Using deposition rate to increase the thermal and kinetic stability of vapor-deposited hole transport layer glasses via a simple sublimation apparatus

Abstract

Deposition rate is known to affect the relative stability of vapor-deposited glasses; slower rates give more stable materials due to enhanced mobility at the free surface of the film. Here we show that the deposition rate can affect both the thermodynamic and kinetic stabilities of N,N-bis(3-methylphenyl)-N,N-diphenylbenzidine (TPD) and N,N-di-[(1-naphthyl)-N,N-diphenyl]-1,1'-biphenyl)-4,4'-diamine (NPD) glasses used as hole transport layers for organic light emitting diodes (OLEDs). A simple, low-vacuum glass sublimation apparatus and a high vacuum deposition chamber were used to deposit the glass. 50 μm thick films were deposited in the sublimation apparatus and characterized by differential scanning calorimetry while 75 nm thick films were prepared in the high vacuum chamber and studied by hot-stage spectroscopic ellipsometry (SE). The thermodynamic stability from both preparation chambers was consistent and showed that the fictive temperature (T) was more than 30 K lower than the conventional glass transition temperature (T) at the slowest deposition rates. The kinetic stability, measured as the onset temperature (T) where the glass begins to transform into the supercooled liquid, was 16-17 K greater than T at the slowest rates. T was systematically lower for the thin films characterized by SE and was attributed to the thickness dependent transformation of the glass into the supercooled liquid. These results show the first calorimetric characterization of the stability of glasses for OLED applications made by vapor deposition and the first direct comparison of deposition apparatuses as a function of the deposition rate. The ease of fabrication will create an opportunity for others to study the effect of deposition conditions on glass stability.

Vapor deposition of a nonmesogen prepares highly structured organic glasses

“…3 A and B, the glass deposited at a lower rate (Fig. 3A) shows a further increase in order as each molecule spends more time at the highly mobile free surface (42,43). Therefore, given a highly orientationally ordered equilibrium free surface, lower deposition rates and higher substrate temperatures result in glasses with more ordered molecules.…”

Ultrastable glasses: new perspectives for an old problem

“…It is remarkable that the influence of the growth rate on the degree of stability is much stronger in metallic glasses than in the case of organic molecules. As an example, Luo et al achieved a variation of onset temperature with respect to T g ( T on /T g ) of 7.1% over one order of magnitude change of growth rate between 1 and 10 nm/min, while in the case of the organic semiconductor TPD, Kearns et al reached a lower T on /T g of 3% over more than two orders of magnitude variation in growth rate, from 0.3 to 70 nm/s [61]. This difference could be related to the fragility of the glass-forming systems [62].…”

Multiple glass transitions in vapor-deposited orientational glasses of the most fragile plastic crystal Freon 113

“…That is, samples vapor deposited directly as a GC, GCVD, are kinetically more stable than samples quenched from the PC, GCFC. Several indirect evidences do support the adscription of this peak to a GC to PC transition and not to the ordered crystal towards the PC (OC to PC transition) that according to Kolesov et al 9 occurs at 82.5 K. i) Films grown within this temperature interval at different growth rates (between 0.5-2.5 nm/s) show different onset temperatures similarly to what is observed in vapor-deposited glasses, 33 as shown in figure S1. On the contrary, the OC should exhibit a well-defined onset temperature nearly independent of the growth rate in this small range variation.…”