Pipkin 1ADVANCEMENT OF PLED TECHNOLOGY: A reviewZachary Pipkin

IntroductionSemiconductors are of great interest to society, and impact our daily lives in countless ways. They are the building blocks to the entire technological revolution. Every electrical technological advice relies on these powerhouses small, lightweight, high speed and low power consumption. Semiconductors are often associated with silicon the most common semiconducting material used in electronics. These types of metal semiconductors are known as inorganic. However there are other types of semiconductors that exist, including those known as organic semiconductors. Organic semiconductors can be either composed of small molecules or large molecules (polymers), with each type having its own advantages8. Organic materials in general and polymers in particular have been extensively researched for decades, the reasoning behind this research varies across scientific disciplines. Some motivations include the ability to mimic biological systems (DNA is a polymer) and implement polymeric properties into electronics8. Organic materials promise technological advantages; polymers in particular are attractive for semiconductor technology due to low production cost and its applications. Polymers can be easily synthesized in labs, printed, and be spun-cast from solutions9; unlike their inorganic counterparts, polymer semiconductors do not require ultra-clean processing environments during any stage of their production. When concerning light and color emittance, these materials can be easily chemically tuned so their emission color can cover the entire visible spectrum4. Therefore polymer semiconductors and organic semiconductors in general are a technology of the future. It expands into many new emerging disciplines such as nanobiotechnology and technologies not even fully realized, such as biocomputers. Why are polymers useful in electronics? When you think of the word polymer, think of plastics. Plastics are polymers too, and are a suitable example as to why polymers are advantageous plastic is: cheaply made, tough but thin, can be transparent or opaque, can have high or low melting point, can be brittle or tough, can be soft or hard; it can exhibit a wide range or properties depending on its chemical subunits. This is the advantage of using polymers in electrical production cheap cost, biological applications and ways to physically apply them that arent achievable with the rigidity of metal semiconductors1. The topic of organic and polymer electronics is so broad, this review emphasizes the development of one type of polymer semiconductor. Light emitting diodes (LEDs) are semiconductors that emit light via photons when exposed to electrical current7. They are used in a variety of technologies for illuminating electronics, food, cosmetics and art while emitting much less heat than other light sources. Organic light emitting diodes (OLEDs) and a type of OLEDs known as polymer organic light emitting diodes (PLEDs) have specific advantages in applications for emitting light. The first OLEDs were demonstrated in 19876, and the first PLEDs were demonstrated in 19901. Since then both have been brought into the market, but there is still plenty of research to be done until a full understanding of the technology is realized. Polymer LEDs have the added benefit of potential printable and flexible displays of electronic devices, which would impact all industries10. The advancement of PLEDs showcased here also tracks the advancement of OLEDs and furthermore organic/polymer semiconductor technology as a whole. Also discussed is the benefits to the emerging science of thin, printable, flexible electronics, and possibilities for future biological and medical applications. Electronic technology could be entirely revolutionized affecting all industries, sciences, health and medicine. The way electronic technology is now actively involved our world means future revolution in light and electronic mediums will change how current technology is used as well as create new technological devices for bettering society. This will require engineers, chemists, physicists and other peoples from a wide range of disciplines to collaborate in understanding and developing this technology.

Emergence of PLEDsBy 1990 solid-state LEDs were integrated into society8. Inorganic LEDs were efficient and found widespread application, and now molecular (organic) LEDs were being developed scientists were finding ways to exhibit a wide array of colors and make them more flexible, but stability was an issue8 in the organic films. In 1989 the first polymer-based light emitting diode (PLED) was discovered using PPV as the emissive layer1. Burroughes et al. explored the advancement of moving from molecular molecules in regular organic semiconductors to macromolecular materials (polymers). Burroughes explained that utilizing polymers for light emission is a good choice in theory since they should be able to produce a high amount of photon emission, provide good transportation of electrons and provide both good charge transport1. Research had been exploring conjugated polymers and their properties as conducting materials, but not as luminescence materials1. Until then polymer research was focused on electrical flow only. As discovered by Burroughes, conjugated polymers that have larger semiconductor gaps can show high yields of photoluminescence. This study used poly (p-phenylene vinylene) or PPV to make high quality luminescent films8. The advantage of polymeric organic semiconductors is the processability of polymers. This work set a foundation for all advancement in polymer LEDs and future lighting technology. Polymers do exhibit luminescence depending on what type of polymers you use and how you chemically alter parts of their chains11. Thus solid-state electroluminescence can be developed from polymers, which allow for large-scale, cost-beneficial production and allows for fine-tuning of the electrical and light emittance properties. This research advanced light emittance technology in OLEDs and opened up a new field known as PLEDs. Polymers are particularly useful since they are composed of chemical subunits and thus can be altered quite easily in labs. The physical aspects of polymers are unique as a macroscopic material, but the chemical properties at small scale describe how chains interact through their chemical bonds and can output changes in color, strength, stiffness, solubility and more. Due to its stability, processability, and electrical and optical properties, PPV has been considered for a wide variety of applications including electron donating in solar powered cells9.

PLEDs Cover the Visible SpectraAs PLEDs began to emerge as commercial products, research began to look for the different ways to fine-tune their optical and electrical properties by varying their polymer structure. Research led by Ego, Friend et al. in 2003 focused solely on developing full color flat-panel displays and tuning the colors that PLEDs emit. Interestingly enough Friend was also a researcher of the aforementioned study. The objective in this study deals with keeping the PLEDs easy to process while making sure they exhibit high luminescence with low turn on and operating voltages and good chemical and electrical stability as well as photostability4. At the time, color tuning was only known to be achieved by substituting chemical groups in the polymers, controlling effective conjugation length, or by blending it with another emissive material (chromophores)4. Chromophores are the group of atoms and electrons forming part of an organic molecule that causes it to be colored11. By blending red-emitting tetraphenylporphyrin into a blue-emitting polyflourene for example, a red-emitting LED is produced. However these dyes have a tendency to show phase separation over time and lead to device instability. The researchs solution is to attach the dye covalently to the polymer instead of blending it in. Ego et al. were able to tune emission colors over the whole visible region by attachment of perylene dyes to chains of polyflourenes as either comonomers in the main chain, endcapping groups at chain termini, or as pendant side groups. Since polymers can be synthesized in labs, the length of chains, type of subunits and atoms present in the large chain molecules can be chosen, giving the technology customized properties. This research further exploits this idea by showcasing the different color emittance potential of PLEDs.

Ultrathin, Highly Flexible and Stretchable PLEDs for a New TomorrowA research study in Nature Photonics in June 2013 produced the first PLEDs that can be stretched and crumpled while lit. Thinner PLEDs can be more flexible. Though todays PLEDs are very thin (just a few hundred nanometers thick) they have to be surrounded by bulky layers of electrode metals12. Also todays commercial PLED materials are sensitive to air and water, and need a protective layer on top that encapsulates them. White et. al produced their PLEDs on a 1.4m thick foil substrate (extremely thin). Its hard to spin-coat PLED materials onto such thin films, but they achieved it by sticking the films onto rigid silicone-coated glass, which holds them in place using van der Waals forces alone, allowing easy post-fabrication removal. Basically, you peel the foil off12. However, the devices electrode materials werent air stable and they only worked for a few hours. Being so thin gave their devices uniquely small bending radii, which allowed them to be crumpled. Attaching the films to extended elastomeric tape made stretchable PLEDs11. When the tape contracts the PLEDs fold, and will pull flat again when stretched. This advancement of light-emitting foils is an important advancement towards PLEDs integrating with materials like biotextiles and artificial skin. Stretchable electronics opens up completely new possibilities we could apply them to a variety of surfaces including our bodies. Materials used in electronics, even conducting and semiconducting polymers, do not currently share this property. It is also compatible with spin coating, screen-printing, ink-jet printing, and a multitude of other processing techniques according to the researchers. The ultrathin PLEDs presented in this work are the thinnest and most flexible electroluminescent devices to date. This work represents a major step towards the realization of newer medical technology. The option to combine flexible, stretchable surface molding electronics could allow us to incorporate electronics into medical fabrics. Integrating OLEDs into Wearable Products with Medical ApplicationsS.K. Attili et al. focus on ambulatory photodynamic therapy (PDT) in their 2009 article. PDT is a popular treatment for nonmelanoma skin cancer with clearance rates between 70% and 100%. Although reported to have superior cosmetic outcome, the inconvenience of hospital visits and discomfort experienced during therapy are PDTs shortcomings. The pain associated with PDT for skin cancer isnt well understood, but the degree of pain depends on the light source used and the intensity of light delivery2. S.K. Attili et al. attempt to study an ambulatory PDT device. Twelve patients invited to participate in the study had lesions prepared via gentle superficial curettage without local anesthesia, and self-adhesive gel layer was applied (to prevent slippage). A small device made from a power supply and patch of containing the OLED was administered while patients sat in a waiting room for the entire treatment period; lesions were assessed for efficacy of treatment at three, six, nine, and twelve months following the surgical treatment. Patients were asked to score maximal pain and discomfort level as well. Overall, the efficiency of the device wasnt statistically shown. A possibility is that the lesions treated with the device were >one-five centimeters in diameter, while the patch the patients applied had only a two centimeter diameter. Perhaps a larger patch wouldve worked and the research suggests that improved outcomes would have been obtained with larger devices. This study has other beneficial information - it still suggests that OLED-PDT is less painful than conventional PDT with the added advantage of being lightweight, and therefore has the potential for more convenient 'home PDT'. It can be incorporated into potentially disposable and lightweight OLEDs so that patients can remain mobile2. Again, polymers wide range of properties allows them to be integrated into future devices such as these. This means that light applications can be applied to stretchable material, and the material itself is responsible for the stretchy properties as well as the light emission.

Deep Blue PLED Emission and Phototherapy PotentialResearch has since been able to manufacture PLEDs that emit lots of different wavelengths due to the knowledge of how to covalently bond chromophores to molecules. In a study conducted by U. Giovanella et al. in 2013, the knowledge of fluorinated phenylene as a alternating copolymer is used to develop blue polymeric emitters. Deep blue light is made in the study and the possibility for biomedical applications is explored. It is challenging to make deep blue and UV efficient emitters with PLEDs6. Polyflourene derivatives have attracted interest due to their high photoluminescent efficiency, but has been limited due by their poor color stability during operation lifetime of the PLEDs10. By introducing fluorinated functional groups, the research team was able to reduce oxidation and provide a smart strategy to improve color purity and durability of the materials in the devices6. They used a method known as Suzuki coupling to achieve this. Since the electroluminescence of their new polymer poly[(9,9-di-n-octylfluorene-2,7-diyl)-alt-tetrafluoro-p-phenylene] (PFOTFP) falls in the border region of the visible spectrum, the PLED performance fulfills the requirements for devices to maintain tissue sterility2. A photostability test was preformed under ultraviolet irradiation or sterilization of the surface to be placed in contact with the wounded skin. The polymer was directly heated by a 100 mW UV lamp at 365 nm for five minutes. The photodegradation of PFOTFP is reduced. This means that deep blues can now be more easily understood and can be applied in full color high-definition displays6. Before, balancing the large band gap between the materials and the charge balance were hard to equalize for optimum properties you need for deep blue light. Besides creating new displays, blue light therapy could be incorporated into biomedical applications. With knowledge from the next previous and forthcoming articles, phototherapy could one day be implemented into medical textiles. Ready-to-use bandages that emit this blue light could be envisioned. Mass production of this PLED phototherapy would allow for transportable therapy. Instead of sitting around for light therapy treatment, people could wear materials with the electrical phototherapy, walk around and enjoy their day. This technology has the potential to revolutionize the way we use phototherapy. Flesh wound treatment on-the-go in materials such as medical gauze can be envisioned, and lower rates of infection are disease spread are possible. This technology can also be envisaged to revolutionize other avenues of phototherapy, such as treating psoriasis with phototherapy embedded textiles, or being able to treat jaundiced babies while they remain mobile embedding the phototherapy technology into blankets, bandages or clothes. PLEDs offer not only direct medical treatment, but indirectly as well. Light itself regulates the biological clock of humans and has an amazing effect on people. It can affect their psycho-physiological processes such as mood, season depression and anxiety levels. By harnessing PLED light and using it effectively in healthcare facilities, one can enhance the healing environment and play a key role in promoting the well being of patients and staff13.

ConclusionsThis article has generalized the study of PLEDs since their discovery and the benefits of organic luminescence, as well its potential benefits: unprecedented physical applications, the possibility of medical applications, and cheaper production of electronic systems overall. Since the technology is fairly new, a more underlying understanding of this technology should be developed and possibilities of advancement in PLED technology will come to fruition alongside the understanding of the complex and variable properties of PLEDs. PLED science has advanced dramatically, yet there is still much research to be done in perfecting the chemistry and physics to achieve the properties we desire, as well as ease of production. Advancing alongside PLED technology, polymer semiconductors as a whole will advance, but for now processes associated with plastic electronics must be run so that they remain significantly less expensive then those associated with the established and more efficient silicon electronics industry. Eventually it will be more accessible and cheaper for large-scale applications. In the world of medicine and disease, any technology from simple adhesive bandages to complex medical devices could benefit. The advantages of PLEDs are numerous, with a major benefit in biomedical textiles and the emerging flexible electronics industry. This would impact the face of medicine. Polymers can have seemingly limitless options for physical characteristics and colors, and applying this to LED technology itself will open the door to an electronic and textile revolution, pushing medicine forward. As polymer science, organic semiconductor science and their engineering techniques develop; further advances will be made in polymer semiconductors to fulfill other uses that inorganics and small organic molecules cannot. Once fully understanding the technology it can be integrated into carriers, such as biomedical textiles and biological systems. For healthcare devices that come into contact with our skin, there are also specific challenges related to avoiding allergic responses, ensuring biocompatibility and sterilization. Invention of devices such as sensor filled adhesive bandages will change how we combat disease and infection. Phototherapy can be more easily employed and accessible. The ability to produce transparent and flexible displays will be noticed, as the capability to blend electronics with fibers and textiles become a reality. Electronic medical textiles could display patient information such as heart rate and oxygen levels on clothing and gowns; electronic papers could automatically change information, and biocomputers could be created inside of living cells to directly interact with cellular processes. Medical device improvement and electrical integration with our clothing and our bodies can advance in ways weve only imagined. Research in the near future will most likely deal with understanding how to make organic semiconductors do exactly what we need by altering their chemical subunits and molecular geometry, and how to eventually move toward wide-scale use of printable electronic technology. As of now, organic thin-film transistors have been developed3. If we are able to easily do this in the future, we will have cheaper material and solution processing as well as lower production cost. Some applications discussed may come to fruition in the next several years, some in the next several decades. Regardless, we are on the cusp of an better technology with polymeric organic semiconductors.

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