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Ultrahigh-Efficiency Solar Cells

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Ultrahigh-Efficiency Solar Cells Powered By Docstoc
					            presented at Fermilab, April 17, 2002




              GaInP/GaAs/Ge
         Ultrahigh-Efficiency
             Solar Cells
D. Friedman, J. Geisz, A. Kibbler, C. Kramer, S. Kurtz,
       B. McMahon, J. Olson, A. Ptak, M. Young

          III-V Materials and Devices Group

       National Renewable Energy Laboratory

                *NREL
               with Spectrolab Inc.
             *NREL – Research Programs
Started in 1977 (as SERI)
1100 staff, mostly in Golden
CO

Renewable-energy research:

•   basic materials
•   photovoltaics
•   wind energy
•   building technologies
•   advanced vehicles
•   solar thermal electric
•   hydrogen
•   superconductivity
•   geothermal power
•   resource assessment
                 Solar Cell Technologies

PARAMETERS                   EXAMPLES
substrates                   glass, crystal wafer
materials                    silicon, CuInSe2, GaAs
amorphous vs crystalline     amorphous silicon vs. GaAs
fabrication method           evaporation, MOCVD


                   Best one-sun Efficiencies:

                  amorphous silicon     13%
                  CuInSe2               19%
                  crystalline silicon   25%
                  GaInP/GaAs/Ge         31%

highest efficiency... also most expensive
          photon                materials                   alloy
        conversion            (III-V alloys)              ordering
         efficiency

  solar               multi-Eg           GaInP/GaAs                  GaInP/GaAs and
spectrum                                   tandem                    GaInP/GaAs/Ge
                      device              concept                       realization




                       demand                   need
$$$                   for higher                1-eV
                     efficiencies              material
        space
      applications
                                 4-junction               development       status at
                                  designs                 of GaInNAs        present…
       terrestrial
      applications



      basic materials science — devices — applications
              Photon Conversion Efficiency

                                               hν


                              p
                              n                           Eg




• hν > Eg: absorbed                                                      Eg



                                  Conversion Efficiency
  hν - Eg wasted (heat)



                                                          not absorbed
• hν < Eg: transmitted
                                                                                  lost as heat
⇒ Conversion to energy
 ideally efficient only for
 hν = Eg
                                                                          Photon Energy
                          Solar Spectrum


The sun is not a laser!                         photon energy (eV)




                             solar spectrum (AM0)
                                              4        1.4    1




                                     dNph/dλ
                                                      color scheme for illustration only!
               Voltage vs. Current Tradeoff
                                  photon                      photons not
                                  energy                      absorbed
What is the optimal Eg            lost as
 for solar cell?                  heat
                            25%                               Efficiency

Power = Volts * Amps

Increasing Eg ⇒
  • increased voltage
  • decreased current
                                                                   Volts
… and vice versa …

inherent limit on single-                                         Current
  junction solar cell
  efficiency
                              0                 1.4 eV
                                            Solar cell band gap
  Solution to Spectrum Problem: Multi-Bandgaps

Divide up spectrum                          photon energy (eV)




                         solar spectrum (AM0)
 into little regions                      4        1.4    1




                                 dNph/dλ
Convert each region
 with a band gap
 tuned for that region

Sounds great in
 principle…

                         solar
                         cells
                                                 color scheme is only for illustration!
          photon                materials                   alloy
        conversion            (III-V alloys)              ordering
         efficiency

  solar               multi-Eg           GaInP/GaAs                  GaInP/GaAs and
spectrum                                   tandem                    GaInP/GaAs/Ge
                      device               concept                      realization




                       demand                   need
$$$                   for higher                1-eV
                     efficiencies              material
        space
      applications
                                 4-junction               development        status at
                                  designs                  of GaInNAs       present…
       terrestrial
      applications
                        Materials Issues

Must work with available materials!

• Band gap

• Lattice constant (if epitaxial)

note - materials like Si, GaAs etc offer limited selection of these


• ease of growth

• materials quality
III-V Alloys: Band gap and Lattice Constant

                      2.4 GaP
                                      AlAs     Ga0.5In0.5P
                      2.0
                                                           AlSb
      Band gap (eV)

                      1.6
                                 GaAs
                      1.2                    InP

                      0.8                                  GaSb
                                 Ge
                      0.4                                          InSb
                                                    InAs
                       0
                           5.4    5.6      5.8      6.0      6.2    6.4
                                        lattice constant (Å)

epitaxy: materials must be at or near lattice-matched
             Designing a Realistic Device

                                                      Tandem Cell Efficency (%) vs Bandgaps
• want monolithic two-
  terminal device                                       30
                                                  2.0          32
• lattice-matched to




                         top-cell band gap (eV)
                                                                        34
  standard substrate
                                                  1.9
• making many-junction                                                36
  device would be very                            GaInP                                 *
  difficult
                                                  1.8       37%              32
  Let’s look at two                                                           30
  junctions:                                              *
                                                        ideal                           22   18
             hν                                                                    26
                                                  1.7                                 GaAs
                                                     1.1         1.2        1.3       1.4
          GaInP                                                 bottom-cell band gap (eV)
          GaAs
              Alloy Ordering and Band Gap
III-V epitaxial alloys (GaInP,              Ga
   InGaAs) tend to show
   cation site ordering                     In

                                            P
ordering is surface-driven

ordering depends on growth
  conditions (Tg, etc)

more ordered ⇒ lower Eg

Can be significant:
 >0.1 eV lowering for
 Ga0.5In0.5P

Gives additional degree of
 freedom in Eg selection
 GaInP/GaAs/Ge Cell Schematic

               hν
                      antireflection
                      coat
                         front grids


                    GaInP Top Cell
                    Eg = 1.85 eV
                                                 tunnel
                                                 junction
  5 µm
               GaAs Middle Cell
               Eg = 1.42 eV
                                                 tunnel
                     Ge third cell; Eg=0.67 eV   junction


200 µm              Ge substrate

                     back contact


monolithic, two terminals — very important!
                          GaInP/GaAs/Ge Cell Development Timeline
                         40                                                                                       *
                                                                commercial             production
                                             R&D100 award:                             levels reach
                                             GaInP/GaAs         production of
                                                                tandem                 400 kW/yr *
                                             2-junction cell
                                                                                                 *
                         30                                                     *        *
 AM1.5G Efficiency (%)




                                                                       *
                                                       *                                                     4-junction
                                                                           tandem-                           device
                                                                           powered      commercial
                                                                           satellite    licensing of
                                                  *   terrestrial                       tandem
                         20                                                flown
                                                      concentrator
                                                      company                                        R&D 100 award:
                                                      collaborations           4-junction cell
                              invention of                                     proposed              GaInP/GaAs/Ge
                              GaInP/GaAs              started                                        3-junction cell
                              tandem
                         10                   *

                                      *      patent
                                             issued
                                                                                                     *NREL
                         0       *
                                     1985    1995     1990  2000          2005
                                          Year
several of the most recent record efficiencies were set by Spectrolab Inc
          photon                materials                   alloy
        conversion            (III-V alloys)              ordering
         efficiency

  solar               multi-Eg           GaInP/GaAs                  GaInP/GaAs and
spectrum                                   tandem                    GaInP/GaAs/Ge
                      device               concept                      realization




                       demand                   need
$$$                   for higher                1-eV
                     efficiencies              material
        space
      applications
                                 4-junction               development        status at
                                  designs                  of GaInNAs       present…
       terrestrial
      applications
 Economic Considerations: What are the Markets?

• III-V multijunction solar cells are most efficient


• However, epitaxial III-V technology is very expensive:
  – substrates
  – epitaxy (complex structures)
  – processing


So, useful where the high efficiency is valuable
                Markets: Space / Satellites

High cell efficiency   GaAs/Ge cells
means less weight
and smaller size:
critical for space
applications!

GaInP/GaAs/Ge
cell is now the
preferred cell for
most space
applications

Yearly roduction rate (in the US) is
~300–800 kW/year (~$100–200 million/year)

Production capacity is ~1 MW/year
                    ...might seem like a small number...
        Terrestrial Applications: Concentrators

Small, efficient, expensive
 cell + large cheap
 concentrating optics (lens
 or mirror)

⇒ potential economic
 viability for terrestrial
 applications (in its infancy)




Remember the ~1 MW/yr production capacity?
At 1000x concentration, it corresponds to ~1 GW!
          photon                materials                   alloy
        conversion            (III-V alloys)              ordering
         efficiency

  solar               multi-Eg           GaInP/GaAs                  GaInP/GaAs and
spectrum                                   tandem                    GaInP/GaAs/Ge
                      device               concept                      realization




                       demand                   need
$$$                   for higher                1-eV
                     efficiencies              material
        space
      applications                 next-
                                                          development        status at
                                 generation                                 present…
       terrestrial                designs                  of GaInNAs
      applications
           Next-Generation III-V Solar Cells

Add a fourth junction      calculated AM0         AM0 solar spectrum
                           efficiencies (ideal):       dNph/dλ
                            35%              41%




                                                                        4
Need new material:




                                                                        photon energy (eV)
                           GaInP          GaInP
                           1.8 eV         1.8 eV
• 1 eV




                                                                            1.8 1.4
                           GaAs           GaAs
• lattice-matched          1.4 eV         1.4 eV
                                         new
  to GaAs / Ge                           material
                           1.0
                          Ge eV          1.0 eV




                                                                                      1
                          0.7 eV          Ge
                                          0.7 eV
• also, good opto-           in            future
  electronic            production     generation
  properties                                   color scheme is only for illustration!
                GaInNAs for 1-eV Junction
Want
• 1-eV band gap
• lattice-matched to GaAs (for materials quality)

                                               1.3         Ga1-xInxNyAs1-y




                               band gap (eV)
 Kondow noted that
 Ga1-xInxNyAs1-y                               1.2
 satisfies these criteria
 (with x≈3y and y≈3%)                          1.1

 Lots of interest for                          1.0
 lasers
                                                     0    1      2      3
                                                         N content y (%)
For us: obvious candidate for 1-eV junction
Adding N to the GaInAsP Materials System

                       GaN
                  3

                                       GaP
                  2
  band gap (eV)




                                             GaAs    InP
                  1

                                                           InAs
                  0


                  -1

                       4.5     5               5.5     6
                             lattice constant (Å)
           GaInNAs Electronic Structure: N level
Interaction between conduction band and narrow resonant N-
  derived state
from: Walukiewicz et al, LBL, Phys. Rev. Lett. 82, 1221 (1999)

• EM is energy of GaInAs                             2.4
                                                         Ga0.95In0.05N0.012As0.988
                                                     2.3
  CB edge                                            2.2
                                                         295 K

• EN is energy of N level                            2.1                                                  EMΓ
                                                     2.0
                                                                        E+
                                                     1.9
• EM and EN interact to                              1.8                                            EMX


                                       Energy (eV)
  give observed E– and                               1.7
  E+                                                 1.6 EN




                                                                                PR signal intens.
                                                     1.5                                                   4.5 GPa
                                                                                                     E–     E+
                                                     1.4
                                                     1.3
                                                                   E–
                                                     1.2
                                                     1.1                           1.4 1.6 1.8 2.0 2.2
                                                                                       Energy (eV)
                                                     1.0
                                                           0   1    2    3   4 5 6 7 8 9 10 11
                                                                             Pressure (GPa)
                 GaInNAs - the Bad News

GaInNAs diffusion lengths are
 very small                                        0
 ... so why do we care?...
                                                   -2




                                current (mA/cm )
                                2
• interesting semiconductor                        -4
  physics question                                               "good"
                                                   -6
                                                                 GaInNAs
• need longer diffusion                            -8
                                                                 junction
  lengths for the solar cell
                                          -10
                                                                        (ideal device)
                                          -12
                                                        0.0   0.2     0.4      0.6
                                                               voltage (V)
      Possible Causes for Low Diffusion Length



Structural?               don’t see anything in TEM


Impurity?                 have tried purifying source, but
                           not conclusive
Native defect?            experimental and theoretical
                           evidence for N interstitials


electronic structure?     band fluctuations due to GaN
                           clustering?
                       Present Status

• Studying poor minority-carrier properties of GaInNAs


• Other 1-eV materials?
  e.g. have tried GaTlP… that one made GaInNAs look good!




                  result of attempting to grow GaTlP
         Summary



                        improved
                        solar cell




semiconductor   solar cell
  properties    properties

				
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