Photogeneration of charges in microcrystalline chlorophyll a

The electric-field and temperature dependence of hole photogeneration in chlorophyll a (Chl a) have been analyzed in terms of electric-field assisted thermal dissociation of charge pairs based on Onsager theory. An excellent agreement between the experimental and theoretical values of the slope-to-intercept ratio, S/ I, for the plot of photogeneration efficiency vs. electric field at low field strengths provides a proof for the applicability of the Onsager approach to the photogeneration of charges in Chl a. A value of 19 nm has been obtained for Coulomb capture radius, r c, from S/ I. From the temperature dependence of photogeneration, the initial separation, r 0, of photogenerated electron-hole has been evaluated, and has a value of 1.24 nm. This smaller r 0 compared to r c leads to a feeble dissociation probability of electron-hole pairs into free carriers, and may, among other factors, explain the low power conversion efficiencies of Chl a photovoltaic cells