Poly-Si thin films by metal-induced growth for photovoltaic applications

Poly-Si films were produced using a metal-induced growth technique by sputtering from an n-type Si target onto a 50 nm thick Co seed-layer at 625degreesC. Silicon grew heteroepitaxially on the CoSi2 layer formed due to the reaction between the sputtered Si atoms and Co at the beginning stage of deposition. A 5 mum thick Si film with grain features up to 1 mum was produced on the thin and flexible tungsten substrate by using a two-step sputtering method. The films also have a natural texture structure on the surface that is strongly recommended in thin-film solar cells in order to obtain high current density by increasing incident light trapping. After post-sputtering annealing at 700degreesC, the measured minority carrier lifetime for poly-Si film was 1.33 mus which shows the film to be suitable for photovoltaic applications. To explore the photovoltaic applications by using MIG poly-Si films, Au/n-Si Schottky photodiodes were fabricated due to the process simplicity. The effects of different parameters, which include film doping density, active-layer thickness, Si film surface conditions and hydrogenation, were studied. It was found that with the increasing of doping density, the open-circuit voltage (V-oc) increased while short-circuit current density (J(sc)) decreased. Increasing the poly-Si active-layer thickness tended to improve the light absorption with an increased J(sc), but the V-oc was decreased due to a higher value of reverse saturation current. Because the metal/semiconductor interface condition facilitates the carrier transport in Schottky devices, the earlier study of modifying the Si surface by polishing showed an improved V-oc. The overall photo response was further improved by plasma hydrogenation. (C) 2004 Elsevier B.V. All rights reserved.