Talk delivered by Dr. T. Veal, University of Liverpool on Nov 12 2014 as part of the CDT-PV TCO Masterclass

1. Transparent conducting oxides
for thin film PV
Rob Treharne, Laurie Phillips,
Jon Major, Sepehr Vasheghani Farahani*,
Tim Veal, Ken Durose
University of Liverpool, UK
*University of Warwick, UK
For more details, see: http://www.slideshare.net/RobertTreharne/the-physics-of-
transparent-conducting-oxides

2. TCOs in solar cells
a - silicon CdTe
CIGS
From Miles et al Materials Today 2007
Before TCOs...
pvpowerway.com

3. , PhD thesis, Durham (2011)
Transparency of metal oxide semiconductors
Transparency is determined:
1) at the high energy, short wavelength end by the
optical gap which may be larger than the
fundamental band gap due to conduction band filling;
and
2) at the low energy,
long wavelength end by
the free carrier
absorption or
Rob Treharne, conduction electron
plasma edge.

4. Transparency of metal oxide semiconductors
Band tailing (Urbach tails) influence absorption
edge and optical gap determination in heavily
doped semiconductors
, PhD thesis, Durham (
2011)
Jacques Pankove, Optical Properties of Semiconductors
(Dover, 1975)
Rob Treharne, Donor states exert attractive force on CB
electrons and repulsive force on VB holes.
Impurities are inhomogeneously distributed so
resultant CB and VB varies in space. Band tails
result. These influence the optical properties,

5. Conductivity of metal oxide semiconductors
Type of material Conductor (metal) Semiconductor Insulator
Example material copper silicon silicon dioxide
Conductivity (Ωcm)-1 108 10-4 10-18
TCOs have conductivities, , of up to 104 (Ωcm)-1 or S/cm
Conductivty,  = ne
where n is the free electron density (cm-3)
e is the electronic charge (1.6  10-19 C)
 is the electron mobility (cm2V-1s-1)
To maximise , we need to maximise n and , but as n,  due to ionized impurity
scattering. Also, increasing n increases p, the plasma frequency, impairing long
wavelength transparency.
Resisitivity, , is 1/ and has units of Ωcm. Sheet resistivity is 1/t where t is film thickness.
So sheet resistivity is /t which gives units of . To distinguish from resistance it is given
the units of / or Ohms per square.

6. Inherent n-type conductivity even in
undoped metal oxide semiconductors is
traditionally attributed to oxygen
vacancies.
With the exception of CdO, this is now in
doubt based on both experimental and
theoretical findings.
Oxygen vacancies are generally now
thought to be deep rather than shallow
donors.
Rob Treharne, PhD thesis, Durham (2011)

7. Why are TCOs inherently n-type?
P. D. C. King and T. D. Veal, JPCM (2011)

8. CdO as an ideal transparent conductor for solar cells
Research on CdO as a transparent
conductor dates back to at least
1907, when Cd was evaporated and
then oxidized in air
K. Bädeker, Ann. Phys. (Leipzig) 22 (1907) 749.
CdO is regularly referred to as the
archetypal or ideal transparent conductor.
A.Wang et al., PNAS 98, 7113 (2001).
Karl Baedeker III (1910)
Y. Yang et al., J. Am. Chem. Soc. 127, 8796 (2005).
K. M. Yu et al., J. Appl. Phys. 111, 123505 (2012)

When doped to increase the optical gap, it
is potentially suitable for thin film solar
cells (such as CdTe/CdS) and full spectrum
multijunction PV
Conductivity >104 S/cm
Transmission >85% from 400 to >1500 nm

9. Epitaxial growth and structure of CdO films
Carrier densities in different samples obtained
by annealing at 600°C in vacuum for different times

10. CdO band structure
Indirect band gap due to pd-repulsion
except at Gamma point where due to
octahedral symmetry it is forbidden.
Indirect gap about 1 eV
Fundamental direct band gap ~2.2 eV,
but exact value subject of this work.
HSE06 DFT
M. Burbano, D. Scanlon et al.,
JACS 133 (2011) 15065.

11. Infrared reflectance from CdO thin films
2
  

0 ( ) *
2
m
ne

p  
Infrared reflectance measurements  p and 
Hall effect measurements  n and Hall

12. “Optical” mobility of CdO thin films
• Intra-grain mobility probed optically is dominated by ionized impurity scattering
• Modelled with degenerate form of Brooks-Herring formula
S. K. Vasheghani Farahani, T. D. Veal et al.,
J. Appl. Phys. 109, 073712 (2011)

13. Transport versus optical mobility of CdO thin films
Mobility from Hall effect is significantly lower than from reflectance measurements
Grain boundary/dislocation scattering?
S. K. Vasheghani Farahani, T. D. Veal et al.,
J. Appl. Phys. 109, 073712 (2011)

14. Transport versus optical mobility of CdO thin films
XRD 002 FWHM 0.27-0.29°  2-4109 cm-2  200 nm average grain size
Mayadas-Shatzkes model used to model grain boundary scattering
S. K. Vasheghani Farahani, T. D. Veal et al.,
J. Appl. Phys. 109, 073712 (2011)

15. Influence of grain size on transport mobility of CdO thin films
Modelling of influence of increased grain size on transport mobility
S. K. Vasheghani Farahani, T. D. Veal et al.,
J. Appl. Phys. 109, 073712 (2011)

16. Conductivity of CdO
CdO
With intentional doping by Ga and
In, compensation is reduced increasing
mobility and giving conductivity up to 20,000
S/cm (5x10-5 cm)
K. M. Yu, W. Walukiewicz et al., J. Appl. Phys. 
111, 123505 (2012)

17. Previous results for the band gap of CdO
• Early measurements found a room temperature band gap of 2.3 eV
and conduction band edge effective masses in the range 0.1-0.3m0
M. Altwein, H. Finkenrath, C. Konak, J. Stuke, and G. Zimmerer, Phys. Stat. Sol. 29, 203 (1968).
R. W. Wright and J. A. Bastin, Proc. Phys. Soc. 71, 109 (1958).
K. Maschke and U. Rossler, Phys. Stat. Sol. 28, 577 (1968).
• Recent room temperature values of 2.3 eV and 2.4 eV reported
K. M. Yu et al., J. Appl. Phys. 111, 123505 (2012)
I. N. Demchenko, K. M. Yu, D. T. Speaks, W. Walukiewicz et al., Phys. Rev. B 82, 075107 (2010).
• One widely cited value of 2.28 eV was recorded at 100 K, but is often
compared with room temperature absorption data and optical gaps
F. P. Koffyberg, Phys. Rev. B 13, 4470 (1976).
• By accounting for conduction band filling effects, we previously found
a room temperature band gap value of 2.16 eV using transmission spectroscopy
and then revised this to 2.20 eV by including reflectance measurements
P. H. Jefferson, T. D. Veal et al., Appl. Phys. Lett. 92, 022201 (2008)
S. K. Vasheghani Farahani, T. D. Veal et al., J. Appl. Phys. 109, 073712 (2011)
But no report of 0 K gap or the T-dependence of the band gap

18. Optical absorption data from CdO – T and n dependence
S. K. Vasheghani Farahani, T. D. Veal et al., APL 102, 022102 (2013).

19. Contributions to observed optical gap in CdO
Fundamental band gap at 0 K
for a hypothetical sample with
zero carrier concentration
Fundamental band gap at
temperature T with Varshni
expression for accounting for
lattice expansion and
electron-phonon coupling
Optical gap for a sample with
finite n – band gap at temp T
increased by B-M shift due to
CB filling and decreased due
to band gap renormalization*
due to e-e and e-ionized
impurity interactions
*F. Berggren and B. E.
Sernelius, Phys. Rev. B 24, 1971
(1981)
Also note the upward valence
band dispersion at Γ due to lack
of p-d repulsion for symmetry of
rocksalt structure
eg. M. Burbano, D. Scanlon et al.,
JACS 133 (2011) 15065.

20. Hall effect measurements of CdO thin films
Free electron density is constant as a function of T, consistent with degenerate doping
Mobility peaks at about 150 K due to T-1 dependence of dislocation scattering and T3/2
dependence of phonon scattering.
S. K. Vasheghani Farahani, T. D. Veal et al., APL 102, 022102 (2013).

21. Infrared reflectance from CdO thin films
2
  

0 ( ) *
2
m
ne

p  
Infrared reflectance measurements of the conduction band plasma edge along with the Hall
effect measurements enable the effective mass dependence on T and n to be determined.
Conduction band non-parabolicity is thereby included in absorption edge modelling.

22. Optical gap versus T for sample with different n
S. K. Vasheghani Farahani, T. D. Veal et al., APL 102, 022102 (2013).

23. CdO fundamental band gap variation with T
• Room temp. band gap has previously generally been overestimated due to band filling effects.
• Parameters now established can be used to model T and n effects for use of CdO in devices.
S. K. Vasheghani Farahani, T. D. Veal et al., APL 102, 022102 (2013).

24. Bose-Einstein modelling of band gap variation with T

25. CdO Conclusions
• Fundamental band gap of CdO is 2.18 eV at 300K (2.31 eV at 0K)
• Optical gap can be increased to 3.2 eV by doping – Burstein-Moss shift
• Grain size found to be limiting factor for Hall mobility of MOVPE films
• Conductivity up to 3000 S/cm undoped, 20,000 S/cm with In doping
• Resistivity down to 5 x 10-5 cm with In doping
• So why is it not used in CdS/CdTe devices?
CdO is hygroscopic making it difficult to handle.
ZnO and SnO2 are not.
Cd2SnO4 (cadmium stannate) has been used to some degree.

26. Transparent conductors for thin film
solar cells
1. TCO effects in solar cells
2. Combinatorial optimisation and physics of ZnO
– Combinatorial method
– Optical dispersion
–– Effective mass
– Band gap
– Optimum doping level results
– Mobility
3. Window layer optimisation
4. Conclusions

27. 1 Optical transparency
ZnO
single film
ZnO/CdS
AM 1.5
AM 1.5
ZnO gap
CdS gap

28. 2 Combinatorial study of ZnO
•For TCO on glass, the market leader is
SnO2, in-line coated on the float line
•ZnO is important for substrate cell designs
AGC flat glass

29. 2a Combinatorial methods
-co-sputtering of ZnO and dopant
thickness (nm)
ZnO SiO2
Non-parabolicity and band gap re-normalisation in Si doped ZnO
RE Treharne, LJ Phillips, K Durose, A Weerakkody, IZ Mitrovic, S Hall
Journal of Applied Physics 115, 063505 (2014)

30. 2a Combinatorial methods
– property mapping instruments
Optical transmission
Shimadzu
Solid Spec 3600
UV-Vis-IR
spectrophotometer
Sheet resistance
CMT
10 x 10 cm2
SR2000N automatic
van der Pauw
Band gap
Woollam
M200DI
Variable angle
ellipsometer
17 x 17 = 289 data points
from each sample

31. 2b Optical transmission
- dielectric modelling
1. Lorentz oscillator
Models response of bound electrons
Gives rise to dielectric background
2. Drude
Models response of free electrons
Important parameter: Plasma Frequency
3. Inter-band transtions
Accounts for behaviour in vicinity of direct band gap

32. 2bi) Optical transmission
ZnO:Si
single film
Parameters
from this film
 d = 518 ± 10 nm
 ε
0.5p
= 0.97 ± 0.02 rads.s-1
 Eg = 3.38 eV
(ellipsometry does
Eg better…)

33. 2c Effective mass
Plasma frequency
(from dielectric model)
e
n e
e
p m
0
2
 



 Linear plot
p e  2 vs n 

34. 2c Effective mass
C = 0.3 ± 0.03 eV-1
me0 = 0.34 ± 0.04 m0
Non-parabolic band
2
2 2
8
k  

h E CE
2
m

35. 2d Band gap vs carrier conc.
ZnO:Si - Band gap from spectroscopic
ellipsometry

36. 2d Band gap vs carrier conc.
Band gap renormalisation
due to many body effects
2
ZnO:Si - Band gap from spectroscopic
ellipsometry
exchange
energy
correlation
energy
electron-ion
interactions
Lu et al., J. Appl. Phys. 101, 083705 (2007)
Jain et al., J. Appl. Phys. 68, 3747 (1990)

37. 2e Optimisation of doping density
carrier carrier concentration
conc.
mobility
mobility
resistivity

38. 2e Optimisation of doping density
carrier concentration
mobility
Clatot TSF 2013 PLD ZnO:Si
carrier
conc.
This work 
sputtered ZnO:Si
resistivity
mobility
resistivity
ne 4.4 x 1020 cm-3
e 16.5 cm2V-1s-1
 8.6 x 10-4 .cm

39. 2f Mobility
E
Grain boundary limited transport
(Seto, polycrystalline silicon)
grain
boundaries
ΦB
F
trap states Nt
Depletion regions
Charge at grain boundary trap states

40. 2f Mobility in ZnO:Si
With additional
term to include
tunnelling
in a highly doped
(degenerate)
semiconductor
Seto model
NB Trap density
~ 1014 cm-2

41. Compositionally graded ZnO:Al
Combinatorial optimization of Al-doped ZnO
films for thin-film photovoltaics
RE Treharne, K Hutchings, DA Lamb, SJC
Irvine, D Lane, K Durose
Journal of Physics D: Applied Physics
45, 335102 (2012)

42. Carrier density and mobility map
Combinatorial optimization of Al-doped ZnO films for thin-film photovoltaics
RE Treharne, K Hutchings, DA Lamb, SJC Irvine, D Lane, K Durose
Journal of Physics D: Applied Physics 45, 335102 (2012)

43. Carrier density and mobility graphs
(81 points from one sample)
ρ = 7.6 ± 0.3 x 10-4 Ω.cm
n = 3.4 ± 0.1 x 1020 cm-3
μ = 24.5 ± 0.5 cm2V-1s-1
R = 14.4 ± 0.2 Ω/□
d = 528 nm
Combinatorial optimization of Al-doped ZnO
films for thin-film photovoltaics
RE Treharne, K Hutchings, DA Lamb, SJC Irvine,
D Lane, K Durose
Journal of Physics D: Applied Physics 45, 335102
(2012)

44. ZnO Conclusions
Combinatorial study of ZnO:Si and ZnO:Al
Effective mass and band shape
Band gap effects
Mobility physics
Grain boundary trap density
is ~ 1014 cm-2
Optical constants
Optimisation of doping