SOLID STATE
LIGHTING (SSL)
Cécile Rosset
MSc. Environmental Engineering
Technische Universität München
1
Introduction
History of Lighting
3 traditional Technologies:
•Fire
Incandescence
Oil lamp
Incandescent bulbs
•Fluorescence & High
Intensity discharge
Fluorescent bulbs
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
1
Introduction
SSL: a new alternative to other lighting
technologies?
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 positively
impacting climate change
• Reduced heat generation
• Use of less power
• Longer life span
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.
2
LED Mechanism
Forward-biased pn junction
Electrons injected
Holes injected
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
2
LED Mechanism
Light extraction
• Snell’s law: n1 sin  2  n2 sin  2
 nt sin 90 

n
i


 C  Arc sin 
c
c
• Light is unable to escape at
angles greater than  c
•  c defines the solid
angle=projection on the
surface area   2 1  cos 
2
LED Mechanism
Different geometries increasing the
extraction of light
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
2
LED Mechanism
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
BUT: Poor efficiency!
• 1993: Efficient GaN-based LEDs
• Today: InGaN-based LEDs,
intensity 5x bigger than with GaN
An ultraviolet GaN LED
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
2 LED Mechanism
White LED
GaN or InGaN LED
Ce:YAG
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
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.
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
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 nlayers 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
4 Towards a better efficiency
Fabrication process: MOCVD
•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
•Aim: To reduce the voltage at the operating current
Exemple: nButyllithium ,
C4H9Li
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
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
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
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 factors
Cost 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
Poor Quantum efficiency
• LEDs are currently limited by poor internal quantum and lightextraction efficiency, but photonic crystals offer a potential solution
to both problems.
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 50 000 hrs of life compared to 1000 hrs with
incandescent light bulbs
Figure 1. LED vs. conventional light sources degradation in light output over time
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!
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
• 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
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
• 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
8
Applications
Common application: Digital clock, battery level indicator, torch
Traffic signals, street light
Buildings
Outdoor:
runway in airports
Residential
Information boards
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 colourmixing technologies for generation of white light
10
Research and
development going on
Projects sponsorised by the DOE:
• The US Department of Energy promotes research on SSL by allocating
$21 million to 13 projects!
• Develop a polymer OLED using advanced polymer synthesis to allow
large-scale manufacturing of p-OLED lamp
• Understand & solve the problem of low radiative emissions in green LEDs
• Create a GaN substrate with very few dislocations in order to improve
blue LED efficiency
• And many more…
10 Research and
development going on
Project : “LED lights might light homes in less than 3
years”
• University of Glasgow
• Investigation of the possibility of making microscopic holes on
the surface of the LED to extract more light, in order to increase
the brighteness without increasing the energy consumption
• Technique used: Nano-printing lithography, direct impression of
the holes
11 Environment
Result doubly environment-friendly
• Less current consumption
(less electricity burned)
• Less heat produced
Less CO2 emissions
Less light pollution
Positive impact on global warming
Incandescent traffic lights
replaced by LEDs in
USA:
economy of
2.5 billion kWhours
= US$ 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 »
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
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
2 LED Mechanism
Ce3+:YAG : phosphor coating for the InGaNGaN 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
Phosphorescence
Luminescence
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 T j
• The maximum junction temperature is
T j max , but the aim is to keep T j low
• Thermal resistance:it’s the ratio of the difference in temperature to the power
dissipated (°C/W)
R1
• Power dissipated: P  U  I
+
R2

• Junction temperature: T j  Ta + Rth j  a  P

R1 +
Rh
+
• 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
R2