Locally placed nanoscale gold islands film within a TiO2 photoanode for enhanced plasmon light absorption in dye sensitized solar cells
© The Author(s) 2016
Received: 8 November 2016
Accepted: 21 November 2016
Published: 7 December 2016
As metal nanostructures demonstrated extraordinary plasmon resonance, their optical characteristics have widely been investigated in photo-electronic applications. However, there has been no clear demonstration on the location effect of plasmonic metal layer within the photoanode on both optical characteristics and photovoltaic performances. In this research, the gold (Au) nano-islands (NIs) film was embedded at different positions within the TiO2 nanoparticulate photoanode in dye-sensitized solar cells (DSSC) to check the effect of plasmon resonance location on the device performance; at the top, in the middle, at the bottom of the TiO2 photoanode, and also at all the three positions. The Au NIs were fabricated by annealing a Au thin film at 550 °C. The DSSC having the Au NIs-embedded TiO2 photoanode exhibited an increase in short circuit currents (Jsc) and power conversion efficiency (PCE) owing to the plasmon resonance absorption. Thus, the PCE was increased from 5.92% (reference: only TiO2 photoanode) to 6.52% when the Au NIs film was solely positioned at the bottom, in the middle or at the top of TiO2 film. When the Au NIs films were placed at all the three positions, the Jsc was increased by 16% compared to the reference cell, and consequently the PCE was further increased to 7.01%.
Many researchers are focusing on the plasmon resonance phenomenon due to their strong absorption and scattering effect . Recently, various nanostructures, including nanoparticles, nanoislands, nanorods, and nanoflowers, have drawn a great attention due to their exceptional surface plasmon resonance phenomenon [2–6]. Various lithographic techniques were employed to design plasmon nanostructures with a controlled size, shape, and arrangement for the surface-enhanced Raman scattering in the field of chemical and biosensors . Among those plasmon nanostructures, nanoparticles have the most effective localized surface plasmon resonance absorption enhancement at visible wavelengths, which can be utilized in energy harvesting, photocatalyst, solar cells or water splitting [8–10]. Furthermore, it has been extensively reported that the plasmon resonance in noble metal nanoparticles can enhance the light trapping within a photovoltaic medium in dye-sensitized solar cells (DSSC) .
Dye-sensitized solar cells was firstly demonstrated by Gratzel in 1991 , which exhibited plenty of advantages such as its transparency, flexibility, low cost and easy fabrication process. DSSC is composed of three parts; photoanode, electrolyte and counter electrode, among which, the dyes adsorbed within a semiconducting photoanode layer absorb photons and generate electron–hole pairs. The efficiency of DSSC increases with the formation of more electron–hole pairs. It is well known that the TiO2 nanoparticulate layer mixed with metallic nanoparticles showed a higher light absorbance due to the localized surface plasmon resonance around the surface of metal nanoparticles, and more electron–hole pairs were generated .
Generally, solid thin films are thermodynamically unstable and easy to be transformed into more stable shapes when heated below their melting temperature due to the solid state thermal dewetting phenomenon [14, 15]. This phenomenon occurs to reduce the surface energy of thin film and interfacial energy between the thin film and the substrate. Therefore, while annealing the metal thin film such as gold (Au), Au nanoislands (NIs) film was formed, which revealed plasmon resonance phenomenon at a specific wavelength depending on the island sizes and shapes [16, 17]. To utilize the plasmon resonance phenomenon for enhancing the DSSC efficiency, researchers have incorporated the Au NIs into the TiO2 semiconducting layer in the photoanode. However, within our knowledge, there has been no clear demonstration on the location effect of plasmonic metal layer within the photoanode on both optical characteristics and photovoltaic performance.
In this research, we fabricated four different configurations of Au NIs film-embedded TiO2 photoanode; solely located at the top, in the middle or at the bottom, and combined at all the three positions, and their plasmon resonance properties were then studied. The size and morphology of Au NIs were optimized by varying the initial Au film thickness. Furthermore, DSSCs having the four different photoanodes were fabricated to study the effect of plasmon resonance location on the DSSC performance.
2 Results and discussion
2.1 Morphology of Au NIs
Average diameter and its standard deviation of the Au NIs after annealing the Au film with a thickness of 2, 4 and 8 nm at 500 °C for 1 h
Initial Au film thickness (nm)
2.2 Optical characterization of Au NIs
The Au NIs film within the TiO2 nanoparticulate layer should be transparent to the light concurrently so that the penetrated light can excite the dyes adsorbed within the rest TiO2 film. Considering the plasmon resonance absorption peak at 550 nm and the appropriate transparency, the 4 nm thick Au film was chosen to fabricate Au NIs film-incorporated TiO2 photoanode in the following DSSC fabrication with the four different configurations; solely located at the top, in the middle, at the bottom, and combined at all the three positions within the TiO2 film, which are named hereafter as top, middle, bottom and all configuration sample, respectively (detailed in the “Experimental details” section). For the DSSC fabrication, the TiO2 nanoparticulate paste was coated onto a fluorine doped tin oxide (FTO) glass by doctor-blade method.
2.3 Comparison of DSSC performance
DSSC performances (Voc, Jsc, FF, and PCE) of DSSCs having the different photoanodes
In summary, the Au NIs film was placed at different positions within a TiO2 photoanode to exploit the effect of Au NIs location on the surface plasmon resonance phenomenon. Au NIs were spontaneously generated from an Au thin film after thermal treatment at 550 °C for 1 h and the average size of Au NIs increased with the initial Au film thickness. Au NIs with a diameter of 33 nm in average were produced from a 4 nm thick Au film and revealed a plasmon resonance absorption at 550 nm, which was well matched with the absorption peak of N719 dye material.
The Au NIs film was incorporated at different locations within a TiO2 film to generate the bottom, middle, top, and all configuration photoanodes for DSSCs fabrication. The DSSCs having the Au NIs-incorporated photoanode exhibited the higher JSC compared to the reference cell owing to the enhanced plasmon resonance light absorption. The all configuration solar cell had the highest Jsc among the cells. Consequently, the PCE was increased from 5.92% for the reference cell to ~6.4% for the single Au NIs film-incorporated cells, and to 7.01% for the all configuration cell. The three single Au NIs film-incorporated photoanodes demonstrated the similar optical properties and solar performances, indicating that there was no specific effect of plasmon resonance location on solar cell performances.
4 Experimental details
4.1 Formation of Au NIs
The Au NIs were formed by thermal annealing process of Au thin film. Prior to the deposition of Au thin film, the target substrate such as glass, silicon or fluorine doped tin oxide (FTO, 16 Ω/cm) was cleaned by sonication in acetone, IPA and deionized water for 15 min, respectively, and dried with a nitrogen gun. The thin Au film was deposited by electron beam (e-beam) evaporator on the substrates and/or on top of the TiO2 nanoparticulate film coated on the FTO substrate at a different thickness and then annealed at 550 °C for 1 h by using a wind furnace to form the Au NIs.
4.2 Fabrication of photoanodes
Fluorine doped tin oxide glass substrates were used to fabricate the photoanode and the counter electrode in a DSSC. Reference photoanode was fabricated by coating the mesoporous TiO2 (TTP-20 N, ENB-T1204051) nanoparticulate paste by doctor blade technique on top of the FTO glass. Immediately, it was baked on a hot plate at 150 °C for 30 min to remove the remaining solvent within the TiO2 film and then sintered at 450 °C for 90 min in air atmosphere. After sintering, 12 μm thick TiO2 photoanode was prepared in the area of 0.25 cm2. As-prepared photoanode was immersed into 0.5 mM N719 dye solution (Ruthenizer 535-bis TBA, Solaronix, Aubonne, Switzerland) in 1:1 (v/v) mixed solution of acetonitirile (ACN) and tert-butanol, for 12 h to adsorb the dye molecules onto the TiO2 nanoparticulate film. The photoanode was then rinsed in ethanol to remove excessive dyes and dried in air.
The Au NIs film-incorporated TiO2 photoanodes having the Au NIs film positioned at the top, in the middle, at the bottom and at all the three positions within the TiO2 nanoparticulate film were fabricated. The top configuration photoanode was fabricated by depositing 4 nm thick Au film on top of the 12 μm TiO2/FTO substrate and then annealed to form the Au NIs. The middle configuration photoanode was fabricated by depositing the Au thin film on the 6 μm TiO2/FTO substrate and then annealed to form the Au NIs. Then, TiO2 paste was again coated on top of the Au NIs and sintered to have another 6 μm thick TiO2 film. The bottom configuration photoanode was fabricated by firstly making the Au NIs film on the FTO substrate and then 12 μm thick TiO2 nanoparticulate film was placed on it. The all configuration photoanode was fabricated by repeating the necessary processes to have three layers of NIs film within the TiO2 photoanode. Each photoanode configuration is illustrated at the bottom of Fig. 4.
4.3 Fabrication of DSSCs
The counter electrode was fabricated by depositing a 20 nm thick platinum on the FTO glass substrate by using e-beam evaporator. Finally, for assembling the DSSC, the fabricated photoanode and counter electrode were attached using a 30 μm thick surlyn spacer (Dupont) and annealed at 90 °C on a hot plate. Before measuring the DSSC performance, the electrolyte, consisted of 0.6 M 1-butyl-3-methylimidaxolium iodide (C6DMI), 0.04 M I2, 0.2 M LiI2 and 0.5 M tert-butyl pyridine (TBP) in a 1:1 (v/v) mixture of acetonitrile (CAN) and 3-methoxy propiontirile (MPN), was injected into the gap between the photoanode and counter electrode.
HL and GYJ conceived the research. THK and SJC fabricated the devices and measured device characteristics. CLL performed IPCE measurement. THK, YHK and GYJ wrote the manuscript. All authors read and approved the final manuscript.
This work was supported by the Pioneer Research Center Program (NRF-2015M3C1A3022548) and by the “GRI (GIST Research Institute)” Project through a Grant provided by GIST in 2016.
The authors declare that they have no competing interests.
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