GaSb Infrared Cells and Modules for Energy Applications
Seth Hettinger1, Lewis Fraas1, Ben Francis1, Jim Avery1
1JX
Crystals Inc. 1105 12th Ave NW, Issaquah, WA 98027
Seth Hettinger hettinger.seth@gmail.com
Abstract:
Thermal photo voltaics is a promising technology that converts infrared heat radiation
into useable electrical power. Gallium antimonide infrared cells can be used to convert heat
radiation into electricity. The cells can be designed into a variety of circuit designs and modules
that would encompass an infrared source to meet the needs of various applications. The heat
radiation of the application can come from a variety of sources as well, ranging from creating
your own heat source with a burner and making a generator to an industrial heat source such
as hot steel billets. The GaSb cells can also offer solar power 24 hours a day with efficiency
levels of 40% with a solar satellite power beaming an infrared laser to a ground array of GaSb
cells. These applications offer clean renewable energy for the planet.
1. Introduction:
Silicon and GaAs related cells are well known solar cells. They convert visible sunlight
into electricity, but they only operate during the daylight hours. Paths to higher efficiency and
nighttime photovoltaic cell operation are desirable. As shown in figure 1, in 1986, the desire for
higher solar efficiency led to the invention of the infrared sensitive GaSb cell as a means to
increase solar cell efficiencies. This concept was improved upon in 2020 with the 44% efficient
four junction cell shown in figure 2.
Figure 1: GaAs/GaSb Stacked Cell
for Higher Efficiency (1986)
Figure 2: 2020: 44% Efficient 4 Junction Solar Cell
Predan, Dimroth, et al PVSC 2020 Combines Fraas 1978 3J GaInP/GaAs/GaInAs Cell with Fraas
1989 2J GaAs/GaSb Stack Cell to make GaInP/GaAs/GaInAs/GaSb 44% 4J Solar Cell [2]
The new GaSb cell then enabled various Thermal PhotoVoltaic (TPV) systems which can
operate at night and for 24 hours per day if needed. TPV is the concept of converting infrared
radiation, commonly known as heat, into electrical energy. Using TPV GaSb cells is then a great
avenue for providing clean renewable electrical power. GaSb cells absorb infrared radiation and
convert it into electrical energy. There are many systems where these cells can be applied as
heat can be supplied from a multitude of sources.
2. GaSb Cells
2.1 P on N Junction Cells
GaSb cells are fabricated using n type tellurium doped single crystal GaSb wafers as a
substrate. The wafers go through a zinc diffusion process creating a p type layer and a p on n
junction. An anti-reflective layer of SiN is applied. The SiN layer is then pattern etched exposing
sections of the GaSb creating openings for grid lines. The wafer then receives a metal
deposition on both the front and back. The front metal is Ti/Pd/Ag and is the positive side of
the cell. The back metal is Sn/Pd/Ag and is the negative side of the cell, figure 3a shows the
structure of the cell. Excess metal is removed on the front of the wafer leaving only metal on
the grid lines and a bus bar for soldering on contacts. This process is simple, and it resembles
the Si solar cell process and can be low cost when scaled up in production. Figure 3b shows
current I vs voltage V flash test result for a P on N GaSb cell in a TPV application.
Fig. 3a: P on N junction GaSb cell structure
Fig 3b: Current I (Amps) vs voltage V (Volts) flash test result for a P on N GaSb cell in a TPV
application. Power is in Watts.
There are several factors that affect the efficiency of the GaSb cells that come from both
the cell and the infrared source. The factors that come from the cell are the IR quantum
efficiency IQE, the temperature of the cell, and the cell band gap. The cell absorbs IR radiations
with wavelengths shorter than 1.8 microns. To get the best efficiency the cell must be kept at a
temperature around 30o C and the IQE is typically 85% and higher [1]. The variables of the
infrared source are the power density and the spectrum that it emits.
2.2 N on P Junction Cells
Inverting the junction of GaSb cells to n on p could offer improved efficiency to the cells
and higher power output. The p on n junction cells IQE typically does not surpass 85% with
wavelengths greater than 1000 nm while an n on p junction cell can theoretically achieve an IQE
of 95% [3]. A large part of the improvement is because p-GaSb material has a longer electron
diffusion length for photon generated minority carriers as is shown in the figure 4.
Fig. 4 Electron diffusion length n vs. p type GaSb cell
The n on p junction GaSb cell was prepared by MOCVD for the 4-junction cell reported
in Figure 2. However, MOCVD may not be the most optimal way of making an n on p GaSb cell.
A heterojunction cell consisting of a p-GaSb substrate with an N+ transparent conducting oxide
layer could prove to be much lower cost which figure 5 shows a basic design of the cell
structure.
Infra Red
Top Grid
Contact
N+ TCO
P GaSb
Back Side
Contact
Fig. 5 Heterojunction N on P cell structure with N+ transparent Conductive Oxide Coating (TCO)
3. Applications
The challenges for GaSb cells now are to find the first niche application and to reduce
costs by scaling up production. Fig 6 shows possible applications.
Fig 6: With IR core tech, launch one application.
3.1 Combined heat and Power for the home - Midnight Sun Stove
The Midnight Sun Stove is a completed TPV unit that is designed to generate both heat
and power. It is designed to be used off the grid in small cabins. The stove is powered by a
burner that used natural gas. It has two circuits inside of it with cells that are mounted in a
shingle style. Each circuit produces 150 W of power for a total of 300 W shown by the IV curve
of one of the circuits in figure 7. The internal temperature of the IR emitter in the stove is 1200o
C. Each circuit consist of 72 GaSb cells and is 5 cm x 26 cm. The electric power produced by the
GaSb cells is 150 W/130 cm2 = 1.15 W/cm2.
Fig 7: The JX Crystals Inc Midnight Sun Stove for combined heat
and power uses two 72 GaSb cell 150 W shingle circuits.
The problem with this stove product is that in low volume production (kWs), it is too
expensive for customers to buy. Volume production is required. This leads potentially to the
steel industry where one customer can potentially buy a MW of product.
3.2 Waist Heat Recovery - TPV for Electric power from Hot steel
One industry that shows potential for using GaSb TPV cells to generate power is the
steel industry. As steel is heated it reaches temperatures over 1100 o C and is left to cool. All this
heat energy is lost to the air and is not being used. The thermal energy can instead be
converted into electrical power by placing a module containing GaSb cells around the hot steel
billets to absorb this energy shown in figure 8. This would provide electricity 24/7 as steel is
manufactured 24/7. The modules would provide a high-power density of 1 W/cm2. It is
projected that a single steel mill could output 20 MW of power and worldwide 10 GW of power
could be produced. The steel mill would be able to produce power for itself as well as the
surrounding area. This application would be a clean and renewable energy source. The payback
could be a short time of 1.5 years at $1 per watt.
Fig. 8 The left image shows a steel billet with 3 GaSb modules surrounding it. The right image is
what the modules could look like and their dimensions. The module could use 9 stove shingle
circuits
The problem for this application is that funding for an initial demonstration is required.
This leads next to the potential for a lightweight quiet battery replacement for military
applications because the military can afford the development costs.
3.3 Light Weight Fuel-Fired TPV DC Cylindrical Generator
A small portable fuel fired TPV cylindrical power generator has been built under an
Army Research Lab contract. The inside of the cylinder contains a Power Converter Array
(PCA) (Fig. 9) of GaSb cells that encompass a Burner Emitter Recuperator (BER). The BER
system consists of a burner which heats an emitter to 1200o C. The emitter is made of
NiO/MgO that has an emissions band that peaks at 1.6 microns which is near ideal for the
GaSb cells. The PCA has fins on the back side. The fins and cells are cooled with a fan.
Fig. 9: Light Weight Battery Replacement - 20 W or 50 W TPV Cylindrical Generator
TPV circuits for this application are described in a companion paper. The power
converter array (PCA) consists of 2 Power Converter Modules (PCM). Each PCM consist of 54
cells. The design consists of cells on 9 facets with fold lines so that a cylindrical PCA can be
made by combining two PCM’s that each make a half circle. Each facet has 6 cells in two groups
of triplet cells. The cells in each triplet are wired in parallel and the top 9 triplets are wired in
series as are the bottom 9 triplets creating 2 strings of 9 triplets per PCM. The PCM wiring
configuration is designed to accommodate the cylindrical emitter which is assumed to be
hottest in the middle and falling off in temperature toward either end.
Each PCM can output 50 watts of power. With 2 PCMs making up a PCA that is inside of
the cylindrical generator, 100 watts of power output can be achieved with an emitter
temperature of 1300o C. Multiple cylinders can be combined to increase the power output of
the generator.
3.3 Solar Satellite Power Beaming an IR Laser to TPV Cells
A very exciting use of these IR GaSb cells could be for solar power satellites [4][5]. A
solar power satellite in geosynchronous orbit could laser beam power 24 hours a day to 40%
efficient GaSb cells on the ground. The satellite would be equipped with 40% efficient
multijunction InGaP/GaInAs/GaSb concentrator cells that have been demonstrated in figure 2
[2]. These cells would be in large wing arrays attached to the satellite. The satellite would have
an eye safe infrared laser attached to it that would emit a beam with a wavelength of 1.6
microns [4]. The laser would be powered by the solar concentrator cell arrays. The beam would
have an intensity of 1 kW per square meter.
The beam would be received by a station on the ground. The ground station would have
an array of concentrator GaSb cells shown in figure 10. Calculations show GaSb concentrator
cells have an efficiency of 45% when receiving 1.6 micron (0.8 eV) IR radiation [5]. (from fig 3b
Vmax = 0.37 V and 0.37/0.8 = 46%)
Fig. 10 GaSb cells in mini dome concentrator array
This key concept was first described in “A Solar Power Satellite Sending an Infrared
Beam from GEO to 40% efficient Concentrating Solar Power Modules on the Ground 24 Hours
per Day”. Harvesting solar energy in space from a GEO orbit and RF beaming it down to earth
has been a dream since the oil crisis in the 1970’s. However, the colossal and expensive first
step required to achieve this goal has stifled its initiation. The problem derives from the
dispersion of the beam associated with the long RF wavelength leading to a multi km size
receiver station and a km size satellite and a costly multibillion dollar development project for a
GW sized satellite. Using a shorter wavelength infrared beam reduces the dispersion and
ground station size and consequently the satellite size from GWs to MWs or less. If a satellite
with a diode pumped Er:YAG laser generating an IR beam is used, then a 40 m diameter ground
station can receive eye safe IR radiation. Modular concentrating IR arrays with GaSb IR
photovoltaic cells can then generate electricity 24 hours per day with an efficiency of 40%. In
Ref 5, the economics for this concept were described and seem promising for national security
niche power markets.
The ground stations would need to be 40 m in diameter. The station would be capable
of producing 400 kW of power 24 hours per day.
Fig 11: Left image is a basic concept design for a power beaming satellite. Right image is of a
broadcast satellite that is in orbit today as an example that remarkably similar satellites are in
use today.
The designed satellite is similar in size to a direct broadcast communication satellite as
show in figure 11. It uses a 40 kW solar array to provide power to a 20 kW IR laser. Because the
dispersion of the eye safe 1.6 micron IR laser beam is much less than the dispersion for a
microwave beam the spot size on earth from GEO is only tens of meters in diameter rather than
several km. Even though the laser intensity on the ground will be low, nevertheless, modules with
concentrating lenses at the ground can boost the intensity on IR GaSb PV cells and could still
deliver 8 KW of solar power 24 hours per day for a first space to ground solar power satellite
demonstration.
In the context of recent extreme heat days here on earth, is the 40% conversion number
the relevant efficiency number? The answer is yes. Even though there are three energy
conversion steps, i.e. the 40% solar to electric on the satellite and the 50% electric to laser
beam on the satellite and the 40% beam energy conversion to electricity on the ground, the
two waste heat steps on the satellite are dissipated in space and do not contribute to global
warming.
So, in conclusion, the idea of solar 24 hours per day with 40% conversion efficiency is
remarkable and rivals natural gas conversion numbers without the burden of CO2 global
warming.
4. Conclusion
GaSb cells are a promising technology for providing electricity in a variety of ways. The
cells IQE of 85% with wavelengths over 1000 nm can be increased to 95% with the development
of n on p junction cells. The cells can be designed into a variety of modules to fit different
applications. They can be mounted on a circuit that encompasses a burner to create generator.
This has been demonstrated with the Midnight Sun and the lightweight Fuel-Fired TPV DC
Cylindrical Generator. GaSb cells can be used in industrial plants such as steel mills to convert
wasted heat into useable electrical power. GaSb cells can also be used to generate solar power
24 hours a day with an efficiency of 40%. This would be achieved with an array of GaSb cells on
the ground receiving an infrared laser that is beamed from a solar power satellite in
geosynchronous. There are many satellites in orbit today, for example a broadcast satellite, that
have similar solar cell wing arrays attached to them. With that in mind GaSb cells offer the
ability to create the first demo system of efficient power beaming from space. The technology
offers clean energy solutions that would pay for itself in a matter of years.
References
[1] L. Fraas, J. Avery, P. Gruenbaum, V. Sundaram, Fundamental Characterization Studies of GaSb Solar
Cells
[2] F. Predan, A. Franke, O. Hoehn, D. Lackner, H. Helmers, G. Siefer, A. Bett, F. Dimroth, Wafer-bonded
GaInP/GaInAs/GaSb four-junction solar cells with 43.8% efficiency under concentration
[3] L. Tank, L. Fraas, Z. Liu, Y. Zhang, H. Duan, C. Xu, N-type vapor diffusion for fabrication of GaSb
thermophotovoltaic cells to increase the quantum efficiency in the long wave range
[4] L. Fraas, M. O’Neil, Terrestrial Electric Power 24 Hours per Day Using 40% Efficient GaSb
Concentrator Photovoltaic (CPV) Cells
[5] L. Fraas, M. O,Neil, A Solar Power Satellite Sending an Infrared Beam from GEO to 40% Efficient
Concentrating Solar Power Modules on the Ground 24 Hours per Day