Stable Inverted Polymer/Fullerene Solar Cells Using a Cationic Polythiophene Modified PEDOT:PSS Cathodic Interface

A cationic and water-soluble polythiophene [poly[3-(6-pyridiniumylhexyl) thiophene bromide] (P3PHT(+)Br(-))] is synthesized and used in combination with anionic poly(3,4-ethylenedioxythiophene):poly(p-styrenesulfonate) (PEDOT:PSS)(-) to produce hybrid coatings on indium tin oxide (ITO). Two coating strategies are established: i) electrostatic layer-by-layer assembly with colloidal suspensions of (PEDOT:PSS)(-), and ii) modification of an electrochemically prepared (PEDOT:PSS)(-) film on ITO. The coatings are found to modify the work function of ITO such that it could act as a cathode in inverted 2,5-diyl-poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) polymer photovoltaic cells. The interfacial modifier created from the layer-by-layer assembly route is used to produce efficient inverted organic photovoltaic devices (power conversion efficiency similar to 2%) with significant long-term stability in excess of 500 h.