Control of molecular orientation of organic p–i–n structures by using molecular templating effect at heterointerfaces

Structural control of the organic intrinsic layer (i-layer), which consists of a mixture of p- and n-type organic semiconductor materials, and the organic p–i–n triple-layer structure has been investigated. Significant modification of these structures, which used metal-free phthalocyanine (H2Pc) and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) as the p- and n-type materials, respectively, was observed by introducing a modification layer deposited on a glass substrate. The orientation of the molecular planes of both H2Pc and PTCDA in the i-layer (H2Pc:PTCDA mixture) and p–i–n structure (H2Pc/i-layer/PTCDA) formed onto a PTCDA modification layer was parallel to the substrate surface. On the other hand, when H2Pc was used as the modification layer, the orientation of the molecular planes in the i-layer was found to be random. These results indicate that the internal structure of the organic mixed-layer is strongly affected by the organic layer-substrate interface. The enhancement in optical absorption observed for the orientation controlled p–i–n structure suggests the importance of the control of molecular orientation for improving the optical properties in organic device structures.