Contacts for semitransparent organic solar cells

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.