2C21
High Performance dye sensitized solar cells based on multilayered
structure of P90 and P25 titania nanoparticles(Inst. Molecular Science, Graduate Univ. for Advanced Studies1, Josai Univ.2)○Deepak Bashyal1, Masakazu Ito2, Koichiro Mitsuke2
1. Introduction
Since the pioneering work of Grätzel and coworkers in 1991, the dye sensitized solar cells (DSSCs)
have attracted remarkable attention due to their promise for low-cost photovoltaics [1]. It has often
been documented that one of the most critical elements in DSSCs is a compact thin film ofmesoporous TiO2 nanoparticles on a conducting substrate [2]. The light harvesting efficiency ofDSSCs depends crucially on the crystalline phase, particle size, and surface area of TiO2 nanoparticles.
Moreover, morphology and porosity of TiO2 films have a direct influence on the efficiency. Herein,
we systematically studied the multilayered TiO2 electrodes made of P90 and P25 nanoparticles,
commercially available from Tegsa Inc. Manipulating the structure of the TiO2 films have led to an
appreciable improvement in the DSSC performance.
2.Experiment
Anatase phase Titania powders of two types, AEROXIDE® TiO2 P25 and VP TiO2 P90 were provided
from Japan Aerosil Inc. Fluoprine-doped SnO2 glass (FTO, 15 Ω/square) and Pt paste (Pt catalyst
T/SP) were purchased from Solaronix SA. Paste containing P25 and P90 was prepared by a bead-millmethod from TiO2 powder, citric acid, ethylene glycol, α-Terpineol, ethanol and distilled water. The
P25 and P90 paste was opaque or semi-transparent, respectively. The TiO2 paste was coated onto FTO
substrates by doctor-blading method, and then sintered at 500 °C for 30 min. In order to ensure thegood light reflection effect, coating the P90 paste as the bottom layer was followed by deposition of
the P25 nanoparticles as the middle layer, and by that of the submicroparticles ~ 400 nm in diameteras the top layers. The mesoporous TiO2 photoelectrodes were preheated at 120 °C for 30 min; after
cooling down, the electrodes were washed with ethanol to remove the remaining small glassfragments. Afterwards, they were immersed in a 0.35 mM N-719 solution in acetonitrile and t-butylalcohol for 24 h at room temperature to allow complete dye adsorption. After taking them out, theexcess of dye molecules on the electrodes were washed away by acetonitrile for several times, andthen the electrodes were dried with a hair dryer. A Surlyn film 25 μm thick was applied between thephotoelectrode and a counter electrode (a platinum-sputtered FTO glass). The cell was sealed byheating at 125 °C. The electrolyte solution was composed of 0.1 M LiI, 0.05 M I2, 0.5 M 1-propyl-3methylimidazolium iodide, 0.1 M GNCS, and 0.3 M 4-tert-butylpyridine in acetonitrile.
The active area of cells is 0.25 cm2. The photocurrent-voltage (I-V) characteristic curves weremeasured by Keithley 2420 under AM1.5G illumination. The morphology of TiO2 film was observedby using a field emission scanning electron microscope (FESEM; JEOL Ltd , JSM-6700F).
3. Results and discussion
Large nanoparticles in a bottom region close to the FTO surface would bring about an unwanted backscattering of light and thus prevents the light from penetrating to dye molecules in a middle and top
regions. To minimize light loss due to such a back scattering, smaller nanoparticles should be placedin the bottom region near the FTO surface. Layer of the larger particles were then loaded onto that of
the smaller nanoparicles. The FESEM images of the top and cross sectional views of the layers ofTiO2 nanoparticles were shown in Figs. 1(a)–(d). It is apparent from these images that average particle
size of P90 is smaller than that of P25.
With the same film thickness the photovoltaic performance of DSSCs is better for P90 than forP25. This is because smaller nanoparticles having higher specific surface area can adsorb greater
amount of dye molecules leading to higher photocurrent and higher external quantum yield.
Therefore, the P90 TiO2 layer in the bottom region absorbs more incident light and producehigher photocurrent. And also there is(c)(a)
negligible back scattering effect at theP90 film. The remaining light that haveescaped from photoabsorption by dye
molecules is transmitted to the upper
layers without any loss owing to lightscattering or reflection at the boundariesof the nanoparticles. This facilitateseffective light absorption of dye(b)(d)
molecules in the middle and top region.
On the other hand, smaller particles sizeof P90 makes it easy to recapture thereflected or back scattering light fromthe top scattering layer [3]. The
hierarchical structure of the threedifferent type of TiO2 made the best useof the incident light to realize highFigure 1. Images of FESEM. The top views ofenergy conversion efficiency.
single-layered TiO2 films prepared with (a) P25 andWe found that the DSSCs with(b) P90 nanoparticles. (c) Top view of the lighthigh energy conversion efficiency canreflection layer made of 400-nm submicroparticles.
be achieved by stacking the smallest(d) The cross sectional view of the triple-layered
P90 particles at the bottom and thestructure of TiO2 piled on an FTO electrode.
largest 400-nm particles at the top. The
photovoltaic parameters of the DSSCs
fabricated in such a manner are summarized in Table 1. DSSCs with 400-nm particles at the top layer
above P25 have shown the highest efficiency. In summary, setting smaller particle size P90 in thebottom layer followed by a bit larger nanoparticle P25, together with the top layer of light-reflection
400-nm particles, has turned out to be the best arrangement for achieving higher solar efficiency.
Table 1. Summary of the results of photovoltaic measurements of DSSCs with multilayered structures: P90 in the bottom, P25 in the middle, and 400-nm particles at the top
as the light reflection layer (LRL).

TiO2Particles
P90
P25
P90+P25
P90+400nm

Jsc (mA/cm2)

Voc (V)

Fill factor

11.8
12.417.21
18.32

0.74
0.67
0.71
0.73

0.72
0.70
0.70
0.71

Efficiency(%)
6.40
5.85
8.55
9.50

References
1. B. O’Regan and M. Gratzel, “A low-cost, high-efficiency solar cell based on dye-sensitized
colloidal TiO2 films,” Nature, 353 (1991) 737–740,.
2. S. Ito, T. N. Murakami, P. Comte et al., “Fabrication of thin film dye sensitized solar cells withsolar to electric power conversion efficiency over 10%,” Thin Solid Films, 516 (2008) 4613–4619.
3. J. Ferber and J. Luther, “Computer simulations of light scattering and absorption in dye-sensitizedsolar cells,” Solar Energy Materials and Solar Cells, 54 (1998) 265–275.