Band-like Charge Photogeneration at a Crystalline Organic Donor/Acceptor Interface

Organic photovoltaic cells possess desirable practical characteristics, such as the potential for low-cost fabrication on flexible substrates, but they lag behind their inorganic counterparts in performance due in part to fundamental energy loss mechanisms, such as overcoming the charge transfer (CT) state binding energy when photogenerated charge is transferred across the donor/acceptor interface. However, recent work has suggested that crystalline interfaces can reduce this binding energy due to enhanced CT state delocalization. Solar cells based on rubrene and C-60 are investigated as an archetypal system because it allows the degree of crystallinity to be moldulated from a highly disordered to highly ordered system. Using a postdeposition annealing method to transform as-deposited amorphous rubrene thin films into ones that are highly crystalline, it is shown that the CT state of a highly crystalline rubrene/C-60 heterojunction undergoes extreme delocalization parallel to the interface leading to a band-like state that exhibits a linear Stark effect. This state parallels the direct charge formation of inorganic solar cells and reduces energetic losses by 220 meV compared with 12 other archetypal heterojunctions reported in the literature.