Cation Off-Stoichiometry Leads to High p-Type Conductivity and Enhanced Transparency in Co2ZnO4 and Co2NiO4 Thin Films

We explore the effects of cation off-stoichiometry on structural, electrical, optical, and electronic properties of Co ₂ZnO ₄ normal spinel and Co ₂NiO 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 ₂ZnO ₄. Experiment confirms high conductivity and high work function in Co ₂NiO ₄ and Zn-rich Co ₂ZnO ₄ thin films grown by nonequilibrium physical vapor deposition techniques. High p-type conductivities of Co ₂ZnO ₄ (up to 5 S/cm) and Co ₂NiO 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 ₂ZnO ₄ and Co ₂NiO 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 ₂ZnO ₄ and Co ₂NiO 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.