(Invited) Challenges and Opportunities in Dye Sensitized Solar Cells Using DBD Plasma Treated Upconversion Nanoparticles
Authors: Kamataki Kunihiro , Chawarambwa Lesly Fadzai , Koga Kazunori , Shiratani Masaharu 
Published: February 2020

1. Introduction
The dye-sensitized solar cells(DSSC) are widely receiving increasing attention as low-cost solar cells. The photoelectrode of the DSSC is composed of a nanoporous TiO2film which has a very large specific area, on which a sensitizing dye is adsorbed [1]. The specific chemical composition, surface area and surface profiles of the TiO2films determine the energy conversion efficiency of the DSSCs. Through surface treatment, these properties of TiO2can be enhanced [2].
DSSCs generally use visible light for photo-electric conversion. In a DSSC, light absorption is conducted by a light sensitive dye. The ability of the dye to capture light in a wide band range of the light spectrum determines the conversion efficiency of the DSSC. The currently employed dyes in DSSC with the highest conversion efficiencies (N-719) have a light spectrum absorption range of 300-750 nm [1]. Consequently, the inability of these dyes to absorb near infrared (NIR) light, which makes up to 48% of energy of the entire solar spectrum, forms the main energy loss mechanism of DSSC. To realize photo-current conversion from these light, rare-earth luminescent phosphors, which have large band gap, high refractive index, low phonon energy are promising to circumvent the photon transmission losses [2].
2. Experimental details
2.1 Sample preparation
A facile urea based homogeneous precipitation method was employed for the fabrication of Y2O3:Er3+/Yb3+nanoparticles. DSSC was fabricated via the conventional doctor blade method. For the preparation of the DSSC photoanode, transparent TiO2 colloids were deposited on the conductive glass substrate (FTO) to form a transparent photoelectrode and then sintered at 500℃ for 2 hours. In the case of the UC photoanode, transparent TiO2 colloids were screen printed on the FTO and sintered at 500 for 2hours. Reflective TiO2colloids mixed with 5 wt% Y2O3:Er3+/Yb3+nanoparticles were subsequently screen-printed on-top of the TiO2film and sintered at 500℃ for 2 hours. The photoanodes were placed on a dielectric barrier discharge (DBD) grounded plate electrode of DBD, then the TiO2film was treated using DBD under room air conditions for 10 minutes. The distance between the surface of the photoelectrode and the DBD electrodes was 5 mm. After the DBD treatment, the photoanodes were finally soaked in the dye (Solaronix N-719 0.2mM/L in ethanol) solution for 24 hours. Eventually the sandwich-type DSSC were fabricated with platinum-coated FTO glass and iodine electrolyte.
2.2 DBD plasma treatments
Plasma treatments of upconverting nanoparticle films were carried out using a scalable dielectric barrier discharge device [3]. The device consisted of 20 electrodes of a stainless rod of 1 mm outer diameter and 60 mm length covered with a ceramic tube of 2 mm outer diameter. The electrodes were arranged parallel to each other at a gap of 0.2 mm. The discharge voltage and frequency were 7.96 kV and 9.2 kHz, respectively. The discharge power, which was obtained from Q-V Lissajous characteristics, was 2.17 W.
3. Results and Discussion
The photoluminescence spectra of upconversion (UC) nanoparticles under IR laser diode excitation (λ= 980 nm) for three different laser power were measured. We also confirmed both green and red UC emissions, corresponding to the Yb (5F5/2-2Fj) and Er (4F7/2-4Ij) transitions. This indicates that the up converters we developed are capable of transforming NIR radiation into visible light thereby contributing to more photogeneration and have the potential for enhancement of photovoltaic performance of DSSCs.
The device characteristics (short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF), of the corresponding DSSC devices are showed in Figure 1. The DSSC with the DBD treated TiO2/Y2O3:Er3+/Yb3+composite double layer had a Jscvalue of 10.08 mA. Also, the voltage of the DSSC increased from 0.70 to 0.72 V. The overall efficiency of the DSSC increased from 4.31 % for the untreated TiO2/Y2O3:Er3+/Yb3+composite double layer to 5.15 % for the treated composite double layer.
The conceivable reasons for the improvement of the plasma DBD treated cells are as follows [4];
a)Plasma treatment causes surface modification on the TiO2/Y2O3:Er3+/Yb3+double composite film, which enhances the light absorption by the film particles (not by the adsorbed dye).
b) Plasma treatment increases the specific surface area of the composite film by changing the surface morphology of the film particles. This increases the amount of the dye adsorbed by the composite film.
4. References
[1] B. O`Regan and M. Gratzel, Nature, 353 (1991) 737.
[2]T. Yuji, et al., Vacuum 83 (2009) 124.
[3] K.Koga, et al., APEX, 9 (2016) 016201.
[4] Shungo Zen, et al., JJAP.50 (2011) 036201.

Figure 1