From the the transfer characteristics of pentacene OTFTs under UV-light illumination , it is important to discuss the degradation of UV-light illumination on OTFTs. Therefore , we try to investigate the pentacene film properties under different UV-light illumination times firstly. In Fig. 4-4 , we show the x-ray diffraction (XRD) curves of pentacene film under different UV-light illumination times. All pentacene films show the (001) signal with a peak value about 5.73° (degree). The higher order signal of (002) and (003) peak is also observed with a values about 11.47°
(degree) and 17.21° (degree), respectively. According to the XRD analysis[47], it implies that the pentacene films under different UV-light illumination times are almost identical. It is clear that pentacene crystallization was not significantly changed after UV-light illumination. Consequently, we also try to verify the pentacene film by the Raman spectrum. With the Raman spectrum analysis, the conformational transition during carrier transport [47]. The pentacene
vibration-modes of C−H (around 1155-1179cm-1) and C−C (around 1353-1380cm-1) bindings are plotted in Fig. 4-5 . According to the Raman spectrum analysis, the result also reveals that the intermolecular coupling was not be changed even if the pentacene films under different UV-light illumination times. Finally, we also show fourier transform infrared spectroscopy (FTIR) spectrum of the pentacene
films under different UV-light illumination times. The FTIR spectrum of 100nm thick pentacene films on SiO2 (see Fig. 4-6 ) displays a number of strong absorption bands, which can be identified according to the literature [12,48]: 729 cm-1 and 903 cm-1 are out-of-plane C−H bending modes, 1296 cm-1 a ring stretching mode, and 3004 cm-1correspond to C−H in-plane stretching modes. According to the FTIR spectrum analysis, the bonding of pentacene films was not be changed even if the pentacene films under different UV-light illumination times. Based on these material analysis, the crystalline, molecular coupling, and the absorptive properties of pentacene film under different UV-light illumination times should be almost identical.
It is clear that deep UV do not damage the pentacene crystalline channel layer from the surface to the channel/gate oxide interface, and not generating excessive defects in the whole layer thickness.
In order to study the effect of UV-light illumination on OTFTs, we verified the effects of UV-light illumination on thin-film properties. In Fig. 4-7, we show the AFM images of pentacene films under different UV-light illumination times. Before illumination, the dendritic pentacene grains could be observed. Beside, the monolayer edges also clearly appeared. However, after illumination, several knobs appeared on the dendritic pentacene grains[12].The monolayer edges were blurred. Pentacene grain slightly changes were observed among these AFM images. Therefore , deep
UV cause a surface damage of organic pentacene which UV-light illumination times increases.
It is not clearly understood why UV cause a degradation of pentacene TFTs.
However , it is conjectured from electron Spectroscopy for Chemical Analysis (ESCA) was used to examine the changes of thin film chemical bonding. The pentacene vibration-modes of carbon (C) signal (around 285eV) and oxygen (O) signal (around 532eV) bindings are plotted in Fig. 4-8(a). The carbon (C) signal of pentacene films is further plotted as a function of binding energy in Fig. 4-8(b), which were illuminated by UV-light with different times. Significantly, the carbon signal significantly decreased, additionally, a carbon-oxygen (O) related signal around 290eV appear after UV-light illumination[49]. We also show the oxygen signal in Fig. 4-8 (c).We found pentacene film without UV-light illumination showed a very low oxygen signal.
However, when pentacene film was exposed to UV-light illumination, the oxygen signal significantly increased. This implied that UV-light illumination may result in a photo-induced oxidation of organic pentacene. This will result in changed bonding between the benzene ring and oxygen. The added oxygen in the pentacene film severely influenced OTFT performance.
To further find where the photo-induced oxidation of organic pentacene is, electron Spectroscopy for Chemical Analysis (ESCA) depth profile was plotted in Fig.
4-9. We can compare the carbon (C) signal and oxygen (O) signal, which were illuminated by UV-light with different times. Significantly, in the thickness of pentacene film thiner than 10 nm , the carbon signal significantly decreased. In contrast , the oxygen signal significantly increased. It is clear that UV-light illumination cause a surface oxidation of organic pentacene , generating deep level defects near the surface in the vicinity of oxygen molecules in air.[13]
According to charge transport mechanisms of pentacene OTFTs[50], all the charge of the conducting channel resides in the first monolayer next to the insulator–pentacene interface; in other words, the charges located close to the interface have the highest mobility, which reinforces their contribution to the conductivity of the conducting channel. Consequently, the conventional field-effect mobility gives a relatively good estimate for the actual mobility at the interface.
However , the electron Spectroscopy for Chemical Analysis (ESCA) depth profile result further verify surface oxidation of pentacene film severely degraded OTFT performance.Therfore , it is clear that the charge of the conducting channel not just located in the first monolayer next to the insulator–pentacene interface ,but also in whole pentacene layer.
4-2 Ultraviolet light protective layer for OTFTs passivation
In order to protect the organic thin-film transistors (OTFTs) from degradation under UV-light illumination , we proposed on the passivation of pentacene TFTs using two kinds of passivation layers : TiO2, N′-bis(naphthalen-1-y)-N,N′-bis(phenyl) benzidine (NPB)/Mg/TiO2/PDMS and NPB.
For UV-light illumination , the TiO2 thin film have strong absorption property.
Therefore , we used the TiO2 thin film to protect the organic thin-film transistors (OTFTs) from degradation under UV-light illumination. However, in figure 4-10(a) ,we shows the performance of the OTFT before and after the passivation with TiO2 . It is clear that there is a significant increment in the off-current after the passivation. We signifies that UV-light illumination on the TiO2 thin film, occurring the chemical reactions in TiO2 thin film. Furthermore, during the deposition process , x rays, and electron beams damages the pentacene active layer, resulting in the degradation of the field-effect mobility from 0.38 to 0.32 cm2/V.s, This result coincides with the previous report on the X-rays and electron beams effect on pentacene OTFT.[45-46]
Therefore , in order to avoid OTFT degradation during the passivation process and the off current decrease, we proposed on the passivation of pentacene TFTs with NPB/Mg/TiO2/PDMS. By using e-gun evaporation system, the multiple thin-film
layers, which made by oxide and metal layers, can be continuously deposited on OTFTs. These multiple thin-film layers were low optical transparency and high gaseous resistance, which will act as the protective layers to avoid OTFT degradation under UV-light (wavelength: 175-254 nm) soaking. However , in figure 4-10(b) ,we shows the performance of the OTFT before and after the passivation with NPB/Mg/TiO2 /PDMS. It is clear that although the off-current does not change after the passivation. But during the deposition process , X-rays, and electron beams damages the pentacene active layer[45-46], resulting in the degradation of the field-effect mobility from 0.44 to 0.35 cm2/V.s, so that the drop was 0.09 cm2/V s.
Therefore , in order to avoid OTFT degradation during the passivation process , we proposed the NPB film as the passivation for pentacene TFTs .Because the NPB film have highly transparent property.[51]. Furthermore , in figure 4-10(c) , we shows the transfer characteristics of the OTFT before and after passivation with NPB layer.
Before passivation , the device shows a mobility about 0.47cm2/v-sec, a threshold voltage about -12.2V, a subthreshold swing about 2.36 V/decade , and a on/off ratio about 106 . On the other hand, the OTFTs with NPB passivation show a similar mobility about 0.48 cm2/v-sec, a lower threshold voltage about -11 V, a larger subthreshold swing about 2.4V/decade, and a lower on/off ratio about 104. It is interesting that with NPB passivation , the threshold voltage will be reduced , the
subthreshold swing will be increased , and the on/off ratio will be reduced . But the field effect mobilities almost remains unchanged..Therefore , it is clear that during the passivation process , the NPB film does not damage on organic pentacene layer.
The transfer characteristics of the OTFTs before and after passivation with TiO2 ,NPB, NPB/Mg/TiO2/PDMS layers compared with literatures which have summarized in Table 2. It is clear that the transfer characteristics of the OTFTs before and after passivation with NPB layer almost remains unchanged. This result for passsivation layer of pentacene OTFTs is our unique finding .Therefore, the NPB film as the passivation layer to protect the organic thin-film transistors (OTFTs) from degradation under UV-light illumination.