B-doped a-Si1-xCx:H films for a window layer of Si thin film solar cells have been prepared by the Cat-CVD method. It is found that C is effectively incorporated into the films by using C2H2 as a C source gas, where an only little C incorporation is observed from CH4 and C2H6 under similar deposition conditions. Using a-Si1-xCx:H films grown from C2H2, heterojunction p-i-n solar cells have been prepared by the Cat-CVD method. The cell structure is (SnO2 Asahi-U)/ZnO/a-Si1-xCx:H(p)/a-Si:H(i)/muc-Si:H(n)/Al. The obtained conversion efficiency was similar to5.4%. (C) 2003 Elsevier Science B.V. All rights reserved.