Efficient large area semitransparent organic solar cells based on highly transparent and conductive ZTO/Ag/ZTO multilayer top electrodes

Abstract

We present transparent multilayer electrodes prepared by RF sputtering of zinc tin oxide (ZTO) and thermal evaporation of silver (Ag) as top contact for organic electronics. Specifically we study the electrical, optical and structural properties of the ZTO/Ag/ZTO (ZAZ) electrode. In the first step, we analyze the origin of high conductivity and high transparency by studying structural, electrical and optical properties of ultra thin Ag films. Secondly, we demonstrate that continuous and highly conductive Ag films can be deposited down to a thickness of 6 nm on top of ZTO. Furthermore we show, that owing to the stabilizing effect of the sputtered ZTO cap, ZAZ is substantially more robust than ZTO/Ag (ZA). As a first result we achieve a low sheet resistance of 5 Ω/sq and a transmissivity of 82% in the visible region of light. These values are compared to common transparent and semitransparent electrode materials deposited at room temperature like indium tin oxide (ITO). As an application we realized an optimized ZAZ structure as highly conductive and transparent top contact for large area semitransparent bulk hetero junction (BHJ) polymer solar cells (OSC) with an inverted device architecture. To expose the outstanding properties of the novel multilayer electrode we compare semitransparent OSCs using common ITO and ZAZ as top electrode cells with a large cell area >2 cm 2.