Functionalized Pentacene Active Layer Organic Thin‐Film Transistors

Abstract

Organic thin-film transistors (OTFTs) have potential for use in many low-cost, large-area electronic applications such as smart cards, radio-frequency (RF) identification tags, and flatpanel displays. To date, OTFTs with the best overall reported performance have been based on the organic semiconductor pentacene, with carrier field-effect mobility > 2 cm 2 V ±1 s ±1 .[1]Molecular ordering is believed to play a large role in the performance of devices based on organic active layer materials, [2,3] especially for small molecule materials such as pentacene. In particular, the amount of p-orbital overlap is expected to have a strong influence on field-effect mobility. Interestingly, investigation of the solid-state order of pentacene reveals ªherringboneº packing with a combination of edge-to-face and face-to-face molecular interactions. This packing arrangement yields limited p-orbital overlap and may limit the field-effect mobility possible in pentacene. One approach to improve pentacene-based device performance is to modify the pentacene molecule to form molecular crystals with increased p-orbital overlap. This paper reports on OTFTs fabricated with pentacene functionalized to improve p-orbital overlap. Previous work on functionalized pentacene focused on using the material as a soluble precursor allowing the fabrication of solution-cast unsubstituted pentacene OTFTs.[4±6] The functional groups were removed from the molecule before completion of device fabrication and did not result in molecular ordering with improved electronic transport potential. This study reports on the use of molecular modifications to engineer improved organic semiconductor carrier-transport properties. The approach reported here is to add bulky functional groups at the 6,13-positions of the pentacene molecule to discourage edge-to-face molecular interactions. The desired result is modified molecular ordering with primarily face-toface interactions and improved p-orbital overlap. In addition to providing modified molecular ordering, the functional groups may also improve the solubility of the material in organic solvents, allowing solution-cast devices, and may also enhance the oxidative stability of the material. Figure 1 shows (left) the molecular structure of the pentacene derivatives investigated in this work and (right) the five functional groups used. The substituent is separated from the pentacene molecule by a rigid alkyne spacer used to hold the bulky groups away from the aromatic core to allow the closest possible approach between the aromatic rings. X-ray crystallography data on triisopropylsilyl (TIPS) pentacene reveals that the material stacks in a two-dimensional columnar array with significantly increased p-orbital overlap and reduced interplanar spacing compared to unsubstituted pentacene. [7,8] OTFTs were fabricated using the five functionalized pentacene derivatives as the active layer. A heavily doped silicon wafer was used as a convenient substrate and gate electrode. Thermally grown silicon dioxide was used as the gate ...

Localization length scales of triplet excitons in singlet fission materials

“…For TIPS-pentacene however, the face-to-face stacking shown in Fig. 4(b) indicates a significantly more anisotropic environment, 41,44 and a greater influence of intermolecular interactions between neighbouring molecules 42,44 (also pointed to by the significant difference between solution and film absorption spectra. 31 ) We note that anisotropy has been shown to play an important role in triplet exciton diffusion in molecular semiconductors, with the experimental characterization typically relying on radiative triplet-triplet annihilation to track energy migration.…”

Molecular Packing and Arrangement Govern the Photo-Oxidative Stability of Organic Photovoltaic Materials

“…Amorphous films of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPSpentacene), 47 PCBM, PC 71 BM, and T1 were prepared by spin-casting the materials from chloroform or chlorobenzene solutions, and crystallinity was induced by thermal annealing. Amorphous films of rubrene were prepared by evaporating at a fast rate, and crystallinity was similarly induced by annealing.…”

Ab initio electronic structure calculations of solid, solution-processed metallotetrabenzoporphyrins

“…[54][55][56][57] Minari et al examined the effects of intermolecular distance and π orbital overlap in a series of metal-substituted octaethylporphyrin OFETs with the molecules arranged face-to-face along the c axis, and found µ FE to be inversely dependent on the intermolecular spacing. 58 XRD measurements on MTBP thin-films have shown that molecules stack in the herringbone arrangement along the b axis, with the molecule face parallel to (114) and the HOMO orbital surface parallel to the molecule face.…”