Full Solar Spectrum Photovoltaic MaterialInGaN

1
Full Solar Spectrum Photovoltaic Material-InGaN

• The direct energy gap of InGaN spans nearly the
  entire energy range of the solar spectrum.
• Multijunction solar cell based on this single
  ternary system could have higher efficiencies.
• Advantages of using InGaN
• Flexibility in choosing the number and the
  bandgaps of junctions to optimize the solar cell
  performance.
• Fabrication process could be greatly simplified
• Superior radiation resistance solar cells
  operated in outer space.

2
Progress and Outstanding Issues

• InN
• Energy gap 0.64 to 0.67 eV _at_ RT
• Mobility electrons (2200 to 4000 cm2/Vs), holes
  (40 cm2/Vs)
• Minority carriers (holes) lifetime (gt 1 to 2 ns)
  diffusion length ( 1mm)
• Doping n-type (3x1017 to 1021 cm-3), p-type (mid
  1019 cm-3 ?)
• Surface electron accumulation (2x1013 to 4x1013
  cm-2)
• InGaN (or
  InAlN)
• Band gap bowing InGaN (1.4 to 1.8 eV), InAlN
  (2.7 to 5.5 eV)
• Small Stokes shift
• Doping n-type should be easier than p-type
• Composition modulation effects
• Surface electron accumulation in In-rich alloys
  (xGalt0.66)
• Charge control at heterointerfaces

3
Solutions??

• No charge accumulation at heterointerfaces
• - low gap cell (InN n/p junction, Eg0.7 eV)
  high gap cell (In0.34Ga0.66N n/p junction,
  Eg2.2 eV)
• Compositionally graded n/p junction from xGa0 to
  0.66
• Heterointerfaces with large work function p-type
  semiconductors (e.g. CuInS2)
• Elimination of the accumulation layer through
  surface treatments

4
Point-Counter Point

• Advantages
• Massive span in PV energies for high efficiencies
  in one material system
• Nitrides generally offer recombination
  insensitivity to dislocations
• Strong band bending is perfect for low surface
  recombination velocity

• Disadvantages
• Massive span in PV energies is not in a lattice
  matched system
• High dislocation density
  .
• Strong band bending has resulted in inability to
  form a solid state junction
• P-type doping undemonstrated
• Tunnel junctions probably not possible
• P-type base is usually preferred due to higher
  mobility of minority electrons
• 3-junction high concentration solar cells are
  already in excess of 37.2 efficient
  (GaInP/GaAs/Ge Spectrolabs King et al)

5
Low-energy optical transitions
plateau
transition near 1.3 1.4 eV
onset (Eg 0.7 eV)
F. Bechstedt et al., phys. stat. sol. (a) 195,
628 (2003)