Cation off-stoichiometry leads to high $p$-type conductivity and enhanced transparency in Co${}_{2}$ZnO${}_{4}$ and Co${}_{2}$NiO${}_{4}$ thin films

Cation off-stoichiometry leads to high $p$ -type conductivity and enhanced transparency in Co $_{2}$ ZnO $_{4}$ and Co $_{2}$ NiO $_{4}$ thin films

A. Zakutayev, T. R. Paudel, P. F. Ndione, J. D. Perkins, S. Lany, A. Zunger, and D. S. Ginley

Phys. Rev. B 85, 085204 – Published 13 February 2012

Abstract

We explore the effects of cation off-stoichiometry on structural, electrical, optical, and electronic properties of Co $_{2}$ ZnO $_{4}$ normal spinel and Co $_{2}$ NiO $_{4}$ inverse spinel using theoretic and experimental (combinatorial and conventional) techniques, both at thermodynamic equilibrium and in the metastable regime. Theory predicts that nonequilibrium substitution of divalent Zn on nominally trivalent octahedral sites increases net hole density in Co $_{2}$ ZnO $_{4}$ . Experiment confirms high conductivity and high work function in Co $_{2}$ NiO $_{4}$ and Zn-rich Co $_{2}$ ZnO $_{4}$ thin films grown by nonequilibrium physical vapor deposition techniques. High $p$ -type conductivities of Co $_{2}$ ZnO $_{4}$ (up to 5 S/cm) and Co $_{2}$ NiO $_{4}$ (up to 204 S/cm) are found over a broad compositional range, they are only weakly sensitive to oxygen partial pressure and quite tolerant to a wide range of processing temperatures. In addition, off-stoichiometry caused by nonequilibrium growth decreases the optical absorption of Co $_{2}$ ZnO $_{4}$ and Co $_{2}$ NiO $_{4}$ thin films, although the 500-nm thin films still have rather limited transparency. All these properties as well as high work functions make Co $_{2}$ ZnO $_{4}$ and Co $_{2}$ NiO $_{4}$ thin films attractive for technological applications, such as hole transport layers in organic photovoltaic devices or $p$ -type buffer layers in inorganic solar cells.

Received 11 October 2011

DOI:https://doi.org/10.1103/PhysRevB.85.085204

Authors & Affiliations

A. Zakutayev^*, T. R. Paudel, P. F. Ndione, J. D. Perkins, S. Lany, A. Zunger^†, and D. S. Ginley

National Renewable Energy Laboratory, Golden, Colorado 80401, USA

^*Corresponding author: 1617 Cole Blvd., Golden, CO 80401-3305; andriy.zakutayev@nrel.gov
^†Current address: University of Colorado, Boulder, CO 80309, USA.

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Vol. 85, Iss. 8 — 15 February 2012

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