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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