solar cell operation
DESCRIPTION
Mapping free carrier diffusion in GaAs with radiative and heat-generating 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 ~. -. -. -. - PowerPoint PPT PresentationTRANSCRIPT
Mapping free carrier diffusion in GaAs with radiative and heat-
generating recombination
Tim Gfroerer and Ryan CrumDavidson College, Davidson, NC
with Mark WanlassNational Renewable Energy Lab, Golden,
CO
~ Supported by the American Chemical Society – Petroleum Research Fund ~
Solar Cell Operation
Conduction Band
Valence Band
PHOTONEN
ER
GY
ELECTRON
E-Field
E-Field
HOLE
E-Field
E-Field
+ +
++
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-
-
CURRENTABSORPTION
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.
HEAT
Light- and Heat-Generating Recombination
Electrons can recombine with holes by releasing light or heat.This loss mechanism also reduces the efficiency of a solar cell.
Conduction Band
Valence Band
DefectLevel
-
+
HEAT
HEAT
Conduction Band
Valence Band
EN
ER
GY
Photon
-
+
Rate ≈ A x n (n = carrier density)Rate ≈ B x n 2 (n = carrier density)
Experimental Setup
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Laser spot ~4 mm diameter
GaAs sample (plan view)
ThermalCamera
LuminescenceCamera
0 100 200 300 400
10-3
10-2
10-1
100
0-33 ms 33-67 ms 67-100 ms 100-133 ms
Tem
pera
ture
Diff
eren
ce (
K)
Distance (m)
Time Window:
Time evolution of thermal profile
Laser on!
Heat loss
Thermal diffusion
Luminescence and Thermal Profiles
0 100 200 300 40010-5
10-4
10-3
10-2
10-1
100
Laser ExcitationLight EmissionT (Heat)
Nor
mal
ized
Lig
ht o
r H
eat
Sig
nal
Distance From Excitation Position (m)
Square-root of the Luminescence
0 100 200 300 40010-5
10-4
10-3
10-2
10-1
100
Laser ExcitationLight Emission
(Light Emission)1/2
T (Heat)
Nor
mal
ized
Lig
ht o
r H
eat
Sig
nal
Distance From Excitation Position (m)
Rate ≈ B x n 2
Rate ≈ A x n
Free-Carrier or Thermal Diffusion?!
0 100 200 300 40010-5
10-4
10-3
10-2
10-1
100
Laser ExcitationLight Emission
(Light Emission)1/2
T (Heat) Thermal Diffusion
Nor
mal
ized
Lig
ht o
r H
eat
Sig
nal
Distance From Excitation Position (m)
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!