PPV (Polyphenylene Vinylene) - Is This the LED Technology of Tomorrow?

Light-emission, resultant from the excitation of electrons in a conjugated polymer, is typically referred to as electroluminescence (1). Discovered first by the Cambridge Research Group, and led by physicist Richard H. Friend at the University of Cambridge; the group was initially trying to create an organic semiconductor out of the polymer, Poly (para-phenylene vinylene) (PPV) (2).1 PPV was already understood to have electrical properties, not unlike semiconductors, because of its conjugated structure (2). It was also understood, at the time, to have a strong material resilient enough to use as a base for thin-filmed devices (2). What the Cambridge Research Group, nor anyone else, had foreseen was, when a small voltage was applied across a purified version of the polymeric material, a bright light was emitted (2). Ultimately, the group went on to demonstrate the practicality of the active emissive properties of PPV in polymeric light-emitting diodes (5), and even started Cambridge Display Technology (CDT), a group that licenses the use of polymeric light-emitting diodes (PLED) to companies around the world (9).
Since the discovery of PPV as an electroluminescent polymer, physicists, chemists, and material scientists, have come together to formulate a wide array of light-emitting polymers (1). Recent research has led to enough enhancements in efficiency and reproducibility for the field to allow for commercialization of LED-based products (1). Largely due to their easy processing, there is an effort in the display industry to create cheap, flexible displays with full color, capable of being used in computer monitors and televisions, among other things (5). CDT, Richard H. Friend’s company, has stated that they intend to enter the marketplace for cathode-ray tubes, light-emitting diodes, fluorescent lights, and Liquid Crystal Display (LCD) technology. As a luminescent technology, it is expected to surpass LCD as the standard form of displayed luminescence. Until now, however, Organic Light-Emitting Diode (OLED) technology, based on polymerized PPV, has emerged as a common replacement for LCD screens in smaller items, like MP3 players.
The structure of PPV and the components that make up its monomer are conducive to its electroluminating abilities. The monomer consists of two compounds, a phenylene ring and a vinyl ethylene. The phenylene ring, a di-substituted benzene ring, is bonded in the para-position (C1 and C4), to the vinyl ethylene (or vinylene) and the adjacent monomer. The vinylene molecule, on the other hand, is bonded to the phenylene ring in the monomer, and sandwiched between two phenylene rings when the monomers are polymerized (7).
The structure of the overall polymer, poly (p-phenylene vinylene), is such that when the monomers are adjoined, the vinylene is connected to its neighbors in a staggered conformation at an angle of 123˚ (7). This affects the planarity of the polymer. The deviation in planarity can affect the excitation of π-electrons in the conjugated regions of the monomer. Extended π-conjugation is required for the best excitation of electrons, but the non-planarity of the polymer diminishes conjugation and lowers spectral emission (7).
The angles between the two compounds are caused by steric hindrance between the vinylene compound and the hydrogens on the phenylene ring (7). The intermolecular steric repulsion forces the non-planer conformation of the polymer to form (7).
Despite the non-planarity caused by the angles, PPV has morphology such that high levels of crystallinity are capable of formation (7).
The synthesis of PPV is important for its influence on electroluminescent properties. The goal becomes to attain as pure a polymer as possible. Increased levels of impurity can alter the luminescent properties of the polymer, and shift the spectrum of emitted visible light away from that emitted by the pure compound. Synthesis methods that cause the highest yield of pure polymer thus become an important factor in formulating electroluminescent polymers. It is also important for the likelihood of adding derivatives to alter the properties of the polymer.
There are several methods for the synthesis of PPV. The first attempt made at synthesizing PPV was a direct chemical polymerization (9). This yielded insoluble crystals that limited the range of applications of the polymer (9). As of 2003, the most popular method for the synthesis of PPV was a base-induced anionic polymerization of sulfonium salts, like p-xylene-bis-(diethylsulfonium chloride) (9). In this method, the polymer is procured by adjoining the monomers via thermal elimination, to remove the diethyl sulfide moieties at the ends of the compound, and the HCl by-product (9). 2
After the formation of the monomer, derivatives can be grafted on the positions ortho- to the 1,4-disubstitutions on the phenylene to alter the electrochemical properties of the polymer. These grafts, depending on what the deposited derivative compound is, can change properties of the lead compound, like voltage intake before degradation, intensity of luminescence, lifetime, and spectral emission, among others (4).
It appears to be difficult to synthesize large amounts of the monomer first, and then polymerize them in a large solution. This is likely because of the conjugation that exists in the vinylene precursor. It would be highly reactive, and end up causing the polymer to not be a homologous chain, but instead a copolymer consisting of separate vinylene and phenylene monomers. It would also prove much more difficult to form an alternating polymer, required for the most efficient and favorable emission. Rather, the compound would likely form a random (or statistical) polymer, resulting in less predictable and less controlled results. The current synthetic methods appear to favor methods that require the elimination of easily removable side moieties and leave units of the required para-phenylene vinylene monomer intact. This promotes more controllable intramolecular reactivity of the units, and thus, more predictable results.
The current method is chemical synthesis via an elimination of diethyl sulfide moiety from p-xylene-bis-(diethylsulfonium chloride). This is among the easiest methods of synthesis, and hence, the most popular. Synthetically, this process is preferred because it forms the lower energy trans-conformations of the chain. It also forms under relatively low thermal conditions of 246ºc for 6 hours (9). Perhaps the most favorable reason for its usage though, lies in the simple fact that this precursor to PPV can be easily handled in a solution (2). According to Richard J. Artley, a consultant with the Generics Group, an investor in Cambridge Display Technologies, it can be “shipped around the world like paint, put it on a substrate, then just heat it up to 200 degrees Celsius.” (2). Because of the ability to put PPV on a surface like one would paint, it is capable of being put on a wide range of surface-types, and as will be explained later, make it a versatile substance when it comes to application.
Compared to electrochemical synthetic measures, where the synthesis and elimination of side moieties are dependent on oxidation-reduction methods, the formation of PPV via chemical synthesis contains higher thermal stability (9). Thermal stability becomes an important property for the application of PPV to electronics, since the conduction that causes luminescence generates heat (9). A higher thermal stability will result in a greater range of colors as well as an ability to heat to higher temperatures without risking degradation, allowing for a longer lifetime for the electronic it is being used in.
From the earliest discoveries of PPV as an electroluminescent polymer, it had been understood that this polymer, and other polymers like it, could be placed between a transparent cathode and anode, and used to transmit light of various color (1). These early applications though, proved to be inefficient at converting injected charges into usable light (1). Naturally, developments continued and derivatives of PPV were made, like PPV/SiO2. New electroluminescent polymers were also synthesized, like poly (9,9dioctylfluorene), and have now led to enough stability and efficiency to allow for broader commercialization of PLED-based products (1).
In its applications, the polymer film is placed between a transparent anode and cathode. The anode is usually made out of indium tin oxide. The cathode, on the other hand, is usually made out of Calcium or Barium (1). The principle involved in the actual luminescence is the sandwiching of the polymer film between the diode, and upon injection of a charge, causing electrons from the Highest Occupied Molecular Orbital (HOMO) to become excited and jump to the Lowest Unoccupied Molecular Orbital (LUMO) (7). The HOMO and LUMO are separated by an energy gap (7). The energy required to cause an electron to jump from one molecular orbital level to the other falls within the range of the visible energy spectrum (7). This means upon excitation it will emit wavelengths somewhere between 400-700nm, allowing for visible light to be produced (7). This is the essence of how electroluminescence works in PLEDs.
Unmodified, pure PPV emits a greenish-yellow light upon injection of charge (2). It was thought that, until a “good” blue was formed by the alteration of PPV, only two-color displays could be created. However, after much intensive experimentation, the discoverer of electroluminescence, Richard H. Friend, and his fellow Cambridge colleague, Andrew B. Holmes, created derivatives of PPV capable of spanning the entire visible spectrum. PPV was also found to be capable of shining up to 10x brighter than the average television screen (1). With brighter luminescence, and the ability to give off any wavelength in the visible spectrum, PLED technology is quickly becoming a viable replacement for other, less efficient technologies, like LCD and Plasma. PLED applications are fast including other areas too, like emergency lights, and signs, since they can shine brighter and can give off more than two colors (1).
The key to widespread commercialization though, is the increase in light-output-to-current-input (efficiency) of the technology. Currently, the goal is to increase the efficiency of electroluminescent polymers as much as possible. So far, four major steps in device operation from which improvement can occur have been looked at in detail, and are considered to be key areas for optimal efficiency; charge injection from the electrodes into the polymer, charge transport, charge recombination, and decay of the excited state (1).
Improvement in efficiency can come from simple precautions, like lowering exposure of PPV to oxygen, especially near the cathode, since photooxidation reactions can cause the reduction of efficiency by up to ~50% (3). The cathode, being made of a compound, like calcium (Ca2+), can be grafted on to a polar locale on the polymer, and work similarly, in principle, to deposition of Ca2+ in PPV. During grafting or deposition, Ca2+ works by diffusing into the PPV and becoming reversibly encapsulated as a single ion. A higher concentration of oxygen near the cathode and near the polymer works to decrease the efficiency of the product by decreasing the ion diffusion coefficient. The reduction in value of this coefficient, by the presence of oxygen, decreases the distance between the potential minima and increases the average binding energy between PPV and Ca2+ (3). Grafting calcium onto the polymer thus decreases the ability of oxygen to interject itself between the two atoms via oxidation reaction. This increases the ability of PPV, or whatever other electroluminescent polymer is between the diodes, to emit light at, or near, its optimal efficiency (3).
While PPV is not the most efficient electroluminescent polymer in its pure form, it is worth noting that electroluminescence of organic polymers were initially expected to reach an efficiency of 10%, according to reports from the 1990s (2). Inorganic polymers have already been concluded to have an efficiency of 1% (2). This is indicative of how important the structure and conductivity of the polymer is in relation to the electroluminescence (2). Perhaps this conductive potential is the very reason why Richard H. Friend and the Cambridge Research Group were using PPV as their test subject in their studies on semiconducting properties or organic compounds.
Derivatives of compounds are another way of increasing the efficiency of electroluminescent polymers. Different derivative groups alter the properties of the polymer and increase efficiency to much higher levels than pure PPV. While it would take too long to go through all the derivatives or even many derivatives and what makes them so efficient, it should just be noted quickly, as a point of comparison, that derivatives like Poly (2,3-dibutoxy-1,4-phenylene vinylene) and 2,5-disilylated PPV have efficiencies of 40% and 60%. This is a significant difference, compared to pure PPV, and indicative of just how well they conduct charge through the interdiode polymeric film and how easily they excite their electrons from the HOMO to the LUMO (4).
Changing the way the world will watch television in the next decade, electroluminescence has currently had its largest effects on screen technology. OLED and PLED are allowing for thinner, brighter, more flexible screens to be made for televisions, digital cameras, cellular phones, computer monitors, MP3 players and billboards, among other things (6). The thin polymeric film allows for brighter displays with the same array of colors as current LCD-based technologies, but with efficiency anywhere from 5x-10x greater (8). This means that they use less electricity, and give off better quality picture. This is important, especially if put in a larger societal context, where energy consumption is only increasing on an annual basis. The employment of electroluminescent technologies will make energy consumption by monitors and televisions lower. This same technology will also allow for longer battery-life in cellular phones, MP3 players and digital cameras that employ display screens (2).
By 1993, the lifetime of PPV had already surpassed 1000 hours, and was expected to surpass 100,000 hours (11 years and 5 months) soon thereafter (2). Similar materials were measured to be stable for up to 10,000 hours (2). Current data on the lifetime of PPV or its derivatives, or its possible replacements were unavailable; however, if these were the numbers for 1993, then it is reasonable to assume that these goals have been surpassed in the 15 years since the predictions were made.
The future of electroluminescent polymer technology lies in the usage of these thin films in PLEDs for screen technology. This appears to be, not only its most practical application, but also its most commercially viable. Because of the flexibility of the material, the future entails monitors and television that are capable of being rolled up like carpets or projection screens, as well as portable battlefield displays for real-time updates, and plastic laminates that could replace lighting fixtures (6).
Looking further into the future, the possibilities are endless for where these developments can lead to. So long as the electrodes are made to be transparent, PPV, or any other electroluminescent polymer can be used to create transparent windows that can be doubled as screens with the flick of a switch (6). Alterations to the windows can also allow them to function as touch screens. This could bring to reality the science fiction concept of a transparent touch-screen, like the one seen in the movie Minority Report. These same screens could very well end up having better color and resolution than the monitors and screens presently in use.
As of right now, most companies are still focusing on the usage of this technology on smaller screens (6). Once they are able to put together a model capable of transferring the same resolution and color display to larger screens, it should not be long before LCD, cathode-ray tubes, and other present technologies become phased out.
It is quite apparent that the future of screen technology and all aspects of television-viewing and computer-using will be dependent on the simple notion of polymeric electroluminescence. It is a marvel that a long chain of a single monomer can be causal in the development of entirely new technologies, and lead to a sub-branch of chemistry that had previously been untouched and largely unforeseen. The fact that these developments are in conjunction with the on-going developments in nanotechnology indicate that there is a great deal of potential in compounds like PPV for widespread commercial application. Just the examples of television and the impact that would have on everyday life would appear to be enough to designate the discovery of electroluminescence a grand scientific discovery. In 1993, when lifetimes of PPV and its electroluminescence was much less than it is today, and its emission of color in the visible spectrum was severely limited, the estimate of commercial profitability was projected at $30 billion (2). With the more recent advances that have lead to it being a viable replacement for current technologies, it is near unfathomable as to just how commercially successful electroluminescent polymers might become.