Optimization of metal-induced growth for the fabrication of microcrystalline Si thin film solar cells

Abstract

Metal-induced growth (MIG) of microcrystalline Si (/spl mu/c-Si) thin films has been proposed and studied due to potentially reduced process cost and decrease in the thermal budget. Large grain size Si films were produced by using Ni or Co as seed layers. The optimization of the process is very important in order to fabricate device quality /spl mu/c-Si thin films. In this paper, three important process optimizations are reported and discussed. Two-step sputtering controls the sputtering power and deposition rate at different stages of Si film deposition to produce large grain Si films with preferred orientation together with improved J/sub sc/ and V/sub oc/ for Schottky solar cells. A double seed layer method uses a thin Co layer on top of a Ni layer to reduce the Ni diffusion into the Si films, which decreased leakage current and increased the open-circuit voltage (V/sub oc/). Oxygen level and resulting thermal donor effect was suppressed by filtering the sputtering gas to 50 ppb oxygen level. The total charge state density in the Si film was reduced so that the J/sub sc/ was improved due to less oxygen impurity in the Si film. With the optimized condition (two-step sputtering, 5 nm-Co/25 nm-Ni double seedlayer, using an oxygen filter and annealing of the Si film at 800 /spl deg/C in forming gas (15% H/sub 2/), Au/n-Si (5 /spl mu/m) Schottky junction solar cells showed a J/sub sc/ and V/sub oc/ of 12 mA/cm/sup 2/ and 0.215 V, respectively. By passivating the MIG /spl mu/c-Si surface with hydrogenated nanocrystalline Si (nc-Si:H), the V/sub oc/ was improved to 0.31 V. Note that Schottky solar cells were used for convenience and may not be the final device of choice.