Transcript Photocapacitance measurements on GaP alloys for high efficiency solar cells

Photocapacitance measurements
on GaP alloys for
high efficiency solar cells
Dan Hampton and Tim Gfroerer , Davidson College, Davidson, NC
Mark Wanlass, National Renewable Energy Lab, Golden, CO
Acknowledgements
Davidson Research Initiative
American Chemical Society Petroleum Research Fund
Jeff Carapella for growing and processing test structures
Solar Cell Operation
Conduction Band
Energy
E=hc/λ
+
Valence Band
Conduction Band
Energy
E=hc/λ
+
Valence Band
Electron-Hole Pair
Energy
Conduction
Band
Conduction
Conduction Band
Band
+
Valence Band
Conduction
Band
Conduction
Conduction Band
Band
Energy
Band Edge
+
Valence Band
Heat
Conduction
Band
Conduction
Conduction Band
Band
Energy
Band Edge
Ephoton – Band Gap Energy = Energy Lost
+
Valence Band
-2
-1
Solar Spectral Irradiance (Wm nm )
2.0eV GaInP
1.50
1.75eV GaAsP
1.25
GaAs bandgap
1.00
0.75
0.50
Visible
0.25
0.00
400
800
1200
1600
Wavelength (nm)
2000
2400
Defects
Defect Levels
Trap Depth
Increasing Energy
Conduction Band
Conduction Band
+
++
Hole
+
+
Valence
Valence Band
Band
+
Defect Levels
+
Capture
Trap Depth
Increasing Energy
Conduction Band
Conduction Band
+
++
Hole
+
+
Valence
Valence Band
Band
Defect Levels
+
Escape
Trap Depth
Increasing Energy
Conduction Band
Conduction Band
+
++
Hole
+
+
Valence
Valence Band
Band
1 mm
Picture of Device
Photocapacitance
- - - - - - - N+ - - - -
+
+
+
+
+
-
-
- +
-
P
-
Depletion with bias
+
+
+
+
+
Photocapacitance
- - - - - - - N+ - - - -
+
+
+
+
+
-
-
-
Depletion without bias
- +
+
P
-
+
+
+
+
Depletion Layer
- - - - - - N+ - - - - - --
+
-
-
+-
+
+P
+ +
+
++ -
Depletion without bias
+
+
Depletion Layer
T = 77 K
- - - - - - N+ - - - - - --
+
-
+-
+
+
+
-
-
+
-
+
Depletion without bias
+-
P
+-
+
+
Depletion Layer
T = 77 K
- - - - - - - N+ - - - - - - - --
+
+
+
+
+
-
-
-
Depletion without bias
- +
P
+
+
- +
+
+
T = 77K
2
Cross Section (cm )
Optical Cross Sections GaInP
-11
10
= A*(h-ETh) /(h)
5/2
3
-12
10
EThreshold = 0.695 (eV)
0.7
0.8
0.9
1.0
1.1
Photon Energy (eV)
1.2
1.3
Capacitance (nF)
Capacitance vs Temperature GaAsP
0.60
0.58
0.56
0.54
0.52
0.50
0.48
0.46
0.44
0.42
0.40
0.38
0.36
0.34
0.32
Thermal Capture
Thermal Escape
Optical Escape
50
100
150
200
250
Temperature (K)
300
Conclusions
• Optical threshold escape energies are higher than
thermal threshold energies → Configuration
Dependent Traps
• GaAsP has a very large trap density, comparable to
the density of dopants. This suggests zinc is
contributing to the formation of the traps.