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Control of the Optical and Electronic Structure Properties of Multi-component Transparent Conducting Oxides from First-principles Calculations NREL is a national laboratory of the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy operated by Midwest Research Institute • Battelle Aron Walsh, Juarez L. F. Da Silva, Yanfa Yan and Su-Huai Wei

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Multicomponent TCOs • Cost: Reduce indium content; less intensive processing. • Stability: Chemical, thermal and surface. • Control: Carrier concentrations; mobility. National Renewable Energy Laboratory Innovation for Our Energy Future • Parent Binary Oxides (Groups 12-14): ZnO, In2 O3 , Al2 O3 , Ga2 O3 , SnO2 • Electronic Band Gaps: Al >> Ga > Sn > Zn > In • Resistivity: Al >> Ga > Sn > Zn > In Goal: Combine multiple cations to enhance pertinent TCO properties.

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Theoretical Approach Quantum-chemical simulations based on Density Functional Theory (DFT). Key Properties: Band structure, optical absorption. Defect and doping effects, structural stability. Programs: Reliable DFT codes (VASP, WIEN2K) along with in-house analysis software. Band Gap Description: Hybrid-DFT within HSE formulism. Screened Fock-Exchange (ω = 0.11 bohr-1). Structure Amorphization: DFT molecular dynamics. ‘Heat and quench’: property average over multiple structures. National Renewable Energy Laboratory Innovation for Our Energy Future

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Optical Peculiarity of In2 O3 11. (a) A. Walsh et al., Phys. Rev. Lett. 100, 167402 (2008); (b) A. Bourlange et al., Appl. Phys. Lett. 92, 092117 (2008). Forbidden gaps in rutile oxides: J. Robertson, J. Phys. C: Solid State Phys. 12, 4767 (1979). National Renewable Energy Laboratory Innovation for Our Energy Future • Inequivalence of fundamental electronic and optical band gaps.1a • Taking into account intrinsic carrier concentrations: Eg ~ 2.7 eV.1b • Provides a basis for understanding the superior performance of In2 O3 .

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• Layered R3/m structure established from XRD (Kasper, 1967). • Alternating InO2 octahedron and (ZnO)n tetrahedron layers. • Modulation of M-O units established from HR-TEM (Kimizuka,1994). • Modulation provides strain release, obeys octet rule and lowers energy.2 InMO3 (ZnO)n (M = In, Ga, Al) Structure 22. J. L. F. Da Silva, Y. Yan and S.-H. Wei, Phys. Rev. Lett. 100, 255501 (2008). National Renewable Energy Laboratory Innovation for Our Energy Future

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In2 O3 (ZnO)Electronic Structure 33. A. Walsh, J. L. F. Da Silva, Y. Yan and S.-H. Wei, In Review (2008). National Renewable Energy Laboratory Innovation for Our Energy Future • Density of states a direct combination of parent oxides (ZnO and In2 O3 ). • Conduction band maintains delocalized cation s character.

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In2 O3 (ZnO)n Electronic Structure National Renewable Energy Laboratory Innovation for Our Energy Future • Band edge localization increases with increasing n. • Matches empirical trend of conductivity decrease.

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In2 O3 (ZnO)n Optical Absorption National Renewable Energy Laboratory Innovation for Our Energy Future • Band edge transitions dipole allowed (Zn-O layers). • In-O layers still result in weak optical absorption below the IZO VBM. • Explains the origin of the IZO optical gap redshift relative to In2 O3 and ZnO.

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Amorphous In2 O3 (ZnO)n National Renewable Energy Laboratory Innovation for Our Energy Future • Compositions around n = 1 correspond to maximum conductivity. • No discontinuity in electronic properties.

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Amorphous In2 O3 (ZnO) National Renewable Energy Laboratory Innovation for Our Energy Future • Local anion-cation coordination environments maintained. • Long range structural disorder. • Bond lengths marginally contracted from crystalline phase.

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Amorphous In2 O3 (ZnO) National Renewable Energy Laboratory Innovation for Our Energy Future • Valence crystal orbitals highly localized. • Good hole transport extremely unlikely in amorphous oxides.

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National Renewable Energy Laboratory Innovation for Our Energy Future Q. Can the delocalized cation s orbitals overcome the absence of symmetry? Key Question

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Amorphous In2 O3 (ZnO) National Renewable Energy Laboratory Innovation for Our Energy Future • Overlap of cation s conduction band provides dispersion despite lack of crystal symmetry.

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In2 O3 • Optical and electronic band gaps inequivalent (forbidden transitions). • Low conduction band ensures excellent n-type conduction properties. In-Zn-O • General bonding features of binary components maintained. • Band edge localization increases with increasing n. • Band edge optical transitions allowed (from Zn-O layers). • Conduction band character maintained after amorphization. Al-In-Zn-O / Ga-In-Zn-O • Ongoing exploration of crystalline and amorphous phases. Conclusions / Current Status CCollaborator Presentations: David Payne (In2 O3 ): Talk Monday 11:30AM B1.7 Graeme Watson (SrCu2 O2 ): Talk Monday 4:30PM B2.8 National Renewable Energy Laboratory Innovation for Our Energy Future David Scanlon (CuAl1-x Crx O2 ): Poster Tuesday 8:00PM B6.3 Russell Egdell (In2 O3 :Cr/Fe): Talk Thursday 4:30PM PP11.9