23P013-O
Effects of Electrolyte on Performance of Quantum Dot-sensitized Solar Cells
Using Si Nanoparticles Synthesized by Multi-hollow Discharge Plasma CVD
Si量子ドット増感型太陽電池の電解液最適化
Yuting Wang1, Muneharu Sato1, Hyunwoong Seo1, Giichiro Uchida1, Naho Itagaki1,2, Kazunori
Koga1, Masaharu Shiratani1
王 玉亭1, 佐藤 宗治1, 徐鉉雄1, 内田 儀一郎1, 板垣 奈穂1,2, 古閑 一憲1, 白谷 正治11

Graduate School of Information Science and Electrical Engineering, Kyushu University
744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan2
PRESTO, Japan Science and Technology Agency, Sanbancho, Chiyoda-ku, Tokyo 102-0075, Japan1
九州大学システム情報科学府, 744 元岡, 西区, 福岡 819-0395
2
JSTさきがけ, 東京千代田区三番町, 102-0075
Effects of electrolyte on performance of Si QDSCs were investigated. We successfully fabricated anefficient polysulfide electrolyte based on the solvent mixed with water and methanol at a volume ratio of3:7. The optimal electrolyte contains 1.0M Na2S, 0.5M S and 0.2M KCl. By introducing different
ingredients with proper concentration, Jsc, Voc and fill factor are much improved. The efficiency of Si
QDSCs using optimized electrolyte is 0.027%.

1. Introduction
Semiconductors show dramatic quantization
effects such as tunable band gap and multiple
exciton generation when their sizes are below
25nm. Based on these characteristics, Shockley
and Queisser limit are expected to be overcome[1]. Therefore, quantum dot-sensitized solar cells(QDSCs) using narrow band gap quantum dotssuch as Si and CdSe, have attracted considerable
interest [2]. Si is a dominant material of solar
cells because of its abundance and absence oftoxicity. As compared with the crystalline Si solarcells, Si QDSCs have one more advantage of low
cost. These backgrounds motivated us to fabricate
QDSCs using Si nano-particles [3]. In ourprevious studies, carrier generation was observedin the wavelength range less than 550 nm in Si
QDSCs [4-6]. However the overall performanceis still low, especially Voc. Since Voc of the cellis given by the difference between the conduction
band edge of TiO2 layer and the redox potentialof the electrolyte, the photovoltaic performance
including Voc was much affected by theingredient and concentration of an electrolyte.
Therefore, the performance of Si QDSC wasinvestigated according to various composition ofan electrolyte in this study.
2. Experimental
2.1 Synthesis of Si nano-particles
Si nano-particles were synthesized bymulti-hollow discharge plasma chemical vapor
deposition [3]. Hydrogen diluted silane gas wasintroduced from the bottom of the reactor. It flows
through the hollows of the electrodes and pumped
out from the top of the reactor. Gas flow rates of H2and SiH4 were 9 and 1.5sccm, respectively. Total
pressure was 798 Pa. High voltage with 60MHz
was supplied to the electrode. The discharge power
was 200 W. In such high H2 dilution discharges,
crystalline Si nano-particles were nucleated andgrown in the discharges. They were transported tothe downstream region by gas flow and collectedwith stainless mesh grids located at the downstream
region. In this experiment, their mean size was 17nm.
2.2 Fabrication of Si QDSCs
Si QDSCs were fabricated as shown in Fig. 1.
FTO substrates were used to make the photo andcounter electrodes. The photo electrode wascomposed of two layers: nano-porous TiO2 layer
and Si nano-particle mixed with TiO2 (Si/TiO2)
layer.

Fig. 1 Structure of Si quantum dot-sensitizedSolar cells.

As to an electrolyte, aqueous solvent wasreplaced by solution mixed with water andmethanol at a volume ratio of 3:7. Different
ingredients were introduced into this solvent andthe donor concentration was also adjusted to findthe optimal composition.
3. Results and Discussion
Figure 2 shows I-V characteristics of Si QDSCs
according to the concentration of Na2S. A higherconcentration led to a higher Jsc. It was ascribed tothe faster hole-recovery with higher S2concentration. The best efficiency reached to 0.01%
with electrolyte containing 1.0M Na2S.
0.05

0.25

concentration of Na2S

0.04

0.2
0.15

1.0M

0.03

0.1

1.5M

0.02

0.05
0.5M
0.3M

0-0.05-0.1

0.01

efficiency (%)

2current density (mA/cm )

0.3

00

0.1

0.2

0.3

0.4

0.5

interface, leading to the suppression of charge
recombination.
Table I. Characteristics parameters of Si QDSCs withdifferent concentration of SS (M)
00. 5
1.0
2.0

Jsc(mA/cm2)
0.16079
0.356110.220810.24099

Voc(V)
0.222
0.219
0.132
0.092

FF
0.27
0.30
0.29
0.27

Eff.(%)
0.00960.02340.00860.0060Since Voc decreases with increasing
concentration of Na2S and S, another ingredient
KCl was introduced. The introduction of KCl only
contributes to the enhancement of Voc, while theJsc and fill factor barely has any significant change.
The highest performance is achieved with
optimized electrolyte containing 1.0M Na2S, 0.5MS and 0.2M KCl.
Table II. Characteristics parameters of Si QDSCs
with different concentration of KCl
KCl (M) Jsc(mA/cm2) Voc(V) FF
Eff.(%)
00.35611
0.219
0.30 0.0234
0.05
0.14453
0.191
0.28 0.0079
0.1
0.32953
0.198
0.30 0.0196
0.2
0.34513
0.252
0.31
0.027
0.3
0.33518
0.233
0.33
0.025

voltage (V)Fig. 2 I-V characteristics of Si QDSCs as aparameter of the concentration of Na2S.

To enhance the performance of Si QDSCs, otheringredients, S and KCl, were introduced into theelectrolyte. I-V characteristics of Si QDSCs withelectrolyte containing different concentration of S
are shown in Fig. 3 and the characteristic
parameters are listed in Table I. Cells withelectrolyte containing 1.0M Na2S and 0.5M S has
the best performance. Super-sulfide (S2-) has afavorable effect on hole-recovery and the presenceof sulfide with S2- leads to the presence of S2- [7].
Therefore, a doubled Jsc of 0.36mA/cm2 is obtained

0.3
0.2
0.1

0.05
Na2S:1.0M S:0.5M
Na2S:1.0M

S:2M

0.04
0.03

Na2S:1.0M S:1.0M 0.02
Na2S:1.0M S:0M
0.01

efficiency (%)

2current density (mA/cm )

0.4

0-0.1

00 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

voltage (V)Fig. 3 I-V characteristics of Si QDSCs as aparameter of the concentration of S.

after S is introduced. Fill factor is improved due tothe modification of photoelectrode/electrolyte

4. Conclusions
The optimization of electrolyte for higher
performance of Si QDSCs was carried out. We
successfully fabricated an efficient polysulfide
electrolyte based on the solvent solution mixed
with water and methanol at a volume ratio of 3:7.
The optimal electrolyte contains 1.0M Na2S,0.5M S and 0.2M KCl. By introducing different
ingredients with proper concentration, Jsc, Voc
and fill factor are much improved. As a result, theefficiency of Si QDSCs using optimized
electrolyte is 0.027%.
Acknowledgment
This work was partly supported by New EnergyandIndustrialTechnologyDevelopment
Organization (NEDO).
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[2] Yuh-Lang Lee, et al., Chem. Mater. 20 (2008) 6903.
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10769.
[4] Y. Kawashima, et al., Trans. Mater. Res. Soc. Jpn. 35(2010) 597.
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[7] S. Licht, Nature 300 (1987) 148.