Exciton diffusion length in microcrystalline chlorophyll a

The measurements of photocurrent as a function of depletion layer or Schottky barrier width have been performed to determine the exciton diffusion length (L) in microcrystalline chlorophyll a (Chl a). This method of determining L is particularly advantageous in that it does not require a prior knowledge of the absorption coefficient of the material as is usually the case with other, e.g., constant surface photovoltage or constant photocurrent techniques. A value of 180 Å has been obtained for L in Chl a. While this value is comparable to those for many organic semiconductors, it is much inferior to that for the minority carriers in inorganic photoconductors. The smaller diffusion length may, in part, explain the feeble power conversion efficiency of Chl a and other organic photovoltaic cells.