The efficiency of photosynthetic molecularly based electronic devices

We present in this paper a detailed treatment of the transport properties of photosynthetic molecularly based electronic devices. A primary problem we address are the conditions that must be obtained for molecular electronic devices to efficiently effect charge separation. We study a general d=1 site-disordered stochastic hopping model consisting of N impurities and a random distribution of fluctuating hopping rates. Each impurity is treated as a symmetric well. A relaxation mechanism is included on the first site to simulate decay of the initially created excitation. Using the formalism recently developed by us [K. Kundu and P. Phillips, Phys. Rev. A (in press)], we derive exact expressions for the quantum yield, transit times, and the moments of the distribution of transit times. We show explicitly that the efficiency of charge separation in molecular electronic devices is determined almost entirely by the efficiency of the first electron transfer step. This result is relevant to recent experimental results on charge separation in the quinone-viologen and chlorophyllic systems. Nanosecond electron transit times are predicted for molecular assemblies consisting of five molecular subunits and an effective activation energy of 0.55 eV. The significance of this result to the feasibility of molecular photodiodes and recent experiments on chlorophyll-like systems is discussed.