Intermediate layers for tandem structure of bulk heterojunction photovoltaic cells

Abstract

For advanced organic thin film photovoltaic cell, stacked structure of single cells, tandem structure, would be a key issue. Many kinds of tandem structure have been already reported. When an appropriate intermediate layer was inserted between the single cells, open circuit voltage (Voc) can be doubled compared with the single cell. For small molecules, vacuum evaporation can be applicable for fabrication. Systematic investigations have been made to reveal the requirement for the intermediate materials. Quite thin, a few nm thick, metal layer can act as intermediate layer[1]. The metal cannot form continuous layer but island lake structure in such small amount. On the other hand, the combination of the metal oxide (such as ZnO, TiO2 and ITO) and PEDOT:PSS are used for the intermediate layer. We need to reveal minimum requirement for the intermediate materials for polymer based bulkhererojunction cells for low-cost high performance organic photovoltaic cells. We have developed a polymer thin film preparation technique, Evaporative Spray Deposition using Ultradilute Solution (ESDUS)[2]. This method has enabled fabricating organic thin films applicable to polymer light-emitting diodes, organic photovoltaics and organic field-effect transistors13 from highly diluted solutions of 1-10 ppm. Moreover, it has been exhibited that a successive polymer layer can be deposited without damaging the preceding polymer layer by use of a same solvent for each layer deposition. We conduct the systematic investigation of the intermediate materials. Onto the bottom cell/intermediate layer, top cell can be deposited by use of ESDUS.