The authorsw ould like to acknowledgteh e continuousfi nancials upportf rom the NetherlandAs gencyfor Energy and the Environmen(tN OVEM) for the researcho f a-Si: H-baseds olarcells. This researchh as been also supportedb y the Netherlands Organisationfo r ScientificR esearch( NWO);
the NetherlandTs echnologyF ounda-tion (STW);
and the EuropeanC ommunityJO ULE ProgrammeT.h e authorsh ighly appreciatteh e co-operationw ith Dr. R.E.I. Schropp from Utrecht University;
w ho providedu s with data on the a-Si: H layers and the tandems olarcell. The authors wish to thank Vladimir Kuznetsovf or valuabled iscussions;