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

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.

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

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 .

• 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

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.

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.

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.

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.

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.

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.

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

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.

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