New Generation Silicon Solar Cells
By Sarah Lindner
Engineering Physics
TUM
New Generation Silicon Solar Cells
23.03.2016
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
New Generation Silicon Solar Cells
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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
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1. Introduction - Photovoltaics on the world market
• „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
Annual installed power grew significantly from 2004
2.1 Electronic band structure
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 Mg with potential well
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Discrete energy levels
Band structure of silicon E(k)
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2.2 Metal – Isolator – Semiconductor
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 ~ 10-12s
Isolator:
Band structure
• at T = 0 the conduction band is empty  very high resistivity
• band gap EG > 3eV
• no conductivity despite doping possible
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2.2 Metal – Isolator – Semiconductor
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
•
(intrinsic semiconductor)
• T ↑  resistivity ↓
• Electrons can stay in the conduction band for
about 10-3s
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Band structure
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2.3 Definition
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.
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2.4 Doping
Doping: Change in carrier concentration  change in electrical properties
Donor - doping
Acceptor - doping
• add an extra electron
• number of e- > number valence e• n – type dopant
• ED right under conduction band EC
• add an extra hole
• number of e- < number valence e• p – type dopant
• EA right above valence band EV
n-type doping
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p-type doping
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2.4 Doping
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2.5 Intrinsic/Extrinsic
Intrinsic
Extrinsic
pure semiconductor
doped semiconductor
n=p
At thermal equilibrium
Self conduction + conduction
because of doping
self conduction
Conductivity depends on T
n≠p
T>0
Conductivity depends on T and on
additional charge carriers (dopant)
Change in EF
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2.5 Intrinsic/Extrinsic
Intrinsic case
Fermi-level for a) T = 0K and b) T > =K
Extrinsic case
Fermi- level for n-doped semiconductor and T > 0K
2.5 Intrinsic/Extrinsic
Switch of the Fermi level with increasing temperature a) n-doped b) p-doped
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2.6 Conductivity
EG 

 i  ni e( e   h )  C  e( e   h )  T exp  

 2kT 
3
2


σi depends strongly on the
temperature and the
charge carrier densities
extrinsic conductivity
depends additionaly on
excitation of dopants into
the conduction band.
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2.7 Direct/indirect band gap
Material
c-Si
a-Si:H
GaAs
Band gap
1,12 eV
(indirekt)
1,8 eV
(„direct“)
1, 43 eV
(direct)
Absorption
coefficient
(hν = 2,2)
[cm-1]
6*103
2*104
5*104
Indirect and direct band gap
Indirect:
• need a photon, a phonon, and a charge
carrier  happens more seldom
 longer absorption length
• recombination at grain boundarys and
point defects
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Direct:
• need just the right photon
for band transition
• higher transition probability
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2.8 Absorptioncoefficient
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3.1 pn-junction
• 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
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3.1 pn-junction
a) Band structur for n-doped
and p-doped semiconductor
before contact
b) Band structure after contact
c) Depletion area
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3.2 pn-junction under radiation
Absorption of light:
If Eph < Eg  no electron-hole-creation
If Eph > Eg  electron-hole-creation  drift
and diffusion  current and voltage
Band structure
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Solar cell under radiation
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3.3 Solar cell characteristics
I  I 0 (e
eU
nkT
 Isc = -Iph
 1)  I ph
for V = 0
 I  I ph 
nk

V
 T  ln 1 
e
I0 

I-V characteristic of a solar cell

Voc 
nkT  I sc 
 ln  
e
 I0 
New Generation Silicon Solar Cells
for I = 0
I0
n
k
Isc
Voc
is the saturation current
is the ideality factor
is the Boltzmann`s constant
is the short circuit current
is the open circuit voltage
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3.3 Solar cell characteristics
Maximum power point (MMP)
depends on:
• Temperature
• Irradiance
• Solar cell characteristics
Wilson s. 209
Fill factor
Efficency coefficent
 Performance of solar cell
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3.4 Equivalent circuit
Equivalent circuit
 e(V  IRS ) 
 e(V  IRS ) 
(V  IRS )




I  I 01 (exp 
 1)  I 02 (exp 
 1)  I ph 


RP
 n1kT 
 n2 kT 
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3.5 Generation and recombination
n0  n0 + ∆n = n
Recombination and generation processes. Generation processes depend on
absorption and on flow of photons
dn
dn
n
0

GRG
dt
dt

Life time of minority carriers:
New Generation Silicon Solar Cells
n
 G = R  i 
Ri
∆n
Ri
n0
n
G
is the surplus concentration
is the rate of recombination
is the concentration at equilibrium
is the charge concentration
is the rate of generation
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3.5 Generation and recombination
Recombination by radiation
Auger-recombination
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3.5 Generation and recombination
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
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3.5 Generation and recombination
radiation
105
Auger
τ [µs]
104
SRH
103
Experimental
• Low p0  SRH is dominant
and τ independet of p0
• High p0  τ ~ p0-2
(Auger recombination)
• radiation recombination
plays no role for silicon
102
 Normal sunlight radiation
the basis of the solar cell
is in the are of the SRH
recombination
101
100
1014
1015
1016
1017
p0 [cm-3]
Low innjection, depenence between hole equilibrium concentration and τ
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3.6 Diffusion length
Is the mean free length of path a charge carrier can travel in a volume of a
crystall lattice before recombination takes place.
D is the diffusion constant
depends on:



The semiconductor material
The doping
The perfection of the crystall lattice
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3.6 Diffusion length
Silicon: (10 μm - 100 μm)
λ < 800nm light absorbed within 10μm
λ > 800nm electron-hole generation all over the volume
Multichristall silicon
 for an effectiv solar cell the diffusion
τeff = 50μs
Leff,n (cm)
Leff,p (cm)
length has to be 2-3 times thicker
p-type
0,037
0,023
than the actual solar cell
n-type
0,040
0,024
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4.1 Dilemma
P=U*I
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.
ideal band gap size, depending on the solar spectrum
 The usuall ideal band gap is supposed to be at EG = 1,5eV
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4.2 Solar basics
AM0 solar spectrum 1353W/m2
Black body curve 5762K
AM1 solar spectrum
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Spectral distribution of solar radiation.
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4.2 Solar basics
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.
Different air mass numbers
Standard Test Conditions (STC):
Temperature of 25°; irradiance of 1000W/m2; AM1.5 (air mass spectrum)
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4.3 Losses
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
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4.3 losses
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)

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4.4 Efficiency values
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
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5.1 Why still silicon?

> 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
Global PV-market
of the materials and the devices
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5.2 Surface passivation
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
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5.2 Surface passivation
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
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5.2 Surface passivation
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%

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Passivierqualität als Funktion der a-Si:H-Schichtdicke
HIT solar cell
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5.2 Surface passivation
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.
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5.2 Surface passivation
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
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5.3 Reflection
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
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5.3 Reflection
Examples of light trapping
 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
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5.3 Reflection
 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
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5.3 Reflection
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
a)
b)
represents the aluminium layer
represents the new version
bounces around inside

Efficiency can be increased up to 37%
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5.4 Laser operations
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
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5.4 Laser operations
• p-doped layer is coated with an outer layer of n-doped silicon to form a large pnjunction
• 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
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5.4 Laser operations
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 generated groves on the cell surface
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%
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Over 1000 rear side local metal point-contacts created per solar cell
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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
5.5 Solar cell contacts
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 monocrystalline 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
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5.5 Solar cell contacts
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)
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EWT/MWT
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
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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.
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5.5 Solar cell contacts
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
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%
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MWT-cell
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5.5 Solar cell contacts
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
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5.5 Solar cell contacts
Cross section of a partially plated laser groove.
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5.5 Solar cell contacts
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%
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5.6 OECO-cell (Obliquely Evaporated COntacts)
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%
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5.6 OECO-cell
Advantages:

reduces the oscuration

easy manufacturing processes and
environmentally friendly

efficiency value > 20%
 Mass production
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Standard OECO solar cell
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5.6 OECO-cell




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
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5.7 Further prospects
There is also high potential in improvents for the manufacturing
process  development of a „solar silicon“
1.
2.
3.
4.
Sawing process has to be improved
Automation processes have to be developed
New contact processes
Fast processes with low cycle time
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5.7 Further prospects
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
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2030 silicon needed per year = 160,000t !
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5.7 Further prospects
Nominal power (crystalline
silicon)
Incline of the modules
Russia – Saint Petersburg
Germany - Munich
1kW
1kW
42°
37°
6,4%
6,5%
2,9%
2,9%
Losses in general
15,0%
15%
Complete losses
24,3%
24,4%
865kWh
1009kWh
Losses because of temp.
Losses because of reflection
Power production out of a
PV constructed for 1kW
per year
By http://re.jrc.ec.europa.eu/pvgis/apps/pvest.php?lang=de
New Generation Silicon Solar Cells
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6. Bibliography

http://www.isfh.de

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New Generation Silicon Solar Cells
23.03.2016
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