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This study investigates the impact of deposition rate on the phase separation of vapor-deposited organic thin films, specifically a blend of TPD and DO37, revealing how kinetic effects dictate the resulting domain structures. By employing power spectral density analysis of AFM images and complementary RSoXS measurements, the authors identify two distinct length scales that emerge from varying deposition rates, demonstrating that surface mobility during deposition allows for controlled phase separation. The findings highlight the potential for tuning domain sizes to optimize the performance of organic electronic devices through strategic deposition techniques.
Varying the deposition rate can finely tune the domain sizes in organic thin films, unlocking new design strategies for organic electronic devices.
The characteristic length scale of phase-separated organic thin film blends is a critical structural parameter governing the performance and functionality of organic electronic devices. The arrested morphologies of vapor-deposited organic thin films result from the interplay between thermodynamic driving forces and kinetic constraints during deposition. Here, we aim to isolate the role of kinetic effects in phase separation by varying the deposition rate at a constant substrate temperature for a co-deposited molecular glass blend of N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD) and Disperse Orange 37 (DO37). The dependence of morphology on deposition rate is quantified using power spectral density (PSD) analysis of atomic force microscopy (AFM) images. Two distinct deposition rate-dependent length scales at the surface reveal how deposition kinetics directly influence domain size and film topography. Complementary Resonant Soft X-ray Scattering (RSoXS) measurements indicate that phase separation extends throughout the film thickness. These observations are consistent with the surface equilibration mechanism previously described for homogeneous vapor-deposited films, in which enhanced surface mobility allows molecules in the growing film to partially equilibrate into distinct surface-templated states during deposition. In the current work, this mechanism allows the multi-component blend to phase separate and coarsen into a structure with multiple length scales before kinetically arresting to an extent that depends on the deposition rate. The demonstration of finely tunable domain size with deposition rate provides strategies to design new organic electronic devices with desired morphologies.
