West SESAPS 2006 poster

1
Thermally activated radiative efficiency
enhancement in a GaAs/GaInP heterostructure Brant
West and Tim Gfroerer, Davidson College Mark
Wanlass, National Renewable Energy Laboratory,
Golden, CO Supported by the American Chemical
Society Petroleum Research Fund
Luminescence Spectra of Low-Band Gap Sample
Abstract When electron-hole pairs are generated
in a semiconductor, recombination proceeds via
radiative and nonradiative events. We measure
the radiative efficiency as a 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 GaAs0.86P0.14/GaInP heterostructure,
we observe a systematic decrease in efficiency
with increasing temperature as predicted by a
simple model. Assuming a temperature-independent
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.
Motivation Lattice-Mismatched Multi-Junction
Solar Cells
Efficiency Results Band Gap Energy 1.43 eV
Some Basic Semiconductor Theory
As hypothesized, a sub-bandgap (SBG) peak
approximately 0.15 eV below the band-to-band
(B-B) recombination is present.
Any photon energy exceeding the band-gap energy
of the semiconductor is lost in the form of heat,
decreasing the conversion efficiency.
Integrated SBG Intensity vs. 1/kT
Absorption of Light in Multilayer Cell
In general, the efficiency should increase with
increasing carrier density and decrease with
increasing temperature.
Efficiency Results Band Gap Energy 1.65 eV
In the 1.43 eV structure an increase in radiative
efficiency is observed from 77K-120K before the
expected decrease in efficiency ensues.
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
Stacking several different semiconductors on top
of one another allows for more efficient
conversion of the broad incident spectrum.
Experimental Setup

• Conclusions and Future Work
• - We observe an unexpected increase in radiative
  efficiency with increasing temperature.
• - We propose thermal depletion of nonradiative
  defect levels as a possible explanation.
• - Temperature-dependent sub-bandgap transitions
  seem to support this hypothesis.
• 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.)

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.
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.
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.