Retarding Ion-Exchange between Conducting Polymers and Ionic Liquids for Printable Top Electrodes in Semi-Transparent Organic Solar Cells.

Semi-transparent organic solar cells (ST-OSCs) are considered to be an influential tool for aesthetic and economic building-integrated photovoltaics (BIPVs), which can be fabricated by printing technology. A poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) and ionic liquid (IL) composite has been considered as an electrode for ST-OSCs because of its high electrical conductivity, high transparency and printability. However, we found that the introduction of IL into PEDOT:PSS solution for enhancing its electrical conductivity results in 1) non-reliable printing of PEDOT:PSS/IL composite films because of gradual gelation of the mixture solution and 2) the production of chemically reactive ion pairs during ion exchange between PSS and ionic liquid, which induces the oxidation of the underlying organic semiconductors during printing. To solve these problems, we developed a sequential printing method using pristine PEDOT:PSS and IL solutions to retard ion exchange, thus preventing chemical doping of organic semiconductors by newly generated ion pairs. Finally, by using only solution processes, we demonstrate efficient ST-OSCs with a printed PEDOT:PSS/IL composite as top electrode, exhibiting a power conversion efficiency of 6.32% at an average visible transmittance of 35.4%.