[Paper Review] Growth of macroscopic-area single crystal polyacene thin films on arbitrary substrates

Randall L. Headrick, Hua Zhou+8 more|arXiv (Cornell University)|Jul 17, 2011

Organic Electronics and Photovoltaics21 references3 citations

TL;DR

This paper presents a solvent-based deposition method to grow macroscopic single-crystal organic semiconductor thin films, including polyacene and its derivative 6,13-bis(triisopropylsilylethynyl)pentacene, on arbitrary substrates. The technique achieves single-crystal domains exceeding 1 cm² for anthracene and >2 mm² for the pentacene derivative—significantly larger than the micron-scale grains from conventional vapor deposition or spin coating—enabling high carrier mobility and reduced defects for advanced organic electronics.

ABSTRACT

Organic electronic materials have potential applications in a number of low-cost, large area electronic devices such as flat panel displays and inexpensive solar panels. Small molecules in the series Anthracene, Tetracene, Pentacene, are model molecules for organic semiconductor thin films to be used as the active layers in such devices. This has motivated a number of studies of polyacene thin film growth and structure. Although the majority of these studies rely on vapor-deposited films, solvent-based deposition of films with improved properties onto non-crystalline substrates is desired for industrial production of devices. Improved ordering in thin films will have a large impact on their electronic properties, since grain boundaries and other defects are detrimental to carrier mobilities and lifetimes. Thus, a long-standing challenge in this field is to prepare largearea single crystal films on arbitrary substrates. Here we demonstrate a solvent-based method to deposit thin films of organic semiconductors, which is very general. Anthracene thin films with single-crystal domain sizes exceeding 1x1 cm2 can be prepared on various substrates by the technique. Films of 6,13-bis(triisopropylsilylethynyl)pentacene are also demonstrated to have grain sizes larger than 2x2 mm2. In contrast, films produced by conventional means such as vapor deposition or spin coating are polycrystalline with micron-scale grain sizes. The general propensity of these small molecules towards crystalline order in an optimized solvent deposition process shows that there is great potential for thin films with improved properties. Films prepared by these methods will also be useful in exploring the limits of performance in organic thin film devices.

Motivation & Objective

• To develop a scalable, solvent-based method for growing large-area single-crystal organic semiconductor thin films on arbitrary substrates.
• To overcome the limitations of conventional vapor deposition and spin coating, which produce polycrystalline films with micron-scale grain boundaries.
• To achieve macroscopic single-crystal domains (>1 cm²) in small-molecule organic semiconductors like anthracene and pentacene derivatives.
• To improve electronic properties such as carrier mobility and device lifetime by minimizing grain boundaries and defects.
• To demonstrate the generality of the method across different substrates and organic semiconductor materials.

Proposed method

• A solvent-based deposition technique is employed to grow thin films from solution, optimizing solvent choice and processing conditions to promote crystalline ordering.
• The method enables controlled nucleation and lateral growth of single-crystal domains by tuning solution concentration, substrate surface energy, and evaporation kinetics.
• The process is applied to anthracene and 6,13-bis(triisopropylsilylethynyl)pentacene, demonstrating its broad applicability to organic semiconductors.
• Film morphology and crystallinity are characterized using optical microscopy, X-ray diffraction, and atomic force microscopy to confirm single-crystal nature.
• Substrate independence is achieved by modifying surface energy or using interfacial layers to promote epitaxial-like growth on diverse materials.
• The technique avoids high vacuum or thermal evaporation, enabling compatibility with flexible and non-crystalline substrates.

Experimental results

Research questions

• RQ1 Can solvent-based deposition produce single-crystal organic semiconductor films larger than 1 cm² on arbitrary substrates?
• RQ2 How does solvent-based processing compare to vapor deposition or spin coating in terms of grain size and crystallinity?
• RQ3 What is the maximum single-crystal domain size achievable for anthracene and pentacene derivatives using this method?
• RQ4 Can this method be generalized across different organic semiconductors and substrate types?
• RQ5 To what extent do macroscopic single crystals improve electronic performance compared to polycrystalline films?

Key findings

• Single-crystal anthracene films with domain sizes exceeding 1×1 cm² were successfully grown on various substrates using the solvent-based method.
• For 6,13-bis(triisopropylsilylethynyl)pentacene, single-crystal domains larger than 2×2 mm² were achieved.
• The grain sizes from this method are significantly larger than those from conventional vapor deposition or spin coating, which produce polycrystalline films with micron-scale grains.
• The method is effective on arbitrary substrates, including non-crystalline and flexible materials, demonstrating broad substrate compatibility.
• The results indicate a strong intrinsic tendency of small-molecule organic semiconductors to form large single crystals under optimized solution processing conditions.
• The films exhibit high structural order, suggesting potential for enhanced carrier mobility and device performance in organic field-effect transistors and photovoltaics.