Microscopic origin of the mobility enhancement at OTS-modified gate dielectrics in pentacene thin-film transistors, and the decoupling of grain size from field-effect mobility

Organic-vapor-phase-deposited pentacene thin-film transistors on SiO2 treated with an octadecyltrichlorosilane (OTS) self-assembled monolayer show hole mobilities up to about $1.6$ cm$^2$/V s despite having far smaller crystalline grains than untreated-SiO2 devices, and the original 2002 report explicitly left the microscopic mechanism open. The question is to determine, quantitatively, what causes this enhancement and the decoupling of grain size from mobility: the role of interfacial molecular orientation and in-plane $\pi$-orbital overlap (the original flat-lying-molecule suggestion), interfacial trap-state energetics at the dielectric, and growth-mode/intergrain-packing effects, together with the origin of source-purity- and storage-related mobility variations. Later orientation studies (x-ray absorption, NEXAFS, and a 2024 infrared pMAIRS study specific to OTS) contradict the flat-lying-molecule premise, and trap spectroscopy attributes the grain-size decoupling to charge traps at the semiconductor/dielectric interface rather than at grain boundaries; however, a generally accepted quantitative decomposition of the enhancement among interfacial trap passivation, dipolar electrostatics, SAM crystallinity/roughness, and growth-mode effects is still missing, so a precise open core survives.

Background

Pentacene, a linear five-ring polycyclic aromatic hydrocarbon, was for years the highest-mobility small-molecule organic semiconductor for thin-film transistors (TFTs). In a TFT, a gate voltage induces a conducting sheet of holes at the interface between the pentacene channel film and the gate dielectric (typically thermally grown SiO2), and the field-effect mobility $\mu_{\text{eff}}$ measures how fast those holes move. Pentacene films are polycrystalline: they consist of crystalline grains separated by grain boundaries, and the molecules pack in a herringbone arrangement with their long axis nearly perpendicular to the substrate ('standing') in the usual thin-film growth on dielectrics, so that the $\pi$-orbital overlap between adjacent stacked molecules lies in the plane of the substrate, which is also the plane of charge transport. Two crystallographic polymorphs coexist in such films: a 'thin-film phase' (interplanar spacing $d \approx 15.5$ Å) favored at dielectric surfaces and the bulk phase ($d \approx 14.4$ Å). Organic vapor phase deposition (OVPD) is a growth technique in which an inert carrier gas transports evaporated molecules through a hot-walled chamber, giving independent control of substrate temperature, chamber pressure, and growth rate. A standard dielectric treatment is an octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) — an ordered carpet of $\text{C}_{18}$ alkyl chains covalently bound to the oxide — which renders the surface hydrophobic, passivates polar silanol (Si–OH) surface groups that can act as charge traps, and alters how pentacene nucleates and grows. Shtein, Mapel, Benziger, and Forrest (Appl. Phys. Lett. 81, 268 (2002)) used OVPD to vary pentacene grain size on plasma-treated SiO2 from about $0.2$ to $5\ \mu$m and found mobilities from $0.05$ to $0.58$ cm$^2$/V s that grew with grain size; but OTS treatment produced devices with $\mu_{\text{eff}} \leqslant 1.6$ cm$^2$/V s, on/off ratios up to $\sim 10^8$, a steeper subthreshold slope, and less hysteresis, while shrinking the grain size to $\leqslant 0.2\ \mu$m. Their x-ray diffraction showed a stronger $2\theta \approx 19^\circ$ reflection (in-plane $(\bar{1}11)/(110)$-type reflections) on OTS, from which they suggested — as a correlation, explicitly not a demonstration — an increased density of flat-lying molecules accompanying the mobility improvement, and left open what role in-plane $\pi$-system overlap plays, whether interfacial orbital overlap, interlayer coupling, or trap-state energetics dominate, why grain size and mobility decouple, and why freshly purified pentacene source material outperformed material stored for more than a day. Adjacent results around the same time showed the same decoupling elsewhere: low-surface-energy polymer dielectrics yield smaller grains yet higher mobility (Yang, Shin, and Park, 2005), and at fixed grain size, dielectric roughness and deposition temperature change mobility independently of morphology (Knipp, Street, and Völkel, 2003). The established experimental toolbox for this problem includes NEXAFS (near-edge x-ray absorption fine structure, which reads molecular tilt from the polarization dependence of the $\pi^*$ resonances), pMAIRS (p-polarized multiple-angle incidence resolution spectrometry, an infrared analogue that separates in-plane from out-of-plane molecular vibrations), grazing-incidence x-ray diffraction, and trap spectroscopy from transistor transfer characteristics.

Problem Statement

Determine, quantitatively and mechanistically, why an octadecyltrichlorosilane (OTS) self-assembled monolayer on the SiO2 gate dielectric of organic-vapor-phase-deposited pentacene thin-film transistors raises the field-effect hole mobility (to $\mu_{\text{eff}} \leqslant 1.6$ cm$^2$/V s, and to $\sim 3$ cm$^2$/V s on optimized crystalline ultrasmooth SAMs) while reducing the crystalline grain size and decoupling mobility from grain size, and why freshly purified source material outperforms stored material. Concretely: (i) establish what role, if any, the interfacial molecular orientation and in-plane $\pi$-orbital overlap play in the transistor behavior — the original suggestion of an increased fraction of flat-lying interfacial molecules is now contradicted by x-ray absorption, NEXAFS, and pMAIRS evidence for standing molecules on SiO2 and on OTS, so this branch asks for the definitive structural interpretation of the OTS-related $2\theta \approx 19^\circ$ diffraction feature and of interfacial packing in the electrically active first layers; (ii) quantitatively establish the relative contributions of interfacial charge-trap passivation at the dielectric, dipolar/electrostatic effects of the SAM, SAM crystallinity and roughness, and growth-mode-induced intergrain packing to the observed mobility increase, the reduced off-current, and the steeper subthreshold slope; and (iii) identify the microscopic origin of the purification- and short-term-storage-related mobility variability. The common objective is predictive control: an interface-design rule that predicts transistor performance from dielectric surface chemistry and structure, in place of the empirical correlations reported in 2002. Parts (i)–(iii) are jointly required — none of them alone resolves the task — and the interface-design rule is the synthesis they together support, not a separate deliverable; within (ii), all four listed mechanisms enter the decomposition and their relative contributions are to be apportioned, rather than the list merely delimiting the class of effects to consider.

References

1. M. Shtein, J. Mapel, J. B. Benziger, and S. R. Forrest, 'Effects of film morphology and gate dielectric surface preparation on the electrical characteristics of organic-vapor-phase-deposited pentacene thin-film transistors', Applied Physics Letters 81, 268-270 (2002). DOI: 10.1063/1.1491009, https://doi.org/10.1063/1.1491009
2. D. Knipp, R. A. Street, and A. R. Völkel, 'Morphology and electronic transport of polycrystalline pentacene thin-film transistors', Applied Physics Letters 82, 3907-3909 (2003). DOI: 10.1063/1.1578536, https://doi.org/10.1063/1.1578536
3. T. Schwieger, X. Liu, D. Olligs, M. Knupfer, and Th. Schmidt, 'Orientation and electronic properties of pentacene molecules on SiO2 and GeS(0001) studied using x-ray absorption spectroscopy', Journal of Applied Physics 96, 5596-5600 (2004). DOI: 10.1063/1.1805183, https://doi.org/10.1063/1.1805183
4. K. Shankar and T. N. Jackson, 'Morphology and electrical transport in pentacene films on silylated oxide surfaces', Journal of Materials Research 19, 2003-2007 (2004). DOI: 10.1557/JMR.2004.0255, https://doi.org/10.1557/JMR.2004.0255
5. S. Y. Yang, K. Shin, and C. E. Park, 'The Effect of Gate-Dielectric Surface Energy on Pentacene Morphology and Organic Field-Effect Transistor Characteristics', Advanced Functional Materials 15, 1806-1814 (2005). DOI: 10.1002/adfm.200400486, https://doi.org/10.1002/adfm.200400486
6. J. W. H. Smith and I. G. Hill, 'Influence of SiO2 dielectric preparation on interfacial trap density in pentacene-based organic thin-film transistors', Journal of Applied Physics 101, 044503 (2007). DOI: 10.1063/1.2437673, https://doi.org/10.1063/1.2437673
7. F. Zheng, B.-N. Park, S. Seo, P. G. Evans, and F. J. Himpsel, 'Orientation of pentacene molecules on SiO2: From a monolayer to the bulk', The Journal of Chemical Physics 126, 154702 (2007). DOI: 10.1063/1.2717161, https://doi.org/10.1063/1.2717161
8. S. D. Wang, T. Miyadera, T. Minari, Y. Aoyagi, and K. Tsukagoshi, 'Correlation between grain size and device parameters in pentacene thin film transistors', Applied Physics Letters 93, 043311 (2008). DOI: 10.1063/1.2967193, https://doi.org/10.1063/1.2967193
9. J. Zhang, J. P. Rabe, and N. Koch, 'Grain-Boundary Evolution in a Pentacene Monolayer', Advanced Materials 20, 3254-3257 (2008). DOI: 10.1002/adma.200703066, https://doi.org/10.1002/adma.200703066
10. E. Gomar-Nadal, B. R. Conrad, W. G. Cullen, and E. D. Williams, 'Effect of Impurities on Pentacene Thin Film Growth for Field-Effect Transistors', The Journal of Physical Chemistry C 112, 5646-5650 (2008). DOI: 10.1021/jp711622z, https://doi.org/10.1021/jp711622z
11. Y. Ito, A. A. Virkar, S. Mannsfeld, J. H. Oh, M. Toney, J. Locklin, and Z. Bao, 'Crystalline Ultrasmooth Self-Assembled Monolayers of Alkylsilanes for Organic Field-Effect Transistors', Journal of the American Chemical Society 131, 9396-9404 (2009). DOI: 10.1021/ja9029957, https://doi.org/10.1021/ja9029957
12. F. D. Fleischli, S. Suárez, M. Schaer, and L. Zuppiroli, 'Organic Thin-Film Transistors: The Passivation of the Dielectric-Pentacene Interface by Dipolar Self-Assembled Monolayers', Langmuir 26, 15044-15049 (2010). DOI: 10.1021/la102060u, https://doi.org/10.1021/la102060u
13. A. A. Virkar, S. Mannsfeld, Z. Bao, and N. Stingelin, 'Organic Semiconductor Growth and Morphology Considerations for Organic Thin-Film Transistors', Advanced Materials 22, 3857-3875 (2010). DOI: 10.1002/adma.200903193, https://doi.org/10.1002/adma.200903193
14. M. M. Islam, S. Pola, and Y.-T. Tao, 'Effect of Interfacial Structure on the Transistor Properties: Probing the Role of Surface Modification of Gate Dielectrics with Self-Assembled Monolayer Using Organic Single-Crystal Field-Effect Transistors', ACS Applied Materials & Interfaces 3, 2136-2141 (2011). DOI: 10.1021/am200349j, https://doi.org/10.1021/am200349j
15. S. Casalini, C. A. Bortolotti, F. Leonardi, and F. Biscarini, 'Self-assembled monolayers in organic electronics', Chemical Society Reviews 46, 40-71 (2017). DOI: 10.1039/c6cs00509h, https://doi.org/10.1039/c6cs00509h
16. B. S. Bhardwaj, T. Sugiyama, N. Namba, T. Umakoshi, T. Uemura, T. Sekitani, and P. Verma, 'Orientation analysis of pentacene molecules in organic field-effect transistor devices using polarization-dependent Raman spectroscopy', Scientific Reports 9, 15149 (2019). DOI: 10.1038/s41598-019-51647-2, https://doi.org/10.1038/s41598-019-51647-2
17. S. Nagai, N. Shioya, T. Hasegawa, and T. Ishibashi, 'Standing Orientation of Pentacene Promoted on Octadecyltrichlorosilane Self-Assembled Monolayers Revealed by pMAIRS', The Journal of Physical Chemistry C 128, 20343-20349 (2024). DOI: 10.1021/acs.jpcc.4c04062, https://doi.org/10.1021/acs.jpcc.4c04062