This document discusses new generation silicon solar cells. It begins with an introduction to photovoltaics and the semiconductor properties relevant to solar cells. It then describes the functionality of solar cells including pn junctions, characteristics, and losses. Methods to optimize silicon solar cells are discussed, including surface passivation techniques to reduce recombination at surfaces and contacts. The document concludes that silicon still has potential for high efficiency due to its abundance and the existing manufacturing infrastructure, and that surface passivation is key to improving solar cell performance.
1. New Generation Silicon Solar
Cells
11.03.2014New Generation Silicon Solar Cells
By Sarah Lindner
Engineering Physics
TUM
2. Table of contents
1. Introduction - Photovoltaics on the world market
2. Semiconductor
2.1 Electronic band structure
2.2 Metal – Isolator – Semiconductor
2.3 Definition
2.4 Doping
2.5 Intrinsic/Extrinsic
2.6 Conductivity
2.7 Direct/indirect band gap
2.8 Absorptioncoefficient
3. Solar cell – functionality
3.1 pn-junction
3.2 pn-junction under radiation
3.3 Solar cell characteristics
3.4 Equivalent circuit
3.5 Generation and recombination
11.03.2014New Generation Silicon Solar Cells
3. Table of contents
3.6 Diffusion length
4. Solar cell – efficiency
4.1 Dilemma
4.2 Solar basics
4.3 Losses
4.4 Efficiency values
5. How to optimize silicon solar cells
5.1 Why still silicon?
5.2 Surface passivation
5.3 Reflection
5.4 Laser operations
5.5 Solar cell contacts
5.6 OECO-cell
5.7 Further prospects
6. Bibliography
11.03.2014New Generation Silicon Solar Cells
4. • „In 2007 the photovoltaic market grew over 40% with ~ 2.3 GW of newly installed
capacity“ (EPIA)
• Germany has the first position on the world market with 50% global market share
• power Installed by region:
80% Europe
16% North America
4% Asia
• Most dynamic market is Spain
• Seven Countries hosting the majority of large photovoltaic
power plants: RoW, Italy, Japan, Korea, USA,
Spain, Germany
• the cumulative power quadrupled
• Installed PV world wide 7300MWP
• Annual growth predicted ~ 25%
• Turnover by modules (2030) ~100billion €/a
• By 2030 a worldwide contribution of 1% is
reached
1. Introduction - Photovoltaics on the world
market
Annual installed power grew significantly from 2004
5. 2.1 Electronic band structure
11.03.2014New Generation Silicon Solar Cells
One single atom discrete energy levels
Bring atoms close together , e.g. crystall lattice
Interaction of the electrons
Energy levels split up
band structure
Band structure of silicon E(k)Band structure of Mg with potential well
Discrete energy levels
6. 2.2 Metal – Isolator – Semiconductor
11.03.2014New Generation Silicon Solar Cells
Metal:
• either the conduction band is partly
filled
• or no seperate conduction and
valence band exist
• electrons can move freely
• T ↑ resistivity ↑
• electrons give their energy to the
phonons
very fast ~ 10-12sIsolator:
• at T = 0 the conduction band is empty very high resistivity
• band gap EG > 3eV
• no conductivity despite doping possible
Band structure
7. 2.2 Metal – Isolator – Semiconductor
11.03.2014New Generation Silicon Solar Cells
Semiconductor:
• isolator for deep temperatures (T = 0)
• conduction band at low temperatures as
good as empty, valence band almost full
• band gap 0,1eV < EG < 3eV
•
• T ↑ resistivity ↓
• Electrons can stay in the conduction
band for
about 10-3s
(intrinsic semiconductor)
Band structure
8. 2.3 Definition
11.03.2014New Generation Silicon Solar Cells
A semiconductor is a material that has electrical
conductivity between that of a conductor and that of an
insulator
Its resistivity decreases with increasing temperature
and therefore its conductivity increases.
9. 2.4 Doping
11.03.2014New Generation Silicon Solar Cells
Donor - doping
• add an extra electron
• number of e- > number valence e-
• n – type dopant
• ED right under conduction band EC
Acceptor - doping
• add an extra hole
• number of e- < number valence e-
• p – type dopant
• EA right above valence band EV
Doping: Change in carrier concentration change in electrical properties
n-type doping
p-type doping
11. 2.5 Intrinsic/Extrinsic
11.03.2014New Generation Silicon Solar Cells
Intrinsic Extrinsic
pure semiconductor doped semiconductor
n = p n ≠ p
self conduction Self conduction + conduction
because of doping
Conductivity depends on T Conductivity depends on T and on
additional charge carriers (dopant)
Change in EF
At thermal equilibrium
T>0
14. 2.6 Conductivity
11.03.2014New Generation Silicon Solar Cells
σi depends strongly on
the temperature and
the charge carrier
densities
extrinsic conductivity
depends additionaly on
excitation of dopants
into the conduction
band.
kT
E
TeCen G
heheii
2
exp)()( 2
3
15. 2.7 Direct/indirect band gap
11.03.2014New Generation Silicon Solar Cells
Indirect:
• need a photon, a phonon, and a charge
carrier happens more seldom
longer absorption length
• recombination at grain boundarys and
point defects
Direct:
• need just the right photon
for band transition
• higher transition probability
Material c-Si a-Si:H GaAs
Band gap 1,12 eV
(indirekt)
1,8 eV
(„direct“)
1, 43 eV
(direct)
Absorptio
n
coefficient
(hν = 2,2)
[cm-1]
6*103 2*104 5*104
Indirect and direct band gap
17. 3.1 pn-junction
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• Equilibrium condition, no bias voltage
• diffusion current opposite to
the E-field
• diffusion voltage V0
with ∆E = eV0
at diffusion force = E-field force
V0 is the electrial voltage at the
equlibrium state = diffusion voltage
18. 3.1 pn-junction
11.03.2014New Generation Silicon Solar Cells
a) Band structur for n-doped
and p-doped semiconductor
before contact
b) Band structure after contact
c) Depletion area
19. 3.2 pn-junction under radiation
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Absorption of light:
If Eph < Eg no electron-hole-creation
If Eph > Eg electron-hole-creation drift
and diffusion current and voltage
Band structure
Solar cell under radiation
20. 3.3 Solar cell characteristics
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Isc = -Iph for V = 0
for I = 0
ph
nkT
eU
IeII )1(0
I0 is the saturation current
n is the ideality factor
k is the Boltzmann`s constant
Isc is the short circuit current
Voc is the open circuit voltage
0
1ln
I
II
T
e
nk
V
ph
0
ln
I
I
e
nkT
V sc
oc
I-V characteristic of a solar cell
21. 3.3 Solar cell characteristics
11.03.2014New Generation Silicon Solar Cells
Maximum power point (MMP)
depends on:
• Temperature
• Irradiance
• Solar cell characteristics
Wilson s. 209
Efficency coefficent
Performance of solar cell
Fill factor
22. 3.4 Equivalent circuit
11.03.2014New Generation Silicon Solar Cells
P
S
ph
SS
R
IRV
I
kTn
IRVe
I
kTn
IRVe
II
)(
)1
)(
(exp)1
)(
(exp
2
02
1
01
Equivalent circuit
23. 3.5 Generation and recombination
11.03.2014New Generation Silicon Solar Cells
n0 n0 + ∆n = n
Recombination and generation processes. Generation processes depend on
absorption and on flow of photons
G = R
Life time of minority carriers:
i
i
R
n
∆n is the surplus concentration
Ri is the rate of recombination
n0 is the concentration at equilibrium
n is the charge concentration
G is the rate of generation
n
GRG
dt
dn
0
dt
dn
24. 3.5 Generation and recombination
11.03.2014New Generation Silicon Solar Cells
Recombination by radiation
Auger-recombination
25. 3.5 Generation and recombination
11.03.2014New Generation Silicon Solar Cells
Recombination by impurity
τSRH depends on:
Number of impurities
Energy level of impurities
Cross section of impurities
Recombination on the surface
Untreated silicon surfaces S > 106 cm/s
Depends strongly on charge carrier injection
and doping
26. 3.5 Generation and recombination
11.03.2014
radiation
Auger
SRH
Experimental
1014 1015 1016 1017
105
104
103
102
101
100
τ[µs]
p0 [cm-3]
Low innjection, depenence between hole equilibrium concentration and τ
• Low p0 SRH is dominant
and τ independet of p0
• High p0 τ ~ p0
-2
(Auger recombination)
• radiation recombination
plays no role for silicon
Normal sunlight radiation
the basis of the solar cell
is in the are of the SRH
recombination
27. 3.6 Diffusion length
11.03.2014New Generation Silicon Solar Cells
Is the mean free length of path a charge carrier can travel in a volume
of a crystall lattice before recombination takes place.
depends on:
The semiconductor material
The doping
The perfection of the crystall lattice
D is the diffusion constant
28. 3.6 Diffusion length
11.03.2014New Generation Silicon Solar Cells
Silicon: (10 μm - 100 μm)
λ < 800nm light absorbed within 10μm
λ > 800nm electron-hole generation all over the volume
for an effectiv solar cell the diffusion
length has to be 2-3 times thicker
than the actual solar cell
Multichristall silicon
τeff = 50μs Leff,n (cm) Leff,p (cm)
p-type 0,037 0,023
n-type 0,040 0,024
29. 4.1 Dilemma
11.03.2014New Generation Silicon Solar Cells
P = U * I
ideal band gap size, depending on the solar spectrum
The usuall ideal band gap is supposed to be at EG =
1,5eV
A small band gap causes
a big short circuit current,
because many photons will
create electron-hole-couples.
A big band gap causes
a larger potential barrier
and therefore a larger
open circuit voltage.
30. 4.2 Solar basics
11.03.2014New Generation Silicon Solar Cells
Spectral distribution of solar radiation.
Black body curve 5762K
AM1 solar spectrum
AM0 solar spectrum 1353W/m2
31. 4.2 Solar basics
11.03.2014New Generation Silicon Solar Cells
AM = air mass = degree to which the atmosphere affects the sunlight
received at the earth`s surface
The factor behind tells you the length of the way when the light passes
through the atmosphere.
Standard Test Conditions (STC):
Temperature of 25°; irradiance of 1000W/m2; AM1.5 (air mass
spectrum)
Different air mass numbers
32. 4.3 Losses
11.03.2014New Generation Silicon Solar Cells
1. Reflection:
the metall circuit path on the front of a solar cell
reflects the light
the solar cell itsself reflects the light
2. Shadow
The metall circuit path obscures the front of the solar cell
3. Recombination
On the surface dangling bonds
Inside the volume
4. Interaction with phonons
33. 4.3 losses
11.03.2014New Generation Silicon Solar Cells
5. Resistance factors
short circuit between the front and the back of the
solar cell
transport of the charge carriers through the cables
and contacts
6. Absorption and Transmission
Other layers of the solar cell (e.g. ARC) can also
absorb
Light can totaly be transmitted trough the solar cell
7. Other factors
Dirt on the solar cell
No ideal conditions (STC)
34. 4.4 Efficiency values
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Material η (laboratory) η (produktion)
Monocrystalline 24,7 14,0 – 18,0
Polycrystalline 19,8 13,0 – 15,5
Amorphous 13,0 8,0
Material Crystalline order Thickness Wafer
Monocrystalline One ideal lattice 50μm - 300μm One single crystall
Polycristalline Many small crystalls 50μm - 300μm grain (0,1mm – Xcm)
Amorphous No crystalline order;
Groups of some
regularly bound
atoms
< 1μm No wafer
35. 5.1 Why still silicon?
11.03.2014New Generation Silicon Solar Cells
> 90% silicon and multisilicon
Silicon has the potential for high efficiency
Silicon is available unlimited
second most element of the earth‘s crust
The involved materials and processes
are non-toxic and do not harm the environment
The silicon technology already exists
and is reliable
Already exists a broad knowledge
of the materials and the devices
Global PV-market
36. 5.2 Surface passivation
11.03.2014New Generation Silicon Solar Cells
1. Thermal oxidation:
Reduction of the density of states on the interface or surface
Oxygen streams over the hot wafer surface and reacts with silicon
to SiO2
This results in an amorphous layer
Temperature of the process ~ 1000°C
Thickness of the layer > 35nm efficiency decreases
Time goes on and the velocity of the growth of the oxidic layer
decreases
37. 5.2 Surface passivation
11.03.2014New Generation Silicon Solar Cells
2. Passivation with SiNx
Reduction of the density of states on the interface
Gases silane SiH4 and methane NH3 form a layer of Si3N4
Temperature of the process ~ 350°C
Passivation quality rises with silane amount
S ~ 20 cm/s – 240 cm/s depending on the refraction index
advantages:
lower production temperature
Nitride seems also to work better as an anti reflection layer for solar
cells
better passivation
38. 5.2 Surface passivation
11.03.2014New Generation Silicon Solar Cells
3. Passivation with only silane
The quality of the passivation is enormous
Passivation layer on the emitter should be very
thin (10nm)
high absorption prefer SiNx-Process on the
emitter
The process temperature is ~225°C
The passivation seems independet of
contaminations of the silicon surface brought in
during the manufacturing process
An example is the HIT-Solar Cell from Sanyo
Layer of monocristalline silicon between
amorphous silicon layers
Efficiency of ~ 18,5%
Passivierqualität als Funktion der a-Si:H-Schichtdicke
HIT solar cell
39. 5.2 Surface passivation
11.03.2014New Generation Silicon Solar Cells
4. Back Surface Field (BSF)
A thin layer of p-doped material to prevent the minorities from moving to the
back contact where they recombinate
e.g. use aluminium for a back contact, which melts (T ~ 500°C) into the silicon
and creates a positive doped BSF. Besides it serves as a reflection layer.
40. 5.2 Surface passivation
11.03.2014New Generation Silicon Solar Cells
Intrinsic gettering:
Contaminations will be collected at one area in the crystall and
afterwards will be removed
Extrinsic gettering:
Contaminations will be transported to the crystall surface and
afterwards be removed
e.g. aluminium
Foreign atom will be freed out of their bonds diffuse into the Al-Si
alloy
30 minutes at T = 800°C to eliminate most of the contaminations,
depends on the diffusion length of the atom
41. 5.3 Reflection
11.03.2014New Generation Silicon Solar Cells
1. Anti reflection layer
One or more layers reduction from 30-35% to 5%-10%
Mainly 600nm transmission
Silicon nitride or transparent layers, e.g. SiO2; TiO2; Ta2O5
ITO can be used as anti reflection layer and at the same time as a transparent
contact
Double anti reflection layers ZnS or MgF2
2. Texturing (light trapping)
Use NaOH, KOH in etching baths
The etching works anisotropic 2μm - 10μm
big pyramids on (100) oriented crystall planes
42. 5.3 Reflection
11.03.2014New Generation Silicon Solar Cells
advantages:
At least second reflection
The effective absorption length of the silicon layer will be reduced
the light way through the layer increases
The area of the surface becomes bigger
Total reflection on the inside of the front layer possible
Reflection can be reduced about 9/10 of the former reflection
Examples of light trapping
43. 5.3 Reflection
11.03.2014New Generation Silicon Solar Cells
disadvantage:
More difficult to form it on multi-/polycrystalline silicon layers no sufficient
reflection reduction
The surface area is increased higher surface carrier recombination
rates
New:
A focused laser scans the
wafer surface to form a dotted matrix
The damage on the surface of the
crystall will be etched away afterwards
Advantage: it is better for the
environment and can be used on
different materials
Reflection can be reduced from ~35% to 20% Laser texturized poly chrystall silicon
44. 5.3 Reflection
11.03.2014New Generation Silicon Solar Cells
3. Back side reflection
Two different layers at the backside:
Patterns of microscopic spheres of glass within
a precisely designed photonic crystall
Capture and recycle the photons
Large-scale manufacturing techniques are
being developed
advantage:
Reflects more light than the aluminium layer
Light reenters the silicon at low angle light
bounces around inside
Efficiency can be increased up to 37%
a) represents the aluminium layer
b) represents the new version
45. 5.4 Laser operations
11.03.2014New Generation Silicon Solar Cells
Why using laser?
All for Si-PV-technology used materials absorb light
A small optical/thermical penetration depth is given for λL < 1µm
Laser can focuse very good (size of structure 10µm – 100µm)
Minimal mechanical demands on the fragile Si-wafer
Screen printing process can be prevented
Laser`s high quality output beams and unique pulse characteristics coupled
with low cost –of-ownership
46. 5.4 Laser operations
11.03.2014New Generation Silicon Solar Cells
• p-doped layer is coated with an outer layer of n-doped silicon to form a large
pn-junction
• n-doped layer coats the entire wafer recombination pathways between
front and rear surfaces
Edge isolation:
groove is continuously scribed completely
through the n-type layer right next to the
edge of the cell
Requirements:
• Rp should be kept high; FF > 76%
• Little waste of solar cell area
• 1000 wafer/h
• Flexibility (thin wafers)
Groove to isolate the front and rear side of the cells
47. 5.4 Laser operations
11.03.2014New Generation Silicon Solar Cells
Front surface contacts:
Burried contacts to minimize the area
obscured by the front contacts
electrodes with a high volume
and collection surface
Depth and width 20μm – 30μm
every 2mm-3mm
Laser Fired Contacts
Electrically and thermo-mechanically
advantageous to include passivation
layer, which is non-conducting
laser creates localized Al/Si- alloys
Efficiency of ~ 21%
Over 1000 rear side local metal point-contacts created per
solar cell
Laser generated groves on the cell surface
48. 5.5 Solar cell contacts
Saturn-solar-cell Laser Grooved Buried Contact (LGBC)
Laser will burn a trench in the front side of the solar cell
Trench is 35µm deep and 20µm wide and has form of a „U“ or a „V“
Trench will chemically be filled up with the front contact material, usually silver
a large metal hight-to-width aspect ratio
allows closely spaced metal findgers
low parasitic resistance losses
advantages:
Shading losses will only be 2% to 3%
Reduction of metall grid and contact
resistance
Reduction of emitter resistance
because of very close fingers
Possible efficiency >17%
LGBC-cell
49. 5.5 Solar cell contacts
11.03.2014New Generation Silicon Solar Cells
Prevent obscuration of the solar cell or high reflection and absorption
of the silver grids.
small and high grids, which will become smaller towards the edge of the cell
COSIMA (Contacts to a-Si:H passivated wafers
by means of annealing):
Amorphous silicon (silane process) on mono-
crystalline silicon
Aluminium on theses layers results in
contacting the monocrystalline silicon
Process temperature ~ 200°C
No photolithography
Solar cell with a-Si:H-rear passivation and COSIMA contacts
50. 5.5 Solar cell contacts
11.03.2014New Generation Silicon Solar Cells
Advantages:
Simplifies thin film manufacturing process
Efficiency values about 20%
Combination with doted contacts:
Screen printed interface layer (little holes) good passivation
Aluminium on the interface layer COSIMA
Advantages:
Can be used on thinner wafers no bending
The passivation abbility of the amorphous layer will be kept after the annealing
process
The contact resistivity is 15mΩcm2
Increase of the quantum yield in the infrared wavelength range
Reduces Seff to 100 cm/s (4% metallization)
51. EWT/MWT
11.03.2014New Generation Silicon Solar Cells
Front (left) and rear (right) of a EWT-solar cell. The front contacts
are brought to the rear of the solar cell by many dots.
Emitter Wrap through (EWT)
• Emitter on the front surface is wraped with the rear surface by little holes
• Edges of the holes are also emitter areas, which transport emitter current
• Power-conveying busbars and the grid are moved to the rear surface
• Use double sided carrier collection (n+pn+) increases the efficiency
• 100µm holes are made by laser
EWT- cell with n+pn+ - structure
52. 5.5 Solar cell contacts
11.03.2014New Generation Silicon Solar Cells
Disadvantage:
Manufacturing process is very complex
Metal wrap throug (MWT)
• Absence of the bus bars (on the rear side) connection by holes
• Less serial resistance losses because of interconnection of the modules
on the back
• FF ~77%; efficiency ~ 16%
Advantages:
• Eliminate grid obscuration no high doping high Isc high efficiency
• n+pn+- structure use lower quality solar grade silicon
• Uniform optical appereance improves asthetics
• Silicon solar cell < 200μm
• Efficiency around 18%
• gain in active cell area
•Diffusion length can be reduced to the half
MWT-cell
53. 5.5 Solar cell contacts
11.03.2014New Generation Silicon Solar Cells
MWT EWT
Voc [mV] 617 596
Jsc [mA/cm2] 36,1 37,7
FF [%] 75,1 72,8
η [%] 16,7 16,3
Area [cm2] 189,5 61,5
54. 5.5 Solar cell contacts
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Cross section of a partially plated laser groove.
55. 5.5 Solar cell contacts
11.03.2014New Generation Silicon Solar Cells
A 300 solar cell:
Negative conducting silcon wafer
Emitter and all contacts on the back side
No obscuration on the front side
Efficiency value > 21%
56. 5.6 OECO-cell (Obliquely Evaporated
COntacts)
11.03.2014New Generation Silicon Solar Cells
Standard OECO cell:
• front contacts are evaporated on the
flanks of the ditch by self obscurance
• flat homogeneous emitter because of
one step phosphor diffusion
• very thin contacts of metall are possible
• development of a ultra thin tunnel oxid
between metal and semiconductor,
which forms high sufficient MIS contacts
• passivation layer on the front and rear
side (SiNX or SiO2)
• efficiency ~ 20%
57. 5.6 OECO-cell
11.03.2014New Generation Silicon Solar Cells
Advantages:
reduces the oscuration
easy manufacturing processes and
environmentally friendly
efficiency value > 20%
Standard OECO solar cell
Mass production
58. 5.6 OECO-cell
11.03.2014
Both contacts are on the rear side
The back of this cell accords to the
standard OECO cell
The front has a texturized surface
Deep phosphorous emitter on almost
the whole back side
Advantages:
Reduction of impurity shunt
resistance and serial resistance
Reduction of obscurance at the front
Double sided light-sensitivity
bifaciale solar cell
efficiency for both sides ~ 22%
possible
Back – OECO - cell
59. 5.7 Further prospects
11.03.2014New Generation Silicon Solar Cells
There is also high potential in improvents for the
manufacturing process development of a „solar
silicon“
1. Sawing process has to be improved
2. Automation processes have to be developed
3. New contact processes
4. Fast processes with low cycle time
60. 5.7 Further prospects
11.03.2014New Generation Silicon Solar Cells
Annual consumption of electricity per person:
1000kWh/a
Annual solar cell power 1000W/m2a
800 – 1200 hours of sun in Germany with 80%
ca. 800kWh/m2a out of a photovoltaic
system
Efficiency of 15% 120kWh/m2a
To cover the annual consumption of electricity
per person you need ~ 8,3m2
Multicrystalline solar cell (15x15x0,03cm3) has
a peak power of 3,5W and is made out of 24g
silicon (+ loss during production) 6,8kg silicon
2030 silicon needed per year = 160,000t !
61. 5.7 Further prospects
11.03.2014New Generation Silicon Solar Cells
Russia – Saint
Petersburg
Germany - Munich
Nominal power
(crystalline silicon)
1kW 1kW
Incline of the modules 42° 37°
Losses because of
temp.
6,4% 6,5%
Losses because of
reflection 2,9% 2,9%
Losses in general 15,0% 15%
Complete losses 24,3% 24,4%
Power production out of
a PV constructed for
1kW per year
865kWh 1009kWhBy http://re.jrc.ec.europa.eu/pvgis/apps/pvest.php?lang=de