Tandem photovoltaic cells based on low-concentration donor doped C60

We demonstrate that enhanced efficiency can be achieved in organic tandem photovoltaic cells using identical bulk heterojunction subcells based on 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane doped C-60 in series. Power conversion efficiencies greater than 4% have been achieved in 2- and 3-stack tandem cells, an improvement of at least 30% over the single-stack cell. (C) 2011 Published by Elsevier B.V.