Mapping free carrier diffusion in GaAs with radiative and heat- generating recombination

advertisement
Mapping free carrier diffusion in
GaAs with radiative and heatgenerating recombination
Tim Gfroerer and Ryan Crum
Davidson College, Davidson, NC
with Mark Wanlass
National Renewable Energy Lab, Golden, CO
~ Supported by the American Chemical Society –
Petroleum Research Fund ~
Solar Cell Operation
Conduction Band
-
E-Field
-
HEAT
ELECTRON
ABSORPTION
PHOTON
CURRENT
HOLE
-
Valence Band
+
E-Field
+
+
+
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.
Light- and Heat-Generating Recombination
Conduction Band
ENERGY
-
Photon
Conduction Band
-
Defect
Level
HEAT
HEAT
+
+
Valence Band
Valence Band
Rate ≈ B x n 2 (n = carrier density)
Rate ≈ A x n (n = carrier density)
Electrons can recombine with holes by releasing light or heat.
This loss mechanism also reduces the efficiency of a solar cell.
Experimental Setup
Laser spot ~
4 mm diameter
-
+
+
+
+
-
-
+
-
+
+-+ -
-
+
Luminescence
Camera
+
GaAs sample (plan view)
-
Thermal
Camera
-
-
+
Time evolution of thermal profile
0
Temperature Difference (K)
10
Time Window:
Laser on!
Heat loss
-1
10
0-33 ms
33-67 ms
67-100 ms
100-133 ms
Thermal diffusion
-2
10
-3
10
0
100
200
Distance (mm)
300
400
Luminescence and Thermal Profiles
0
Normalized Light or Heat Signal
10
Laser Excitation
Light Emission
T (Heat)
-1
10
-2
10
-3
10
-4
10
-5
10
0
100
200
300
Distance From Excitation Position (mm)
400
Square-root of the Luminescence
0
Normalized Light or Heat Signal
10
Laser Excitation
Light Emission
1/2
(Light Emission)
T (Heat)
Rate ≈ A x n
-1
10
-2
10
-3
10
Rate ≈ B x n 2
-4
10
-5
10
0
100
200
300
Distance From Excitation Position (mm)
400
Free-Carrier or Thermal Diffusion?!
0
Normalized Light or Heat Signal
10
Laser Excitation
Light Emission
1/2
(Light Emission)
T (Heat)
Thermal Diffusion
-1
10
-2
10
-3
10
-4
10
-5
10
0
100
200
300
Distance From Excitation Position (mm)
400
Conclusions
• We use optical and thermal imaging to
map the free-carrier density near a
localized photo-excitation source.
• The density profiles agree when we
account for the bimolecular nature of
radiative recombination.
• BUT: a thermal diffusion calculation also
mimics the temperature profile …
• So what have we measured?!
We’ll figure it out!
Download