Improved electron collection in fullerene via caesium iodide or carbonate by means of annealing in inverted organic solar cells

Inverted organic photovoltaic cells (IOPVCs), based on the planar heterojunction C-60/CuPc, were grown using MoO3 as anode buffer layer and CsI or Cs2CO3 as cathode buffer layer (CBL), the cathode being an ITO coated glass. Work functions, Phi(f), of treated cathode were estimated using the cyclic voltammetry method. It is shown that Phi(f) of ITO covered with a Cs compounds is decreased. This decrease is amplified by the annealing. It is shown that the thermal deposition under vacuum of the CBL induces a partial decomposition of the caesium compounds. In parallel, the formation of a compound with the In of ITO is put in evidence. This reaction is amplified by annealing, which allows obtaining IOPVCs with improved efficiency. The optimum annealing conditions is 150 degrees C for 5 min.