Nanoscale Dimples for Improved Absorption in and Efficiency of Organic Solar Cells

Organic solar cells (OSC’s) have attracted much attention in the past years due to their low costs, light weight and mechanical flexibility. A promising method for improving the power conversion efficiencies of the devices is by incorporating structured electrodes in the solar cell architecture. That way light absorption in the active layers of the devices can be improved. A cheap and large-scale production compatible method for structuring the electrodes in OSC’s is by the use of Anodic Alumina Oxide (AAO) membranes. Here, nano-scale pores of controlled dimensions are formed through anodic oxidation of sputter deposited high purity Al films. The Al deposition conditions are controlled in order to modify the roughness and the grain size of the Al layers, as those parameters critically affect the subsequent pore formation during the anodization process. The anodization of the Al layers occurs in an electrochemical cell in H2SO4, H2C2O4 and H3PO4 solutions, in order to tune the AAO pore diameter and interpore distance.
Following anodization, the fabricated AAO is selectively etched away in H2CrO4/H3PO4 mixtures, in order to reveal the underlying Al nanoscale dimples, which are present at the bottom of the pores. The light-trapping properties of these dimples are investigated as a function of their dimensions and ordering. The optical properties of the dimples are characterized mainly via reflection measurements, supported by laser ablation based measurements of the field enhancement [1]. The experiments are compared to FDTD calculations to further explain the mechanisms of light-trapping in these structures. The nanoscale dimples as light-trapping nanostructure are integrated into inverted P3HT/PCBM organic solar cell devices. The impact from the different dimple structures on the resulting power conversion efficiency of the devices is investigated and optimized for obtaining the highest device performance.
[1] Fiutowski J., Maibohm C., Kjelstrup-Hansen J., and Rubahn H.-G. Appl. Phys. Lett. 98, 193117 (2011)