Quantum auto-ionization of molecular excitons and photovoltaic conversion

This paper explores a novel way of charge separation (auto-ionization) of molecular excitons, by quantum tunneling through nano-size $p{-}n$ junction. This mechanism can dominate the standard one (i.e., when Frenkel exciton is ionized at donor or acceptor impurity sites) for very short, nano-size, $p{-}n$ junction, where the junction electric field can be strong for relatively small (on the order of $1$ V) voltage drop. Within a simple one-dimensional model for the depletion region of the $p{-}n$ junction (donor and acceptor reservoirs connected by a short molecular wire) we compute the quantum yield $Y_b$ for the tunneling exciton auto-ionization in the “bulk” of the depletion region. For modern organic photo-sensitive materials with $p{-}n$ junction size on the order of $10$–$20$ nm, $Y_b$ could be close to $1$. Such a high efficiency of the charge separation (one of the main factor entering figure of merit, indicating how good are photovoltaic conversion cells) makes this new mechanism potentially very perspective for the applications.