Polymer-based LEDs

Following the encouraging results demonstrated by metal complex-based phosphorescent OLEDs [5,6,26], several groups started investigating a possibility to attain electrophosphorescence in solution-processible polymer-based LEDs. The first report on using... 

This book represents an attempt to illustrate the state-of-the-art of the basic physics and materials science of conjugated polymers and their interfaces, as related more-or-less to a specific application, present polymer-based LEDs, as of the time of writing (March 1995). It is intended to illustrate the basic fundamental physical (and the specifically associated chemical) principles that apply to these materials, which in many instances are different than those encountered in their inorganic counterparts. The references provided are not exhaustive, but rather are representative of the state-of-the-art as such, they should be sufficient to enable the interested reader to delve deeper into the area. Although by doing so some duplication occurs, references are grouped at the end of each chapter for... 

The nature of organic molecular solid surfaces and interfaces with metals. This is a summary containing a digest of the results of the investigations, where our view of polymer surfaces and interfaces , in the context of polymer-based LEDs, is summarized in a direct way. Sub-divisions include polymer surfaces polymer-on-metal interfaces and polymer-polymer interfaces. Different (ideal) models of the interfaces are outlined. 

Although polythiophene may not be presendy one of the most attractive materials for polymer-based LEDs, it has played an important role in LED development. For example, the first polymer-LED with a significant degree of polarized light was fabricated from a substituted polythiophene15. From the experimental interface standpoint, two of the more promising thiophene systems... 

Finally, it should be mentioned that the results of studies of the interaction of metal atoms other than those listed above with polythiophenes have been reported31-33. These other metals are mainly noble metals, however, which are not presendy under active consideration for use in polymer-based LEDs. 

Consider the polymer-on-metal interface, which might be prepared by coating a thin metal film with polymer in a polymer-based LED. The case of the counter electrode, formed by vapor-deposition, is discussed subsequently. First, assume that the substrates have clean surfaces hydrocarbon and oxide free, or naturally oxidized but still hydrocarbon free (pointed out as necessary). Typically, in connection with polymer-based LEDs, the metallic substrate could be gold, ITO (indium tin oxide) coated glass, the clean natural oxide of aluminum ( 20 A in thickness), the natural oxide which forms upon freshly etched Si( 110) wafers ( 10 A), or possibly even a polyaniline film. Dirt , which may be either a problem or an advantage, will not be taken up here. Discussions will alternate between coated polymer films and condensed model molecular solid films, as necessary to illustrate points. 

Further advances in the chemistry of processible conjugated polymers and focused work on the physics of electroluminescence in these materials have led to the development of flexible, almost entirely metal-free LEDs [55]. These polymer-based LEDs could be competitive in display applications because of the potential ease, low cost of fabrication, and large surface area of devices based upon processible polymers. 

PAVs have emerged as one of the most important classes of conjugated polymers. Ironically, while their electrical properties, which were the original motivation for their study, have proved disappointing, their electro-optical properties are such that they comprise the active material in the first commercially available polymer-based LEDs, and they have been used in some of the best performing polymer-based solar cells yet tested. While considerable development remains before polymer-based electronic devices can represent more than a tiny share of the electronics market, there is every reason to believe that when they do, PAVs will form a significant fi action of the materials used. 

One reason that organic LEDs fabricated by thermal evaporation of small molecules operate more efficiently than polymer-based LEDs is that thermal evaporation allows the deposition of multilayered structures with different molecular composition. The ability to create heterostructured molecular thin films by thermal evaporation allows one to tailor the electronic structure of the active layer such that carriers are efficiently transported to and confined within the light emissive region. The ability to create heterostructures within PLED active layers is not feasible, because most polymers are soluble in the organic solvents used for spin-coating, and thus the solvent used for casting a second layer will dissolve the underlying film. 

But in no case is the interface ever without some intermediate layer, which must ultimately be included in models of charge injection in such devices. Second, studies of the vapor deposition of calcium in the presence of Oi indicate an optimum in device yield fora pressure of about 10" mbar. This indicates that not only is UHV not necessary in the fabrication of polymer-based LEDs, but it is actually detrimental [101]. 

Photovoltaic cells were prepared with a polymer layer sandwiched between electrodes with different work functions, as in polymer-based LEDs (Fig. 29.2). In the dark, the device exhibits a rectification ratio of 10 at 3.5 V. Under illumination, devices of this type show a strong photoresponse, with open-circuit voltages of 0.6 V and short-circuit currents that correspond to quantum efficiencies of up to 6%. Under forward or reverse biases, the quantum efficiencies rise rapidly, reaching 15% at a reverse bias of 3.5 V, 40% at 10 V. and considerably higher values under forward bias. These performance figures are very much better than those for devices made with aluminum electrodes and either MEH-PPV or CN-PPV alone. Thus, Halls et al. report values for the quantum yield of the short-circuit photocurrent in the MEH-PPV of 0.04% at the peak response wavelength (2.2 eV) and on the order of 10 %... 

The events following the astute observation by Jeremy Burroughes of light emission from a layer of PPV held under an electrical potential that led to the description of the use of such materials as the electroluminescent layer in polymer-based LEDs, and the enormous amount of research and development associated with this area has now entered the folk law of the organic electronics field. [2.2]Paracyclophane-1,9-diene, the monomer shown on the left-hand side in Figure 27 (where R=H), is the obvious monomer to use in ROMP for the synthesis of such materials and Thom-Csanyi and coworkers were the first to show that this was possible. 

As presented in the Introduction, one major aspect of conjugated polymer-based LEDs is the presence of interfaces between the metal electrodes and the active polymer layer. In particular, the electron-injecting contact is made of a metal with a low workfunction such as aluminum. As this metal is known to be reactive, it is important to determine to which extent the phenomena occurring at the interface with the polymer can affect the geometric and electronic structure of the 7r-conjugated system and how the interface interactions can influence the operation of the device. 

Another experiment of time-resolved photo-induced absorption spectroscopy is necessary to characterize the dynamics of triplet-triplet absorption in detail. After the addition of Si02 Au nanoparticles into the PPV films, the dynamics of the triplet-triplet photo-induced absorption charateristic would be monitored. In addition, Si02 Au nanoparticles used in this work will not work directly in light-emitting diodes (LED), because typical conjugated polymer-based LED active regions are only 100 nm thick. 

---