Report

New Generation Silicon Solar
Cells
11.03.2014New Generation Silicon Solar Cells
By Sarah Lindner
Engineering Physics
TUM

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

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

• „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

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 silicon E(k)Band structure of Mg with potential well
Discrete energy levels

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

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

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.

2.4 Doping
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

2.4 Doping

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

Fermi-level for a) T = 0K and b) T > =K Fermi- level for n-doped semiconductor and T > 0K
Intrinsic case Extrinsic case

Switch of the Fermi level with increasing temperature a) n-doped b) p-doped

2.6 Conductivity
 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

2.7 Direct/indirect 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
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

2.8 Absorptioncoefficient

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

3.1 pn-junction
a) Band structur for n-doped
and p-doped semiconductor
before contact
b) Band structure after contact
c) Depletion area

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
Solar cell under radiation

 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

Maximum power point (MMP)
depends on:
• Temperature
• Irradiance
• Solar cell characteristics
Wilson s. 209
Efficency coefficent
 Performance of solar cell
Fill factor

3.4 Equivalent circuit
P
S
ph
SS
R
IRV
I
kTn
IRVe
I
kTn
IRVe
II
)(
)1
)(
(exp)1
)(
(exp
2
02
1
01
Equivalent circuit

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

Recombination by radiation
Auger-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

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

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

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

4.1 Dilemma
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.

4.2 Solar basics
Spectral distribution of solar radiation.
Black body curve 5762K
AM1 solar spectrum
AM0 solar spectrum 1353W/m2

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.
Standard Test Conditions (STC):
Temperature of 25°; irradiance of 1000W/m2; AM1.5 (air mass
spectrum)
Different air mass numbers

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

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)

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

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
of the materials and the devices
Global PV-market

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

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

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

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.

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

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

5.3 Reflection
 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

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

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
bounces around inside
 Efficiency can be increased up to 37%
a) represents the aluminium layer
b) represents the new version

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

• 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

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

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

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

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)

EWT/MWT
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

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

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

Cross section of a partially plated laser groove.

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%

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%

5.6 OECO-cell
Advantages:
 reduces the oscuration
 easy manufacturing processes and
environmentally friendly
 efficiency value > 20%
Standard OECO solar cell
 Mass production

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

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

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 !

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

6. Bibliography
 http://www.isfh.de
 http://www.fv-sonnenenergie.de
 http://www.solarserver.de
 http://www.laser-zentrum-hannover.de
 http://www.hmi.de
 http://www.coherent.com
 http://www.bine.de
 http://www.lexsolar.de
 http://www.diss.fu-berlin.de
 http://www.energieinfo.de
 http://www.wikipedia.de
 Rudden M.N., Wilson J., „Elementare Festkörperphysik und Halbleiterelektronik“, Spektrum
Akademischer Verlag, ©1995, 3. Auflage
 Würfel P., „Physik der Solarzellen, Spektrum Akademischer Verlag, ©2000, 2.Auflage
 Kaltschmitt M., Streicher W., Wiese W. (Hrsg.), „Erneuerbare Energien Systemtechnik, Wirtschaftlichkeit,
Umweltaspekte“, Springer Verlag, ©1993, 3. Auflage

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