Optimal energy levels and offsets of an organic donor/acceptor binary-type solar cell have been analyzed using the classic Marcus electron transfer theory to identify the most efficient photo-induced charge separation. This study reveals that, an exciton quenching parameter (Y-eq) yields one optimal donor/acceptor energy offset where the photo-induced exciton is converted to charges most efficiently, and a recorribination quenching parameter (Y-rq) yields a second optimal donor/acceptor energy offset where the ratio of charge separation rate constant over charge recombination rate constant becomes maximum. Another energy offset is also identified where charge recombination becomes most severe. This information would be useful for evaluating and fine tuning frontier energy levels of a donor/acceptor pair for optimum solar cell applications. (C) 2004 Elsevier B.V. All rights reserved.