Low-Temperature Growth of Amorphous Silicon Films and Direct Fabrication of Solar Cells on Flexible Polyimide and Photo-Paper Substrates

Direct deposition of hydrogenated amorphous silicon (a-Si:H) thin films and fabrication of solar cells on polyimide (PI) and photo-paper (PP) substrates using a rf-plasma-enhanced chemical vapor deposition technique is reported. Intrinsic amorphous silicon films were deposited on PI and PP substrates by varying the substrate temperature (T (s)) over 70-150A degrees C to optimize the deposition parameters for best quality films. The films deposited on both PI and PP substrates at a temperature as low as 70A degrees C showed a photosensitivity (sigma (ph)/sigma (d)) of nearly 4 orders of magnitude which increased to 5-6 orders of magnitude when the substrate temperature was increased to 130-150A degrees C. The increase in sigma (ph)/sigma (d) is due to the presence of a few nanometer-sized crystallites embedded in the film. Solar cells (n-i-p) were fabricated directly on PI, PP and Corning 1737 glass (Corning) at 150A degrees C for different thicknesses of an intrinsic amorphous silicon layer (i-layer). With the increase in i-layer thickness from 330 nm to 700 nm, the solar cell efficiency was found to increase from 3.81% to 5.02% on the Corning substrate whereas on the flexible PI substrate an increase from 3.38% to 4.38% was observed. On the other hand, in the case of cells on PP, the i-layer thickness was varied from 200 nm to 700 nm and the best cell efficiency 1.54% was obtained for the 200-nm-thick i-layer. The fabrication of a-Si (n-i-p) solar cells on photo-paper is presented for the first time.