Solar Cells

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Solar cell device physics (1)

Prof. Richard R. King

Solar Cells

EEE 565

Arizona State University

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What does a typical solar cell look like?

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Typical commercial Si solar cell structure

Simplified schematic of main features

Basic solar cell structure

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Simplified schematic of main features

Basic solar cell structure

General parts of a typical solar cell

Base – the region of semiconductor on the side of the p-n junction away from the sun

In conventional solar cells the base is often also the Absorber – the thickest region of the solar cell that absorbs most of the sunlight

Emitter – the region of semiconductor on the side of the p-n junction closest to the sun

Window – a front barrier layer that suppresses recombination of minority charge carriers at the front surface, i.e., passivates the front surface of the emitter

Back-surface field (BSF) layer – a back barrier layer that suppresses recombination of minority charge carriers at the back surface, i.e., passivates the back surface of the base

Front (gridded) and back metal contacts

Anti-reflection coating

p-n junction

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FF strongly affected by parasitic series (Rs) and shunt (Rsh).

Solar cell efficiency

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(1) Maximize short circuit current, Isc:

Absorb maximum amount of light above bandgap (100%)

Collect light-generated carriers (100%)

(2) Generate a large open-circuit voltage, Voc (minimize dark current, i.e., recombination)

(3) Minimize parasitic power loss mechanisms (particularly series and shunt resistance), i.e., maximize fill factor, FF

Good solar cell operation

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Fundamental energy losses in solar cells:

1. photon energy below bandgap

2. carrier thermalization to bandedges

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insufficient energy to reach Ec

thermalization of carriers

Energy Transitions in Semiconductors

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Entropy of charge carriers in semiconductor and metal contacts

→ Another fundamental loss in solar cells:

3. quasi-Fermi level splitting < bandgap

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Not all of bandgap energy is available to be collected at terminals, even though electron in conduction band has energy Eg

Only qV = qφp – φn is available at solar cell terminals

Due to difference in entropy S of carriers at low concentration in conduction band, and at high concentration in contact layers:

G = H – TS S = kB ln Ω Ω = (# of microstates)

EFC = -qφn

EFV = -qφp

V = voltage of solar cell

= quasi-Fermi level splitting

= φp - φn

Energy Transitions in

Semiconductors

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Energy Transitions in

Semiconductors

EFC = -qφn

EFV = -qφp

V = voltage of solar cell

= quasi-Fermi level splitting

= φp - φn

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h =

Iinc

Voc

Jsc

h = solar cell efficiency (unitless)

Voc = open-circuit voltage (V)

Jsc = short-circuit current (A/cm2)

FF = fill factor (unitless)

Iinc = incident light intensity (W/cm2)

Voc

Solar Cell Efficiency

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Recombination of electrons and holes determines solar cell voltage, and ultimately, solar cell efficiency

Fundamental recombination processes:

Radiative recombination

Auger recombination

Non-fundamental recombination processes – can be reduced in principle to zero:

Shockley-Read-Hall (SRH) recombination mediated by traps in the energy gap, due to defects

Recombination can take place in the semiconductor bulk or, especially for SRH recombination, at semiconductor interfaces

Recombination in Semiconductors

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Solar cell current J is simply photogenerated current Jph minus recombination current Jrec :

diode ideality

factor

Recombination in Semiconductors

Recombination rate term is proportional to the concentration of the “reactants,” the electrons and holes, as in a chemical reaction

Generation term, ensures that net recombination rate Jrec is zero at equilibrium when pn = ni2 .

net recombination current density

Note that for g = 1:

In general:

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Solar cell current J is simply photogenerated current Jph minus recombination current Jrec :

Recombination current increases exponentially with V

Jog is the key parameter expressing the dependence of recombination current on voltage

Jog is strongly dependent on g: Jog is much higher for g = 2 than for g = 1 at the same voltage

= diode ideality

factor

In forward bias (normal operation for solar cells):

Simplified Diode Equation

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For g = 1, Voc increases 60 mV = (kT/q)ln(10) for every decade increase in Jph , or decrease of Jog

An increase of a factor of 2 in Jph corresponds to a Voc increase of 18 mV = (kT/q)ln(2)

For g = 2, Voc increases 120 mV = (2kT/q)ln(10) for every decade increase in Jph , or decrease of Jog

for

Simplified Diode Equation

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AR coating

window (n-type)

emitter

quasi-neutral region

(n-type)

back-surface field (BSF) (p-type)

p-side tunnel junction

emitter

space-charge region

base

space-charge region

base

quasi-neutral region

(p-type)

n-side tunnel junction

When L is comparable to the emitter or base thickness:

When L is large, this reduces to:

 Excellent motivation to work on

semiconductors where L is large !

Jo From Combined Bulk and Surface Recombination

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Diode Equation with Non-Ideal Shunt, Series Resistance Effects (series after shunt)

rs = specific series resistance [Wcm2]

ssh = specific shunt conductance [W-1cm-2]

Go crazy, include everything:

1-diode model, with series, with shunt

1-diode model, with series resistance

2-diode model, no series, no shunt

1-diode model, no series, no shunt

ρs

hν

σsh

Joγ

(V + Jρs )

Jo1

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nkT

100

200

300

400

500

600

700

0.5

1.5

2.5

3.5

Photon Energy (eV)

Intensity per Unit Photon Energy

(W/m

2 .

eV)

0.2

0.4

0.6

0.8

1.2

1.4

Photon Utilization Efficiency

AM1.5D, ASTM G173-03, 1000 W/m2

< E

100

200

300

400

500

600

700

0.5

1.5

2.5

3.5

Photon Energy (eV)

Intensity per Unit Photon Energy

(W/m

2 .

eV)

0.2

0.4

0.6

0.8

1.2

1.4

Photon Utilization Efficiency

AM1.5D, ASTM G173-03, 1000 W/m2

Utilization efficiency of photon energy

to bandgap Eg = 1.424 eV

> E

100

200

300

400

500

600

700

0.5

1.5

2.5

3.5

Photon Energy (eV)

Intensity per Unit Photon Energy

(W/m

2 .

eV)

0.2

0.4

0.6

0.8

1.2

1.4

Photon utilization efficiency

AM1.5D, ASTM G173-03, 1000 W/m2

Utilization efficiency of photon energy

to bandgap Eg = 1.424 eV

to Voc at 1000 suns

to Voc at 1 sun

(

)

rec

(

)

rec

cosh

sinh

cosh

sinh

cosh

shunt

rec

(

)

(

)

(

)

(

)

(

)

(

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