Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.
This journal is © The Royal Society of Chemistry 2016
CdS-CdTe Heterojunction Nanotube Arrays for Efficient Solar Energy Conversion
W. P. R. Liyanage,a and M. Natha,*
Supplementary images
Figure S1 SEM image of a CdTe tube grown by pulsed electrodeposition. Compared to the continuous
deposition, lateral growth has minimized and tubes can be grown beyond the thickness of the polymer by
pulsed deposition. (a)(
Figure S2 PXRD pattern of the electrodeposited CdTe layer before deposition of CdS layer. This pattern
matched with the standard CdTe pattern [JCPDS 15-0770]. * indicates ITO peaks from the substrate.
(a)
(b)
(c)
Figure S3 SEM image of one nanotube pattern written in three different sizes to show
the versatility of the protocol to fine tune the nanotube diameter and wall thickness
without changing the feature size of the original pattern used in EBL. When the pattern
is written in a larger area (a) the tube diameter as well as the distance between adjacent
tubes (pitch) becomes larger and the tube wall thickness become thinner. When the
same pattern is written in a small area (c) the tube pore diameter as well as pitch
become smaller and thickness of the nanotube wall become larger.
Te 3d5/2
Te 3d3/2
CdTe
Te-O
590
585
580
575
Binding energy (eV)
Cd 3d5/2
(b)
Intensity (a. u.)
Intensity (a.u.)
(a)
570
CdTe
417
Cd 3d3/2
412
407
Binding energy (eV)
402
Figure S4 XPS analysis of the CdTe nanotube arrays. (a) Binding energy spectrum of Te 3d3/2 and
3d5/2 (d) Binding energy spectrum of Cd 3d3/2 and 3d5/2 confirms the presence of CdTe.
9
5.0
(a)
9
x 109
14
(b)
5
12
4.0
(αhν)2 (ev/cm2)2
(αhν)2 (eV/cm2)2
x 10
4
3
2
(αhν)2 (eV/cm2)2
6
x 10
3.0
2.0
1.0
1
Eg 1.51 eV
0
Eg 2.31 eV
0.0
0.5
1.0 1.5 2.0 2.5
Photon energy (eV)
10
8
6
4
2
0
0.0
1.0 2.0 3.0 4.0
Photon energy (eV)
0.5
1.0 1.5 2.0 2.5
Photon energy (eV)
Figure S5 Optical band gap determination of the (a) as-prepared nanotube arrays, (b) cadmium sulfide
thin film and (c) CdS/CdTe nanotube combined device
Figure S6 Stability of the photocurrent response of the CdTe on CdS device under photoelectrochemical
testing conditions. Initially, the device response was monitored by periodically turning on and off the
light source. Then light source was kept on and photocurrent was recorded. It can be seen that the
photocurrent response was stable for more than 2 h of continuous illumination.
Figure S7 Comparison of light on-off response of the CdTe nanotube array with CdS/CdTe
nanotube arrays to see the effect of having a CdS layer. It was seen that there is ~25% increase of
current density in the presence of n-type CdS layer.
8
7
Rs
4
RCT
3
2
1
CPE
0
0
5
10
Z'/kohm
Rs
4
RCT
2
CPE
0
15
-Z"/kohm
6
5
-Z"/kohm
-Z"/ kohms
6
300
0
4
Z'/ kohm
200
Rs
100
CPE
0
8
RCT
0
100
200
Z'/kohm
300
Figure S8 Nyquist plots of (a) CdS/CdTe nanotube arrays (b) CdS/CdTe nanorod arrays and (c)
CdS/CdTe thin film. The inset shows the equivalent circuit diagram.
Table ST1 Evaluated EIS parameters of different geometries of the fabricated devices
CdTe
morphology
Nanotube
array
Nanorod array
Thin film
Rs/ohm
Rct/kohm
CPE/Fcm-2
0.28
15.02
5.80 x 10-2
0.38
0.44
18.52
819.10
7.02 x 10-6
1.65 x10-6