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Prepared by Lawrence Kok

Option C, Photovoltaic Cell, Semiconductor, DSSC cell and Electron Conjugation

Silicon Semiconductor

Conductivity

Ionization energy Band Energy Gap

Low IE - free delocalized elec Good conductor

Resistance increase with Temp↓

Vibration +ve ion in lattice with elec ↓

Resistance increase↓

Conductivity decreaseTemp ↑ – Conductivity ↓

metal

Free delocalized elec

- +

Resistance increase with Temp

Atom come together, atom orbital energy

mix forming molecular orbital energy . Energy level overlap forming energy

band(Valence / Conduction band)

Conductor

band overlap electron flow to conduction band

InsulatorSemiconductorConductivity bet metal/insulator

Band gap too large

No electron can flow into conduction band

Delocalized elec

band overlap can be made closer by doping

depends on

doping with n type elec donors

doping with p type elec acceptor

N type-

P type+

electron

Closer gap by doping

Silicon Semiconductor

dope with n type elec donors

dope with p type elec acceptor

n-type semiconductorsDope with Gp 15, (P, As, Sb)

Donor they “donate” elec to conduction band

Energy gap small, elec jump to conduction band

Conductivity due to -ve carriers

4 valence elec

Dope with gp 15

Dope with gp 13

elec from P jump to conduction band

free elecmobile –ve

charge carrier

Elec from donor have energy level close to conduction band.

Delocalized elec

p-type semiconductorsDope with Gp 13, (B, AI, Ga)

Acceptor they “accept” elec into holeEnergy gap small, elec jump to holes

Conductivity due to +ve carriers (holes)

free holemobile +ve

charge carrier

elec from valence band jump to holes of Boron

Conductivity due to +ve carriers (holes)

Gp 13 Gp 15

Semiconductor – conductivity increase with temp↓

More elec released and move to conduction band

P type+

N type-

- - - - - -

pn junction

Photovoltaic cell – Light to electricity

Holes (+)

free holemobile +ve

charge carrier

P type+

N type-

Electron (-)combine both junction togetherpn junction

+ + + + + + + + + + + +

- - - - - - - - - - - - - - - -

elec and hole combine

Depletion region Depletion region

p type and n type side by sidedepletion region form when

-ve elec flow to +ve hole+ve hole flow to –ve elec

n type - elec donor P atom become +ve immobilize ion when lose elec

+ + + + + + + +

- - - - - - - -

p type Holes (+) n type Elec (-) + + + + + +

Electric fieldprevent –ve elec from

crossing pn junction

p type - elec acceptor B atom become -ve immobolize ion when accept elec

Click here pn junction

Click here photovoltaic cell Electric field

free elecmobile –ve

charge carrier

e-e-

e-e-

- - - -

+ + + + + + + +

+ + + + + +

- - - - - -

pn junction

Photovoltaic cell – Light to electricity

Holes (+)

free holemobile +ve

charge carrier

P type+

N type-

Electron (-)combine both junction togetherpn junction

n type - elec donor P atom become +ve immobilize ion when lose elec- - - -

- - - - p type Holes (+) n type Elec (-)

Connect to external wire

p type - elec acceptor B atom become -ve immobolize ion when accept elec

Electric field

+ + + + + +

+ + + + + + + +

- - - - - - - -

Elec excited from depletion regionElec attracted by electric field and pull to

n typeHoles attracted to p type

Elec combine with holes at p type

n type Elec (-)p type hole (+)e-e-e-

- - - - - -

e-e-

e-e-

e-e-

Electron – excited by sun photon due to small band gap

Elec and hole move in opposite direction due to electric field in p–n junction

+ +

+ +

free elecmobile –ve

charge carrier

Silicon Semiconductor

Conductor

band overlap elec flow to conduction band

InsulatorSemiconductorConductivity bet metal/insulator

Band gap too large

No elec can flow into conduction band

Delocalized elec

band overlap can be made closer by doping

doping with n type elec donors

doping with p type elec acceptor

N type-

P type+

electron

Closer gap by doping

Compare property of semiconductor with metal and insulator and relate property to ionization energy

Semiconductor- electrical conductivity bet metal and insulator.Metal,- low ionization energy result in free-moving elecInsulator, strong covalent bond NO elec able to move

Semiconductor, lower ionization energy mean elec be removed/excited by light photon

Advantages Disadvantages

Renewable source from sun

Only 10-20% efficient

Reduce fossil fuel usage

No input fuel

Dependent on sun/storage

Clean, no pollution Large area needed to build panel

Advantages/Disadvantages of photovoltaic cell

Suggest why Si is semiconductor, while diamond is an insulator.

- Pure Si covalently bond with each Si tetrahedrally. - Si non-conductive unless elec can be excited by photon. - Solar energy excite elec .- Absent elec, or ‘hole’, move and carry charge. –ve carriers (elec) and +ve charge carrier (hole) - Ionization energy diamond too high to form holes and mobile electron

Extra elec (-ve) Holes (+ve)

Dye sensitized solar cell (DSSC)

Click here DSSC construction

Making DSSC cell. Click here view

Solar radiation – excite elec from Dye (D) ↓

Elec from photosensitive dye (conjugated structure)(alternating single/double bond)

↓Excited elec flow to TiO2 semiconductor

↓Elec flow through circuit back to electrolyte (E)

↓E receive elec to form E- ion

↓E- ion pass elec back to oxidized dye (D+)

Cathode (+) - metal with carbon/platinumAnode (-) -Nanoparticle TiO2

with dye (D)

Anode CathodeCarbonplatinum

TiO2

Dye (D)Electrolyte (E)I2 + I- → I3

- (I2)

e-e-

Dye (D)

e- e- e- e- e- e-

Dye → Dye+ + e

e flow to TiO2 → Anode(-ve) → Circuit → Cathode(+ve)

alternating double/single bond

E + e → E-

(I2 + e → 2I-)

E- + Dye+ → E + Dye

Dye coat with TiO2NanoparticlesIncrease surface area – absorption light/photon

Use nanotechnology – TiO2 nanoparticles

Advantages Disadvantages

Efficient, dye effective in

absorbing photon

Low Current

Cheap/low cost Dye degrade over time

Use light low energy(visible region)

Liquid electrolyte

freeze at low temp

Nanoparticle – provide high surface

area for photon absorption

Low density/light/thin structure/flexible

Compare the working of photovoltaic cell with DSSCin terms of light absorption and charge separation

Dye sensitized solar cell (DSSC)

Advantages/Disadvantages of DSSC cell

Photovoltaic DSSC

Elec source Silicon atom Organic dye (conjugated structure)

Light absorption

Silicon atom– excited elec and hole pair (charge carrier)

Elec excited from conjugated organic dye

Charge separation

Elec and hole move in opposite direction due to

electric field in pn junction

Elec produced by Dye Dye → Dye+ + e

Dye+ receive elec from Electrolyte

Dye+ + e (from E-) → Dye

1,3-hexadiene

Explain whether 1,3 hexadiene or 1,5 hexadiene absorb longer wavelength

More conjugated system↓

More alternate single/double bond↓

Absorb longer wavelength (visible)

Less conjugated system↓

Less alternate single/double bond↓

Absorb shorter wavelength (UV)

1,5-hexadiene

1,3-hexadiene absorb longer wavelength as 1,5-hexadiene doesnt undergo conjugation but 1,3-hexadiene does

Indicator has red colour (Acid) and yellow (Alkali)Predict which of two colour is due to molecule with

higher degree conjugation

RED

Red seen–complementary colour absorb Green( 540nm)Yellow seen – complementary colour absorb Violet

(410nm)Longer wavelength Green (540nm)– more conjugation

Red colour – More conjugation

vs YELLOW

C C

Absorption of UV by organic molecule and chromophores

Absorption UV radiation by C = C, C = O, N = N, N =O gp

C = C /N = N (π bond)C = O: (lone pair

electron)NO2 (lone pair electron)

Chromophores gp

Ground

Higher empty orbital

π electron

Absorb UV to excite π/lone pair e to higher empty orbital

C O lone pair electron:

Chromophores – organic molecule with conjugated double bond

Absorb radiation to excite delocalized e to empty orbital

alternating double/single bond

Filled orbital Bonding orbital

empty orbital antibonding orbital

Biological Pigments (Anthocyanins)Coloured – extensive conjugation of elec alternating single and double bond

Porphyrin Chlorophyll Heme (hemoglobin)

Anthocyanin

Carotene

absorb absorb absorb absorb

C C

Absorption UV radiation by C = C, C = O, N = N, N =O gps

C = C /N = N (π bond)C = O: (lone pair

electron)NO2 (lone pair electron)

Ground π electron

Absorpb UV to excite π/lone pair e to higher empty orbital

C O lone pair electron:

alternating double/single bond

Carotene

Diff bet UV and Visible absorption

Colourless - Absorption in UV rangeElectronic transition from bonding to antibonding

orbital (involve pi / lone pair e)

UV visible

Organic molecule/chromophore

Biological Pigments (Anthocyanins)Coloured – extensive conjugation of electron

Alternating single and double bond Electron in pi orbital delocalized through single and

double bond. π elec excited by absorbing long wavelength in visible

region

Anthocyanin

Chlorophyll

absorb absorbHigher empty orbital

Chromophore λ max/nm

C = C 175

C = O 190

C = C – C = C 210

- NO2 270

190- 260Benzene ring – conjugated system

Absorb radiation to excite delocalized e to empty orbital

Filled orbital

empty orbital

Carotene

Colourless – Absorption in UV range Electronic transition from bonding to antibonding

orbital (involve pi / lone pair e)

UV visible

Anthocyanin

Absorption of UV/vis by organic molecule/ pigment

Less conjugated system↓

Less alternating single/double bond↓

Absorb shorter wavelength (UV)↓

Colourless compound

More conjugated system↓

More alternating single/double bond↓

Absorb longer wavelength (visible)↓

Colour compound

alternating double/single bond

More conjugation → More delocalization → Absorption in visible rangeExtensive conjugation of double bond allow more delocalization of π elec

More conjugation → More delocalization → Less energy to excite electron → ↓ E lower ( absorb at visible region (colour )

How number of conjugation led to colour formation from UV to visible?

Biological Pigments (Anthocyanins)Coloured – extensive conjugation of electron

Alternating single/double bond Elec in pi orbital delocalized through single/double

bond. π elec excited by absorbing long wavelength in

visible region

UV visible

Absorption of UV/vis by organic molecule/pigments

More conjugation → More delocalization → Absorption in visible rangeExtensive conjugation of double bond allow more delocalization of π electron

More conjugation → More delocalization → Less energy to excite electron → ↓ E lower ( absorb visible region (colour )

How number of conjugation led to colour formation from UV to visible?

More conjugation – splitting energy less ∆E ↓ – wavelength increase (visible range)

Filled orbital

empty orbital

100 200 300 400 700nm Wavelength λ

C – C C = C C = C – C = C C = C – C = C – C = C

∆E ↓with more conjugationabsorb from UV to visible

∆E ↓with more conjugationAbsorb at ↓ lower energy (↑ longer λ)

Absorb UV – sunblock Absorb visible region – food dye (Azo dye)Acid/base indicator

alternating double/single bond

CaroteneAnthocyanin Chlorophyll Heme (hemoglobin)

Wavelength - absorbed

Visible light

Colour seen RED – RED reflect to eyes - Blue absorb (complementary colour)

absorbed

REDtransmitted

Carotenoids absorb λ at 460 nm

Colour – extensive conjugation of elec. Alternating single/double bond

π elec delocalized through single/ double bond. π elec excited by absorbing long wavelength in visible

region

700 600 500 400

Biological Pigment

alternating double/single bond

CaroteneAnthocyanin Chlorophyll Heme (hemoglobin)

Wavelength - absorbed

Visible light

Colour seen GREEN– GREEN reflect to eyes - Red/Blue absorb (complementary colour)

absorbed

Greentransmitted

Chlorophyll absorb λ at 400 and 700nm

Biological Pigment

Colour – extensive conjugation of elec. Alternating single/double bond

π elec delocalized through single/ double bond. π elec excited by absorbing long wavelength in visible

region

700 600 500 400

C6H5–(CH=CH)6–C6H5

↓More conjugate

↓Absorb blue

↓Complement colour reflect

Orange

C6H5–(CH=CH)5–C6H5

↓Less conjugate

↓Absorb violet

↓Complement colour reflect

Yellow

Anthocyanins – used as acid/base indicatorIdentify λ max which correspond to max absorbance at

diff pH and suggest colour in acid/base condition.

pH Max Colour absorb

Colour pigment

1 550 Green Red

12 475 Blue Yellow/orange

wavelength wavelength

Anthocyanins – used as acid/base indicatorIdentify λ max which correspond to max absorbance at

diff pH and suggest colour in acid/base condition.

pH Max Colour absorb

Colour pigment

1 550 Green Red

7 350 None visible Colourless

Describe relationship bet n and λ max

Suggest which series absorb in visible regionSuggest colour of C6H5–(CH=CH)5–C6H5 and C6H5–

(CH=CH)6–C6H5 Increase n or conjugation → Absorption to longer wavelength λmax increaseAbsorption from 400 – 700nm ( visible region) when n > 4

n = 5 n = 6

Tetracene - Greater delocalization elec (Higher conjugation bond) - Absorb longer wavelength – visible light (colour)

Organic compounds shown anthracene and tetracene.Predict with reference to conjugation double bond, which absorb visible light (colour)

Carotene absorb light in blue/green region, so complementary colour (red and orange) are transmitted

Anthracene Tetracene

Absorption spectrum of carotene was shown. Explain why carotene have colour.

Carotene

700 600 500 400

RED

Absorption spectrum of anthrocyanin is shown.Explain what effect, the absorption at 375 and 530 nm have on colour of anthrocyanin

At 375 nm - No effect, lies outside visible spectrum (UV region) At 530 nm - Visible colour, red, complementary to blue-green - Absorb green – Reflect Red

700 600 500 400 300 200

AnthocyaninRED

CaroteneAnthocyanin Chlorophyll Heme (hemoglobin)

Wavelength - absorbed

Colour seen RED – RED reflect to eye - GREEN absorb

Anthrocyanin – acid base indicator - absorb λ 550nm at pH 1 (acid)

Colour seen Yellow – yellow reflect to eye - Blue absorb

Wavelength - absorbed

Anthrocyanin – acid base indicator - absorb λ 470nm at pH 12 (alkali)

+ H+

+ OH-

Add acid

Add base

Change in number OH gpChange in number conjugationAbsorb at diff wavelength

RED YELLOW

Number conjugation increase ↓

Absorb longer wavelength

Number conjugation decrease ↓Absorb shorter wavelength

Colour – extensive conjugation of elec. Alternating single/double bond

π elec delocalized through single/ double bond. π elec excited by absorbing long wavelength in visible

region

Acknowledgements

Thanks to source of pictures and video used in this presentationhttp://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/nucnot.htmlhttp://www.m2c3.com/chemistry/VLI/M3_Topic2/M3_Topic2_print.htmlhttp://www.universityneurosurgery.com/index.php?srchttp://www.medwow.com/med/cobalt-linear-accelerator/radon/tr-cobalt-60/42865.model-spechttp://endocrinesurgery.ucla.edu/patient_education_adm_tst_radioactive_iodine_uptake_test.html

Thanks to Creative Commons for excellent contribution on licenseshttp://creativecommons.org/licenses/

Prepared by Lawrence Kok

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