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  1. 1. New Generation Silicon Solar Cells 11.03.2014New Generation Silicon Solar Cells By Sarah Lindner Engineering Physics TUM
  2. 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. 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. 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. 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. 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. 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. 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. 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
  10. 10. 2.4 Doping 11.03.2014New Generation Silicon Solar Cells
  11. 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
  12. 12. 2.5 Intrinsic/Extrinsic Fermi-level for a) T = 0K and b) T > =K Fermi- level for n-doped semiconductor and T > 0K Intrinsic case Extrinsic case
  13. 13. 2.5 Intrinsic/Extrinsic 11.03.2014New Generation Silicon Solar Cells Switch of the Fermi level with increasing temperature a) n-doped b) p-doped
  14. 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. 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
  16. 16. 2.8 Absorptioncoefficient 11.03.2014New Generation Silicon Solar Cells
  17. 17. 3.1 pn-junction 11.03.2014New 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
  18. 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. 19. 3.2 pn-junction under radiation 11.03.2014New 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 structure Solar cell under radiation
  20. 20. 3.3 Solar cell characteristics 11.03.2014New Generation Silicon Solar Cells  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. 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. 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. 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. 24. 3.5 Generation and recombination 11.03.2014New Generation Silicon Solar Cells Recombination by radiation Auger-recombination
  25. 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. 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. 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. 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. 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. 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. 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. 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. 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. 34. 4.4 Efficiency values 11.03.2014New 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
  35. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 54. 5.5 Solar cell contacts 11.03.2014New Generation Silicon Solar Cells Cross section of a partially plated laser groove.
  55. 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. 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. 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. 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. 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. 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. 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
  62. 62. 6. Bibliography 11.03.2014New 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
  63. 63. Thank you for listening!

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