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7/27/2019 Solar cell operating principles
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Solar cell operation is based on the photovoltaic effect:
The generation of a voltage difference at the junction of two differentmaterials in response to visible or other radiation.
1. Absorption of light - Generation of charge carriers
2. Separation of charge carriers
3. Collection of the carriers at the electrodes
Solar cell operating principles
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p-type Si absorber
p-n junction
back contact
front contact
Semiconductor based solar cells
p++-type Si membrane
n-type Si membrane
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p-type Si
n-type Si
back contact
front contact
Solar cell materials
Transparent adhesive
Cover glass
Encapsulant/glass
Passivation layer/ARC
p++-type Si
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Solid materials crystalline (regular atomic structure: long range order) amorphous (amorphous network: short range order)
Electrical conductivity based upon mobile electrons conductors ( > 104 -1 cm-1)
semiconductors (104 -1 cm-1 > > 10-8 -1 cm-1)
insulators ( < 10-8 -1 cm-1)
Thermal behavior of electrical conductivity
Control of electrical conductivity by doping
General properties
Semiconductors
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Optical properties
band gap absorption coefficient
index of refraction
Carriers concentration
Concentration of dopant atoms
Transport properties mobility of carriers (drift)
diffusion coefficient (diffusion)
Recombination lifetime of minority carriers and diffusion length
distribution of density of energy states
Important properties for a solar cell:
Semiconductors
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Atomic number: 14
Atomic weight: 28.08
Ground state
electron configuration:
4 valence electrons
Covalent bond
Basic unit: 5 Si atoms
Crystal lattice:
diamond lattice unit
lattice constant 5.4
Atom Crystal
Silicon
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Si atom
Unit cell
Silicon
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Si atom electron
Bonding model
Silicon
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n = p
holeCovalent bond
Bonding model
Silicon
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Introducing PhosphorusPhosphorus atom:
5 valence electrons
Introducing BoronBoron atom:
3 valence electrons
n-type p-type
Doping
Silicon
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n > p n < p
P atom B atom
Bonding model
Silicon
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Fermi-Dirac distribution function
( ) ( ) kTEE FeEf
+=
1
1
A plot of the allowed electron energy states in a material as
a function of position along pre-selected direction.
n = p = nini = 1.5 10
10 cm-3
=
kT
EENn FC
C exp
=
kT
EENp VFV exp
NC = 3.21019 cm-3
NV = 1.81019 cm-3
Concetrantion of electrons and holes
Energy band diagram
Silicon
EC
EV
EF EG=1.1 eV
CB
VB
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n ND (T=300 K)
EDEC
EV
EF
n > p
n-type Si
np = ni2 n < p
EC
EV
EF EA
p-type Si
np = ni2
p NA (T=300 K)
ND = n = 1.01017 cm-3
p = ni2/n = 2.25103 cm-3
EC - EF = 0.14 eV
NA = p = 1.01020 cm-3
n = ni2/p = 2.25 cm-3
EF - EV = 0.04 eV
Silicon
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About 55% of solar energy is not usable by PV cells
EC
EV
EphEG
Creation of anelectron-hole pair
EC
EV
EphEG
Thermalization Eph>EG
Band gap (EG)
SemiconductorsEC
EV
EphEG
Non absorption Eph
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Band gap (EG)
Semiconductors
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Drift: Charged-particle motion in response to electric field
Drift current:
Mobility:
Phonon scattering
Ionized impurity scattering
EC
EV
EF
ndriftN nqJ = pdriftP pqJ =
( )+ + AD NNT 23
Transport
Semiconductors
Electrical conductivity:
pqnq pn +=
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Diffusion current:
EC
EV
EF
dx
dn
DqJ ndiffN =
dx
dpDqJ pdiffP =
Diffusion coefficient:
nnq
kTD =
ppq
kTD =
Diffusion:A process whereby particles tend to spread out fromregions of high particle concentration into regions of low particle
concentration as a result of random thermal motion.
Transport
Semiconductors
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Recombination: A process whereby electrons and holes (carriers) areannihilated or destroyed.
Generation: A process whereby electrons and holes are created.
Phonons
Heat
EC
EV
ET
R-G Center recombination Minority carrier lifetime:
Tn
nNC
1=
Tp
pNC
1=
Diffusion length:
nnn DL = ppp DL =
Semiconductors
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Heat
Light
EC
EV
EC
EV
Light
(photons)
Band-to-band recombination
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EC
EV
ET
Heat
(phonons)
R-G center- dominant recombination-generation mechanism