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Transparent conducting oxidesfor thin film PV

Rob Treharne, Laurie Phillips,Jon Major, Sepehr Vasheghani Farahani*,

Tim Veal, Ken DuroseUniversity of Liverpool, UKUniversity of Liverpool, UK*University of Warwick, UK

For more details, see: http://www.slideshare.net/RobertTreharne/the-physics-of-transparent-conducting-oxides

TCOs in solar cellsa - silicon CdTe

From Miles et al Materials Today 2007

CIGS

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Before TCOs...

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Transparency of metal oxide semiconductorsTransparency is determined:1) at the high energy, short wavelength end by theoptical gap which may be larger than thefundamental band gap due to conduction band filling;

and2) at the low energy,long wavelength end bythe free carrierabsorption or

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absorption orconduction electronplasma edge.

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Transparency of metal oxide semiconductorsBand tailing (Urbach tails) influence absorptionedge and optical gap determination in heavilydoped semiconductors

Jacques Pankove, Optical Properties of Semiconductors(Dover, 1975)

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Donor states exert attractive force on CBelectrons and repulsive force on VB holes.Impurities are inhomogeneously distributed soresultant CB and VB varies in space. Band tailsresult. These influence the optical properties,

Conductivity of metal oxide semiconductors

Type of material Conductor (metal) Semiconductor Insulator

Example material copper silicon silicon dioxide

Conductivity (cm)-1 108 10-4 10-18

TCOs have conductivities, , of up to 104 (cm)-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 impurityscattering. Also, increasing n increases p, the plasma frequency, impairing longwavelength transparency.

Resisitivity, , is 1/ and has units of cm. 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 giventhe units of / or Ohms per square.

Inherent n-type conductivity even inundoped metal oxide semiconductors isundoped metal oxide semiconductors istraditionally attributed to oxygenvacancies.

With the exception of CdO, this is now indoubt based on both experimental andtheoretical findings.

Oxygen vacancies are generally nowthought to be deep rather than shallowdonors.

Rob Treharne, PhD thesis, Durham (2011)

Why are TCOs inherently n-type?

P. D. C. King and T. D. Veal, JPCM (2011)

CdO as an ideal transparent conductor for solar cells

CdO is regularly referred to as thearchetypal or ideal transparent conductor.A. Wang et al., PNAS 98, 7113 (2001).

Research on CdO as a transparentconductor dates back to at least1907, when Cd was evaporated andthen oxidized in airK. Bdeker, Ann. Phys. (Leipzig) 22 (1907) 749.

Karl Baedeker III (1910)A. Wang et al., PNAS 98, 7113 (2001).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, itis potentially suitable for thin film solarcells (such as CdTe/CdS) and full spectrummultijunction PV

Conductivity >104 S/cmTransmission >85% from 400 to >1500 nm

Karl Baedeker III (1910)

Epitaxial growth and structure of CdO films

Carrier densities in different samples obtainedby annealing at 600C in vacuum for different times

CdO band structure

Indirect band gap due to pd-repulsionexcept at Gamma point where due tooctahedral symmetry it is forbidden.

Indirect gap about 1 eV

Fundamental direct band gap ~2.2 eV,but exact value subject of this work.

HSE06 DFTM. Burbano, D. Scanlon et al.,JACS 133 (2011) 15065.

Infrared reflectance from CdO thin films

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Infrared reflectance measurements p and Hall effect measurements n and Hall

Optical mobility of CdO thin films

S. K. Vasheghani Farahani, T. D. Veal et al.,J. Appl. Phys. 109, 073712 (2011)

Intra-grain mobility probed optically is dominated by ionized impurity scattering Modelled with degenerate form of Brooks-Herring formula

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)

Transport versus optical mobility of CdO thin films

XRD 002 FWHM 0.27-0.29 2-4109 cm-2 200 nm average grain sizeMayadas-Shatzkes model used to model grain boundary scattering

S. K. Vasheghani Farahani, T. D. Veal et al.,J. Appl. Phys. 109, 073712 (2011)

Influence of grain size on transport mobility of CdO thin films

S. K. Vasheghani Farahani, T. D. Veal et al.,J. Appl. Phys. 109, 073712 (2011)

Modelling of influence of increased grain size on transport mobility

Conductivity of CdO

CdO

With intentional doping by Ga andIn, compensation is reduced increasingmobility and giving conductivity up to 20,000S/cm (5x10-5 cm)

K. M. Yu, W. Walukiewicz et al., J. Appl. Phys. 111, 123505 (2012)

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 0.1-0.3m0M. 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 reportedK. 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 One widely cited value of 2.28 eV was recorded at 100 K, but is oftencompared with room temperature absorption data and optical gapsF. P. Koffyberg, Phys. Rev. B 13, 4470 (1976).

By accounting for conduction band filling effects, we previously founda room temperature band gap value of 2.16 eV using transmission spectroscopyand then revised this to 2.20 eV by including reflectance measurementsP. 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

Optical absorption data from CdO T and n dependence

S. K. Vasheghani Farahani, T. D. Veal et al., APL 102, 022102 (2013).

Contributions to observed optical gap in CdO

Fundamental band gap at 0 Kfor a hypothetical sample withzero carrier concentration

Fundamental band gap attemperature T with Varshniexpression for accounting forlattice expansion andelectron-phonon coupling

Optical gap for a sample withfinite n band gap at temp Tincreased by B-M shift due toCB filling and decreased dueto band gap renormalization*due to e-e and e-ionizedimpurity interactions*F. Berggren and B. E.Sernelius, Phys. Rev. B 24, 1971(1981)

Also note the upward valenceband dispersion at due to lack of p-d repulsion for symmetry ofrocksalt structureeg. M. Burbano, D. Scanlon et al.,JACS 133 (2011) 15065.

Hall effect measurements of CdO thin films

Free electron density is constant as a function of T, consistent with degenerate dopingMobility 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).

Infrared reflectance from CdO thin films

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Infrared reflectance measurements of the conduction band plasma edge along with the Halleffect measurements enable the effective mass dependence on T and n to be determined.

Conduction band non-parabolicity is thereby included in absorption edge modelling.

Optical gap versus T for sample with different n

S. K. Vasheghani Farahani, T. D. Veal et al., APL 102, 022102 (2013).

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).

Bose-Einstein modelling of band gap variation with T

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.

Transparent conductors for thin filmsolar cells

1. TCO effects in solar cells2. Combinatorial optimisation and physics of ZnO

Combinatorial method Optical dispersion Effective mass Effective mass Band