Epitaxial Strain-Induced Growth of CuO at Cu2O/ZnO Interfaces

Cu2O/ZnO has been envisaged as a potential material system for the next-generation thin film solar cells. Thus far, the experimental efforts to obtain conversion efficiencies close to the theoretically predicted value have failed. Combining aberration-corrected (scanning) transmission electron microscopy and density functional theory modeling, we studied the interfaces between single-crystal taxis -oriented ZnO and high-quality magnetron-sputtered Cu2O films. Strikingly, our study shows that the first similar to 5 nm of the Cu oxide films has the structure of the monoclinic CuO phase. The CuO layer is textured with the (111), (11(1) over bar), ((1) over bar 11), (1 (11) over bar) ((11) over bar1), (1(1) over bar 11), ((1) over bar1 (1) over bar) and (100) planes parallel to the (0001) and (000 (1) over bar) ZnO interfaces. The ionic arrangement on these planes resembles the hexagonal arrangement of the ZnO interface, and epitaxy exists across the interface. A continued epitaxial growth of [111]-oriented Cu2O follows resulting in epitaxial 180 degrees rotation twins in the Cu2O layer. For the case with the (100)(Cuo) interfacial plane we have (111)[1 (1) over bar0](Cu2O)parallel to(100)[0 (1) over bar1](cuo)parallel to(0001)[11 (2) over bar0](zno). Because of a closer lattice matching of CuO with ZnO and Cu2O, the total strain and energy is reduced compared to a pure (111)cu(2)o parallel to(0001)(zno) interface. The existence of CuO is anticipated to be a contributing factor for the low conversion efficiencies obtained experimentally.