Materials studies of copper oxides obtained by low temperature oxidation of copper sheets

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Abstract

Inexpensive and stable solar cells based on oxide materials are currently strongly investigated. The surest candidates are copper oxides, which unlike to other metal oxides display strong absorption across the visible range. Although, copper oxides thin film can be deposited by the number of methods, cost effective techniques applicable for large scale production are necessary. Therefore, the main focus of this paper was turned to the simple and time effective thermal oxidation of copper sheets using relatively low temperatures 200–400 °C. The surface pretreatment was selected in order to support homogeneous growth conditions. The basic material research presented here included the morphological, compositional and microstructural aspects of grown copper oxides. The surface roughness of manufactured layers for investigated temperature range was described either by AFM microscopy where Ra was assessed between 8.9 and 67.6 nm and also by the optical method where calculated haze factor ranged from 34 to 79.3%. Microstructure and phase composition studies were performed by means of TEM and XRD supplemented by XPS surface analysis confirming the dominating phase of copper (I) oxide capped with thin surface CuO layer. Finally, the optical properties were described allowing to determine the sample absorptivity and band gap energy varying in the range 2.43–1.93 eV.

Introduction

One of the most important challenges for modern science is the possibility of producing solar cells with the highest effectiveness value, i.e. the ratio of cell efficiency to its price using cheap and non-toxic materials such as copper and its oxides. Copper is a relatively cheap material with low resistivity (1.676 × 10-6 Ωcm at 20 °C) [1] and extremely good flexibility which makes it an attractive component used in many electronics devices. Among them a large potential in the area of heterostructural solar cells based on copper oxides is noticed [[2], [3], [4]]. Copper can form three types of oxides namely: cuprite (Cu2O), tenorite (CuO) and paramelaconite (Cu4O3) [2]. Two stable phases are cuprite and tenorite, which are a p-type semiconductors. Their properties are consistent with a structural model based on the presence of cation vacancies as the predominant ionic defects [5]. It is confirmed that the p-type conductivity of CuO thin films is the direct result of Cu vacancies in lattice structure which leads to the formation of holes in the valence band [6]. Literature data claim that the tenorite CuO and cuprite Cu2O have a band gap energy of 1.21–1.51 eV and 2.10–2.60 eV respectively [7]. According to the study, the Cu2O film is obtained at 200 °C, whereas the CuO is formed above the annealing temperature of 300 °C. It is not trivial to determine the exact temperature transition due to progressive oxidation in the temperature range of 200–300 °C [8].

Besides thermal oxidation of copper sheet in a wide temperature range 200 °C–1050 °C [9]. Copper oxides can be also deposited by many different method. The most popular of them are: magnetron sputtering [[10], [11], [12]], rf magnetron sputtering [13,14], successive ionic layer adsorption and reaction (SILAR) [15], chemical spray pyrolysis technique using copper (ΙΙ) chloride hydrate [16], chemical vapor deposition from copper dipivaloylmethanate [17], atomic layer deposition (ALD) with the exafluoroacetyl-acetonate Cu(I) 3,3-dimethyl-1-butene [18] or copper (II) acetate Cu(OAc)2 and water vapor as precursors [19], chemical deposition using copper sulfate pentahydrate (CuSO4·5H2O) with sodium thiosulfate (Na2S2O3) and sodium hydroxide (NaOH) [7], electrodeposition method with copper (II) sulfate CuSO4 and l-lactic acid dissolved in distilled water [20], sol-gel technique with copper (II) chloride dehydrate dissolved in methanol and trimethylamine [21], hydrothermal electro-deposition technique [22] and pulsed laser deposition (PLD) [23].

As shown in Table 1, the influence of the production methods and deposition parameters on the layer stoichiometry, defect and microstructure is responsible for the broad dispersion of the most important parameters of the obtained materials, namely, carrier mobility, carrier concentration, resistivity and band gap energy. Also the post-deposition annealing of copper oxide may affect its properties. For instance, the Cu2O layer obtained by reactive sputtering at 400 °C after heating at 900 °C for 3 min in a vacuum, changed the majority carrier mobility from 10 to 50 cm2/V and the resistivity from 560 Ωcm to 200 Ωcm [14]. Another method of shaping the parameters of copper oxide layers is their doping. Barakat improved electronic and optical properties of sol-gel deposited CuO thin film for hetero-junction solar cell by lithium (Li) doping using lithium chloride monohydrate [21].

After all, the easiest and cheapest method of obtaining copper oxide remains thermal oxidation which has already been widely described in the literature. Scientific reports mainly concerned either the thin copper films deposited on different substrates [8,24] and oxidation of copper sheets of several hundred micrometers in thickness. In the second - more applicative approach – oxidation process of the copper sheets is highly temperature dependent. At very high temperatures, above 1000 °C, the high quality Cu2O phase is formed according to Toth-Trivich method [25]. Despite the millimeter grains size and crystallite size beyond 100 nm supporting the outstanding mobility, this highly energy-consuming process is less attractive from the industrial perspective. Additionally, due to the high oxidation rate the entire sheet is subject to oxidation, which is much more demanding from optical point of view and requires additional steps for contacting. Castrejón-Sánchez et al. [26] performed long annealing (hour or its multiple) of relatively thick Cu sheets within the broad temperature range 200–1000 °C. Beside the highest temperature in the range of 200–900 °C they found the fragile layers with mixed oxide phases. J. Liang et al. explored oxidation of Cu foil at 700–900 °C for 2 h, and found better crystallinity and thick Cu2O only when the water vapor was introduced into nitrogen atmosphere [27]. Also in the case of the thermal oxidation, the process parameters have a key influence on the composition, morphology and parameters of the layer. The mean grain size of Cu2O and CuO increases with a temperature increasing. The tenorite starts to nucleate above 250 °C with crystallite diameters of about 9 nm, and it grows with annealing temperature up to 40 nm at 1000 °C [8]. At higher annealing temperatures such as 950 °C causes high porosity and cracks on the oxide surface. The kinetics of the oxidation mechanism of Cu depends on many factors such as Cu surface condition, kind of heating source, temperature, ambient atmosphere, oxygen partial pressure and time of annealing. Oxidation at temperatures higher than 350 °C leads to the growth of a fragile copper oxide layer characterized with poor adherence to the Cu surface what is a results of different values of the thermal expansion coefficient α(T) for crystalline Cu and copper oxides. A monocrystalline copper has a α(T) = 31 × 10-6 K-1, while CuO characterized by 90 nm crystallites exhibits α(T) = 5.1 × 10-6 K-1 [28]. The above-mentioned difference in parameters causes great problems with the adhesion of thermally produced copper oxide layers to a solid Cu substrate. Despite the unfavorable values of some material parameters, copper oxides are very promising p-type semiconductor for solar cells application.

This paper presents technological aspects and materials studies of oxidation products of copper sheets at relatively low temperatures between 200 and 400 °C and time in terms of minutes. We believe that compromise between the light absorption ability and charge carrier transport in imperfect material leads through the control over a layer thickness and morphology. Hence, this investigation is a good platform for further studies on recrystallization and doping in order to enhance the material properties.

Section snippets

Experimental

Copper sheet (ETP 3 N grate) of thickness of 0.3 mm were pretreated prior the oxidation stage. Three different kinds of treatment were compared and one of them was selected for further analysis: a) chemical etching, b) coarse polishing and c) fine polishing. Chemical treatment was performed using mixture of orthophosphoric, nitric and acetic acids. The copper samples were immersed in etching solution for 60 s and rinsed in isopropyl alcohol. For coarse polishing special polishing paste was

Results

At first the proper surface preparation was considered. The topography SEM images of the samples prepared by chemical etching, coarse and fine polishing were presented on Fig. 1. After chemical treatment the surface area was considerably developed as correlated with the matt appearance and the high value of haze factor H - close to 90%. Additionally, the oxidation process performed on chemically treated samples resulted in highly non-uniform growth of oxide layer. Hence, this kind of treatment

Summary and conclusions

In this studies the low temperature thermal oxidation process of copper sheets was investigated. The temperatures of 200, 250, 300 and 400 °C were considered retaining the constant time within the heating zone (10 min) At first the surface pretreatment was verified resulting in selection of coarse polishing with polishing paste as the proper choice for uniform oxide growth. As the temperature was the most significant factor the performed analysis enable to make the following observations and

CRediT authorship contribution statement

Z. Starowicz: Investigation, Conceptualization, Methodology, Resources, Formal analysis, Writing - original draft, Writing - review & editing, Validation, Visualization. K. Gawlińska – Nęcek: Investigation, Conceptualization, Resources. R.P. Socha: Investigation. T. Płociński: Investigation. J. Zdunek: Investigation. M.J. Szczerba: Formal analysis. P. Panek: Conceptualization, Writing - review & editing, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This publication is co-founded by the National Centre for Research and Development in the frame of the project No. TECHMATSTRATEG/2/409122/3/NCBR/2019 entitled: “Development of technology for manufacturing of functional materials for application in non-silicon photovoltaic cells".

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