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 Co2ZnO4 normal spinel and Co2NiO4 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 Co2ZnO4. Experiment confirms high conductivity and high work function in Co2NiO4 and Zn-rich Co2ZnO4 thin films grown by nonequilibrium physical vapor deposition techniques. High p-type conductivities of Co2ZnO4 (up to 5 S/cm) and Co2NiO4 (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 Co2ZnO4 and Co2NiO4 thin films, although the 500-nm thin films still have rather limited transparency. All these properties as well as high work functions make Co2ZnO4 and Co2NiO4 thin films attractive for technological applications, such as hole transport layers in organic photovoltaic devices or p-type buffer layers in inorganic solar cells.