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					Crystalline Si Photovoltaics
     Arthur Weeber
    Outline

    • Short introduction ECN
    • Introduction ECN Solar Energy
    • General Si solar cells
    • Crystalline Si Photovoltaics
       - Feedstock
       - Wafering
       - Cell processing
       - Module technology
       - Costs and environmental
    • Summary



2
    Petten: ECN; NRG; JRC; TYCO




3
    Targets ECN research

    Transition to renewable energy supply:

    efficiency improvement                 maximum reliability
                                      minimum environmental burden
    development of renewable energy     optimal cost effectiveness

    clean use of fossil fuels




4
    ECN Programme units
    Strategically            Policy Studies

    Energy savings           Energy Efficiency in Industry
                             Renewable Energy in the Built Environment

    Renewable energy         Solar Energy
                             Wind Energy
                             Biomass, Coal & Environmental research

    Clean use fossil fuels   Hydrogen & Clean fossil fuels

    Support                  Engineering & Services


5
    Turnover share per unit (2005)

                Fuel Cell        Biomass
               Technology          13%
                  19%

                                            Policy Studies
                                                 11%


                                               Energy Eff. in the
    Clean Use of                                   Industry
    Fossil Fuels                                      7%
        16%


                                           Solar Energy
           RE in the Built                     15%
           Environment       Wind Energy
                 8%             12%


6
ECN Solar Energy
    Solar Energy

    • Silicon Photovoltaics
    • Thin-Film Photovoltaics
    • PV Module Technology

    Objective:
    • Price of solar electricity in 2015 the same as consumer
      electricity price, and after that even lower
       - High efficiency
       - Reduction of material use
       - Cost effective and environmental friendly processes and
         products
       - Long lifetime of the modules

8
                               PV technology development:
                               no revolution, but evolution
                          4                                                                                tf-Si = thin-film silicon

                                                     am   ilies




                                                                                      tec
                                            o   gy f                                                       CIGSS = copper-indium/
                                        nol                                  2004




                                                                                         hn
                                   t ech                                                                   gallium-selenium/sulfur




                                                                                           o lo
    module price (€/Wp)




                                                                  c-Si
                          3




                                                                                            gy
                                                     i
                                                tf-S                                                       c-Si = wafer-type




                                                                                               gen
                                                   S S
                                          &   C IG                                                         crystalline silicon
                                                                               2010




                                                                                                  era
                                                                                                     t
                                                                                                   ion
                                                                                                           OSC = organic solar cells




                                                                                                      s
                          2
                                                                                                           new concepts =
                               OSC                                                                         advanced versions of
                                                                                      2020
                                                                                                           existing technologies
                          1                                                                                & new conversion principles


                                                              new concepts                   >2030             (free after
                                                                                                               W. Hoffmann)
                          0
                           0   5             10          15                         20                    25
                                          module efficiency (%)

9
      ECN Solar Energy
Thin-film photovoltaics
  • Sensitised oxides
        – efficiency, stability, manufacturing technology
        – solid state version: in 2015 η=8% for 10x10 cm2
          device with >10 year outdoor stability




 10
                  ECN Solar Energy
Thin-film photovoltaics
  • Organic solar cells
     – device fabrication, efficiency and stability
     – in 2015 η=8% for 10x10 cm2 device
        with >10 year outdoor stability
                  2.0
                                                                                     OC H21
                                                                                       10
                                  50
                        EQE (%)




                  1.5
 Efficiency (%)




                                  25
                                                                          H3CO
                                  0
                  1.0             400        600        800
                                        wavelength (nm)                          +
                                                                                 S            OR1
                  0.5                                                                            CN

                                                                     R2   R2         NC                   n
                                                                                       MeO            S



                  2002                  2003 2004             2005
                                            Year
                                                                                 Collaboration of ECN, TNO

11
    ECN Solar Energy

Thin-film photovoltaics
  • Thin-film silicon
      – R-2-R deposition of (n,i,p) silicon on foils
      – Development of thin-film Si tandems
      – In 2015 a 0.3x1 m2 PV module η=12% at 0.8€/Wp




  12
                                                                                           AM1.5
                    ECN Solar Energy
                    New concepts                                                                        1
                                                                                                                 Re
                       •improved spectrum utilization
                                                                                                            Ri
                       •flat plate concentrator
                                                                                       2
                                                                                           a
                                                                                   2               Fr

                     UV visibleinfrared
              1.6                                                                          1
                                    solar spectrum (Air Mass 1.5; 1000 W/m2)

                                      available for conversion in crystalline Si
power [W/(m2.nm)]




              1.2


                                          1100 nm ∼1.1 eV = Si bandgap
              0.8


              0.4


              0.0
                      400     800      1200        1600        2000        2400
                                      wavelength [nm]


  13
     ECN Solar Energy

PV systems
  • grid interaction
  • system design & monitoring
  • standards and guidelines




14
     Crystalline Si PV technology

     Objective:
     • Price of solar electricity in 2015 the same as consumer
       electricity price, and after that even lower
        - High efficiency
           - 18% module efficiency for crystalline Si PV
        - Reduction of material usage
           - Thin wafers (<150 µm compared to current >240 µm)
        - Cost effective and environmental friendly processes and
          products
        - Long lifetime of the modules (>30 yr for crystalline Si)

          - Energy Pay Back Time < 1 yr
15
     Cell structure
     Crystalline silicon solar cell (minority carrier device)

     • Base: B doped Si (p-type)
     • Emitter: P doped layer (n-type)
        - Recombination losses in base and emitter        ohmic contact
                                                                                               -

        - Voltage over pn junction
                                                                                                       external load
                                                                   ARC
                                                                                                   +
                                                         emitter
     • Metallization for contacts                             base         - +
                                                                                      -

        - Shading losses                                                          +


        - Resistance losses                                          back ohmic contact/ BSF


                                                       collection
     • Antireflection coating to
       enhance current
                                                                     recombination
     • Surfaces: recombination losses
16
     Cell structure
     Crystalline silicon solar cell
                                                                   recombination
     • Base: B doped Si (p-type)
                                                          photon             Back
     • Emitter: P doped layer (n-type)
                                                                             Surface
        - Voltage over pn junction
                                                                             Field
        - Recombination losses
                                           collection
     • BSF: doped layer
             p+                       EC
        - Highly doped                EF
        - Reduced
          recombination               EV
                                                        junction
                                           n-type                  p-type      p+-type
17
     Cell structure
     Losses in crystalline silicon solar cell

     • Colour mismatch                                    recombination
                                          η≤30%
     • Fundamental recombination

     • Additional recombination                   Eband
                                                                                       hν
        - Impurities, defects, surfaces
     • Shading
     • Reflection, absorption and transmission
        - Absorption at the rear                                          generation
     • Resistance
     • Non-ideal band gap

     Crystalline Si solar cell: η=13-20%


18
     Cell characterization
     • Current Voltage (IV curve)
        - Open circuit voltage Voc                           30
                                                                                               standard

        - Short circuit current Jsc
        - Efficiency




                                                J (mA/cm2)
                                                             20        Rshunt

        - Resistance losses
                                                             10
                                                                                    Rserie


                                                              0
                                  -0.4   -0.2                      0       0.2       0.4        0.6       0.8
                                                                                 Voltage (V)
                                                             -10




19
     Cell characterization
     • Internal Quantum Efficiency IQE
        - IQE=collected carriers / absorbed photons
        - Depth profile cell quality
                                 100


                                 80
                                                                           Rear side            cell
                                                                           reflection
                                 60
                                                                                        light
                       IQE (%)




                                       Front side /            Bulk /
                                 40    emitter                 rear side


                                 20


                                  0
                                   300         500       700       900       1100
                                                      wavelength (nm)


20
     Crystalline Si PV technology

     • Feedstock
        - Effect impurities on cell output
     • Wafers
        - Monocrystalline Si
        - Multicrystalline Si
     • Cell technology
        - High efficiency with industrial in-line processing
     • Module Technology
        - Module design integrated with cell concept
        - Simple interconnection and encapsulation
     • Costs and environmental aspects

21
     Crystalline Si PV technology

     • Feedstock
        - Effect impurities on cell output
     • Wafers
        - Monocrystalline Si
        - Multicrystalline Si
     • Cell technology
        - High efficiency with industrial in-line processing
     • Module Technology
        - Module design integrated with cell concept
        - Simple interconnection and encapsulation
     • Costs and environmental aspects

22
     Feedstock production

                                            existing; 900.000 ton/y
      quartz, carbon                                                                              mg-Si




                                                                                       d
                                                                                   ite
                 un




                                                                                lim
                   der




                                                                              un
                       dev




                                                                            ll y
                                                                          ua
                          elo




                                                                                                              refining



                                                                      i rt
                                                                                                                              existing;




                                                                    ,v
                             pm




                                                                                                                              26.000 ton/y




                                                                  nt
                                                                                                              (silanes)
                                ent




                                                                   e
                                                                pm
                                   ,
                                 vir




                                                             elo                           under
                                     tua




                                                           ev
                                                                                           development
                                                         rd
                                        lly




                                                                                           high cost
                                                       de
                                            un


                                                     un
                                              lim




                                                                                                                     eg-Si
                                              ited




                                                                                                 semi-prime
                                       solar grade Si                                            4500 ton/y

                                  demand                                                         off-grade
                                                                                                                          semiconductor
                                  13.000 ton (2004)                                              2000 ton/y                  industry
                                  ~200.000 ton (2020)




23
     Feedstock production
     • Direct route: SOLSILC process

     • plasma furnace: SiC from pure SiO2 and pure C
      pellets of SiC and SiO2          Si(L)

                                       CO          SiO2
                                                   SiC


                                                          SiO+SiC   2Si+CO
                                                          Si+SiO2   2SiO




                                                          Si(liq)


24
     Feedstock: ingot growth
     • Multicrystalline Si
       ingot growth




25
     Feedstock: effect impurities
                                  dissolution evaporation from melt
     • Feedstock
        - Melting
        - Crystallization                                             Decreasing:
     • Ingot                                                                [Oi]
                                                                      Increasing:
        - Sawing                                                            dopant conc.
                                       segregation
     • Wafer                                                                metal conc.
                                                                            [Cs]



                                            contamination from crucible
                            small grain bottom



26
     Feedstock: effect impurities
      feedstock               ingot         wafer
                     ppmw
          Fe         1.E+01



                     1.E+00



          Ti         1.E-01


          Al         1.E-02
       (tentative)
                     1.E-03



                     1.E-04



                     1.E-05



                     1.E-06

                                      85%
27
                Feedstock: effect impurities
                    Impurities added to feedstock
               35

               30

               25
                                                                       Effect Ti and O on cell output
Jsc (mA/cm2)




               20
                                                                       clearly visible
               15
                                clean reference ingot
               10               10 ppmw Ti and high O
                                10 ppmw Ti and high Fe, C
               5

               0
                    0     20       40          60           80   100
                               position in ingot (%)


28
     Feedstock: effect impurities
     Impurities added to feedstock
     • Ingot growth                                                                                                         14.5% cell techn.
     • Wafering                                                                                                             17% cell techn.




                                                           relative to high-purity feedstock
                                                                                               1.0
     • Cell processing


                                  cell efficiency (%rel)
     • Characterization
     • Model development                                                                       0.9
                                                                                                             Higher                      our results:
        - 1/Leff2∝1/τ ∝Cimp
                                                                                                             efficiency                  5 ppmw Al or
        - Segregation                                                                          0.8                                       10 ppmw Ti
            during growth                                                                                       reduction
        -   Solar cell modeling
                                                                                               0.7
                                                                                                      1           10           100
     • Needed to define                                                                              impurity concentration (a.u.)
       Solar Grade Si

29
                      Wafer technologies
                Pulling of Single Crystals                            Casting of Silicon Blocks                                                   Bridgman Solidification
                      (Czochralski)


     heating                       single crystal
                                                                                                                  columnar
                                                                                                                  crystallised silicon   inductive heating                                        liquid
                                                                                                planar liquid /                                                                                   silicon
                                                                                                solid interface




                                                            Inductive heating
                                                                                                                                         columnar                                             liquid / solid
                                                            liquid Silicon                                                               crystallised silicon                                 interface



          crucible                   liquid silicon




                  Heat Exchange Method                       Edge defined Film fed Growth                                                     Ribbon Growth on Substrate
                                                                                          pulling speed
                                                                                                                                                    speed of
                                                                                                                                                    solid/liquid       casting frame    Si foil
     liquid / solid                                                                                                                                 interface plane
     interface                                                                                        Si foil
                                              liquid
                                                                                                      (octagon)
                                                                                                                                                             Vk
                                              silicon            solid/liquid
                                                                 interface plane
                                             columnar
                                             crystallised
        heating
                                             silicon


                                                                                                    casting frame
                                                                                                                                                                                          casting speed
                                 heat sink                                                                                                                                                          V   Z
                                                                                   liquid silicon
                                                                                                                                                liquid silicon        solid/liquid         Substrate
                                                                                                                                                                      Interface plane




30
     Wafer technologies
     • Multicrystalline Si
       ingot growth




31
     Wafer technologies
     • From ingot to
      mc-Si wafer




32
     Wafer technologies

     • High quality monocrystalline Si material
        - Low impurity concentration
        - Low defect concentration
        - Higher efficiency (15-17% in industry, 20% pilot)
        - Higher costs per cell

     • Lower quality multicrystalline Si material (mc-Si)
        - Higher impurity concentration
        - More defects
        - Lower efficiency (13-15% in industry, >16% pilot)
        - Lower costs per cell

     • For both technologies: high sawing losses (about 50%!)

33
     Wafer technologies
     • Ribbon technologies
      (multicrystalline Si)

     • Substrate growing and
      crystallization in the same
      direction




                                    Edge defined Film-fed Growth   String Ribbon
                                    (SCHOTT Solar)




34
     Wafer technologies
     • ECN’s ribbon technology
      Ribbon Growth on Substrate
      RGS

     • Substrate growing
      perpendicular to
      crystallization Casting frame
                       (cut)
                                                Gassing      Annealing



                                    Direction
                                                                           Finished foils



                            Preheating              Continuous substrate




35
     Wafer technologies
     Ribbons:
     • Better use of Si material (about factor 2)
     But
     • Lower initial material quality
     • Lower efficiencies
                                                Material      Pull   Through- Furnaces
                                                             Speed      put    per 100
     • EFG/SR: about 14% (industry)                                      2
                                                            [cm/min] [cm /min]  MW
                                                EFG            1.7      165      100
     • RGS: about 13% (lab)
                                                SR             1-2     5-16     1175
        - Very high throughput                  RGS           600      7500      2-3
                                                *[J. Kalejs, E-MRS 2001 Strasbourg]




36
     Wafer Technology
     RGS cell efficiencies using industrial process
       • Average efficiency 12.5%.
       • Current top efficiency 13%confirmed
       • High efficiency lab processing 14.4%confirmed

     • ~100 µm thin RGS wafer made
        • Efficiency around 11%
        • 2.9 g Si/Wp (nowadays ~10 g Si/Wp)




37
     Cell processing
     • Saw damage removal
        - Texturing for enhanced light coupling
            (better efficiencies)
     •   Emitter diffusion
          - Material improvement by gettering
     •   SiNx deposition as antireflection layer
          - Material improvement by passivation
          - Reduced surface recombination
            (surface passivation)
     •   Metallization
          - Ag front side
          - Al rear side (so-called Back Surface Field)
     •   Sintering for contact formation

38
        Cell processing
        Batch processing
        • Wafers in carriers
        • Each process step well controlled
        • Used for high efficiency processing

        etching     junction      ARC           contacts




 load                                                      unload


             transport
39
     Cell processing

     ECN’s inline processing
     Horizontal wafer transport on belts (wafer in; cell out)
       • No wafer carriers
       • Large and thin wafers easier to handle (cost reduction)




              etching        junction         ARC          contacts




40
     Cell processing

     Examples from industry
     Batch processing BP Solar




     In-line processing SCHOTT Solar




41
     Cell processing

     ECN Baseline process
       • Multicrystalline p-type Si
       • Acidic texturing / saw damage removal
       • P diffusion using belt furnace
       • Deposition of SiNx                       Wet chemical etching
       • Metallization (Ag front, full Al rear)
       • Simultaneous sintering both contacts

     Results
     Processing complete columns of wafers
     during two years
       • Average 16%
       • In industry about 15%
                                                   Sintering contacts
42
     Texturing
     Acidic texturing of mc-Si




                                 Alkaline and acidic etch




43
     Texturing
     Acidic texturing of mc-Si

                            HNO3 drip
     exhausts                              controller
                                            belt speed
        inlet

                              rinse
                etch bath             destaining
     cooler                                        rinse
                                                           drying
                                                                    outlet




44
     Texturing
     Acidic texturing of mc-Si

     • Lower reflection, higher efficiency
        - About 0.5% absolute
     • Better appearance




45
     Texturing
     Surface structure texturing Si

     • Monocrystalline Si
        - Alkaline etching (NaOH or KOH)
        - Anisotropic etching
           - (111) planes slowest etching rate
           - Pyramids on (100) substrates


     • Multicrystalline Si
        - HF/HNO3 etching
        - Isotropic etching
           - Random structure


46
     Texturing
                                                Micro structure
     Alkaline etching of Si
        Si + OH– + H2O      SiO32– + H2 gas

     • Higher concentrations and higher T
        - Almost isotropic etching
        - High etching rate
        - Used to remove saw damage (5-10 µm)
                                                Full size wafer
        - High reflectance (~30%)




47
     Texturing
     Alkaline etching of Si

     • Lower concentrations and lower T
        - Anisotropic etching
           - (111) planes slowest etching rate
           - Pyramids as texture on (100) substrates
           - Low reflectance (~10%)
        - But, low etching rate




48
     Texturing
     Acidic etching of Si

     Mixture HF/HNO3

     Oxidation
       3 Si + 4 HNO3   3 SiO2 + 4 NO gas + 2 H2O
     Oxide removal
       SiO2 + 4 HF   SiF4 gas + 2 H2O

     • Obtained surface morphology depends on
       composition
        - Polishing
        - Defect etching
        - Texturing

49
     Emitter processing
     Needed to form p-n junction
     • Apply P source
     • Diffusion at ~900 C for about 10 minutes                     16.5




     • Depth about 0.5 µm                                           16.0




                                                   efficiency (%)
     • P concentration at surface: > 2×1020 cm-3
        - Higher concentration needed
          for good contacting                                       15.5


        - However, it will result in additional
          recombination losses                                      15.01.00E+02   1.00E+03   1.00E+04   1.00E+05   1.00E+06   1.00E+07

                                                                                              emitter losses


     Improved emitter/front side processing can give an efficiency gain of
       more than 0.5% absolute

50
                           Emitter processing
                           Effect dopant concentration on IQE
                           • Improved blue response (up to 550 nm) for lower dopant
                             concentration
                           • Higher Voc and higher Jsc: higher efficiency!

                        1E+21
                                                                                  Thicker diffusion barrier
atomic density [cm-3]




                                                     barrier
                                                                         1.0
                                                     no barrier
                        1E+20
                                                                         0.8

                                                                   IQE
                                                                                                            20%
                        1E+19                                            0.6
                                                                                                            15%
                                                                                                            10%
                                                                                        no barrier          5%
                                                                                                            0%

                        1E+18                                            0.4
                                0   0.1     0.2       0.3    0.4            350      400     450      500     550
                                    depth in silicon [um]                               wavelength [nm]

              51
     Emitter processing
     Additional effect of emitter processing

     • So-called gettering
        - Diffusion of impurities to P rich layers (P-gettering)
        - Impurities will not affect efficiency in those P rich layers

     • Improved bulk quality and, thus, higher efficiency


                                                  diffusion
                                  impurity



52
     SiNx deposition
     Applied using chemical vapour deposition
     • Low pressure chemical vapour deposition (only surface
       passivation, ~700 C)
     • Plasma enhanced chemical vapour deposition
       (different systems, ~400 C, 0.5-10 nm/s)
     • Sputtering (several nm/s)

     Functions SiNx:H layer
     • Antireflection coating (70-80 nm)
     • Surface passivation (reduced recombination at the surface)
     • Bulk passivation (improved material quality)
        - During anneal H diffuses into bulk and makes defects/impurities
          electrically inactive

53
     SiNx deposition
     Plasma Enhanced Chemical Vapour Deposition (PECVD)
     • Parallel plate system
       Direct plasma
        - Wafers as electrodes
        - Ion bombardment dependent
          on plasma frequency
        - Damaged layer

                                                          Aberle et al.

     • Remote PECVD
        - No ion bombardment




54
     SiNx deposition
     ECN MicroWave Remote PECVD
     • Deposition rate about 1 nm/s



                                                NH3 or N2


                                                     MW plasma




                                         SiH4


                                      substrates

55
     SiNx deposition
     Expanding Thermal Plasma (ETP)
     • Developed by TU/e
     • Deposition rate 5-10 nm/s
     • TU Delft: for thin film Si depositions       argon gas inlet



                            Plasmabron              electrode
                                                    isolating plates
                            Expansie plasma
                                                }   cascade plates (4x)


                                                    anode plate
                                                    nozzle with NH3 injection

                                                    Ar/NH3 plasma expansion
                            Substraat
                                                    SiH4 injection ring




56
     SiNx deposition
                                                SiNx: n1; d1   Air: n0
     Optical specifications SiNx:H layer
     • Refractive index: n=2.1
       higher n causes absorption at lower wavelength          Si: n2

                                     λ0
       n1 = n 0 n 2           d1 =
                                     4 n1

     • Ideal for air-Si: n=1.9; d=~80 nm
     • Ideal for air-glass-Si: n=2.3; d=~65 nm
       (absorption SiNx too high)

     • n can be tuned with gas composition
     • Higher n: more Si (SiH4)
     • Lower n: more N (NH3)

57
     SiNx deposition
     Optical specifications SiNx:H layer
     • Different layer thickness: different colour




58
                      Gettering and bulk passivation (emitter and SiNx:H)
                      Improved bulk quality using gettering and passivation
                      • Lifetime>100 µs will hardly affect cell efficiency (diffusion length 2
                        times cell thickness)
                      • Besides higher efficiency, gettering and passivation will result in a
                        narrower efficiency distribution.
                     120                                                                               18
                               emitter and double
                     100       sided H passivation                                                     17


                     80                                                                                16




                                                                                      efficiency (%)
     lifetime (µs)




                                                               emitter and H
                     60                                        passivation (SiNx:H)                    15

                     40                                                                                14

                     20                              initial                                           13

                      0                                                                                12
                           0        20        40          60           80       100                         1   10                   100   1000
                                                                                                                     lifetime (µs)
                                          position ingot (%)

59
     Metallization
     Screen-printing process and sintering in belt furnace




60
     Metallization
     Principle screen-printing process
     • Metallization paste is ‘pressed’ through pattern in screen
     • Paste contains metal particles and oxides (etches Si at higher T)




                  Metallization line

61
     Metallization
     Ag front side metallization
     • Fine line metallization printed through patterned screen




62
     Metallization
     Fine line printing
     • Reduced shading losses
     • Contact resistance might be critical




       emulsion line opening (∼100µm)         fine line   slumping
            wire gauze
63
     Metallization
     • Other techniques:

     • Plating (electroless)

     • Dispensing



     • Pad printing




     • Roller printing


64
     Metallization
     Al rear side (Back Surface Field to reduce recombination at surface)
     • After sintering step (around 800 C, few seconds) highly doped layer
     • Better BSF when thicker and higher doped




65
     Efficiency ECN process

     Results ECN Baseline process
     Processing two complete columns (different ingots) of wafers during 2 years
       • Average 16.0%
       • In industry about 15%
                      17.0
                                               column 2004/5
                      16.5                     column 2005/6     Wafer size: 125x125 mm2
     efficiency (%)




                      16.0                                       Thickness: ~300 µm
                      15.5                                       Material: p-type mc-Si
                      15.0

                      14.5

                      14.0
                             0   100    200      300       400
                                  position in ingot

66
               Efficiency ECN process
               • High-efficiency (17%) in-line process (300 µm thick; 156 cm2 mc-Si)
                  - 50 cells processed (best efficiency 17.1%; average 16.8%)
                  - Module made using cover glass with ARC
                    Full area efficiency 14.8%; encapsulated cell eff: 16.8%

             12

             10

              8
                                                                                VOC=22.2 V;
                                                                                ISC=5.76A;
     count




              6
                                                                                FF=0.738;
              4                                                                 P=94.3 Wp
              2

              0
                          .5


                                  .6


                                          .7


                                                  .8


                                                          .9


                                                                  .0


                                                                            0
                  .5




                                                                         7.
                       16


                               16


                                       16


                                               16


                                                       16


                                                               17
                16




                                                                       >1
             =<




                                        efficiency class
67
                      Efficiency ECN process
                      • From 2000 up to now



                 18
                                                                                                          100
                                                                          17

                                                             16
                                                                                     ECN 2005
                                                                                                           80
Efficiency (%)




                 16                                                                                                                                   rear side
                                                                                                                                                      reflection
                                                   14.9




                                                                                                IQE (%)
                                                                                                           60

                 14                     13.7                                                                                           bulk and
                                                                                                           40
                                                                                                                                       rear side
                      12.5
                                                                                                           20
                 12                                                                                             front side
                                                                                                                and emitter
                                                   BSF




                                                          and emitter
                                        lifetime




                                                                        management
                                                          passivation
                      initial (<2000)




                                                           front side




                                                                                                            0
                                                                           light




                                                                                                             300       500       700       900     1100
                                                                                                                              wavelength (nm)




         68
     Future improvements
     Thin wafers
     • Rear side critical

                                                  1.2E+14
                                                                                                                  15.7%
                                                                Front                                 Rear        15.3%
     Minority carrier                             1.0E+14
                                                                                                                  15.9%
       density                                                                                                    17.0%
                                                  8.0E+13
     • Combination of             density (cm )                                                                   14.3%
                                  -3
                                                                               Good bulk quality
       generation and                             6.0E+13
       recombination                                              Low bulk quality
                                                                                                           Good rear side
                                                  4.0E+13                                                     li
     17.0%: good bulk and rear
     15.9%: good bulk, low rear
     15.7%: low bulk, good rear                   2.0E+13
                                                                Thin cell                                  Low rear side
     15.3%: low bulk and rear
     14.3%: as 15.9%, but thin                    0.0E+00
                                                            0         50       100     150   200     250    300
                                                                            Distance from front (µm)

69
     Future improvement
     Al rear side (Back Surface Field to reduce recombination at surface)
     • 17% reached on 300 µm thick wafers
     However:
     • Bowing for thinner wafers
     • Recombination losses too high for high efficiencies (>18%)
     • Internal reflection too low (~70%) for high efficiencies




70
     Future improvements rear side
     Thin wafers
     • Rear side critical (bowing, reflection, BSF)
     • New rear side processing using for example SiNx
        - Higher efficiencies for thinner wafers
                                                          17.0
                                                                                     modelling
                                                          16.5




                                          efficency (%)
                                                          16.0



                                                          15.5
                                                                                    current rear side
                                                                                    future rear side

      SiNx for rear side passivation                      15.0
                                                                 0   100          200           300
      Local rear contacts / BSF                                        cell thickness (µm)


71
     Future improvements rear side
     Thin wafers
     • New rear side processing using SiNx
        - 16.4% obtained by ECN with baseline-like processing
        - About 1% absolute higher than reference with Al BSF
          (obtained efficiency depends on Si material quality)




72
     Future improvements
     Thin wafers (less dependent on material quality)
     • Improved light management
        - Texturing
        - Light trapping
     • Improved emitter (reduce losses)
     • Perfect surface passivation
        - Both surfaces
     • Less metallization losses
        - Series resistance (contact and line resistance)
        - Reduced shading losses

     20% mc-Si cell efficiency should be possible! (long term)


73
     Other industrial cell concepts
     Laser Grooved Buried Contacts
     BP Solar
     • Monocrystalline




     Rear side contacted cell
     SunPower
     • ~20%!
     • High quality
       and expensive
       material


74
     Other industrial cell concepts
     SunPower
     • Cell 21.8%
     • n-type material
     • Module: full area 18.1%



     Sanyo
     • HIT cell: 21.8%
     • n-type material
     • Emitter deposited




75
     Record efficiencies
     Monocrystalline (4 cm2):       24.7%
     Monocrystalline (149 cm2):     21.5%
     Multicrystalline (1 cm2):      20.3%
     Multicrystalline (137 cm2):    18.1%

     ECN multi (156 cm2):            17.0%
     Single layer ARC; homogeneous emitter; inline processing
       Highest efficiency with completely inline processing




76
     Module technology
     Conventional module technology (soldering)

                                                  Series connection
                    Glass

                    EVA

                    Solar cells


                    EVA

                    Tedlar foil               Interconnection strips (tabs)




77
      Module technology
      Conventional module technology




     interconnection                   lamination




78
     Module technology

     Pilot-line tabber-stringer for interconnection




79
     Module technology
     Pilot line to be built at ECN

     Fully automated and realibility-tested interconnection process for
     back-contact cells and suitable for thin and fragile cells




80
      Module technology
      New module technology:                  MWA
      • New cell designs needed
         - Back contacted
         - Simple interconnection
         - Can be used for thin cells




                                                    EWT



     MWT
                                        PUM


81
     Module technology
     Emitter Wrap Through:
     • No metallization on the front
     • Thousands of holes




82
     Module technology
     ECN’s PUM concept:
     • More energy from attractive cells
     • 2-3% less shading
     • Resistance losses independent
       on cell size (only on size unit cell)
     • Standard cell processing except:
        - Laser drilling holes
        - Junction isolation around holes


     Mother Nature’s
     water lily



83
            Module technology
            ECN’s PUM concept:
            • Single shot interconnection
              and encapsulation


Single step module assembly

                              Glass plate

                                 PUM cell

                              Adhesive




Rear side foil



    84
     Module technology
     ECN’s PUM process:
     • Foil preparation
     • Apply conductive adhesive
       instead of soldering
       (lower stress)
     • Pick and place cells
     • One step curing
       and encapsulation




85
     Module technology
     ECN’s PUM result:
     • Full size module (71×147 cm2)
     • 128 Wp (15.8% encapsulated
       cell efficiency)
     • 0.6-0.8% absolute efficiency gain

     Best PUM cell result up to now:
     • 16.7% (225 cm2)



     At this moment PUM is the
      only integrated concept for
      cell and module

86
         PV market
         Annual market growth more than 40%


                                  PV Cell/Module Production

            2000
             1800                                                           ROW
             1600                                                           Europe
             1400                                                           USA
             1200                                                           Japan
         MWp 1000                                                           Total
              800
               600
               400
               200                                                               Growth rate 2004: 67%
                 0
                                                                    Japan
                                                                                 2005: 1720 MWp (+44%)
                    92

                           94




                                                                   ROW
                                  96
                  19




                                         98
                         19




                                                00
                                19




                                                       02




                                                                                 Japan: ~50% market share
                                       19




                                                              04
                                              20

                                                     20

                                                            20




Photon International, 2006

   87
         PV market
         More than 90% crystalline Si technology

                                    100%                                6.2    5.9    6.5
                                                  10.1    9.6    7.3
                                            13
                                    90%
          Technology market share




                                    80%
                                    70%
                                           46.2   52.5          56.2           58    55.2
                                                         55.8          61.5
                                    60%
                                                                                            thin film
                                    50%                                                     multi and ribbon Si
                                                                                            mono Si
                                    40%
                                    30%
                                    20%    40.8   37.4   34.6   36.4          36.2   38.3
                                                                       32.2
                                    10%
                                     0%
                                           1999   2000   2001   2002   2003   2004   2005
                                                                Year
Photon International, 2006

   88
     PV market
     Expected market: solar the most important primary energy source




                                              PV and solar
                                              thermal power




        Wissenschaftliche Beirat 2003

89
     Costs PV
     Contributes wafer is about 45%!
     Thinner wafers, or better ribbons, important!

     Price solar electricity:
     0.20-0.50 €/kWh
                                                                       sg-Si
       (depending on location)                              9%
                                                                       wafer
                                                     25%
                                                                       cells
                                                                       module

     NL: ~0.50 €/kWh
                                                                 36%




                                                      30%




90
     Cost reduction PV
     • Less material use
        • Thin ribbons
        • Less module materials
     • High efficiencies for the same process costs
        • Advanced processing
        • New cell design
     • Easy manufacturing
        • Automation
        • Easy module manufacturing
     • High lifetime
     • Improved yearly system output



91
     Cost reduction PV
     • Expected costs

                             1000 GWp worldwide
                         200 GWp in EU (200,000 jobs)
                    5                               BOS
        Typical     4
         typische                                   modules
        turn-key
         turn-key   3
        system
        systeem-
                    2
        price
           prijs
          (€/Wp)    1
        (€/Wp)
                    0
                        2004   2010   2020   2030   2050

                                        jaar




92
     Cost reduction PV
     • Expected costs based on learning curves (EU project Photex)
        - Combined effect of technology development, experience, ….
        - Progress ratio PR should be around 80%
           [2001 $]                                     Power Modules (1976-2001)
            100

                      1976




                                                                          1983

                                                                                           1990

             10                                        1981


                                                                           1987

                                                                                                           2001
                             Price of Power Modules (2001 $)

                             Estimate 1976 - 2001: PR = 80.0±0.4%

                             Estimate 1987 - 2001: PR = 77.0±1.5%
              1
                  0                      1                     10                 100               1000          10000
                                             Cumulative Shipments [MW p] power modules (SU, 2003)



93
     Cost reduction PV
     • Expected costs                                                     €/kWh
                                                                               1.0
                                                                                                                       900 h/a:
                                                                                                                     0,60 €/kWh
                                                                               0.8
                                                                                                                         1800 h/a:
                                     Photovoltaics                                                                      0,30 €/kWh
                                                                               0.6
                                     Utility peak power
     • Solar competitive             Bulk power                                0.4
       between 2010-2020
                                                                               0.2

                                                                               0.0
                                                                                   1990               2000       2010          2020        2030   2040


                           Source: RWE Energie AG and RSS GmbH


                                Towards an Effective European Industrial Policy for PV.ppt / 05.06.2004 / Rapp   @ RWE SCHOTT Solar GmbH




94
     Environmental aspects
     • Energy Pay Back Time 2005


                                                                Energy Pay-Back Time
                                                              (grid-connected, roof-top PV system;
                                                            irradiation 1700 resp. 1000 kWh/m2/yr)
            Energy Pay-Back Time (yr)




                                        5
                                        4
                                        3                                                                   BOS
                                        2                                                                   frame

                                        1                                                                   laminate

                                        0
                                            ribbon ribbon            multi multi            mono mono
                                            S-Eur. M-Eur.           S-Eur. M-Eur.           S-Eur. M-Eur.




95
     Environmental aspects
     Energy Pay Back Time 2005 and 2010+
     • Low energy consumption especially for Solar Grade Si
     • Low material use
       (abundance)
     • High efficiency           3.00




                                 Energy Pay-Back Time (yr)
     • High lifetime modules     2.50

     • Environmental friendly    2.00
                                                                                                                          BOS
       processes                 1.50                                                                                     frame
     • Recycling                 1.00                                                                                     laminate


                                                             0.50

                                                             0.00
                                                                    ribbon    multi   mono   future ribbon future multi
                                                                    11.5%    13.2%    14%        15%           16%




96
     Conclusions
     • Solar Grade Silicon needed for growing market
        - Effect of impurities on cell efficiency should be known
     • Less Si use with ribbons
     • Improved processing has led to 17% mc-Si efficiency using in-line
       processing
     • New processes for thin wafers/ribbons under development
     • Integrated cell and module design like PUM needed
     • High module lifetime
     Then
     • Cost reduction possible
        - Will be competitive with bulk electricity price
     • Energy Pay Back Time can be reduced to <1 year
     • Solar energy will be the most important primary energy source in
       2100

97
     Applications at ECN




98
     Applications




99
  Thank you for your kind attention


                         Information / internship
                         www.ecn.nl
                         weeber@ecn.nl
Floriade (2.3 MWp PV)
 100

				
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