Nanostructured semiconductors for solar cells...Applications of mesoporous materials Metal powders...

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"