Transparent Conducting Oxides for Thin Film PV

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

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Before TCOs...

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Transparency of metal oxide semiconductors

Transparency 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 semiconductors

Band 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 (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 impurityscattering. Also, increasing n increases p, the plasma frequency, impairing longwavelength 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 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. Bädeker, 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 600°C 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?


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


Influence of grain size on transport mobility of CdO thin films


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 eVand 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 Γ 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.


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


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.


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 gap– Optimum doping level results– Mobility

3. Window layer optimisation4. Conclusions

1 Optical transparency

ZnOsingle film

ZnO/CdS

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2 Combinatorial study of ZnO

•For TCO on glass, the market leader isSnO2, in-line coated on the float line

•ZnO is important for substrate cell designs

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2a Combinatorial methods-co-sputtering of ZnO and dopant

SiO2ZnO

Non-parabolicity and band gap re-normalisation in Si doped ZnORE Treharne, LJ Phillips, K Durose, A Weerakkody, IZ Mitrovic, S HallJournal of Applied Physics 115, 063505 (2014)

2a Combinatorial methods– property mapping instruments

Optical transmissionShimadzuSolid Spec 3600UV-Vis-IRspectrophotometer

Sheet resistanceCMT

10 x 10 cm2

CMTSR2000N automaticvan der Pauw

Band gapWoollamM200DIVariable angleellipsometer

17 x 17 = 289 data pointsfrom each sample

2b Optical transmission- dielectric modelling

1. Lorentz oscillatorModels response of bound electronsGives rise to dielectric backgroundGives rise to dielectric background

2. DrudeModels response of free electronsImportant parameter: Plasma Frequency

3. Inter-band transtionsAccounts for behaviour in vicinity of direct band gap

2bi) Optical transmission

ZnO:Sisingle film

ParametersParametersfrom this film

d = 518 ± 10 nm ε

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= 0.97 ± 0.02 rads.s-1

Eg = 3.38 eV

(ellipsometry doesEg better…)

2c Effective mass

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2c Effective mass

C = 0.3 ± 0.03 eV-1C = 0.3 ± 0.03 eVme0 = 0.34 ± 0.04 m0

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Non-parabolic band

2d Band gap vs carrier conc.

ZnO:Si - Band gap from spectroscopicellipsometry

2d Band gap vs carrier conc.Band gap renormalisationdue to many body effects

2

ZnO:Si - Band gap from spectroscopicellipsometry

exchangeenergy

correlationenergy

electron-ioninteractions

Lu et al., J. Appl. Phys. 101, 083705 (2007)Jain et al., J. Appl. Phys. 68, 3747 (1990)

2e Optimisation of doping density

mobility

carrier concentrationcarrierconc.

mobility

mobility

resistivity

2e Optimisation of doping density

mobility

carrier concentrationcarrierconc.

Clatot TSF 2013 PLD ZnO:Si

This worksputtered ZnO:Si

mobility

resistivity

mobility

resistivity

ne 4.4 x 1020 cm-3

e 16.5 cm2V-1s-1

8.6 x 10-4 .cm

2f Mobility

E

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Grain boundary limited transport(Seto, polycrystalline silicon)

EF

trap states Nt

Charge at grain boundary trap states

Depletion regions

2f Mobility in ZnO:Si

With additionalterm to includetunnellingin a highly doped(degenerate)semiconductor

Seto model

semiconductor

NB Trap density~ 1014 cm-2

Compositionally graded ZnO:Al

Combinatorial optimization of Al-doped ZnOfilms for thin-film photovoltaicsRE Treharne, K Hutchings, DA Lamb, SJCIrvine, D Lane, K DuroseJournal of Physics D: Applied Physics45, 335102 (2012)

Carrier density and mobility map

Combinatorial optimization of Al-doped ZnO films for thin-film photovoltaicsRE Treharne, K Hutchings, DA Lamb, SJC Irvine, D Lane, K DuroseJournal of Physics D: Applied Physics 45, 335102 (2012)

Carrier density and mobility graphs(81 points from one sample)

ρ = 7.6 ± 0.3 x 10-4 Ω.cmn = 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 ZnOfilms for thin-film photovoltaicsRE Treharne, K Hutchings, DA Lamb, SJC Irvine,D Lane, K DuroseJournal of Physics D: Applied Physics 45, 335102(2012)

ZnO Conclusions

Combinatorial study of ZnO:Si and ZnO:Al

Effective mass and band shapeBand gap effectsMobility physicsGrain boundary trap densityis ~ 1014 cm-2is ~ 1014 cm-2

Optical constantsOptimisation of doping

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Transparent Conducting Oxides for Thin Film PV