Improvement of Transparent Metal Top Electrodes for Organic Solar Cells by Introducing a High Surface Energy Seed Layer

We present highly transparent and conductive silver thin films in a thermally evaporated dielectric/metal/dielectric (DMD) multilayer architecture as top electrode for efficient small molecule organic solar cells. DMD electrodes are frequently used for optoelectronic devices and exhibit excellent optical and electrical properties. Here, we show that ultrathin seed layers such as calcium, aluminum, and gold of only 1 nm thickness strongly influence the morphology of the subsequently deposited silver layer used as electrode. The wetting of silver on the substrate is significantly improved with increasing surface energy of the seed material resulting in enhanced optical and electrical properties. Typically thermally evaporated silver on a dielectric material forms rough and granular layers which are not closed and not conductive below thicknesses of 10 nm. With gold acting as seed layer, the silver electrode forms a continuous, smooth, conductive layer down to a silver thickness of 3 nm. At 7 nm silver thickness such an electrode exhibits a sheet resistance of 19 / and a peak transmittance of 83% at 580 nm wavelength, both superior compared to silver electrodes without seed layer and even to indium tin oxide (ITO). Top-illuminated solar cells using gold/silver double layer electrodes achieve power conversion efficiencies of 4.7%, which is equal to 4.6% observed in bottom-illuminated reference devices employing conventional ITO. The top electrodes investigated here exhibit promising properties for semitransparent solar cells or devices fabricated on opaque substrates.