Presentation on theme: "SOLID STATE LIGHTING (SSL)"— Presentation transcript:
1SOLID STATE LIGHTING (SSL) Cécile RossetMSc. Environmental EngineeringTechnische Universität München
21 Introduction History of Lighting 3 traditional Technologies: Fire IncandescenceFluorescence & High Intensity dischargeIncandescent bulbsFluorescent bulbsOil lamp
31 Introduction The fourth lighting technology SSL: Creation of first light emitting diodes (LED)Solid: Light emitted by a solid: a piece of semiconductorAt that time, LEDs were used for showing the time in an alarm clock or as a battery indicator
41 Introduction SSL: a new alternative to other lighting technologies? corresponds to 19% of the worldwide energy consumption.Reducing energy consumptionby using LEDs will significantlyreduce the level ofCO2 emissions, therefore positivelyimpacting climate changeWorld Lighting PollutionReduced heat generationUse of less powerLonger life span
52 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.
62 LED Mechanism Forward-biased pn junction Electrons injected Holes injected
72 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
82 LED Mechanism Light extraction Snell’s law: Light is unable to escape at angles greater thandefines the solid angle=projection on the surface area
92 LED MechanismDifferent geometries increasing the extraction of light
102 LED Mechanism Wavelentgh and colour The wavelentgh, and therefore the colour depends on the band gap of the semiconductor material.Red: GaAlAsBlue: InGaNUV: InGaAs
112 LED Mechanism UV & blue LEDs Colours available:(Blue and white are much harder to obtain)UV & blue LEDsFor a long time unavailable; relied on blue coating!The first blue LED was designed several years ago using SiCAn ultraviolet GaN LEDBUT: Poor efficiency!1993: Efficient GaN-based LEDsToday: InGaN-based LEDs, intensity 5x bigger than with GaN
122 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 InstitutStart with LED with an active layer made of InGaNCover 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.PhosphorescenceLuminescence
132 LED Mechanism White LED 4000-4500 K, Incandescant or warm white GaN or InGaN LEDCe:YAG3 kinds of white light, depending on the temperature:K, Incandescant or warm whiteK, Pure whiteK, Cool white
142 LED Mechanism Other techniques of creating white LEDs 1. Coat near ultra-violet (NUV) with europium-based red and blue emitting phosphorsTransfer NUV radiation to visible light via the photoluminescence process in phosphor materialsMethod 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.
152 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
163 LED Fabrication process LED GrowthGrowth 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) orAl2O3 substrates (diameter 50mm)Further fabrication steps, comparable to silicon device fabrication
173 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 spreadingmaximizes light emission and minimizes turn-on voltage and series resistanceBecause most of the light generated at the junction escapes the device through the top surfacethe large-area p-contact has to be made as transparent as possible outside the area where electrical bond wires are attached
184 Towards a better efficiency Fabrication process: MOCVDMOCVD: metal-organic chemical vapor depositionExemple: n-Butyllithium , C4H9LiChemical process used to grow quantum wells, thus to produce high purity, high performance solid materialsIt’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 surfaceAim: To reduce the voltage at the operating current
194 Towards a better efficiency Quantum efficiencyQuantum 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 regionthe internal QE of double heterostructure can be greater than 99%Double Heterostructure: change in bandgap
204 Towards a better efficiency Lowering the operating temperature:Flip chipNormal LED:Big thermal resistance in thermal conduction pathLarge amount of heat transferred from active layer through front face of LED and the encapsulating material and then dissipated into the airFlip Chip LED:designed with a thermal conductive submount and metal interconnections to conduct most of the heat through submount
214 Towards a better efficency State-of-the-artThe 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 generationTunability of the final emission spectrum, by controlling the particle size distribution and/or surface chemistry thus white light colour is betterquantum efficiency of 76% (in solution) for a blue emission, in GAN led
225 Different types of LEDs Organic LED & Polymer LEDThese 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 intrinsicallyProblems: degradation in air & easily damaged by exposure to water
236 Limiting factors Cost Competitiveness Narrow angle of emission Now, LED prices are 10 times higher than of incandescent light bulbsBUT: Better efficiency and longer lifetimeNarrow angle of emissionTo use LEDs in ambient lighting, multiple LEDs are asembled in a single fixture. This leads to sharp shadows.High quality variationInexpensive LEDs have inconsistent colour temperature and light outputPoor Quantum efficiencyLEDs are currently limited by poor internal quantum and light-extraction efficiency, but photonic crystals offer a potential solution to both problems.
247 Perspectives & Advantages Good efficiency & durabilityAssociated with perfect material and devices, LEDs would require only 3 Watts to generate the light obtained with a 60-Watt incandescent bulbLEDs can provide hrs of life compared to 1000 hrs with incandescent light bulbsFigure 1. LED vs. conventional light sources degradation in light output over time
257 Perspectives & Advantages Good stabilityDue to their solid state, they can withstand vibrations better and have no filament that might breakThey 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!
267 Perspectives & Advantages Reduction of energy consumptionLEDs require less current than incandescent bulbsDDP® LED LampIncandescent Bulb6S6L120-CWX11mA6S6/120V50mA120PSBL-NWX5.8mA120PSB25mA387L-X116mA38740mA1819L-X-CX17mA1819Comparison 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.
277 Perspectives & Advantages Reduction of heat emissionLess heat emission Lens stays cooler Less energy wastedSome 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
287 Perspectives & Advantages Allows wide variety of lightingArtificial lighting similar to daylightMore control of the colour and intensitySSL can be coupled to light pipes Light can be efficiently and flexibly distributedInteresting design possibilities: they can be placed on floors, walls, ceilings or furniture!
297 Perspectives & Advantages White LED: The Future Lighting TechnologyIn the past 6 years: Tremendous gain in energy efficiency, brightness and lifespanFor 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
308 ApplicationsCommon application: Digital clock, battery level indicator, torchTraffic signals, street lightBuildingsOutdoor:runway in airportsResidentialInformation boards
319 Challenges Phosphor challenge Better understanding of the physics of semiconductors used; AlGaInP and AlGaInN materials and nanostructuresFor white light: Improved wavelength-conversion and colour-mixing technologies for generation of white light
3210 Research and development going on Projects sponsorised by the DOE:The US Department of Energy promotes research on SSL by allocating $21 million to 13 projects!Develop a polymer OLED using advanced polymer synthesis to allow large-scale manufacturing of p-OLED lampUnderstand & solve the problem of low radiative emissions in green LEDsCreate a GaN substrate with very few dislocations in order to improve blue LED efficiencyAnd many more…
3310 Research and development going on Project : “LED lights might light homes in less than 3 years”University of GlasgowInvestigation of the possibility of making microscopic holes on the surface of the LED to extract more light, in order to increase the brighteness without increasing the energy consumptionTechnique used: Nano-printing lithography, direct impression of the holes
3411 Environment Result doubly environment-friendly Incandescent traffic lights replaced by LEDs in USA:economy of2.5 billion kWhours= US$ 200 million= 3 billion kilos of CO2 released in the atmosphereLess current consumption(less electricity burned)Less heat producedLess CO2 emissionsLess light pollutionPositive impact on global warming
3512 Future & perspectivesBased on semiconductor material, like microprocessorImportant research and progress for these materialsIn 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
392 LED Mechanism Semi-conductor diode Solid material that is between insulators and conductor:Behaves like an Insulator (large bandgap) at room temperatureBehaveslike conductor (no bandgap) when applying electric field
402 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 holen-type: enhanced conductivity with valence electron-enriched dopants, thus the donor imprity contribute free electron
412 LED MechanismCe3+:YAG : phosphor coating for the InGaN-GaN structureCerium-doped yttrium aluminum garnet: phosphor or scintillatorHere 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 wavelengthOutpur colour strongly dependant on current and temperatureLuminescencePhosphorescence
424 Towards a better efficiency Thermal ManagementReduce 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 temperatureThe maximum junction temperature is , but the aim is to keep lowThermal resistance:it’s the ratio of the difference in temperature to the power dissipated (°C/W)R R2Power dissipated:R Rh R2Junction temperature:()PRthTaj+=-Heatsink: aborbs the heat and dissipates it by conduction or convectionBut by adding a Heatsink we add a resistanceneed to determine its maximum value