N-type silicon photocathodes with Al-doped rear p+ emitter and
Al2O3-coated front surface for efficient and stable H2 production
Supplementary Material
Experimental Section
The fabrication of samples
Single-crystalline-Si wafers (15.615.60.18 cm3, p-type Si, n-type Si, specific
resistance ρ = 1-3 Ωcm, GCL company, China) were used for this work. The
pyramid surface texture was produced by a standard process of alkaline etching on a
mass production line of crystalline-Si solar cells in the Suzhou company of Canadian
Solar Inc. Al-doped rear p+ emitter was fabricated on n-type Si to form np+
photocathode by means of a simple screen-printing process. A non-fritted Al paste is
screen-printed onto the rear surface. After drying the screen-printed samples at 150 0C
for 5 min to vaporize the organic solvents in the Al paste, the p+ region is formed in an
infrared conveyor belt furnace at 900 0C for about 13s. For comparison, the n+p
photocathode was also fabricated on the p-type Si, using a standard process of
phosphorus doping on a mass production line of crystalline-Si solar cells.
The Al2O3 protective layer on the top np+ and pn+ electrode surface was
deposited by a standard atomic layer deposition (ALD) process. The setup is a
InPassion ALD (SoLayTec) using tri-methyl-aluminium (Al(CH3)3) (Shanghai
PuJiang Special Gas Co., Ltd., semiconductor grade, 99.999%) and water. The
process temperature is controlled to be 200 0C and the thickness of Al2O3 is controlled
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to be ~2 nm. The Si samples were then transferred to a solution containing H2PtCl6
(5mM) (Sigma-Aldrich, reagent grade) for 3 min for optimum energy conversion
efficiency. The electrode was photo-irradiated at a negative potential (0.1V vs.
Ag/AgCl), allowing for reduction of the Pt salt to Pt metal. The samples for PEC
measurements were laser-cut into 1.51.5 cm2. In order to establish an ohmic contact
between the copper wire and the unpolished side (back side) of Si, tinned Cu wire was
connected to the Al bottom electrode by gallium-indium eutectic (Sigma-Aldrich).
The exposed backside, edges, and some part of the front of the electrodes (except for
the intended illumination area 0.50.5 cm2) were sealed with an industrial epoxy
(PKM12C-1, Pattex).
PEC Measurement
Prior to the PEC measurement for the samples without Al2O3 protection, native
oxide on the Si surface was removed with 5% HF (Enox, Guaranteed Reagent). A 300
W Xe lamp (Oriel, Newport Co.) with an IR cutoff filter was used as a light source.
During the PEC measurement, the light intensity was carefully maintained at 100
mW/cm2, measured using an optical power meter (Newport Co.) just before the light
enters into the PEC cell. PEC experiments were performed in a one-compartment
quartz cell with a size of 150mm100mm70mm. To minimize the sticking of
hydrogen bubbles on the Si surface, violent stirring of the electrolyte was performed
during the measurement. No additional surfactant was introduced, because the harsh
stirring can remove the bubbles effectively. The measurements were conducted in a
solution containing 0.5mol K2SO4 (Sinopharm Chemical Reagent Co., Ltd.,
2
Analytical reagent) and H2SO4 (pH=1) (Enox, Analytical reagent), using the Si
photocathode as working electrode, an Ag/AgCl (3M KCl) as reference electrode, and
a Pt as auxiliary electrode. The potentials were re-scaled to the ones versus the
reversible hydrogen electrode (RHE) according to the following equation:
E(RHE)=E(Ag/AgCl)+0.197V at pH=0. The potential of the Si electrode was
controlled using a potentiostat (CHI600D, CH Instrument).
Sample characterization
The surface morphology of sample surface was analyzed by Field-Emission
SEM (SU8010, Hitachi). In order to avoid surface charging, Pt was coated by the
sputtering method for 120s using the BAL-TEC/SCD 005 model prior to the SEM
analysis.
Specimens for cross-sectional TEM analysis were conducted by transmission
electron microscope operating at 200 kV (Tecnai G220 S-TWIN, FEI).
The electrochemical impedance spectroscopy measurement was performed using
an IM6 electrochemical workstation in the frequency range from 1 Hz to 100 MHz.
Capacitance versus voltage (C–V) measurement was carried out with a precision
impedance analyzer (4294A, Agilent). Before the measurements, the upper electrode
was fabricated on the electrode surface by coating and drying the Ag paste.
The minority carrier lifetimes were measured using a silicon wafer lifetime tester
(WT-1000, Semilab Semiconductor Physics Laboratory, Inc.).
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0
+
Al2O3-np -Si-0h
+
J (mA/cm2)
Al2O3-np -Si-138h
-10
-20
-30
-40
-1.0 -0.8 -0.6 -0.4 -0.2 0.0
0.2
V (vs. RHE)
Figure S1. PEC J-V curves of Al2O3 coated np+ photocathode before and after a 138 h
long time test. The photocurrent is stable at about 34 mA/cm2 throughout the 138h run
under -0.9V vs. RHE in H2SO4 and 0.5 M K2SO4 solution.
Figure S2. C-2-V plots of np+ and n+p electrodes with and without 2 nm Al2O3 layer.
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Figure S3. Potentiostatic photocurrent density traces for the Pt-loaded Al2O3-protected
np+ photocathodes during the reduction reaction to produce H2 biased 0.1V vs. RHE in
H2SO4 and 0.5 M K2SO4 solution under 100 mW/cm2 Xe lamp. The photocurrent is
stable at about 30 mA/cm2 throughout a 73 h run under 0.1V vs. RHE.
0
-10
+
(b)
np -Si
+
+
n np -Si
+
Al2O3-np -Si
+
10
Pt-Al2O3-n np -Si (=9.01%)
+
+
Pt-Al2O3-np -Si (=8.68%)
+
0
J (mA/cm2)
J (mA/cm2)
(a)
+
Al2O3-n np -Si
-20
-30
-10
-20
-30
-1.2
-0.8
-0.4
0.0
0.0
0.1
0.2
0.3
0.4
0.5
V (V vs. RHE)
V (vs. RHE)
Figure S4. (a) PEC current-voltage (J-V) curves of np+ and n+np+ photocathodes with
and without 2 nm Al2O3 layer. The samples were scanned at 10 mV/s from left to right
potentials. (b) The PEC J-V measurements for the Pt-impregnated and
Al2O3-protected n+np+ and np+ photocathodes measured under 100 mW/cm2 Xe lamp
illumination with an AM1.5 filter.
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