分子發光二極體(molecular light emitting diode)包含了有機發光二極體(organic light emitting diode)和高分子發光二極體(polymer light emitting diode)兩類，其結構是由陽極和陰極中間夾有單層或多層之共軛分子或高分子層所組合而成，當將此結構施加以一電壓時，電子和電洞會分別由陰極和陽極注入到分子層中，其中部分之電子和電洞會在此分子層中進行再結合而發光。
本篇論文研究的焦點是放在分子發光二極體的電極改良，希望能夠藉由電極改良來提高電子與電洞注入到分子層的效率，進而提高發光的亮度並降低其起始電壓(threshold voltage)。電極改良的方法包含了使用有較低功函數(work function)之陰極或較高功函數之陽極，或者是在電極和分子層之間加入一層非常薄之電極改良層以提升電子或電洞的注入效率。
在使用全共軛硬桿式poly-p-phenylenebenzobisthiazole (PBT) or poly-p-phenylenebenzobisoxazole (PBO)高分子為發光層之單層高分子發光二極體中，當陰極使用了擁有低功函數的鎂替代一般通用的鋁作為陰極，可使起始電壓降至3 V即開始發光。另外在鋁陰極和分子層中間加入一層很薄的氟化鋰(LiF)或氧化鋁(Al2O3)的陰極改良層，來增進電子注入效率，藉以提升分子發光二極體的發光亮度並將其起始電壓降至2.8 V。
在陽極的改良部分， PBO高分子被用來作為陽極的電極改良層材料，當在氧化銦錫(indium-tin-oxide，ITO)陽極和分子層中間加入一層很薄的PBO層時，可有效地增進電洞注入效率，並使數種不同之單層或多層分子發光二極體的起始電壓明顯下降至1 V∼3 V，且發電致光強度也隨之增強。此外，因經過酸洗的ITO基材會較一般僅用超音波震盪洗滌的ITO基材擁有較高之功函數，故在此研究中，也使用酸洗之ITO基材來改良陽極，以降低電洞注入到分子層之能障，可有效提高分子發光二極體之發光亮度並降低其起始電壓。

Molecular light emitting diode, including organic light emitting diode (OLED) and polymer light emitting diode (PLED), is commonly consist of one or several molecular layer(s) sandwiched between an anode and a cathode. When electrons and holes are injected respectively from cathode and anode into the molecular layer by a bias voltage, these two types of carriers migrate towards each other and a fraction of them recombine to form light emission.
The focus of this study is electrode modifications of molecular light emitting diode. The electrode modifications include using a low work function cathode material, a high work function anode material or inserting a very thin electrode modifier between molecular layer and electrode for enhancing the electron or the hole injection efficiency leading to higher electroluminescence emission and/or lower threshold voltage.
Low work function metal, Mg, could effectively reduce the electron injection barrier between molecular layer and cathode leading to better emission brightness and threshold voltage. A monolayer rigid-rod poly-p-phenylenebenzobisthiazole (PBT) or poly-p-phenylenebenzobis- oxazole (PBO) PLED with Mg cathode demonstrated a low threshold voltage of 3 V. Besides, a very thin layer of LiF (or Al2O3) inserted between molecular layer and Al cathode was applied to enhance the electron injection efficiency leading to a stronger electroluminescence intensity and a low threshold voltage of 2.8 V.
On anode modification, a thin PBO layer was inserted between molecular layer and the indium-tin-oxide (ITO) substrate for improving the electroluminescence emission brightness and the threshold voltage. The PBO modified anode could effectively enhance the electro- luminescence intensity and lower the threshold voltage to 1 V~ 3 V on several mono- or multi-layer molecular light emitting diodes. Besides, a novel ITO substrate cleaning method via acid treatment was applied for increasing the work function of ITO to effectively enhance the hole injection efficiency.

TABLE OF CONTENTS

LIST OF FIGURES…………………………………………………………III
LIST OF TABLES…………………………………………………….….VII

CHAPTER 1 INTRODUCTION………………………………...................1
1.1 Development of Molecular Light Emitting Diodes...……………….1
1.2 Structure of Molecular Light Emitting Diodes………………...…….....3

CHAPTER 2 THEORY AND FABRICATION………………………….....4
2.1 Luminescent Materials…………………………………………………4
2.1.1 Small Dye Molecules…………………………………………..4
2.1.2 Conjugated Polymers………………………………………..…5
2.2 Energy Band Theory…………………………………………………...6
2.3 Luminescence Theory………………………………………………….9
2.4 Fabrication of Device………………………………………………....10
2.4.1 ITO Glass Cleaning………………………………………...…10
2.4.2 Spin-coating of Solution………………………………………11
2.4.3 Electrode Deposition by Thermal Evaporation………………12
2.5 Instruments and Measurements……………………………………….13
2.5.1 UV-Vis Absorption Spectrum………………………………....13
2.5.2 Photoluminescence Spectrum………………………………....16
2.5.3 I-V Curve…………………………………………………...…16
2.5.4 Electroluminescence Spectrum……………………………….17
2.5.5 Scanning Electron Microscopy……………………………….17
2.5.6 Spectral Reflectance…………………………………………..18

CHAPTER 3 EXPERIMENTS……………………..……………………21
3.1 Instruments and Light emitting Devices…………………………..21
3.1.1 Rigid-rod Polymer Solution……………………………….….21
3.1.1.1 MSA Solution Preparation…………………………21
3.1.1.2 Lewis Acid Solution Preparation……………………24
3.1.2 Spin-coating of Polymer………………………………………24
3.1.3 PL Spectrum…………………………….………………...…25
3.1.4 FE-SEM Imaging….…………………………………..…...…25
3.1.5 I-V Measurement…………………………………………...…25
3.1.6 EL Spectrum.……………………………………………...…26
3.1.7 Spectral Reflectance………………………………………..…27
3.1.8 UV-Vis Spectrum………………………………….………..…27
3.1.9 Thermal Evaporation Deposition……………………………..28
3.2 LED Design and Experiment…………………………………………29
3.2.1 ITO Substrate Cleaning……………………………………..29
3.2.2 Spin-coating and Thickness Control…………………………29
3.2.3 Cathode Design and Modifications…………………………30
3.2.3.1 LiF/Al Cathode…………………………………..…30
3.2.3.2 Mg Cathode………………………….…………….30
3.2.3.3 Al2O3/Al Cathode……...……………………….…31
3.2.4 Anode Design and Modifications……..………………………31
3.2.4.1 ITO/PBO Anode………………………………….…31
3.2.4.2 Acid Treated ITO………………………….………32

CHAPTER 4 RESULTS AND DISCUSSION………………….………...37
4.1 Spun Film Thickness………………………………………………….37
4.2 UV-Vis and PL Spectra……………………………………………..…38
4.3 Refractive Index Measurement………………….…………………….46
4.4 Thickness Influence in Light emitting Devices……………………….46
4.5 Cathode Modifications…………………………….………………….46
4.5.1 LiF/Al Cathode………………………………………………..46
4.5.2 Mg Cathode………………………………………………….53
4.5.3 Al2O3/Al Cathode……………………………………………..57
4.6 Anode Modifications………………………………………………….61
4.6.1 ITO/PBO Anode……………………………………………..61
4.6.1.1 Anode Modification Using PBO……………………61
4.6.1.2 Both Anode and Cathode Modifications……………63
4.6.2 Acid Treated ITO……………………………………………63

CHAPTER 5 CONCLUSIONS…………………………………………...73

List of Reference….……...…………………………………………………76

List of References

1.C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 51, 913, (1987).
2.D. D. C. Bradley J. H. Burroughes, A. R. Brown, R. N. Marks, K. Mackay, R.
H. Friend, P. L. Burn, and A. B. Holmes, Nature, 347, 539, (1990).
3.S. Miyata, "Organic Electroluminescent Materials and Devices," Gordon and
Breach Publishers, Tokyo, 1997.
4.M. A. Baldo, and S. R. Forrest, Phys. Rev. B, 62, 10958, (2000).
5.S. W. Pro, S. P. Lee, H. S. Lee, O. K. Kwon, H. S. Hoe, S. H. Lee, Y. K. Ha,
Y. K. Kim , and J. S. Kim, Thin Solid Films, 363, 232, (2000).
6.H. Cao, X. Gao, and C. H. Huang, Appl. Surf. Sci, 161, 443, (2000).
7.L. Zugang, and H. Nazare, Synth. Met., 111-112, 47, (2000).
8.B. M. Krasovitskii and B. M. Bolotin, "Organic Luminescent Materials," VCH
Publishers, New York, 1988.
9.J. A. Osaheni and S. A. Jenekhe, Macromolecules, 27, 739, (1994).
10.J. A. Osaheni and S. A. Jenekhe, Chem. Mater., 7, 672, (1995).
11.R. F. Pierret, "Semiconductor Device Fundamentals," Addison-Wesley
Publishing Company, Inc., New York, 1996.
12.J. S. Kim F. Cacialli, T. M. Brown, J. Morgado, M. Granstrom, R. H. Friend,
G. Gigli, R. Cingolani, L. Favaretto, G. Barbarella, R. Daik, and W. J.
Feast, Synth. Met., 109, 7, (2000).
13.E. W. Forsythe Q. T. Le, F. Nuesch, L. J. Rothberg, L. Yan, and Y. Gao,
Thin Solid Films, 363, 42, (2000).
14.L. J. Tothberg F. Nuesch, E. W. Forsythe, Q. T. Le, and Y. Gao, Appl. Phys.
Lett., 74, 880, (1999).
15.D. Braun G. L. J. A. Rikken, E. G. J. Staring, and R. Demandt, Appl. Phys,
Lett., 65, 219, (1994).
16.K. Sugiyama H. Ishii, E. Ito, and K. Seki, Adv. Mater., 11, 605, (1999).
17.R. Swanepoel, Opt. Soc. Am. A, 2, 1339, (1985).
18.Michio Matsumura, Keiichi Furukawa, and Yukitoshi Jinde, Thin Solid Films,
331, 96, (1998).
19.X. H. Yang, T. Kietzke, F. Jaiser, and D. Neher, Synth. Met., 137, 1503,
(2003)
20.H. Fujikawa T. Mori, S. Tokito, and Y. Taga, Appl. Phys. Lett., 73, 2763,
(1998).
21.G. E. Jabbour, S. E. Shaheen, M. M. Morrell, Y. Kawabe, B. Kippelen, and N.
Peyghambarian, J. Appl. Phys., 84, 2324, (1998).
22.C. W. Tang L. S. Hung, M. G. Mason, P. Raychaudhuri, and J. Madathil, Appl.
Phys. Lett., 78, 544, (2001).
23.C. W. Tang L. S. Hung, and M. G. Mason, Appl. Phys. Lett., 70, 152, (1997).
24.K. L. Wang, B. Lai, M. Lu, X. Zhou, L. S. Liao, X. M. Ding, X. Y. Hou, and
S. T. Lee, Thin Solid Films, 363, 178, (2000).
25.F. Li, H. Tang, J. Anderegg, and J. Shinar, Appl. Phys. Lett., 70, 1233,
(1997).
26.C. H. Lee, Synth. Met., 91, 125, (1997).
27.Zuang Liu, O. V. Salata, and Nigel Male, Synth. Met., 128, 211, (2002).
28.A. J. Makinen W. H. Kim, N. Kikolov, R. Shashidhar, H. Kim, and Z. H.
Kafafi, Appl. Phys. Lett., 80, 3844, (2002).
29.L. J. van Ijzendoorn M. P. de Jong, and M. J. A. de Voigt, Appl. Phys.
Lett., 77, 2255, (2000).
30.D. C. Muller M. Gross, H. G. Nothofer, U. Scherf, D. Neher, C. Brauchle,
and K. Meerholz, Nature, 405, 661, (2000).
31.J. S. Kim T. M. Brown, R. H. Friend, and F. Cacialli, Appl. Phys. Lett.,
75, 1679, (1999).
32.L. M. Hung X. M. Ding, L. F. Cheng, Z. B. Deng, X. Y. Hou, C. S. Lee, and
S. T. Lee, Appl. Phys. Lett., 76, 2704, (2000).
33.I-Min Chan, Tsung-Yi Hsu, and Franklin C. hong, Appl. Phys. Lett., 81,
1899, (2002).
34.Stephen R. Day, Ross A. Hatton, Michael A Chesters, and Martin R. Willis,
Thin Solid Films, 410, 159, (2002).
35.Satoshi Hoshino and Hiroyuki Suzuki, Appl. Phys, Lett., 69, 224, (1996).
36.E. W. Forsythe, M. A. Abkowitz, and Yongli Gao, J. Phys. Chem. B, 104,
3948, (2000).
37.F. Nuesch, M. Carrara, M. Schaer, D. B. Romero, and L. Zuppiroli, Chem.
Phys. Lett., 347, 311, (2001).
38.S. A. Jenekhe, L. R. de Paor, X. L. Chen, and R. M. Tarkka, Chem. Mater.,
8, 2401, (1996).
39.Yasuhiko Shirota, Yoshiyuki Kuwabara, Hiroshi Inada, Takeo Wakimoto,
Hitoshi Nakada, Yoshinobu Yonemoto, Shin Kawami, and Kunio Imai, Appl.
Phys, Lett., 65, 807, (1994).
40.M. A. Baldo and S. R. Forrest, Phys. Rev. B, 62, 10958, (2000).