Transparent Conducting Oxides for Thin Film PV
- 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° 2-4109 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 0.5p = 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