Cupric Oxide Thin Films for Photovoltaic Applications

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Cupric Oxide Thin Films for Photovoltaic Applications Patrick J. M. Isherwood, Biancamaria Maniscalco, Fabiana Lisco, Piotr M. Kaminski, Jake W. Bowers and John M. Walls CREST, School of Electrical, Electronic and Systems Engineering, Loughborough University, Loughborough, Leicestershire, LE11 3TU UK Email: P.J.M [email protected]

ABSTRACT Cupric oxide thin films were sputtered onto soda-lime glass slides from a single preformed ceramic target using a radio-frequency power supply. The effects of oxygen partial pressure and substrate temperature on the optical, electrical and structural properties of the films were studied. It was found that increasing temperature resulted in increased crystallinity and crystal size but also increased film resistivity. The most conductive films were those deposited at room temperature. Increasing oxygen partial pressure was found to reduce resistivity dramatically. This is thought to be due to higher charge carrier concentrations resulting from increased copper vacancies. Increasing oxygen partial pressure causes an increase in the optical band gap from a minimum of 0.8eV up to a maximum of 1.42eV. Oxygen-rich films display reduced crystallinity, becoming increasingly amorphous with increased oxygen content. These results show that the optical, electrical and structural properties of sputtered cupric oxide films can be controlled by alteration of the deposition environment.

INTRODUCTION Transparent conductive oxides (TCOs) are wide-band gap semiconductors which when doped to a sufficiently high degree begin to show metal-like conductive behaviour [1]. They are easily formed, and common techniques such as chemical vapour deposition (CVD), magnetron sputtering and chemical bath deposition (CBD) are regularly employed [1]. They have good electrical properties, are relatively easy to produce and exhibit good chemical and thermal stability [1], [2]. They are used for a very wide range of purposes, including electrical contacts in computer screens, smartphone displays, LEDs and photovoltaic cells [1], [3]. TCOs in common usage are almost exclusively n-type. There are various p-type materials currently being investigated, but so far none have been developed with the same order of magnitude of electrical conductivity as n-type materials such as aluminium-doped zinc oxide (AZO) or tin-doped indium oxide (ITO) [4–6]. The most significant problem for p-type TCOs is that oxygen ions tend to localise holes (the majority charge carriers), preventing them from moving readily through the material [6]. There are a limited number of cationic species which are able to counteract this effect, notably copper, silver and gold [6–9]. Of these, copper is the most interesting largely because it is both relatively cheap and commonly available. Cupric oxide (CuO) is one of two principal oxides of copper, and has the chemical formula CuO. It is an indirect band gap semiconductor, with reported experimental band gap values of 1.2 to 2eV [10], and theoretical values of around 1eV [11]. It is naturally p-typ

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