Organic Leds.Indd

Organic LEDs Small lights, big impression Andy Extance goggles at the display revolution, the culmination of 30 years of research into organic light emitting diodes

Gown-clad students smile, posing for photos. Fluorescent compound Alq (left) enabled the It is part of a life-defining moment for them, 3 first generation of OLEDs but it is also one that recently brought the but has a maximum University of Michigan’s Stephen Forrest’s internal quantum career achievements into focus. He saw two efficiency of 25%, while women taking pictures of a third, one using a Ir(ppy)3 (right) can reach smartphone with an liquid crystal display (LCD) 100% screen. But the other used a Samsung Galaxy, whose organic light emitting diode (OLED) display exploits technology Forrest developed. ‘I had a clear shot at a 1:1 comparison,’ he enthuses. ‘The difference was breathtaking.’ The Korean electronics giant has shipped over 300 million products featuring OLED displays, generating an estimated $7 billion (£4.5 billion) in global revenue in 2012. That success evolved over more than 30 years, with careful molecule selection and design making OLEDs the market-leading displays. But with companies moving from pocket-sized displays electroluminescence: the recombination of laboratories in Rochester, New York in the to 55-inch-plus TVs, manufacturing problems an excited electron with a ‘hole’ left by the US, Ching Tang was struggling to develop remain to be solved. electron’s absence. On meeting in the molecule, organic photovoltaic cells. A mixture of LCDs typically shine a single white the excited electron and hole sit in the material’s polymer, absorbing dye and hole-transport ‘backlight’ at viewers. Its light passes through a previously lowest unoccupied molecular molecules segregated into two separate liquid crystal layer and the thin-film transistor orbital (LUMO) and highest occupied molecular phases gave slightly better results, and so he (TFT) that controls the image signals, orbitals (HOMO), respectively. The electron and pursued the idea. polarisers, diffusers and colour filters. By hole bind together, becoming an exciton. When And when Tang, now at the University of contrast, manufacturers can deposit separate the electron fills the hole, the size of the energy Rochester, connected up a later, more efficient 100nm-thick OLEDs directly onto a TFT. Each gap between the LUMO and HOMO determines two-layer cell to study current flow through it, one lights up independently, without other the colour of the light it releases. he saw a dim glow that sparked his curiosity. optical components. That brings more vivid One layer was transporting holes, and the colours and truer black, faster response times seeing the light other electrons, to the interface between the that avoid blurring and suit 3D images better, While scientists saw organic molecules materials. There they recombined efficiently, and lower power consumption. ‘They knock electroluminesce in the 1950s, perhaps allowing Tang to make light from organic your eyeballs out of your head,’ Forrest says. the first faint sign that it could be usefully compounds at much lower voltages than OLEDs emit light through a process called harnessed came in 1979. At Eastman Kodak’s ever before. ‘This is today called an organic

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heterojunction, and almost all organic By 1989 the Kodak organic semiconductor programme, wanted electronic devices have one,’ Tang says. ‘I was amazed that scientists2 had to patent their discovery first. That was not Unsure how to use the technology, Kodak doubled the internal common at the time. ‘When we told the hired Steven Van Slyke to help Tang. They with those three quantum efficiency university that’s what we wanted to do, the chose materials to increase light output, components, (IQE) of electrons response was “Why?”’ Bradley remembers. lower drive voltage and therefore power the device lasted producing photons The Cambridge team persisted, filing patents consumption, and produce a stable device. overnight’ to 2.5%. To do this, before writing a conventional paper describing ‘Kodak had a huge library of chemicals made they modified a green/yellow device much simpler than over 100 years,’ says Van Slyke, now chief their original Kodak’s small molecule versions, simply PPV 3 technology officer at Kateeva in Menlo Park, design by doping the Alq3 ‘host material’ sandwiched between two electrodes. They California, US. ‘They allowed me in with a UV with fluorescent coumarin or conjugated spun their patents off into a company called light. Some bottles were clear and some had dicyanomethylene molecules. ‘Introducing a Cambridge Display Technology (CDT), which powder around the neck, so you could see the small amount of highly luminescent materials now also includes polyethylenedioxythiophene compounds getting excited.’ in a host matrix, we could change colour and (PEDOT) hole transport layers in its polymer improve device efficiency and lifetime,’ Tang OLED sandwiches. O for organometallic explained. That sandwich of hole and electron Roaming this luminescent library, transport layers with a host–dopant emissive Brightening dim hopes one uniquely fluorescent compound layer filling is still used. As the 21st century approached, OLEDs was obvious to Van Slyke: tris-8- Also in 1989, unaware of Kodak’s OLEDs, remained inefficient in spite of their early hydroxyquinolinatoaluminium, or Alq3. Jeremy Burroughes was working at the hopes, partly because fluorescence has a

Tang and Van Slyke found an Alq3 layer both University of Cambridge, UK, to find an fundamental quantum mechanical limit. transported electrons and emitted light. insulator to put in a prototype conjugated Singlet excitons, made when holes and Putting an electropositive, or low work polymer transistor. His colleague, Donal electrons with opposite spins meet, are function, 10:1 magnesium:silver electrode Bradley suggested polyphenylene vinylene allowed to fall to the singlet ground state and on one side injected electrons into Alq3 well. (PPV), which he had worked with in his PhD. emit light. Triplet excitons, made by holes On the other side, they put an aromatic But they were also interested in its optical and electrons with the same spin, are not. amine hole transport layer, and finally a high properties, for use in telecommunications. Recombination produces singlet and triplet work function transparent indium tin oxide And when Burroughes applied an electrical excitons in a 1:3 ratio, limiting IQE at 25%. (ITO) electrode to inject holes.1 Together, field to PPV to test these properties, he saw The US scientists Stephen Forrest, then at these elements powered ‘a huge jump’ in the light emission. Princeton University in New Jersey, and Mark resulting green OLEDs’ lifetime. ‘Most devices ‘We then had to invent a way to use that Thompson, at the University of Southern then lasted just minutes,’ Van Slyke says. ‘I discovery,’ recalls Bradley, who is currently the California in Los Angeles, thought they could remember being amazed that with those Director of the Centre for Plastic Electronics at dodge this rule. First they made an unusually three components, we could turn the device Imperial College London in the UK. But rather bright red OLED using a platinum-based on overnight and sometimes in the morning it than write a paper, Burroughes, Bradley and dopant.4 They realised that near the large would be a little brighter.’ Richard Friend, who headed Cambridge’s metal nuclei, excitons experience spin–orbit

www.chemistryworld.org | June 2013 | Chemistry World | 51 Organic LEDs © xuhua wang Organic and polymer LEDs have the additional benefit of allowing flexible displays

make OLEDs more stable. ‘Excitons are the biggest destructor of these materials,’ Forrest says. ‘They collide with an electron or another exciton, doubling their energy. If that energy couples with a bond on a molecule, it destroys that molecule. In a fluorescent material, the triplet can last for seconds, wandering around dying to do some damage.’

Making choices Now that OLED displays are a practical reality, Germany company Novaled helps lower their power consumption further, bringing other benefits as it does so. Their transport layer effects. Triplet excitons’ motion and spin ‘[Iridium] is the key to everything,’ Forrest materials, also omnipresent in Galaxy phones, interact with each other, nudging them to fall says ‘That one blew the lid off the field. It contain small molecule organic redox dopants to singlet ground states and emit light through wasn’t long before we were demonstrating that smooth the journeys through devices phosphorescence. 100% IQE. At that point the OLED field took taken by electrons and holes. Then, in 1998, Thompson saw a green off as a practical solution for displays.’ Forrest ‘Electrode materials’ work functions electrophosphorescent material’s optical and Thompson’s industrial partner Universal are mismatched with the transport properties presented at an American Chemical Display Corporation, also based in Princeton, materials’ HOMO and LUMO levels,’ explains Society meeting. The team immediately knew has commercialised phosphorescent dopants, Sven Murano, vice president of product the molecule, fac-tris(2-phenylpyridine) selling them for red and green subpixels in management for Novaled in Dresden. ‘We add

iridium (Ir(ppy)3), was an ideal every Galaxy phone. ‘There’s a direct lineage strong electron donors on the electron side, phosphorescent dopant. And when Forrest’s between those molecules and the ones that we and strong electron acceptors on the hole student Marc Baldo, today a professor at the developed,’ Forrest says. side, producing free charge carriers in the Massachusetts Institute of Technology, made Beyond brightness, returning triplet layer. That provides much greater freedom in an OLED with it, it smashed the IQE limit.5 excitons to ground state electrons also helps choosing electrodes, and controlling charge

52 | Chemistry World | June 2013 | www.chemistryworld.org Organic LEDs

Even though OLEDs are a mature enough technology to feature on many commercial products, there is still more research into them © lg

carrier transport and energy level. Our is developing inkjet equipment to do just versus small molecule, but solution versus technology allows new power consumption, this. ‘Manufacturers like this because all the vacuum,’ Bradley says. ‘At the Consumer colour quality and lifetime optimisation, and material ends up on the substrate, wasting Electronics Show in Las Vegas in January, therefore better compromises.’ far less, with the same appearance and same Panasonic presented its 56-inch OLED TV, The chemical sandwich in the most power consumption as shadow masking,’ he alongside Samsung and LG’s 55-inch models. common OLED displays is layered by says. But it is difficult to control a printed film’s The principal difference is that Panasonic’s is evaporation in high vacuum tools onto thickness and uniformity. ‘All the solvent has inkjet printed.’ Despite these announcements, electrode pads in TFTs previously deposited to evaporate, and he stresses that there’s more research to be on glass substrates. That includes Novaled’s ‘The key question there’s a lot, because done, for example on getting emission from doped transport layers, formed by heating you’re often at 1–3% triplets without using expensive iridium. compounds until they sublime or evaporate, isn’t polymer concentration,’ Van ‘You might imagine that with a 55-inch TV before they condense on a cooler target. versus small Slyke says. ‘The commercial product there wouldn’t be much However, that process can be difficult to molecule, but resultant film must more science to do,’ Bradley says. ‘In fact manage. meet uniformity and there’s a lot more.’ ‘There is a small temperature gap between solution versus thickness targets OLED innovations also benefit other evaporation and decomposition,’ Murano vacuum’ over the whole aspects of organic electronics, most notably explains. ‘Manufacturers want to raise substrate. This is photovoltaics. And even though that’s where temperature to increase deposition rate all possible with inkjet, but people are still Tang started out, he cautions success there and production speed, but at a certain level working on it.’ could be harder than the display market. decomposition sets in. We are trying to widen One ink supplier is Sumitomo Chemical, ‘Organic solar cells compete with silicon,’ that gap, which takes molecular design to find based in Tokyo, Japan, which bought CDT in Tang says. ‘OLED doesn’t. Other uses will out where the molecule is weak, and propose 2007. CDT’s polymer OLEDs are constructed have to find space, but OLEDs are finding real new core structures.’ from solution, enabling inkjet printing commercial applications. One thing you can Samsung’s displays contain red, green since 2004, including on flexible films. That be sure of: OLEDs will set the display quality and blue subpixels side by side. It makes requires layers coated later not to damage standard for future televisions.’ them by vapour deposition, applying OLED previous ones. ‘The chemistry is designing materials in uniform films across a series of the polymer’s electronic structure,’ Donal Andy Extance is a science writer based in fine metal mask (FMM) stencils, one for each Bradley comments. ‘But it’s also designing Exeter, UK colour. ‘That wastes material, and the FMM how peripheral polymer components control can cause damage,’ Steven Van Slyke says. But its processability. They control which solvents References for making large TVs, the biggest problem is you can use and how soluble the polymer is 1 C W Tang and S A Van Slyke, Appl. Phys. Lett., 1987, 51, 913 (DOI: 10.1063/1.98799) that FMM cannot get to larger mother glass in different solvents, to give layers you can 2 C W Tang et al, J. Appl. Phys., 1989, 65, 3610 substrates that can produce several 55-inch deposit on top of each other.’ (DOI: 10.1063/1.343409) displays simultaneously. Though displays so far favour vacuum- 3 J H Burroughes et al, Nature, 1990, 347, 539 deposited small molecule OLEDs over (DOI: 10.1038/347539a0) Film stars 4 M A Baldo et al, Nature, 1998, 395, 151 polymers, printing’s potential means that (DOI: 10.1038/25954) One alternative is to print the layers across researchers are also developing it for small 5 M A Baldo et al, J. Appl. Phys., 1999, 75, 4 larger glass sizes, and at Kateeva, Van Slyke molecules. ‘The key question isn’t polymer (DOI: 10.1063/10.1063/1.124258)