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Materials 4N03: Final Project

Solid State Lighting

Materials 4N03 Final Project Submitted by: Jeffery Clare, Catherine Pereira, Heather Smith & Mitch Tuckey

December 06, 2011.

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Table of Contents Introduction .................................................................................................................................................. 3

How does Semiconductor LED Technology Work? ....................................................................................... 4

Creation of Semiconductor LEDs .............................................................................................................. 6

LED Material Issues and Solutions ................................................................................................................ 7

Organic Light Emitting Diodes ....................................................................................................................... 9

How does OLED Technology Work? .......................................................................................................... 9

Types of OLEDs ........................................................................................................................................ 10

OLED Material Issues and Solutions............................................................................................................ 10

SMOLEDs ................................................................................................................................................. 11

PLED ........................................................................................................................................................ 12

Common Host Polymers for PLEDs ..................................................................................................... 12

Common Polymers Utilized in PLEDs .................................................................................................. 12

Phosphorescent Materials ...................................................................................................................... 13

Conclusions ................................................................................................................................................. 14

References .................................................................................................................................................. 15

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Introduction Solid state lighting, light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs) have

the potential to be a significantly more efficient and effective method of lighting our homes and

buildings. The current efficiency of LED and OLED bulbs are significantly greater than that of

incandescent and compact fluorescent lamps, however it can be improved further and there are several

materials issues with the technology, currently preventing it from gaining widespread popularity.

Wide spread use of LEDs in lighting applications has numerous benefits which can help to

address the energy crisis by consuming less electricity in everyday lighting uses. Table 1 below clearly

shows that LED light bulbs last five times longer than compact fluorescent light bulbs (CFL) and forty-two

times longer than incandescent light bulbs. The electricity consumed over the lifespan of a single LED

bulb (over 50,000 hours of use), is ten times less than the electricity consumed by an incandescent light

bulb and is two and a third times less than the electricity consumed by a CFL. [1] By switching to LED

light bulbs, society will experience an overall reduction in energy consumption resulting in a decrease in

electricity costs. From a consumer’s point of view, this is very attractive. However, the initial cost of an

LED light bulb is quite high and this is something which must be reduced in order for this form of lighting

to further gain wide-spread use.

On a larger scale, in 2001 the total annual energy consumption in the United States was

approximately 9 200 terawatt-hours (TWh). Thirty-eight percent of the energy was consumed as

electricity and of that twenty-two percent was consumed by lighting. This means that in the United

States in 2001 over eight percent of total energy consumption was due to lighting which is

approximately 765 TWh or electricity. It is also estimated that worldwide, in 2005 grid-based electric

lighting was responsible for about nineteen percent of global electricity consumption. [2] This is clearly

a significant percentage and addressing the need for more efficient lighting sources can greatly help

decrease our global demand for electricity.

Table 1: Comparison of LED lamps, compact fluorescent bulbs, and incandescent light bulbs in terms of lifespan, electricity consumption and cost. [1]

LED CFL Incandescent

Lifespan (hours) 50 000 10 000 1200

Cost per bulb $35.95 $3.95 $1.25

KWh used over 50 000 hours 300 700 3000

Cost of electricity(@ $0.09/KWh)* $27 $63 $270

Total cost for 50k hours $62.95 $82.75 $322.50

KWh per household** 7500 17 500 75 000

Cost per household** $675 $1575 $6750

Savings by switching from incandescent** $6075 $5175 $0

67 500 KWh 57 500 KWh 0 KWh

*Electricity cost based on an average of current Horizon Utilities smart meter pricing. **Household electricity use and savings

based on an assumed 25 light bulbs per house switched from incandescent bulbs.

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How does Semiconductor LED Technology Work? A semiconductor light-emitting diode (LED) is essentially a semiconductor p-n junction. The p-

type semiconductor is doped to have excess amounts of holes and the n-type is doped to have excess

electrons. When a current is applied to the semiconductor, the excess holes from the p-type

semiconductor and the excess electrons n-type semiconductor will travel towards the p-n junction

where they recombine. This electron-hole recombination releases a photon thereby producing

electroluminescence, as shown in Figure 1. [3]

Figure 1: Diagram of a basic semiconductor LED.

The energy released in this process is described by the equation,

, where λ is the wavelength of

the emitted light, h is Planck’s constant, c is the speed of light, and Eg is the band gap energy of the

material. Since the electroluminescence effect generates a specific wavelength of light, the emission

from an LED is very narrow compared to incandescent lighting, which emit light well into the UV

spectrum.[3]

A common LED device can be seen in Figure 2, but the size and shape of LEDs is widely varied.

The anode and cathode carry the electrons and holes, respectively, to the semiconductor chip. The

semiconductor chip is located on top of the cathode, as shown in Figure 2, but can be place on top of

the anode. The semiconductor location depends on whether the substrate is p-type or n-type. The

semiconductor chip is inside of a cone of reflective material. The semiconductor will emit light out of

any surface, to maximize the amount of light that is directed forward the reflective cone is place around

the semiconductor. Attaching from the anode to the top of the semiconductor in Figure 2 is a

conduction wire. This wire is made very small to reduce the amount of shadowing, thereby increasing

the amount of light emitted by the LED. A protective epoxy coating is applied surrounding the LED

components. Epoxy has a high fracture toughness which makes it ideal material to utilize as a protective

coating. This is why LEDs are more durable compared to glass incandescent bulbs, which are known to

scatter easily.

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Figure 2: Common LED device. [4]

Many applications use coloured LEDs as indicators, but for house hold lighting white light is

preferred. The human eye can detect light in the 400-800nm range; it is within this range that white

light emission is needed for lighting applications.[5] RGB and phosphor coatings are the two

predominant methods of achieving white light emission in modern semiconductor LEDs.

The most common method is the RGB (Red, Green and Blue) method. These three colour LEDs

emitting three corresponding wavelengths and are mixed via an optical mixer to produce a broad

spectrum white light, as shown in Figure 3.[6]

Figure 3: The broad spectrum emission obtained from red, green, and blue LEDs. [6]

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Phosphor coatings are another method for obtaining white light emission. The phosphor

coating is applied to the semiconductor chip inside the epoxy, such that the photon released after

electron-hole recombination will pass through the phosphor coating which converts it to visible light.

The most common types of phosphor coated LEDs emit in the blue-UV range. Commonly, GaN LED

devices are used in conjunction with yttrium aluminum garnet (YAG) phosphors. The YAG phosphor

emits yellow after absorbing blue from the GaN. The combination of the yellow and blue can create a

white light emission.[7] The different wavelengths and their corresponding electroluminescence (EL)

intensities are shown in Figure 4.

Figure 4: The broad spectrum obtained from the combination of GaN in conjunction with YAG phosphor. [7]

Both the RGB and phosphor coating methods are able to produce white light emissions.

Unfortunately, both methods experience a quality issue with respect the white light emitted. As can be

seen in Figure 3 and Figure 4, there is an EL distribution of white light produced. This variation causes a

quality issue with the white light emitted by the LEDs.

Creation of Semiconductor LEDs Semiconductor LEDs are created by epitaxial growth. There are four major method of

conducting epitaxial growth, liquid phase epitaxy, vapour phase epitaxy, metalorganic vapour phase

epitaxy, and molecular beam epitaxy. Liquid phase epitaxy uses a solution saturated with the desired

compound that is to be grown. The substrate is placed in contact with the solution and cooled. As the

solution cools the compound will grow on the substrate. This method is the simplest of the epitaxy

methods. The disadvantage is the controlling the composition is difficult and growing multiple layers is

difficult. Vapour phase epitaxy uses gaseous form reactants of the compounds desired in the layers.

The gases are added to a quartz chamber with the substrate. This method is very easy to conduct at

large scale. The composition of the growth layers and thickness can be controlled easily. The

disadvantage of this process that aluminum compounds cannot be used. Aluminum will degrade the

chamber and not deposit onto the substrate. Metalorganic vapour phase epitaxy is similar to vapour

phase epitaxy, but uses metal organic gasses. The substrate is heated and the metalorganics

decompose on the surface of the substrate and deposit the layers. Unlike vapour phase epitaxy metal

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organic vapour phase epitaxy is able to deposit aluminum on the substrate. Molecular beam epitaxy

uses a vacuum chamber to evaporate compounds and deposits them onto the substrate. This method is

very slow and expensive, but is able to create very thin layers with high precision of composition.[4]

LED Material Issues and Solutions There are three characteristics of white light: the Commission Internationale d’Eclairage (CIE)

coordinates or chromaticity, the colour temperature (CT) and the colour rendering index (CRI). Figure 5

below shows the (x,y) plot of the CIE chromaticity and as can be seen, the coordinates for the white

point on this chart are defined as (0.33, 0.33). Generally the desired coordinates for ambient light lie on

the black body curve with colour temperatures between 3000-10,000 K. [8]

Figure 5: CIE (x,y) chromaticity diagram. All the colours in the visible spectrum lie within or on the boundary of this diagram The internal arc is the Planckian locus, which is the plot of the coordinates of black body radiation at the temperatures shown described as colour-correlated temperatures.[8]

There are currently three methods for producing white light using LEDs: multi-chip LEDs, blue LEDs combined with one or more phosphors, and UV LEDs combined with multiple phosphors. It is thought that the most efficient white LEDs will be produced through the mixing of multiple different coloured LEDs in the same packaging however this method does present some challenges currently.[9] Red, blue and green LED need to be combined together so that the colour mixing produces a white light. One of the main problems with this is that green LEDs are significantly less efficient than red or blue LEDs; this means the efficiency of the overall bulb would be limited by the efficiency of the green LED. A second problem with this method is that the three different coloured LEDs change efficiency over time at different rates.[2]

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In the second method of white light production a blue LED is used with a phosphor which absorbs some of the blue light and fluoresces yellow. The combination of the remaining blue light and the yellow light from the phosphor creates a white light. The main problem with this technique is getting proper colour mixing; there is often an issue of different coloured light depending on the angle one views the LED from.[9]

The major issue with the third method, UV LEDs and multiple phosphors is the reduced efficiency. This is because you are absorbing a UV photon and reemitting a lower energy visible photon. Another problem associated with this is that the high flux of UV radiation can cause serious degradation of packaging and phosphor material.[9] In this case improvements and innovations in phosphor and packaging materials which can withstand the UV radiation must be made in order for this form of white LED to be used wide scale.

Figure 6: Internal quantum efficiency versus wavelength. The crosses plotted represent the highest IQE values found in

literature and the triangles are the highest IQE from results on Cambridge grown structures tested at Manchester[7].

The best white LEDs currently have an efficiency of about 30% and efficacy of 100 lm/W. Table 2 shows the efficiencies of various forms of light. The factors affecting the efficiency of GaN LEDs are not well understood. The internal quantum efficiency versus wavelength for LEDs, is shown in Error! Reference source not found.. The theoretical basis for this curve is not understood. In order to further improve the efficiencies of these types of LEDs a better understanding of this curve. While it is clear that LED lamps are more efficient than traditional incandescent and fluorescent light sources this comes at a financial cost which is preventing them from being widely used in commercial and household applications.[2]

Table 2: Efficiencies and Efficacies of various forms of commercially available lighting in 2007[2].

Type of Light Source Efficiency (%) Efficacy (lm/W)

Incandescent light bulb 5 15 Long flourescent tube 25 80 Compact fluorescent lamp (CFL) 20 60 High power white LEDs 30 100 Low power white LEDs 50 150 Sodium lamp (high pressure) 45 130

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White LEDs (10 year target) 60 200

One of the major reasons that LED lighting has not obtained wide-scale usage is due to cost.[2] While the lifetime cost of LED bulbs is significantly less than incandescent and CFL bulbs (as seen in Table 1 above), the initial investment in the bulb is quite high.[1] This cost is continually declining as improvements to the processing methods of white LEDs are made. This will make LED a more financially viable option in the future. One way of doing this is by growing the GaN LED on larger wafers made of different materials. Currently, these LEDs are grown on two inch diameter sapphire or SiC substrates, if this diameter is increased to four or even six inch wafers significantly more LEDs can be acquired per wafer. In addition, growing LEDs on Si wafers have lower manufacturing costs than growing LEDs on sapphire or SiC wafers.

Organic Light Emitting Diodes Organic light emitting diodes (OLEDs) are part of an emerging lighting technology that is

currently area of growing interest. OLEDs can be utilized to produce lighting panels, where light is

produced evenly over an area of a panel rather than from a point source such as a light bulb. This

particular application promises to significantly decrease the electricity usage of household lighting while

producing evenly distributed light from flexible panels up to 1mm in thickness.[10] OLED lighting panels

could also decrease our reliance on other efficient lighting technologies such as compact fluorescents,

which contain high amounts of the hazardous heavy metal mercury. Many creative and ingenious

applications of OLED lighting panels have been purposed including OLED wallpaper and OLED window

coverings.

How does OLED Technology Work?

OLEDs operate very similarly to LEDs. Essentially, a voltage is applied to a sandwich structure of

semiconductor materials and as electrons are excited and eventually return to their ground state a

photon is emitted to produce light.[10] OLEDs differ from LEDs in the fact that organic compounds are

utilized as the semiconductors in the sandwich structure to produce photons. The organic compounds

used in OLEDs can replace the reliance on rare and expensive semiconductor materials found in LEDs.

OLEDs consist of a sandwich structure of six main components: the cathode, electron transport

layer, emissive layer, hole injection layer, anode and a glass substrate. As a voltage of about 2 to 10 volts

is applied to the cathode and anode a build-up of electrons occurs at the electron transport layer and a

build-up of electron-holes occurs at the hole injection layer.[11] This electron-hole gradient occurs due

to electrons that are constantly being added at the cathode and removed at the anode. As the anode

removes the electrons an electron hole is left behind. In order to be filled by an electron, the holes move

to the emissive layer where they recombine with an electron. When this recombination occurs, the

electron will return to the lower energy state of the atom that is missing an electron. The excess energy

of the electron will then be emitted as a photon and in turn produce light.[12]

When manufacturing an OLED, a substrate is used as a base to build all other thin film layers of

the OLED on top of. The substrate usually consists of glass or plastic, as it must be transparent for light

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to be emitted and durable because the substrate will act as the outer surface and must protect the inner

components of the OLED. The next layer to be applied to the substrate is the anode. The anode is

usually made out of indium tin oxide due to its high electrical conductivity and transparency. This allows

the anode to easily emit photons produced within the cell and act as the anode by conducting

electricity. The next layers of the OLED cell include the organic layers. The organic compounds used in

these layers vary by the type of OLED being produced. The first type of OLED that was produced utilized

small organic molecules for the organic layers (SMOLED). This OLED structure utilized copper

phthalocyanine as the hole injection layer, doped Alq3 (Tris-(8-hydroxyquinoline) aluminum) as the

emissive layer and Alq3 as the electron transport layer.[11] The final thin film to be applied is the

cathode layer. This layer does not need to be transparent because it is the back of the OLED but needs

to efficiently conduct electricity to act as an electrode. Calcium and aluminum alloys have commonly

been used as the cathode. Calcium however, has been found to produce photons from an OLED cell

more efficiently due to its lower work function, meaning less energy will be required to inject an

electron from the cathode into the organic material.[13]

Types of OLEDs There are three main types of organic molecules that can be used in OLEDs; SMOLEDs, polymer

LEDs (PLEDs) and phosphorescent materials. The first consists of small organic molecules such as Alq3.

The second type consists of polymer molecules, which are long chains of organic molecules linked

together. PLEDs are currently being heavily researched and will likely enter the consumer market in the

near future. PLEDs significantly decreased the cost of manufacturing due to the fact that polymers can

be printed on a substrate using a process similar to inkjet printers as opposed to the expensive method

of vacuum depositing thin films on a substrate.[14]These two types of organic materials have a

maximum theoretical efficiency of 25% due to the fact that only singlet electrons are allowed to decay

and emit photons. Phosphorescent organic materials can be utilized to increase the maximum

theoretical efficiency to 100% due to the fact that they allow for the decay of both singlet and triplet

electrons to produce photons.[12] Research into phosphorescent materials is ongoing and many

technological hurdles must be overcome including the relatively short lifespan of phosphorescent

organics.[15]

The wavelength and colour produced from emitted photons depends on the organic material

used. Alq3 is widely used in SMOLEDs to produce green light. Alq3 is often dyed with fluorescent organic

compounds in order to produce light of other wavelengths. For production of PLEDs, the polyaniline has

been used for the conducting layer and polyfluorene for the emissive layer. Different side chains added

to the polymer affect the wavelength of light emitted from these polymers.[15] Research is currently

being performed to use doped organometallic compounds for phosphorescent LEDs (PHLEDs).

OLED Material Issues and Solutions The following sections will explore various widely used materials for SMOLEDs, PLEDs and

phosphorescent materials.

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SMOLEDs There are two types of SMOLEDs; those that use fluorescent material and those that use

phosphorescent material. [16]Fluorescent material produces singlet excitons while phosphorescent

material produces triplet excitons. One of the major differences between the two is quantum

efficiencies. The SMOLDs that use fluorescent material are ‘…limited by spin statistics to only ~25%

[meanwhile] the recently developed phosphorescent SMOLEDs...[can] achieve[] quautum efficiencies

approaching 100%.’[16]

The most commonly used ‘fluorescent blue host materials are anthracene and distyryl-based

compounds…These materials have good phase-compatibility with their guest blue emitters, which in

turn also belong to the anthracene, distyryl amine compounds, perylene, and fluorine derivatives. These

materials have large band gaps and are thermally stable with high Tg,[16] (glass transition temperature)

which will preventing changes in material properties from occurring during device operation, that would

occur as the operating temperature approaches Tg. ‘Solution-processible polymers with large band gaps

and relatively high triplet energies may also be suitable for use as host materials. Examples of these

include polyalkylfluorene…polyvinylene…and rigid polycarbazoles.’[16]

There are a wide range of dopants that can be used for SMOLEDs. Each of these must be highly

fluorescent, match the highest occupied and lowest unoccupied molecular orbitals, have fast energy

transfer processes and be phase compatible with the host materials. If the dopant is phosphorescent,

then its triplet energy level normally should be lower than that of the host; [16] the reasoning for which

is explained later during the discussion of phosphorescent materials.

The simplest SMOLED structure for producing white emission colour consists of three primary

emission colours (blue, green and red) with the structure ITO/TPD/p-EtTAZ/Alq3/Alq3:Nile

Red/Alq3/Mg:Ag, in which the TPD layer produces a blue emission, the Alq3 layer produces a green

emission , and a red emission is produced from the Nile red dye. The white emission of this device is

‘sensitive to the operating voltage and device structure parameters such as active layer thickness and

doping concentration.’ [16] In addition, the device fabrication process is tedious since ‘today’s devices

have a total of 7-9 layers, some of which are deposited by different techniques. Some of these

techniques require humidity- and oxygen-free conditions, which contributes to the OLEDs’ high

manufacturing costs.’ Although these costs can be decreased by reducing the number of layers in a

device, improving and/or developing new manufacturing processes which is the focus of much research

at this time. [17]

The main issue with using devices with the three primary emission colours is that the colours

degrade at different rate overtime. This leads to a decrease brightness and colour rendering abilities in

the SMOLED. The degradation mechanisms are relatively unstudied. Recent research has found other

SMOLED devices that can be utilized to obtain white light that avoids this type of degradation.

Very recently, a white emitting diode was fabricated from ITO/TECEB/BCP/Alq3/Mg:Ag. This

structure is able to emit white light by depending on emission from only one layer, that of TECEB.

Another recent study found that white emission could be obtained from a device containing two blue

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phosphorescent dopants Firpic and plantinum(II) [2-(4,6-difluorophenyl)pyridinato-N,C2’] (2,4-

pentanedionato) (FPt1) as excimers. With each dopant having a concentration of 6%, BCP

(bathocuproine) can produce a bright white emission with a maximum quantum efficiency of 4.4%. [16]

It appears that the future of white emission SMOLEDs may lay with the utilization of

phosphorescent dopants. Although more research is still required in order to overcome the major

challenges of lifetime and colour stability under operating conditions for phosphorescent OLEDs.[16]

PLED

Common Host Polymers for PLEDs

Similar to that of SMOLEDs, white emission PLED can be obtained through the layering of red,

blue and green emitting polymer layers. The colour of the EL obtained from PLED devices can be

changed as desired by simply ‘modifying the chemical structure of the polymer either through the main-

chain molecular structure or through the side-chain structures, as in PPV derivatives.’[16] By doping the

host polymer with luminescent emitters, the EL colour can be tuned. Fluorescent dyes, phosphorescent

emitters or other luminescent polymers are common emitters. These doped host polymers are referred

to as blend systems. Typically in a blend system, ‘the host polymer has a wider energy gap while the

dopant has a smaller energy.’ [16] When the host is excited, this excitation energy is transferred to the

dopant molecules ‘through the dipole-dipole interaction (Förster energy transfer), or the direct quantum

mechanical electrons transfer (Dexter energy transfer). By selecting appropriate host and guest

materials, and adjusting the weight ratio of the guest to the host, the white [P]LEDs have also been

successfully demonstrated.’ [16]

Common Polymers Utilized in PLEDs

Great success has been achieved in Poly(p-Phenylene Vinylene)-, polyfluorene- and poly(p-

pheylene) -based PLED devices when using the single-layer (single semiconducting layer between the

bilayer anode and metal cathode) structure. Although, it is difficult to utilize this device structure for

polymers with energy gap larger than 2.9eV.[16]

Poly(p-Phenylene Vinylenes)

Poly(p-Phenylene Vinylene) (PPV) and its derivatives popular materials for PLED due to their

relatively good ability to donate electrons and ability to be chemically doped by strong oxidizing agents

and strong acids, producing highly conductive p-doped materials (with conductivity up to ~104 S/cm).

[16]

Polyfluorenes

Polyfluorenes (PF) are an attractive class of materials for PLEDs because of their excellent optical

and electronic properties, and high thermal and chemical stability. However, there is a major problem

encountered with colour instability of blue PLEDs utilizing PFs. Upon thermal annealing of the polymer

film or during device operation, the pure blue emission of PFs can be contaminated by the undesired

contribution of a green emission band (at ca. 530 nm). It is believed that this green emission band is

caused by the formation of fluorenone during these two events. The exact mechanism of the fluorenone

formation is not known at this time, although research continues to search for an explanation. There

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have been studies that have yielded different methods that are effective at reducing the contribution of

this green emission band. The introduction of bulky substituents in PF has been shown to ‘hinder the

energy transfer onto fluorenone defect sites,’ [16] thereby stopping the quenching of the blue

fluorescence of PF via production of green emission band. A second study has shown that after

preventing the fluorenone green emission via introduction of bulky substituents to PF, the stability of

the blue colour can be further increased by introducing a buffer layer between the PF and the cathode.

This buffer layer inhibits the formation of fluorenone defects.[16]

Currently, research is focusing on fabricating PLED devices (and that of SMOLEDs) that produce

triplet excitons due to their potential higher quantum efficiencies than that obtained with singlet

excitons. This is because the lifetime of triplet excitons is much longer, typically in the range of 10-7 to

10-3s, than that of singlet excitons, typically in the range 10-10 to 10-9s. (The importance of singlet verse

triplet excitons lifetime is expanded upon in the following section). ‘A challenge in this approach is to

prevent the long life triplet excitons from interacting with impurities in the organic layers. More rigorous

requirements on material purity, charge blocking and device encapsulation are anticipated.’ [16]

Phosphorescent Materials There has been a continuous trend of shifting research endeavours toward heavy transition

metal based emitters for OLED applications. Organometallic complexes possessing a heavy transition-

metal element produce strong EL by harvesting both singlet and triplet excitons.[18] This ability gives

heavy transition metal emitters ‘internal phosphorescence quantum efficiency as high as ~100% can be

theoretically achieved,’ and thus are superior to their fluorescent counterparts which can only harvest

singlet excitons. Osmium-, ruthenium-, iridium and platinum-based emitters are popular examples of

heavy transition metal based emitters.[18]

The main problematic issue encountered with ‘phosphorescent emitters in OLED applications is

the saturation of emission sites, which are caused by excessively long lifetimes [and] triplet-triplet

annihilation…’ [18] The issue of excessively long lifetimes is not unique to phosphorescent materials as it

is encountered with SMOLEDs and PLEDs as well. ‘The time is takes for the device carrier injection,

conduction, and recombination to come to steady state, such that the rate of formation of excitons

within the device reaches equilibrium, is the RC time constant. Typically, OLEDs have RC time constants

of 200-500ns.’ [18] This means that phosphorescent dopants with ‘long lifetimes will be promoted into

their excited states orders of magnitudes faster than they can relax. In the meantime, the OLED will

continue to generate triplet excitons, which will not be effectively trapped by the excited dopant. The

net results are that these dopants are inefficient at trapping triplet excitons,’ [18] as the emission sites

have become saturated. This causes OLED efficiency to decrease as the device current is increased. This

efficiency drop can be prevented by selecting dopants and host combinations so that the host always

has the higher triplet energy. Meanwhile, triplet-triplet annihilation can be avoided by increasing the

radiative decay rate of the ‘phosphorescent dopant, such that the triplet excitons relaxes before coming

into contact with another exciton…thereby decreasing the likelihood that the excitons will undergo

annihilation…’ [18] By preventing excitons annihilation, this will actually increase the luminescent

efficiency.[18]

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Platinum octathylporphyrin (PtOEP) is a popular dopant due to its high intersystem crossing

rate, giving PtOEP the added ability of trapping singlet excitons and rapidly converting them to a triplet,

thus producing only one type of emission – phosphorescence. This ability allows for PtOEP devices with a

‘CBP[4,4’-di(N-carbazolyl)biphenyl] host to have internal efficiencies of 30% and those with an Alq3 host

have internal efficiencies of over 20%.’ [18] OLED devices containing other platinum porphyrins have

produced similar results. [18]

The Cyclometallated Ir complex, facial-tris(2-phenylpyridinato,N,C2’)iridium(III), fac-Ir(ppy)3 is an

example of a ‘ phosphorescent dopant with a comparatively short radiative lifetime and high luminance

efficiency used in OLEDs.’[18] When CBP was doped with an optimal doping level of 6% fac-Ir(ppy)3, the

external efficiencies close to 8% (internal efficiency >40%). When the CBP host material was replaced

‘with a transporting triazole host material, TAZ, the external EL efficiency increased to >15%,

corresponding to an internal efficiency of >75%. The improved efficiency for the TAZ-doped device is

due to an increase in the host triplet energy, leading to a lower level of dopant emission quenching by

the host.’ [18] This is further proof that in order to obtain a high external efficiency, it is important for

the host materials to have higher triplet energy than the dopants.

Other than triplet energies, the efficiency of phosphorescent devices can also be improved by

adjusting device architecture. Since phosphorescent dopants produce triplet excitons, more complex

device architectures are required than those for fluorescent dopants. This is due to the differences in

diffusion lengths between singlet and triplet excitons. Triplet excitons can readily diffuse > 1000Å due to

their long excitons lifetimes, whereas singlet excitons can diffuse only 10-100Å. Thus the increased

architecture complexity is required to confine the excitons within the luminescent layer. By doing so, the

external efficiency of the device will increase. [18]

This has been successfully achieve with both PtOEP and Ir(ppy)3-based OLEDs. PtOEP doped into

CBP, as in (ITO/NPD/CBP:PtOEP/Alq3/Mg-Ag) is an example of a successful execution of this. It is

energetically favourable for the PtOEP excitons to diffuse into the adjacent electron transport layer.

Without a blocking layer the external efficiency of the device was 4.2%. After the addition of a blocking

layer (CBP), the resulting device was (anode/NPD/CBP:PtOEP/BCP/Alq3/cathode) with an external

efficiency of 5.6%. [18]

Conclusions LED CONCLUSION HERE

OLED technology makes many promises of a brilliant and brighter future where energy efficient

OLED panels distribute light evenly throughout office buildings and houses. There are many

technological hurdles yet to be overcome. Some of these hurdles include increasing the lifetime of

organic compounds in order to increase the expected lifetime of panels and finding more efficient and

cost effective techniques of mass-producing larger panels. In order to increase the maximum external

efficiencies of OLED lighting, new ways to improve existing SMOLEDs, PLEDs and PHLEDs along with new

organic materials must be further researched and developed. Once this is achieved, OLED lighting can be

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economically brought to the market the energy consumption of such devices would be a fraction of

current compact fluorescent and incandescent light bulbs.

As solid state lighting technology gains widespread adoptability for various technological and

lighting applications our reliance on the electrical system is reduced, particularly during peak electricity

usage times when lighting is required the most.

References [1] G. Seaman. “LED Light Bulbs: Comparison Charts.” EarthEasy: Solutions for Sustainable Living. <http://eartheasy.com/live_led_bulbs_comparison.html> 28 November 2011. [2] C. Humphreys. “Solid State Lighting,” MRS Bulletin, vol. 33, pp. 459-470, May. 2008. [3] B. Streetman & S. Banerjee, “Solid State Electronic Devices,” New Jersey, USA, Pearson, 2006. [4] R. Haitz & M. Crawford, “Light Emitting Diodes,” San Jose, USA, McGraw Hill, 2010. [5] C. Humphreys. “Solid-State Lighting,” MRS Bulletin, Cambridge, UK. Cambridge University, 2008. [6] P. Schlotter et al. “Luminescence Conversion of Blue Light Emitting Diodes,” Applied Physics, Freiburg, Germany, Springer-Verlag, 1997. [7] J. Sheu et al. “White-Light Emission From Near UV InGaN-GaN LED Chip Precoated With Blue/Green/Red Phosphors,” IEEE Photonics Technology Letters, Vol 15, No 1, 2003. [8] K. Kamtekar et al. “Recent advances in white organic light-emitting materials and devices (WOLEDs).,” Advanced materials (Deerfield Beach, Fla.), vol. 22, no. 5, pp. 572-82, Feb. 2010. [9] S. Johnson & J. Simmons. “Materials for Solid State Lighting,” in Materials Research Society Spring Meeting, 2002, pp. 1-10. [10] C. Freudenrich. "How OLEDs Work" 24 March 2005. HowStuffWorks.com. <http://electronics.howstuffworks.com/oled.htm> 18 November 2011. [11] M. Gaillet. "OLED – Organic Light Emitting Diodes" June 2003. Horiba Group. <http://www.horiba.com/fileadmin/uploads/Scientific/Documents/TFilm/se-01.pdf> 19 November 2011. [12] “Introduction to OLED Technology” 2011. OSRAM. <http://www.osram.com/_global/pdf/Professional/LED/OLED/Introduction_to_OLED_technology_04-2011.pdf> 20 November 2011. [13] D. Braun. "Semiconducting Polymer LEDs" June 2002. California Polytechnic State University. <http://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1039&context=eeng_fac> 18 November 2011. [14] F. Zhu. "OLED Activity and Technology Development" 07 May 2009. Institute of Materials Research and Engineering Singapore. <http://www.pshk.org.hk/Activity%20DOC/2009/Green%20Automotive/Day%202%2008%20May,%202009/20090508-07DrFuzongZHU%20for%20web%20uploading.pdf> 25 November 2011. [15] H. Antoniadis. "OLED Product Development” 15 January 2004. OPTO Semiconductors. <http://www.ewh.ieee.org/soc/cpmt/presentations/cpmt0401a.pdf> 19 November 2011. [16] Z. Li & H. Meng. Organic Light-Emitting Materials and Devices. Boca Raton, FL, USA: Taylor & Francis Group, 2007, pp. 1-655. [17] M. Stolka. “Organic Light Emitting Diodes (OLEDs) For General Illumination. Internet: http://lighting.sandia.gov/lightingdocs/StolkaMOLEDRoadmap200103.pdf, March, 2001 [25 November 2011]. [18] H. Yersin. Highly Efficient OLEDs with Phosphorescent Materials. Weinheim, GER: Betz-Druck GmbH, 2008, pp. 1-417.