Zn-Ni-Co-O wide-band-gap p-type conductive oxides with high work functions
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Zn–Ni–Co–O wide-band-gap p-type conductive oxides with high work functions A. Zakutayev, J.D. Perkins, and P.A. Parilla, National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, Colorado 80401 N.E. Widjonarko, National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, Colorado 80401; Department of Physics, University of Colorado, Boulder, Colorado 80309 A.K. Sigdel, National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, Colorado 80401; Department of Physics and Astronomy, University of Denver, Denver, Colorado 80208 J.J. Berry and D.S. Ginley, National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, CO 80401 Address all correspondence to A. Zakutayev at [email protected] (Received 24 June 2011; accepted 4 August 2011)
Abstract Co3O4-based spinels are a new class of wide-band-gap p-type conductive oxides with high work functions. We examined the structures, conductivities, work functions, and optical spectra of quaternary Zn–Ni–Co–O thin films across the entire spinel region of the ZnO–NiO–Co3O4 diagram using a high-throughput combinatorial approach. We found that the conductivity of as-deposited films is maximized (100 S/cm) and optical absorption (at 1.8 eV) is minimized in different regions of the diagram, while the work function of annealed films is high and relatively constant (5.8 ± 0.1 eV). These properties made Zn–Ni–Co–O thin films applicable as p-type interlayers in solar cells. As an example, amorphous Zn–Co–O hole transport layers had good performance in bulk heterojunction organic photovoltaic devices.
High-performance p-type transparent conductive oxides (TCOs) remain an elusive goal of materials research. If developed, TCOs with high optical transparency, high p-type conductivity, large work functions, and low processing temperatures would be of considerable interest as p-type electrodes in thin-film photovoltaics (PVs). Unfortunately, no existing p-type TCOs[1–4] meet all these requirements. Other potential applications for p-type TCOs include flat panel displays, lightemitting devices, and transparent electronics.[5] One promising and largely unexplored area to search for new p-type TCOs are the quaternary oxides, similar to previously studied quaternary chalcogenide p-type transparent conductors.[6,7] Quaternary materials have additional chemical degrees of freedom compared with binaries and ternaries, which may allow for independent tuning of their relevant physical properties. However, such chemically complex materials are time consuming to study, and hence require a highthroughput approach to make surveying the large composition space tractable. In this letter, we report on a high-throughput combinatorial study of structural, optical, transport properties, and work functions of quaternary Zn–Ni–Co–O p-type oxides on the pseudoternary ZnO–NiO–Co3O4 stoichiometry map. Zn–Ni–Co–O can be viewed either as Co3O4 spinel co-doped with Zn and Ni, or as a mixture of Co3O4 with Co2NiO4 inv
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