Use of an Underlayer for Large Area Crystallization of Rubrene Thin Films

Abstract

In this work, we discovered a very efficient method of crystallization of thermally evaporated rubrene, resulting in ultrathin, large-area, fully connected, and highly crystalline thin films of this organic semiconductor with a grain size of up to 500 μm and charge carrier mobility of up to 3.5 cm2 V–1 s–1. We found that it is critical to use a 5 nm-thick organic underlayer on which a thin film of amorphous rubrene is evaporated and then annealed to dramatically influence the ability of rubrene to crystallize. The underlayer property that controls this influence is the glass transition temperature. By experimenting with different underlayers with glass transition temperatures varying over 120 °C, we identified the molecules leading to the best crystallinity of rubrene films and explained why values both above and below the optimum result in poor crystallinity. We discuss the formation of different crystalline morphologies of rubrene produced by this method and show that field-effect transistors made with films of a single-domain platelet morphology, achieved through the aid of the optimal underlayer, outperform their spherulite counterparts with a nearly four times higher charge carrier mobility. This large-area crystallization technique overcomes the fabrication bottleneck of high-mobility rubrene thin film transistors and other related devices and, given its scalability and patternability, may lead to practical technologies compatible with large-area flexible electronics.

Hall Effect in Polycrystalline Organic Semiconductors: The Effect of Grain Boundaries

“…Figure a,c,e shows polarized optical microscopy images of these films, revealing their polycrystalline microstructure. In particular, a homogeneous birefringence within the grains suggests that each grain is a single crystalline domain, which has been recently confirmed by electron backscattering diffraction . We therefore expect that the intragrain charge transport in these samples should be close to fully coherent .…”

“…To further test our model, we have employed another type of organic semiconductor, large‐area polycrystalline rubrene thin films, recently developed by Fusella et al These polycrystalline thin films feature single‐crystal grains of rubrene with a very large grain size of up to ≈1 mm, with the surface of the grains corresponding to the ( a , b ) facet of orthorhombic rubrene, separated by crystallographically well‐defined grain boundaries. Such films allow for a few or even a single GB in the channel of OFETs, with known crystallographic orientation of each grain.…”

“…The Hall effect is called underdeveloped, when the Hall mobility, μ Hall , is substantially smaller than the longitudinal field‐effect mobility, μ FET , (μ Hall < μ FET ), and, respectively, the Hall carrier density, n Hall , is systematically greater than the capacitively defined (total) carrier density, n FET , induced in the accumulation channel ( n Hall > n FET ) . To understand the mechanism of this effect, besides blends, we have also investigated high‐quality polycrystalline rubrene thin films with a large grain size previously developed by Fusella et al These films can be viewed as an ideal polycrystalline organic semiconductor due to the very large grain size of up to ≈1 mm and crystallographically well‐defined grain boundaries, with a highly compact film morphology extended over macroscopically large areas (up to 1 cm 2 ). In OFETs, such rubrene films ensure just a few discrete GBs in the channel and result in relatively high field‐effect mobilities, μ FET = 1–4 cm 2 V −1 s −1 .…”

“…To understand the mechanism of this effect, besides blends, we have also investigated high‐quality polycrystalline rubrene thin films with a large grain size previously developed by Fusella et al These films can be viewed as an ideal polycrystalline organic semiconductor due to the very large grain size of up to ≈1 mm and crystallographically well‐defined grain boundaries, with a highly compact film morphology extended over macroscopically large areas (up to 1 cm 2 ). In OFETs, such rubrene films ensure just a few discrete GBs in the channel and result in relatively high field‐effect mobilities, μ FET = 1–4 cm 2 V −1 s −1 . As the result of our study, we conclude that GBs are the main cause of an underdeveloped Hall effect observed in polycrystalline OFETs.…”

The Impact of Local Morphology on Organic Donor/Acceptor Charge Transfer States

“…Besides varying the donor‐to‐acceptor blend ratio in bulk heterojunctions, others have sought to probe the limits of interfacial crystallinity on CT state properties in planar heterojunctions by using a postdeposition annealing method that converts as‐deposited amorphous thin films into highly crystalline films. Because of its morphological variability, rubrene has served as the model donor for numerous studies using this technique . When this thickness‐tunable technique was first applied to rubrene‐based solar cells, the films revealed a remarkably long exciton diffusion length of over 200 nm, paralleling the impressive properties of bulk single crystal rubrene, where exciton diffusion lengths of up to 8 µm have been reported .…”