S. M. Sawde 1, A. M. Bhake 2, R. R. Patil 3, S. J. Dhoble 4, S. V. Moharil 5
1,2 Asstt.Prof. & Applied Physics Department, Priyadarshini Institute of Engg. & Technology,
RTM Nagpur University, Nagpur, India
3 Department of Physics, Institute of Forensic Science, Nagpur, India
4,5 Department of Physics, RTM Nagpur University, Nagpur, India
Recent research is characterized by strong interaction among other branches of solid state and between different areas of luminescence using organic and inorganic materials. Luminescence is observed in inorganic as well as organic materials. The coupling of organic and inorganic semiconductors allows the most favourable properties of the inorganic component, e.g. the high electrical conductivity, with the most favourable properties of the organic component, e.g. the high photoluminescence yield across the visible spectrum. Luminescence in organic molecules originates from the excitation of Pi electrons. In inorganic materials activators plays an important role in imparting luminescent properties. The activators used are mostly rare earth ions or 3d elements. Luminescence phenomena in both materials can be classified depending on the excitation source used and the time for which emission persists. In this paper, best outcomes of organic and inorganic luminescence about LEDs and OLEDs are discussed.
Keywords— Phosphors, OLEDs, LEDs, Organic Luminescence, Inorganic Luminescence
Luminescence is an interdisciplinary subject as it can be studied and is applicable in various fields like physics, chemistry, biological science, medical science, forensic science, geology, material science, engineering technology, etc. Recent research is characterized by strong interaction among other branches of solid state and between different areas of luminescence using organic and inorganic materials . Both experimental as well as theoretical approaches have been made for it. Luminescence means emission of light by appropriate materials when they are relatively cool. Luminescence is defined as the emission of radiation of the light by bodies, which is in excess of that attributable to the black radiation and persist for longer duration than the period of electromagnetic radiation after the excitation stops. Traditionally, luminescence is classified as fluorescence and phosphorescence. The word “Luminescence” which includes both fluorescence and phosphorescence was first used by Eilhardt Wiedemann, a German Physicist; in 1888 material science, engineering technology, etc. If the emission from the material persists for a shorter duration (<10-8 s) then it is called as fluorescence while phosphorescence persist for quite longer duration (upto several seconds). With respect to organic molecules, the term phosphorescence means light emission caused by electronic transitions between levels of different multiplicity (explained below in detail), whereas the term fluorescence is used for light emission connected with electronic transitions between levels of like multiplicity. The situation is far more complicated in the case of inorganic phosphors. The term phosphorescence was first used to describe the persistent luminescence (afterglow) of phosphors. The mechanism described above for the phosphorescence of excited organic molecules fits this picture in that it is also responsible for light persistence up to several seconds. Fluorescence, on the other hand, is an almost instantaneous effect, ending within about 10−8 second after excitation. The term fluorescence was coined in 1852, when it was experimentally demonstrated that certain substances absorb light of a narrow spectral region (e.g. blue light) and instantaneously emit light in another spectral region not present in the incident light (e.g. yellow light) and that this emission ceases at once when the irradiation of the material comes to an end. The name fluorescence was derived from the mineral fluorspar, which exhibits a violet, short-duration luminescence on irradiation by ultra-violet light.
II. LUMINESCENCE IN ORGANIC AND INORGANIC MATERIALS
Material science is a key technology, to make it possible to design ecofriendly materials for a scientific need. Therefore, ample luminescent materials were investigated till date as per the requirement. There are basically two types of luminescent materials viz. organic and inorganic.
Luminescence in organic molecules originates from the excitation of Pi electrons. The organic materials are held together by Vander Walls forces between molecules and are therefore molecular solids. The consequence of this molecular nature is that the luminescence processes in organic materials are associated with the excited states of molecules. Those hydrocarbons which contain double or triple bonds between the carbon atoms, that is the unsaturated hydrocarbons, commonly give rise to strong luminescence emission. It is the excited states of π-electrons systems which are of interest for organic luminescence and in particular double bonded molecules such as aromatic hydrocarbons. These π-electrons are less tightly bound to their parent carbon nuclear than the localized 6 electrons and those require less energy to excite them. It is very easy to excite the pi electrons when the host forms complex with metal ions resulting the efficient luminescence from the formed complexes. The metal ions could be alkali – alkaline earth ion, transition metal ion or lanthanide ion .
The inorganic materials are held together by strong covalent bonds between atoms and then molecules and are therefore molecular solids. In inorganic materials activators plays an important role in imparting luminescent properties. The activators used are mostly rare earth ions or 3d elements. There is a great amount of experimental evidence showing that the ability of a material to luminescence is associated with presence of so called-‘activators’. These activators may be special impurity atoms occurring in relatively small concentrations in the host material, or a small stoichiometric excess of one of the self-activation. Several inorganic phosphors have been developed and are routinely used in day to day life.
A. Luminescence Phenomenon
Luminescence of phosphors is carried out by two processes such as Excitation and Emission. Luminescence emission occurs after certain material has absorbed energy from a source such as electron beams, ultraviolet or X-ray radiation, chemical reactions etc. The energy lifts the atoms of the material into an excited state, and then, because excited states are unstable, the material undergoes another transition, back to its unexcited ground state, and the absorbed energy is liberated in the form of either light or heat or both (all discrete energy states, including the ground state, of an atom are defined as quantum states). The excitation involves only the outermost electrons orbiting around the nuclei of the atoms. Luminescence phenomena in materials can be classified depending on the excitation source used and the time for which emission persists. In short, it includes;
- absorption of excitation energy and stimulating the system into an excited state,
- transformation and transfer of the excitation energy and
- emission of light and attenuation of the system into an unexcited condition.
Several types of luminescence centre consists of transition metal ions, (Mn2+, Cr3+, Fe3+), rare earth elements (RE2+/3+), actinides (uranyl UO22+), heavy metals (Pb2+,Tl+), electron–hole centre’s (S2−, O2−, F− centre’s), more extended defects (dislocations, clusters).
Evidently it is a two-step process; the excitation of the electronic system of the substance and the subsequent emission of the photons; these steps may not be separated by intermediate processes. However it is a broad term; several categories of luminescence are possible in crystals, depending upon the means employed to excite the electrons. The area of the application of materials is vast and varied .
III. CLASSIFICATION OF LUMINESCENCE
Classification of luminescence based on the source of excitation and their various applications
The classification of luminescence based on the source of excitation and their various applications are summarised in Table 1.
However, inorganic phosphors have certain disadvantages. They are not cost effective. Large displays cannot be made easily as it is very difficult to make large size films. The electroluminescent display requires more power. With the miniaturization of electronic devices it is desired to have displays which are cost effective, requires less power to operate, efficient and colourful. For such applications phosphors based on organic materials are suitable.
B. Important Processes in Organic and Inorganic Luminescence
Charge Transfer Transitions
An electron may jump from a principally ligand orbital to a principally metal orbital, giving rise to a ligand-to-metal charge transfer (LMCT) transition. These can occur easily when the metal is in a high oxidation state. For example, due to LMCT transitions the color of chromate, dichromate and permanganate ions, mercuric iodide, HgI2, is red. This example shows that charge transfer transitions are not restricted to transition metals . A metal-to-ligand charge transfer (MLCT) transition will most likely occur when the metal is in a low oxidation state and the ligand is easily reduced.
d– d transitions
In complexes, the transition metals the d-orbitals do not all have the same energy. The splitting pattern of the d orbitals can be designed using crystal field theory. The scope of the splitting depends on the particular metal, its oxidation state and the nature of the ligands. In this case, an electron jumps from one d-orbital to another. The ions that are inert to luminescent emission are of yttrium, lanthanum, gadolinium and lutetium. It is well-known that rare earth metal chelates are characterized by highly competent to intra-energy conversion from the ligand singlet (S1) into the triplet (T1), and hence to the excited state of the central rare earth metal ion [4, 5]. The metal ions exhibit sharp spectral bands corresponding to 5Dx→7Fx transitions. This mechanism is characterized by the molecules suspended in the dilute solution or by high photoluminescence efficiency of about 20-95% [6, 7]. Unlike common fluorescent and phosphorescent compounds, rare earth complexes exhibit high luminescence efficiency with sharp spectral bands relating electrons associated with inner 4f orbitals of the central rare earth metal ions.
C. Decay Characteristics
An investigation of the decay properties of the luminescent materials indicates that they fall into two broad categories. In first type, the decay equation is given by:
I (t) = Io exp (-α t)
where Io is the initial intensity at time t and α is a constant.
This closely resembles to the process governing the growth of monomolecular reaction . This behaviour suggests that in these cases the luminescence takes place by simple excitation with subsequent optical emission in the active centre, the excitation energy remaining closely localized in the centre between excitation and emission. The decay constant is small and is independent of temperature.
Most of the luminescent materials which are valued for their long decay characteristics, obey a decay equation of second type:
I (t) = Io /(βt+1) n
Where Io is the initial intensity, I(t) is the intensity at time t, β and n are constants.
This equation is similar to the rate equation for their bimolecular reaction. The constant β is dependent on temperature. The atoms or clusters of atoms become ionized during the excitation and luminescent radiation is emitted during recombination of the free electrons and the ionized centres. Johnson  has suggested that essentially all centres become ionized during excitation and that a majority of free electrons are recaptured into a state, which has a very long lifetime (of the order of milliseconds) because the optical transition to the ground state is forbidden. These electrons contribute an exponential components to the decay curve. The remaining electrons are captured at the trapping centres and are released over a period of a time that is long compared to the life times of the excited state of the fluorescing centre. The second class electrons is responsible for the bimolecular component of the decay curve.
D. Concentration quenching
Luminescence efficiency depends on the degree of transformation of excitation energy into light, and there are relatively few materials that have sufficient luminescence efficiency to be of practical value. In order to enhance the luminescence efficiency of a phosphor it seems that the activator concentration in the host should be as high as possible. However, it is found that the luminescence efficiency decreases if the activator concentration exceeds the specific value known as critical concentration. This effect is called as concentration quenching. If the concentration of the activator becomes so high that the probability of energy transfer exceeds that for emission then the excitation energy repeatedly goes from the one activator to the other and eventually lost at the surface, dislocations or impurities. Thus, it makes no contribution to the luminescence. The efficiency then decreases in spite of the increase of the activator concentration. Fig. 1gives the idea about the concentration quenching.
E. Thermal Quenching
Since the interaction with lattice will be temperature dependent, it is quite understandable that the positions, splitting and the life times of various energy levels of an activator can be temperature dependent. It is quite common to find that at lower temperatures the host lattice offers conditions conducive for luminescence while at high temperatures, the non-radiative processes become dominant. This has been termed as thermal quenching. For many applications it is of prime importance. It determines the operating temperature of the device based on the luminescent material. In some cases (e.g.Y2O3: Eu) increase in luminescence efficiency at high temperatures has been observed. This occurs due to the thermal quenching of the processes, which compete with the desired emission.
F. Stokes Shift
Absorption and emission in gaseous state consist of sharp line while in solids they occur as bands. Transitions that are forbidden in Free State become allowed in crystals with smaller probability. In solids the emission in condensed media occurs on longer wavelength than the absorption due to the influence of host lattice. This is termed as Stokes shift. Greater the interaction of impurity ion with the host media, greater is the Stokes shift and the width of the emission line. Stokes shift occurs due to the difference in the interionic separation in the ground state and excited state. The luminescence centre expands on excitation of cation electrons and contracts on anion electron excitation in charge transfer transitions. Phosphors that show an emission with a large Stokes shift usually exhibit a low quenching temperature. It is as shown by following graph;
This phenomenon is same as radio-luminescence. It is named scintillation because it is used as a technique to detect individual light pulses generated by the incidence of each X-ray or gamma ray photon or a nuclear particle. Such light pulses are called scintillations, since like a spark they are very short lived. Thallium activated sodium iodide is a well-known scintillation detector used for gamma ray spectrometry. The intensity of the scintillation (light pulse) is directly proportional to the energy of the incident gamma ray photon (when it is totally absorbed). The measurement of this pulse intensity, therefore, provided the means for knowing the gamma ray energy .
H. Stokes Shift for Scintillators
An important, general concept to keep in mind for all scintillators is that:
- emitted photons are at longer wavelengths (smaller energies) than the energy gap of the excitation called the “Stokes shift”
- the processes that produce the Stokes shift are different in different scintillating materials
- this allows the scintillation light to propagate through the material and
- emitted photons can’t be self-absorbed by exciting the material again.
I. Organic Scintillators
The scintillation mechanism is determined by the chemistry and physics of the benzene ring. An organic scintillator will thus scintillate whether it’s in a crystal form, a liquid, a gas or embedded in a polymer. All organic scintillators in use employ aromatic molecules (i.e. have a benzene ring) chemical bonds in the benzene ring, σ-bonds in the plane, bond angle 120° from sp2 hybridization. π-orbitals are out of the plane; they overlap and the π-electrons are completely delocalized .
J. Self-Activated Scintillating Crystals
Chemically pure crystal has luminescence centres (probably interstitial) due to stoichiometric excess of one of the constituents. Example: PbWO4 and CdWO4. Extra tungstate ions are the activator centres in PbWO4 crystals.
K. Doped and Exciton Luminescence in Crystals
- For doped crystals: the decay time primarily depends on the lifetime of the activator excited state. Examples of doped crystals: NaI(Tl), CsI(Tl), CaF2(Eu).
- For crystals with exciton luminescence: electron-hole pairs stay somewhat bound to each other forming an exciton. The exciton moves together in the crystal impurities or defects (w/o activator). It is a site for recombination. Example of exciton luminescence: BGO (bismuth germanate Bi4Ge3O12).
L. Organic luminescent materials
Although the inorganic phosphors are industrially produced in far higher quantities (several hundred tons per year) than the organic luminescent materials, some types of the latter are becoming more and more important in special fields of practical application. Paints and dyes for outdoor advertising contain strongly fluorescing organic molecules such as fluorescein, eosin, rhodamine and stilbene derivatives. Their main shortcoming is their relatively poor stability in light, because of which they are used mostly when durability is not required. Organic phosphors are used as optical brighteners for invisible markers of laundry, banknotes, identity cards, and stamps and for fluorescence microscopy of tissues in biology and medicine. Their ‘invisibility’ is due to the fact that they absorb practically no visible light. The fluorescence is excited by invisible ultraviolet radiation (black light).
M. When to Use organic and Inorganic scintillator ?
- For spectroscopy, use inorganic (or better still semiconductor) e.g. NaI has high density, high Z, good photopeak
- For timing applications, use organic
- Most common inorganic scintillating crystals have longer decay times than organic scintillators which have scintillation lifetimes of approximately few ns for e.g. for cosmic ray detector, use sheets of plastic or tanks of liquid to cover a large area cheaply (compared to many crystals) since you might not care about energy resolution since a cosmic ray crossing your detector deposits a known amount of energy but rather care when the cosmic ray hit your detector or use time-of-flight spectrometry to determine a particle’s mass/ momentum/energy.
- For neutron detection, having C and H is favourable hence use an organic.
N. OLED and LED
Light emitting diodes (LEDs) are inorganic solid-state lighting source increasingly being used in display backlighting, communications, medical services, signage, and general illumination. OLED is a series of organic films that, when activated with electricity, will emit light.
An organic light-emitting diode (OLED) is a light emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compound that emits light in response to an electric current being used in OLED technology. It is used in commercial applications such as displays for mobile phones and portable digital media players, car radios and digital cameras among others.
The biggest technical problem for OLEDs is the limited lifetime of the organic materials. In particular, blue OLEDs historically have had a lifetime of around 14,000 hours to half original brightness (five years at 8 hours a day) when used for flat-panel displays. This is lower than the typical lifetime of LCD, LED or PDP technology each currently rated for about 60,000 hours to half brightness, depending on manufacturer and model .
Luminescence is observed in organic as well as inorganic materials. The coupling of organic and inorganic semiconductors allows the most favourable properties of the inorganic component with the most favourable properties of the organic component. Luminescence in organic molecules originates from the excitation of Pi electrons. In inorganic materials activators plays an important role in imparting luminescent properties. The activators used are mostly rare earth ions or 3d elements. Important processes in organic and inorganic Luminescence with LEDs and OLEDs are studied.
- Cees Ronda “Luminescence: From Theory to Applications” WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, ISBN: 978-3-527-31402-7, 2008.
- S.M. Sawde, R.R.Patil, S.V Moharil, “Fundamentals of Organic Luminescence” International Journal of Novel Research in Engineering, Science & Technology vol.1, Issue-1, ISSN No: 2455-7935, April, 2016 .
- I. Fujii, N. J. Hirayama, K. Kodama Ohtani, Anal. Sci. vol. 12, 153, 1996.
- R. E. Whan, G. A. Crosby, J. Mol Spectrosc; vol. 8, 315, 1962.
- M. L. Bhaumik, M. A. El-Sayed, J Chem Phys; vol. 42, 787, 1965.
- H. G. Huang, K. Hiraki,; Y. Nishikawa; Nippon Kagaku Kaishi (Internal Report); vol. 1, 66, 1981.
- G. L. Rikken, J. A. Phys Rev A; vol. 51, 4906, 1995.
- F.Seitz, “Luminescent crystals”, Preparation and characteristics of solid Luminescent materials, Symp.Crnell Univ., JohnWiley & Sons, INC, NY, vol. 9, 1948.
- (a) R.P. Johnson, J. Opt. Soc.Amer., vol. 29, 387, 1939. (b) R.P.Johnson, Phys.Rev., vol. 55, 881, 1939. M. Kobayashi (KEK), Introduction to scintillators, 17 November, 2003.
- M. Kobayashi (KEK), Introduction to scintillators, 17 November, 2003.