1 The effect of spectral variations on the performance parameters of single and double junction amorphous silicon solar cells R. Gottschalg 1* , T. R Betts 1 , D.G. Infield 1 and M.J. Kearney 2 1 Centre for Renewable Energy Systems Technology, Department of Electronic and Electrical Engineering, Loughborough University, Loughborough, Leicestershire, LE11 3TU, UK 2 Advanced Technology Institute, School of Electronics and Physical Sciences, University of Surrey, Guildford, Surrey, GU2 7XH, UK * corresponding author: Tel.: +44-(0)1509-228148, Fax: +44-(0)1509-610031, e-mail: R.Gottschalg@lboro.ac.uk