Tricks and traps in the latest recordefficiency solar cells Tim Gfroerer Davidson College, Davidson,

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Tricks and traps in the latestrecord-efficiency
solar cells  Tim GfroererDavidson College,
Davidson, NCwith Mark WanlassNational
Renewable Energy Lab, Golden, CO Supported by
the American Chemical Society Petroleum
Research Fund
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Experiments and Analysis by . . .
Malu Fairley (Spelman 03)
Brant West (08)
Patten Priestley (03)
Peter Simov (08)
Adam Topaz (08)
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Outline

• Semiconductors, solar cells, and defects
• Radiative efficiency and dependence on defect
  level distributions
• Diode capacitance and the DLTS experiment
• Non-exponential behavior and a new model for
  carrier transport during DLTS

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Semiconductors
Periodic Potential Physlet
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Solar Cell Operation
Conduction Band
E
-
Field
E
-
Field
HEAT
ELECTRON
ENERGY
ABSORPTION
CURRENT
PHOTON
HOLE
E
-
Field
E
-
Field
Valence Band
When a photon is absorbed, an electron is excited
into the conduction band, leaving a hole behind
in the valence band.  Some heat is lost, reducing
efficiency. Then an internal electric field
sweeps the electrons and holes away, creating
electricity.
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The Solar Spectrum at the Surface of the Earth
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A Trick Multi-Junction Solar Cells
Higher energy photons are absorbed in higher
bandgap alloys, reducing the heat loss caused by
excess photon energy relative to the gap.
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A Trap Lattice Matching
Growing a stack of defect-free alloys usually
requires lattice matching. The dashed vertical
line is a common triple-junction lattice target.
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The Solar Spectrum with Triple-Junction Bandgaps
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A Trick Lattice-Mismatched InGaAs
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Semiconductor Defects
Lattice-Mismatch Applet
Defect Level Physlet from Physlet Quantum
Physics An Interactive Introduction by Mario
Belloni et al. (2006).
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Defect-Related Trappingand Recombination
Conduction Band
Defect Level
HEAT
ENERGY
HEAT
Valence Band
Electrons can recombine with holes by hopping
through defect levels and releasing more heat.
This loss mechanism also reduces the efficiency
of a solar cell.
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One More Trick Step-Grading

Typical sample structure (not to scale).
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Equilibrium Occupation in a Low Temperature
Semiconductor
Holes
Electron Trap
Hole Trap
Electrons
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Photoexcitation
Photon
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Photoexcitation
Photon
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Photoexcitation
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Photoexcitation
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Band-to-Band Radiative Recombination
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Band-to-Band Radiative Recombination
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Band-to-Band Radiative Recombination
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Electron Trapping
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Electron Trapping
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Defect-Related Recombination
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Defect-Related Recombination
Note Sub-bandgap photons may also be emitted
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Defect-Related Recombination
Note Sub-bandgap photons may also be emitted
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Luminescence Spectra
Shifted Vertically For clarity
Radiative recombination can reveal defect-related
transitions that lie below the usual band-to-band
(B-to-B) emission.
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Thermally Activated Escape
E
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Thermally Activated Escape
E
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Radiative Efficiency
heat
light in
light out
light in heat light out radiative efficiency
light out / light in
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Photoluminescence Experimental Setup
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Some data with conventional theoretical fits

• Assumptions
• Defect levels clustered near the middle of the
  gap
• no thermal excitation out of traps
• ( of electrons) ( of holes) n
• Theoretical Efficiency

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A Better Model and a Different Plot

• Improvements
• Defect level distribution can be tailored to
  achieve the best fit
• Theory accounts for thermal excitation out of
  traps
• ( of e-s in conduction band) n can differ from
  
• ( of holes in valence band) p
• Theoretical Efficiency

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Defect-Related Density of States
Valence Band
Conduction Band
The distribution of defect levels within the
bandgap can be represented by a density of states
(DOS) function as shown above.
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The Defect-Related Density of States (DOS)
Function
Conduction Band
Defect States
Energy
Ev
Ec
Energy
Valence Band
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New Theoretical Fit With Improved Defect Level
Distribution Analysis

• Improvements in fit
• Asymmetric DOS produces shallow slope at low
  carrier concentration
• Thermal activation out of traps gives comparable
  temperature dependence
• References (students in red)
• T. H. Gfroerer, L. P. Priestley,
• F. E. Weindruch, and M.W. Wanlass,
• Appl. Phys. Lett. 80, 4570 (2002).
• A. Topaz, B. A. West, T. H. Gfroerer,
• and M. W. Wanlass, Appl. Phys. Lett.
• 90, 092110-1 (2007).

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DLTS Experimental Setup
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p/n Junction Formation

P
N
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Bias-Dependent Depletion

P
N
Depletion Layer
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Diode Capacitance
d1
No bias
Vbuilt-in
C DQ/DV eA/d
ENERGY
d2
Vbuilt-inVapplied
Reverse bias
Reverse bias increases the separation between the
layers where free charge is added or taken away.
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Defect characterization via DLTS

P
N
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Typical DLTS Measurements
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Device Structure and Band Diagram

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Exponential transient analysis
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Reciprocal Analysis
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Hopping between traps

N
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Hopping between traps

N
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Hopping between traps

N
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Hopping between traps

N
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Hopping between traps

N
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Hopping between traps

N
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Discussion of DLTS Results

• Non-exponential transient rates are incompatible
  with conventional thermal activation analysis
• Reciprocal of the capacitance varies linearly
  with time, and the slope yields a single thermal
  activation energy
• Hopping? (thermally-activated reciprocal
  behavior is a characteristic of hopping
  transport).
• Test dependence on transport distance by varying
  magnitude of pulse. (in progress!)

DLTS Reference

• T.H. Gfroerer, P.R. Simov ('08), B.A. West ('08),
  and M.W. Wanlass, 33rd IEEE Photovoltaics
  Specialists Conference (to be presented in May,
  2008).

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Conclusions

• Further improvements in solar cell efficiency
  will depend on better lattice-mismatched designs
• Lattice-mismatch introduces defects which can
  degrade solar cell performance
• Understanding the impact of defects will
  facilitate better designs
• Photoluminescence and DLTS are powerful tools for
  characterizing defect properties in semiconductors