Figure 1.

The operational principle of bulk-heterojunction polymer solar cells: formation of photo-induced excitons in D and A, respectively (1 & 1′); intra-molecular electron-hole recombination (2 & 2′); the excitons diffusion and dissociation at the D/A interface (3 & 3′); generation of charge-transfer (CT) states and these CT states dissociate into free charge carriers (electrons and holes) with an ultrafast quasi-adiabatic charge transfer process (4 & 4′); charge carriers that are transported through either D or A (5 & 5′) and then collected by the respective electrodes (6 & 6′); the separated charge carriers may recombine with each other (7, geminate recombination) before dissociation; the separated charge carriers may also collide and be recombined (8, bimolecular recombination or non-geminate recombination) before being collected by the respective electrodes (6 & 6′).