Observation of Phase Transition from Dissolved Organic Semiconducting Materials to Solid Film during Drying

We studied the formation dynamics of low-molecular-weight organic semiconducting molecules of N,N'-bis[4-[bis(3-methylphenyl)amino]phenyl]-N,N'-diphenylbenzidine (DNTPD) during solvent evaporation. We dropped the DNTPD-toluene solution on a glass substrate and performed in situ measurements of mass, temperature, scattered light intensity, and photoluminescence (PL). We found that both light scattering and PL measurements were able to detect the onset of solid film formation from dissolved organic semiconducting materials. When a solid thin film forms during drying, sudden changes in scattered light and PL are observed. Furthermore, we also found that a period of time after the onset was necessary for completion of thin solid film formation. Observations and in situ PL measurement during thin-film formation by spin coating revealed that this period of time affected the optical properties of the film. This result indicates that quantitative information on the film formation process is quite important to obtain thin films with desired properties by coating and drying. Our in situ measurements were simple and practical approaches to monitor the formation dynamics of organic thin films during drying.