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
Capacitance vs Temperature
Capacitance (nF)
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.
GaInP Results
Photocapacitance: Optically Stimulated Escape
T = 77K
GaAs bandgap
0.75
0.50
Visible
0.25
+
-
+-
-
+
-
+-
P
+ +
+
++ - Optical Escape
+
+
0.00
400
800
1200
1600
2000
2400
Wavelength (nm)
If higher energy photons are absorbed in higher bandgap alloys, the heat loss
caused by excess photon energy relative to the gap is reduced.
Depletion without
bias
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.
Photocapacitance Experimental Setup
Optical Escape
-11
10
100
150
200
250
300
Temperature (K)
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).
975nm
1350nm
1150nm
1050nm
1800nm
Example Raw Data
850
800
750
Capacitance (pF)
- - - - - - N+ - - --
1.25
Cross Section (cm )
1.75 eV GaAsP
1.00
Depletion Layer
77 K
2
-2
-1
Solar Spectral Irradiance (Wm nm )
2.0 eV GaInP
Thermal Escape
50
Cross Sections GaInP
1.50
Thermal Capture
-12
10
EThreshold = 0.695 (eV)
0.7
0.8
0.9
1.0
1.1
1.2
1.3
Energy (eV)
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.
Working Model for Observed Threshold Energies
700
650
600
550
500
450
0
20
40
60
80
100
Optical Cross Sections
Time (s)
Valence Band
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.
High Energy 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.
0.60
0.58
0.56
0.54
0.52
0.50
0.48
0.46
0.44
0.42
0.40
0.38
0.36
0.34
0.32
Low Energy Light
Motivation: Multi-junction solar cells
Defect Levels
Conduction Band
Thermal Capture (0.370 eV)
Capture
Trap Depth
+
Hole
Defect
Levels
+
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.
Optical Escape
+
Thermal Escape
Increasing Energy For Holes
Thermal Escape (0.394 eV)
Valence Band
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.
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.
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
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.