Pentacene films

In an attempt to combine band-Uke charge carrier motion realized in an -inevitably fragile - crystalline FET structure with structural robustness and flexibility, Sakanoue and Sirringhaus [167] prepared FETs using spin coated films of 6,13-bis(triisopropylsilylethynyl)(TIPS)-pentacene films in contact with a perfluorinated, low dielectric-constant polymer gate electrode. The (linear) hole mobility at room temperature is 0.8 cm /V s with tendency of an apparent band-like negative temperature coefficient of the mobility (d/i/dT < 0). 

D. J. Gundlach, T. N. Jackson, D. G. Schlom, S. F. Nelson, Solvent-induced phase transition in thermally evaporated pentacene films, Appl. 

Aromatic hydrocarbon pentacene is important for application in OTFTs as it has superior field effect mobility, good semiconducting behavior, and stability [153], The chemical structure is shown in Fig. 6.10. Pentacene semiconductor films can be fabricated by sublimation in a vacuum deposition system. Optimization of the fabrication parameters, such as the substrate temperature and deposition rate, can yield a highly ordered pentacene film with improved device performance. Oriented films have optical and electrical anisotropies. 

Crystal size, crystal disorder, and field effect mobility of pentacene films grown on the surface of ion-beam treated and native Si02- Dichroic ratio signifies alignment of pentacene perpendicular to the ion-beam direction. 

Figure 8 The absorption spectrum and its decomposition into Gaussian profiles for a pentacene film deposited at 80 K, and the spectrum recorded at 240 K. The main S0 S1 transition hv0(1) is accompanied by the upper Davydov component hv0(2), in the crystal spectrum, its first vibronic band, hvovlbr, and a defect band hvoD. Adapted from Ref. 67.

Duffy. C. M. Andreasen. J. W. Breiby. D. W. Nielsen. M. M. Ando. M. Minakata. T. Sirringhaus. H. (2008). High-Mobility Aligned Pentacene Films Grown by Zone-Casting. Chemistry of Materials, vol. 20, no. 23,7252-9. 

In order to exclude the appearance of rotational domains in the initial stage of growth we have used a Cu(llO) substrate and have studied the growth of pentacene films on this substrate quite extensively. In fact, a highly ordered submonolayer and a saturated monolayer phase are found which exhibit a uniform alignment of the flat lying molecules with their long axes orientated parallel to the (110 )-azimuth direction of the substrate [46, 52]. 

Figure 11.6 Evolution of pentacene films on Cu(l 10). After completion of the first monolayer revealing a predominant (6.5 X 2) phase and occasionally a coexisting c(13 X 2) phase (a) an intermediate phase A is formed whereas for thickness above 2 nm the molecules continue in an upright orientation a-dopting the well known thin film phase B (b).

Figure 11.7 Stmcture and morphology of pentacene films grown on Au(l 11). (a) SEM micrograph of 2 nm grown at room temperature together with (b) STM data (U= -2Y, I = 15 pA) showing the stmcture of the first monolayer (indicated by I) between the islands formed (indicated by II). A similar morphology was also obtained for films grown at various conditions, (c) 30 nm (15 nm/min). The resulting islands reveal characteristic orientations relative to the substrate as shown schematically in (d).

Figure 11.10 Morphology and structure of thin pentacene films on a SAM pre-covered Au(l 11) substrate (a) SEM micrograph of a 2 nm film together with corresponding STM data (b), (c) showing a layered arrangement of upright standing molecules. XRD data recorded for a 30 nm film (d) clearly reveal the presence of (001) oriented films revealing the thin film phase while at larger thickness the bulk phase is adopted.

D. Kafer and G. Witte, Evolution of pentacene films on Ag(l 11) Growth beyond the first monolayer, Chem. Phys. Lett. 442,376-383 (2007). 

The reason for pentacene being superior for the production of TFT devices [7, 8] when compared with other molecules [9] is still not obvious. In this chapter, we will discuss to what extent the peculiar growth properties [10] of pentacene on metallic contacts and gate dielectrics contribute to the device performance. For this purpose, first the early growth state of pentacene films and the molecular structure of the so called thin film phase is reviewed. Then, major sources of crystal defects in thin films as determined by advanced synchrotron diffiaction techniques are discussed. The relation of these defects to the frequently discussed electronic traps that strongly influence transport properties of TFTs [6, 11, 12] is indicated. Finally, the spatially resolved photo response of pentacene OTFTs will be discussed in the context of injection barriers and contact homogeneity. 

Figure 15.1 AFM amplitude micrograph of a pentacene OTFT stmcture. (a) Gold contacts on Si02 have been covered by a 50 nm pentacene film, (b) A zoom reveals a grainy stmcture and terraces with step heights of ca. 15.4 A.

The most simple pentaeene OTFT test structure used in many labs is based on a Si wafer piece covered with a thermal oxide. Here, the heavily doped Si wafer takes the role of the back gate electrode, and the Si02 takes the role of the gate dielectric. A pentacene thin film is deposited as the semiconducting layer. Source and drain electrodes are deposited either on the silicon oxide (bottom contact) or on top of the pentacene film (top contact). 

The observed threshold shift of AVj = 2.0 V results in a trap density t= 2.4 X lO" traps/cm. This number is not so small eompared with the number of eharge carriers Wj, indicating that traps largely influence the device performance. One may speculate whether structural defects within the pentacene film contribute to trap states. An X-ray analysis of the defect densities in pentacene films revealed defect densities in the order of t=2xl0" defects/cm [44], in good agreement with the trap density inferred from the electronic characterisation. Thus, the observed structural defects apparently contribute to... 

Pentacene shows a strong absorption in the visible (see Figure 15.8), more optieal properties of pentacene films can be found in [48]. If pentacene is eom-bined with a proper n-conductor such as C60, it can be used as the aetive region in a solar eell [49]. Transient photoconductivity experiments using optieal... 

We observe a strong photo response localised at the anode [52]. The response is inhomogenous along the contact indicating variations in the transport or injection properties of the device. These variations may be due to local variations of the contact work function, or due to bad physical contact of the pentacene grains adjacent to the electrode, or due to a local enhancement of defect densities in the pentacene film. Thus, a systematic study of the photo response in combination with the respective characteristic transistor curves allows visualising problematic regions of an OTFT, which is a key prerequisite for device optimisation. 

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