1. SOLID STATE LIGHTING (SSL)
Cécile Rosset
MSc. Environmental Engineering
Technische Universität München

2. 1 Introduction History of Lighting 3 traditional Technologies:
Fire Incandescence
Fluorescence & High Intensity discharge
Incandescent bulbs
Fluorescent bulbs
Oil lamp

3. 1 Introduction The fourth lighting technology
SSL: Creation of first light emitting diodes (LED)
Solid: Light emitted by a solid: a piece of semiconductor
At that time, LEDs were used for showing the time in an alarm clock or as a battery indicator

4. 1 Introduction SSL: a new alternative to other lighting technologies?
corresponds to 19% of the worldwide energy consumption.
Reducing energy consumption
by using LEDs will significantly
reduce the level of
CO2 emissions, therefore positively
impacting climate change
World Lighting Pollution
Reduced heat generation
Use of less power
Longer life span

5. 2 LED Mechanism Process of emitting Light
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.

6. 2 LED Mechanism Forward-biased pn junction Electrons injected
Holes injected

7. 2 LED Mechanism Direct / Indirect bandgap:
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

8. 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

9. 2 LED Mechanism
Different geometries increasing the extraction of light

10. 2 LED Mechanism Wavelentgh and colour
The wavelentgh, and therefore the colour depends on the band gap of the semiconductor material.
Red: GaAlAs
Blue: InGaN
UV: InGaAs

11. 2 LED Mechanism UV & blue LEDs
Colours available:
(Blue and white are much harder to obtain)
UV & blue LEDs
For a long time unavailable; relied on blue coating!
The first blue LED was designed several years ago using SiC
An ultraviolet GaN LED
BUT: Poor efficiency!
1993: Efficient GaN-based LEDs
Today: InGaN-based LEDs, intensity 5x bigger than with GaN

12. 2 LED Mechanism White LEDs
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.
Most simple method but not often used nowadays.
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.
Phosphorescence
Luminescence

13. 2 LED Mechanism White LED 4000-4500 K, Incandescant or warm white
GaN or InGaN LED
Ce:YAG
3 kinds of white light, depending on the temperature:
K, Incandescant or warm white
K, Pure white
K, Cool white

14. 2 LED Mechanism Other techniques of creating white LEDs 1.
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. 2.
Coat blue LEDs with quantum dots, which absorb the blue light and emit a warm white light.

15. 2 LED Mechanism Color temperature and color mixing
An LED gives a pure monochromatic colour (on the edges of the CIE diagramm)
By mixing these colours a new one can be obtained.
CIE diagram of human vision

16. 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

17. 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

18. 4 Towards a better efficiency
Fabrication process: MOCVD
MOCVD: metal-organic chemical vapor deposition
Exemple: n-Butyllithium , C4H9Li
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
Aim: To reduce the voltage at the operating current

19. 4 Towards a better efficiency
Quantum efficiency
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
the internal QE of double heterostructure can be greater than 99%
Double Heterostructure: change in bandgap

20. 4 Towards a better efficiency
Lowering the operating temperature:
Flip chip
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

21. 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.
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.
Avantages of QDs (CdSe) over phosphor for white light generation
Tunability of the final emission spectrum, by controlling the particle size distribution and/or surface chemistry thus white light colour is better
quantum efficiency of 76% (in solution) for a blue emission, in GAN led

22. 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

23. 6 Limiting factors Cost Competitiveness Narrow angle of emission
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
Poor Quantum efficiency
LEDs are currently limited by poor internal quantum and light-extraction efficiency, but photonic crystals offer a potential solution to both problems.

24. 7 Perspectives & Advantages
Good efficiency & durability
Associated with perfect material and devices, LEDs would require only 3 Watts to generate the light obtained with a 60-Watt incandescent bulb
LEDs can provide hrs of life compared to 1000 hrs with incandescent light bulbs
Figure 1. LED vs. conventional light sources degradation in light output over time

25. 7 Perspectives & Advantages
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!

26. 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
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.

27. 7 Perspectives & Advantages
Reduction of heat emission
Less heat emission
 Lens stays cooler
 Less energy wasted
Some LED lamps are designed with series resistors to limit the operating current, resulting in no cold filament current variation.
Room temperature stays cooler, so we don’t need further air conditioning

28. 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!

29. 7 Perspectives & Advantages
White LED: The Future Lighting Technology
In the past 6 years: Tremendous gain in energy efficiency, brightness and lifespan
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!
Although they are still expensive, they could come in the market for residential lighting in the next 10 or 15 years

30. 8 Applications
Common application: Digital clock, battery level indicator, torch
Traffic signals, street light
Buildings
Outdoor:
runway in airports
Residential
Information boards

31. 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

32. 10 Research and development going on
Projects sponsorised by the DOE:
The US Department of Energy promotes research on SSL by allocating $21 million to 13 projects!
Develop a polymer OLED using advanced polymer synthesis to allow large-scale manufacturing of p-OLED lamp
Understand & solve the problem of low radiative emissions in green LEDs
Create a GaN substrate with very few dislocations in order to improve blue LED efficiency
And many more…

33. 10 Research and development going on
Project : “LED lights might light homes in less than 3 years”
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

34. 11 Environment Result doubly environment-friendly
Incandescent traffic lights replaced by LEDs in USA:
economy of
2.5 billion kWhours
= US$ 200 million
= 3 billion kilos of CO2 released in the atmosphere
Less current consumption
(less electricity burned)
Less heat produced
Less CO2 emissions
Less light pollution
Positive impact on global warming

35. 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

36. 13 Interesting websites http://lighting.sandia.gov http://www.loe.org

37. « LEDs could soon be the light of the world »
Thank you!

39. 2 LED Mechanism Semi-conductor diode
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

40. 2 LED Mechanism Doping to enhance the conductivity
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

41. 2 LED Mechanism
Ce3+:YAG : phosphor coating for the InGaN-GaN structure
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
Luminescence
Phosphorescence

42. 4 Towards a better efficiency
Thermal Management
Reduce LED’s temperature in order to have an incresed output and life time:
The maximum ambient temperature at which LEDs can work is determine by the PN junction temperature
The maximum junction temperature is , but the aim is to keep low
Thermal resistance:it’s the ratio of the difference in temperature to the power dissipated (°C/W)
R R2
Power dissipated:
R Rh R2
Junction temperature: ( ) P
Rth T a j + = -
Heatsink: aborbs the heat and dissipates it by conduction or convection
But by adding a Heatsink we add a resistanceneed to determine its maximum value