A roll-to-roll fabrication concept for manufacturing cost-effective thin film solar cells was developed. The model involves a combination of electroless and electrolytic depositions, and in this work, Polymer film Kapton®, electroless nickel (Ni), copper indium diselenide (CuInSe2, referred to as CIS), and ZnO were used to demonstrate the feasibility of the proposed concept.
Typical flexible solar cell consists of polymer substrate, back contact (BC) and photovoltaic layers. Polymer film Kapton® was metallized with electroless Ni, which served as the BC. The ideal duration for the electroless Ni deposition was determined to be between 2 to 5 minutes. The product was heat treated at 245oC for 45 minutes. The photovoltaic layer (CIS) was plated onto the electroless Ni back contact at various potentials (–0.5V to –1.1V vs. Ag/AgCl), and at different electrolyte circulating rate (0.3ml/s to 6.2ml/s). The CIS were also deposited onto different roughness back contacts to study the effect of surface roughness on the deposition quality. The deposited CIS were subjected to heat treatment at 390oC up to 2 hours in argon furnace. The materials were characterized using Atomic Force Microscope (AFM), Environmental Scanning Electron Microscope (ESEM), Energy Dispersive X-Ray Spectroscopy (EDS), and X-ray Diffraction (XRD). The results provided the basis for studying the effects of deposition parameters on the quality of the thin film.
The Ni produced by the electroless method used in this investigation was amorphous. Although annealing did not improve its crystallinity, it affected the nano-scale cluster packing and the roughness of the Ni. Deposition potentials and electrolyte transport properties were proven to influence the atomic composition and morphology of produced film, while the effects of back contact roughness was inconclusive. Indium was found to favor lower flow rates and lower deposition potentials. Stoichiometric composition of CuInSe2 was achieved with electrolyte recirculating rate of 1ml/s. The CIS crystallinity was improved by annealing. The major diffraction peaks were identified to be CIS (112), (220), and (116). The lattice parameter were found to be a= b=5.77Å, c=11.54Å. This work clearly demonstrated that electroless and electrolytic methods can be used to mass produce thin film for solar cells.
