A new kind of n-type material N,N'-di-(2,4-difluorophenyl)-3,4,9,10-perylenetetracarboxylic diimide (D24DFPP) was co-deposited in vacuum with p-type copper phthalocyanine (CuPc) to obtain the composite material. Using this composite material, a photovoltaic (PV) device with the structure of ITO/CuPc/composite layer/D24DFPP/Al was made. A higher energy conversion efficiency (ECE) was obtained when compared with a traditional two layered PV device, and a certain tendency changed with varied relative proportions of the two materials. Through UV-Vis, XRD and AFM characterizations, it was found that the changes could be ascribed to the aggregate structure of the composite layer: the two semiconductors formed a structure of micro-phase separation after co-deposition. When the contents of the two molecules were almost equal, the size of the micro-zones formed by phase separation reached the minimum, resulting in a larger donor-accepter contact area and thus a higher charge separation efficiency; if the content was dominated by one component, conglomeration and phase separation increased, giving a lower charge separation efficiency and ECE. Further more, it was disclosed that the thickness of the composite layer also had an influence on ECE which would be optimized only if the factors of the light absorbance and the charge carrier diffusion came to a balance.

Key words: fluorinated perylene diimide, aggregate structure, photovoltaic device, micro-phase separation