Environmental Engineering Reference
In-Depth Information
16.5.2
Defect-Induced Photoconductivity in Nanowire Films
Optoelectrical properties of ZnO can be extracted by measuring photo-
conductivity of the sample. When ZnO nanowire i lms are excited with
UV light, the valence band electron gets excited and contributes in electri-
cal conductivity. h e ef ect of defect states on photoconductivity of ZnO
nanowire has been studied and it has been observed that high defect states
in ZnO nanowire i lms enhances photocurrent [25]. As seen in Figure 16.6,
that dark current was higher in annealed NWs, however photocurrent was
higher in as-grown sample. h ese results reveal that defect induces high
photocurrent in solution grown NWs i lms. h is can be explained through
the schematic of the band diagram of ZnO nanowire under dark and pho-
toexcited conditions (Figure 16.8). Photo-excitation creates electron-hole
pairs in NWs. h e excited electrons i ll the conduction band and contrib-
ute in NWs i lm conductivity. Simultaneously, surface adsorbed oxygen
molecules (O
2-
) capture the photogenerated holes available at the nearby
surface and release oxygen. h is leads to change in band bending by an
amount with respect to the dark condition. h e amount of band bending
(
a
)
Under Dark condition
(
b
)
Under UV photon
O
2
O
2
±
±
±
±
±
±
±
±
O
-
O
-
O
2
V
0
+2
V
0
+2
O
2
V
0
0
V
0
0
O
-
O
-
O
-
O
-
O
-
O
2
V
0
0
V
0
0
O
2
V
0
+2
V
0
+2
O
-
O
2
O
2
V
0
+2
: Conducting defect state
CBM
V
0
2
: Non-conducting defect
s
Metastable state
-Electron
-Hole
VBM
Figure 16.8
Energy band diagram of ZnO nanowire (a) under dark condition and (b) under
UV light illumination condition [25].
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