The optical absorption in organic solar cells, and hence their
efficiency to convert sunlight into electricity, is limited
by both a confined spectral absorption range and the very thin active layers which are required since polymers have short charge carrier diffusion
lengths. We propose to make the most of this
apparent 'drawback' by using transparent contacts on both sides of
the diode, leading to brilliant red-coloured semitransparent solar
cells. Such cells are interesting for new, aesthetically appealing
applications where coloured glasses or foils with the additional
benefit of providing electric power are desired. Maybe even more
importantly – such semitransparent solar cells are essential for
mechanically stacked tandem arrays where two subcells are
combined. We investigated different cathodes based on Al-doped ZnO and thin LiF and Al layers. Apparently, the different cathode layer preparation conditions strongly influence the cell
performance. Our standard LiF/Al cells do
not usually require annealing for improved efficiencies, in
contrast to cells with sputtered cathodes. For the latter, a
thermal post-treatment can greatly enhance the performance,
depending on the sputtering process. We could demonstrate power conversion efficiencies of up to 3.4% for single cells and up to 4.2% for stacked tandem arrays.