Organic Electronics

Conjugated polymers are a novel class of materials that combine the optical and electronic properties of semiconductors with processing advantages and mechanical properties of polymers. Conjugated polymers consist of alternating single and double bonds along the polymer chain (sp-2-hybridised carbons). The pi-electrons form a common pi-electron system along the whole polymer chain. This pi-electron system gives the polymers semiconducting properties. Semiconducting polymers can be used to make light-emitting devices, solar cells and field effect transistors. Our research in this area covers design and synthesis of new organic semiconducting materials, material processing, and interface and device engineering.

 

1. Polymer Light-Emitting Diodes (PLEDs)

Organic light-emitting diodes (PLEDs) consist of semiconducting polymer layers which are in total only 100 nm-thick and lie between two electrodes, an anode and a cathode, respectively. When voltage is applied to the electrodes, current flows through the organic layers and by the mechanism of electroluminescence-electric energy is directly converted to light. PLEDs have been developed for flat panel display applications, e.g., mobile phones, laptops and televisions, and with the tremendous advances in performance and stability, they are very promising candidate as the light source for the solid-state lighting, which offers significant gains in power efficiency, color quality, and life time at lower cost and less environmental impact than traditional incandescent and fluorescent lighting.

 

 

Our research in PLEDs addresses a number of issues, including tuning of charge carrier injection and conduction and exploration of new materials and novel device architectures for PLEDs. Our highly efficient solution-processed LEDs have been achieved through the “end-to-end” approach including the electrode interface modification and material engineering.  

 

2. Organic/Polymer Field Effect Transistors

In recent years, research on organic thin film transistors (OTFTs) has gained momentum due to their use in niche applications including drivers for large area flat panel displays, disposable radio frequency identification tags, and various sensors. Organic pi-conjugated materials used as the semiconductor demonstrate particular advantages over their silicon counterparts that can be exploited for the above applications, including: low cost, low temperature all solution processing, and compatibility with flexible substrates.

Of primary importance for high- performance OTFT device is the material design of the organic semiconductor. The ideal material would be free of defects, make an ohmic contact at the source and drain electrodes, and possess high electron or hole charge carrier mobilities for complimentary circuit design. Our group is currently exploring small molecule n-type and p-type organic semiconductors that exploit the interplay between p-p , chalcogen, dipole, hydrogen bonding, and van-der Waals interactions to facilitate ordered intermolecular interactions for optimum device performance.

OTFT performance hinges on the quality of the material interfaces in the device. By modifying the electrodes and dielectric with organic self assembled monolayers (SAMs) the organic semiconductor will have improved charge carrier injection and transport at these interfaces.

Another piece of the OTFT puzzle that our group is directing effort towards is the manipulation and modification of the insulating dielectrics. We are replacing the traditional dielectric materials (typically metal oxides) with crosslinkable polymers as well as ultra-thin SAM dielectrics. These materials are tailored to perform a number of beneficial tasks in the OTFT structure, such as improve interfacial arrangement, reduce leakage current and decrease operating voltage.

 

3. Organic/Polymer Photovoltaic Cells

Harvesting energy directly from sunlight using photovoltaic technology is considered as one of the most important way to address the growing global energy needs with a renewable resource while minimizing detrimental effects on the environment by reducing atmospheric emissions. In the past decade, research on organic-based photovoltaic devices has attracted tremendous scientific and industrial interest due to their potential in achieving the goal of PV technology that is economically viable for large scale power generation. Some key advantages for organic-based solar cell are that pi-conjugated small-molecule and polymeric materials are inherently inexpensive; they are compatible with flexible plastic substrates; and they can be fabricated using high-throughput and low-temperature process. Moreover, the versatility of the synthesis of organic materials allowing the alternation a wide range of properties including molecular weight, bandgap, energy levels, optical properties, electrical properties, structural properties and wetting properties. This ability to design and synthesize molecules and then integrate them into organic-organic or organic-inorganic composites provides a unique pathway in the design of materials for novel devices.

Nowadays, organic-based photovoltaics with power conversion efficiency reaching values in the 5% regime are achieved by the continuously development of new materials, improved materials engineering, and more advanced device concepts. However, many formidable challenges still have to be overcome before OPV devices can be considered as a truly practical technology. Those challenges include:

Research in our group is focusing on the development of new functional materials, the improvement of material processing and interfacial engineering, and the development of new concept for innovative photovoltaic devices. Our ultimate goal is to design and demonstrate advanced and efficient organic-based solar cells through the optimization of the closely interconnected parameters among the molecular structure, morphology and device properties.

 

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Relevant Refereed Papers

  1. “Self-Assembled Monolayer Modified ZnO/Metal Bilayer Cathodes for Polymer:Fullerene Bulk-Heterojunction Solar Cells”  Yip. H.-L.; Hau, S. K.; Baek, N. S.; Jen, A. K.-Y. Appl. Phys. Lett., 2008, 92, 193313.
  2. “Polymer Solar Cells That Use Self Assembled-Monolayer-Modified ZnO/Metals as Cathodes” Yip. H.-L.; Hau, S. K.; Baek, N. S.; Ma, H.; Jen, A. K.-Y. Adv. Mater.,2008, (in press).
  3. ”Air-Stable Inverted Flexible Polymer Solar Cells Using Zinc Oxide Nanoparticles as an Electron Seletive Layer” Hau, S. K.; Yip, H.-L.; Baek, N. S., Zou, J.; O’ Mellay, K.; Jen A. K.-Y. Appl. Phys. Lett., 2008, (in press).
  4. "Highly efficient white polymer light-emitting diodes based on nanometer-scale control of the electron injection layer morphology through solvent processing” Zhang, Y; Huang, F; Chi, Y.; Jen, A. K.-Y.  Advanced Materials, 2008, 20, 1565.
  5. “Highly efficient white polymer light-emitting diodes based on nanometer-scale control of the electron injection layer morphology through solvent processing.”  Zhang, Y; Huang, F; Chi, Y.; Jen, A. K.-Y.   Appl. Phys. Lett., 2008, 92, 063303.
  6. “Colloidal CdSe quantum dot electroluminescence: ligands and light-emitting diodes.” Munro, A. M.; Bardecker, J. A.; Liu, M. S.; Cheng, Y.-J.; Niu, Y.-H.; Plante, Ilan Jen-La; Jen, A. K.-Y.; Ginger, D. S  Microchimica Acta, 2008, 160, 345.
  7. “Thermally Cross-Linkable Hole-Transporting Materials on Conducting Polymer: Synthesis, Characterization, and Applications for Polymer Light-Emitting Devices.”  Cheng, Y.-J.; Liu, M. S.; Zhang, Y.; Niu, Y.; Huang, F.; Ka, J.-W.; Yip, H.-L.; Tian, Y.; Jen, A. K.-Y, Chemistry of Materials, 2008, 20, 413.
  8. ”High-efficiency and color stable blue-light-emitting polymers and devices”,  Huang, F.; Zhang, Y.; Liu, M. S.; Cheng, Y.-J.; Jen, A. K.-Y. Advanced Functional Materials, 2007, 17, 3808.
  9. “Improved performance from multilayer quantum dot light-emitting diodes via thermal annealing of the quantum dot layer”, Niu, Y.-H.; Munro, A. M.; Cheng, Y.-J.; Tian, Y.; Liu, M. S.; Zhao, J.; Bardecker, J. A.; Plante, I. J.-L.; Ginger, D. S.; Jen, A. K.-Y.    Advanced Materials, 2007, 19, 3371
  10. “A conjugated, neutral surfactant as electron-injection material for high-efficiency polymer light-emitting diodes”,  Huang, F.; Niu, Y.-H.; Zhang, Y.; Ka, J.-W.; Liu, M. S.; Jen, A. K.-Y., Advanced Materials 2007, 19, 2010.
  11. “Highly efficient UV-violet light-emitting polymers derived from fluorene and tetraphenylsilane derivatives: molecular design toward enhanced electroluminescent performance”,  Zhou, X.-H.; Niu, Y.-H.; Huang, F.; Liu, M. S.; Jen, A. K.-Y.,  Macromolecules 2007, 40, 3015.
  12. “Efficient CdSe/CdS Quantum Dot Light-Emitting Diodes Using a Thermally Polymerized Hole Transport Layer”, J. Zhao, J. A. Bardecker, A. M. Munro, M. S. Liu, Y. Niu, I-K. Ding, J. Luo, B. Chen, A. K-Y. Jen, and D. S. Ginger, Nano Lett., 2006, 6(3), 463.
  13. “Crosslinkable Hole-Transport Layer on Conducting Polymer for High Efficiency White Polymer Light-Emitting Diodes”, Yu-Hua Niu, Michelle S. Liu, Jae-Won Ka, Julie Bardeker, Melvin Zin, Richard Schofield, Yun Chi, and Alex K.-Y. Jen, Adv. Mater., (in press).
  14. “High-efficiency Polymer Light-emitting Diodes using Neutral Surfactant Modified Aluminum Cathode”, Y. H. Niu and A. K-Y. Jen, J. Phys. Chem. B.,2006, 110(12), 6010.
  15. “Material and Interface Engineering for Highly Efficient Polymer Light Emitting Diodes”, M. S. Liu, Y. H. Niu, J. D. Luo, B. Q. Chen, T. D. Kim, J. Bardecker and A. K.-Y. Jen, Polymer Reviews, 2006, 46(1), 7.
  16. “ Thermally Crosslinkable Hole-transporting Polymers for Efficient Blue Phosphorescent Polymer Light-emitting Diodes”, Y. H. Niu, M. S. Liu, and A. K-Y. Jen, Appl. Phys. Lett., 2006, 88, 093505.
  17. “Efficient Ultraviolet-Blue Polymer Light-Emitting Diodes Based on a Fluorene-based Non-conjugated Polymer”, F. Huang, Y. H. Niu, M. S. Liu, X. H. Zhou, Y. Q. Tian, and A. K-Y. Jen, Appl. Phys. Lett., 2006, 89(8), 081104.
  18. “Highly Efficient Red-Electrophosporescent Devices Based on Polyfluorene Copolymers Containing Charge-Transporting Pendent Units”, F. I. Wu, P. I. Shih, Y-H. Tseng, G-Y. Chen, C-H. Chien, C. F. Shu, Y-L. Tung, Y. Chi, and A. K-Y. Jen, J. Phys. Chem. B., 2005, 109(29), 14000.
  19. “A Highly Electroluminescent Molecular Square”, L. Zhang, Y. Niu, A. K.-Y. Jen, and W. Lin, Chem. Commun., 2005, 1002.
  20. “Highly Efficient Electrophosphorescent Devices with Saturated Red Emission from a Neutral Osmium Complex”, Yu-Hua Niu, Yung-Liang Tung, Yun Chi, Ching-Fong Shu, Joo Hyun Kim, Baoquan Chen, Jingdong Luo, Arthur J. Carty, and Alex K.-Y. Jen, Chem. Mater., 2005, 17, 3532.
  21. “High Efficiency Light-emitting Diodes using Neutral Surfactants and Aluminum Cathode”, Y. H. Niu, H. Ma, Q. Xu, and A. K-Y. Jen, Appl. Phys. Lett.,2005, 86(8), 083504.
  22. “Efficient Green Polymer Light-Emitting Diodes with Microcavity Effect in Electroluminescence Spectrum but with Constant Quantum Efficiency”, X. Jiang, P. Herguth, T. Sassa, and A. K-Y. Jen, J. Appl. Phys., 2004, 96(6), 3553.
  23. “Bright White Light Electroluminescent Devices Based on a Dye-dispersed Polyfluorene Derivative”, J. H. Kim, P. Herguth, M-S. Kang, A. K–Y. Jen, and C-F. Shu, Appl. Phys. Lett., 2004, 85(7), 1116.
  24. “Synthesis and Optoelectronic Properties of Starlike Polyfluorenes with Silsesquioxane”, W. J. Liu, W. C. Chen, W. C. Wu, Y. H. Niu, and A. K-Y. Jen, Macromolecules, 2004, 37, 2335.
  25. “Investigation of Polymers and Marine-Derived DNA in Opto-electronics”, J. G. Grote, J. A. Hagen, J. S. Zetts, R. L. Nelson, D. E. Diggs, M. O. Stone, P. P. Yaney, E. Heckman, C. Zhang, W. H. Steier, A. K-Y. Jen, L. R. Dalton, N. Ogata, M. J. Curley, S. J. Clarson, F.. K. Hopkins, J. Phys. Chem., B., 2004, 108(25), 8584.
  26. “Highly Efficient Red Electrophosphorescent Devices Based on Ir(III)-complex with CF 3-substituted Pyrimidine Ligand”, Y. Niu, B. Chen, S. Liu, H. Yip, J. Bardecker, A. K.-Y. Jen, Appl. Phys. Lett., 2004, 85(9), 1619.
  27. “Efficient and Stable Blue Light-emitting Diodes Based on an Anthracene Derivative Doped Poly(N-vinylcarbazole)”, Y. Niu, B. Chen, T. D. Kim, M. S. Liu, and A. K.-Y. Jen, Appl. Phys. Lett., 2004, 85(21), 1.
  28. “ Nanorheological Approach for Characterization of Electroluminescent Polymer Thin Films”, T. Gray, C. Buenviaje, R. M. Overney, S. Jenekhe, L. Zheng, A. K-Y. Jen, Appl. Phys. Lett., 2003, 83(13), 2563.
  29. “Electrophosphorescence from a Conjugated Copolymer Doped with Iridium Complex: High Brightness and Improved Operational Stability”, X. Gong, J. Ostrowski, G. C. Bazan, A. J. Heeger, M. S. Liu, and A. K-Y. Jen, Adv. Mater., 2003, 15(1), 45.
  30. “A Novel Oxadiazole-Containing Polyfluorene with Efficient Blue Electro-luminescence”, F-I. Wu, D. S. Reddy, C. F. Shu, M. S. Liu, A. K-Y. Jen, Chem. Mater., 2003, 15, 269.
  31. ‘Highly Efficient Blue Light-Emitting Diodes from Polyfluorene Containing Bipolar Pendent Groups”, C. F. Shu, R. Dodda, F. I. Wu, M. S. Liu, and A. K-Y. Jen, Macromolecules, 2003, 36, 6698.
  32. “Efficient Green Light-Emitting Diodes from A Silole-Containing Copolymer”, M. S. Liu, J. Luo and A. K-Y. Jen, Chem. Mater., 2003, 15, 3496.
  33. “High-Performance Polymer Light-Emitting Diodes Fabricated with a Novel Hole Injection Layer”, X. Gong, S. Liu, A. K-Y. Jen, D. Moses, and A. Heeger, Appl. Phys. Lett., 2003, 83(1), 183.
  34. “Bright Red-Emitting Polymeric Electrophosphorescent Device from Osmium Complex Doped as a Triplet Emitter’, J-H. Kim, M. S. Liu, A. K.–Y. Jen, B. Carlson, L. R. Dalton, C-F. Shu, and R. Dodda, Appl. Phys. Lett., 2003, 83(4), 776.
  35. “Effect of Cyano-Substituents on Electron Affinity and Electron-Transporting Properties of Conjugated Polymers”, M. S. Liu, X. Jiang, S. Liu, P. Herguth, and A. K-Y. Jen, Macromolecules, 2002, 35, 3532.
  36. “Novel Europium and Osmium Complexes for Pure Red Light-Emitting Diode Applications”, X. Jiang, G. Phelan, B. Carlson, S. Liu, L. R. Dalton, and A. K-Y. Jen, Macromol. Symp., 2002, 186, 171.
  37. “Divalent Osmium Complexes: Synthesis, Characterization, Strong Red Phosphorescence and Electrophosphrescence”, B. Carlson, L. Dalton, X. Jiang, S. Liu, and A. K-Y. Jen, J. Am. Chem. Soc., 2002, 124, 14162.
  38. “Highly Efficient Fluorene- and Benzothiadiazole-Based Conjugated Copolymers for Polymer Light-Emitting Diodes”, P. Herguth, X. Jiang, M. S. Liu and A. K-Y. Jen, Macromolecules, 2002, 35, 6094.
  39. “Perfluorocyclobutane-Based Arylamine Hole-Transporting Materials for Organic and Polymer Light-Emitting Diodes”, X. Jiang, S. Liu, M. S. Liu, P. Herguth, A. K-Y. Jen, H. Fong and M. Sarikaya, Adv. Func. Mater., 2002, 12(11-12), 745.
  40. “Red Emitting Electroluminescent Devices Based on Osmium Complexes Doped Blend of Poly(vinylnaphthalene) and 1,3,4-Oxadiazole derivative”, X. Jiang, A. K-Y. Jen, B. Carlson and L. R. Dalton, Appl. Phys. Lett., 2002, 81(17), 3125.