Tuning the Transparency of Cu2O with Substitutional Cation Doping

Aug 13, 2008 - can be used to tune the band gap. Unlike many oxides, in which the band gap is reduced by the appearance of dopant induced states in th...
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Chem. Mater. 2008, 20, 5522–5531

Tuning the Transparency of Cu2O with Substitutional Cation Doping Michael Nolan* and Simon D. Elliott Tyndall National Institute, Lee Maltings, Prospect Row, Cork, Ireland ReceiVed NoVember 29, 2007. ReVised Manuscript ReceiVed June 30, 2008

Materials derived from copper oxide, Cu2O, are potential p-type transparent conducting oxides. Cu2O displays some unusual features, including p-type semiconductivity and the importance of cation-cation interactions in determining the band gap and transparency. We apply first-principles density functional theory to investigate how dopants with a range of ionic radii, oxidation states, and electronic structure can be used to tune the band gap. Unlike many oxides, in which the band gap is reduced by the appearance of dopant induced states in the host band gap, the band gap of Cu2O can be both increased or decreased by a suitable choice of dopant. Two effects dominate: (i) dopant-induced changes to the Cu-Cu interactions through structural distortions around the dopant site and (ii) the alignment of the dopant electronic states with the valence band or conduction band of Cu2O. Dopants with ionic radii larger than Cu+ (Ba2+, Sn2+, Cd2+, In3+, La3+, and Ce4+) produce strong structural distortions around the dopant site. Dopants with ionic radii smaller than Cu+, such as Al3+, Ga3+, Ti4+, and Cr4+, show no structural distortions. Structural distortions disrupt Cu-Cu interactions and for Sn2+ and La3+, this opens up the band gap, potentially improving the transparency. However, if dopant electronic states interact with the valence or conduction band of Cu2O, e.g., In3+ or Cd2+, or produce defect states in the Cu2O band gap, e.g., Ce4+, the band gap is reduced, regardless of dopant-induced disruption of the Cu-Cu interactions. We present a set of materials design guidelines to be used for choosing potential dopants in Cu2O.

1. Introduction Substitutional cation doping of metal oxides is used to modify optical and electrical properties and modern deposition techniques enable unprecedented control. To understand how dopant concentration and distribution impact on the properties of an oxide, first-principles calculations, usually using density functional theory (DFT), are invaluable. There is great interest in applying Cu2O as the starting material for novel p-type transparent conducting oxides (TCO).1-4 TCOs are unusual materials, displaying simultaneous transparency to visible light and semiconducting behavior.2-4 These properties open up interesting technological possibilities, such as transparent electronics and displays. However, Cu2O suffers from a relatively small band gap of 2.17 eV, limiting its transparency. An aim of our work is to investigate if the transparency of Cu2O can be improved by cation doping. Even among TCO materials, Cu2O is unusual. The crystal structure5 consists of two interpenetrating three-dimensional Cu2O networks, which have been examined in refs 6 and 7

so that Cu2O is a network oxide, in contrast to the close packed nature of many oxides. The two networks are held together by metallic Cu-Cu internetwork interactions, which stabilize the crystal structure.6,7 In addition to the internetwork Cu-Cu interactions, Cu-Cu interactions along -O-Cu-O- ribbons (which we term intranetwork) are also important, but the latter interactions have generally been neglected in discussing the properties of Cu2O. These interactions are not simply closed-shell or d10-d10 interactions,8 but rather must be more generally referred to as Cu-Cu interactions, because of the role played by the participation of Cu 4s and 4p states in the stability of Cu2O.7-13 From previous work, disrupting the internetwork interactions (e.g., by lowering the dimensionality of the network) has been implicated in the enhancement of the band gap in alloyed Cu2O materials, such as CuAlO2 and SrCu2O2,1,7,9,10 and is expected to be a general feature of materials derived from Cu2O. As a consequence of this apparent band gap dependence on Cu-Cu interactions, it is possible that cation doping14,15 of Cu2O could induce structural distortions, disrupting Cu-Cu interactions around the dopant, potentially

* Corresponding author. E-mail: [email protected].

(1) Kawazoe, H.; Yasukawa, M.; Hyodo, M.; Kurita, M.; Yanagi, H.; Hosono, H. Nature 1997, 389, 939. (2) Banerjee, A. N.; Chattopadhyay, K. K. Prog. Cryst. Growth Charact. 2005, 50, 52. (3) Ingram, B. J.; Gonzalez, G. B.; Kammler, D. R.; Bertoni, M. I.; Mason, T. O. J. Electroceram. 2004, 13, 167. (4) Sheng, S.; Fang, G.; Li, C.; Xu, S.; Zhao, X. Phys. Status Solidi A 2006, 203, 1891. (5) Werner, A.; Hocheimer, H. D. Phys. ReV. B 1982, 25, 5929. (6) Angels Carvajal, M.; Alvarez, S.; Novoa, J. J. Chem.sEur. J. 2004, 10, 2117.

(7) Filippetti, A.; Fiorentini, V. Phys. ReV. B 2005, 72, 035128. (8) Pyykko¨, P. Chem. ReV. 1997, 97, 597. (9) Kudo, A.; Yanagi, H.; Hosono, H.; Kawazoe, H. Appl. Phys. Lett. 1998, 73, 222. (10) Buljan, A.; Lunell, M.; Ruiz, E.; Alemany, P. Chem. Mater. 2001, 13, 338. (11) Nolan, M.; Elliott, S. D. Phys. Chem. Chem. Phys. 2006, 8, 5350. (12) Laskowski, R.; Blaha, P.; Schwarz, K. Phys. ReV. B 2003, 67, 075102. (13) Ruiz, E.; Alvarez, S.; Alemany, P.; Evarestov, R. A. Phys. ReV. B 1997, 56, 7189. (14) Kikuchi, N.; Tonooka, K. Thin Solid Films 2005, 486, 33.

10.1021/cm703395k CCC: $40.75  2008 American Chemical Society Published on Web 08/13/2008

Tuning Cu2O Transparency with Cation Doping

increasing the band gap over the undoped oxide. This would be in contrast to many oxides, where doping usually decreases the band gap.16 The potential importance of both types of Cu-Cu interactions is seen in alloys of Cu2O. The delafossite-structured CuAlO2 alloy has only internetwork Cu-Cu interactions and it is the density of Cu-Cu interactions that is affected.1,10 In SrCu2O2, the structure has the Cu-Cu interactions along one-dimensional ribbons (reduced dimensionality of intranetwork Cu-Cu interactions), compared to ribbons directed in all three dimensions in Cu2O.9 However, much of the work in this area has been based on computations using extended Hu¨ckel theory and simple molecular orbital analysis10 or making assumptions about the correlation between the structure of the alloy and the observed band gap increase.1,9 Cu+ in Cu2O is thus behaving as a typical soft Lewis acid,17 with metallic Cu-Cu interactions producing highlying valence band (VB) states and low-lying conduction band (CB) states, and resulting in a higher conductivity and lower transparency than the oxides of harder Lewis acids (e.g., Na2O). We therefore suggest that one way to achieve higher transparency will be through doping Cu2O with cations that are harder Lewis acids. In this paper, we use first principles density functional theory (DFT) to investigate doping with Ca2+, Sn2+, La3+, and Ti4+, among others, which are typical hard Lewis acids, i.e. electronegative cations that form oxides with wide band gaps. We find this strategy to be a useful starting point. However, as our analysis of a range of dopants shows, the hard/soft concept is too simple for quantitative properties such as transparency. It turns out that the impact of a particular dopant depends on the dopant ionic radius (which has also been studied for CdO18), oxidation state, and crucially, the detailed electronic structure of the dopant and Cu2O host. Considering the interplay between these factors, we propose some general design guidelines. This study enhances our understanding of the electronic structure and transparency of Cu2O derived TCO materials. 2. Methods We use the Vienna ab initio Simulation Package (VASP),19 in which the valence electronic states are described using periodic plane waves. To describe the core-valence interaction, we use the projector augmented wave (PAW) approach,20 with a [He] core on O; a [Ne] core on Mg and Al; an [Ar] core on Ca, Ga, Ti, Cr, V, Zn, and Cu; a [Kr] core on Sr, In, Cd, and Sn; and a [Xe] core on Ba, La, Hg, and Ce. The cutoff energy for the plane wave expansion is 396 eV. The generalized gradient approximation to the exchange-correlation functional of Perdew-Burke-Ernzerhof (15) Nolan, M.; Elliott, S. D. Thin Solid Films 2008, 516, 1468. (16) Stoneham, A. M. Theory of Defects in Solids; Oxford University Press: Oxford, U.K., 1972. (17) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533. (18) Yang, Y.; Jin, S.; Medvedeva, J. E.; Ireland, J. R.; Metz, A. W.; Ni, J.; Hersam, M. C.; Freeman, A. J.; Marks, T. J. Am. Chem. Soc. 2005, 127, 8796. (19) (a) Kresse, G.; Furthmu¨ller, J. Comput. Mater. Sci. 1996, 6, 15. (b) Kresse, G.; Furthmu¨ller, J. Phys. ReV. B 1996, 54, 11169. (20) (a) Blo¨chl, P. E. Phys. ReV. B 1994, 50, 17953. (b) Joubert, D.; Kresse, G. Phys. ReV. B 1999, 59, 1758.

Chem. Mater., Vol. 20, No. 17, 2008 5523 (PBE)21 is used. The Monkhorst-Pack k-point sampling scheme is applied, with 20 irreducible k-points for the unit cell of Cu2O and 4 k-points for the (2 × 2 × 2) cell expansion. A single cation dopant in the (2 × 2 × 2) cell expansion gives a 3% concentration, with formula Cu31O16M1 (M ) dopant). Our earlier work on Cu vacancies in Cu2O11 shows that the (2 × 2 × 2) cell expansion and larger (3 × 3 × 3) cell expansions give similar results. However, to be sure of the suitability of the smaller supercell, we repeat the calculations with Cu vacancies and with Sn-doping using the larger supercell. We relax the ionic positions and the lattice constant of bulk cubic Cu2O (space group No. 224)5 using the Murnaghan equation of state,22 in which energy-volume pairs are generated through relaxation at a series of lattice volumes and the Murnaghan equation of state is used to fit to the energy-volume curve. This approach surmounts the Pulay error that arises from a possible change in the lattice dimensions with a fixed plane wave basis. The accepted mechanism for p-type conductivity in Cu2O is formation of Cu vacancies, which are predicted to have very small formation energies.11,23 Doping can strongly affect the formation of defects such as vacancies and hence affect the electrical characteristics of the system, as well as impacting on the transparency. In the doped systems, the formation energy of a neutral Cu vacancy is computed as

ECuvac)[E(Cu30O16M1) + Eref(Cu)] - E(Cu31O16M1) (1) where M is the cation dopant. The reference energy for Cu is the computed energy of metallic Cu, which is at the limit of reduced Cu and provides an upper bound on the vacancy formation energy.11 A negative energy ECuvac signifies that the formation of Cu vacancies is favored. For all calculations involving dopants and Cu vacancies, the system is spin-polarized with spin relaxation allowed. The ionic positions are relaxed with no constraints until the resulting forces on the atoms are less than 0.02 eV/Å, with fixed cell parameters. Experimental ionic radii24 are used to rationalize our results and are discussed in section 4.

3. Results 3.1. Bulk Cu2O. For bulk Cu2O, the computed lattice constant is 4.2892 Å, compared to 4.2696 Å5 from the experiment. There is a direct band gap at Γ, which we compute to be 0.47 eV; despite the underestimation of the band gap with DFT (2.17 eV from experiment 25), trends in band gap are properly described and in this paper, it is the question of increasing or decreasing the band gap by doping that is of interest, rather than the absolute magnitude of the band gap. In the cubic Cu2O structure, shown in Figure 1, there are two interpenetrating Cu2O networks, which are stabilized by Cu-Cu interactions, with the computed stabilization energy being 0.46 eV per Cu2O, consistent with previous studies.7 Nearest neighbor Cu-Cu distances are 3.04 Å. To demon(21) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (22) Murnaghan, F. D. Proc. Natl. Acad. Sci. U.S.A. 1994, 30, 244. (23) Raebiger, H.; Lany, S.; Zunger, A. Phys. ReV. B 2007, 76, 045209. (24) Shannon, R. D.; Prewitt, C. T. Acta Crystallogr., Sect. B 1969, 25, 925. (b) Acta Crys. 1970, B26, 1046. (25) Ghijsen, J.; Tjeng, L. H.; van Elp, J.; Eskes, H.; Westerink, J.; Sawatzky, G. A.; Czyzyk, M. T. Phys. ReV. B 1988, 38, 11322.

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Figure 1. (a) Atomic arrangement of Cu2O in a (2 × 2 × 2) expansion of the unit cell. The gray spheres are copper and the red spheres are oxygen. (b) Cu2O cell rotated to show the two interpenetrating networks, each indicated with different coloring; one network is black and the other is yellow. (c) Cu 3d and 4s electronic density of states, where the zero of energy is the Fermi level. (d) Detail of the Cu 4s PDOS.

strate the effect of Cu-Cu interactions on the band gap of Cu2O, we remove one Cu2O network, thus suppressing internetwork Cu-Cu interactions. As a result, the computed DFT band gap increases from 0.47 to 1.00 eV. Similarly, disrupting Cu-Cu interactions along a ribbon also opens up the band gap (with the increase depending on the change in the ribbon angle). Throughout the remainder of the present paper, we will see how doping impacts on these Cu-Cu interactions. The Cu 3d and 4s partial electronic density of states (PDOS) is also shown in figure 1. There is a small gap between the top of the valence band, which is derived from Cu 3d states and the bottom of the conduction band, which is derived from Cu 4s states, and it is this energy gap that we seek to modify with cation doping. We have previously investigated the formation of single Cu vacancies11 and two Cu vacancies26 in a (2 × 2 × 2) cell. The latter case is important for this work, where we have multiple Cu vacancies. The results in ref 26 using the (2 × 2 × 2) supercell show that when two Cu vacancies form, their preference is generally to be isolated. To make a further check on this point, we have studied a number of distributions for two Cu vacancies in a (3 × 3 × 3) supercell, which allows for a greater number of distributions of the vacancies compared to the smaller supercell. In this supercell, vacancy-vacancy distances can range from 3.04 Å (vacancies separated by nearest neighbor Cu-Cu distance), which we term clustered, to 8.04 Å, which we term isolated. We find for the larger supercell that isolated Cu vacancies are more (26) Nolan, M. Thin Solid Films 2008, doi:10.1016/j.tsf.2008.04.020. (27) Kroger, F. The Chemistry of Imperfect Crystals; North-Holland: Amsterdam, 1974; p 14.

stable than clustered vacancies, giving confidence in using the distributions found for the (2 × 2 × 2) supercell in this work. 3.2. Compensation of Donor Dopants. In this work, Cu2O is doped with aliovalent dopants, i.e. dopants with a different oxidation state to Cu+. Isovalent doping with Ag+/ Au+ has been discussed in ref.15 In Cu2O, aliovalent dopant compensation is achieved by forming Cu vacancies. For a dopant of oxidation state n+, we require (n - 1) Cu vacancies to compensate. The subsequent, nth, Cu vacancy dopes the system p-type.11 For M2+ doping, we indicate this schematically as follows, using Kroger-Vink notation27 and showing only cations CuCux–CuCux-CuCux-CuCux f MCu/–CuCux-CuCux–CuCux (2) MCu/–CuCux-CuCux–CuCux f MCux-VCux-CuCux–CuCux (3) MCux-VCux-CuCux–CuCux f MCux-VCux-VCux–CuCu· (4) In this notation, an ion is represented by its elemental symbol, a subscript indicates that lattice site at which it is found, e.g. CuCu indicates Cu at a Cu lattice site in Cu2O and the superscript indicates the charge state. An “x” is neutral, a “/” indicates a negative charge and a “ · ” indicates a positive charge relative to the host lattice. A “V” indicates a vacancy site. Thus in eq 2, M2+ substitutes for Cu, in eq 3, a neutral (28) Guerts, J.; Rau, S.; Richter, W.; Schmitte, F. J. Thin Solid Films 1984, 121, 217.

Tuning Cu2O Transparency with Cation Doping

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Table 1. Ionic Radius and Cu Vacancy Formation Energies (Evac) and Band Gaps (Eg) for Cation-Doped Cu2O

dopant

ionic radius (Å)

experimental Eg of pure oxide (eV)

En-1vac (eV)

Envac e(V)

computed Eg of doped Cu2O (eV)

Cu2O Sn2+ Zn2+ Mg2+ Ca2+ Sr2+ Ba2+ Cd2+ Hg2+ Al3+ Ga3+ In3+ La3+ Ti4+ Cr4+ V4+ Ce4+

0.93 0.93 0.40 0.49 1.00 1.16 1.36 0.84 0.96 0.39 0.47 0.79 1.06 0.61 0.44 0.36 0.80

2.1725 3.0528 3.3029 7.8030 7.0331 5.9032 4.1033 2.3534 2.2035 8.8036 4.9537 3.5738 5.40-5.8039 3.0540 3.2541 0.7042 2.4043

0.36 -0.95 -0.78 -0.46 -0.62 -0.65 -0.66 -0.28 0.06 -0.30 -0.08 0.16 -0.76 0.12 0.22 -0.02 -1.26

0.24 0.15 0.22 0.26 0.23 0.14 -0.12 0.18 0.13 0.36 0.14 0.29 -0.57 0.42 0.34 0.15 0.51

0.52 (1 Cu vacancy) 0.63 (2 Cu vacancy) 0.49 (2 Cu vacancy) 0.56 (2 Cu vacancy) 0.63 (2 Cu vacancy) 0.65 (2 Cu vacancy) 0.62 (2 Cu vacancy) 0.45 (2 Cu vacancy) 0.30 (2 Cu vacancy) 0.36 (3 Cu vacancy) 0.11 (3 Cu vacancy) 0.13 (3 Cu vacancy) 0.67 (3 Cu vacancy) 0.27 (4 Cu vacancy) 0.04 (4 Cu vacancy) 0.04 (4 Cu vacancy) 0.05 (4 Cu vacancy)

In the column headed En-1vac, we present the Cu vacancy formation energy for the first compensating Cu vacancy for dopant with oxidation state Mn+; in the column headed Envac, we present the Cu vacancy formation energy for the p-type Cu vacancy for dopant with oxidation state Mn+; the band gaps quoted are for this case, with the number of Cu vacancies indicated. Data for Cu2O are given for comparison. The superscripts in the oxide band gap column are the references from which these data are obtained, which for Sn2+-doping refers to the oxide SnO. For V4+ and Cr4+, we have also tested DFT+U, with (U - J) ) 3.5 eV for V4+ and Cr4+, which change the vacancy formation energies by at most 0.02 eV and give band gaps of 0.03 and 0.05 eV for V4+ and Cr4+ doping with 4 Cu vacancies.

Cu vacancy forms to compensate the dopant, and in eq 4, another Cu vacancy forms to dope the system p-type. In the doping-compensation mechanism, we assume formal oxidation states for Cu and M, e.g., in eq 4, we denote one lattice Cu as positively charged; in reality, the hole is delocalized over Cu atoms around the vacancy.11 For dopants with other oxidation states, a similar scheme is followed, with the number of compensating Cu vacancies dependent on the dopant oxidation state. Table 1 shows the formation energies for the first compensating Cu vacancy for doped Cu2O, denoted E(n vac 1) . Relative to the dopant site, there are a number of ways to distribute Cu vacancies, which we will elaborate upon in the following. In this section, we discuss Sn2+ and Zn2+ doping in some detail and the discussion for other dopants will include necessary trends, as our general findings for Sn and Zn are also valid for other dopants. (29) Lu, J. G.; Fujita, S.; Kawaharamura, T.; Nishinaka, H.; Kamada, Y.; Oshima, T.; Ye, Z. Z.; Zeng, Y. J.; Zhang, Y. Z.; Zhu, L. P.; He, H. P.; Zao, B. H. J. Appl. Phys. 2007, 101, 3705. (30) Mather, P. G.; Read, J. C.; Buhrman, R. A. Phys. ReV. B 2006, 73, 205412. (31) Seth, U.; Chaney, R. Phys. ReV. B 1975, 12, 5923. (32) Rao, A. S.; amey., R. T. Phys. Status Solidi 1979, 95, 243. (33) Saum, G. A.; Hensley, E. B. Phys. ReV. B 1959, 113, 1019. (34) Dakhrl, A. A.; Henari, F. Z. Cryst. Res. Technol. 2003, 38, 979. (35) Zhou, T.; Schwarz, U.; Hanfland, M.; Liu, Z. X.; Syassen, K.; Cardona, M. Phys. ReV. B 1998, 57, 153. (36) French, R. H.; Jones, D. J.; Loughin, S. J. Am. Ceram. Soc. 1994, 77, 412. (37) Kokubun, Y.; Miura, K.; Endo, F.; Nakgomi, S. Appl. Phys. Lett. 2007, 90, 031912. (38) Prathap, P.; Subbaiah, Y. P. V.; Devika, M.; Ramakrishna Reddy, K. T. Mater. Chem. Phys. 2006, 100, 375.

Figure 2. Structures for compensated Sn2+/Zn2+ dopants in Cu2O: (a) Sn2+ dopant and Cu vacancy in the same network, (b) Zn2+ dopant and Cu vacancy in the same network; (c) Sn2+ dopant and Cu vacancy in different networks; (d) Zn2+ dopant and Cu vacancy in different networks. In (a) and (b), the vacancy, indicated with a black “V”, is in the black colored network, whereas in (c) and (d), the vacancy indicated with a yellow “V” is in the yellow colored network, which lies behind the black network in this view.

Because Sn and Zn have a +2 oxidation state, one Cu vacancy compensates the dopant. This Cu vacancy can be found in either the same Cu2O network as the dopant or in the other Cu2O network, as shown in Figure 2. The structures in Figure 2 and all subsequent figures show the immediate region around the dopant and the vacancy. In the (2 × 2 × 2) supercell, the computed Cu vacancy formation energy for Sn2+ doping is -0.95 eV (dopant and vacancy in different networks) and -0.92 eV (same network) and -0.78 eV (different network) and -0.66 eV (same network) for Zn2+ doping. For Sn2+, the (3 × 3 × 3) supercell with the same vacancy-dopant distribution also results in the distribution with the dopant and vacancy on different network being most stable. The sign of the energies above and those given in Table 1 means that compensation of the potential n-type Sn2+ and Zn2+ donors through formation of neutral Cu vacancies is thermodynamically favored, disfavoring n-type doping of Cu2O with these dopants. While Hg2+, In3+, Ti4+, and Cr4+ have positive formation energies for the compensating Cu vacancy the energy is rather small and is substantially reduced compared to undoped Cu2O. (39) Hattori, T.; Yoshida, T.; Shiraishi, T.; Takahashi, K.; Nohira, H.; Joumori, S.; Nakajima, K.; Suzuki, M.; Kimura, K.; Kasiwagi, I.; Ohshima, C.; Ohmi, S.; Iwai, H. Microelectron. Eng. 2004, 72, 283. (40) Tang, H.; Prasad, K.; Sanjine`s, R.; Schmid, P. E.; Le´vy, F. J. Appl. Phys. 1994, 75, 2042. (41) Cox, P. A. Transition Metal Oxides: An Introduction to their Electronic Structure and Properties; Clarendon Press: Oxford, U.K., 1995. (42) Shin, S.; Suga, S.; Taniguchi, M.; Fujisawa, M.; Kanzaki, H.; Fujimori, A.; Daimon, H.; Ueda, Y.; Kosuge, K.; Kachi, S. Phys. ReV. B 1990, 41, 4993. (43) Fangxin, L.; Chengyun, W.; Qingde, S.; Tianpeng, Z.; Guiwen, Z. Appl. Opt. 1997, 36, 2796.

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Figure 3. Cu vacancy distributions for doped Cu2O with 2 Cu vacancies per (2 × 2 × 2) supercell. In this figure, one network is indicated with black spheres, the other is indicated with yellow spheres, and the dopant is the large gray sphere. The Cu vacancy sites are indicated with a black “V” for a vacancy in the black colored network and a yellow “V” for a vacancy in the yellow colored network.

Regardless of the distribution of the dopant and the Cu vacancy, the impact of doping on the structure around the dopant is the same. For Zn2+, there is little perturbation to the structure around the dopant and vacancy sites, whereas Sn2+ doping results in large distortions of the geometry around the vacancy, as is clear in panels a and c in Figure 2, where (black) Cu atoms around the Sn dopant have moved off their lattice sites. Sn-O distances are 2.12 and 2.39 Å, which are significantly larger than the Cu-O distance of 1.85 Å and Zn-O distances of 1.89 Å. When doping with Sn2+, the displacement of Cu atoms around the dopant site from their lattice sites, results in increased internetwork Cu-Cu distances, which are in the range 3.26-3.29 Å (3.04 Å in undoped Cu2O). When doping with Zn2+, the same distances are 3.08 Å. A detailed analysis of other dopants shows the same correlation between distortion and vacancy formation energies and therefore we suggest that the origin of the large difference in the vacancy formation energies for different dopants is tied to the different structural distortion in the relaxed structures upon doping. 3.3. p-Type Doped Cu2O. In this section, we investigate the formation of p-type M2+-doped Cu2O, by forming a second Cu vacancy, giving chemical composition Cu29O16M1 for an M2+ dopant, Cu28O16M1 for an M3+ dopant (with 3 Cu vacancies), and Cu27O16M1 for an M4+ dopant (with 4 Cu vacancies), all in a (2 × 2 × 2) supercell. The major questions to be investigated are (i) how does doping impact on formation of p-type Cu2O, (ii) are there dopant-dependent structural distortions, (iii) how is the band structure (and transparency) modified, and (iv) what effect is there on the effective hole masses and conductivity? 3.3.1. Vacancy Distributions and Energetics of p-Type Doped Cu2O. We continue the detailed analysis for Sn2+ and Zn2+ doping. With a dopant and 2 Cu vacancies, there are a number of ways of distributing the Cu vacancies and in figure 3, we show five representative distributions, numbered 1-5, and described as follows:

Table 2. Formation Energies for the Second Cu Vacancy in Sn2+and Zn2+-Doped Cu2O with Different Vacancy Distributions in the (2 × 2 × 2) Supercell configuration

dvac-vac (Å)

Evac (eV) (Sn2+)

Evac (eV) (Zn2+)

1 2 3 4 5

4.295 5.260 5.260 3.037 3.037

0.15/0.18 0.24/0.27 0.44 0.86 0.45

0.22/0.33 0.41/0.53 0.37 0.90 0.59

The formation energy for this vacancy is given by Evac ) E(Cu29O16M1) - [E(Cu30O16M1) + E(Cu)]. For configurations 1 and 2, there are two ways to form the pair of vacancies, which depends on the distribution of the dopant/vacancy pair for a single Cu vacancy. The lower formation energy is the structure in which the dopant and the first Cu vacancy are on different networks. The column headed dvac-vac presents the vacancy--vacancy distances.

• In configuration 1, the Cu vacancies are clustered, i.e., separated by the internetwork Cu-Cu nearest neighbor distance (see Table 2), with one vacancy on the same Cu2O network as the dopant and the second vacancy on the other Cu2O network. • Configuration 2 is the same as 1, except the Cu vacancies are isolated, i.e., separated by the next-nearest-neighbor internetwork Cu-Cu distance (see Table 2). • In configuration 3, the vacancies are separated by the next-nearest-neighbor internetwork Cu-Cu distance and are on a different Cu2O network to the dopant, but the same as each other. • Configuration 4 is the same as 3, except the vacancies are separated by the nearest-neighbor intranetwork Cu-Cu distance. • In configuration 5, both vacancies are separated by the nearest neighbor intranetwork Cu-Cu distance and are found on the same Cu2O network as the dopant. These distributions were also investigated within the (3 × 3 × 3) supercell. Distribution 1 is the most stable, with Evac ) 0.15/0.18 eV for Sn2+ doping and Evac ) 0.22/0.33 eV for Zn2+ doping, Table 2, which is slightly reduced compared to Evac for the same vacancy distribution in undoped Cu2O, 0.24

Tuning Cu2O Transparency with Cation Doping

eV.26 Distribution 1 is used for subsequent calculations with other dopants.44 The least stable distribution is 4 for both dopants, with Cu vacancy formation energies of 0.86 eV for Sn2+ and 0.90 eV for Zn2+. The results in Table 2 make it clear that the stability of vacancy structures depends strongly on the distribution of the vacancies and the dopant atom. It is more favorable for Cu vacancies to be found on different Cu2O networks and in configurations that minimize vacancy clustering. In the (3 × 3 × 3) supercell expansion with Sn doping, with the same dopant-vacancy distributions, the same energetic ordering of the distributions is found. The band gaps of the doped materials are discussed in section 3.3.4, but here we mention how the distribution of Cu vacancies can impact on the band gap of Sn2+-doped Cu2O, with a band gap of 0.63 eV for configuration 1 (the most stable), while configurations 2 and 3 have band gaps of 0.59 and 0.56 eV, respectively. For other dopants, the formation energy of the vacancy that dopes the system p-type is shown in Table 1. For M3+ and M4+ dopants, a number of vacancy/dopant distributions were tested, some of which are supplied as Supporting Information.44 The most stable structures are as follows. For three Cu vacancies, two of the three Cu vacancies are in the same Cu2O network. For four Cu vacancies, three of the four Cu vacancies are found in the same Cu2O network. From table 1, the formation energy for the p-type vacancy is similar to that found for undoped Cu2O. Exceptions to this are some M4+ transition metals with 4 Cu vacancies, which have vacancy concentrations of 12.5% in a (2 × 2 × 2) supercell. This may be the limit of applicability of the (2 × 2 × 2) supercell model and a lower formation energy may be found in a larger supercell. There is a relative lack of importance of vacancy clustering. While such vacancy concentrations can include clustering in the (2 × 2 × 2) supercell, the impact of clustering is not so great. It is possible that although this Cu2O phase can accommodate a high level of Cu vacancies, a vacancy concentration as high as 12.5% could be accompanied by structural changes in the crystal, although we have not investigated this point because it is not of importance for the findings of this paper. 3.3.2. Structural Distortions Due to Doping. Details of the structure around the dopant site are shown in panels a and b in Figure 4 for Sn2+ and Zn2+ p-type doping. As in the compensated case (Figure 2), there is a strong distortion around the dopant site for Sn2+ doping compared to Zn2+. With Sn2+ doping, Cu ions neighboring the dopant are displaced from their lattice sites, lengthening the internetwork Cu-Cu distances for these Cu ions to 3.20-3.30 Å, and giving a wide range of Sn-O distances (2.12-2.41 Å). The ribbon angles around the dopant site increase to 111.7-116.1°, which is a substantial opening up over undoped Cu2O, for which the ribbon angle is 109°. With Zn2+ doping Cu2O, there is little distortion to the local structure around the dopant site, irrespective of the vacancy distribution. Internetwork Cu-Cu distances involving Cu atoms neighboring the dopant site are 3.01-3.08 Å and Zn-O distances range (44) Pictures of dopant and vacancy configurations studied are available in the Supporting Information that accompanies this manuscript.

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Figure 4. Structure around the dopant for doped Cu2O with Cu vacancies that makes the system p-type. The coloring of the ions and the “V” are the same as Figure 2 and the dopant elemental symbol is given in each case.

from 1.85-1.91 Å. The same ribbon angles are in the range 108.3-112.6°, a smaller change. We attribute the difference in structural distortion between Sn2+ and Zn2+ to the larger ionic radius of Sn2+ (6 coordinate, 0.93 Å as calculated following ref 24) compared to 6-coordinate Zn2+ (0.40 Å) and Cu+ (0.46 Å). The larger Sn2+ ion strongly perturbs the local atomic structure in its vicinity. The impact of these structural modifications on the band structure and band gap will be discussed in section 3.3.4. In Figure 4, the relaxed structures for n Cu vacancies in examples of Cu2O doped with selected Mn+ cations are presented. Those dopants with a larger ionic radius than Cu+ induce strong structural distortions around the dopant site. For instance with Sr2+ doping, the Cu-Cu distances around the dopant site are strongly elongated, to 3.37-3.41 Å. With Cd2+ (and Hg2+), the Cu-Cu distances around the dopant site are 3.14 - 3.25 Å and the intranetwork ribbon angles are also distorted, to as large as 116°. The ionic radius of Al3+ is smaller than Cu+, that of Ga3+ is similar to Cu+ and both In3+ and La3+ have larger ionic radii than Cu+. Consistent with this, Al3+ and Ga3+ do not distort the oxide host, showing Cu-Cu distances of 2.96-3.00 Å (Al) and 2.97-3.05 Å (Ga) around the dopant site. The larger ionic

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Figure 5. Excess spin density for (left) Sn2+- and (right) Zn2+-doped Cu2O with 2 Cu vacancies.

radius of In3+ and La3+ results in a distortion of Cu atoms around the dopant site, with Cu-Cu distances in the range 3.12-3.18 Å for In3+ and 3.11-3.48 Å for La3+. We find the same trend when doping with the early transition metals Ti4+, Cr4+ and V4+ and the rare earth Ce4+. In these oxidation states, Cr4+ and V4+ have smaller ionic radii than Cu+ and show little distortion around the dopant site (Cu-Cu distances are 3.03 Å for Cr4+ and 2.98-3.02 Å for V4+). Ti4+ and Ce4+ show larger ionic radii than Cu+ and Cu-Cu distances around the dopant site are correspondingly elongated to 3.06 Å for Ti4+ and 3.09 Å-3.39 Å for Ce4+. For dopants that distort the local atomic arrangement, not only are the Cu-Cu distances around the dopant site affected but also the -O-Cu-O- ribbon angles. In this way, the intranetwork Cu-Cu interactions are modified. For example, in Al3+- and La3+-doped Cu2O, the ribbon angles around the dopant site are in the range 110.3-111.0° (Al3+) and 112.1-155.7° (La3+), consistent with the different ionic radii. For Ce4+-doped Cu2O, the same angles are in the range 113.6-146.0°. We propose that the negative formation energy for the second and third Cu vacancies in Ba2+- and La3+-doped Cu2O arise from the very strong structural distortions introduced by these dopants. 3.3.3. How Doping Impacts ConductiVity. To see if doping impacts the conductivity of the host oxide, we examine the delocalization property of the electronic hole state in Sn2+and Zn2+-doped Cu2O and the impact of doping on the computed effective hole mass. Isosurfaces of excess spin density (which is the difference between the up and down spin densities) for Sn2+- and Zn-doped Cu2O, with two Cu vacancies (i.e., p-type) are shown in Figure 5. The resulting hole state is delocalized over the Cu atoms in the structure, similar to a Cu vacancy in undoped Cu2O.11 We find that doping has little impact on the nature of these delocalized hole states. This is also seen in the band structures for the most stable vacancy structure, in which hole states are present at the top of the valence band, similar to undoped Cu2O.11 The computed effective hole masses are -10, -1.28, and -0.96 me for Sn2+ doping and -8.0, -1.50, and -0.93 me for Zn2+ doping (in the most stable structures). The effective mass of the lightest hole is increased over Cu2O, for which computed effective masses are -12.1, -4.0, and -0.5 me, suggesting slightly poorer conductivity in the doped cases. However, there is a second hole that has become light compared to Cu2O, indicating a possible positive effect of doping on the electrical characteristics.

Nolan and Elliott

3.3.4. How Doping Impacts the Band Gap of Cu2O. We turn the focus to the impact of doping on the band gap and hence transparency of Cu2O. At a 3% doping level, Sn2+ doping results in a computed band gap of 0.63 eV and Zn2+ doping gives a band gap of 0.49 eV, so that Sn2+ doping enhances the band gap and may therefore enhance the transparency, whereas Zn2+ doping makes little change. The PDOS plots are shown in Figure 6. For Sn2+ doping, Sn 5s states are found below the VB edge, but for Zn2+ doping, unoccupied Zn 4s states are at the bottom of the conduction band (CB). The Zn 4s states shift the bottom of the CB down and this is the origin of the reduced effective band gap of Zn-doped Cu2O. These data confirm that Sn and Zn are present in the +2 oxidation state. Turning to the other dopants, the trend for alkaline earth doping on going down the column is to increase the band gap of Cu2O, see table 1. The small decrease in the band gap upon Ba2+ doping compared to Sr2+ doping is consistent with the behavior of alloyed oxides of copper with alkaline earths.45 The increased band gap over Cu2O for alkaline earth doping correlates with the larger ionic radius and accompanying structural distortion. However, we see that although Cd2+ and Hg2+ are larger ions than Cu+, the band gap is reduced (0.45 and 0.30 eV for Cd2+ and Hg2+) compared to stoichiometric Cu2O. Al3+, Ga3+, and In3+ also reduce the band gap compared to Cu2O, even though In3+ has a larger ionic radius than Cu+, whereas La3+ doping increases the band gap. Finally, for the transition metals, the band gap is always reduced over Cu2O. We examine the PDOS to clarify the origin of this behavior. Panels a and b in Figure 7 show the Cu and dopant PDOS for Sr and Cd doping. In Figure 7, the Cu 3d PDOS is reduced by a factor of 10 in order to enhance the remaining contributions and the PDOS is shown over a narrow energy range around the VB-CB band gap. The PDOS for Sr shows that the VB and CB are dominated by Cu2O; the dopant is “electronically inert”, in that no dopant electronic states are seen to interact strongly with the Cu2O VB or CB, nor are there any dopant electronic states in the Cu2O band gap. The band alignments of the host oxide and the dopant do not facilitate band hybridization. For Cd, unoccupied Cd 5s states are found below the original Cu 4s derived CB edge, which has the effect of reducing the effective band gap, despite the introduction of Cu vacancies and the distortions induced by the dopant. Continuing this analysis, the PDOS plots for In and La doping with 3 Cu vacancies are shown in panels c and d in Figure 7. (The Al and Ga PDOS are similar to In and are not shown). With In doping, an unoccupied In 5s (Al 3s, Ga 4s) state is present above the valence band and is expected to reduce the effective band gap and Table 1 shows that this is the case. The band gap reduction is despite the larger ionic radius of In and distortion of the host structure. On the other hand, for La, the band gap increase is similar to that of the alkaline earth metals. The PDOS for La shows no dopant derived states in the Cu2O band gap and the change in band gap is determined solely by the structural distortion induced by the large dopant cation. (45) Nie, X.; Wei, S.-H.; Zhang, S. B. Phys. ReV. B 2002, 65, 075111.

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Figure 6. Spin-polarized PDOS for (a) Sn- and (b) Zn- doped Cu2O with 2 Cu vacancies. The zero of energy is the Fermi level and the Cu 3d PDOS only is reduced by a factor of 10 relative to the Cu 4s and dopant PDOS.

The PDOS plots for Ti- and Ce-doped Cu2O with 4 Cu vacancies are shown in panels e and f in Figure 7. For these dopants (as well as Cr and V), we find dopant-derived defect states in the Cu2O band gap. The in-gap states reduce the band gap of Cu2O and thus the transparency. For Ti/Cr/V, the smaller dopant ionic radius compared to Cu results in little structural distortion and would not be expected to enhance the band gap in any case, but the dopant electronic states are key. Ce doping illustrates the competition between dopant ionic radius and electronic structure. Ce has a larger ionic radius than Cu and produces strong structural distortions around the dopant site. However, Ce 4f states are found in the band gap, reducing the effective band gap. These results indicate that the early transition metals and other cations with partially filled d or f shells, such as Ce, will not be suitable dopants to enhance the transparency of Cu2O. 4. Discussion In many metal oxides, defect formation and structural distortion results in a narrowing of the band gap,15,46 cation-cation interactions are usually neglected 47,48 and the structure is close-packed, e.g., rock salt structured alkaline earth oxides.46 A well-known example of an oxide that is doped to narrow the band gap is TiO2, which is usually doped so that it absorbs in the visible part of the spectrum rather than in UV.49-51 However, our results show that Cu2O behaves in an atypical fashion compared to most oxides. Cu2O (like TiO2) is a network oxide and is held together by intranetwork Cu-O bonding and Cu-Cu interactions between the interpenetrating Cu2O networks, and these interactions are key for understanding the electronic structure of Cu2O.6-10 Our studies also show another unusual feature of Cu2O: Cu vacancies are accommodated with little structural distortion in the network,11 facilitating p-type conductivity. (46) West, A. R. Basic Solid State Chemistry; Wiley: Chichester, U.K., 1999. (47) Woodley, S. M.; Battle, P. D.; Gale, J. D.; Catlow, C. R. A. Phys. Chem. Chem. Phys. 1999, 1, 2535. (48) Lewis, G. V.; Catlow, C. R. A. J. Phys. C 1985, 18, 1149. (49) Umbeyashi, T.; Yamaki, T.; Itoh, H.; Asai, K. J. Phys. Chem. Solids 2003, 63, 1909. (50) Yates, H. M.; Nolan, M. G.; Sheel, D. W.; Pemble, M. E. Thin Solid Films 2006, 179, 213. (51) Serpone, N. J. Phys. Chem. B 2006, 110, 24287.

We are therefore interested to see if the unusual properties of Cu2O can be exploited for tuning the band gap and hence transparency, without impairing the conductivity, which will be of great interest for TCO applications. We have therefore computed vacancy formation and band gap change with doping, using DFT. There are some errors present in the computations through using DFT and a periodic supercell model, which we now discuss. The first error is the wellknown underestimation of the magnitude of the band gap of a metal oxide with DFT. However, we are interested in the trend of how the band gap of the host oxide changes with doping, which is expected to be reliable. For example, Nie et al. have discussed this point for the series of alloys MgCu2O2, CaCu2O2, SrCu2O2, and BaCu2O2.45 Other sources of error impact on vacancy formation energetics and include (i) the computed energy of Cu metal, which accumulates as the number of vacancies increases, and (ii) the use of the (2 × 2 × 2) supercell, which for a high vacancy concentration can necessarily result in clustering of Cu vacancies and lead to too high a vacancy formation energy. We have checked with a larger supercell that the impact of clustering on our results is not of importance and we estimate an error of 0.1 eV per supercell in the computed vacancy formation energies. However, the formation energy errors are the same within a given dopant oxidation state and our interest is in trends in vacancy formation energies and band gap. From our calculations, the origin of the band gap modifications for doped Cu2O are attributed to two factors; (i) the dopant ionic radius and (ii) the dopant electronic structure. We believe that the impact of cation doping of Cu2O could be tested experimentally by doping Cu2O with substitutional dopants that go down a periodic column, e.g., the alkaline earths. In the first factor, disrupting the Cu-Cu interactions through cation doping is possible, depending on dopant ionic radius. In a simple interpretation, the ionic radius attempts to give a measure to the “size” of an ion in a crystal and depends on coordination number (CN) and oxidation state,24 meaning that there is no unique ionic radius. We have used different references to arrive at a set of ionic radii for dopants in this work that are consistent as far as possible with their oxidation state and coordination in doped Cu2O,24,46 which may be different to the coordination and oxidation state in their native oxide. Many of the dopants studied in this work have more than one CN, usually 4-6, and the

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Figure 7. Spin-polarized PDOS for (a) Sr2+-, (b) Cd2+-, (c) In3+-, (d) La3+-, (e) Ti4+-, and (f) Ce4+-doped Cu2O with n Cu vacancies for cations with oxidation state Mn+. The zero of energy is the Fermi level and the Cu 3d PDOS only is reduced by a factor of 10 relative to the Cu 4s and dopant PDOS.

cation coordination in Cu2O is 2, which is unusual. Many dopants are displaced off the substitutional lattice site toward a group of Cu and O atoms, with the resulting CN appearing to be around 4. Thus, for each dopant we use the CN available in the literature that is closest to 4,25 giving the ionic radii in Table 1. The dopant-induced distortions in Cu2O are consistent with the dopant ionic radii relative to 2 coordinate Cu+. The ionic radius also depends on oxidation state and there is usually more than one possibility. For most dopants, the choice of oxidation state is clear and consistent with the computations, whereas for the transition metals and Ce (with potentially variable oxidation states) the computed charges indicate that these ions have a formal +4 oxidation state. Thus, the ionic radii used in this paper are sufficiently robust. Dopants with a larger ionic radius than Cu (e.g., Sn, Sr, Ba, In, La, and Ce) induce distortions around the dopant site, leading to elongated internetwork Cu-Cu distances and larger intranetwork O-Cu-O ribbon angles. For some of these dopants, the band gap is increased, e.g., Sn, Sr, and La. Dopants with ionic radii similar to or smaller than Cu, e.g., Zn, Al, Ti, lead to little or no disruption to the Cu-Cu interactions and the band gap is little changed or decreased.

However, although some dopants, such as In and Ce, result in structural distortions, they lead to a decrease in the band gap. The other property of interest for TCO materials is the conductivity and doping could also modify this property. However, we have shown for Sn and Zn doping that the doped material still retains a light hole, which maintains p-type semiconducting behavior and we assume that this remains the case for the other doping scenarios considered herein. While clustering of Cu vacancies can degrade the effective hole mass, e.g., by forming hole traps, the most stable structures in this work show isolated Cu vacancies and delocalized hole states. The first design guideline for enhancing transparency of Cu2O is as follows: 1. A dopant that introduces structural modifications that distort the Cu2O network and reduce three-dimensional Cu-Cu interactions will open the band gap (maintaining p-type behavior). This is achieved by the dopant being of a larger ionic size than Cu+. The second design guideline is as follows: 2. The dopant should have an oxidation state greater than Cu+ in Cu2O. With an oxidation state different to Cu+, Cu vacancies will be generated to compensate the extra electrons.

Tuning Cu2O Transparency with Cation Doping

Cu vacancies also contribute to the band gap increase,11 because of a reduction in the number of Cu-Cu interactions. The final, and most important issue in tuning the band gap of Cu2O is the alignment of the dopant and host oxide electronic states. Ideally, the most favorable dopants will be hard cations that are “electronically inert”. By this, we mean that dopant electronic states will not hybridize with the Cu2O valence (VB) or conduction bands (CB) or will not appear in the band gap of Cu2O. A good example of this is the enhanced band gap for SrCu2O2, where the VB and CB of the alloy are essentially those of Cu2O and the electronic states of SrO play no role.45,52 From these considerations, the final, and most important, design guideline is as follows: 3. The dopant must be “electronically inert”. This means that the dopant electronic states must not hybridize with the Cu2O-derived VB/CB edge states or introduce in-gap states, both of which reduce the effective band gap. A naı¨ve approach to meeting this criterion is to assume that doping with a “hard” cation, such as Sr2+, Ba2+, Al3+, La3+, and Ce4+, will increase the band gap. The band gaps of the corresponding oxides (SrO, BaO, Al2O3, La2O3, and CeO2) are 5.9,32 4.10,33 8.8,36 5.4-5.8, 39 and 4.2 eV,43 respectively. We find that Sr, Ba, and La introduce no dopant derived electronic states in the band gap of Cu2O, nor is there hybridization with the Cu2O VB/CB. These dopants fit our guidelines and are suitable dopants for enhancing the band gap of Cu2O. In contrast, Al and Ce doping result in dopant-derived electronic states (Al 3s and Ce 4f) in the band gap, reducing the effective band gap. As further examples, Ga and In doping (with oxide band gaps of 4.9537 and 3.57 eV 38) show a similar trend, despite In being a larger cation than Cu. The reduced band gap for these ions arises from the misalignment of dopant states, namely Ga 4s and In 5s, with the band edges of Cu2O (guideline 3). These results demonstrate that both ionic size and electronic structure effects, particularly the alignment of the dopant and host oxide electronic states, must be considered. In summary, to improve the transparency of Cu2O, a simplistic understanding is that cations forming wide band gap oxides (“hard cations”) are suitable. In fact, dopants must meet the dual criteria of a larger ionic radius than Cu+ and no electronic states in the Cu2O band gap, which cannot be predicted simply by comparing the band gaps of the oxides (52) Modreanau, M.; Nolan, M.; Elliott, S. D.; Durand, O.; Servet, B.; Garry, G.; Gehan, H.; Huyberechts, G.; Papadopoulou, E. L.; Androulidaki, M.; Aperathitis, E. Thin Solid Films 2007, 515, 862.

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of the dopant. These criteria severely narrow the range of possible dopants to those cations whose electronic states are correctly aligned with those of the host oxide, such as alkaline earth metals. 5. Conclusions In seeking an understanding of how doping can be used to tune the band gap of Cu2O, we have elucidated the key effects. The existence of Cu-Cu interactions in the cubic Cu2O structure plays a key role in the unusual behavior of Cu2O, in particular the opening up of the band gap upon disruption of these interactions through, for example, doping. There are two conditions related to the dopant that are key for increasing the band gap. The first is that dopants with larger ionic radii than Cu+ will distort the local structure around the dopant site and hence Cu-Cu interactions and for La3+, Sr2+, and Ba2+ dopants, this distortion results in a larger band gap over stoichiometric Cu2O. Other dopants with ionic radii smaller than Cu+ show no enhancement of the band gap. The second condition is avoiding misalignment of dopant electronic states with Cu2O bands, which turns out to be the most important aspect of doping. If dopant electronic states hybridize with the valence or conduction band of Cu2O or form defect states in the band gap, then the band gap is reduced compared to undoped Cu2O, potentially reducing transparency. This band gap reduction will occur irrespective of the ionic radius of the dopant, even if it results in strong structural distortions. It is not necessarily the case that dopants which form wide band gap oxides will also increase the band gap of Cu2O, as demonstrated for Aldoping. These results provide useful guidelines for choosing cation dopants to enhance the transparency of Cu2O. Acknowledgment. We acknowledge support from the European Commission through the 6th Framework project “Novel Advanced Transparent Conducting Oxides” (NATCO, IST-FP6511925). We acknowledge Science Foundation Ireland (SFI) funded computer resources at Tyndall and at the SFI/Higher Education Authority funded Irish Centre for High End Computing (ICHEC). Supporting Information Available: Pictures of dopant and vacancy configurations for 2 Cu vacancies, 3 Cu vacancies, and 4 Cu vacancies (PDF). This information is available free of charge via the Internet at http://pubs.acs.org. CM703395K