Cell and module construction. Photovoltaic effect and basic solar cell parameters To obtain a potential difference that may be used as a source of electrical.

Solar cell physics and technology

Vítezslav Benda Dept. of Electrotechnology, Czech Technical University in Prague

Technická 2, 166 27 Praha 6, Czech Republic, e-mail: [email protected]

Cell and module construction

Photovoltaic effect and basic solar cell parameters

To obtain a potential difference that may be used as a source of electrical energy, an inhomogeneous structure with internal electric field is necessary.

Suitable structures may be:• PN junction • heterojunction (contact of different materials).

Vmp VOC

• open circuit voltage VOC, • short circuit current ISC

• maximum output power VmpImp

• fill factor

• efficiency

SCOC

mpmp

IV

IVFF

Parameters VOC, ISC, FF and η are usually given for standard conditions:

• spectrum AM 1.5

• radiation power 1000 W/m2

• cell temperature 25°C.

in

SCOC

in

mpmp

P

FFIV

P

IV

Important solar cell electrical parameters

I

IPV D Rp

Rs

RL V

p

sssPVill R

IRV

kT

IRVqI

kT

IRVqIJAI

12

exp1exp 0201

Modelling I-V characteristics of a solar cell

Series resistance RS

Parallel resistance Rp

1exp01 kT

qVJJ j

1

2exp02 kT

qVJ j

PN junction I-V characteristics

n0p

p

p0n

n2i01

11

nL

D

pL

DqnJ

sc

i02

dqnJ

Aill – illuminated cell area

A - total cell area

Output cell voltage V = Vj- RsI

To maximise current density JPV it is necessary• maximise generation rate G• minimise losses

losses

optical recombination electrical

• reflection

• shadowing

•not absorbed radiation

• emitter region

• base region

• surface

• series resistance

• parallel resistance

Electrical losses

Series resistance Rs consists of:

·R1 – contact metal-semiconductor on the back surface

·R2 – base semiconductor material·R3 – lateral emitter resistance between two contact grid fingers· R4 – contact metal-semiconductor on the grid fingers ·R5 – resistance of the grid finger·R6 – resistance of the collector bus

AHR Si /2

bh

lR M

35

Series resistance Rs influences strongly solar cells efficiency

j

N

x

dR

~3

B

BM

hb

lR

~6

R3 – lateral emitter resistance between two contact grid fingers

j

N

x

dR

~3

Decrease of ρN is connected with increasing ND Auger recombination rate increases

Decrease of the finger distance d results in a decrease of illuminated area Aill

It is very important to optimise xj

N

Lp

P

x = Hx = 0

d

Ln

xj xPN xj+d

hν

Optimising PN junction depth

The photo-current density JPV consists from carriers collected by the electric field in the space charge region of the PN junction, i.e. from carriers generated in a distance of about diffusion length from the PN junction.

The PN junction depth xj should be less than 0.5 μm (0.2 m is desirable).

Construction of solar cells

To obtain high efficient solar cell it is necessary

• maximise IPV and Rp

• minimise Rs

In the illuminated area generated excess carriers diffuse towards the PN junction. The density JFV is created by carriers collected by the junction space charge region

)()()()( OPNPVPPVNPV JJJJ

• in the N-type region

• in the P-type region

jj

j

dx

x

OPN dxGqJ )()( • in the PN junction space charge region

)0()()(0 0

sr

x x

pPVN Jdx

pqdxGqJ

j j

)()()( HJdxn

qdxGqJ sr

H

dx

H

dx nPVP

j j

More detail information can be obtain from solution of continuity equations.

Jp - current density generated in N-type layer

Jn - current density generated in P-type base

JOPN- generated in the PN junction space charge layer opnj

in

OPNαdαxR eJ exp1exp1

nnn

nn

n

nnn

nn

n

opnjn

nin

opnj

pnn

LH

LH

LτS

αHαLLH

αHLH

LτS

αL

dxαLα

Lα R e

dxdx

dneDJ

coshsinh

expsinhexpcosh

exp1

122

jp

p

j

p

j

p

pp

p

j

p

j

p

ppjp

p

pp

p

pin

j

npp

αxαL

L

x

L

x

L

τS

Lx

Lx

L

τSαxαL

L

τS

Lα

L R e

xdxdp

eDJ

exp

coshsinh

sinhcoshexp

1

122

Results of simulations for different wavelength of incident light

Losses due to recombination may considerably limit the cell parameters, especially, losses connected with the recombination in the P-type region.

To maximise current density JPV it is necessary• maximise generation rate G• minimise recombination rate

To maximise generation rate it is necessary to minimise surface reflection, especially in the visible region of solar spectrum

x

xdt

ndxG

gen

)(exp)()()(

);()()();(

0

Φ0 = Φin (1 – R)

R is the surface reflexivity

da

n0

n1

n2

For a monochromatic light, the minimum reflexivity Rmin occurs when the optical path is equal to a quarter of wavelength, i.e. the layer thickness is

.

Antireflection coating

14nda

(n12 + n0n2)

2 Rmin = (n12 – n0n2)

2

From that follows for

201 nnn Silicon refraction index

Rmin = 0

Thin film material of n2 2 is

desirable for silicon solar cells (Si3N4 or TiO2 layers of d 75 nm are usually used for antireflection coating).

1960 -1980

Theoretical efficiency limit – 18%

- plane surface

- SiO2 (TiO2) antireflection layer

- photolithography

- vacuum contact deposition

Surface texturing

If the surface has a pyramidal structure it is possible to decrease reflection on about one third of that on a plane surface.

texturised

Both principles (surface texturing and antireflection coating) can be combined to minimise losses by surface reflection

PEARL structure (1994)

New construction principles

- antireflection coating improvement

- high contact quality

- high quality starting (FZ) Si

- minimising the structure thicknessmicroelectronics technology

with several photolithographic processes principles of construction and technology

were simplified for mass production

Present construction used in mass production

- etching monocrystalline (1,0,0) Si in KOH

-Surface texturing without photolithography

- acid etching in the case of other crystallographic orientation of Si

A single solar cell……~0.5 V, about 30 mA/cm2

For practical use it is necessary connect cells in series to obtain a source of higher voltage and in parallel to obtain a higher current

Solar cell

PV module

PV field

Rp

RsRs

RpRpRp

RsRs

0

1

2

3

4

5

6

7

8

9

10

0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7

V (V)

I (A

)

Optimum situation: all cells have the same VMP

If characteristics of individual cells in parallel differ, efficiency decreases

Cell connection in parallel

Cells in series….. the same current flows through all cells voltage does sums

I

V

n cellsn + 1 cells

Optimum situation:

all cells have the same IMP

If characteristics of individual cells in series differ, efficiency decreases

Rp

RsRs

RpRpRp

RsRs

Effect of partial shading – one cell (a decrease of illuminated area Aill)

p

sssPVill R

IRV

kT

IRVqI

kT

IRVqIJAI

12

exp1exp 0201

01

0102012

0202

2

42

I

JAIIIII

q

kTV PVill

OC

)(ln

In the case of one cell

• a decrease of the output current (proportional to area illuminated)

• a decrease of the output voltage

• a decrease of the output power

Rp

RsRs

RpRpRp

RsRs

Effect of partial shading – cells in series

In the case of cells in series increases series resistance

• a decrease of the output current

• a decrease of the output voltage

• a considerable decrease of the output power

One cell shaded

A practical example of the shading effect – a bottom of the field 2 is shaded in January by the field 1

antireflection coating

N-type

P-type

contact

Crystalline silicon cells Thin film cells

Suitable materials

CuInSe2

amorphous silicon

amorphous SiGe

CdTe/CdS

Basic types of solar cells:

Basic problem: cost…...

Glass

Molybdenum

Cu(InGa)Se2

CdSITO/ZnO

MgF2

NiAl0.5

- 1.5

m1.5 -

2.0 m

0.03 -

0.05

m0.1 m

0.5 -

1 m

CIS Cadmium Telluride(Copper Indium Selenide)

Contact

CdTe

CdSTCO

Glass

0.5

- 1.5

m

0.03 -

0.2 m

A simple structure of a thin film solar cell from amorphous silicon

transparent substrate

transparent conducting electrode

a-Si:H p+ layer (20 - 30 nm)

a-Si:H (non-doped layer – 250 nm)

a-Si:H n+ layer (20 nm)

Transparent conducting electrode (diffusion barrier)

Silver or aluminium back reflector

1%1%

<12%<12%

600nm600nm

substrate

TCO

TCO

Ag or Al contact

Light trapping

Tandem cells

irradiation

Wg1> Wg2

“Micromorf” cells

AIIIBV triple cells

The highest efficiency from all solar cell types

30%

High efficiency solar cell application Concentrator modules