分析成果介绍lecjon 111

CHM

11500Lecture

29Advanced

MaterialsSection

12.7and

pp.

249-252EnergyWorld

annual

energy

consumption

is

=

1.22

x

1014

kWhKnown

reserves

of

petroleum

will

provide

2.14

x

1015

kWhNote

that

energy

comes

from

oil,

coal,

gas,

nuclear,

biosources(e.g.,

wood)

and

renewables-

not

just

oil.Still,

these

numbers

may

help

us

to

see

why

renewable

sources

of

energyare

needed,

from

a

supply

perspective.Plus

there

are

also

environmental

issues

at

play.Let’s

talk

about

solar

energySolar

EnergySolar

energy

available

in

U.S.

in

1

year

is

about10

times

total

energy

available

in

ALL

known

oilreserves

on

the

planet.Solar

radiation

hitting

Earth’s

surface(for

overhead

midday

sun)

=

925

W/m2(power

=

energy

per

time, 1

W

=

1

J/s)Possibilities

for

Solar

Power4Current

solar

cells

operate

between20-30%

efficiency

(i.e.

100

J

in

assunlight,

20-30

J

out

as

electricity)Cost

is

currently

about

35¢/kWh

(versus

2¢/kWh

from

fossil

fuel-basedelectricity)Pollution-free

during

use. But

production

yields

toxic

waste.Grid

connected

solar

electricity

can

be

used

locally,

minimizingtransmission

losses

and

excess

generated

can

be

sold

to

powercompanies

(to

be

used

later.)Requires

energy

storage

through

battery

systems

(for

times

of

no

sun)Solar

cells

produce

DC

which

must

be

converted

to

AC

(for

existingpower

grids). This

incurs

an

energy

loss

of

4-12%Capturing

Solar

Energy“Passive”Allow

solar

radiation

to

be

captured

as

heat

–

such

as heating

water

or

bricks

or

other

“heat

sinks”. Heat

is then

released

as

needed.“Active”Allowing

solar

radiation

to

strike

a

solar

cell

which converts

it

to

an

electric

current. Store

the

electricity in

batteries

to

use

later

or

use

it

do

work

(example, powering

a

motor)5 Passive

Solar

Panels

“Solar

Thermal”

panels

–

the

original

solarpanelsGenerally

used

toheat

water6Passive

Solar

Home

DesignHeat

absorbing

materialwhich

will

radiate

heatback

out

at

night.Example:

brick

orcertain

tiles.8 Solar

Cells

(Photovoltaics)

Overview

of

how

they

work….To

understand

how

solar

cells

work,

wemust

understand

“n”

and

“p”

typesemiconductors

at

the

“band”

level.How

Does

Electrical

ConductionHappen

in

Solids?9Electronic

structure

of

solidsclasses:conductorssemiconductorsinsulatorsBand

TheoryRecall

atomic

energy

levels

and

orbitalsRecall:“valenceorbitals”=

outermostorbitals

withhighest

n

value101

valence

electron

/

Li

atom)Band

TheoryAs

the

number

ofatoms

in

a

crystallinesolid

increases,

thenumber

of

availableenergy

levelsincreases–

The

number

ofenergylevels

in

a

cluster

ofatoms

=

the

totalnumber

of

levelsavailable

from

theindividual

atoms(for

Li

2s

orbitals;11As

the

number

of

atoms

increases,

the

energy

levelsget

closer

together,

until

they

eventually

make

acontinuous

“band”Electrons

will

fill

energy

levels

from

the

bottom

up,forming

a

filled

band

and

an

unfilled

band.12Band

TheoryThe

closeness

of

theenergy

levels

results

in

afilled

band

(a

“valenceband”)

and

an

emptyband,

called“conduction

band”.Band

Theory13EgThey

are

separated

by

anenergy

gap

called

the“band

gap”,

Eg.Band

Gap14The

energy

gap,

Eg,

between

the

valence

band

andconduction

band

determines

the

electrical

propertiesof

the

material:

metal

(conductor),

semiconductor,insulator(NOTE:

the

bands

do

not

get

smaller

as

the

gap

gets

bigger

although

the

figurebelow

suggests

that)ConductivityTransfer

anelectron

fromvalence

band

toconduction

band–

This

creates

apositive

“hole”

inthe

valence

band(a

virtual

positivecharge)Electron

andhole

move

inoppositedirections

whenconducting

(inresponse

to

anapplied

voltagedifference)15Energy

Required

=

Eg16Need

to

put

in

Eg

of

energy

to

induce

acurrentIn

metals,

this

is

essentially

zero

(just

needto

apply

a

potential

difference).

Theelectrons

can

travel

freely

at

room

tempIn

insulators,

it

is

very

large

-

no

mobileelectrons

at

room

tempIn

semiconductors,

it

is

in

the

IR/Visiblerange.

A

small

number

of

electrons

aremobile

at

room

temp.Characteristics

of

TypicalSemiconducting

MaterialsMaterialEgdiamond(insulator)9.61

x

10-19

J(206

nm,

UV)puregermanium1.05

x

10-19

J(1886

nm,

mid-IR)pure

silicon1.79

x

10-19

J(1110

nm,

near

IR)pure

galliumarsenide2.27

x

10-19

J(874

nm,

near

IR)Band

gaps

often

reported

in

eV;1

eV=

1.602

x

10-19

J17QuestionIt

is

useful

for

solar

cells

to

be

responsivetolight

in

the

visible

portion

of

the

spectrum.Which

of

these

materials

would

be

mostuseful

for

such

an

application?Material

A:

bandgap

=

5.5

x

10-19

JMaterial

B:

bandgap

=

1.5

x

10-19

JMaterial

C:

bandgap

=

1.2

x

10-18

J18Extrinsic

and

IntrinsicSemiconductorsintrinsic

semiconductors

–

the

pure,crystalline

material

has

semiconductingproperties

(such

as

pure

Si

or

Ge)extrinsic

semiconductors

––

material

is

“doped”

with

a

small

amount

ofimpurity

of

a

similar

material

(about

1

in

106

to1

in

108

atoms)Doped

Semiconductors20decreased

Eg

(band

gap)

relative

to

intrinsicmaterialp-type:doped

with

an

element

with

fewer

electronsprovides

positive

charge

carriers

(“holes”)n-type:doped

with

element

with

more

electronsprovides

negative

charge

carriers

(electrons)Extrinsic

and

IntrinsicSemiconductorsintrinsic

silicon

(group

IV

element)silicondopedwithP

atoms

(groupV);

additionalelectronsavailable; smaller

bandgapsilicon

doped

with

Ga

atoms

(group

III);

additional

holes;

smaller bandgap21Doped

Semiconductor

BandStructureNote:Fermilevel:averageenergy

ofunpairedelectronsLevelsare

ofunequalenergyThe Big

PictureEnergy

bands

result

when

large

numbers

ofatoms

combine

to

form

a

crystal.The

spacing

between

the

filled

(“valence”

and

unfilled(“conduction”)

bands

is

the

“bandgap”

with

energy

Eg.The

size

of

Eg

willdetermine

the

properties

of

the

material.Nanomaterials

are

composed

of

substances

withstructures

on

the

nanometer

scale.Integrated

Circuits(Computer

“Chips”)put

n-type

and

p-typematerial

together

to

get

ap-n

rectifier,

or

“diode”current

can

only

move

inone

direction

across

the

p-njunctionother

combinations

of

pand

n

can

be

used

tomake

other

devices.combinations

can

beprinted

on

silicon

tomakesolid-state

integratedcircuits24How

an

LED

WorksLight

Emitting

Diodep-n

junctionwhen

an

electron

and

hole

get

to

the

junction,they

can

“combine”

and

emit

light. (This

isactually

the

electron

losing

energy

by

fallinginto

the

hole.)Electricity

in,

light

out2526Fermi

levels

areequal

at

the

junction.When

electron

andhole

get

to

junction,they

can

“combine”and

emit

light.Electricity

in,

lightoutLight

Emitting

DiodeSolarenergy27bodySolar

cellPlantsHeat,

steamBlackWind

(windmills)ElectricityHydrogenFossil

fuelsBiofuelsConversion

of

Solar

EnergyrenewableHow

a

Solar

Cell

WorksOpposite

of

an

LED. Apply

the

voltage

in

reverse.Light

in,

electricity

outSolar

panels

are

made

out

of

hundreds

of

solarcells

(photovoltaic

cells)

in

series.28DemoA

solar

car2930New

Materials

for

Solar

PowerTraditional

solar

cells

are

brittle

and

bulky.New

flexible

materials

are

being

developedand

researched.A

silicon

nanoparticle-basedflexible

solar

material….31New

Materials

for

Solar

PowerOthers

are