Supplementary Information
Photovoltaic Device Based on TiO2 Rutile/Anatase Phase Junctions Fabricated in Coaxial Nanorod Arrays
Pengli Yana, b, Xiang Wangb, c, Xiaojia Zhengb, c, Rengui Lic, Jingfeng Hanb, c, Jingying Shib, Ailong Lib, c, Yang Gana, *, Can Lib, *
a
School of Chemical Engineering & Technology, Harbin Institute of Technology, Harbin,
150001, China. b
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy
of Sciences; Dalian National Laboratory for Clean Energy, Dalian, 116023, China. c
University of Chinese Academy of Sciences, Beijing, 100049, China.
*Corresponding Author. E-mail:
[email protected] (Can Li);
[email protected] (Yang Gan).
S1
Experimental Section Rutile TiO2 nanorod arrays (NRAs): The rutile TiO2 NRAs were grown on a FTO glass substrate by the hydrothermal method.[1] Tetrabutyl titanate (Alfa Aesar, 99%), 0.1 mL in volume, was added dropwise into 10 mL HCl (6M) solution then the solution was kept vigorously stirred for 1 h. Then the solution was transferred into a 30 mL Teflon-lined stainless autoclave. A piece of FTO substrate was cleaned by ultrasonic sequentially in acetone, isopropanol, ethanol and water. This cleaned FTO substrate was immersed into the solution. The autoclave was then put into an oven and kept at 180 oC for 2 h. The FTO substrate was left naturally cooled down at ambient conditions, removed from the autoclave and rinsed with copious deionized water. The NRAs were annealed at 450 oC for 2 h in a muffle furnace under air atmosphere. Rutile/anatase core-shell NRAs: Anatase were deposited over the TiO2 nanorod arrays by the DC reactive physical vapor deposition (PVD) method using a commercial sputtering system (Kurt J. Lesker Company).[2] A titanium metal disk (99.99% purity) was used as the target. The system was pumped down to 10-7 Pa firstly, the argon and oxygen mixed gas was introduced into the chamber through the mass flow controller over the target. The total sputtering pressure was 1.0 Pa with the oxygen partial pressure of 0.25 Pa. The sputtering power was kept constant at 400 W. The target-substrate distance was 140 mm. The thickness of the sputtered film was about 200 nm. The resulting films were annealed at 450 oC for 2 h to improve crystallinity. For comparison, anatase single phase films were deposited over FTO substrate by PVD method. Assembly of TiO2 phase junction solar cell: The counter electrode was prepared by sputtering an ITO film (thickness: 250 nm, diameter: 2 mm) onto the core-shell NRAs with a vacuum deposition mask. Two types of TiO2 single phase devices—FTO/rutile NRAs/ITO and FTO/anatase/ITO were fabricated by directly sputtering ITO films onto the rutile NRAs or sputtered anatase nanofilm. S2
Measurements and characterization: The morphology of rutile TiO2 NRAs and TiO2 rutile/anatase coaxial NRAs were examined using a field emission scanning electron microscopy (FE-SEM, Quanta 200 FEG). The thickness of films was determined with a Stylus Profilometry (Dektak XT). The phase identification was carried out with a powder X-ray diffractometer (Rigaku Rotaflex Ru-200 B) equipped with a Cu-Kα radiation source (λ=0.1541nm) with operating voltage of 40 kV and current of 200 mA. Raman scattering measurements were recorded on a scanning double monochromator (Jobin–Yvon U1000) with a spectral resolution of 4 cm−1 and a 532 nm single-frequency laser (DPSS 532 Model 200) was used as the excitation source. The thickness of the film was determined with a Stylus Profilometry (Dektak XT). Current-voltage (J-V) characteristics of the TiO2 phase junction PV device and TiO2 single phase devices were measured using a Keithley model 2400 digital source meter and a 3 W LED UV LED as light source. The illumination intensity was adjusted to 25 mW/cm2 by a certified reference solar cell (Newport 81388). The Mott-Schottky tests were conducted with an electrochemical work station (2273, Princeton) and a conventional three-electrode system. The single phase TiO2 electrode, a saturated calomel electrode (SCE) and platinum plate were used as working, reference and counter electrodes, respectively; 0.5 M Na2SO4 aqueous solution (pH 6.8) was used as electrolyte. Mott-Schottky plots were evaluated at DC potential range of -0.4~ 0.4 V vs. RHE at a frequency of 1 kHz in dark.
S3
Figures
Figure S1. SEM cross-sectional view of a) the TiO2 rutile/anatase coaxial nanorod arrays, b) the deposited anatase individual columns (detached from nanorods during SEM sample preparation).
S4
2
Current Density (mA/cm )
3 2
ITO Positive FTO Positive
1 0 -1 -2 -3 -0.2
-0.1
0.0
0.1
0.2
Voltage (V)
Figure S2. Photocurrent density-voltage (J-V) curve of the solar cell based on TiO2 phase junction, FTO connect the positive electrode (black line) or ITO connect the positive electrode (red line), the tests were done under UV illumination.
S5
2
Current Density (mA/cm )
1.0 0.5 0.0 -0.5 -1.0
illumination from FTO side illumination from ITO side
-1.5 -0.10
-0.05
0.00
0.05
0.10
0.15
0.20
Voltage (V)
Figure S3. Photocurrent density-voltage (J-V) curve of the solar cell based on TiO2 phase junction, illuminate from the FTO side (black line) or the ITO side (red line), the tests were done under UV illumination.
S6
415
Intensity (a. u.)
362 375 383 392
330
360
390
420
410nm 395nm 385nm 375nm 365nm
450
480
Wavelength (nm) Figure S4. The emission spectrum of the LED light source in the 365–410 nm spectral range
S7
0.09 FTO/rutile/ITO
2
Current Density (mA/cm )
a)
dark condition under illumination
0.06
0.03
0.00
-0.03
-0.06 -0.10
-0.05
0.00
0.05
0.10
0.15
Voltage (V) 0.9
FTO/anatase/ITO
2
Current Density (mA/cm )
b)
0.6 0.3 0.0 -0.3
dark condition under illumination
-0.6 -0.9 -1.2
-0.10
-0.05
0.00
0.05
0.10
0.15
Voltage (V) Figure S5. Photocurrent density-voltage (J-V) curves of TiO2 single phase devices, a) FTO/rutile/ITO (ITO sputtering over as-grown rutile NRAs), b) FTO/anatase/ITO (anatase sputtering on FTO by sputtering method), the test were done under UV illumination
S8
5
3
2
1/C2 (1010 F-2cm4)
1/C2 (1012 F-2cm4)
4
5 4 3 2 1 0 -0.4
-0.2
0.0
0.2
anatase rutile NRAs
1
0 -0.4
0.4
Ptential (V vs RHE)
-0.2
0.0
0.2
0.4
Ptential (V vs RHE) Figure S6. Mott–Schottky plots collected at a frequency of 1 kHz in the dark for the pristine rutile NRAs and anatase fabricated over FTO substrate by PVD method. Inset: Mott-Schottky plots of the anatase electrode collected under the same conditions. The intercept (flat band) of the rutile NRAs plot is about -0.04 V and that of anatase plot is about -0.09 V. The slop of the rutile NRAs plot is about two orders magnitude enhancement than that of the anatase, indicating the carrier density of anatase is about two orders magnitude enhancement than that of the rutile NRAs.
S9
Table S1. PV properties of TiO2 coaxial phase junction solar cell.
Number
Voc (mV)
Isc (mA/cm2)
FF (%)
1
145
0.876
32.3
2
134
0.980
28.0
3
154
1.763
28.7
4
126
1.773
26.2
5
166
0.742
27.6
6
190
0.701
28.9
7
206
0.866
29.1
8
186
0.719
26.9
9
139
1.282
27.1
10
156
0.993
26.6
Average
160
1.07
28.0
S10
Reference [1] B. Liu, E.S. Aydil, Growth of Oriented Single-Crystalline Rutile TiO2 Nanorods on Transparent Conducting Substrates for Dye-Sensitized Solar Cells, J. Am. Chem. Soc., 131 (2009) 3985-3990. [2] L. Meng, A. Ma, P. Ying, Z. Feng, C. Li, Sputtered Highly Ordered TiO2 Nanorod Arrays and Their Applications as the Electrode in Dye-Sensitized Solar Cells, J. Nanosci. Nanotechno., 11 (2011) 929-934.
S11
