Photovoltaic Properties of a
Revolutionary New Solar Cell
Drew A. Mazurek
andrew.mazurek@yale.edu
Department of Electrical
Engineering
Yale University
15 Prospect Street
New Haven, CT 06511
Abstract
A region of space approximately 3,200 km
from the Earth's surface called the Van Allen Belt provides an ideal location
for communications satellites. It does
not carry the large communications delay of the distant geosynchronous orbits,
nor does it require the number of satellites needed for full Earth coverage at
lower Earth orbits. However, the Van
Allen Belt is a high radiation area in which current radiation-hard solar cell
technology would degrade and become useless within days. At Yale, we have designed a novel InP
drift-based solar cell with cell lifetimes expected to be in the decades under
those high-radiation conditions.
Additionally, our solar cells have a high power-to-weight efficiency,
reducing the cost necessary to launch the satellites into space.
Introduction
Solar cell technology has found its way
into our lives in many different places.
Solar cells provide an excellent source of power in locations distant
from the power grid, where constructing power plants or running transmission
lines would be too costly or inefficient.
The cells themselves also have no moving parts, making them one of the
lowest-maintenance power supplies available.
By being such a low maintenance, long lasting power source, solar cells
are ideal for remote areas such as water pumping stations, telecommunications stations,
and lighthouses. Solar cells are also
finding their way to remote villages in many countries. For example, in Cacimbas, a South American
village in Brazil, the Brazilian Energy Ministry with support from the U.S.
Department of Energy is installing 50-watt solar cells on the roofs of houses
to provide the homes with fluorescent lighting.
In their generation of power, solar cells
also produce no waste. They are
extremely environmentally-friendly and are powered from the sun, our most
readily available renewable energy source.
Of all the renewable energy sources, only the sun is capable of
supplying more power than is currently being used. In addition, through implementing global solar power generation,
we can become less reliant on fossil fuels and other “dirty” sources of
power. The sun provides a sustainable
source of power, and as more of our energy usage comes from the sun, we can
begin to slow global warming and reduce pollution.
The above solar cells, however, may work
well in terrestrial environments, but they would not survive very long in the
harsh, high-radiation environment of outer space. At Yale, we have designed a new solar cell that is optimized for
outer space applications. It is
lightweight, inexpensive to manufacture, and also radiation-hard, making it
ideal for use on communications satellites.
Our new solar cell, when outfitted on the satellites, will be able to
withstand the high radiations encountered at orbits of 3,200 km in the Van
Allen Belt. This report provides a brief
overview of our current work.
The report is organized as follows. First I will provide a short and simple
introduction to current solar cell technology.
After that, I will present our new InP drift-based design, followed by
measurements we obtained from our solar cell.
I will then discuss the implications of our work before I conclude.
Current
Solar Cells
All solar cells to date are simply
pn-junction devices that operate on the principles of diffusion. The basic idea behind solar cells is that as
photons hit the cell, those possessing more energy than the band gap of the
semiconductor generate hole-electron pairs.
These holes and electrons diffuse toward the metallurgical junction,
crossing it when they arrive. Once they
have crossed the junction, a net positive charge appears on the p-doped side,
and a net negative charge appears on the n-doped side. When the cell is attached to an external
circuit, a voltage is set up between the two sides, and current flows.
A measure of diffusion is the minority
carrier lifetime: the average amount of time a free hole on the n-side of the
junction or a free electron on the p-side of the junction can exist before it
is lost to recombination. Recombination
is a process opposite to generation in which a hole and electron combine and
return to the semiconductor's crystal lattice.
In well-made high-efficiency solar cells, minority carrier lifetimes are
very large; for the most part, all holes and electrons created by incoming
photons will have enough time to diffuse to the other side of the junction and
be available as power. However, the
minority carrier lifetime is based on physical properties of the
semiconductor. Impurities in the
material can add additional states which will decrease the carrier lifetimes. Radiation damage is also another source for
reduced minority carrier lifetimes.
As minority carrier lifetimes are
diminished, more holes and electrons will recombine and be lost before they can
be extracted from the cell as power.
This causes a drop in the solar cell’s efficiency. In the zero-maintenance environment of outer
space, this is unacceptable. Solar
cells must be able to last at least years, and preferably decades. Our new drift-based design can withstand
high doses of radiation and proves to be an ideal candidate for use in
communications satellites at high-radiation altitudes.
While previous solar cells have been based
on diffusion to move charge carriers, our solar cell relies on drift currents
established by an electric field. For
our design, we have used molecular beam epitaxy (MBE) to grow cells with a
graded n+-n doping to create a strong electric field of
approximately 104 V/cm. The
cell structure can be seen in Figure 1.
The electric field region starts at the surface of the cell and extends 0.2 mm deep. As hole-electron pairs are photo-generated in this region, the holes are swiftly swept to the other side of the pn-junction. This drift field can be seen in the band diagram in Figure 2.
|
|
632 nm (red) |
532 nm (green) |
|
Ideality factor (n) |
4.22 |
4.22 |
|
Reverse bias current (I0) |
616nA |
616nA |
|
Short circuit current (Ish) |
558 mA |
644 mA |
|
Open circuit voltage (Voc) |
0.695V |
0.710V |
|
Fill factor (FF) |
52.32% |
59.67% |
|
Spectral response |
338mA/W |
230mA/W |
|
Internal Quantum Efficiency |
97% |
83.21% |
|
Efficiency
(h) |
12.32% |
10% |
As can be
seen from the data in Table 1, our cells have an extremely high quantum
efficiency. Figure 5 shows a comparison
of quantum efficiencies of various other solar cells in comparison with
ours. It is of note that our InP design
has extremely high quantum efficiencies all the way from ultraviolet at 150nm
to infrared at approximately 825nm. For
almost the entire ultraviolet and visible spectrum, our cell operates at
quantum efficiencies above 90%!
We chose InP for our design for several reasons. As was already shown, its quantum efficiency across the solar spectrum is excellent. Figure 6 shows the amount of power absorbed for various thicknesses of solar cells. At only 1mm thick, InP absorbs 85% of the power available from incoming photons. This outperforms GaAs cells, which can never attain that level of absorption, and Si cells, which would require approximately 30mm thick Si to absorb the same amount of light. Thus, our cells are gravimetric-efficient; i.e., they have a large power-to-weight ratio.
