Surface Equilibration Mechanism Controls the Stability of a Model Codeposited Glass Mixture of Organic Semiconductors

Abstract

While previous work has identified the conditions for preparing ultrastable single-component organic glasses by physical vapor deposition (PVD), little is known about the stability of codeposited mixtures. Here, we prepared binary PVD glasses of organic semiconductors, TPD (N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine) and m-MTDATA (4,4′,4″-Tris[phenyl(m-tolyl)amino]triphenylamine), with a 50:50 mass concentration over a wide range of substrate temperatures (T sub). The enthalpy and kinetic stability are evaluated with differential scanning calorimetry and spectroscopic ellipsometry. Binary organic semiconductor glasses with exceptional thermodynamic and kinetic stability comparable to the most stable single-component organic glasses are obtained when deposited at T sub = 0.78–0.90T g (where T g is the conventional glass transition temperature). When deposited at 0.94T g, the enthalpy of the m-MTDATA/TPD glass equals that expected for the equilibrium liquid at that temperature. Thus, the surface equilibration mechanism previously advanced for single-component PVD glasses is also applicable for these codeposited glasses. These results provide an avenue for designing high-performance organic electronic devices.

Generic Behavior of Ultrastability and Anisotropic Molecular Packing in Codeposited Organic Semiconductor Glass Mixtures

“…From this perspective, highly stable codeposited glass mixtures might be expected at deposition conditions, where both components can individually form ultrastable single-component PVD glasses. Recent reports on PVD glass mixtures of isomers 15 and a pair of organic semiconductors with similar T g values 16 support this viewpoint. However, it is unclear whether this result holds generically for organic semiconductors.…”

“…Notably, the T g,mixture values obtained here are in good agreement with those of their corresponding bulk mixtures with a 50:50 mass ratio, as shown in Figure S2. The DSC result of the bulk TPD/ m-MTDATA mixture can be found in ref 16. For each panel in Figure 1, the heat capacity is normalized to the heat capacity change, ΔC p , of the liquid-cooled glass mixtures during the glass-to-liquid transition.…”

“…It is worth noting that the difference in glass-transition temperatures of the two components, ΔT g , is as large as 96K, which is five times larger than a recent work. 16 Enthalpy. As shown in Figure 2, the codeposited glasses show a pronounced endothermic peak during the glass-toliquid transition, while their corresponding liquid-cooled glasses exhibit a step-like change in heat capacity with a negligible endothermic peak.…”

On the Orientation Mechanism of Nonpolar Dyes in Light-Emitting Guest–Host Systems

“…This observation shows that one cannot simply assume the validity of the surface equilibration mechanism for such cohost systems, which are actually widely used in modern OLEDs. In a recent publication, Cheng et al performed one of the first systematic studies into this question on co-evaporated mixtures of two organic semiconductors (m-MTDATA and TPD) with a relatively small difference in their T g ’s of only 20 °C . They suggest three criteria as prerequisites for the validity of this mechanism: (i) surface equilibration is valid for the two neat materials, (ii) both components are well miscible, and (iii) their T g ’s are not too different.…”

“…In a recent publication, Cheng et al performed one of the first systematic studies into this question on co-evaporated mixtures of two organic semiconductors (m-MTDATA and TPD) with a relatively small difference in their T g 's of only 20 °C. 53 They suggest three criteria as prerequisites for the validity of this mechanism: (i) surface equilibration is valid for the two neat materials, (ii) both components are well miscible, and (iii) their T g 's are not too different. For our cohost system of mCP and TCTA, the latter condition is clearly not fulfilled, so we can expect very different surface diffusivities of the two components, with the mCP molecules being probably orders of magnitude more mobile on the surface of the films grown at T S = 300 K. Thus, it is understandable that emitter alignment in our cohost mixtures is given by the orientation parameter they have in a neat mCP host.…”