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J. Korean Ceram. Soc. > Volume 53(2); 2016 > Article
Noh, Choi, Kim, and Song: Properties of Working Electrodes with Nano YBO3:Eu3+ Phosphor in a Dye Sensitized Solar Cell

Abstract

We added 0 ~ 5 wt% YBO3:Eu3+ nano powders in a scattering layer of a working electrode to improve the energy conversion efficiency (ECE) of a dye sensitized solar cell (DSSC). FESEM and XRD were used to characterize the microstructure and phase. PL and micro Raman were used to determine the fluorescence and the composition of YBO3:Eu3+ phosphor. A solar simulator and a potentiostat were used to confirm the photovoltaic properties of the DSSC with YBO3:Eu3+. From the results of the microstructure and phase of the fabricated YBO3:Eu3+ nano powders, we identified YBO3:Eu3+ having particle size less than 100 nm. Based on the microstructure and micro Raman results, we confirmed the existence of YBO3:Eu3+ in the scattering layer and found that it was dispersed uniformly. Through photovoltaic properties results, the maximum ECE was shown to be 5.20%, which can be compared to the value of 5.00% without YBO3:Eu3+. As these results are derived from conversion of light in the UV range into visible light by employing YBO3:Eu3+ in the scattering layer, these indicate that the ECE of a DSSC can be enhanced by employing an appropriate amount of YBO3:Eu3+.

1. Introduction

As one of the next-generation solar cells, the dye sensitized solar cell (DSSC) has the advantage of low-cost, large scale fabrication, and flexible substrate applicability.1) However, whereas energy conversion efficiency (ECE) of the Si solar cell is about 20%,2) that of DSSC is about relatively low at about 11%,3) requiring further research into solving this problem.
The DSSC consists of working electrode (WE), electrolyte, and counter electrode (CE).4-6) In case of WE, dye plays a role as forming electrons and holes upon absorbing light, and the formed electrons are delivered to an external circuit through TiO2 layers.7) Hence, WE engineering significantly affects the ECE of DSSCs. In particular, in order to improve the ECE, absorbing light from outside as much as possible is important.
To enhance light energy absorption, two major ways are utilized: one is to improve the quantity of absorption through WE engineering; and the other is to enhance the absorb range of ultra-violet rays and infrared rays as well as the existing visible-light range.
First, as for the way of light energy absorption through WE engineering, D. Huang et al. 8) have reported an increase in the ECE to 5.25% compared with the exiting 3.81% by adding diatoms into the mesoporous TiO2 layer in order to increase specific surface area. Y. Noh et al. 9) reported that an existing ECE of 5.25% increased to 6.35% by using a scattering layer which reflects penetrated light and makes re-absorption possible.
Second, in order to absorb the range of ultra-violet rays and infrared rays as well as the existing visible-light range, the up-conversion and down-conversion method is available. The up-conversion method converts long waves in the infrared range into visible rays while the down-conversion method converts short waves in the ultraviolet range into visible rays. Since the down-conversion method is easier, utilizing phosphor that absorb the ultraviolet range is more effective.10) N. Yao et al.11) applied ZnO: Eu3+, Dy3+ phosphor that absorbs short waves of 338 nm and 395 nm and then emits waves of 458 nm and 611 nm to DSSCs, which improved the ECE up to 4.48% higher than the existing 1.3%. S. Bai et al.12) added perylene into an electrolyte that absorbs short waves of 350 ~ 440 nm and emits a visible-light waves as wide as 450 ~ 550 nm so that it can absorb the ultraviolet range of light further, which improved the ECE to 7.99% higher than the existing 6.89%. However, the commercialized phosphor have a particle size of 10 μm, and it is necessary to adopt nano-size phosphor in order to use DSSCs with 8 ~ 9 μm thick TiO2 layer.
YBO3:Eu3+ phosphor is a type of red phosphor, which absorbs short wave of 245 nm and emits the visible light wave of 580 nm.13) Since the size of particles may be varied depending on the thermal treatment temperature in the synthesis process, it is possible to produce nano-size phosphor by optimizing the thermal treatment temperature.14)
In addition, it is reported that the transmittance of ultraviolet rays in a DSSC device is as much as 30 ~ 40% of the incident light. It is expected that the ECE can be improved by adopting the scattering layer with YBO3:Eu3+ phosphor that absorbs ultraviolet rays that would be lost in existing ways.15)
In this study, we adopted YBO3:Eu3+ phosphor having a nano particle size on scattering layers in DSSC to improving ECE.

2. Experimental Procedure

To examine the change of opto-electrical characteristics in a DSSC upon addition of YBO3:Eu3+ phosphor, 0 ~ 5 wt% of YBO3:Eu3+ whose particles were as small as 100 nm was added to the scattering layer and then distributed.
Nano-size phosphor particles were mixed for 5 h at 80°C after Y(NO3)3·6H2O 3.8345 g(0.1 M), H3BO3 0.6493 g(0.105 M), Eu(NO3)3·5H2O 0.2142 g(0.005 M) were dissolved in 100 ml of distilled water with 13 ml of ammonia solution (25%, NH4OH) at pH 9. They were then cleansed by means of distilled water and ethanol and went through a thermal treatment at 800°C for 2 h to produce YBO3:Eu3+.
We analyzed the surface microstructure of nano-size phosphor by field emission scanning electron microscope (FESEM, S-4300, Hitachi) at an acceleration voltage of 15 kV.
To check the composition of producing phosphor, the high resolution X-ray diffraction analysis method (HRXRD, X′ pert-pro, PANalytical) was used. The X-ray source was CuKα with a nickel filter. The tube current was 30 mA, and the acceleration voltage was 40 kV. The phosphor composition was examined in the 2θ range of 20° to 80°.
PL (Hitachi, F-4500) was utilized to check the fluorescence characteristics and emitted waves in the range of 200 to 800 nm. The excitation wavelength was set to 250 nm, the scan mode to emission, and the data mode to fluorescence respectively.
We produced 300 nm-thick blocking layers by mixing titanium( IV)bis(ethyl aceto acetato)-diisopropoxide and 1-butanol into a solution, followed by spin-coating for 500 rpm-10 sec. and 2000 rpm-40 sec., and then heat treating at 500°C for 15 min.
We fabricated the 10 μm-thick TiO2 films by coating TiO2 paste having particle size of 20 nm (DSL 18NR-T of 10, Dyesol) via doctor blade method, and by heat treatment at 500°C for 30 min. The scattering layer with 0 ~ 10 wt% of phosphor was coated by doctor blade method, and then it went through a thermal treatment at 500°C for 30 min to produce a 10 μm-thick scattering layer.
For a visual analysis, dark-field illumination on the sides and bottom of the sample was conducted in the range of 10 ~ 60 magnifications by means of a GIA optical microscope in addition to the overhead light and UV illumination at the top of the sample. The image was taken by means of a digital camera (Nikon, Coolpix4500).
The micro-Raman spectrometer (UniThink, UniRaman) was utilized to measure the major components: as the phosphor powder was put on a glass. At this time, characteristic peaks were checked for the range of 200 ~ 2000 cm−1 by conducting scans for 60 times/sec by using an accumulation mode at the center value of 1000 cm−1 for each sample.
We absorbed 0.5 mM cis-vis bis-ruthenium (II) bis-tetra-butylammonium (N719) to complete the working electrode of the glass/FTO/blocking layer/TiO2/scattering layer + YBO3:Eu3+ / dye(N719).
The CE was prepared by RF sputter (MHS-1500, Moohan, 300 W, 13.56 MHz) to form a 100 nm-Pt film on a glass substrate using 99.99% Pt as a target. A flow of 40 sccm Ar at pressure of 5 mtorr at RT was set for the process.
With the produced working electrode and counter electrode fixed with clips, electrolytes were put in to complete the DSSC consist of glass / FTO / blocking layer / TiO2 / scattering layer + YBO3:Eu3+ / dye(N719) / electrolyte / 100 nm Pt / glass with active area of 0.45 cm2.
The impedance of DSSC was determined by solar simulator (PEC-L11, Peccell) and potentiostat (Iviumstat, Ivium) to verify interfacial resistance. The analysis was carried out in the frequency range of 10 mHz ~ 1 MHz applying AC voltage and collecting the current responses.
I-V (current-voltage) characteristic of DSSC was measured by the same instruments under a setup; a 100 W Xenon lamp was the illumination source at 1 sun (100 mW/ cm2) condition. From the I-V curves, short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and ECE were evaluated. To keep the quantity of light constant, a 0.45 cm2-large mask was made with black insulation tape and attached on the DSSC.

3. Results and Discussion

Figure 1 shows the XRD results of the YBO3:Eu3+ phosphor. The upper right inset is the FESEM image of the YBO3:Eu3+ phosphor at 80,000 magnifications. The result of XRD analysis shows that the plane directions of YBO3:Eu3+ are (100), (102), (004), (110), (112), (200), and (114) for 26.80, 33.70, 41.00, 47.70, 52.35, 55.80, and 64.60°, respectively. This result indicates that the YBO3:Eu3+ having nano-particle size successfully produced.16) In addition, the FESEM image in the upper right inset shows that the particles size of YBO3:Eu3+ phosphor are from 50 nm to 100 nm.
Figure 2 shows the PL analysis results in the wavelength of the produced YBO3:Eu3+ from 570 nm to 660 nm. The PL analysis shows that emission peak of YBO3:Eu3+ was shown in the wavelengths of 595, 610, 625, and 650 nm, which corresponds to the report of Dubey et al.16) We confirmed that YBO3:Eu3+ phosphor absorbs short wave of 245 nm and emits light to the area of red visible rays with range of 595 ~ 650 nm. Therefore, our results imply that YBO3:Eu3+ phosphor will improve the ECE of DSSC because of increasing light absorption in the area of red visible rays.
Figure 3 shows that the surface images of the scattering layers with (a) 0 wt% and (b) 5 wt% of YBO3:Eu3+ phosphor under UV light. As for (a) 0 wt% phosphor employed to the scattering layer, the coating was uniform and no fluorescence was observed under UV irradiation. This result indicates that the only TiO2 (scattering layer) does not have the fluorescence. As for (b) 5 wt% phosphor employed to the scattering layer, the spherical phosphor particles were uniformly dispersed compared to (a). In addition, the red light emission was observed from dispersed phosphor particles under UV irradiation. Thus, our results imply that YBO3:Eu3+ phosphor was successfully dispersed into the scattering layer.
Figure 4 shows that the FE-SEM images of WE with (a) 0 wt% and (b) 5 wt% of YBO3:Eu3+ phosphor at 500 magnifications. The upper right inset is a cross sectional FESEM image of the same WE at 4,000 magnifications. As for (a) 0 wt% of phosphor, the surface was uniformly coated without agglomeration, which corresponded to the result of previous Fig. 3. The cross sectional image in the upper right inset shows that both the scattering layer and TiO2 layer were coated properly as thick as 10 μm. As for (b) 5 wt% of phosphor, spherical particles were uniformly dispersed compared to (a). The YBO3:Eu3+ nano particles were condensed as small as 3 μm, as already shown in the Fig. 3. The cross sectional image in the upper right inset shows that both the scattering layer and TiO2 layer were coated in the same thickness with (a). As indicated by the dotted circle, we confirmed the size of agglomeration was 3 μm. Thus, the FESEM analysis result demonstrates that YBO3:Eu3+ phosphor was successfully dispersed into the scattering layer as intended.
Figure 5 shows micro-Raman data of the WE samples with 0 ~ 5 wt% of YBO3:Eu3+ phosphor. The YBO3:Eu3+ phosphor showed Raman characteristic peaks at 1569, 1865, 1914, and 1948 cm−1. In contrast, the Raman characteristic peak of YBO3:Eu3+ was not observed in the WE with 0 wt% of YBO3:Eu3+. In case of WEs with 1 ~ 5 wt% of YBO3:Eu3+ phosphor, the Raman characteristic peak of YBO3:Eu3+ was observed. As the amount of YBO3:Eu3+ increased, the intensity increased linearly. Thus, the Raman analysis result indirectly implied that the addition of YBO3:Eu3+ on the scattering layer was successfully dispersed as intended.
Figure 6 shows the Nyquist plot consisting of real part (Z′) and imaginary part (Z″) obtained at an applied frequency for DSSC devices employing 0~10 wt% YBO3:Eu3+. As internal resistances of the common DSSC, three semicircles (R1, R2, R3) were observed. R1 values are the interface resistance that is related to the CE at 103-105 Hz, which is about 1.6 Ω within the error range. This is because the same CE was used in this study. R2 values are related to the electron-transfer resistance in TiO2 at 1-103 Hz, and it is shown to linearly decrease upon increasing the amounts of 0, 1, 3, 5 wt% YBO3:Eu3+, respectively. As phosphor was dispersed on the scattering layer, ultraviolet rays were converted into visible rays, and as a result, light absorbance increased. However, R2 values increased when the amounts of phosphor were more than 5 wt%, and this is attributed to a relative decrease in the fraction of TiO2 (scattering layer). R3 value represents the Warburg impedance, which is related to the diffusion of oxidation-reduction species in the electrolyte at above 106 Hz. R3 value was approximately 1.6 Ω and was within the error range. The same electrolyte used for measurements should be responsible for this result. Thus, we confirmed that a proper amount of phosphor into scattering layer increased in additional light absorption and excited electrons, reducing the electron transport resistance.
Figure 7 shows the I-V data for the DSSC device with the structure of glass/FTO/blocking layer/TiO2/scattering layer with 0 ~ 5 wt% YBO3:Eu3+/dye(N719)/electrolyte/100 nm Pt/ glass. The Jsc of DSSC without phosphor was smaller than that of DSSC with phosphor. Especially as for 5 wt% phosphor, the value of Jsc was the largest.
Table 1 shows the I-V result of Fig. 7 in detail. Voc showed similar values within the error range, and the similar values of Voc considered to be attributable to the use of the same electrolyte in all Fermi levels as an element related to the oxidation-reduction reaction of the electrolyte. FF was affected by the interface resistance of elements, and was measured within the error range since the same TiO2 paste, electrolyte, and CE were used in this study. Jsc of DSSC device employing 0 wt% phosphor was 10.96 mA/cm2, and those of 1, 3, and 5 wt% phosphor were 11.18, 11.71, and 11.96 mA/cm2, respectively. This indicates that as the amount of addition increased, the value of Jsc increased accordingly. As the result of the previous impedance analysis, employing 1 ~ 5 wt% phosphor makes possible additional light absorption in the ultraviolet range. As a result, the number of generated electrons increases. When a large amount of 8, 10 wt% phosphor was added, the values of Jsc decreased down to 11.02 and 10.97 mA/cm2 due to a relative decrease in the fraction of TiO2(scattering layer). Thus, the final ECEs obtained by employing 5 wt% phosphor were 5.20% depending on the increase in Jsc. Therefore, we successfully proposed DSSC devices with an improvement in ECE by employing phosphor of a proper quantity into scattering layer.

4. Conclusions

We employed YBO3:Eu3+ with particle size having 100 nm or smaller into the scattering layer of DSSC. YBO3:Eu3+ with particle size having 100 nm or smaller were produced and were uniformly dispersed into the scattering layer. As a result of analyzing the composition and fluorescence, it was demonstrated that YBO3:Eu3+ absorbed waves of 245 nm and emitted light in the range of visible rays. As for the ECE of DSSCs, DSSC device of employing 5 wt% phosphor was increased resulting from an increase in Jsc. In contrast, when a large amount of 8, 10 wt% phosphor was added, the values of ECE decreased due to a relative decrease in the fraction of TiO2(scattering layer). Thus, by employing the appropriate amount of YBO3:Eu3+ phosphor, we have demonstrated an improvement in the ECE of DSSC.

Fig. 1
XRD peak of fabricated YBO3:Eu3+. Inset is FESEM image.
jkcs-53-2-253f1.gif
Fig. 2
Photoluminescence data of fabricated YBO3:Eu3+ in visible ray region.
jkcs-53-2-253f2.gif
Fig. 3
Macro optical images of scattering layers with phosphor of (a) 0 wt%, and (b)5 wt% under UV irradiation.
jkcs-53-2-253f3.gif
Fig. 4
FESEM images of scattering layers with phosphor of (a) 0 wt%, and (b) 5 wt%. Insets are cross sectional images.
jkcs-53-2-253f4.gif
Fig. 5
Raman spectrums of working electrodes with scattering layers with phosphor of 0 wt% ~ 5 wt%.
jkcs-53-2-253f5.gif
Fig. 6
Nyquist plot of DSSCs employing scattering layers with phosphor of 0 wt% ~ 10 wt%.
jkcs-53-2-253f6.gif
Fig. 7
Current-voltage (I-V) characteristic of DSSCs employing scattering layers with phosphor of 0 wt% ~ 10 wt%.
jkcs-53-2-253f7.gif
Table 1
Photovoltaic Properties and ECE of DSSC with Phosphor Addition
Sample Voc (V) FF Jsc (mA/cm2) η (%)
0 wt% 0.69 0.69 10.96 5.00
1 wt% 0.69 0.66 11.18 5.12
3 wt% 0.68 0.65 11.71 5.17
5 wt% 0.68 0.64 11.96 5.20
8 wt% 0.69 0.66 11.02 5.07
10 wt% 0.68 0.66 10.97 4.98

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