Active Layer Morphological Study of Small Molecules Vacuum Deposited Single Planar, Multilayer or Bulk Heterojunction Organic Solar Cells
Wassim Hojiej, Roshanak Radbeh, Benoit Brousse, Bernard Ratier and Andre Moliton
Solar cells achieved by vacuum sublimation of the CuPc-C60 donor-acceptor couple are compared according to their active layer structure: single planar, bulk or multilayer heterojunction. The best photovoltaic properties, measured in open air, are obtained with the single planar heterojunction ITO/PEDOT-PSS/CuPc/C60/Al with an optimisation of the donor and acceptor active layers and of the PEDOT-PSS hole injecting layer which acts as an optical spacer. The lower results obtained with the bulk heterojunction are rather attributed to a too intimate mixture of the donor and acceptor molecules giving rise to too small donor and acceptor domain size. In order to circumnavigate this problem, alternated donor-acceptor multilayer devices have been realised that show photovoltaic properties attributable rather to a bulk heterojunction than to stacked cells. Optimisation of these structures is more sensible to the number of bilayers and the sublimation growth rate than to the thickness of each layer.