紅熒烯與香豆素-6 多晶薄膜之制備與特性量測

Fabrication and Characterization of Rubrene and Coumarin-6 Thin Films

陳克倫(Ko-Lun Chen)

指導教授 ： 邱寬城

中原大學/理學院/物理研究所/碩士(2018年)

https://doi.org/10.6840/cycu201800165

摘要

本實驗探討以物理汽相沉積法成長香豆素- 6 與紅熒烯多晶薄膜之製備與特性分析。第一部分，固定粉末端溫度 Tsub = 185℃，改變基板溫度Tsub = 106 ~ 178℃ 成長一系列香豆素-6 多晶薄膜。在沉積過程中因為分子結構對稱性低，因此需要足夠的能量 EA = 0.93 ± 0.06 eV，來跨越堆疊排列時遭遇到的空間位阻。使用 SEM 觀察薄膜表面形貌，在 125℃  Tsub < 150℃ 沉積之薄膜表現出扭曲且粗糙的圖案；直到 Tsub  150℃ 結晶開始出現清楚的刻面。從 XRD 可以很明顯觀察到 (010) 特徵峰值，表示在 ITO 基板上成長香豆素-6 薄膜具有優選成長方向，並且平行於分子晶體的長軸。從半強度頻寬分析得知， Tsub = 150℃ 的薄膜具有最高的結晶度。此外，將晶格常數增加一倍來敘述兩個可見的峰值 2 = 7.260 o 和 9.007 o 出現的原因，說明分子固體中N- 和 S- 兩種異構體共同存在的可能。接著從光吸收譜量測 Tsub = 150℃ 的薄膜之吸收閥值約為 2.392 eV。光致發光譜做高斯擬合可以獲得四個分量，其中主要峰值為 Frenkel exciton (FE) 複合放光；分子存在振動能態下有一個能量低於 FE 的放光；另外兩個高能量的峰值則歸類為 charge transfer excitons (CT1 and CT2)。
第二部分，固定粉末端溫度 Tsub = 240℃，基板溫度 Tsub = 151 ~ 186℃ 成長平面 (Planar rubrene) 或扭曲 (Twisted rubrene) 之紅熒烯分子薄膜。在 Tsub > 170℃ 下，薄膜主要以平面分子組成；Tsub < 170℃ 則是以扭曲分子為主。由 XRD 觀察在 Tsub = 184℃薄膜的結晶品質高，沿著 (200) 具有優選生長方向。在 300 K 交流頻率 20 Hz 的條件下介電常數與損失率分別為 εr = 7.2、D = 0.16， Debye relaxation 熱活化機制之 EA = 41 meV。然而，對於在 Tsub < 170℃ 下成長的薄膜品質較差，優選成長方向則是沿 (002) 方向。介電相關參數 εr = 5.8、D = 0.25，EA = 83 meV。由空間電荷限制電流量測電洞載子遷移率隨溫度的變化，在 270 K 下 Tsub = 176℃ 的薄膜 載子遷移率可以來到 ~ 7 10-5 (cm2V-1s-1) ，熱活化 EA = 0.18 eV； Tsub = 157℃ 為 ~2 10-5 (cm2V-1s-1)， EA = 0.13 eV。由平面分子所構成的薄膜不但具有較高載子遷移率也同時需要更大的活化能。最後，對紅熒烯多晶薄膜量測變溫持續性光電導，以平面紅熒烯分子組成的薄膜相較於扭曲分子具有較短衰減長數。且兩者皆展現出衰減常數隨量測溫度上升而增加的行為，為異常之持續性光電導。

關鍵字

物理汽相沉積法 ； 香豆素- 6 ； 紅熒烯 ； 構相異構物

並列摘要

In this work, the fabrication and characterization of coumarin-6 and rubrene polycrystalline films from vacuum deposition were studied. In the first part, coumarin-6 polycrystalline films were deposited at substrate temperatures Tsub = 106 to 178℃ with a fixed source temperature Tsou = 185℃. Because of its slenderer and more asymmetric structure, the coumarin-6 molecule encounters a higher steric hindrance to occupy the appropriate location and correct orientation, and hence possesses a larger steric energetic barrier of 0.92 eV. From top-view SEM pictures, for 125℃  Tsub < 150℃, the as-deposited coumarin-6 thin films exhibit a twisted pattern and a kinematic roughness; while for Tsub  150℃, clear facets emerge. From XRD spectra, a protruding peak assigned to (010) peak suggests that the preferred growth direction of these films on ITO substrate is parallel to the long axis of coumarin-6 molecule, and from the analysis of full width at half maximum the polycrystalline film deposited at Tsub = 150o has a highest crystallinity among other films. In addition, two noticeable peaks at 2 ~ 9.007o and 7.260o reveal that lattice constants are doubled due to the co-existence of two (namely, N- and S-isomer) coumarin-6 isomers within the crystalline grains. Furthermore, the bandgap from optical transmission on the coumarin-6 films deposited at Tsub = 150°C is around 2.392 eV. From photoluminescence spectra, four components are obtained by a Gaussian fit, with the main peak assigned to the Frenkel exciton (FE) emission; a weak peak with energy lower than that of FE recognized to the participation of the molecular vibrational states; and the other two with energies higher than that of FE attributed to the charge transfer excitons (CT1 and CT2).
In the second part, rubrene polycrystalline films were deposited at Tsub = 151 to 186℃ with a fixed Tsou = 240℃. For Tsub > 170℃, the as-deposited films consist of planar rubrene molecules; while for Tsub < 170℃, the films are dominant by twisted rubrene molecules. For the rubrene polycrystalline films deposited at Tsub = 184℃, a preferred growth direction along (200) with high crystallinity was observed from XRD. The dielectric constant εr = 7.2 and a dielectric loss D = 0.16 were estimated at 300 K with 20 Hz. The relaxation frequency obeyed Debye model with a thermally activated energy EA = 41 meV. However, for the rubrene polycrystalline films deposited at Tsub < 170℃, the preferred growth direction was along (002) with low crystallinity, εr = 5.8, D = 0.25 (at 300 K with 20 Hz), and EA = 36 meV were obtained. In addition, the temperature-dependent hole mobility (μSCLC) of rubrene polycrystalline film was measured from space-charge limited current, μSCLC ~ 7×10-5 cm2V-1s-1 at T = 270 K with EA = 0.18 eV for rubrene film deposited at Tsub = 176℃ and μSCLC = 2×10-5 cm2V-1s-1 at T = 270 K with EA = 0.13 eV for Tsub = 157℃. The film consisted of planar rubrene possessed a higher mobility as well as a higher EA. Finally, temperature-dependent persistent photoconductivity decay measurement was performed on rubrene polycrystalline films. The decay time constant (τ) for planar rubrene films was much shorter than that for twisted rubrene films. Both of them exhibited an abnormal behavior, i.e. τ increased with increasing T.

並列關鍵字

Physical vapor deposition ； Coumarin- 6 ； Rubrene ； Conformational isomers

參考文獻

[1] S. R. Forrest, The path to ubiquitous and low-cost organic electronic appliances on plastic, Nature 428(2004) 911.

[2] H. Shirakawa, E. J. Louis, A. G. MacDiarmid, C. K. Chiang, and A. J. Heeger,Synthesis of electrically conducting organic polymers: halogen derivatives of polyacetylene, (CH)x, J. Chem. Soc. Chem. Comm. (1977) 578.

[3] A. Tsumura, H. Koezuka, and T. Ando,Macromolecular electronic device: Field- effect transistor with a polythiophene thin film,Appl. Phys. Lett. 49(1986) 1210.

[4] C. W. Tang, Two‐layer organic photovoltaic cell, Appl. Phys. Lett. 48(1986) 183.

[5] C. W. Tang and S. A. Van Slyke, Organic electroluminescent diodes, Appl. Phys. Lett.51(1987) 913.