Inverted semi-transparent organic solar cells with spray coated, surfactant free polymer top-electrodes

Depositing a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) buffer layer or top-electrode from aqueous solution on a non-polar active layer in inverted organic solar cells is commonly considered a very challenging task. In this work we utilize spray deposited PEDOT:PSS seeds to effectively reduce the surface free energy atop the active layer in poly-hexylthiophene polymer solar cells before applying a PEDOT:PSS top-electrode. Though aqueous PEDOT:PSS is repelled from non-polar surfaces, very small droplets can adhere to the surface. The distribution of the sprayed PEDOT:PSS droplets can be controlled via the substrate temperature and the material flow rate. The less time the droplets have to contract along the surface before drying, the better is the surface seed coverage and the more homogenous is the formation of a subsequently deposited PEDOT:PSS electrode. The respective solar cells are semi-transparent and exhibit an overall power conversion efficiency η≈2%.