Morphology and Thickness Control of Thin Copper Films Prepared by Electrochemical Deposition onto Mo-sputtered Stainless Steel Substrates

Abstract

Chalcopyrite thin film solar cells of copper (Cu), indium (In), gallium (Ga) and selenium (Se) (CIGS) are receiving much attention, because they show high efficiency and longterm stability. 1 Methods to prepare CIGS light absorber layers include co-evaporation of elemental sources, 2 physical vapor deposition, 3 printing and electrochemical deposition. 4 Among those, electrochemical deposition is considered as an excellent technique for large area processing with good uniformity, low cost, and high material yield. 5 Electrodeposition of CIGS precursor films can be performed by using either single step or multistep process. Although Bhattacharya et al. have reported the development of a 15.4% efficiency CIGS cell by employing a single-bath electrochemical deposition following physical vapor deposition, 6 no other laboratory has reproduced it. The multistep process can be two or three stage process; some examples of such stacks are like Cu/In/Ga, Cu-Ga/In/Se, Cu/In/Ga-Se, Cu/In-Se/Ga-Se, etc. 7-9 For successful industrialization of CIGS solar cells, control of metal layers is required in morphology, thickness, and production speed. The objective of this study was to fabricate the targeted copper thin film onto Mo/SUS substrates by electrochemical deposition for CIGS thin film solar cell precursor. Morphology and thickness of copper thin film onto the Mo/SUS are very important factors because film’s surface condition can affect electrochemical deposition of the next layers of In, Ga, and Se before thermal selenization process. In this work, we wish to report our preliminary work on the electrochemical conditions that can produce a uniform thin copper metal layer onto the Mo sputtered stainless steel (Mo/SUS) flexible substrate for short time (sec). Mo/SUS substrate has the advantages of weight lighter than soda lime glass, of installation to the building integrated photovoltaic and of roll-to-roll manufacturing leading to substantial cost reductions due to enhanced productivity. All chemicals were analytical grade. Double-distilled water was used throughout. The electrochemical experiment was performed in a freshly prepared 0.1, 0.5 and 1 M CuSO4 baths containing various concentrations of (0.1, 0.5, 1, 1.5 M) H2SO4. Two-electrode configurations were used for deposition of Cu thin film. Hull cell test was performed in order to find the current density. The 100 × 100 mm 2 Mo/