Efficient photon harvesting at high optical intensities in ultrathin organic double-heterostructure photovoltaic diodes

We demonstrate a method for efficient photon harvesting in organic thin films, thereby increasing the efficiency of organic photovoltaic cells. By incorporating an exciton-blocking layer (EBL) inserted between the photoactive organic layers and the metal cathode, we achieved an external power conversion efficiency of 2.4%+/- 0.3% in vacuum-deposited ultrathin organic bilayer photovoltaic (PV) cells employed in a simple light trapping geometry. Ultrathin (similar to 100 Angstrom) cells incorporating the transparent, conductive EBL have an internal quantum efficiency as high as 33%+/- 4% over a spectral region matched to the solar spectrum. The very thin organic layers have a low series resistance, allowing for efficient power conversion in organic PV cells under intense (> 15 suns) AM1.5 illumination. This device structure demonstrates that control of exciton diffusion in solid-state organic devices leads to a significant increase in the photon-to-carrier conversion efficiency. (C) 2000 American Institute of Physics. [S0003-6951(00)03419-7].