Evidence of thermal transport anisotropy in stable glasses of vapor deposited organic molecules

Abstract

Vapour-deposited organic glasses are currently in use in many optoelectronic devices. Their operation temperature is limited by the glass transition temperature of the organic layers and thermal management strategies become increasingly important to improve the lifetime of the device. Here we report the unusual finding that molecular orientation heavily influences heat flow propagation in glassy films of small molecule organic semiconductors. The thermal conductivity of vapour-deposited thin-film semiconductor glasses is anisotropic and controlled by the deposition temperature. We compare our data with extensive molecular dynamics simulations to disentangle the role of density and molecular orientation on heat propagation. Simulations do support the view that thermal transport along the backbone of the organic molecule is strongly preferred with respect to the perpendicular direction. This is due to the anisotropy of the molecular interaction strength that limit the transport of atomic vibrations. This approach could be used in future developments to implement small molecule glassy films in thermoelectric or other organic electronic devices.

Thermal Properties of TPD-Based Organic Glasses

“…setting (κ xx +κ yy + κ zz )/κ T=320K =1. The resulting picture is consistent with experimental evidence [62] as shown in Fig. 4 where the thermal conductivity of the investigated TPD films is reported for three the different directions.…”

“…The calculated values of thermal conductivity resulted somewhat larger than those obtained experimentally. More specifically, at room temperature we get for the ISO sample 0.85 W m −1 K −1 to be compared with the experimental value 0.15 W m −1 K −1 [62]. This is very likely related to the adopted force-field, describing both intra-and intermolecular forces just approximately.…”

“…According to Fourier law, the heat flux J is proportional to the temperature difference ∆T through J=G∆T, where G is the thermal conductance. Considering the in-plane and through-plane values of the thermal conductivity obtained for the experimental samples [62] at T = 300 K (i.e. κ || = 0.175 W m −1 K −1 and κ ⊥ = 0.110 W m −1 K −1 ), we get the following results: G vdW,|| ∼ 292 MW m −2 K −1 and G vdW,⊥ ∼ 240 MW m −2 K −1 .…”

Kinetic arrest of front transformation to gain access to the bulk glass transition in ultrathin films of vapour-deposited glasses

“…23 To carry out this study, we have chosen the organic semiconductor TPD (N,N 0 -bis(3-methylphenyl)-N,N 0 -diphenylbenzidine), and the pharmaceutical molecule IMC (1-(4-Chlorobenzoyl)-5methoxy-2-methyl-3-indoleacetic acid), as they form glasses with enhanced stability and have been widely characterised as a function of deposition temperature. 2,3,15,16,22,[24][25][26][27][28][29][30] IMC shows a maximum of stability when deposited around 266 K (0.85T g ) 26 and a transition into the supercooled liquid governed in the first stages by a transformation front whose velocity depends on the deposition conditions of the IMC glass. 15,26 Spectroscopic ellipsometry measurements of thin film TPD glasses have shown that TPD also transforms via a propagating front under isothermal conditions.…”

Extreme Elasticity Anisotropy in Molecular Glasses

“…Analogously, it was recently reported that thermal conductivity in vapor‐deposited glasses is also correlated with molecular orientation, with the most efficient thermal transport occurring along the molecular long axes. [ 45 ]…”