Interplay of cleaning and de-doping in oxygen plasma treated high work function indium tin oxide (ITO)

Indium tin oxide (ITO) is extensively used as a transparent electrode in photovoltaic cells and organic light emitting diodes. High surface work function (WF) of ITO is a crucial parameter for enhanced device performance. The ITO WF is usually around 4.3 eV without any surface treatment. With surface treatments ITO WF, as high as 5.4 eV has been reported. We designed a surface treatment method with which we achieved substantially high ITO surface work function of over 6.1 eV. We observed changes in valence electronic structure and core levels, apart from surface cleaning. We also investigated interface formation of copper phthalocyanine (CuPc) on the high WF ITO. In the proximity of the interface the highest occupied energy level of CuPc was observed to be almost pinned to the Fermi level. We fabricated three simple devices with no to high treatment. The device results were observed to be consistent with the findings of electronic energy level alignment.

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► We have achieved high ITO work function up to 6.15 eV with oxygen plasma treatment.
► The enhancement is found to be an interplay of surface cleaning and de-doping.
► Cleaning increases work function up to 5.5 eV and is very beneficial for devices.
► De-doping replenishes oxygen vacancies and so increases work function over 6 eV.
► Devices from no to high treatments are fabricated to show agreement with the model.