*Corresponding author. Tel.: +45-46-77-47-99; fax: +45-46-77-47-91. E-mail address: frederik.krebs@risoe.dk (F.C. Krebs). 0927-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2004.02.031 1. Introduction Polymer-based solar cells have been studied for the past decade, [1 –8]and the major promise in polymer-based solar cells as opposed to small molecule solar