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

Vapor 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 vapor 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 limits 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.

Figure 1

Measurement procedure of the in-plane thermal conductance. Scheme of the procedure followed to evaluate the thermal conductance of a multilayer stack. The substrate was set to the deposition temperature (in blue) and subsequently cooled or heated at the reference temperature of 295 K. The conductance value is obtained from the differential value before and after the measurement (in red).

Figure 2

Heater/sensors for 3ω-Völklein thermal conductivity and measured values for TPD. (a) Thermal conductivity vs deposition temperatures in TPD glasses. Data points correspond to two different measurement procedures as further explained in Methods. Blue-square points were determined from single independent evaporations and cleaning the sensor after each measurement. Triangles and circles correspond to data obtained with a continuous method using two different devices, in which multilayers are deposited sequentially and the differential conductance provides values for each individual layer. (b) Schematic design of the sensor and the direction of the heat flow. (c) Optical images of membrane (brown) and the heater/sensor (central metallic line) and upper view of the chip (d).

Figure 3

Thermal conductivity compared to the density of the stable glasses. Thermal conductivity and density variation in TPD as a function of deposition temperature relative to the glass transition temperature. Density data taken from Walters et al. [14] assuming a density of $1.08\mathrm{g}/\mathrm{c}{\mathrm{m}}^3$ for the conventional liquid. (Inset) MD simulations of the role of density in the thermal conductivity of an isotropic glass.

Figure 4

Correlation between thermal conductivity and molecular orientation. Comparison between the relative variation of the thermal conductivity and the order parameter obtained from Dalal et al. [13] plotted against the deposition temperature scaled by $T_{\mathrm{g}}$.

Figure 5

Thermal anisotropy ratio for experimental and simulated samples. Schematics showing the relative variation of thermal conductivity for the various experimental (left) and simulated (right) samples. Sample grown at 220 K (approximately 0.66 $T_{\mathrm{g}}$) is roughly equivalent to the simulated xy-ISO. The upper panel illustrates the structure of the experimental and simulated samples. The colored arrows indicate the direction of the heat flux. The yellow arrows in the simulated samples indicate the long molecular axis.

Figure 6

Thermal conductivity and van der Waals energy of simulated quasi-1D structures. (Top) Backbone and stacking quasi-1D structures. (Bottom) (a) Thermal conductivity vs length for the quasi-1D structures. (b) Configurational energy due to van der Waals interactions vs intermolecular spacing.

Figure 7

Thermal resistive network. Schematics of thermal resistance network in the in-plane direction (a) and the through-plane direction (b). $R_{\mathrm{VdW}}$ accounts for the strength of the intermolecular interactions and $R_{\mathrm{mol}}$ represents the intramolecular thermal resistance.