SOLID STATE SOLID STATE LIGHTING (SSL)LIGHTING (SSL)

Cécile Rosset

MSc. Environmental Engineering

Technische Universität München

History of Lighting

1 Introduction

3 traditional Technologies:

•Fire Incandescence

•Fluorescence & High Intensity discharge

Oil lamp Incandescent bulbs Fluorescent bulbs

The fourth lighting technology

1 Introduction

Solid: Light emitted by a solid: a piece of semiconductor

SSL: Creation of first light emitting diodes (LED)

At that time, LEDs were used for showing the time in an alarm clock or as a battery indicator

SSL: a new alternative to other lighting technologies?

1 Introduction

• Reduced heat generation • Use of less power • Longer life span

World Lighting Pollution

Lighting corresponds to 19% of the worldwide energy consumption. Reducing energy consumption by using LEDs will significantly reduce the level of CO2 emissions, therefore positivelyimpacting climate change

Process of emitting Light

2 LED Mechanism

•n-type & p-type semiconductors are combined in one device.

• With the application of a voltage between the p-side and the n-side, free electrons from the n-type side go to the p-type side through the junction.

•When an electron meets a hole, it recombines and thus releases its energy by emitting a photon.

Forward-biased pn junction

2 LED Mechanism

Holes injected

Electrons injected

Direct / Indirect bandgap:

2 LED Mechanism

• LEDs are made of direct bandgap semiconductors with bandgaps corresponding to near infrared, visible, or near ultraviolet light. The minimum of the conduction band lies directly above the maximum of the valence band.

•Silicon has an indirect bandgap: For the recombinqtion of electrons and holes; the participation of a phonon (or a defect) is needed to conserve momentum

2 LED Mechanism

Light extraction

• Snell’s law:

• Light is unable to escape at angles greater than

• defines the solid angle=projection on the surface area

c

cc

cos12

i

tC n

nArc

90sinsin

2221 sinsin nn

c

2 LED MechanismDifferent geometries increasing the extraction of light

Wavelentgh and colour

2 LED Mechanism

• The wavelentgh, and therefore the colour depends on the band gap of the semiconductor material.

Red: GaAlAs

Blue: InGaN

UV: InGaAs

2 LED Mechanism

UV & blue LEDs

For a long time unavailable; relied on blue coating!

Colours available:

(Blue and white are much harder to obtain)

• The first blue LED was designed several years ago using SiC

BUT: Poor efficiency!

• 1993: Efficient GaN-based LEDs

• Today: InGaN-based LEDs, intensity 5x bigger than with GaN

An ultraviolet GaN LED

White LEDs

2 LED Mechanism

Most simple method but not often used nowadays.

• 1st technique: Found in 1993, when the first blue LED was produced.

By juxtaposing at a certain distance blue, red, and green LEDs, white light was obtained.

• 2nd technique: found in 1996 by Nichia Corp. and Fraunhofer Institut

Start with LED with an active layer made of InGaN

Cover this structure is covered with a yellow phosphor crystal coating (Ce3+:YAG).

The LED chip emits blue light, which is converted to yellow light by the phosphor.

LuminescencePhosphorescence

White LED

2 LED Mechanism

3 kinds of white light, depending on the temperature:

• 4000-4500 K, Incandescant or warm white

• 5000-6500 K, Pure white• 7000-8000 K, Cool white

GaN or InGaN LED Ce:YAG

Other techniques of creating white LEDs

2 LED Mechanism

• Coat near ultra-violet (NUV) with europium-based red and blue emitting phosphors

• Transfer NUV radiation to visible light via the photoluminescence process in phosphor materials

• Method less efficient then with the blue LED because of photodegradation of the epoxy resin used in LED packaging.

• Coat blue LEDs with quantum dots, which absorb the blue light and emit a warm white light.

1.

2.

Color temperature and color mixing

2 LED Mechanism

CIE diagram of human vision

• An LED gives a pure monochromatic colour (on the edges of the CIE diagramm)

• By mixing these colours a new one can be obtained.

3 LED Fabrication process

LED Growth

Growth of a thin layer of semiconductor (InGaN or AlGaInP, depending on the colour we want):

•For AlGaInP, GaAs substrates are used (typ. diameter 150 mm)

•For InGaN, SiC substrates (diameter 75 mm) or Al2O3 substrates (diameter 50mm)

Further fabrication steps, comparable to silicon device fabrication

3 LED Fabrication process

Final structure of an GaN-LED:

• electrical contacts to the p- and n-layers are both on the top surface of the device because of the insulating sapphire substrate.

• the area of the contact to the p-layer has to be maximized to promote current spreading

maximizes light emission and minimizes turn-on voltage and series resistance

Because most of the light generated at the junction escapes the device through the top surface

the large-area p-contact has to be made as transparent as possible outside the area where electrical bond wires are attached

Fabrication process: MOCVD

•Aim: To reduce the voltage at the operating current

Exemple: n-Butyllithium , C4H9Li

•MOCVD: metal-organic chemical vapor deposition

•Chemical process used to grow quantum wells, thus to produce high purity, high performance solid materials

•It’s a CVD process that uses metalorganic source gases(chemical compound containing bonds between carbon and metal)

•Reduction of dislocation density at the GaN epitaxial surface

4 Towards a better efficiency

Quantum efficiency

4 Towards a better efficiency

the internal QE of double heterostructure can be greater than 99%

• Quantum wells are potential wells that confine particles, which were originally free to move in three dimensions, to two dimensions, forcing them to occupy a planar region

Double Heterostructure: change in bandgap

Lowering the operating temperature: Flip chip

4 Towards a better efficiency

Normal LED:

• Big thermal resistance in thermal conduction path

•Large amount of heat transferred from active layer through front face of LED and the encapsulating material and then dissipated into the air

Flip Chip LED:

• designed with a thermal conductive submount and metal interconnections to conduct most of the heat through submount

4 Towards a better efficency

State-of-the-art The present state-of-the-art is 30% external efficiency in AlGaAs-based LEDs, employing a thick transparent semiconductor superstrate, and total substrate etching in a particularly low-loss optical design.

Tunability of the final emission spectrum, by controlling the particle size distribution and/or surface chemistry thus white light colour is better

• Avantages of QDs (CdSe) over phosphor for white light generation

quantum efficiency of 76% (in solution) for a blue emission, in GAN led

Efficiency of 43% attained with an efficient NUV LED. The device structure consists of an MQW on a lateral epitaxy on a patterned surface and is flip-chip mounted on a silicon substrate.

5 Different types of LEDs

Organic LED & Polymer LED

These are LEDs whose emissive electroluminescent layer is composed of an organic compound or polymer that will luminesce blue, green and red, and are covered with a transluscent material.

Advantages: -more easily integrated with other electronic components -it can emit white light intrinsically

Problems: degradation in air & easily damaged by exposure to water

6 Limiting factorsCost Competitiveness

• Now, LED prices are 10 times higher than of incandescent light bulbs

BUT: Better efficiency and longer lifetime

Narrow angle of emission• To use LEDs in ambient lighting, multiple LEDs are asembled in a single fixture. This leads to sharp shadows.

High quality variation• Inexpensive LEDs have inconsistent colour temperature and light output

• LEDs are currently limited by poor internal quantum and light-extraction efficiency, but photonic crystals offer a potential solution to both problems.

Poor Quantum efficiency

Good efficiency & durability

7 Perspectives & Advantages

• LEDs can provide 50 000 hrs of life compared to 1000 hrs with incandescent light bulbs

• Associated with perfect material and devices, LEDs would require only 3 Watts to generate the light obtained with a 60-Watt incandescent bulb

Figure 1. LED vs. conventional light sources degradation in light output over time

Good stability

• Due to their solid state, they can withstand vibrations better and have no filament that might break

• They are capable of functioning in many environments (except OLEDs!)

• An experiment made by ilight showed that a LED sign still worked after a blast of shotgun!

7 Perspectives & Advantages

Reduction of energy consumption

• LEDs require less current than incandescent bulbs

DDP® LED Lamp Incandescent Bulb

6S6L120-CWX 11mA 6S6/120V 50mA

120PSBL-NWX 5.8mA 120PSB 25mA

387L-X1 16mA 387 40mA

1819L-X-CX 17mA 1819 40mA

• Comparison with incandescent bulbs: When cold, an incandescent filament draws ten times as much current as it does during normal operation. The initial powering of hundreds of incandescent bulbs simultaneously causes significant voltage surges that lead to lamp failures.

7 Perspectives & Advantages

Reduction of heat emission

• Less heat emission

Lens stays cooler

Less energy wasted

• Room temperature stays cooler, so we don’t need further air conditioning

• Some LED lamps are designed with series resistors to limit the operating current, resulting in no cold filament current variation.

7 Perspectives & Advantages

Allows wide variety of lighting

• Artificial lighting similar to daylight

• More control of the colour and intensity

• SSL can be coupled to light pipes

Light can be efficiently and flexibly distributed

• Interesting design possibilities: they can be placed on floors, walls, ceilings or furniture!

7 Perspectives & Advantages

White LED: The Future Lighting Technology

• In the past 6 years: Tremendous gain in energy efficiency, brightness and lifespan

•Although they are still expensive, they could come in the market for residential lighting in the next 10 or 15 years

• For now, between 25 & 50% efficiency, but some researchers think it’s possible to have 90% efficiency! Contrary to the traditional light bulb which has 5% efficiency and no perspective to do better!

7 Perspectives & Advantages

Traffic signals, street light

8 Applications

Residential

Information boards

Buildings

Common application: Digital clock, battery level indicator, torch

Outdoor: runway in airports

9 Challenges

Phosphor challenge

Better understanding of the physics of semiconductors used; AlGaInP and AlGaInN materials and nanostructures

For white light: Improved wavelength-conversion and colour-

mixing technologies for generation of white light

Projects sponsorised by the DOE:

10 Research and development going on

• Develop a polymer OLED using advanced polymer synthesis to allow large-scale manufacturing of p-OLED lamp

• Create a GaN substrate with very few dislocations in order to improve blue LED efficiency

• Understand & solve the problem of low radiative emissions in green LEDs

• And many more…

• The US Department of Energy promotes research on SSL by allocating $21 million to 13 projects!

Project : “LED lights might light homes in less than 3 years”

10 Research and development going on

• University of Glasgow

• Investigation of the possibility of making microscopic holes on the surface of the LED to extract more light, in order to increase the brighteness without increasing the energy consumption

• Technique used: Nano-printing lithography, direct impression of the holes

11 Environment

Result doubly environment-friendly

• Less current consumption (less electricity burned)

• Less heat produced

Less CO2 emissions

Less light pollution

Positive impact on global warming

Incandescent traffic lights replaced by LEDs in USA:economy of2.5 billion kWhours= US$ 200 million= 3 billion kilos of CO2 released in the atmosphere

12 Future & perspectives

Based on semiconductor material, like microprocessor

Important research and progress for these materials

In comparison with processors becoming faster and cheaper each year, we can expect the LED to become brighter, more energy efficient, have a higher longevity and become cheaper

13 Interesting websites

http://lighting.sandia.gov

http://www.loe.org

http://www.enn.com

http://www.netl.doe.gov

http://www.nichia.com

http://cree.com

www.lumileds.com

« LEDs could soon be the light of the world »

Semi-conductor diode

2 LED Mechanism

Solid material that is between insulators and conductor:

Behaves like an Insulator (large bandgap) at room temperature Behaveslike conductor (no bandgap) when applying electric field

Doping to enhance the conductivity

2 LED Mechanism

Addition of impurities in the lattice:

• p-type: enhanced conductivity with valence electron-deficient dopants where the acceptor impurity creates a hole

• n-type: enhanced conductivity with valence electron-enriched dopants, thus the donor imprity contribute free electron

Ce3+:YAG : phosphor coating for the InGaN-GaN structure

2 LED Mechanism

• Cerium-doped yttrium aluminum garnet: phosphor or scintillator

• Here used as scintillator as it is in pure single cristal form (semiconductor): absorbs the high energy blue photons and in response, fluoresces photons at a longer wavelength

•Outpur colour strongly dependant on current and temperature

LuminescencePhosphorescence

Thermal Management

4 Towards a better efficiency

• Reduce LED’s temperature in order to have an incresed output and life time:

• Thermal resistance:it’s the ratio of the difference in temperature to the power dissipated (°C/W)

• The maximum ambient temperature at which LEDs can work is determine by the PN junction temperature

• Power dissipated: IUP

• The maximum junction temperature is , but the aim is to keep lowjTmaxjTjT

• Junction temperature: PRthTT ajaj

R1 + R2

• Heatsink: aborbs the heat and dissipates it by conduction or convection

R1 + Rh + R2

• But by adding a Heatsink we add a resistanceneed to determine its maximum value