Open-circuit voltage limit caused by recombination through tail states in bulk heterojunction polymer-fullerene solar cells

The output open-circuit voltage of bulk heterojuntion polymer-fullerene solar cells exhibts an offset of Delta approximate to 0.3 V with the difference between the donor highest-occupied molecular orbital (HOMO) and the acceptor lowest-unoccupied molecular orbital (LUMO), as discussed by Scharber [Adv. Mater. (Weinheim, Ger.) 18, 789 (2006)]. We show that the energetic disorder of the electronic density-of-states (DOS) of acceptor and donor materials reduces the splitting of the electron and hole Fermi levels in bulk heterojunction solar cells, by restricting the accumulation of carriers into the tails of the DOS. A simple model based on the kinetic balance between photogeneration and recombination fluxes provides a lower limit to the energy offset Delta>(sigma(2)(n)+sigma(2)(p))/k(B)T in terms of the disorder parameters.