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New Generation Silicon Solar Cells
19.04.23New Generation Silicon Solar Cells
By Sarah LindnerEngineering 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
19.04.23New Generation Silicon Solar Cells
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
19.04.23New Generation Silicon Solar Cells
• „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
19.04.23New 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
2.2 Metal – Isolator – Semiconductor
19.04.23New 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
2.2 Metal – Isolator – Semiconductor
19.04.23New 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
2.3 Definition
19.04.23New 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.
2.4 Doping
19.04.23New 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
2.4 Doping
19.04.23New Generation Silicon Solar Cells
2.5 Intrinsic/Extrinsic
19.04.23New 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
2.5 Intrinsic/Extrinsic
Fermi-level for a) T = 0K and b) T > =K Fermi- level for n-doped semiconductor and T > 0K
Intrinsic caseExtrinsic case
2.5 Intrinsic/Extrinsic
19.04.23New Generation Silicon Solar Cells
Switch of the Fermi level with increasing temperature a) n-doped b) p-doped
2.6 Conductivity
19.04.23New 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
ETeCen G
heheii 2exp)()( 2
3
2.7 Direct/indirect band gap
19.04.23New 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)
Absorption coefficient(hν = 2,2) [cm-1]
6*103 2*104 5*104
Indirect and direct band gap
2.8 Absorptioncoefficient
19.04.23New Generation Silicon Solar Cells
3.1 pn-junction
19.04.23New Generation Silicon Solar Cells
• 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
19.04.23New Generation Silicon Solar Cells
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
19.04.23New Generation Silicon Solar Cells
Absorption of light:
If Eph < Eg no electron-hole-creation
If Eph > Eg electron-hole-creation drift
and diffusion current and voltage
Band structureSolar cell under radiation
3.3 Solar cell characteristics
19.04.23New Generation Silicon Solar Cells
Isc = -Iph for V = 0
for I = 0
phnkT
eU
IeII )1(0
I0 is the saturation currentn is the ideality factork is the Boltzmann`s constantIsc is the short circuit currentVoc is the open circuit voltage
0
1lnI
IIT
e
nkV ph
0
lnI
I
e
nkTV scoc
I-V characteristic of a solar cell
3.3 Solar cell characteristics
19.04.23New 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
3.4 Equivalent circuit
19.04.23New Generation Silicon Solar Cells
P
Sph
SS
R
IRVI
kTn
IRVeI
kTn
IRVeII
)()1
)((exp)1
)((exp
202
101
Equivalent circuit
3.5 Generation and recombination
19.04.23New 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:
ii R
n
∆n is the surplus concentrationRi is the rate of recombinationn0 is the concentration at equilibriumn is the charge concentrationG is the rate of generation
n
GRGdt
dn 0
dt
dn
3.5 Generation and recombination
19.04.23New Generation Silicon Solar Cells
Recombination by radiation
Auger-recombination
3.5 Generation and recombination
19.04.23New Generation Silicon Solar Cells
Recombination by impurityτSRH 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
3.5 Generation and recombination
19.04.23
radiation
Auger
SRH
Experimental
1014 1015 1016 1017
105
104
103
102
101
100
τ [µ
s]
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
3.6 Diffusion length
19.04.23New 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
3.6 Diffusion length
19.04.23New 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
4.1 Dilemma
19.04.23New 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 causesa big short circuit current,because many photons will create electron-hole-couples.
A big band gap causesa larger potential barrierand therefore a larger open circuit voltage.
4.2 Solar basics
19.04.23New Generation Silicon Solar CellsSpectral distribution of solar radiation.
Black body curve 5762K
AM1 solar spectrum
AM0 solar spectrum 1353W/m2
4.2 Solar basics
19.04.23New 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
4.3 Losses
19.04.23New 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. ShadowThe 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
19.04.23New 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)
4.4 Efficiency values
19.04.23New Generation Silicon Solar Cells
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?
19.04.23New 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
5.2 Surface passivation
19.04.23New 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
5.2 Surface passivation
19.04.23New 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
5.2 Surface passivation
19.04.23New 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
5.2 Surface passivation
19.04.23New 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.
5.2 Surface passivation
19.04.23New 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
5.3 Reflection
19.04.23New 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
5.3 Reflection
19.04.23New 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
5.3 Reflection
19.04.23New 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
5.3 Reflection
19.04.23New 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
5.4 Laser operations
19.04.23New 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
5.4 Laser operations
19.04.23New 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
5.4 Laser operations
19.04.23New 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 ContactsElectrically 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
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
19.04.23New 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
5.5 Solar cell contacts
19.04.23New 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)
EWT/MWT
19.04.23New 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
5.5 Solar cell contacts
19.04.23New 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
5.5 Solar cell contacts
19.04.23New 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
5.5 Solar cell contacts
19.04.23New Generation Silicon Solar Cells
Cross section of a partially plated laser groove.
5.5 Solar cell contacts
19.04.23New 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%
5.6 OECO-cell (Obliquely Evaporated COntacts)
19.04.23New 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%
5.6 OECO-cell
19.04.23New Generation Silicon Solar Cells
Advantages: reduces the oscuration easy manufacturing processes and
environmentally friendly efficiency value > 20%
Standard OECO solar cell
Mass production
5.6 OECO-cell
19.04.23
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
5.7 Further prospects
19.04.23New 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
5.7 Further prospects
19.04.23New Generation Silicon Solar Cells
Annual consumption of electricity per person:1000kWh/a
Annual solar cell power 1000W/m2a800 – 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 electricityper person you need ~ 8,3m2
Multicrystalline solar cell (15x15x0,03cm3) has a peak power of 3,5W and is made out of 24gsilicon (+ loss during production) 6,8kg silicon
2030 silicon needed per year = 160,000t !
5.7 Further prospects
19.04.23New 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
6. Bibliography
19.04.23New Generation Silicon Solar Cells
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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