Photocapacitance measurements on GaP alloys for high efficiency solar cells

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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
GaAsP Results
Motivation Multi-junction solar cells
Abstract
Large bandgap materials are an essential
component of ultra-high efficiency solar cells.
While multi-junction devices harness more of the
solar spectrum for electricity production,
challenges in the construction of a
lattice-mismatched system lead to defects that
trap charge carriers and inhibit overall
efficiency. Characterization of these defects
may enable device engineers to minimize their
effects. This work uses photocapacitance
measurements to obtain optical escape energies
from defect levels in high-bandgap GaInP and
GaAsP alloys. We combine these optical results
with previous thermal measurements to construct a
working model that incorporates a lattice
configuration-dependent energy structure. The
model explains why the optical escape energy is
significantly larger than the thermal capture and
escape energies, and in the GaAsP device, it
helps explain why the number of escaping carriers
depends on the energy of the incident light.
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.
GaInP Results
Photocapacitance Optically Stimulated Escape
As the device cools, charge carriers become
frozen into the defects (thermal capture), which
extends the depletion region and causes the
capacitance to decrease. The charge carriers
escape when the device is exposed to light. On
the warming cycle the thermal capture occurs at a
lower temperature than the thermal escape (the
thermal capture energy is smaller than the
thermal escape energy).
Depletion Layer
77 K

PHOTON
N
P
Optical Escape
Depletion without bias
If higher energy photons are absorbed in higher
bandgap alloys, the heat loss caused by excess
photon energy relative to the gap is reduced.
Deep level transient spectroscopy (DLTS) employs
transient capacitance measurements on diodes
during and after the application of a bias pulse
to monitor the capture/emission of carriers
into/out of defect-related traps. The
photocapacitance experiment is a modified version
of DLTS that uses light (rather than heat) to
excite the charge carriers out of traps.
The optical escape threshold energy of 0.695 eV
is significantly higher than the thermal escape
energy of 0.322 eV for GaInP found in previous
work. These results support the model described
below.
Optical Cross Sections
Photocapacitance Experimental Setup
Valence Band
Working Model for Observed Threshold Energies
Enlarged View
High Energy Light
Low Energy Light
While stacking materials of different bandgaps
will increase the efficiency of solar cells, it
will also create defects within the device
because of lattice-mismatching.
Defect Levels
Thermal Capture (0.370 eV)
Conduction Band
Thermal Escape (0.394 eV)
When the energy of the light is reduced, the
optical escape rate decreases as expected, but
the transient amplitude is also reduced. The
reduced amplitude indicates that carriers in some
traps are not optically activated by the lower
energy light. This result implies the defects in
our GaAsP device have a range of energies as
shown in the diagram above. For a given defect
energy, the optical cross section, which control
the optical escape rate, depends on the energy of
the incident light. The diagram also shows how
low energy light only activates traps in the
upper portion of the energy range.
PHOTON
PHONON


Defect Levels
Thermal Escape
Optical Escape
Trap Depth
Increasing Energy For Holes
Capture
The computer operates the temperature controller
and retrieves data from the digital oscilloscope.
The pulse generator applies the reverse and
pulse biases to the sample while the capacitance
meter reads the resulting change in capacitance
as a function of time. During this process, the
sample is illuminated with a particular
wavelength of light from the monochromator.

Valence Band
Hole
In the configuration-dependent model, the energy
of the system changes as a function of the local
configuration of the lattice. This means that
lattice vibrations, or phonons, can push the
defect level closer to the valence band, but
photons, which carry very little momentum , have
no effect on the defect level. As a result, the
optical escape energy is much larger than the
thermal escape and capture energies.
Acknowledgements
Defects provide energy levels that restrict the
movement of charge carriers. This inhibits the
production of electricity. Once a carrier
becomes trapped, it must be excited out of the
defect state either thermally or optically.
We thank Jeff Carapella for growing and
processing the test structures. We also thank
the Donors of the American Chemical Society
Petroleum Research Fund and the Davidson Research
Initiative for supporting this work.