidly split into electrons and holes. Electrons are injected into the photoelectrode, and holes leave the opposite side of the device by means of redox species (traditionally the I−/I3− couple) in the liquid or solid-state electrolyte used in DSSCs to ensure efficient electron transfer to the redox couple [8–11]. It is important to apply different materials and structures to enhance light photon interaction with dye molecules to achieve a higher proportion of excitons.
Recent research shows that the implementation of a UV-absorbing luminescent wavelength converter (which is emitted at longer wavelengths) not only remarkably improves the photochemical stability of the DSSCs but also enhances the efficiency of the cell [3, 11–14].
LaPO4:Ce, Tb (G4) and (Mg, Zn)Al11O19:Eu (G2) have been widely used in tricolor phosphor lamps and PDP displays as highly effective green phosphor additives [15–18]. YVO4:Bi3+, Ln3+ (Ln = Dy, Er, Ho, Eu, and Sm) phosphors are proposed to be promising UV-absorbing spectral converters for DSSCs as they possess broad absorption band in the whole UV region of 250 to 400 nm and could emit intense visible lights. When excited by ultraviolet light, G4 emits 550 nm of light in the green region. Considering this point, the doping of green phosphors LaPO4:Ce, Tb or (Mg, Zn)Al11O19:Eu into TiO2 photoelectrodes could lead to higher efficiency in dye-sensitized solar cells. Field emission-scanning electron microscopy (FE-SEM) was used to determine the morphology of this hybrid photoelectrode. The absorption and luminescence properties of dye and green phosphor ceramics were investigated using UV spectrophotometry and photoluminescence spectrometry. Electrochemical measurements were used to optimize the weight percentage of fluorescent materials doped in TiO2 photoelectrode, which had higher conversion efficiency (η), fill factor (FF), open-circuit voltage (Voc), and short-circuit current density (Jsc) as a result.
Anhydrous LiI, I2, poly(ethylene glycol) (mw = 20,000), nitric acid, and 4-tertiary butyl pyridine were obtained from Sigma-Aldrich (St. Louis, MO, USA), and TiO2 powder (P25) was obtained from Nippon Aerosil (EVONIK Industries AG, Hanau-Wolfgang, Germany) and used as received. Ethanol was purchased from Daejung Chemicals & Metals Co. (Shiheung, Republic of Korea), and water molecules were removed by placing molecular sieves (3 Å) in the solvent. Commercially sourced bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)-bis-tetrabutyl ammonium (N719 dye) and 1,2-dimethyl-3-propylimidazolium iodide were obtained from Solaronix SA (Aubonne, Switzerland). Green phosphors LaPO4:Ce, Tb and (Mg, Zn)Al11O19:Eu were obtained from Nichia Corporation (Tokushima, Japan). The electrolyte solution consisted of 0.3 M 1,2-dimethyl-3-propylimidazolium iodide, 0.5 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine in 3-methoxypropionitile.
Fabrication of DSSC
TiO2 powder was thoroughly dispersed for 10 h at 300 rpm using a ball mill (Planetary Mono Mill, FRITSCH, Oberstein, Germany), adding acetyl acetone, poly(ethylene glycol), and a Triton X-100 to obtain a viscous TiO2 paste. The doped green phosphors were added to the TiO2 paste and mixed in a ball mill for 2 h. The TiO2 and green phosphor-doped TiO2 pastes were coated onto fluorine-doped SnO2 conducting glass plates (FTO, 8 Ω cm−2, Pilkington, St. Helens, UK) using squeeze printing technique, followed by sintering at 450°C for 30 min. Approximately 8- to 10-μm-thick TiO2 film was deposited onto a 0.25-cm2 FTO glass substrate. Glass-FTO/TiO2 and phosphor-doped TiO2 electrodes were immersed overnight (ca. 24 h) in a 5 × 10−4 mol/L ethanol solution of Ru(dcbpy)2(NCS)2 (535-bis TBA, Solaronix), rinsed with anhydrous ethanol, and dried. A few drops of the liquid electrolyte were dispersed onto the surface, and a full cell assembly was constructed for electrochemical measurements. A Pt-coated FTO electrode was prepared as a counter electrode with an active area of 0.25 cm2. The Pt electrode was placed over the dye-adsorbed TiO2 thin film electrode, and the edges of the cell were sealed with 5-mm wide strips of 60-μm-thick sealing sheet (SX 1170–60, Solaronix). Sealing was accomplished by hot-pressing the two electrodes together at 110°C.
Characterization of DSSC
The surface morphology of the film was observed by FE-SEM (S-4700, Hitachi High-Tech, Minato-ku, Tokyo, Japan). A 450-W xenon lamp was used as light source for generating a monochromatic beam. Calibration was performed using a silicon photodiode, which was calibrated using an NIST-calibrated photodiode G425 as a standard. UV-visible (vis) spectra of the TiO2 film and TiO2 electrode with green phosphor powder added were measured with a UV–vis spectrophotometer (8453, Agilent Technologies, Inc., Santa Clara, CA, USA). Photoluminescence spectra were recorded on Avantes BV (Apeldoorn, The Netherlands) spectrophotometer under the excitation of Nd:YAG laser beam (355 nm). Electrochemical impedance spectroscopies of the DSSCs were measured with an electrochemical workstation (CHI660A, CH Instruments Inc., TX, USA). The photovoltaic properties were investigated by measuring the current density-voltage (J-V) characteristics under irradiation of white light from a 450-W xenon lamp (Thermo Oriel Instruments, Irvine, CA, USA). Incident light intensity and active cell area were 100 mW cm−2 and 0.25 cm2, respectively.
Results and discussion
where Jsc is the short-circuit current density (mA cm−2), Voc is the open-circuit voltage (V), Pin is the incident light power, and Jmax (mA cm−2) and Vmax (V) are the current density and voltage in the J-V curve at the point of maximum power output, respectively.
Photovoltaic properties of pristine TiO 2 -based DSSC and those doped with G2 and G4
Doped with G2
Doped with G4
In summary, we have successfully introduced a 5-wt.% ratio of green phosphors G4 or G2 into the TiO2 photoelectrodes of dye-sensitized solar cells. The enhanced percentage of conversion efficiencies of devices doped with G4 or G2 were 30% and 16% with the open-circuit voltages of 0.67 and 0.68 V and the short-circuit currents of 17.8 and 16.5 mA cm−2, respectively. The fill factors were 0.67 and 0.64, respectively. The 5-wt.% doping ratio of green phosphor contributed to the reduction of the resistances of the surface and the interface of the photoelectrode and enhanced the absorption spectrum in the UV–vis and near-infrared regions. The internal resistances and absorbance of the photoelectrode directly affected the power conversion efficiency. Green phosphor plays an important role towards the realization of high-efficiency dye-sensitized solar cells.
dye-sensitized solar cells
field emission-scanning electron microscopy
fluorine-doped SnO2 conducting glass plates
incident photon-to-current conversion efficiency
short-circuit current density
energy conversion efficiency.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (2012010655). This work was also supported by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009–0094055).
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