Phosphorescent organic light-emitting diodes (PHOLED) are a type of

organic light-emitting diode An organic light-emitting diode (OLED), also known as organic electroluminescent (organic EL) diode, is a type of light-emitting diode (LED) in which the emissive electroluminescent layer is an organic compound film that emits light in respon ...

(OLED) that use the principle of

phosphorescence Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. Unlike fluor ...

to obtain higher internal efficiencies than

fluorescent Fluorescence is one of two kinds of photoluminescence, the emission of light by a substance that has absorbed light or other electromagnetic radiation. When exposed to ultraviolet radiation, many substances will glow (fluoresce) with color ...

OLEDs. This technology is currently under development by many industrial and academic research groups.

Method of operation

Like all types of OLED, phosphorescent OLEDs emit light due to the

electroluminescence Electroluminescence (EL) is an optical phenomenon, optical and electrical phenomenon, in which a material emits light in response to the passage of an electric current or to a strong electric field. This is distinct from black body light emission ...

of an

organic semiconductor Organic semiconductors are solids whose building blocks are pi-bonded molecules or polymers made up by carbon and hydrogen atoms and – at times – heteroatoms such as nitrogen, sulfur and oxygen. They exist in the form of molecular crystals o ...

layer in an electric current. Electrons and

holes A hole is an opening in or through a particular medium, usually a solid body. Holes occur through natural and artificial processes, and may be useful for various purposes, or may represent a problem needing to be addressed in many fields of en ...

are injected into the organic layer at the electrodes and form

exciton An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb's law, Coulomb force resulting from their opposite charges. It is an electrically neutral quasiparticle regarded as ...

s, a bound state of the electron and hole. Electrons and holes are both

fermion In particle physics, a fermion is a subatomic particle that follows Fermi–Dirac statistics. Fermions have a half-integer spin (spin 1/2, spin , Spin (physics)#Higher spins, spin , etc.) and obey the Pauli exclusion principle. These particles i ...

s with half integer

spin Spin or spinning most often refers to: * Spin (physics) or particle spin, a fundamental property of elementary particles * Spin quantum number, a number which defines the value of a particle's spin * Spinning (textiles), the creation of yarn or thr ...

. An exciton is formed by the

coulombic attraction Electrostatics is a branch of physics that studies slow-moving or stationary electric charges. Since classical antiquity, classical times, it has been known that some materials, such as amber, attract lightweight particles after triboelectric e ...

between the electron and the hole, and it may either be in a

singlet state In quantum mechanics, a singlet state usually refers to a system in which all electrons are paired. The term 'singlet' originally meant a linked set of particles whose net angular momentum is zero, that is, whose overall spin quantum number s=0. A ...

or a

triplet state In quantum mechanics, a triplet state, or spin triplet, is the quantum state of an object such as an electron, atom, or molecule, having a quantum spin ''S'' = 1. It has three allowed values of the spin's projection along a given axis ''m''S = � ...

, depending on the spin states of these two bound species. Statistically, there is a 25% probability of forming a singlet state and 75% probability of forming a triplet state. Decay of the excitons results in the production of light through

spontaneous emission Spontaneous emission is the process in which a Quantum mechanics, quantum mechanical system (such as a molecule, an atom or a subatomic particle) transits from an excited state, excited energy state to a lower energy state (e.g., its ground state ...

. In OLEDs using

organic molecules only, the decay of triplet excitons is quantum mechanically forbidden by

selection rules In physics and chemistry, a selection rule, or transition rule, formally constrains the possible transitions of a system from one quantum state to another. Selection rules have been derived for electromagnetic transitions in molecules, in atoms, in ...

, meaning that the lifetime of triplet excitons is long and phosphorescence is not readily observed. Hence it would be expected that in fluorescent OLEDs only the formation of singlet excitons results in the emission of useful radiation, placing a theoretical limit on the internal

quantum efficiency The term quantum efficiency (QE) may apply to incident photon to converted electron (IPCE) ratio of a photosensitive device, or it may refer to the TMR effect of a magnetic tunnel junction. This article deals with the term as a measurement of ...

(the percentage of excitons formed that result in emission of a photon) of 25%. However, phosphorescent OLEDs generate light from both triplet and singlet excitons, allowing the internal quantum efficiency of such devices to reach nearly 100%. This is commonly achieved by doping a host molecule with an

organometallic Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...

complex Complex commonly refers to: * Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe ** Complex system, a system composed of many components which may interact with each ...

. These contain a heavy metal atom at the centre of the molecule, for example platinum or iridium, of which the green emitting complex Ir(mppy)₃ is just one of many examples. The large

spin–orbit interaction In quantum mechanics, the spin–orbit interaction (also called spin–orbit effect or spin–orbit coupling) is a relativistic interaction of a particle's spin with its motion inside a potential. A key example of this phenomenon is the spin– ...

experienced by the molecule due to this heavy metal atom facilitates

intersystem crossing Intersystem crossing (ISC) is an isoenergetic radiationless process involving a transition between the two electronic states with different spin multiplicity. Excited singlet and triplet states When an electron in a molecule with a singlet grou ...

, a process which mixes the singlet and triplet character of excited states. This reduces the lifetime of the triplet state, therefore phosphorescence is readily observed.

Applications

Due to their potentially high level of energy efficiency, even when compared to other OLEDs, PHOLEDs are being studied for potential use in large-screen displays such as computer monitors or television screens, as well as general lighting needs. One potential use of PHOLEDs as lighting devices is to cover walls with large area PHOLED light panels. This would allow entire rooms to glow uniformly, rather than require the use of light bulbs which distribute light unequally throughout a room. The

United States Department of Energy The United States Department of Energy (DOE) is an executive department of the U.S. federal government that oversees U.S. national energy policy and energy production, the research and development of nuclear power, the military's nuclear w ...

has recognized the potential for massive energy savings via the use of this technology and therefore has awarded $200,000 USD in contracts to develop PHOLED products for general lighting applications.

Challenges

One problem that currently hampers the widespread adoption of this highly energy efficient technology is that the average lifetimes of red and green PHOLEDs are often tens of thousands of hours longer than those of blue PHOLEDs. This may cause displays to become visually distorted much sooner than would be acceptable for a commercially viable device.

References

{{Display technology Optical diodes Display technology Molecular electronics