Nanostructured semiconductors for solar cells
Dr. I. Nandhakumar
University of Southampton, UK
Research rationale
• ‘High quality’ nanostructured semiconductors using electroplating methods (e.g. CdTe, ZnO) with novel optical and electrical properties
Mesoporous materials
0.1 1 10 100 1000 nm
Mesoporous materials (2-50 nm)
Surfactants
CH3(CH2)15(OCH2CH2)10OH Hydrophobic tail
Hydrophilic headgroup
Amphiphile self-assembly
T /°C
40
60
30 80 70 40 50 60 Amphiphile concentration /wt%
HI I1
La
V1
Lamellar Hexagonal
Micellar cubic
Micellar
Cubic Ia3d
Mz+
Mz+
electrodeposited!semiconductor film!
LC template
conducting substrate!
Electroplating
Surfactant removal
nanostructured!semiconductor film!
Mz+
Two-step templating
Applications of mesoporous materials
Metal powders
Metal films
Silicas and metal oxides
Polymers
Catalysis
Chemical sieves
Batteries Fuel cells Chemical capacitors
Sensors Semiconductors
solar cells optoelectronic devices
!
!
Phasediagram C16EO8/Cd/Te/water/H2SO4
Polarised Optical Microscopy
Liquid Crystal Template Mesoporous CdTe
Nanoporous CdTe
hexagonal arrangement d-spacing 6 nm
d
• TEM gives direct evidence of a hexagonal array with a d-spacing of 6 nm
end-on view: pore size ~ 3 nm pore-to-pore distance 7 nm
Low-angle XRD
0
50
100
150
200
250
300
350
1 1.5 2 2.5 3 3.5 4 4.5
2θ (degrees)
d100
Inte
nsity
(arb
. uni
ts)
d100
Interchannel spacing = d100 / cos30°
nλ = 2 dhkl sinΘ; λCuKα = 1.54 Å
!CdTe!• d100 = 58 ± 2Å !• 69 Å pore to pore !distance!
• Temp. 25°C
!• 17 domains showing 2 spots • largest domains 3 mm2
Measuring domain sizes by synchrotron SAXS
d100 = 60 Å Pore-to-pore = 70 Å
Optical Data
• Interference fringes => optically flat surfaces • Strong absorption above band gap
100
80
60
40
20
0
Ref
lect
ance
%
1600140012001000800600Wavelength nm
Experiment perpendicular Experiment parallel CdTe RT band gap
Chem. Comm., 12, 1374 (2004)"
ZnO
a
d b
c
a
f d
e
b
c a
a
b
c
d
h
g
f
e
j
i
SEM micrographs of three films electrodeposited at -0.845 V vs. SCE at 50 C The template mixture contained a 45 wt.% solution of 0.1 M zinc nitrate and 55 wt.% Brij®56.
Summary & Conclusions
• high quality semiconductors"• CdTe, PbTe, ZnO, Te….""
• 40 % of all the atoms are at or near a surface:!– Enhanced electronic + optical properties for e.g. solar cells, ultra-fast
photodiodes, non-linear optical elements!"
• Unique 3D nanostructuring"
– Exploration of quantum-size effects"• Bandgap compares welotodiode efficiency 14% to ?? i"
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