Structure Property Relationships of. Organic Light-Emitting Diodes. Michael Kochanek May 19, 2006 MS&E 542 Flexible Electronics

Transcription

1 Structure Property Relationships of Organic Light-Emitting Diodes Michael Kochanek May 19, 2006 MS&E 542 Flexible Electronics

2 Introduction Many of today s solid-state inorganic microelectronic devices are reaching their theoretical limits. The increasing cost of smaller component fabrication combined with the increased power dissipation of higher component density devices beckons the need for an alternative. Although this fact is best illustrated in the realm of microprocessors, its effects are still felt in the arena of active displays one in which organic light-emitting diodes (OLEDs) look to supplant the present standard of liquid crystal displays (LCDs) and solid-state light-emitting diodes (LEDs). Advantages: The optoelectronic properties of organic materials match well with the requirements necessary for electroluminescent (EL) devices. One of the clearly distinct advantages of OLEDs is the practically unlimited number of molecules that can be taylormade to any application one can conceive. 1 This enormous scope of OLEDs far outreaches that of both LCDs and LEDs. Many organic materials also possess high luminescent efficiencies, making them ideal candidates for active displays. 2 Another considerable advantage that OLEDs boast is the economy of their fabrication (ppt). The processing of OLEDs is not only less expensive than LCDs but also less wasteful in terms of materials. Polymer materials can be deposited over large films very easily as well as a variety of substrates. 1 Unlike its counterpart, OLEDs can even be processed on mechanically flexible substrates; consequently offering a vast new range of applications. The conservative fabrication of OLEDs is the main driving force toward the production of viable devices based upon this technology.

3 Disadvantages: The mechanical instability of the organic materials also presents some drawbacks to these devices. For example, the low glass transition temperature of these molecules forces the fabrication of the electrodes to be done under soft conditions. 3 Instability is also a factor when an organic EL device is operated for an extended period of time. 2 In a paper written by Van Slyke, Chen and Tang, 4 the authors report that most organic EL devices reported so far have a short operational lifetime, ranging from a few hours to several hundred hours. This instability has been attributed to the deterioration of the organic as well as the electrode layers. On the other hand, the lifetime of LCDs is on the order of 50,000 hours. 5 In comparison to solid-state devices, the low charge mobility of organic materials limits the maximum performance of these devices. 1 Finally, the structure and performance of the organic material may be compromised by any impurities present in the environment. An extra measure of encapsulation is therefore needed to ensure that organic material does not become contaminated. 1 Basic Principles of Operation In tradition electronic devices polymer materials normally serve as insulators around wires or dielectrics within a coaxial cable. Obviously this class of organic molecules will not serve as basis for OLEDs. Conjugated polymers are the special group of organic molecules which can serve as carbon-based molecular semiconductors. 1 The conjugation is derived from the alternating single and double bonds between carbon

4 atoms at the polymer backbone. This structure causes the nonhybridized p z orbitals to overlap and thus resulting in delocalized p orbitals. As a consequence, the filled π bonding orbitals form the valence states, and the empty π* antibonding orbitals form the conduction states. 1 In general, this structure is only common to conjugated polymers with the exception of fullerene molecules. The need for well-delocalized valence and conduction states is a necessity for predictable semiconductor behavior. In the conjugated polymer the σ bonds between carbon atoms is strong enough to ensure the structural stability of the molecule even under excited π-electron states. If a π-electron is under a photo-excited state, the Coulombic attraction between the electron and holes binds them close together and thus creating an exciton. Since an exciton is strongly localized to the molecule a significant energy difference is present between a spin-singlet and spin-triplet exciton. It is impossible to obtain radiative emission from a triplet exciton without spin-orbit coupling. In comparison, singlet excitons are much more delocalized in the molecule than triplet excitons. It is important to note how disorder affects conduction in these materials. As disorder in the molecule increases, charges and excitons are more likely to be bound at lattice sites. 1 The structure of effective organic semiconductors can be seen in the figures below.

5 The principles discussed thus far fundamentally describe the science behind the operation of OLEDs. Organic LEDs operate through electron and hole injection from opposite electrodes, electron-hole capture within the bulk of the semiconductor film to form bound excitons, and subsequent radiative emission from those excitons. 1 The engineering of such devices aims to improve the stability of the exciton at the recombination center as well as minimize losses due to non-radiative emission. In the next section an overview of a novel OLED structure with efficient light emission is discussed Typical OLED Structure Before the research done by Tang and Van Slyke, the structure of an OLED consisted of a single layer of organic material. 2 Two characteristics inherent to a single layer of organic material made this device very inefficient at emitting light. First of all, organic materials have a much greater hole mobility than electron mobility. This feature of organic materials causes the recombination of holes and electrons to occur near cathode contact and thus leading to an increase in the probability of nonradiative emission. A second factor is that an uneven injection of charges will cause a considerable amount of holes to move across the device without recombination. 6 Unfortunately, these drawbacks to the single layer model required the device to be run at a high bias of 100 V in order to attain any substantial light output. 2 An incredible break through was made in 1987 when Tang and Van Slyke reported that a bilayer organic structure was able to efficiently emit light while running

6 the device at a low voltage. 2 From the bottom up, an indium tin oxide (ITO) layer, which served as the anode, was deposited on top of a glass substrate. The first organic material, an aromatic diamine, was the next film to be set down on the structure. The second layer of organic material, Alq 3, served as the electroluminescent film. The diamine layer has a high hole mobility and a low electron mobility, while the Alq 3 layer has a low hole mobility and a high electron mobility; therefore, the recombination of holes and electrons is effectively confined to the diamine/alq 3 interface. Through the use of transmission electron microscopy, Tang and Van Slyke determined that a smooth and continuous interface between the two organic layers is a necessity for efficient emission of light. 2 Finally, the top of the structure of the OLED consisted of a low work function metal to serve as the cathode. In the case of Tang and Van Slyke, an Mg 90 Ag 10 alloy was utilized as the cathode. The composition that they selected sought to reduce the atmospheric oxidation of Mg as well as increase the sticking coefficient of the alloy to the Alq 3 layer. The structure of the bilayer OLED is shown below. 2 The work of Tang and Van Slyke was an important step forward in the field of OLEDs. Ever since they fabricated an OLED with a quantum efficiency of 1% and a power conversion efficiency of 0.46% comparable to commercially available LEDs at time researchers have been looking to improve upon their original bilayer design. 2 Performance Improvement through Structural Variations

7 ITO-on-top Organic Light-Emitting Diodes: 3 Indium tin oxide (ITO) is currently the most popular material serving as the anode for OLEDs. Sputtering deposition and pulsed laser deposition are two different processes which are able to create a smooth film of ITO. For the most part, Vaufrey examined the effects that sputtering deposition had on the structure and transport properties of ITO. Through transmission electron microscopy, Vaufrey was also able to witness that the structure of the ITO was dependent upon the deposition conditions. 3 When the ITO was deposited by an unbiased low temperature sputtering technique, the structure of the ITO obtained was amorphous. Furthermore, the amorphous phase of ITO exhibits lower charge mobility than crystalline ITO but at the same time a very smooth interface with the organic film. The regular ITO/organic interface imply that the conditions were soft enough so that deterioration of the organic film did not occur. 3 Under high power or high substrate voltage conditions, the film of the ITO was of a crystalline structure with an expected decrease in the resistivity. The crystallization of the ITO at the organic interface is apparently induced by the -100 V bias of the substrate. The biased substrate does have its drawbacks. The increase in surface roughness at the ITO/organic interface suggests that degradation of the occurred due to the violent bombardment of ions in the sputtering process. The figure below depicts an atomic force microscopy (AFM) image of the ITO deposited with (right) and without (left) a substrate bias. 3

8 In a paper by Hong et al, 7 the group obtained an increase in the charge injection of the ITO layer by the addition of a thin film of buckminsterfullerene molecules. The research group was able to demonstrate that a thin film of C 60 between the ITO and NPD (a hole-transport material) layer significantly decreases the hole injection energy barrier. The shift in the energy barrier is also dependent on the thickness of the C 60 layer. The graphs below display the current density-voltage (J-V) relationship of a variety of thicknesses. 7 Addition of a Hole Injection Layer: 8

9 This research investigated the influence that a hole injection layer (HIL) has on the performance of Alq 3 based OLEDs. A total of four different HILs were tested to determine which offered the greatest enhancement to the device. The four different materials were: 4,4, 4 -tris{n,(3-methylphenyl)-n-phenylamino}-triphenylamine, (m- MTDATA); 4,4, 4 -tris{n,-(2-naphthyl)-n-phenylamino}-triphenylamine (2T-NATA); copper phthalocyanine (CuPc); and oxotitanium phthalocycanine (TiOPc). The thin film of the m-mdata provided the greatest increase in performance to the OLED. The images obtained by an AFM of the four different HILs suggest that the performance of the m-mdata is due to its superior uniform interface with the ITO layer. Below are graphs depicting the current density-voltage and luminescent-current density relationships of the four different HILs. 8 Addition of an Exciton-Block Layer: 9 The purpose of this research project sought to confine the triplet excitons which are created within the emitting layer of the device. The incorporation of starburst perfluorinated phenylenes (C 60 F 42 ) in different configurations either X or Y shaped was able to increase the quantum efficiency of phosphorescent OLEDs. For effective blockage of excitons and holes, these molecules were placed between an emitting layer and an electron transport layer such as Alq 3. There are three factors which make the C 60- F 42 molecules ideal for OLED applications are their low sublimation temperature and strong thermal and chemical stability. The figure below shows the structure of both the X and Y configuration of C 60 F 42. 9

10 Stability Enhancement through Structural Variations Improved Stability of Organic Electroluminescent Devices: 4 In 1996 Tang and Van Slyke along with the help of another scientist, Chen, published another paper in regard to improving the stability of the OLED they had developed in If OLEDs are going to be a practical alternative to active displays they must exhibit strong reliability. There are two main differences between their device in 1996 and the first one they created in First of all, the hole transport layer (HTL) has changed from the diamine, TPD, to a naphthyl doped benzidine, NPB. The naphthyl substituents in NPB molecule serve to enhance the thermal stability of the HTL. The second difference in the structure of the newer device is the incorporation of CuPc, a hole injection layer (HIL), between the ITO and NPB layers. The CuPc layer allows the forward bias voltage to either remain steady or rise very slowly under constant current excitation. Before the insertion of this layer, the voltage would increase steadily until an eventual dielectric breakdown. 4 In their study, they noticed that there are two distinct sections to the aging behavior of the OLED. During the first 50 hours of operation the performance of the device actually increases by a total of 10%. Afterwards, once the so-called forming

11 period has ended, the luminescence output consistently decreases at a rate of 0.15 cd/m 2 per hour for the next hundred hours. Through the collection of data from similar OLED devices, Tang and Van Slyke determined that the source of the degradation was due to charge injection and thus making it irreversible. Overall, the multilayer device, shown below, can operate for 4000 hours before its performance is cut in half. 4 Incorporation of a Bipolar Transport Layer in OLEDs: 10 This research group took a new approach to the structure of the OLED. Instead of following the traditional heterojunction first achieved by Tang and Van Slyke, they sought to induce charge recombination within a single bipolar transport layer. They realize that the heterojunction structure was crucial toward improving the performance of an OLED; however, the structure may also limit the reliability of the device. While the accumulation of charges at the heterojunction interfaces serves to distance the recombination zone from the metal electrodes, the high local field these charges generate may adversely affect the device reliability. 10 The goal of the group was to find a viable material to serve as a bipolar transport and emitting layer (BTEL) to replace the concept of the heterojunction. The device fabricated by Choong is shown below. 10

12 The BTEL consisted of a mixture of NPB and Alq 3 which was doped with methyl quinacridone (mqa). By changing the amounts of NPB and Alq 3, and thus its composition, the efficiency of the device is easily optimized. For example, a low composition of NPB would effectively increase the hopping distance for holes and thus lowering their mobility. The same effect is true for the Alq 3 its effect on the electron mobility. The inclusion of dopants is used to further improve the efficiency as well as the location of recombination. Overall, the utilization of BTEL has shown efficiencies comparable to heterojunction devices while also increasing the device s reliability. 10 Conclusion Ever since the initial developments made by Tang and Van Slyke, interest in the field of OLEDs has only increased. Furthermore, the broad applications almost any display one can imagine for OLEDs make it a very popular area of research for universities as well as high-tech companies. It is also exciting to note that some of the most influential research in this field is being done by Cornell s very own Professor George Malliaras. Presently, these devices are at the verge of widespread commercialization.

13 It was interesting to note that just as in solid-state LEDs a heterostructure was necessary to improve the performance of OLEDs. On the other hand, since the research done by Choong in 1999, it would be interesting to know the present progress of bipolar transport layers in OLEDs.

14 References 1 G.G. Malliaras and R.H. Friend, Physics Today 58, 53 (2005). 2 C. W. Tang and S. A. Van Slyke, Appl. Phys. Lett (1987). 3 D. Vaufrey et al, Institute of Physics Publishing. 18, 253 (2003). 4 S. A. Van Slyke, C. H. Chen, and C. W. Tang, Appl. Phys. Lett. 69, 2160 (1996) B. K. Crone et al, J. Appl. Phys. 87, 1974 (2000). 7 I. Hong et al, Appl. Phys. Lett. 87, 3502 (2005). 8 S. Chen and C. Wang, Appl. Phys. Lett. 85, 765 (2004). 9 M. Ikai and S. Tokito, Appl. Phys. Lett. 79, 156 (2001). 10 V. Choong et al, Appl. Phys. Lett. 75, 172 (1999).