Cell and module construction. Photovoltaic effect and basic solar cell parameters To obtain a...
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Transcript of Cell and module construction. Photovoltaic effect and basic solar cell parameters To obtain a...
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