EE4401 – OPTOELECTRONICS

SOLID STATE LIGHTING: THE WHITE REVOLUTION

LIM FANG JENG

U076372R

NATIONAL UNIVERSITY OF SINGAPORE

2010/2011

Table of Contents

1 Introduction ........................................................................................................................ 1

1.1 Light and Life .............................................................................................................. 1

1.2 Definition of Quality of Light ..................................................................................... 1

1.3 Evolution of Lighting Devices: The Emergence of Solid-State Lighting ................... 2

2 Technological Overview of White LED ............................................................................. 3

2.1 Working Principles and Type of Semiconductor White LED ..................................... 3

2.2 Phosphor-Converted White LED ................................................................................ 4

2.3 RGB Color-Mixed White LED ................................................................................... 6

3 Market Overview of White LED ........................................................................................ 6

3.1 Current LED market .................................................................................................... 6

3.2 Market Survey on LED Production ............................................................................. 7

4 Conclusion .......................................................................................................................... 9

Term Paper: Solid State Lighting EE4401 Optoelectronics U076372R

Lim Fang Jeng

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1 Introduction

1.1 Light and Life

Lighting has far become a development critical for our daily lives. Light is not only

important, but an essential element to enable us to navigate, search and to discover different

things. The best natural light sensor of the universe – our eyes are responsible to receive and

process the information to produce an image or a vision. The structure of eyes have enabled

human to receive light within an illumination range and different spectral qualities (colors).

The most important part in our eyes which give us the ability to receive lights at different

ranges and spectral qualities is to be attributed to the retina. The retina is made of different

light-sensitive photoreceptive cells used for different type of vision – rod cells for scotopic

(dark) vision and cone cells for photopic (bright) vision. The scotopic vision has the highest

luminous efficiency at 507 nm where photopic vision achieves maximum efficiency at 555

nm). The density of rod cells in an eye is much higher compared to that of cone cells. Thus,

the eye muscles will be weary more easily when one tries to see under low light condition;

this suggests the importance of lighting applications to human lives, human’s vision is at its

most comfortable state when the surroundings is bright and illuminated which cone cells are

used as the primary light receptor.

1.2 Definition of Quality of Light

To meet the need for better lighting, various light sources have been developed aims

to increasing the efficiency of light vision to the highest, such development ranges from the

use of fire, invention of the famous Edison incandescent light and fluorescent light as well as

the applications of solid-state lighting. In current state, the light-emitting diodes (LED) which

uses the solid-state lighting technology holds the highest potential to achieve ever growing

quality. The quality of light can be quantified by two different quantities: luminous efficiency

and luminous efficacy, as further defined in the table below.

Table 1 - Definition of luminouse efficiency and luminous efficacy

Luminous efficiency Luminous efficacy

Both luminous efficiency and luminous efficacy share the same unit (i.e. lm/W) and

provide a mean to measure the efficiency of a light source. Luminous efficiency measures

efficiency based on the radiant flux while luminous efficacy measures based on the electrical

power input. The spectrum of the sunlight at wavelength 555 nm gives the maximum possible

Term Paper: Solid State Lighting EE4401 Optoelectronics U076372R

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luminous efficiency available of 683 lm/W, corresponds to 100% efficiency. In all lighting

applications, improvements will be made progressing towards this theoretical limit.

1.3 Evolution of Lighting Devices: The Emergence of Solid-State Lighting

For the past century, the development of lighting was mainly limited by

incandescence, fluorescence and high-intensity discharge (HID). However, none of these

technologies were able to exceed the maximum efficiency of 25%. It was not until the mid-

20th

century when the first commercialized light-emitting diodes (LED) was brought to the

market, the development of the efficiency of solid-state lighting (SSL) far overrides the

incandescent light bulbs and fluorescent tubes. SSL is forecasted in the next few decades to

achieve a white light efficiency of 50%. Figure 1 shows the development of lighting

technologies in the past 200 years [2].

SSL is a technology in which it makes use of the electroluminescent properties of

semiconductors to produce light-emitting devices such as LED. The underlying reason

explaining the potential of SSL LEDs of achieving high efficiency is their energy-efficient

nature. Its electroluminescence principle and semiconductor properties have made the light

produced having low energy consumption, longer lifetime, faster response and small in size.

LED is very versatile in its applications ranging from smallest mobile devices, traffic lights,

computer monitor screens to general lighting [1] (see in Figure 2). If all the lighting

applications are to be replaced with LEDs, the global consumption of electricity would have

to decrease by 10% [3]. Furthermore, its gradual decrease of its costs have also made it more

economical competitive than the various existing lighting technologies. The LED

technologies have already reached a stage at which governments of different nations

Figure 1 - Historical Development and Forecast of the luminous efficacy of lighting technologies in 200 years (1800 to

2050) (Compiled by JY Tsao (Sandia Labs) using various datas) [2]

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including the European Union, Japan, United States, Canada, Australia and the Republic of

China are aggressively pursuing the elimination of incandescent bulbs.

In Singapore, the LED market is also growing rapidly and application of white LEDs

as general lighting is slowly emerging. Many global SSL companies such as Philips, Seoul

Semiconductor, Epistar, Toyoda Gosei from various nations are also starting to invest into

Singapore’s market. JTC Corporation and Foxsemicon Integrated Technology from Taiwan

has started to install new LED streetlights at parts of street roads including CleanTech Park,

One-North and few more proposed stretch of roads as test-beds. It is estimated that a large-

scale LED streetlight replacement project will take place in 2 to 3 years in the nation [4]. This

technological shift has been made more and more significant since the past few years. As a

result, an understanding on white LED is crucial and important to keep track with the

technological and economic advancement.

In this paper, technological and market overviews will be presented. Firstly, the

principles of LEDs will be studied, followed by overview of different types of white LED.

Secondly, current LED market and market survey on white light LEDs production by 3 main

LED companies will be presented and evaluated.

2 Technological Overview of White LED

2.1 Working Principles and Type of Semiconductor White LED

The working principles of LED is simple: when a forward biased connection is made

across a p-n junction, electrons and holes will recombine and giving photon with certain

energy, if the photon is extracted out from the chip, an LED is made. The energy of the

Figure 2 - Applications of LED in different scales and range

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photon is limited by the bandgap of the semiconductor and different energy of photons will

give rise to different color of light emission and by making use of these colors, white light

LEDs can be made.

In practice, there are 2 methods to produce white light LEDs: phosphor-down

conversion and color-mixing method. Phosphor-down conversion involves conversion of

LED of UV/purple color to excite phosphor or other down-conversion materials to give a

wavelength-down appearance of white light; color mixing method involves the mixing of

multiple colored LEDs to create a RGB white light. Figure 3(a) and (b) show the basic lamp

structure of phosphor-converted white LED and color-mixed white LED respectively. Both

phosphor-down and color-mixed LEDs have their own pros and cons in producing white light

and they will be discussed accordingly as the paper proceeds.

2.2 Phosphor-Converted White LED

As mentioned in the above section, the phosphor-converted white LED (pc-LED)

makes use of technology of down conversion. This method of producing white light has been

commonly used in fluorescence, only with the replacement of a higher efficiency and light

source with better quality. Under phosphor conversion, the blue light experiences a Stokes

shift from shorter wavelengths to longer. Different types of material can be used as the

phosphor layer depending on the original LED color. If several phosphor layers of distinct

colors are applied, the emitted spectrum can be broadened and effectively raising the color

rendering index (CRI) of a given LED [5]. A typical spectral characteristics produced by a

pc-LED has two peaks, the first higher peak is attributed to the LED light source (blue or UV)

while the second peak attributes to the Stokes shift of the LED light, which gives rise to white

light.

(b)

Figure 3 - (a) Lamp Structure for Phosphor-down White LED; (b) Lamp Structure for Color-Mixed White LED

(a)

White Light White Light

RGB Phosphor

UV/purple

LED

Mixing Optics

RGB LEDs

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To manufacture a white pc-LED, a InGaN blue LEDs can be encapsulated inside a

common yellow phosphor coated epoxy such as cerium-doped yttrium aluminium garnet

(Ce3+

:YAG). On the other hand, a near ultraviolet (NUV) LED source can also be used as the

lamp source, coated with a mixture of high efficiency europium-based (red and blue emitting)

phosphors and copper and aluminium doped zinc sulfide (ZnS:Cu, Al, green emitting)

phosphor. Using a blue LED with Ce3+:YAG phosphor coated is not as efficient as using the

UV LED with the three-color mixed phosphors because the Stokes shift of the former is

larger, causing more heat losses, but produces light with higher CRI due to its broad spectral

broadening. However, using UV LED is still not preferred due to safety concerns; UV light

may leak out and causes harm to human eyes or skin.

Since the nature of this design is to convert the original UV/Purple LED to a broader

spectrum by additional coated layers, certain loss is to be expected. Depending on the design

of the lamp, certain reflective losses and absorption losses can affect the CRI of the white

light produced. This essentially means that there will be a loss in power when light is

delivered. Besides, heat loss due to the Stokes shift, phosphor degradation of the device such

as oxidation, formation of deep electron taps, thermal quenching, etc and lack of tunability to

produce whole array of efficient white light are also the factors affecting the low efficiencies

and low versatility of while light produced by pc-LEDs.

However, due to the simpler and cheaper design and production of a pc-LED

compared to a complex RGB system, the phosphor method is still the most popular method

for making high intensity white LEDs. In fact, majority of high intensity white LEDs on the

market are still manufactured by using phosphor light conversion [6-8]. Different ways can

be employed to increase the efficiency of the light, for instance, a better package design or by

Stokes Conversion

(By Phosphor Layer)

Blue/ UV LED

Figure 4 - Typical Spectral Characteristics of pc-LED

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using a more suitable type of phosphor having lower Stokes energy loss as well as lower

degradation, the efficiency can be increased. A more homogeneous white light is claimed to

be produced by Philips Lumileds by employing a conformal coating process, which can make

the phosphor thickness uniform [9]. With development ongoing, the efficiency of phosphor

based LEDs generally rises with each new product announcement.

In overall, due to the low system complexity of pc-LED and the existence of

technology of using phosphor with UV/purple LEDs, albeit with improvements to be desired,

is the clear current design choice for various lighting applications.

2.3 RGB Color-Mixed White LED

The design of RGB color-mixed LED is, however, different from the pc-LED. It

involves the theory of color-mixing in forming white light, hence no down conversion

processes is present as in the pc-LED. The fundamental color to form white light with

optimal CRI is essentially red, green and blue (RGB). Hence, making use of this principle, a

high efficiency and high intensity white light can be produced. To make the color mixing

works, an

Theoretically, color-mixed LEDs should give a higher efficiency compared to pc-

LED because there is no down-conversion loss; this eventually will save energy and also

lower the cost of LED per lumen, as will be discussed in the later section. However, color

mixed LED is still unable to be produced in general lighting. Based on the current technology,

the ‘Green Gap’ problem is still the hurdle and bottleneck for producing a high efficiency

white color-mixed LEDs [10]. Furthermore, due to its principle of additive color mixing in

producing white light, they could not provide richly saturated and natural looking white lights

[11]. Nevertheless, in a long run, this design is still more favorable than pc-LED due to the

absence of down-conversion losses and a lower capital and production costs.

3 Market Overview of White LED

3.1 Current LED market

According to the most recent report by Strategies Unlimited, the global market for

high brightness LED in 2010 is $10.8 billion and an expected increase to $18.9 billion by

2015, with dominant region in Japan and Korea (61% of the market, shown in Figure 5) [12].

LED is expected to have a dominant market by the year 2014 due to the ever rising demand

for energy efficiency and phasing out of incandescent bulbs.

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Figure 5 - Regional breakout of world wide high-brightness LED supply by Revenue

Major high brightness LED manufacturers such as Nichia, Cree, Lumiled, Osram

Opto Semiconductors, Seoul Semiconductor, Epistar and Toyoda Gosei have developed the

LED technologies in different applications such as backlight illumination, general lighting,

vehicle lighting, etc. All these development are expected to meet the Solid State Lighting

LED (SSL-LED) roadmap from U.S Department of Energy (DoE) in reaching a power

conversion efficiency of 50% by 2020 (Table 2). Apart from the improvement of power

conversion efficiency, improvement of lifetime and production cost is also vital. By 2020,

SSL-LEDs are expected to have a lifetime of 100,000 hours (~ 10 years), 100 times and 10

times longer than that of incandescence and florescence respectively. Though the production

cost of SSL-LEDs are more costly than that of the conventional incandescence and

fluorescence, their longevity, power usage and efficiency clearly outweigh the conventional

lighting technologies. The roadmap seems aggressive, but with rapid development of LED

technologies, it is not impossible for LED technologies to reach a power conversion

efficiency of 50% by the year 2020.

Table 2 - SSL-LED Roadmap with comparisons to traditional lighting technologies [2]

SSL-LED 2012 SSL-LED 2020 Incandescent Fluorescent

Luminous Efficacy (lm/W) 150 200 16 85

Power Conversion efficiency (%) 37.5 50 4 21.25

Flux (lm) 1,000 1,500 1,200 3,400

Input Power (W) 6.7 7.5 75.0 40.0

Lifetime (hr) 100,000 100,000 1,000 10,000

Lamp Cost ($/klm) 5.0 2.0 0.4 1.5

Lamp Cost ($/lamp) 5.0 3.0 0.5 5.0

Color Rendering Index (CRI) 80 80 100 75

3.2 Market Survey on LED Production

A market survey was done by comparing the most recent highest performance LED

by three major LED production companies, i.e. Nichia Corporation, Cree and Seoul

33%

28%

14%

2%

23%Japan

Korea

Taiwan

China

US/Europe

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Semiconductor. Narukawa et.al. from Nichia Corporation from Japan have claimed to

successfully develop a white LED with very high luminous efficacy of 183 lm/W with

luminous flux of 203 lm at 350 mA, achieving a power conversion efficiency of 45.8% [13].

Cree Research Center from the U.S has also successfully developed a high efficacy warm

white LED using their own unique phosphor deposition technology that reaches as high as

138 lm/W at 350 mA, with power conversion efficiency of 34.5% [14]. Acriche, Seoul

Semiconductor’s main product has announced the launch of 4-chip RGB LED (Z-power Z6

and Z7) which could produce 440 lm with 1 W of power for each LED chip and they claim

the LED could produce a diverse temperature of white light if all the 4 chips are used [15].

The performances of LEDs in all these companies are tabulated in Table 3 below.

From Table 3, we can see that the Nichia Corporation apparently sits at the highest

position in luminous efficacy, with the highest power conversion efficiency. However, the

capability of such ultra high brightness LED to be commercialized is still not revealed, so is

the cost of such LED. Though Seoul Semiconductor seems to have the lowest LED efficacy

among the 3 manufacturers, their low-cost technology might earn them higher revenues. High

luminous efficacy should not be the sole evaluation for a best LED, apart from power usage

and efficiency, the cost of LED should also be considered. A good LED should be a balance

between performances and cost in order for it to be commercially available to consumers. It

can also be seen that except for Seoul Semiconductor, most LEDs are that of a phosphor-

converted (pc-LED). This shows the pre-maturity of color mixed LED in current market and

more challenges are to come in the future. Nevertheless, in terms of efficacy, most companies

have moved closer or exceeded the scenario of SSL-LED for 2012 (see Table 2); it is indeed

possible for LED technology to reach a state as projected by SSL-LED 2020.

Table 3 - Comparison of highest performance LED of three majore LED companies

Nichia Corporation [13] Cree [14] Seoul Semiconductor [15, 16]

Luminous Efficacy (lm/W) 183.00 138.00 110.00

Forward Current (mA) 350 350 350

Wall-plug power usage (W) 1 0.97 4

Power Conversion efficiency (%) 45.8 34.5 27.5

LED type pc-LED pc-LED 4-chip color-mixed RGB White

LED

LED Package Surface Mount Surface Mount Ceramic Circuit

Applications General Lighting

General Indoor and

Outdoor lighting,

directional lighting

General Lighting

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4 Conclusion

By the motivation of have comfortable lighting to enhance the photopic vision of

human, various lighting technologies ranging from candles to fluorescence have been

developed since the last 200 years. The emergence of solid state lighting has made a giant

leap towards a low-cost, energy efficient lighting source. The energy saving, high efficient

and easily manipulated nature has made solid state white LED to have versatile applications

ranging from mobile phone backlight to general lighting. In this paper, overview of two main

types of white LED, pc-LED and color-mixed LED, were evaluated and presented. The

exploration to future projection of solid state lighting LED to 2020 and market survey on

current ultra brightness LED market by Nichia, Cree and Seoul Semiconductor were also

documented. It is believed that with the current state of rapid development, the market will

not merely aiming to reach a high performance LED lighting, but the needs for human in

pleasing the vision could also be met. With these technological advancements, surely a

‘bright’ future will be one more step closer, when the ‘sun’ never sets.

Term Paper: Solid State Lighting EE4401 Optoelectronics U076372R

Lim Fang Jeng

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