Transcript West SESAPS 2006 poster

Thermally activated radiative efficiency
enhancement in a GaAs/GaInP
heterostructure*
-
Heat
ENERGY
ELECTRON
E-Field
-
GaAs0.86P0.14/GaInP heterostructure, we observe a systematic decrease in efficiency with increasing temperature as predicted by a simple model. Assuming a temperatureindependent rate of nonradiative defect-related recombination, the decrease in radiative efficiency is attributed to the theoretical decrease in the band-to-band (B-B)
radiative rate. In contrast, we observe an increase in radiative efficiency with temperature between 77K and 120K in a 1.43 eV bandgap GaAs/GaInP heterostructure. Above
120K, the efficiency levels off and then slowly decreases as the temperature is raised to 300K. We hypothesize that a defect level lies close in proximity to one of the
bands, such that the thermal energy at low temperatures is insufficient to activate trapped carriers to the band where they can participate in B-B recombination. Above
120K, the thermal energy is sufficient to facilitate these transitions. Low-temperature, sub-bandgap spectra reveal a weak, radiative defect-related transition approximately
0.15 eV below the B-B emission, which subsides with increasing temperature. An Arrhenius plot of the escape rate yields an activation energy of approximately 0.09 eV.
These energies are comparable, but the magnitude of the difference suggests that a more sophisticated model may be required to fully explain our results.
ELECTRON
-
1.0
Valence Band
+
HOLE
+
+
+
E - Field
light out
light in
radiative_ rate
B(T )n
efficiency 

2
total _ rate
An  B(T )n
Absorption of Light in Multilayer Cell
High Band-Gap
A = defect-related recombination coefficient
B = radiative coefficient (B decreases with increasing Temp)
n = photoexcited carrier density
Conduction Bands
ENERGY
Med. Band Gap
0.6
Valence Bands
-
Stacking several different semiconductors on top of one another allows
for more efficient conversion of the broad incident spectrum.
Experimental Setup
0.8
0.6
1.3
10
Integrated SBG Intensity vs. 1/kT
21
22
10
23
10
10
9.3
3.6
1.1
3
2
W/cm
2
W/cm
2
W/cm
10
avg slope = 93 meV
2
10
escape_ rate  e
 Ea / kT
-3 -1
recombination rate (cm s )
90
105
120
135
150
This Arrhenius plot of the thermal quenching of the SBG emission
indicates that the defect level is approximately 0.09 eV below the
band edge. Deviation from the spectral analysis suggests that a
more sophisticated model may be required.
A Possible Explanation
Low Temp.
High Temp.
Conduction Band
0.4
0.2
10
1.6
1/ kT (1/eV)
0.0
23
10
24
10
- - - - - Defect Related
Recombination
kT
Defect
Level
Conclusions and Future Work
Conduction Band
-
-
-
ELECTRON
-
Radiative
Recombination
- We observe an unexpected increase in radiative
efficiency with increasing temperature.
kT
- We propose thermal depletion of nonradiative
defect levels as a possible explanation.
- Temperature-dependent sub-bandgap transitions
seem to support this hypothesis.
25
10
-3 -1
recombination rate (cm s )
The laser light is incident upon the semiconductor sample, producing
luminescence. We collect this emitted light and focus it onto a photodiode
for efficiency measurements, or into the spectrometer for spectral analysis.
10
1.5
As hypothesized, a sub-bandgap (SBG) peak approximately
0.15 eV below the band-to-band (B-B) recombination is present.
0.2
20
1.4
Energy (eV)
In the 1.43 eV structure an increase in radiative efficiency is observed
from 77K-120K before the expected decrease in efficiency ensues.
Temperature:
effic77K
effic120K
effic165K
effic207K
effic250K
effic300K
22
SBG
0.4
19
ENERGY
-
Efficiency Results:
Band Gap Energy = 1.65 eV
radiative efficiency
-
0.15 eV
10
0.0
1.0
3
2
In general, the efficiency should increase with increasing
carrier density and decrease with increasing temperature.
Low Band Gap
77K
90K
102K
120K
10
effic77
effic120
effic165
effic207
effic250
effic290
0.8
2
Any photon energy exceeding the band-gap energy of the semiconductor
is lost in the form of heat, decreasing the conversion efficiency.
4
10
Temperature:
HOLE
+
B-B
10
radiative efficiency
ELECTRON
2
1
light in = heat + light out
radiative efficiency = light out / light in
heat
CURRENT
Excitation = 3.6 W/cm
Temperature:
Efficiency Results:
Band Gap Energy = 1.43 eV
Some Basic Semiconductor Theory
PHOTON
ABSORPTION
5
10
SBG Integrated Emission
Conduction Band
Luminescence Spectra of LowBand Gap Sample
PL Intensity (a.u.)
Motivation: Lattice-Mismatched
Multi-Junction Solar Cells
Brant West and Tim Gfroerer, Davidson College
Mark Wanlass, National Renewable
Energy
Laboratory,
Golden,
CO
Abstract
When
electron-hole
pairs are generated
in a semiconductor,
recombination
proceeds viaSociety
radiative and nonradiative
events. We measure
the radiative efficiency
as a
*
Supported
by
the
American
Chemical
–
Petroleum
Research
Fund
function of laser excitation intensity and temperature to explore recombination mechanisms in alloys that may be useful for multi-junction solar cells. In a 1.65 eV bandgap
In the 1.65 eV band-gap energy sample, the downward shift in
radiative efficiency with increasing temperature is readily observed.
The solid curves are fits using the theory described above.
Valence Band
Valence Band
A possible explanation for this increase in radiative efficiency with
temperature is thermal excitation from a nonradiative defect level. The
presence of this level may be evident in the luminescence spectrum.
- A more sophisticated model may be required to
fully explain the results …
(See SESAPS abstract CB.00008: Modeling
defect level occupation for recombination
statistics by Topaz, et al. for more information.)