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 Insulator Semiconductor 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
Transcript
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
Insulator Semiconductor 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
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
-
- - - -
- -
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
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
Silicon Semiconductor
Conductor
band overlap elec flow to conduction band
Insulator Semiconductor 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)
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 Cathode Carbon 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 + 2e → 2I-)
E- + Dye+ → E + Dye
Dye coat with TiO2Nanoparticles Increase surface area – absorption light/photon
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
Absorb 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 λ max/nm
C = C 175
C = O 190
C = C – C = C 210
- NO2 270
190- 260
Benzene 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 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
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 700nm
Wavelength 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)
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
