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

Abstract

Vapor-deposited glass mixtures of organic semiconductors commonly serve as active layers in organic electronic devices, whose lifetime and performance are strongly influenced by the stability and structure of these mixed glasses. Here, we study the stability and anisotropic molecular packing of six codeposited organic semiconductor glass mixtures with a 50:50 weight ratio by differential scanning calorimetry and spectroscopic ellipsometry. We find that all six binary systems exhibit high kinetic stability and significantly reduced enthalpy relative to the corresponding liquid-cooled glassy mixtures (ultrastable behavior), even for systems where the glass-transition temperatures of the components differ by more than 90 K. Furthermore, we demonstrate that the birefringence of a codeposited glass mixture, a measure of its anisotropic packing, can be predicted from the birefringence of glasses of the two pure components. These results for stability and structure are expected to be applicable to other codeposited organic semiconductor glass mixtures, so long as the two components mix well in the glass and individually can form ultrastable glasses. Therefore, our findings are significant for the design of novel electronic devices with enhanced device lifetime and increased operational efficiency.

High Density Two-Component Glasses of Organic Semiconductors Prepared by Physical Vapor Deposition

“…In our comparison, we utilized the weight fraction of the two materials rather than the volume fraction, but for the NPD/TPD system this is inconsequential. It is notable that the model is working not only for the 50:50 mixtures (as expected based upon the results of ref ) but also for the dilute mixtures.…”

“…In this work, we show that molecular orientation (as inferred from the birefringence) in NPD:TPD mixtures with different compositions is very similar to orientation in the neat materials, as long as the comparison is made at the same value of T sub / T g . Previous reports on PVD glasses of two or more components fall mostly into two categories: (1) focusing only on dilute mixtures (less than 10%) for emitter and host systems, ,,,, or (2) PVD glasses with 50:50 ratio. , In terms of PVD glasses with 50:50 mol ratios, our efforts extend the recent work of Cheng and co-workers, , by studying dilute mixtures as well as 50:50 mixtures. We showed that the birefringence mixing rule that they proposed (based upon the surface equilibration mechanism) works in both regimes.…”

“…In previous reports on multicomponent PVD glasses, ellipsometric data have only been utilized to characterize kinetic stability; no previous reports provide density measurements for multicomponent PVD glasses. In contrast, for single component PVD glasses, in situ ellipsometry has frequently been employed for detecting the kinetic stability and density increases for as-deposited thin glassy films, in comparison to liquid cooled glasses. ,− ,,,, In our experience using ellipsometry to study other pairs of organic semiconductors, , we have observed that the onset temperature was reproducible, while the relative density was not. We assume that this somehow results from the failure of the ellipsometric model for multi component films.…”

“…A recent study by Cheng et al found that codeposition of organic semiconductors, TPD and m-MTDATA, in 50:50 ratio yielded glasses with exceptional kinetic and thermodynamic stability; these results were interpreted based on the surface equilibration mechanism . In a second paper, Cheng et al identified an additional five codeposited systems of organic semiconductors (all 50:50 mixtures) that exhibited high kinetic stability in spite of differences in T g of up to 96 K . In contrast, K i et al.…”

“…A recent paper from Cheng et al developed a prediction for birefringence of codeposited glasses based upon the surface equilibration model where Δ n represents the birefringence and φ means volume fraction of each components: normalΔ n normalA normalB ( T s u b , A B T g , A B ) = φ normalA * normalΔ n normalA ( T s u b , A T g , A ) + φ normalB * normalΔ n normalA ( T s u b , B T g , B ) This approach assumes that the orientation of each component in the mixture is controlled by the normalized substrate temperature but is otherwise independent of the composition. Cheng et al compared this prediction to the observed birefringence of 50:50 codeposited mixtures of organic semiconductors and found good agreement.…”