Measuring the exciton diffusion length of C 60 in organic planar heterojunction solar cells

Planar copper phthalocyanine (CuPc)/C 60 heterojunction solar cells with a 2 nm layer of bathocuproine (BCP) inserted into the C 60 layer were fabricated and characterized. The 2 nm BCP layer in the devices was used as an electronically selective sieve allowing the electron current through but blocking the excitons in the C 60 layer. By combining the experimental results with the optical modeling, the effective triplet exciton diffusion length in C 60 was confirmed to be 30–35 nm under the device working condition. We demonstrate a simple, useful method to determine the exciton diffusion lengths of organic electron acceptors.