Summary

The surface roughness of pentacene at the pentacene/gold interface was decreased after the annealing treatment, leading to a decrease of contact area. This explained the increase of contact resistance. Through the XRD analysis, the lattice distortion became seriously after thermal annealing. The serious lattice distortion gave rise to the increase of bulk resistance.

After thermal annealing, the AFM images showed that the GB density was decreased, and the NGB was proved to be decreased from Equation (5.2) In addition, the Nit(max) at pentacene/insulator interface was also decreased. The reduction of traps must lead to a decrease of the channel resistance. Furthermore, the reduction of DC stress degradation also explained the reduction of defects and traps. The decrease of Nit(max)

was more distinct than that of NGB, showing that the post-annealing effect on pentacene/insulator interface was the most critical to the device performance.

Chapter 6 CONCLUSIONS

In this study, we controlled the morphology of pentacene through the post-annealing treatment, followed by studying the correlation between the thin-film morphology of pentacene and TFT device performance. The aims of our research are to find out the effect of post-annealing treatment on the device performance, pentacene crystal structures, and their correlations.

After the post-annealing treatment, the device mobility increased from 0.42 to 0.80 cm²V^-1sec^-1 and the device became more stable. After investigating the mechanism, the results indicate that thermal annealing treatment reduces the number of defects in the grain boundaries and pentacene/PαMS-modified-SiO2 interface, leading to the reduction of channel resistance and trap density. Further, results reveal that surface modification of PαMS not only improved the device performance but also enhanced the degree of the improvement induced by the post-annealing effect.

After post-annealing, the decrease of film roughness leads to a raised contact resistance, and the lattice distortion leads to an increased bulk resistance. The increased contact and bulk resistance prohibit the efficiency of charge injection from gold to pentacene and charge transport inside the bulk of pentacene thin-film (perpendicular to the device substrate), respectively. Further, the problem of the increased resistance will

become more noticeable when decreasing the channel length of OTFTs. Therefore, it is necessary to find ways to decrease the bulk and contact resistance.

If we combine post-annealing treatment and other methods, like modification of the pentacene/gold interface, [46] which can enhance the charge injection from gold to pentacene, a better device performance and device mobility higher than 0.80 cm²V^-1sec^-1 can be expected.

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APPENDIX

The program defines the density of grain boundary from the N × N matrix of AFM images. The authors thank Miss. Ying-Pin Chen for programming.

function GB_count (file)

% the pink value should be modulated

% to a proper image.

% for user to judge the pink value

% to define a space for Fourier transform

% highpass filter

% to show the pattern obviously

% to count how many white pixels is

for i=1 : dimension(1,1) for j = 1 : dimension(1,2) if image_i(i,j) == 1

target = target + 1;

end;

end;

end;

ratio = target/(dimension(1,1)*4) % 4 makes the result to a unity