Polycrystalline silicon (poly-Si) thin films prepared by the solid phase crystallization (SPC) method were studied for photovoltaic materials. To improve the properties of the poly-Si thin film, a-Si films suited to the solid phase crystallization were investigated. It was found that TA/TO (peak height ratio of TA peak and TO peak) in Raman spectra of a-Si films had the good correlation with the average grain size, and a factor for the enlargement of grain size was an increase of distortion energy stored in a-Si films. The first adoption of a textured substrate was also performed, which had effects on the enlargement of grain size in poly-Si thin films be the SPC method. By applying the a-Si films with large TA/TO value on textured substrate, the n-type poly-Si thin-film with the grain size of 6 micrometers was fabricated and this film showed the Hall mobility of 623 cm2/Vs (electron density: 3.0 X 1015 cm-3). A new heterojunction technology, which was called by 'Artificially Constructed Junction (ACJ)', was developed by depositions of thin a-Si films on single- crystalline silicon (c-Si). In a solar cell using this technology, a high conversion efficiency of 18.7% was achieved. This is the highest value ever reported for solar cells in which the junctions were fabricated at a low temperature of less than 200 degree(s)C. In a thin-film poly- Si solar cell (thickness: 10 micrometers ) applying this technology, a conversion efficiency of 6.3% was also obtained and a collection efficiency of 51% was achieved at a wavelength of 900 nm. This high value attributes to the hole diffusion length of 11 micrometers , which is longer than the poly-Si thickness.