ansunMansunMHong Kong University of Science & Technology, Department of Electrical & Electronic Engineering

photon Eg~1.1eV

• optical illumination with sufficient energy generates electron/hole pairs

• recombination of electron/hole generates optical photons

• a silicon PN diode in the forward bias region generates infrared (IR) radiation

Electron/Photon Interaction [1]Interaction of silicon with optical radiation

ansunMansunMHong Kong University of Science & Technology, Department of Electrical & Electronic Engineering

photon

• When electrons and holes are generated, they tend to move together

• effectively, the electron current and hole current cancel each other resulting in zero net current

• eventually, the electrons and holes will recombine• a mechanism to separate the electrons and holes is need to

move them in different directions

Electron/Photon Interaction [2]Motion of generated carriers

ansunMansunMHong Kong University of Science & Technology, Department of Electrical & Electronic Engineering

light

EC

EV

EF

optical luminescence

electron-holepairs generated

n

metal

p

PN junction

V

I

€

i = i0 eqVAnkT −1

#

$ %

&

' ( − Iphoto

increasing optical power

Voc

Icc

Electron/Photon Interaction [3]

• There is a built-in electric field in a PN junction to separate the electron hole pairs

ansunMansunMHong Kong University of Science & Technology, Department of Electrical & Electronic Engineering

Solar Cell Basics [1]PN junction again• similar to a photo-diode, but operates in the forward bias

region (4th quadrant) to deliver energy

Iout

VoutV

I

Voc

Iscoutput power

• limitation: maximum output voltage < VBi• it has to operate for more than 10 years to recover the energy

needed for production• high cost due to required material (Si is relatively expensive)• rough number: 10-100mA for an area of 1cm2, and V < 1V• require a power management system due to unstable input

ansunMansunMHong Kong University of Science & Technology, Department of Electrical & Electronic Engineering

Solar Cell Basics [2]Solar energy conversion requirements• Larger surface area (where to launch?) • Material (need to be readily available)• Manufacturing (easy to produce)• Low energy payback time (recover the energy for

production)• Low carbon footprint (low emission of CO2)• Efficiency (currently below 20% for solar cells)• Low cost• Fast turn-a-round

ansunMansunMHong Kong University of Science & Technology, Department of Electrical & Electronic Engineering

Solar Cell Basics [3]Production cost

Production cost of Electricity(http://nsl.caltech.edu/energy.ppt)

polysilicon (46.5%)single crystal silicon (43.4%)

amorphous silicon (4.7%)

CdTe (2.7%)

silicon compound (2.6%)

CIS battery (0.2%)

• silicon cost: $40-$70USD/kg (rised to $100/kg in 2006 and drop to around $30 since 2012)

• about 70% of the cost• A large part of the silicon are useless and just for support

ansunMansunMHong Kong University of Science & Technology, Department of Electrical & Electronic Engineering

Solar Cell Basics [4]Single crystal silicon solar cell• Solar electric effect first discovered in

1839 by A. E. Becquerel

• First prototype in 1950 by Bell Lab with 6% efficiency

• high temperature to form molten silicon make it energy intensive and polluting (~1000kWh/kg)

• high quality, low resistance but expensive

• for 500GW of annual deployment require 10% but contribute less than 3% of the global electricity supply

• Typically needs to operate over 10 years to recover the production energy

ansunMansunMHong Kong University of Science & Technology, Department of Electrical & Electronic Engineering

Solar Cell Basics [5]2nd generation thin-film solar cell

SiH4 polysilicon

glass substrate

• Use of deposition to form amorphous or polysilicon thin film on low-cost substrate

• Later extend to other material systems like cadmium telluride, CIGS etc.

• Reduce energy cost at lower temperature manufacturing (400oC versus 2000oC)

• High resistance and lower generation efficiency due to grain boundaries

• Defects act as recombination centers to reduce the efficiency• Some instability always exists

SiH4 a-siliconglass substrate

ansunMansunMHong Kong University of Science & Technology, Department of Electrical & Electronic Engineering

Solar Cell Basics [6]3rd generation solar cells• Many new material system

has been explored to further reduce film thickness and manufacturing temperature

• Organic and polymer solar has become a hot topic for study with a large number of publications (but very few products)

• It is low cost, fast turn-a-round, can be used on a flexible substrate, but always unstable

ansunMansunMHong Kong University of Science & Technology, Department of Electrical & Electronic Engineering

Optimization for solar cell (device perspective)Solar Cell Basics [7]

• only optical power entering the depletion region is effectively absorbed (some are reflected and transmitted)

p+ n+i

EC

EV

EFextended depletion region

• adding anti-reflection coating to the surface to reduce reflection• maximize the depletion width by using PIN structure • top layer has to be thin to improve photon absorption• contact has to be formed at the edge (prevent blocking of light

by metal) and fingers are used to reduce series resistance

top view of solar celllight

n+

p+i

ansunMansunMHong Kong University of Science & Technology, Department of Electrical & Electronic Engineering

• efficiency of current silicon based solar cell is only about 20%

• more efficient material can be used, but even higher cost

• Using organic material, but suffer from reliability issues

• efficiency can be improved by focusing or using multiple junctions

Increasing EfficiencySolar Cell Basics [8]

Taking solar to high value applications?• for example, fashion

ansunMansunMHong Kong University of Science & Technology, Department of Electrical & Electronic Engineering

Best reported solar cell efficiencySolar Cell Basics [9]

ansunMansunMHong Kong University of Science & Technology, Department of Electrical & Electronic Engineering

Light Emitting Diode (LED) [1]PN junction again

photon Eg~1.1eV

• recombination release photons, especially in forward bias

EC

EV

EF

• want all e-/h+ to recombine before they reach the contact• long base diode preferred• but how to release the light inside a thick material?• optical emission frequency depends on bandgap, and silicon can only emit infra-red radiation

• need a different material system

ansunMansunMHong Kong University of Science & Technology, Department of Electrical & Electronic Engineering

Light Emitting Diode (LED) [2]

0.4 0.6 0.8 1.0 1.2 1.4 1.6

infraredIn0.49AlxGa0.51-xP

AlxGa1-xAs In1-xGaxAsyP1-yx=0.43GaAs1-yPyIn

GaN

SiC

/GaN GaP

(N)

y=0.45

indirect bandgap

GaA

s

InP

GaS

b

Choice of material

• Traditionally only red and green LED, until the development of blue LED using GaN by Nichia

• After that, full color from LED becomes possible and significantly increase its applications

• Note that the white color produced by LED is artificial and does not contain the full spectrum

ansunMansunMHong Kong University of Science & Technology, Department of Electrical & Electronic Engineering

Light Emitting Diode (LED) [3]Packaging

substraten+

p

substraten+ p or

substraten+

p

plastics

• radiation in all directions that require a thin layer p region to avoid re-absorption

• dome-shape semiconductor/plastics to avoid total internal reflection (qc~16o)

Improving Efficiency• band engineering using hetero-structures to encourage recombination within the semiconductor material

pGaAs

pn+

AlGaAs

pn+

electrons

holes

ansunMansunMHong Kong University of Science & Technology, Department of Electrical & Electronic Engineering

Light Emitting Diode (LED) [4]Real efficiency of LED• higher than incandescent light bulbs

• very focus and directional • still a small market for for primary lighting

• efficient for low current, but efficiency drops at high current (brightness)

• heat generation is a very important issue and require significant improvement in cooling technology

Francisco G. Montoya, “Indoor lighting techniques: An overview of evolution and new trends for energy saving”, Energy and Buildings, Volume 140, 1 April 2017, Pages 50-60