chapter 6 solar cell part 1

CHAPTER 6

SOLARCELL

SOLAR CELL (PART I)

WO

RLD

EN

ERG

Y C

ON

SU

MPT

ION 2010

2014Source : http://www.ren21.net/REN21Activities/GlobalStatusReport.aspx

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10

PHOTOVOLTAIC CELLS

PV effect :

the conversion of light (photon) into electrical energy

Types:

Inorganic Solar cell ( Single-crystal silicon, a-Si, GaAs)

Widespread

Efficiency : 15 20%

Expensive to manufacture

Organic solar cell

ultra thin, tunable colour

Efficiency : 12%

not yet to be commercialize

PHOTOVOLTAIC CELLS

Electrochemical Dye- solar cell

Newer, less proven

Inexpensive to manufacture

Flexible

(-) : degradation issues

PHOTOVOLTAIC CELL CONSTRUCTION


Solar cell : pn junction with light shining on it

To maximize efficiency , we must:- maximize e--h+ pairs - minimize recombination of e--h+ pairs


ACTIVE LAYER

Consist of excess electron

Consist of positively charge hole

Depletion region avoid themovement of electron andhole across region


As energy of light is absorbed by semiconductor e--h+ pairswill be created

Create extra mobile electron and hole.

Electric field occur as electron flow to n-doped layer and holeflow to p-doped layer photogeneration of charge carrier More electron in n-doped layer More hole in p-doped layer

Connect to electrical appliances i.e. lamp electric flow turn on light

One PV Cell is approx 150mm indiameter

In bright sunshine it produces 0.4V d.c.

One Module is an array of approx 30cells

Connected together in series/parallel fordesired voltage

Life span is about 25 yrs

Cost varies : 300 - 600 per m2


Solar panel rating :

Max voltage in single PV Cell isapprox 0.4V (D.C.)

Current depends on sun intensity(Max of 2.5A)

Average voltage from a module is20V (D.C.)


Various factors effect power output from

panels :

Shade or Clouds

Panel position or angle

Active panels can track the sun

Temperature and solar irradiance variations

Air gap required for cooling

Partial shading will reduce performanceand can cause damage

PHOTOVOLTAIC CELL

SOLAR CELL (PART II)

21

SOLAR ENERGY SPECTRUM

The intensity of radiation emitted from the sun has a spectrum that resemblesa black body radiation at a temperature of about 6,000 K

This spectrum is modified by:

Effects of the solar atmosphere

Fraunhofer absorption (absorption by hydrogen)

Temperature variations on the surface of the sun facing us

The actual intensity spectrum on Earths surface depends:

on the absorption and scattering effects of the atmosphere

on the atmospheric composition

the radiation path length through the atmosphere

22


0

Black body radiation at 6000 K

AM0

AM1.5

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.00

0.5

1.0

1.5

2.0

2.5

Wavelength (m)

Spectral

Intensity

W cm-2 (m)-1

The spectrum of the solar energy represented as spectralintensity (I) vs wavelength above the earth's atmosphere

(AM0 radiation) and at the earth's surface (AM1.5radiation). Black body radiation at 6000 K is shown for

comparison (After H.J. Mller, Semiconductors for Solar

Cells, Artech House Press, Boston, 1993, p.10)

1999 S.O. Kasap, Optoelectronics (Prentice Hall)

23

Light intensity variation with wavelength is typically represented byintensity per unit wavelength, called spectral intensity I, so that I is theintensity in a small interval

Integration of I over the whole spectrum gives the integrated or totalintensity, I

The integrated intensity above Earths atmosphere gives the total powerflow through a unit area perpendicular to the direction of the sun

This quantity is called the solar constant or air-mass zero (AM0) radiationand it is approximately constant at a value of 1.353 kW/m2


24

Atmospheric effects depend on the wavelength

Clouds increase the absorption and scattering of the sun light andhence substantially reduced the incident intensity

On a clear sunny day, the light intensity arriving on Earths surface is roughly 70% of the intensity above the atmosphere

Absorption and scattering effects increase with the sun beams paththrough the atmosphere

The shortest path through the atmosphere is when the sun is directly above that location and the received spectrum is called air-mass one(AM1) as shown in the figure


25

All other angles of incidence ( 90 increase the optical paththrough the atmosphere, and hence the atmospheric losses

Air-mass m (AMm) is defined as the ratio of the actualradiation path h to the shortest path h0, that is m=h/h0

Since h = h0sec, AMm is AMsec

It is apparent that the spectrum has several sharp absorptionpeaks at certain wavelength which are due to thosewavelengths being absorbed by various molecules in theatmosphere, such as ozone, air and water vapor molecules


26


Direct Diffuse

(a) Illustration of the effect of the angle of incidence on the ray path length and the

definitions of AM0, AM1 and AM(sec ). The angle between the sun beam and the horizon

is the solar latitude (b) Scattering reduces the intensity and gives rise to a diffused radiation

Atmosphere

AM0

AM1

AM(sec)

h0h

(a) (b)

Tilted PV deviceEarth


27

In addition, the atmospheric molecules and dust particles scatter the

sun light

Scattering not only reduces the intensity on the radiation towards

Earth but also gives rise to the suns rays arriving at random angles as

shown in the figure

Consequently the terrestrial light has a diffuse component in addition

to the direct component

The diffuse component increases with cloudiness and suns position


28

Solar Energy Spectrum

Direct

Direct Radiation is solar radiation reaching the Earth's surface after without having

been scattered

Diffuse

Diffuse radiation is solar radiation reaching the Earth's surface after having

been scattered from the direct solar beam

by molecules.

The diffuse component increases with cloudiness and suns position

Direct and Diffuse solar radiation

The scattering of light increases with decreasing wavelength so that shorter wavelengths in the original sun beam experience more scattering than longer wavelengths

On a clear day, the diffuse component can be roughly 20% of the total radiation and significantly higher on cloudy days

29


Example

30

solution

31

solution

32

Photovoltaic device: Structure

Consider a pn junction with a very narrow and more heavily doped n-region

Illumination is through the thin n-side

Depletion region (W) or the space charge layer (SCL) extends primarily into the p-side

There is a built-in field E0 in this depletion layer

33

n-type semiconductor

p-type semiconductor

+ + + + + + + + + + + + + + +- - - - - - - - - - - - - - - - - -

Physics of Photovoltaic Generation

Depletion Zone

35

Photovoltaic device: Structure

Electrodes attached to the n-side must: allow illumination to enter the device

at the same time result in a small series resistance

They are deposited on to n-side to form an array of finger electrodeson the surface

A thin antireflection coating on the surface reduces reflections and allows more light to enter the device

37

Finger electrodes

p

n

Bus electrode

for current collection

Finger electrodes on the surface of a solar cellreduce the series resistance


38

Photovoltaic device: EHP

n-side is very narrow so most of the photons are absorbed within:

the depletion region (W)

the neutral p-side (lp)

(Photogeneration also occur in these regions)

EHPs photogenerated in the depletion region are immediately separated by the built-in field E0 which drifts them apart

The electron drifts and reaches the neutral n+ side whereupon it makes this region negative by an amount of charge e

Similarly the hole drifts and reaches the neutral p-side and thereby makes this side positive

39

Photovoltaic device: EHP

Open circuit voltage develops between the terminals of the device with the p-side positive with respect to the n-side

If an external load is connected then the excess electron in the n-side can travel around the external circuit, do work, and reach the p-side to recombine with the excess hole there

Without the internal field E0 it is not possible to drift apart the photogenerated EHPs and accumulate excess electrons on the n-side and excess holes on the p-side

40

Photovoltaic device: EHP by long wavelengths

EHPs photogenerated by long wavelength photons that are absorbed in the neutral p-side can only diffuse in this region as there is no electric field

If the recombination lifetime of the electron is e, it diffuses a mean distance Le given by

where De is its diffusion coefficient in the p-side

Electrons within a distance Le to the depletion region can readily diffuse and reach this region whereupon they become drifted by E0 to the n-side

eee DL 2

41


Only EHPs photogenerated within the minority carrier diffusion length Le to the depletion layer can contribute to the photovoltaic effect

Again the importance of the built-in field E0 is apparent

Once an electron diffuses to the depletion region it is swept over to the n-side by E0 to give an additional negative charge there

Holes left behind in the p-side contribute a net positive charge to this region

42


Those photogenerated EHPs further away from depletion region than Leare lost by recombination

It is therefore important to have the minority carrier diffusion length Le as long as possible

This is the reason for choosing this side of a Si pn junction to be p-type which makes electrons to be the minority carriers; the electron diffusion length in Si is longer than the hole diffusion length

43

Photovoltaic device: EHP by short wavelengths

The same ideas also apply to EHPs photogenerated by short-wavelengthphotons absorbed in the n-side

Holes photogenerated within a diffusion length Lh can reach the depletionlayer and become swept across to the p-side

The photogeneration of EHPs that contribute to the photovoltaic effecttherefore occurs in a volume covering Lh + W + Le

If the terminals of the device are shorted, then the excess electron in then-side can flow through the external circuit to neutralize the excess holein the p-side

44

Le

Lh W

Iph

x

EHPs

exp(x)

Photogenerated carriers within the volume Lh + W + Le give rise to a photocurrent Iph. The

variation in the photegenerated EHP concentration with distance is also shown where is theabsorption coefficient at the wavelength of interest.


45

Photovoltaic device

Current due to flow of photogenerated carriers is photocurrent

EHPs photogenerated by energetic photons absorbed in the n-side near the surface region or outside the diffusion length Lh to the depletion layer are lost by recombination as the lifetime in the n-side is generally very short (due to heavy doping)

The n-side is therefore made very thin, typically less than 0.2 m or less

46

Photovoltaic device

Indeed, the length ln of the n-side maybe shorter than the hole diffusion length Lh

The EHP photogenerated very near the surface of the n-side however disappear by recombination due to various surface defects acting as recombination centers

47

Photovoltaic device

At long wavelengths, around 1-1.2 m, the absorption coefficient of Si is small and the absorption depth (1/) is typically greater than 100 m

To capture these long wavelength photons we therefore need a thick p-side and at the same time a long minority carrier diffusion length Le

Typically the p-side is 200-500 m and Le tends to be shorter than this

48

Photovoltaic device: Losses

Worst part of the efficiency limitation comes from the high energy photons becoming absorbed near the crystal surface and being lost by recombination in the surface region

Crystal surfaces and interfaces contain a high concentration of recombination centers which facilitate the recombination of photogenerated EHP near the surface

49

Photovoltaic device: Losses

Losses due to EHP recombinations near or at the surface can as high as 40%

These combined effects bring the efficiency down to about 45%

In addition, the antireflection coating is not perfect which reduces the total collected photons by a factor of about 0.8-0.9

When we also include the limitations of the photovoltaic action itself , the upper limit to a photovoltaic device that uses a single crystal of Si is about 24-26% at room temperature

wavelength Photon energy

Short-wavelength infrared

1.4-3 m 0.40.9 eV

Mid-wavelength infrared 38 m 150400 meV

Long-wavelength infrared 815 m 80150 meV

50

pn JUNCTION PHOTOVOLTAIC I-V CHARACTERISTICS

Guess what it is ??

Iph

R

I

V V = 0

Iph

I = Id I

ph

V

Id

Isc

= Iph

R

(a) (b) (c)

(a) The solar cell connected to an external load R and the convention for the definitions ofpositive voltage and positive current. (b) The solar cell in short circuit. The current is thephotocurrent, Iph. (c) The solar cell driving an external load R. There is a voltage V and current

I in the circuit.

Light


PHOTOVOLTAIC I-V CHARACTERISTICS

Iph

R

I

V V = 0

Iph

I = Id I

ph

V

Id

Isc

= Iph

R

(a) (b) (c)


I in the circuit.

Light


The voltage V and current I above define the convention for the direction of positive current and voltage.


Consider an ideal pn junction photovoltaic device connected to a resistive load R.

I and V define the convention for the direction of positive current and positive voltage.

If the load is short circuit the only current in the circuit is due to photogenerated (photocurrent),Iph.

Iph

R

I

V V = 0

Iph

I = Id I

ph

V

Id

Isc

= Iph

R

(a) (b) (c)


I in the circuit.

Light


PHOTOVOLTAIC I-V CHARACTERISTICSPHOTOVOLTAIC I-V CHARACTERISTICS

Photovoltaic I-V Characteristics

If I is the light intensity, then the short circuit current is

The photocurrent does not depend on the voltage across the pn jucntion, because it always some internal field to drift the photogenerated EHP.

If R is not short circuit the positive voltage V appears across the pn junction as a result of the current passing through.

Iph

R

I

V V = 0

Iph

I = Id I

ph

V

Id

Isc

= Iph

R

(a) (b) (c)


I in the circuit.

Light


KIIIphsc

K is constant that depends on particular device


Under short circuit conditions (figure b), the only current in the circuit is the photocurrent Iph generated by the incident light.


K= Ratio of nonradiative to radiative losses in a solar cell material

OrK is constant that depends on particular device


If a load R is now inserted as in the figure (c), a positive voltage V appears across the diode which forward biases the diode.

There is now minority carrier injection and the resulting diode current.

Where I0 = reverse saturation current

Photovoltaic I-V CharacteristicsPHOTOVOLTAIC I-V CHARACTERISTICS

Thus, the total current (solar cell current),

n is the ideality factor that depends on the semiconductor material and fabrication

Photovoltaic I-V CharacteristicsPHOTOVOLTAIC I-V CHARACTERISTICS

Iph

R

I

V V = 0

Iph

I = Id I

ph

V

Id

Isc

= Iph

R

(a) (b) (c)


I in the circuit.

Light


The I-V characteristics of a typical Si

solar cell (Fig.).

Normal dark characteristics being shifted

down by photocurrent Iph (short circuit),

which depend on light intensity, I.

The open circuit voltage, Voc, is given by

the point where the I-V curve cuts the V-

axis (I = 0). V

I (mA)

Dark

Light

Twice the light

0.60.40.2

20

20

0

Iph

Voc

Typical I-V characteristics of a Si solar cell. The short circuit current is Iphand the open circuit voltage is Voc. The I-V curves for positive current

requires an external bias voltage. Photovoltaic operation is always in thenegative current region.



VI (mA)

0.60.40.2

20

0

Voc

10

Isc= Iph

V

The Load Line for R = 30

(I-V for the load)

I-V for a solar cell under an

illumination of 600 Wm-2.

Operating Point

Slope = 1/R

P

I

(a) When a solar cell drives a load R, R has the same voltage as the solar cellbut the current through it is in the opposite direction to the convention thatcurrent flows from high to low potential. (b) The current I and voltage V inthe circuit of (a) can be found from a load line construction. Point P is theoperating point (I, V). The load line is for R = 30 .

LightI

R

V

I

(a) (b)



Since the open circuit voltage is the maximum voltage possible, andshort circuit current Isc is the maximum current possible, theunachievable maximum power = Isc Voc. It is therefore useful tocompare the maximum power output, Im Vm.

The fill factor is a measure of how close to the maximum power is aparticular operating point.


The efficiency of the solar cell

The input sun-light power is

Pinput = (Light Intensity)(Surface Area)

The power delivered to the load is


Two different Vsc

Two different circuit current Isc

Voc 2 Voc1 nkBT

eln

I2I1

0.551 0.0259 ln 0.5

n is the ideality factor that depends on the semiconductor material and fabrication

Isc2 Isc1I2I1

50 mA

50 W m2

100 W m2


A Si solar cell of area 4 cm2 is connected to drive a load R as in Figure 1 . Suppose that the load is 20 and it is used under a light intensity of 1 kW m-2(Figure 1). What are the current and voltage in the circuit? What is the power delivered to the load? What is the efficiency of the solar cell in this circuit?

What is FF?

EXAMPLE

Figure 1

EXAMPLE

Figure 2

EXAMPLE

Series resistance and equivalent circuit Practical devices can deviate substantially from the ideal pn

junction solar cell behavior due to a number of reasons

Consider an illuminated pn junction driving a load resistance RLand assume that photogeneration takes place in the depletion region

As illustrated in the next figure, the photogenerated electron has to traverse a surface semiconductor region to reach the nearest finger electrode

All these electron paths in the n-layer surface region to finger electrodes introduce an effective series resistance Rs into the photovotaic circuit as indicated in the figure

If the finger electrodes are thin, then the series resistance of the electrodes themselves will further increase Rs

There is also a series resistance due to the neutral p-region but this is generally small compared with the resistance of the electron paths to the finger electrodes

Neutral

n-region

Neutral

p-region

Finger

electrode

Back

electrode

Depletion

region

RL

Rs

Rp

Series and shunt resistances and various fates of photegenerated EHPs.

1999 S .O. Kasap, Optoelectronics (Prentice Hall)

Series Resistance and equivalent circuit

AIph Rp RLV

IIph

Id

Solar cell Load

B

Rs

The equivalent circuit of a solar cell



EXAMPLE Consider the equivalent circuit of a solar cell as shown in Figure below.

Show that;

[Hint: Kirchoffs current law]I Iph Id

V

Rp Iph Io exp(

eV

nkBT) Io

V

Rp

Figure shows the equivalent circuit with the series resistance removed. The currents flowing into node A sum to zero (Kirchoffs current law).

Currents into a node are positive and those leaving a node are negative.

Thus,

which is the required equation

EXAMPLE

I (mA)

V

00

0.2 0.4 0.6

5

10

Voc

Isc

Rs = 0

Rs = 20

Rs = 50

Iph

The series resistance broadens the I-V curve and reduces the maximumavailable power and hence the overall efficiency of the solar cell. The exampleis a Si solar cell with n 1.5 and Io 310-6 mA. Illumination is such thatthe photocurrent Iph = 10 mA.



0.60.40.20246

5

15

Voltage (V)Power (mW)

Current (mA)

20

10

1 cell

2 cells in parallel

Current vs. Voltage and Power vs. Current characteristics of one cell and twocells in parallel. The two parallel devices have Rs/2 and 2Iph.



AIphV

Iph

Id

B

Rs

RL

I/2

Id

Iph

I

RsI/2

Two identical solar cells in parallel under the same illumination anddriving a load RL.



Consider two identical solar cells with the properties Io = 2510

-6 mA, n = 1.5, Rs = 20 W, subjected to the same illumination so that Iph = 10 mA. Derive the corresponding current , I and voltage V.

Example

6.4 Open circuit voltage A solar cell under an illumination of 100 W m-2

has a short circuit current

Isc of 50 mA and an open circuit output voltage Voc, of 0.55V. What are the short circuit current and

open circuit voltages when the light intensity is halved?

Solution

The short circuit current is the photocurrent so that at

Isc2 Isc1I2I1

50 mA

50 W m2

100 W m2

= 25 mA

Assuming n1, the new open circuit voltage is

Voc 2 Voc1 nkBT

eln

I2I1

0.551 0.0259 ln 0.5 = 0.508 V

Assuming n2, the new open circuit voltage is

Voc2 Voc1 nkBT

eln

I2I1

0.55 2 0.0259 ln 0.5 = 0.467 V

Temperature Effects Temperature decreases Output voltage and efficiency increase.

Solar cell operate best at lower temperature.

The output voltage Voc, when Voc >> nkBT/e,

I0 is reverse saturation current and strongly depend on temperature, because it depends on square of ni.

If I is light intensity,

0

lnI

I

e

TnkV

phB

oc

lnor ln00

I

KI

Tnk

eV

I

KI

e

TnkV

B

ocB

oc

Temperature Effects Assuming n = 1, at two different temperatures T1 and T2 but the

same illumination level, by subtraction,

Substitute,

Thus,

Rearrange for Voc2,

TkENNnBgvci

exp2

2

2

2

1

02

01

01021

1

2

2 lnlnln- lni

i

B

oc

B

oc

n

n

I

I

I

KI

I

KI

Tk

eV

Tk

eV

121

1

2

211

TTk

E

Tk

eV

Tk

eV

B

g

B

oc

B

oc

1

2

1

2

121

T

T

e

E

T

TVV

g

ococ

Temperature Effects Example, Si solar cell has Voc1 = 0.55 V at 20

oC (T1 = 293 K), at 60 oC

(T2 = 333 K),

V 475.0293

3331)V 1.1(

293

333)V 55.0(

2

ocV

Solar Cells Materials, Devices and Efficiencies For a given solar spectrum,

conversion efficiency depends on the semiconductor material properties and the device structure.

Si based solar cell efficiencies 18% for polycrystalline and 22 24% for single crystal devices.

About 25% solar energy is wasted not enough energy unable to generate EHPs.

Considering all losses, the maximum electrical output power is ~20% for a high efficiency Si solar cell.

100% Incident radiation

Insufficient photon energy

h < Eg

Excessive photon energy

Near surface EHP recombination

h > Eg

Collection efficiency of photons

Voc (0.6Eg)/(ekB)

21%

FF0.85

Overall efficiency

Accounting for various losses of energy in a high efficiency Sisolar cell. Adapted from C. Hu and R. M. White, Solar Cells(McGraw-Hill Inc, New York, 1983, Figure 3.17, p. 61).


chapter 6 solar cell part 1

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