Small molecule organic semiconductors have garnered significant attention due to their potential for high charge carrier mobility, particularly in single-crystal form. Among these, rubrene (C42H28) stands out as a benchmark material, exhibiting exceptional performance in organic field-effect transistors (OFETs). The growth of small molecule single crystals and their charge transport properties are critical to understanding their anisotropic behavior and device applications.

Single crystals of small molecules like rubrene are typically grown using physical vapor transport (PVT) techniques. In this method, purified source material is sublimed in a controlled atmosphere, often under an inert gas flow, and recrystallized in a cooler zone of the furnace. The growth conditions—temperature gradient, gas flow rate, and pressure—must be finely tuned to obtain high-quality crystals. Rubrene single crystals grown via PVT exhibit well-defined facets, with the (001) plane being the most prominent for device fabrication. The absence of grain boundaries in single crystals eliminates scattering sites that would otherwise hinder charge transport in polycrystalline films.

The crystalline structure of rubrene is orthorhombic, with herringbone packing that facilitates strong π-π interactions along specific crystallographic directions. This anisotropic arrangement results in direction-dependent charge transport properties. Charge carrier mobility in rubrene single crystals is highest along the [100] direction, where π-orbital overlap is maximized, reaching values exceeding 40 cm²/V·s at room temperature. In contrast, mobility along the [010] and [001] directions is significantly lower, often by an order of magnitude, due to reduced electronic coupling.

The high mobility in rubrene is attributed to several factors. First, the absence of grain boundaries eliminates trapping sites that would otherwise scatter charge carriers. Second, the tight molecular packing reduces reorganization energy, facilitating efficient charge transfer. Third, the low energetic disorder in single crystals results in band-like transport at room temperature, a phenomenon rarely observed in organic semiconductors. Temperature-dependent measurements reveal that mobility follows a power-law dependence (μ ∝ T^(-n)), where n typically ranges between 1 and 2, indicating a combination of acoustic phonon scattering and shallow trapping effects.

Device fabrication using rubrene single crystals requires careful handling to preserve crystal integrity. Top-contact OFETs are commonly fabricated by laminating crystals onto dielectric-coated substrates, followed by deposition of source and drain electrodes. The choice of dielectric is crucial; polymers like parylene or inorganic oxides such as SiO2 are often used to minimize interface traps. Bottom-contact configurations are less common due to challenges in aligning electrodes with high-mobility crystal directions. Contact resistance remains a limiting factor, with gold being the preferred electrode material due to its favorable work function alignment with rubrene’s highest occupied molecular orbital (HOMO).

The anisotropic nature of rubrene necessitates precise crystal alignment for optimal device performance. Misalignment between the [100] direction and the channel can drastically reduce measured mobility. Techniques such as polarized optical microscopy and X-ray diffraction are employed to verify crystal orientation prior to device fabrication. Additionally, mechanical strain can modulate charge transport; uniaxial strain along the [100] direction has been shown to enhance mobility by further improving π-orbital overlap.

Environmental stability is a concern for rubrene-based devices due to oxidation of the tetracene backbone. Encapsulation with inert materials like alumina or glass is essential to prevent degradation. Despite this vulnerability, the operational stability of rubrene OFETs under controlled conditions is excellent, with negligible threshold voltage shifts over thousands of cycles.

Beyond rubrene, other small molecule single crystals such as pentacene, tetracene, and C60 derivatives have been explored, though none match rubrene’s combination of high mobility and processability. Modifications to the molecular structure, such as side-chain engineering or doping, can further enhance performance but often at the cost of increased disorder.

The potential applications of high-mobility small molecule single crystals extend beyond OFETs. They are promising candidates for flexible electronics, high-frequency circuits, and even spintronic devices due to their long spin diffusion lengths. However, challenges remain in scaling up crystal growth and integrating them into complex circuits without compromising performance.

In summary, small molecule single crystals like rubrene exhibit exceptional charge transport properties due to their defect-free, anisotropic structure. Their growth via PVT ensures high purity and crystallinity, while their electronic properties are dominated by directional π-π interactions. High-mobility devices fabricated from these crystals demonstrate the potential of organic semiconductors for advanced electronic applications, provided that challenges in alignment, contact resistance, and environmental stability are addressed. Future research may focus on discovering new materials with similar or superior properties while improving fabrication techniques for large-scale integration.