Nanoscale aluminum dimples for light-trapping in organic thin-films

Integration of nanostructures in organic solar cells (OSCs) has been investigated intensively in the past few years as an alternative way for enhancing the power conversion efficiency of the devices. Incorporating structured electrodes in the solar cell architecture holds potential for light absorption improvement in the active layer of the devices. A prospective, cheap and large-scale compatible method for structuring the electrodes in OSCs arises by the use of anodic aluminum oxide (AAO) membranes.
In the present work, aluminum films of high purity and low roughness are formed via e-beam evaporation of a few nanometers of aluminum followed by a micrometer layer of aluminum formed via sputter deposition. The samples are then anodized to form nano-scale pores of controlled sizes. The anodization of the prepared samples occurs in an electrochemical cell in H2SO4, H2C2O4 and H3PO4 solutions. Electrolyte solution variation and anodization parameters (sample temperature, voltage) control, allows for AAO pore diameter and interpore distance tuning. The fabricated AAO is selectively etched in H2CrO4/H3PO4 mixtures, in order to reveal the underlying aluminum nanoscale dimples, which are present at the bottom of the pores.
For the characterization of their light-trapping properties, the dimples are covered with a thin layer of PMMA, and the impact from different dimple dimensions is investigated experimentally via laser ablation based measurements of the field enhancement, which is compared to FDTD calculations to further explain the mechanisms of light-trapping in these structures. These dimples can potentially serve as nanostructured electrodes in P3HT/PCBM bulk heterojunction organic solar cells.