Photogeneration of charges in microcrystalline chlorophyll a

The electric-field and temperature dependence of hole photogeneration in chlorophyll a (Chla) 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 Chla. A value of 19 nm has been obtained for Coulomb capture radius, rc, from S/I. From the temperature dependence of photogeneration, the initial separation, r0, of photogenerated electron-hole has been evaluated, and has a value of 1.24 nm. This smaller r0 compared to rc 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 Chla photovoltaic cells.