Substrate-driven switchable molecular orientation in bulk heterojunction films identified using infrared reflection absorption spectroscopy
This research evaluated the substrate effects of two organic molecules, zinc phthalocyanine (ZnPc) and coronene on a glass/Au/coronene substrate (coro sub) and coronene:C60 BHJ film on a glass/Au/ZnPc substrate (ZnPc sub). The interaction strength of the co-evaporated ZnPc:C60 bulk heterojunction (BHJ) films was measured. The molecular orientation angles of each molecule in the BHJ films were detected by in situ infrared reflection absorption spectroscopy. Coronene molecules in the coronene:C60 film on the ZnPc sub could be precisely controlled to attain a lying-down orientation owing to the strong π–π interaction between the coronene–ZnPc molecules. In contrast, the ZnPc molecules in ZnPc:C60 adopted a standing-up orientation on the coro sub owing to the weak π–π interaction between the ZnPc sub and ZnPc molecules. The present work presents an important finding which indicates that substrate-dependent molecular orientation switches may be possible for both transistor and solar cell applications.