Photonic Devices - Georgia Institute of Technologyww2.chemistry.gatech.edu/class/8873/Photonic-Device… · · 2004-10-041 Photonic Devices Human eye: about 400 nm to 700 nm []

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PhotonicPhotonicDevicesDevices

Human eye: about 400 nm to 700 nm

[ ] [ ]eVEnm 1240

=λ

S. M. Sze, “Physics of Semiconductor Devices“

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Sensitivity of the human eye

Two LEDs:

red (λ = 660 nm)

green (λ = 560 nm)

emitting the same light intensity

Which one is brighter ?

green LED appears 16 !!! times brighter

V(λ) … “standardized“ sensitivity of the human eye by day

V‘(λ) … “standardized“ sensitivity of the human eye at night

Hering, Martin, Stohrer, “Physik für Ingenieure“

Radiative Transitions3 processes for interactionbetween a photon and an electron

• Absorption

• Spontaneous emissionno external stimulus; quantum-electrodynamic process

• Stimulated emissionphoton interacts with excited“system“→ second photon emitted with same energy and phase (coherent)

example: simple two-level system

S. M. Sze, “Semiconductor Devices– Physics and Technology“

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Dominating processes:

Light emitting diode (LED): spontaneous emission

Laser: stimulated emission

Photodetector and solar cell: absorption

LightLight--emittingemitting diodediodep-n junction that can emit spontaneous radiation

Visible LEDs

S. M. Sze, “SemiconductorDevices – Physicsand Technology“

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most important SC for visible LEDs: GaAs1-yPy


Problem: y > 0.45 bandgap becomes indirect

Introduction of recombination centers

• replace small fraction of P by N in GaAs1-yPy

• isoelectric center

→ periodicity broken

→ localized trap level close to the bottom of theconduction band

→ greatly enhances probability of a radiative transition

measure for the “recombination efficiency“

quantum efficiency

number of photons generated per injected electron-hole pair

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Consequence for GaAs1-yPy LEDquantum efficiency emission color


graded alloy to reducedefects, which would actas nonradiativerecombination centers

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Loss mechanisms:

• nonradiative recombination current

• absorption within the LED material

• reflection loss

• total internal reflection (n = 3.66 in GaAs → θtotal=16° at interface to air)

up to an order of magnitude increase in outcouplingefficiency by clever design …


S. M. Sze, “Semiconductor Devices – Physics and Technology“

Colored plastic lens as optical filter and to enhance contrast

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Infrared LEDs

Why ?

Opto-isolatorselectrically decouple two circuits throgh an IR-LED and a photodiode


Optical fiber communications

Materials: GaAs (0.9 µm) GaxIn1-xAsyP1-y (1.1 µm to 1.6 µm)S. M. Sze, “Physics of Semiconductor Devices“

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LEDs vs lasers for optical communications

LEDs:• higher temperature operation

• smaller dependence of the emitted power on Tsimpler devicestructure

• simpler drive circuit

Lasers:

• higher brightnes

• higher modulation frequency

• smaller spectral width (0.1 Å to 1 Å, compared to 100 Å to 500 Å in an LED)

OrganicOrganic LEDsLEDs Jean-Luc Brédas will talk about them …

Just a small appetizer …

Courtesy of M. Collon

Courtesy of E.J.W. List

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SemiconductorSemiconductor LasersLasersAdvantages over “conventional“ lasers:

small (0.1 mm), cheap, easily modulated at high frequencies by modulating the bias

Materials → as discussed in the heterostructures chapter …

first SC lasers: GaAs

Other materials: AlxGa1-xAsySb1-y, GaxIn1-xAsyP1-y, InP, GaxIn1-xAsyP1-y, …

Requirements for laser (stimulated emission):

gain (population inversion, optical confinement, feedback (“mirrors“))

Laser structures


• homojunction laser

• double-heterojunction laser

Pair of parallel planes cleaved or polished →mirrors (Fabry-Perot cavity)

Other “vertical sides“ :

roughened

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Population inversion

Gain: probability for a stimulated emission event needsto be higher than that for an absorption event

→ high injection condition (large concentration of e- and h+ injected into transition region)

“roughly“ speaking: for the photon energy range of interest, more than half of the valence bend states needto be empty and more than half of the conduction band states need to be occupied

More rigorously speeking: Fe- + Fh+ > 1 in the region of space, where the laser-action is supposed to occur

How can we achievethat ?

Two

degenerately dopedsemiconductors

Apply a voltage so that EFC-EFV > Eg

S. M. Sze, “Semiconductor Devices –Physics and Technology“

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Crucial parameter: Threshold current density (minimumcurrent density required for lasing)

S. M. Sze, “Semiconductor Devices –Physics and Technology“

homojunctionlaser:

carriers canmove away fromactive region

heterojunctionlaser:

carriers areconfined

Also optical confinement through waveguiding


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Spactral narrowing of the emission spectrum upon lasing

PhotodetectorsPhotodetectorsConvert optical signal into electrical signal

• Photon absorption

• Exciton dissociation

• Carrier transport through SC to external circuit

Quantum efficiency

number of electrons ( = number of holes) created(and transported out of the device) per incident photon

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Photoconductor

slab of semiconductor with Ohmic contacts at the end

Electron-hole pairs are created by absorbed photons →increased number of free carriers → increased conductivity


Photodiodep-n junction operated under reverse bias

Electric field in the depletion region sparatesphotogenerated e-h pair → electric current in external circuit

Quantum-efficiency:depletion region very thick to absorb as many photons aspossible in a region with an electric field !

Response speed (high frequency applications):depletion region as thin as possibleto reduce transit time !

→ compromise depending on application

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Quantum-efficience of photodiodes

Long wavelength cutoff: bandgap

Short wavelength cutoff: very large absorption coefficient at shortwavelengths → radiation absorbed near surface (short recombination time, no field)


p-i-n photodiaode

depletion region thickness “artificially increased“ byan additional intrinsic layer

tuning the thickness of the intrinsic region → tune quantum efficiency and response speed


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Other photodetectors:

•Metal-semiconductor photodiode

•Heterojunction photodiode

•Avalanche photodiode (multiplication effect)S. M. Sze, “Semiconductor Devices – Physics and Technology“

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Solar Solar cellscellsmain difference to standard photodetectors:

need to efficiently detect solar radiation (created bynuclear fusion in the sun)

no external bias applied

Current use:

• remote Power Supplies• portable Power Supplies• replacement of Batteries

Solar Radiation

Nuclear fusion in sun:hydrogen converted to He ⇒ mass loss ⇒ E=m*c2

⇒ primarily emitted as electromagnetic radiation in the UV to IR region

solar constant (intensity of solar radiation in free space at average distance of Earth): 1353 W/m2

Projected: nearly constant radiative-energy output of over10 billion years

at 0°: 925 W/m2

at 45°: 844 W/m2

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Influence of the Atmosphere

air mass(UV absorption in O3; IR in water vapor; scattering by dust and aerosols)

M.P. Thekaekara, Suppl. Proc. 2th Ann. Meet. Inst. Environ. Sci., 21 (1974)

AM0: outside atmosphere (sattelites ….)AM1: at surface, when sun is overheadAM1.5 ….. see sun at 48°

K. Petritsch, Dissertation TUG 2000

Power conversion efficiencyOutput electrical power (i.e. I×V) per incident power

• theoretical limit depends on Eg

• Under AM1.5, maximum efficiency is 31% at Eg =1.35 eV• for two band-gaps in series: max. efficiency is 50%

Amorphous silicon 12% AM1.5Single-crystal silicon 24.4%GaAs (crystalline) 25.1 %

How much power can be converted ?

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Organic solar cells have generally only a few %power conversion efficiency efficiency

So why organic solar cells ?

• cheap• large area• flexible• less energy in production process …

HowHow muchmuch lightlight do do wewe getget intointo thethe cellcell ??

Reflection at interfaces (refractive index contrast)⇒ “anti-reflection” coating

More sophisticated design employing „inverted pyramids“:

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Optical concentrationOptical concentration

• reducing area needed for solar cell• increasing efficiency !

Organic solar cells:

S. M. Sze, SemiconductorDevices- Physics and Technology

What determines magnitude of α ?

Inorganic semiconductor: direct vs. indirect band-gap

Conjugated organic molecule: conjugation length, side chains, texturing, …

absorbtion of virtually all incident light desired

⇒ α determines layer thickness

3.0 3.5 4.0 4.5 5.0 5.50.0

0.1

0.2

0.3

0.4

0.5

0.6

4.91

3.78

3.59

3.38

3.17 4.40

film D

film C

Abso

rptio

n c

oeffi

cien

t (10

6 c

m-1

)

Energy (eV)

Texture dependent optical absorptionin poly-crystalline sexi-phenyl (E. Zojer et al. Phys. Rev. B, 2000)

Absorption

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What is the proper energy gap (for a single-layer cell) ?Ephoton < Egap ⇒ no contribution to cell output

Ephoton > Egap ⇒ excess energy wasted as heat

“Ideal” single-layer cell:maximum efficiency 31% at Eg =1.35 eVEg between 1eV and 2 eV OK• good news for inorganic SC• bad news for organics !

C. H. Henry et al. J. Appl. Phys. 51, 4494 (1980)

Structure of inorganic solar cells

shallow p-n junction close to surface

S. M. Sze, SemiconductorDevices- Physics and Technology, Wiley (1985)

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Single-layer organic solar cell

Graphics by: E.J.W. List

Apart from lack of band-bending similar to inorganic solar cell, butcharge separation needs to be discussed in more detail !

Electron-hole pairs are strongly bound in conjugated polymers !

Charge separation in conjugated molecules

Dissociation mediated by defect sitesTrap sitesCarbonyl groups (Rothberg)Molecular oxygen (Frankevich)Conformational defectsGrain boundaries (crystals)Dopant sites

and high local fields (eg. Interfacial layers – in the case of the build up of space-charge regions)

(What can be detrimental for an LED can be helpful for a solar cell !)

⇒ Insert “defects” and interface sin a controlled manner !

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Double-layer organic solar-cell

Charge- vs. energy transfer (simplified model)

ITO Al

D

A

ITO Al

D

A

excited state diffuses to interface

The “Tang” Cell (1986)

C.W. Tang; Appl. Phys. Lett. 48, 183 (1986)• Power conversion efficiency of about 1% reported• Bilayer cell, evaporated molecules• CuPc - Perrylene as a donor/acceptor pair• Drawbacks: Sharp interface - fewer excitons hit the interface• Advantages: Molecules exhibit higher mobility

30 nm copper phthalocyanine +50 nm perylene - derivative

K. Petritsch Dissertation TUG 2000

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Numerous other material combinations

Problem with solution-processing:Polymers must not besoluble in the samesolvents

Laminated devices

Each layer can be treatedindividually

e.g., doping, heat treatment

During lamination control of interdiffusion through

pressure

temperature,

exposure to vapor …

K. Petritsch, Dissertation TUG 2000

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M. Granström et al., Nature 395, 257 (1998)

• Power conversion efficiency of 1.9% (at AM1.5)• Power conversion efficiency of 4.8 % (at peak wavelength)

Choice of materials:„The red-shifted form (of POPT) is preferred here as it providesthe best match to the solar spectrum“

„MEH-CN-PPV … large electronaffinity as a result of the electron-withdrawing cyano-groups“

• QE of 29% at peak wavelength

Blend Structures

For excited state charge transfer compare N.S. Sariciftci, L. Smilowitz, A.J. Heeger, and F. Wudl, Science 258, 1474 (1992)

• Distribute active interfaces throughout the bulk• Mix electron acceptor and hole acceptor together• All excitons are within a diffusion range of an interface (10 nm)• Electrons transferred to one component, holes to the other


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Charges travel to respective electrodes (percolation) (11 %)

Problem: Phase Separation

Requirement: Interpenetrating network

Possible solutions:

Self-organizing structures

Link C60 covalently to conjugated backbone

DyeDye--sensitized solar cellsensitized solar cell

• Invented 1991 by Grätzel (best:11%)

Compare e.g.: http://www.sta.com.au

Sponge of nano-crystalline TiO2Coated with dye molecules⇒ huge effective surface area

Dye excited⇒ e- transferred into conduction band of TiO2⇒e- transmitted through semiconducting TiO2⇒ dye reduced by electronfrom redox couple

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DeviceDevice chracteristicschracteristicsEquivalent circuit: constant current source parallel to diode (constant current source parallel to diode (pp--nn junction)junction)

“ideal” current voltage characteristics:

Organic SC - Device Physics/Device Parameters

a)Reverse External Bias

b)0 Bias = Short Circuit Current

c)0 Current = Open Circuit Voltage

d)Forward External Bias


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Power conversion efficiency

Filling Factor (FF)

Power Conversion Efficiency

External Load

RL=VP/IP

P = I * V

What is Pmax ?( )

OCSC

max

VIVIFF×

×=

( ) ( ) ( )( )

( ) ( ) ( )( )λ

λ×λ×λ=

=λλ×

=λη

light

OCSC

light

max

PFFVI

PVI


Examples for I-V characteristics in organic solar cells

• Power conversion efficiency of 4.8 %(at peak wavelength)

• QE of 29% at peak wavelength

Tang – double-layer cell Laminate solar cell

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ConcludingConcluding remarksremarksPV is about the 3 fundamental steps:

1. Light absorption2. Charge separation3. Charge collection

This can in organic Solar Cells be achieved by 1. Single layer structures2. Blend structures3. Hetero structures4. Laminated hetero structures5. Dye sensitized structures

Documents

Photonic Devices - Georgia Institute of Technologyww2.chemistry.gatech.edu/class/8873/Photonic-Device… · · 2004-10-041 Photonic Devices Human eye: about 400 nm to 700 nm []