The document discusses semiconductors and photovoltaic cells. It explains that semiconductors have an electrical conductivity between metals and insulators due to their lower ionization energy. This allows electrons to be excited from light photons. Photovoltaic cells convert light to electricity using a pn junction. When light is absorbed, electrons and holes move in opposite directions due to the electric field, generating a current. Dye-sensitized solar cells (DSSC) also use light absorption but with an organic dye rather than silicon. The dye's electrons are excited and transferred to TiO2, generating a current through the external circuit.

1. http://lawrencekok.blogspot.com
Prepared by
Lawrence Kok
Option C, Photovoltaic Cell, Semiconductor, DSSC cell and Electron
Conjugation

2. 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 decrease
Temp ↑ – 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
InsulatorSemiconductor
Conductivity 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

3. Silicon Semiconductor
dope with n type
elec donors
dope with p type
elec acceptor
n-type semiconductors
Dope 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 elec
mobile –ve
charge carrier
Elec from donor have
energy level close to conduction band.
Delocalized elec
p-type semiconductors
Dope with Gp 13, (B, AI, Ga)
Acceptor they “accept” elec into hole
Energy gap small, elec jump to holes
Conductivity due to +ve carriers (holes)
free hole
mobile +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
-

4. - -
- -
- -
pn junction
Photovoltaic cell – Light to electricity
Holes (+)
free hole
mobile +ve
charge carrier
P type
+
N type
-
Electron (-)combine both junction together
pn junction
+ + + + + +
+ + + + + +
- - - - - - - -
- - - - - - - -
elec and hole
combine
Depletion region
Depletion region
p type and n type side by side
depletion 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 field
prevent –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 elec
mobile –ve
charge carrier
e-
e-
e-
e-
- -
- -

5. + + + +
+ + + +
+ +
+ +
+ +
- -
- -
- -
pn junction
Photovoltaic cell – Light to electricity
Holes (+)
free hole
mobile +ve
charge carrier
P type
+
N type
-
Electron (-)combine both junction together
pn 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 region
Elec attracted by electric field and pull to n type
Holes 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 elec
mobile –ve
charge carrier

6. Silicon Semiconductor
Conductor
band overlap elec flow
to conduction band
InsulatorSemiconductor
Conductivity 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 elec
Insulator, 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)

7. 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/platinum
Anode (-) -Nanoparticle TiO2
with dye (D)
Anode CathodeCarbon
platinum
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 TiO2Nanoparticles
Increase surface area – absorption light/photon
Use nanotechnology – TiO2 nanoparticles

8. 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 DSSC
in 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

9. 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

10. 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 range
Electronic 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 absorb
Higher empty orbital
Chromophore λmax/nm
C = C 175
C = O 190
C = C – C = C 210
- NO2 270
190- 260
Benzene ring – conjugated system

11. 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 range
Extensive 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

12. UV visible
Absorption of UV/vis by organic molecule/pigments
More conjugation → More delocalization → Absorption in visible range
Extensive 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 700nmWavelength λ
C – C C = C C = C – C = C C = C – C = C – C = C
∆E ↓with more conjugation
absorb from UV to visible
∆E ↓with more conjugation
Absorb at ↓ lower energy (↑ longer λ)
Absorb UV – sunblock Absorb visible region – food dye (Azo dye)Acid/base indicator

13. alternating double/single bond
CaroteneAnthocyanin Chlorophyll Heme (hemoglobin)
Wavelength - absorbed
Visible
light
Colour seen RED – RED reflect to eyes
- Blue absorb (complementary colour)
absorbed
RED
transmitted
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

14. 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
Green
transmitted
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

15. 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 indicator
Identify 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 indicator
Identify 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 region
Suggest colour of C6H5–(CH=CH)5–C6H5 and C6H5–(CH=CH)6–C6H5
Increase n or conjugation → Absorption to longer wavelength λmax increase
Absorption from 400 – 700nm ( visible region) when n > 4
n = 5 n = 6

16. 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
Anthocyanin
RED

17. 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 gp
Change in number conjugation
Absorb 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

18. Acknowledgements
Thanks to source of pictures and video used in this presentation
http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/nucnot.html
http://www.m2c3.com/chemistry/VLI/M3_Topic2/M3_Topic2_print.html
http://www.universityneurosurgery.com/index.php?src
http://www.medwow.com/med/cobalt-linear-accelerator/radon/tr-cobalt-60/42865.model-spec
http://endocrinesurgery.ucla.edu/patient_education_adm_tst_radioactive_iodine_uptake_test.html
Thanks to Creative Commons for excellent contribution on licenses
http://creativecommons.org/licenses/
Prepared by Lawrence Kok
Check out more video tutorials from my site and hope you enjoy this tutorial
http://lawrencekok.blogspot.com