The Principle Behind OLEDS

Throughout our exciting journey together, we have explored the wonderfully innovative and unpredictable OLED industry in depth all across the spectrum.  We first focused on the bare bones of the basic layers of an organic light emitting diode.  Subsequently, took a figurative trip to the factories, where we learned about the tedious methods of production that were necessary to create such a complex technology.  This led to a discussion on the current research focuses within OLEDs and the possibilities of the future of OLED being introduced to the everyday household.  Subsequently, we delved into the industry itself, and highlighted a few notable products and applications that the OLED industry is involved in.  Now, we will finally take a close look at what makes an OLED what it is and the underlying principle that allows it to emit light.  After all, we’re interested in just how it identifies itself as a light emitting diode.  In this post, we will explain just how an OLED generates light and makes it such a potential powerhouse technology that we will no doubt look forward to in the future.

The placement of the heterojunction within an OLED

As we stated earlier, the typical OLED structure is composed of layers of organic material between two electrodes which are all deposited on a substrate.  The layers of organic material are considered semiconductors.  This is due to the delocalization of electrons and the structure of the organic molecules; they are electrically conductive throughout the molecule.  What results are levels of conductivity ranging from insulators to conductors.  Like all other molecules, organic material is full of unoccupied molecular orbitals.  In this case, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are two orbitals most similar in energy.  The energy difference between the HOMO and LUMO is known as the HOMO-LUMO gap and allows electrons to move freely within the diode.

 The first OLED was devised by J.H. Burroughes in 1990.  It consisted of a single organic layer of polyphenylene vinylene, a popular conducting polymer.  However, as OLED technology began to evolve, it became clear that a single layer did not provide the efficiency desired.  Thus, contemporary OLEDs now contain two or more layers, usually a conductive layer with an emissive layer.  Further information about this specific structure and the materials used in its production can be found in Blog 2.  The most recent levels OLED efficiency (19%) was achieved by utilizing a graded heterojunction.  A heterojunction is an interface between two layers of semiconductor, such as the organic material being used in the OLED.  The unequal HOMO-LUMO band gaps of a heterojunction allows for greater operating frequencies as well as a high forward gain.  Additionally, the heterojunction structure allows for the hole and electron-transport materials to vary within the emissive layer.  This improves charge injection and balances charge transport at the same time.

In blog 2, we briefly mentioned the purpose of the anode and cathode within the structure of the OLED.  When a voltage is applied across the OLED, a current of electrons begins to flow through the device from the cathode to the anode.  During this process, electrons are injected into the LUMO of the cathode and ejected from the HOMO of the anode.  This can also be described as the injection of electron holes into the HOMO.  As described earlier, an electron hole is the theoretical opposite of an electron and describes a local where an electron can exist.  The ejected electron from the anode is now an electron hole.  Afterwards, electrostatic forces within the OLED bring the electrons and the electron holes together to form anexciton near the emissive layer.  An exciton is considered condensed matter that is able to transport energy without affecting the net electrical charge.  Electrons and electron holes are both fermions with half integer spin.  When they combine to form an exciton, it can be in a singlet or triplet state.  The decay of the singlet state exciton produces the light we see.

A graphical representation of how OLEDs emit light

As mentioned in blog 2, indium tin oxide (ITO) is a popular anode because it is transparent to visible light, high work function, and electrical conductivity.  This reduces the energy required for hole injection within the HOMO.  On the cathode side, barium and calcium are often used due to their low work functions which allows for an easier injection of electrons into the LUMO.  The materials chosen for the anode and cathode for an OLED are fundamental in reaching high efficiency, performance and extension of lifetime.  Thus, it is vital to produce as smooth as possible anode surface to increase adhesion, decrease electrical resistance and reduce dark spots.  While procedures to decrease anode surface roughness have been developed, potential alternate substances for the anode/cathode are also being researched.  Current candidates include single crystal sapphire substrates treated with gold.

The process in which the OLED emits light is the essence of why they exist. This method is simply unrivaled and and absolutely original in regards to anything else in the technology sector, and it brings such a unique and supreme performance experience that the OLED market is predicted to soar to unimaginable heights. It would be unwise to hazard a guess at what holds for the future for such a limitless concept. Over the course of our series covering the OLED technology, we have touched hands with all corners of chemistry, ranging from electrochemistry to organic chemistry and even ventured slightly into materials science.  We hope that by covering such an integral part of the evolving nature of the industry, we have sparked your interest in the technology revolution that has taken over the world. The current rate of innovation requires constant learning in order to stay up to date with each modification.   As Arthur C. Clarke once said, “Any sufficiently advanced technology is indistinguishable from magic.”  OLED technology has the potential to become that magic.

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