Unusually High Optical Transmission in Ca:Ag Blend Films: High-Performance Top Electrodes for Efficient Organic Solar Cells

Highly transparent electrodes are demonstrated based on thermally evaporated calcium:silver blend thin–films, which show unusually high transmission well above the expectations from bulk material properties and thin film optics. These electrodes exhibit a low sheet resistance of 27.3 Ω/, combined with an extraordinarily high mean transmittance of 93.0% in the visible spectral range (σdc/σopt = 186.7), superior to the commonly used inorganic electrodes made from indium tin oxide (ITO). Additionally, the metal blend electrode is flexible, showing a constant sheet resistance down to a bending radius of 10 mm and can be employed on top of organic devices without causing damage to the organic material. The spontaneously formed unique microstructure of a polycrystalline Ag network with randomly distributed nanoapertures, surrounded by a calcium shell, enables broadband transmittance enhancement due to amplified plasmonic coupling. Consequently, top‐illuminated organic solar cells using such metal blend electrodes achieve a power conversion efficiency of 7.2% (which defines a new record for top illuminated organic solar cells) and even exceed the efficiency of similar bottom‐illuminated reference solar cells (6.9%) employing common ITO electrodes. Calcium:silver blend electrodes show extraordinarily high mean transmittance of 93.0% in the visible spectral range, low sheet resistance of 27.3 Ω/□, and excellent flexibility, superior to commonly used indium tin oxide. The unique microstructure enables broadband transmittance enhancement due to plasmonic coupling. Top‐illuminated OPV cells using such metal blends as top electrode achieve an outstanding power conversion efficiency of 7.2%.