Tunable molecular orientation and elevated thermal stability of vapor-deposited organic semiconductors

Physical vapor deposition is commonly used to prepare organic glasses that serve as the active layers in light-emitting diodes, photovoltaics, and other devices. Recent work has shown that orienting the molecules in such organic semiconductors can significantly enhance device performance. We apply a high-throughput characterization scheme to investigate the effect of the substrate temperature (T_substrate) on glasses of three organic molecules used as semiconductors. The optical and material properties are evaluated with spectroscopic ellipsometry. We find that molecular orientation in these glasses is continuously tunable and controlled by T_substrate/T_g, where T_g is the glass transition temperature. All three molecules can produce highly anisotropic glasses; the dependence of molecular orientation upon substrate temperature is remarkably similar and nearly independent of molecular length. All three compounds form “stable glasses” with high density and thermal stability, and have properties similar to stable glasses prepared from model glass formers. Simulations reproduce the experimental trends and explain molecular orientation in the deposited glasses in terms of the surface properties of the equilibrium liquid. By showing that organic semiconductors form stable glasses, these results provide an avenue for systematic performance optimization of active layers in organic electronics.Glasses (or amorphous solids) of low molecular weight organic compounds exhibit desirable properties for organic electronics. Because these materials are made from organic molecules, properties that depend on chemical identity such as optical absorptions, bandgap, and glass transition temperature can be tuned via chemical synthesis. These glasses have solid-like mechanical properties similar to those of crystalline materials, but offer morphological homogeneity, greater ease of processing, and nearly unlimited compositional tunability. An underappreciated feature of these materials, a result of their nonequilibrium nature, is that many different glasses can be prepared with the same chemical composition.There has been considerable recent interest in controlling molecular orientation in organic semiconducting glasses (1–7). Whereas one might expect all glasses to be isotropic because of their structural disorder, Yokoyama et al. and other groups have shown that molecular orientation in vapor-deposited glasses can be quite anisotropic (3, 4, 8, 9) and depend upon deposition conditions (3). It has recently been suggested that orientation resulting from deposition could be used as a figure of merit to identify promising compounds for these applications (10). Oriented materials can increase light outcoupling by a factor of 1.5 by directing emission out of the plane of the device (10–14). It has also been shown that oriented layers can improve device lifetime (15) and charge mobility (16–18). Given the potential utility of controlling molecular orientation in device layers (4, 5, 7), it is desirable to understand the extent to which molecular orientation can be tuned in glasses made from a particular compound and the mechanistic origins of this effect. Anisotropic glassy solids are also of interest for applications in optics and optoelectronics (19).Concurrently, other investigators have shown that vapor-deposited glasses can have desirable physical properties unobtainable by any other means, when the substrate temperature during deposition (T_substrate) is held somewhat below the glass transition temperature (T_g). Discovered using model glass formers and labeled “stable glasses,” these glasses have lower enthalpies (20), higher densities (21), and resist structural reorganization to higher temperatures than is possible with any other preparation route (22–24). The properties of stable glasses are explained by the high mobility of the free surface during the vapor deposition process (20, 25). Because of lowered constraints to motion (26), molecules near the free surface can adopt near-equilibrium packing arrangements during deposition even at temperatures where the bulk structural relaxation time is thousands of years (21, 27). Subsequent deposition traps this efficient packing into the bulk solid. Like organic semiconductors, stable glasses can be birefringent (21) and also anisotropic in wide-angle X-ray scattering (28, 29).Here we show that organic semiconductors form stable glasses, and that surface mobility during vapor deposition governs bulk molecular orientation in these materials. Using a high-throughput experimental scheme, we are able to efficiently characterize the effect of T_substrate on three organic compounds used in semiconducting devices: TPD, NPB, and DSA-Ph [Fig. 1E; N,N’-Bis(3-methylphenyl)-N,N’-diphenylbenzidine, N,N’-Di(1-napthyl)-N,N’-diphenyl-(1,1’-biphenyl)-4,4’-diamine, and 1–4-Di-[4-(N,N-diphenyl)amino]styryl-benzene, respectively]. We find that these compounds form stable glasses, and we show that the orientation of the vapor-deposited molecules is controlled by T_substrate/T_g and is nearly independent of the molecular aspect ratio. Using simulations, we show that anisotropic molecular orientation in the glass can be understood in terms of molecular orientation and mobility near the free surface of the equilibrium liquid. By connecting two apparently disparate bodies of work, we develop avenues for research on organic devices and the physics of glasses, and further the development of “designer” anisotropic solids.Open in a separate windowFig. 1.Schematic illustration of the experimental procedure. (A) Organic molecules are vapor-deposited in a vacuum chamber. A silicon substrate with a controlled range of temperatures allows simultaneous deposition of many glasses with different properties but identical chemical composition. (B) After vapor deposition, each glass is independently interrogated using spectroscopic ellipsometry with a focused beam. (C) Example optical constants for TPD at T_substrate = 215 K. The optical constants for light polarized normal to (z) and in the plane of the substrate (xy) can be independently determined (49). (D) Using the optical constants, the orientation order parameter, S_z, can be computed at each T_substrate. θ_z is the angle of the long molecular axis relative to the substrate normal and P₂ is the second Legendre polynomial. (E) Structures and glass transition temperatures for the three compounds studied.     

Towards the Bisbenzothienocarbazole Core: A Route of Sulfurated Carbazole Derivatives with Assorted Optoelectronic Properties and Applications

Ladder-type molecules, which possess an extended aromatic backbone, are particularly sought within the optoelectronic field. In view of the potential of the 14H-bis[1]benzothieno[3,2-b:2’,3’-h]carbazole core as a p-type semiconductor, herein we studied a set of two derivatives featuring a different alkylation patterning. The followed synthetic route, involving various sulfurated carbazole-based molecules, also resulted in a source of fluorophores with different emitting behaviors. Surprisingly, the sulfoxide-containing fluorophores substantially increased their blue fluorescence with respect to the nearly non-emitting sulfur counterparts. On this basis, we could shed light on the relationship between their chemical structure and their emission as an approach for future applications. Considering the performance in organic thin-film transistors, both bisbenzothienocarbazole derivatives displayed p-type characteristics, with hole mobility values up to 1.1 × 10⁻³ cm² V⁻¹ s⁻¹ and considerable air stability. Moreover, the role of the structural design has been correlated with the device performance by means of X-ray analysis and the elucidation of the corresponding single crystal structures.