Today we will describe our first Organic Light Emitting Diode, or OLED. Electroluminescence refers to the phenomenon when light is emitted from an electrical current, flowing through a solid. The first report on electroluminescence in organic solids dates back to 1963. It was observed in single crystals, that emitted a blue light. Unfortunately, the crystals were quite thick so the voltage required for lighting the machine amounted to several hundreds of volts, which render any practical application very unlikely. All attempts to reproduce the effect on thin films were unfruitful. It is ony 25 years later than Ching Tang from Kodak succeeded in producing on OLED with reasonable efficiency. By using a two layered device, three years later another break through came from the University of Cambridge in the UK, with amoled using polymers. The first commercial application came quite early, in a K radio receiver by the Japanese society pioneer, in 1997. Since then, onades have known spectacular growth. They are currently found in the display of many smartphones, and also for lighting and in television sets. Basically, an onade has a structure of the organic dought. We have this crap earlier. The process of light emission can be decomposed in four steps. Injection of electron holes. Transport of electron holes inside the semiconductor. Recombination of electron holes to form an exciton. And finally radiative decay of the with emission of a photon. An important point is that organic counterparts, organics from the connectors are very low density of charged carriers of both kinds. Hence, the necessity to inject both electrons and holes. Let's first focus on the recombination statement. The process can be described in terms of right equations for electrons and holes. The first equation states, that the time evolution of the density of electrons n is the difference between the generation and a decay term. Generation comes from the injection of electrons, it is proportional to the derivative of the electron columns, decay corresponds to the recombination of electrons and the holes, it is proportional to the density of electrons n and the host p, times second order right cancels gamma. An equivalent equation holds for the density of holes b. If we assume steady-state, that is a density of electrons and holes are constant with time, and integrate the question of whole thickness of the organic diode. We arrive to the conclusion that complete recombination requires that holes and electrons currents are equal. In other words, any excess current of one of the charge carriers will be lost, will only lead to heeding the diet [INAUDIBLE]. This requirement is a primary element for the optimization of [INAUDIBLE] The mainly [INAUDIBLE] in all that is charge [INAUDIBLE] injection. The first modern was invented by Tang et van Slyk from Kodak. In 1987, the big advance was to use a two-layer diode with diamine which behaves as a p-type organic semiconductors and aluminum Tris (8-hydroxyquinoline) or Alq3 which is of the n-type semiconductor. We recall that p-type and n-type refers to easy hole, or easy electron injection. To further reduce injection barriers, the anode is indium tin oxide, or ITO. Transferring conductor with high work function, and the cathode is magnesium, a low-work function metal. Another advantage of the is that the energy barrier that forms at the interface between the two organic layers tends to confine the charge carriers in their way to the other electrons. So the electrons are confined in the n-type semiconductor and holes in the p-type semiconductors. Such a double confinement would favor electron hole recombination in the set of the diode. Further improvement was brought with the three layer structure. With an emitting layer EL, inserted between a P-Type and an N-Type semiconductors, now called hole transport layer HTL and electron transport layer ETL. And electrode as usual. Parent include other refinements with an increased number of layers. The efficiency of all that can be at various levels. The quantum yield, is the ratio of the number of emitted photons to that of injected charge carriage. Power efficiency relates to the ratio of light to electrical power. While luminous efficiency corresponds to the ratio of the luminous flux to the electrical power. Let's now look at this more closely. The quantum efficiency can be decomposed into four components. The first one is re-combination. As stated earlier it greatly depends on the balance between electron and hole currents. For a well-balanced diet, this parameter can arrange a value of 1. Probability of radiative decay refers to a quantum effect linked to the spin of the electron. It is one force if the emesis process fluorescent but reached 1, if the died used Phosphorescence. We shall see that in more details soon. Fluorescence or Phosphorescence quantum yield, is the ratio of radiative to non-radiative decay in the immersive layer. One of the advantage of organic solids, used to have high limited yields. The last term relates to the capacity of the emitted light to escape from the. Let's now look in more details at what we have called the probability of radiative decay We recall that electroluminescence consist of the current recombination of an electron and a hole to form an exciton. Now, depending on the respective speed of the electron and the hole, the exciton can be singlet or triplet. Now one singlet state, and three is triplet states. Hence the total of four states. Because electron holds the combine at random, they statistically one chance out of four to have a singular exciton. If the word emits through fluorescence, that is from the excited singlet state. Kai is only 1/4. The solution found to improve kai, is to convert all the singlet state to triplets via intersystem conversion. So know them the emissive layer emits by phosphorescence and ki equals one. Let's now turn to the limit distance quantum yield. Early devices used compounds with limited yield. Like the polymer, polyphenalinvanilin\g and it's soluble derivative, MEH-PPV. This was also the case with Alq3. With s yield of only 25%. The problem resides in the fines that compile with high Have usually poor trash transfer properties. So trade off, are found by dumping good organic cement connectors like >> With a die of high luminescence sealed, like dimethylquinacridone, which emits in the green, or rubrene, which emits in the red. The last contribution to the concert yield, is a fraction of flights that actually escapes from the device. The issue is related to an optical phenomenon called total reflection. When light passes from the medium with high refractive index to another medium with lower refractive index. All the light emitted at another lower at a given number, is totally reflected. That is, it doesn't get out. Light emitted by the OLED, has to pass through media of various refractive indices. The organic part has a refractive index of around 1.8. The glass substrate, a refractive index of 1.5. And the rref activates for air is one. When passing from the organic diet to the glass abstract around 50% of the light is lost, and when going out the glass there is an additional loss of 30%. All in all, only 20% of the emitted light is retrieved. The energy efficiency is defined as the ratio of light energy to the electrical energy. That is the number of emitted quantum energy h u. Divided by the electrical charge to pass through the diet times the ply voltage V. This can be returned as the quantum yield, times the quantum energy of the photons divided by the elemental charge times the voltage. So the lower the applied voltage the higher the energy efficiency. Note that because power is an energy per unit time, the energy efficiency is strictly equivalent to the power efficiency. The number of interest for lighting application, is the so-called luminous efficacy, eta sub L. Given in lumen per watt. The interest of this physical quantity, is that the light intensity is rated as the of the human eye, given by the eye sensitivity function V. The constant K of m represents the emission of a candle. Set to 1 over 683 watts per steradian. This spectrum, gives the photopic eye sensitivity function. Photopic stands for daylight vision. It was established by the company at Commission International CIE in 1978, the curve peaks at 555 nanometer located in the green region of the spectrum. In summary, the emergence of as a viable commercial device started with the use of multi in which each layer Takes in charge a particular step of the process. The chemical compound for each layer must be carefully selected in terms of its electrical and optical properties. This is where the versatility of organic chemistry turns to a big benefit. Chemical companies now offer an impressive variety of chemical compound suited for each part of the device. Advantage of also reside in their compatibility with other plastics. Thus offering the possibility to make them light, flexible, and low-energy-intensive. I thank you for your attention.
