Construction and Optimization of the Dye Sensitization Solar Cell Using a Au Particle and a Natural Vegetable Pigment
Authors: Komatsu Masaharu , Shinkuma Sei , Sakakibara Takayuki , Okaniwa Yuto , Kiyota Takuya 
Published: February 2020

Light of the sun is inexhaustible and clean. The study of the solar cell using light of the sun is currently conducted. Dye sensitization solar cell (DSSC) uses a metal complex having rare and expensive ruthenium (Ru), or organic dye. Therefore, in this study, a pigment from a naturally occurring plant for cost reduction was extracted and was used for sensitizing dye. However, the conversion efficiency of DSSC made using a nature plant pigment is low. Therefore, with reinforced light electric field occurred by the plasmon absorption of metal nanoparticles, the improvement of the conversion efficiency of DSSC using the natural plant pigment was tried. In this session, current density-voltage (i-V) characteristics is reported with construction of DSSC which put Au nanoparticles and the natural plant pigment together.

In a 100 mL measuring flask, HAuCl4·4H2O, trisodium citrate dihydrate, and NaBH4 was added to this order to become 0.2 mM, 0.7 mM and 0.3 mM each. Au nanoparticles dispersion liquid was prepared in this way, and was added for triethylene glycol to become 30 vol%. In addition, polyethylene glycol, TiO2, and the sodium dodecyl sulfate aqueous solution were added for this Au nanoparticles dispersion liquid. Resulted mixture was sonicated. The paste, which was prepared by the method mentioned above, was applied on conductive face of an ITO substrate by the squeegee method. Substrate casted was calcined at 500 degrees Celsius for 1 hour. A layer prepared on ITO substrate was assumed as AuTiO2 film. On the other hand, after crushing a grape pericarp or 5 g of Phytolacca americana fruit in the mixer with "an aqueous solution which included 0.993 % by volume TFA in methanol 63.8 % by volume (63.8 vol% acidization methanol)", resulting mixture was filtered, centrifuged, and supernatant solution was provided with pigment solution. TiO2 film on ITO substrate was immersed in the pigment solution which was extracted, and was left at rest in a cool incubator of 5 degrees Celsius for 1 day. After dipping for 1 day, a substrate was washed in ultra-pure water and was dried in air. TiO2 film which was manufactured from the paste which did not include Au nanoparticles dispersion liquid. The AuTiO2 film and the TiO2 film modified with pigment were used as a working electrode. The ITO substrate which sputtered Pt was used as a counter electrode. Using these working and counter electrode, i-V characteristics was measured under the halogen bulb irradiation that transmitted a UV cut filter and a heat-rays absorption filter.

By the UV-vis absorption spectrum in the pigment solution which was extracted from Phytolacca americana fruit and Kyoho grape pericarp, the maximum absorption wavelengths was confirmed about two pigment solution in a visible light region of 541 nm and 532 nm, respectively. In addition, in DSSC using the pigment that was extracted from Phytolacca Americana, as a result of i-V characteristics measurement, Light energy conversion efficiency and short circuit photocurrent density of approximately 2.4 times of DSSC using the pigment extracted from Kyoho grape were confirmed. As for this, a hydroxyl group of the TiO2 surface and one of the anthocyanin-based pigment forms hydrogen bonding in DSSC using Kyoho grape pigment, but it is thought that this is because electron transfer from the pigment to TiO2 was performed more effectively than a case of Kyoho grape because the carboxy group of the Betalain series dye and the hydroxyl group of the TiO2surface form ester bond in DSSC using the Phytolacca americana pigment. At present, to identify a nature pigment, structure analysis are being examined.

Au nanoparticles dispersion liquid and the Phytolacca Americana pigment solution respectively showed the maximum absorption wavelength of 526 nm and 549 nm by UV-vis absorption spectrum. The conversion efficiency of DSSC which was assembled with AuTiO2 film and TiO2 film was respectively 0.05287 % and 0.01669 %. The difference in this conversion efficiency depends on magnitude relation of the short circuit photocurrent density. Charge transfer resistance decreases by adding the Au nanoparticles which are a metal to TiO2 which is a semiconductor, and it is thought that short circuit photocurrent density increased. In addition, it is thought that this is because the excitation of the pigment became easy to happen by reinforced light electric field occurred by the plasmon absorption of metal nanoparticles. The improvement of the conversion efficiency was confirmed, but performance is largely low in comparison with general DSSC. Therefore, by ac impedance measurement, the resistance ingredient in the cell is identified and the construction condition of the cell is being examined.