Supporting Information

Copper Sulfide Catalyzed Porous Fluorine-doped Tin Oxide Counter Electrode for

Quantum Dot-Sensitized Solar Cells with High Fill Factor

Authors: Satoshi Koyasu, Daiki Atarashi, Etsuo Sakai, Masahiro Miyauchi* (Tokyo Institute of

Technology)

1. Confirmation of free electrons in FTO (Fig. S1)

Fig S1 shows absorbance of non-doped SnO2 and fluorine doped SnO2 (FTO) powder. FTO powder absorbs

wide range of visible light and near IR region because of the existence of free electrons.

2. Photos of FTO/CuS counter electrodes (Fig. S2 and S3)

Fig S2 shows photographs of the fabricated porous-FTO/CuS counter electrode. Each sample was changed the

times of SILAR cycle number. In case of the sample of 40 SILAR cycle, porous-FTO was broken by inner-

pressure of CuS crystal.

Fig S3 shows photographs of the fabricated porous-FTO/CuS counter electrodes. One electrode was

constructed with an Au film between the porous-FTO and commercial FTO glass substrate, whereas another was

constructed without an Au layer.

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3. Crystal phase of Copper Sulfide (Fig. S4)

Since the amount and crystallite size of CuS on porous FTO electrodes were too small to determine the crystal

phase of CuS, a powder form of CuS was prepared using the same solutions with the SILAR method for XRD

analysis. Fig S4 shows the XRD pattern for CuS powder, and almost diffraction peaks were assigned to Covellite

CuS crystal phase (ICDD No. 00-006-0464, space group: P63/mmc). As a impurities, this powder contain copper

oxide (CuO) and copper hydroxyl-sulfate (Cu3(SO4)(OH)4 )

4. Equivalent circuit of porous-FTO/CuS electrode for impedance analysis (Fig S5) and crystal structure of CuS

(Fig. S6).

5. Catalytic properties of various metal sulfide electrodes prepared by SILAR method (Fig. S7)

Fig S7 shows polarization curve for CuS, FeS2, and PbS electrodes. In this measurement, scan rate of voltage

is 100 mV/sec. Among these metal sulfides, CuS exhibited the highest current under the both cathodic and anodic

bias conditions, revealing the efficient redox catalytic property of CuS. In case of PbS, its anodic current was

higher than that under cathodic bias, suggesting that its reduction activity is not so efficient as compared to its

oxidative ability. In contrast, FeS2 exhibited higher cathodic current than anodic side. The efficient redox ability

of CuS is attributed to its crystal structure. Fig S6 which was created by VESTA is crystal structure of CuS and

FeS2 and PbS[1]. CuS has two types of sulfur sites in its crystal, namely S2- and S22- ions. Notably, the solid-state

S22- site has structural similarity with the polysulfide ion in the electrolyte used in our system and plays an

important role in the catalytic reactivity with sulfur redox couples in the electrolyte. FeS2 also has the S22- sites in

its crystal, while PbS does not. Therefore, the cathodic current of CuS and FeS2 was superior to that of PbS.

8. Comparison of porous-FTO/CuS and Pt electrode (Fig. S8)

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Fig S8 shows Nyquist plot of porous-FTO/CuS electrode and conventional Pt electrode. Pt electrode has high

charge transfer resistance (about 7519 Ω), on the other hand porous-FTO (5μm)/CuS (5 times) has row charge

transfer resistance (about 26.2 Ω).

7. Stability of electrode under repeated redox reaction (Fig. S9)

Fig S9 shows the polarization curve of CuS modified porous FTO electrode (thickness of porous-FTO is 5μm,

SILAR cycle is 20 times) in polysulfide electrolyte (Na2S:1M + S1M). Cyclic measurement was repeated for 20

times, but the performance of the electrode kept stable, suggesting the high stability of our CuS modified FTO

electrode under the repeated redox reactions.

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Fig S1. UV-Vis spectra of SnO2 and FTO powder recorded by a diffuse reflectance method.

Fig S2. Photos of porous-FTO/CuS electrode (SILAR cycle number is 1,5,10,20,40 times respectively)

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Fig S3. Photos of FTO/CuS counter electrodes prepared with (left) and without (right) a sputtered layer of Au.

Fig S4. XRD pattern of CuS powder synthesized by using the same solutions with the SILAR method

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Fig S5. Equivalent circuit of porous-FTO/CuS electrode for impedance analysis. Rs: series resistance, Rin: internal resistance, Rct: charge transfer resistance, L1: inductance, C1: capacitance, CPE1 and CPE2: constant phase elements, Wo1: Warburg impedance, respectively.

Fig S6. Crystal structure of CuS (a), and FeS2 (b), and PbS (c).

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Fig S7. Current-voltage properties of various metal sulfide electrodes (scan rate : 100 mV/sec). Red line: FeS2,

green line: CuS, and blue line: PbS electrode, respectively.

Fig S8. Nyquist plots for the Pt electrode and porous-FTO/CuS electrode (0.25 cm2) in S2-/Sx2- electrolyte.

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Fig S9. Repeated current-voltage property of CuS/FTO electrode (SILAR cycle 20 times)

Reference

1. Momma, K. and F. Izumi, VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. Journal of Applied Crystallography, 2011. 44(6): p. 1272-1276.

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