In situ measurements and modeling of
triple-junction photovoltaic response and degradation can reduce
development cost and evaluation time.
15 April 2014, SPIE Newsroom. DOI: 10.1117/2.1201404.005421
In general, the electrical characteristics of solar cells
are evaluated at a measurement facility before the cells are brought to
an accelerator facility for irradiation (by protons or electrons) and
subsequently returned to the measurement facility for remeasuring.
However, this ‘sequential method’ needs relatively high quantities of
samples that can be irradiated by different amounts of electrons/protons
with different accelerating energies to fully reveal the degradation
behavior of solar cells.
Solar cells are used as the primary energy generators for devices in space satellites
and are thus one of the most important parts. Modern satellites need
more electricity than previous ones because they have more challenging
missions and require more equipment. Solar cells with high conversion
efficiencies are required to keep weight and launch costs low. They are
also required for outer space missions (i.e., far away from the Sun)
where light intensity is lower.
Modern ‘triple-junction’ (3J) space solar cells
contain three kinds of solar cells (subcells) made from different
materials, stacked in layers.1 Their degradation behavior is more complicated than conventional ‘single-junction’ solar cells, which consist of one material.2
Consequently, many costly and time-consuming irradiation experiments
are required, with different amounts of electrons/protons with a range
of energies.
Solar cell manufacturing companies or space
agencies do not usually have their own accelerators. Instead, they use
specialist accelerator facilities for irradiation. Such facilities
usually pay attention only to qualities of proton/electron beams, such
as uniformity and stability. As a result, no advanced irradiation
techniques to evaluate radiation degradation of solar cells have been
developed. In addition, to minimize the number of irradiation
experiments, it would be useful to model radiation degradation. The US Naval Research Laboratory
has modeled degradation for single-junction solar cells by calculating
the radiation energy deposited for damage (non-ionizing energy loss,
NIEL).3 However,
until now there has been no degradation model of 3J solar cells. We
have been working to develop both an advanced irradiation technique and a
model of degradation in 3J solar cells.
We have proposed and tested an in situ evaluation
technique by which the electrical characteristics of solar cells can be
measured under simulated sunlight during proton/electron irradiation.4 Figure 1
shows the proton irradiation chamber for this ‘simultaneous’ method at
the Japan Atomic Energy Agency. A cryogenic system is installed in the
irradiation chamber to mimic conditions of low light and temperature,
similar to space conditions farther away from the Sun than the
Earth/Mars region. We measured the short-circuit current (ISC), open-circuit voltage (VOC), and maximum power (PMAX) of a 3J solar cell as a function of 10MeV-proton fluence (see Figure 2).
All 18 data points were collected from one solar cell by the
simultaneous method, which represents a great saving of time and cost
compared with the 18 samples required for that number of data points
when using the previous (sequential) method.
Figure 2. Short-circuit current (ISC), open-circuit voltage (VOC), and maximum power (PMAX) of a triple-junction (3J) solar cell as a function of 10MeV-proton fluence.
We have also devised and tested a method to model
the radiation degradation of 3J solar cells using a 1D photovoltaic
device simulator.5
We first obtain the minority carrier diffusion length and majority
carrier concentration by fitting the external quantum efficiency for
each of the 3J subcells before and after irradiation. We then estimate
the damage coefficient of the minority carrier diffusion length (KL) and majority carrier removal rate (RC) from the fitting results. The degradation of solar cells is described by the ‘displacement damage dose’ (based on NIEL), and KL and RC are also described as a function of NIEL.
Higher conversion efficiency, higher radiation
resistance, lighter weight, and flexible shape are key concepts for the
development of space solar cells. In future work, we will modify the
simultaneous method and degradation modeling to optimize future
generations of these cells.
Takeshi Ohshima, Shin-ichiro Sato
Japan Atomic Energy Agency (JAEA)
Taksaki, Japan
Takeshi Ohshima received a PhD in engineering in
March 1994 from the University of Tsukuba in Japan. He currently leads
the Semiconductor Analysis and Radiation Effects Group at JAEA, and
lectures part time at Gunma National College of Technology, Japan.
Taishi Sumita, Tetsuya Nakamura, Mitsuru Imaizumi
Japan Aerospace Exploration Agency (JAXA)
Tsukuba, Japan
References:
1. M. Yamaguchi, III-V compound multi-junction solar cells: present and future, Sol. Energy Mater. Sol. Cells 75(1-2), p. 261, 2003. doi:10.1016/S0927-0248(02)00168-X
2. T. Sumita, M.
Imaizumi, S. Matsuda, T. Ohshima, A. Ohi, H. Itoh, Proton radiation
analysis of multi-junction space solar cells, Nucl. Instr. Methods Phys. Res. B 206, p. 448, 2003. doi:10.1016/S0168-583X(03)00791-2
3. S. R. Messenger,
G. P. Summers, E. A. Burke, R. J. Walters, M. A. Xapsos, Modeling solar
cell degradation in space: a comparison of the NRL displacement damage
dose and the JPL equivalent fluence approaches, Prog. Photovolt.: Res. Appl. 9(2), p. 103-121, 2001. doi:10.1002/pip.357
4. R. D. Harris, M.
Imaizumi, R. Walters, J. Lorentzen, S. Messenger, J. Tischler, T.
Ohshima, S. Sato, P. Sharps, N. Fatemi, In situ irradiation and
measurement of triple junction solar cells at low intensity, low temperature (LILT) conditions, IEEE Trans. Nucl. Sci. 55(6), p. 3502-3507, 2008. doi:10.1109/TNS.2008.2006971
5. S. Sato, T.
Ohshima, M. Imaizumi, Modeling of degradation behavior of InGaP/GaAs/Ge
triple-junction space solar cell exposed to charged particles, J. Appl. Phys. 105(4), p. 044504, 2009. doi:10.1063/1.3079522
