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SPECIAL TALK

Putting solar cellresearch in perspective
As of the end of 2015, the cumulative amountof solar cells installed in Japan has reached 30
gigawatts (GW). This amount is projected to grow to
100 GW by 2050, and there are high expectations
that solar energy will serve as a core energy
resource. However, to meet these expectations, it iscritical to develop groundbreaking solar cells that arehighly efficient, reliable and capable of generating
electricity at low cost. What actions need to be taken
toward that end? Professor Makoto Konagai, who
has been leading solar cell R&D with focuses on
thin-film silicon and crystalline silicon solar cells,
and NIMS Fellow Kenjiro Miyano, the leader of the
Ad hoc Team on Perovskite PV Cells, discuss thecurrent situation and prospects of solar cell research.

Kenjiro Miyano

Professor, Advanced Research Laboratories,
Tokyo City University

Fellow, National Institute for Materials Science

Beginning of solar cell research inJapan
Miyano Professor Konagai, I believe you
are very familiar with the history of solar
cell research in Japan from the early days.
Konagai I started solar cell research in1972 when I was a first-year master’s
student at Tokyo Institute of Technology.
I have been in this career for nearly 45
years. The first solar cell material I studied
was gallium arsenide (GaAs). Since
then, I have dealt with many different
materials. The “Sunshine Project” began
in 1974 as Japan’s long-term project for
R&D of new energy technologies. Then,the inauguration of the New Energy
Development Organization (NEDO, predecessor of New Energy and Industrial
Technology Development Organization)

took place in 1980, which has been promoting collaboration between the industry
and academia in R&D of solar cells.

To make more efficient silicon solar
cells
Konagai At present, 40 GW of solar
cells are manufactured worldwide annually. Silicon (Si) is the dominant solar
cell material, accounting for 90% of allmaterials used. However, the energy conversion efficiency of crystalline Si has notbeen improved since it reached around25%. I think we need to take bold action
to achieve conversion efficiency of 28 to
29%, which is considered to be the theoretical maximum.
Miyano At NIMS, we are conducting
research on Si from the economic view-

point. For instance, we developed a new
method to grow single crystal Si froma seed crystal. This method greatly
reduced production cost compared to theprevious method which was originallydeveloped to grow crystals for integrated
circuits. The resulting conversion efficiencies are very similar between thetwo methods.
Konagai I agree that cost reduction is avery important issue. Also, it is extremely
difficult to dramatically increase conversion efficiency using Si alone. In thisregard, we have high expectations fortandem (multijunction) solar cells, consisting of Si cells stacked with several
semiconductors that are capable of absorbing sunlight at various wavelengths
that cannot be absorbed by Si cells.
Rapidly growing Chinese industries are

Miyano The latest trend of solar cell
R&D is to develop materials that aresuperior to Si in terms of cost and conversion efficiency.
Accordingly, NIMS has been conducting
research on III–V compound semiconductors, which are a combination between
group III elements in the periodic tablesuch as aluminum (Al), gallium (Ga) andindium (In), and group V elements suchas arsenic (As) and nitrogen (N). Because
group III and V elements absorb light at
different wavelengths, the combinationof these two groups enables utilizing
sunlight in a wide range of wavelengths
and thus increasing power generation
efficiency. However, it is difficult to control interfaces between different stacked
semiconductors. We are currently developing a technology to solve this issue.
This technology is critical to successfully
fabricate tandem solar cells.
We are also working to develop quantum dot solar cells. The idea behind thistechnology is to achieve high conversion
efficiency taking advantage of quantum
effects by fabricating a nanosize structure called a quantum dot in a crystal.
Konagai It is not appropriate to focus
our research efforts exclusively on Si for

Makoto Konagai

the reason that Si is the dominant solar
cell material. To create highly superior
solar cell products, it is important for us
to develop new materials by constantly
applying several new concepts.

Discovery of perovskite materials
Konagai About three years ago, perovskite-based materials suddenly became known to photovoltaics research
field. What we call perovskite is essentially ionic crystals of lead iodide having
a crystal structure called perovskite. I
was utterly surprised by the fact thatthese materials marked a conversion
efficiency of 12% in initial studies. If I
were a little younger, I definitely would
work on perovskite. It is a big mystery tome that materials of such high potential
remained undiscovered until recently.
Miyano Actually, perovskite materials had been studied in Japan sincethe 1990s. But they were perceived aslight-emitting materials at that time.

A systematic
approach is vital toincrease conversion
efficiency. Fortunately,
all necessary
characterization
technologies areavailable at NIMS.
Kenjiro Miyano

However, if the materials emit light, thelight can be used to generate electricity,
but no one thought of that then.
NIMS has been taking two approaches
in perovskite research. The first approachis to aim for high conversion efficiencies. In fact, Dr. Liyuan Han’s group has
achieved a conversion efficiency of morethan 18%. The second approach is tofocus on basic research in the frameworkof GREEN – the Global Research Center
for Environment and Energy based on
Nanomaterials Science (see page 6 forresearch at GREEN). The secret to highpower generation efficiencies of perovskite materials must lie in the characteristics of the materials themselves. In
this view, we aim to identify the mechanism, and then establish a roadmap for
its application.
Konagai Reliability plays a major role
in reducing power generation cost. Thatis because if solar cell products break
down easily, cost will increase over thelong run.
Miyano It is generally believed that perovskite materials do not last very long.
However, based on a series of basic
research conducted at NIMS, we haveidentified factors related to stability inconsiderable detail. In subsequent studies, we will try to formulate strategies toachieve long-term stability.

More ideas through interdisciplinary interactions
Konagai My impression of Japanese research institutes is that they are conducting solar cell research in isolation withoutcoordination. I hope NIMS will play aleading role in promoting collaboration.

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Makoto Konagai

New concepts to create innovative
materials

It is more challenging
than those ontransistors and LEDs.
That is the very reason
that solar cell research
is so much fun.

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also interested in tandem solar cells, andaccording to report on solar cells titled“International Technology Roadmap for
Photovoltaic (ITRPV),” the prevalenceof tandem solar cells is expected to growto 10% of all Si solar cells by 2025.

SPECIAL TALK

Putting solar cell research in perspective

Miyano Different solar cell materials arestudied separately by researchers withdifferent specialties, and almost no interaction takes place between the groups. It
is a shame that this is the common practice. In fact, there are cases where groups
in certain fields are dealing with some unsolved issues, while groups in other fields
already have found a solution to the same
issues. Researchers should interact withother scientists with diverse backgrounds
across different disciplines. I hope NIMS
will provide researchers with opportunities for vigorous interactions.

Also, to increase the conversion efficiency of solar cells, it is necessary totake systematic measures rather thanrandom and unorganized measures. Fortunately, a broad array of characterization technologies is available at NIMS.
As such, when we create new sample
materials, we can even analyze the distribution of elements in these materials
immediately. When we detect sudden
high conversion efficiencies, we often
find unexpected structural features in thesample materials. By understanding therelationships among parameters of sam-ple fabrication, the resulting conversion
efficiencies and the structure of sample
materials, it may be possible to purposefully create materials with desirable
structures. I want to take full advantage
of such strength NIMS offers.
Konagai Solar cells already have beenin use by many households, but to makesolar energy a core energy resource,
more innovative ideas need to be incorporated. I hope that young people will
engage in solar cell research with new
ideas and dreams.
(by Shino Suzuki, PhotonCreate)

GREEN’s effort in basic research on perovskite solar cells

The research contributes to the enhancement of cell performanceand the understanding of the mechanism

Yasuhiro Shirai (left) and Masatoshi Yanagida (right), members of the Ad hoc Team on Perovskite PV Cells, GREEN.

possible to form a perovskite layer witheven thickness and a smooth surface.
With this new method, it is now feasible
to fabricate perovskite solar cells withhigh reproducibility and high stability.
Team members Yasuhiro Shirai and
Masatoshi Yanagida enthusiasticallysay, “Since perovskite layers can be
fabricated at a low temperature rangeof less than 140°C, they are compatible
with lightweight and flexible substrates
such as plastics. For this reason, theindustrial community is very much interested in them. However, further basic research is necessary as the mechanismsof perovskite solar cells generating
electricity have not even been understood. We will intensify our research
efforts in hope of realizing practical useof the cells as soon as possible.”(by Kumi Yamada)

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rial is covered on a substrate. In conventional methods, the solution must
be applied onto a titanium oxide layer
with a complex porous structure in order to facilitate crystallization. However,those methods are associated with low
stability and low reproducibility causedby moisture and oxygen entering into
the perovskite layer. The team successfully fabricated perovskite layers withenhanced performance by strictly controlling the surrounding environment,
as controlled during the fabricationof organic thin film solar cells, and byeliminating a porous structure in thesubstrate.
In addition, the team made modifications to the perovskite material
application process. It developed a
new fabrication method involving theaddition of chlorine, which made it

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Perovskite solar cells are drawing
much attention as next-generation solar cells due to their high efficiency andlow production cost. However, before
they can be put to practical use, there
are many issues to overcome such as
low reproducibility, low stability andlow durability. To tackle these issues,
GREEN launched the “Ad hoc Team on
Perovskite PV Cells” in October 2014.
Consisting mainly of researchers withexperience in R&D of organic solar
cells, the team has been carrying out
basic research on perovskite solar
cells, taking full advantage of GREEN’s
specialty in calculation/measurement
techniques and materials development
technology. Here are some accomplishments made by the team.
A perovskite layer is formed after asolution containing a perovskite mate-Band structure of single-junction crystalline silicon solar cell

Improvement of Existing Material

New method to create high-quality
single-crystal silicon at low cost
Aiming to regain control of the price-competitive solar cell market
In recent years, the solar cell market has been dominated by multicrystalline
silicon solar cells that are inexpensive but are of low quality. Amid the situation, an industry-government-academia joint research team led by NIMS’s
Takashi Sekiguchi developed a method to create high-quality single-crystal
silicon solar cells with high energy conversion efficiency at lower cost.

· Many dislocations in borders
between seed crystals· Expensive due to the use of many
seed crystals

Modifying the method for creating
multicrystalline silicon
Sekiguchi explains the historical background associated with the project. “Japanese industries used to lead the solar cellmarket before 2006. But other countries
eventually gained a lead as low-priced
multicrystalline silicon solar cells. The
NEDO project was launched under suchcircumstances in the hope of restoring
Japan’s competitiveness in this market
by developing mono-crystal silicon solar
cells with high conversion efficiencies at

low cost.”
Sekiguchi’s research team considered
modifying “cast methods” used to produce multicrystalline silicon.
Normally, single-crystal silicon is produced through the following steps: first,
melt the high purity silicon feedstock ina crucible at about 1,600°C. After that,hang a small piece of single-crystal silicon (seed crystal) above the crucible andplace it in contact with the molten silicon. Then allow the seed crystal to growand expand. Finally, slowly pull up thegrowing crystal mass. This crystal growthmethod is called the Czochralski (CZ)
method. It is time-consuming and requires
advanced techniques and, therefore, iscostly to implement.
On the other hand, multicrystalline
silicon is produced by melting silicon in asquare mold set in an electric furnace, andallowing the molten silicon to solidify
from the bottom. This method was modified to develop a seed cast method thatallows crystal seeds to grow into monoSingle-seed cast method· Low dislocation densities· Low cost

Figure 1 : Conventional multi-seed cast method (left) and the newly developed
single-seed cast method (right).

Figure 2 : Mono-crystal silicon created using the single-seedcast method (left). Unlike the conventional product (right), thenew product has a metal-like smooth and shiny surface.

crystals (We call them “mono” rather than“single” to distinguish this crystal from
CZ). Conventionally, mono crystals aregrown in a setting in which the bottomof a crucible is lined with 20 × 20 cm
single-crystal silicon plates. Silicon feedstock is melted above the plates and thensolidified. This type of seed cast method
is called a multi-seed cast method, as
multiple crystal seeds are used (Figure 1).
This method is easier to perform than the
CZ method, as all you need to do is cooland solidify molten silicon. However, themethod also has some drawbacks—it isexpensive to conduct due to the use ofmultiple crystal seeds and because crystal
defects are created in borders between
seed crystals.
In light of these drawbacks, Sekiguchi’s
research team attempted to develop a new
type of seed cast method which employs
only one crystal seed. The team set asingle-crystal silicon plate holding a lone
crystal seed in the center of the mold togrow silicon crystal. This method eliminatedthe issue of defect formation between
adjacent seeds (Figure 2).
Sekiguchi says, “To reduce production
cost, we used a creative approach to thermal design and temperature control of theelectric furnace. It was especially important to optimize the cooling process of silicon ingot after solidification as a means
to prevent crystal defects (dislocations)from generating and multiplying due tothermal stress.”

European research institutes also conducted similar studies, but many of themwere unable to produce desirable results
and ended up their projects immaturely.
Sekiguchi’s team has built up expertisein crystal defects through many years ofexperience at NIMS in studying defects
in semiconductors. The team took fulladvantage of its skills, knowledge andknow-how for the successful developmentof solar cells.
“Also, the Kyushu University groupplayed a vital role in designing the internal structure of the electric furnace using
its special talent in computer simulationof crystal growth,” added Sekiguchi.
Sekiguchi’s research team also worked tocontrol contamination during the growth ofcrystals with impurities, which was a problem associated with seed cast methods.
“In conventional cast methods, the surface of the mold is coated with silicon
nitride as release agents to prevent thesilicon from sticking. In addition, due
to the fact that the heater and insulationmaterials are made of carbon, and themold quartz, it is very difficult to preventthe incorporation of these materials into
silicon ingots. For this reason, during
the growth of silicon ingots, the precipitation of silicon nitride, silicon carbide,or silicon dioxide occurs, degrading theperformance of silicon solar cells. We
dealt with this issue by minimizing
contamination with impurities, andsucceeded in the creation of high-quality

mono-crystal silicon,” says Sekiguchi.
The resulting solar cells achieved
conversion efficiencies nearly as highas solar cells made of CZ silicon. Furthermore, Sekiguchi said that an oxygen
concentration in the new silicon products
was only about 6 ppm as compared tothe oxygen concentration of about 20
ppm in CZ silicon.
“At present, we estimate that the production cost using the new method falls
under the mid-range between the costs ofimplementing the CZ and multicrystalline silicon. We believe we can achieve
higher conversion efficiencies than thatof CZ silicon by further optimizing our
method. If this vision comes true, thevalue of our products will increase evenmore,” says Sekiguchi.
Hope is growing that high-quality andlow-priced single-crystal silicon solar
cells will become available in the near
future and play an active role in slowing
global warming.
(by Kumi Yamada)Figure 3 : This research project was conducted bystaff members from Japanese silicon manufacturers aswell as researchers and students from several countries
including China, Germany, France, South Korea, and Fiji.

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Multi-seed cast method

method for creating single-crystal siliconthat consistently yields high conversion
efficiencies at lower cost. This achievement was made while implementing
the NEDO (New Energy and Industrial
Technology Development Organization)
project titled “Development of nextgeneration, high-performance technology
for photovoltaic power generation
systems” from FY2010 through FY2014.

Managing Researcher,International Center for Materials
Nanoarchitectonics (MANA)NIMS NOWNIMS NOW08

At present, most of commercial solar cells
are made by silicon. Due to high-quality
requirements, silicon used as semiconductor devices, such as computer CPUs andmemory is single-crystal. On the other
hand, less expensive multicrystalline
silicon is primarily used for solar cells
due to priority on low production cost overthe quality of end products. However, theenergy conversion efficiency of multicrystalline silicon, about 18%, is one-tenthlower than that of single-crystal silicon,
which is about 20%. To address theseissues, inexpensive ways to create single-crystal silicon were pursued.
One of the pursuers was an industry-government-academia research teamconsisting of NIMS, including a teamleader, Takashi Sekiguchi, who serves asa managing researcher at the International
Center for Materials Nanoarchitectonics(MANA); Kyushu University; Japanesemanufacturers; Toyota Technological
Institute; and Meiji University. The teamsucceeded in the development of a

Takashi SekiguchiToward practical use

Achieving Conversion Efficiency Over 18%
With Large-size Perovskite Solar Cells.
Aiming to realize practical use of “all-inorganic” cells by 2020
Expectations are growing for perovskite solar cells to serve as the next-generation solar cells due to their low production cost and high conversion efficiencies.
Competition is intensifying among many countries in the development of thesecells toward practical use. Amid the situation, a research group led by NIMS’s
Liyuan Han is emerging as a groundbreaker in this endeavor.

Using inorganic materials to overcome the most challenging issue—
durability

Perovskite is essentially a mineralogical name for calcium titanate, andcompounds having the same perovskite
crystal structure are collectively called
perovskite-type compounds.
As shown in Figure 2, a perovskite solar
cell consists of a glass substrate, a transparent conductive film, a hole extraction
layer, a perovskite layer, an electron transport layer, an electron extraction layer,
and a back electrode. The cell generates
electricity as the perovskite layer absorbs
light, which then induces charge (electrons
and holes) through photoexcitation.
Currently, an organic-inorganic hybridmaterial with a perovskite structure—amethylammonium lead iodide (CH3N-

Figure 1: Current-voltage characteristics of perovskite solar cells measured by the Calibration, Standards andMeasurement Team at the AIST Research Center for Photovoltaics. The underlined measurement in red
represents a conversion efficiency of 18.2% achieved using the newly developed perovskite solar cells.

Back electrode
Electron extraction layer
Electron transport layer

Perovskite layer
Hole extraction layer
Transparent conductive film
Glass substrate

Figure 2 : Structure of a perovskite solar cell.

H3PbI3) film—is used as a light absorption material. This hybrid material isprocessed into films by adding the material
into a solution and then print. The film
product can be mass-production throughprinting, and the conversion of the substrate from a glass to a film form makes it
possible to create flexible solar cells.
However, the existing perovskite solar
cells have several problems in terms of
performance, such as that power generation characteristics vary among cells
depending on the conditions under which
the films are fabricated, and energy conversion efficiencies fluctuate in relation
to the voltage applied to the cells. In addition, these cells also have stability anddurability issues as they deteriorate easily
when exposed to continuous sunlight.
To deal with these issues, Han focusedon the electron and hole extraction layersof the cells. Up to that time, these twolayers were made of organic materials. He
thought that the stability and durability ofthese layers would dramatically increase
if inorganic materials were used insteadof organic ones. He paid particular attention to two kinds of inorganic materials,
nickel oxide and titanium oxide, as they
are compatible with the energy level ofperovskite materials.
On the other hand, to use these inorganic materials as extraction layers, theyneeded to be transformed into the thinnest
possible layers due to their high electricalresistance. Making them into thin layers,
however, would also cause an increased
occurrence of “pinhole” defects, which reduces the conversion efficiencies of cells.
To overcome this dilemma, Han’s groupreduced pinhole defects by making these
inorganic layers thicker. Also, to increase
the conductivity of these layers, he added
high concentrations of niobium ions to the
inorganic electron extraction layers andhigh concentrations of lithium and magnesium ions to the inorganic hole extraction
layers. Experiments demonstrated that theconversion efficiencies and durability ofthese layers in fact increased as expected.
Moreover, the use of thicker inorganic
layers with reduced pinhole defects
allowed Han’s group to expand cell sizes.
Han explains several issues like the process of fabricating the inorganic layers.
“At present, organic materials are used forelectron transport layers. When we created a thick inorganic layer on top of anorganic electron transport layer, we werevery careful not to damage the organic

layer. Also, when we pasted the inorganic materials, we were fully cautious incompletely removing moisture from theworking environment, as perovskite layers break down in the presence of water.”In addition, the amounts and ratios of
niobium, lithium, and magnesium ions
to be added to inorganic films had to be
optimized according to the thickness ofthe films. Han’s knowledge and uniqueknow-how developed over years wereonce again very helpful for his researchgroup to tackle this challenge.
According to Han, the most critical issue
to be addressed toward the practical use
of perovskite solar cells is to increase
their conversion efficiencies and durability. To increase their durability, it is reallynecessary to replace currently used organic
materials with inorganic materials. Accordingly, we aim to create “all-inorganic”
perovskite solar cells by additionallyreplacing organic electron transport layerswith inorganic layers. Also, I expect thata conversion efficiency of 20% can beachieved by FY2016 through improvement of perovskite materials. Furthermore, I believe it is possible to achievea conversion efficiency of even 30% or
higher by using perovskite solar cells intandem with silicon solar cells by placing
the former on top of the latter. Whatever
approach might be taken, I predict thatperovskite solar cells will be in practicaluse by around 2020.”(by Kumi Yamada)

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small cells are prone to large measurement errors, and thus the efficiencies
measured in this manner are unreliable. In
contrast, Han’s group successfully createdcells of large areas (≥1 cm2) by making
modifications in materials constituting
the layer structure of the perovskite solar
cells and in fabrication methods. The
group also demonstrated the excellent
durability of the cells as the deteriorationof their performance was only less than
10%, even after 1,000 hours of continuous exposure to sunlight.

Managing Researcher,
Center for Green Research on
Energy and Environmental Materials

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Perovskite solar cells are drawing attention rapidly as their energy conversion
efficiencies are nearly as high as those ofsilicon solar cells, they are made of cheapmaterials, and they can be produced inlarge scale at low cost without using hightemperatures/pressure and a high vacuum
process. On the other hand, they still have
many performance issues to be resolvedin terms of reproducibility, stability, anddurability. Consequently, intense competition is ongoing internationally toachieve practical use of these solar cells.
Liyuan Han of the Photovoltaic Materials Group at the Center for Green
Research on Energy and Environmental
Materials, NIMS, is among the firstresearchers to have taken the initiative inthe development of perovskite solar cells.
The perovskite solar cells developed by
Han’s research group marked a conversion efficiency of 15% on February 27,
2015. This was the world’s first official
record verified by an internationallyrecognized independent organization forsolar cell evaluation. Furthermore, Han’sgroup achieved a conversion efficiency of
18.2% in October, 2015 (Figure 1). Thisefficiency—comparable with efficienciesof single-crystal/multicrystalline siliconsolar cells—has attracted much attention
globally.
In most previous reports, conversion
efficiencies of perovskite solar cells weremade using small cells (0.1 cm2) without
disclosing measurement methods. Such

Han Liyuan

Solar cells of the future

Ultimate conversion efficiencies bymeans of “intermediateband concept”
Conversion efficiency of solar cells is determined by the energy required toconvert solar energy into electricity (bandgap). NIMS is currently conducting
research aiming to convert sunlight at more wide range of wavelengths into
electrical energy by forming an “intermediate band” in the bandgap.

Utilization of sunlight at all
wavelengths may be achieved by
III–V nitride semiconductors

abovementioned nitride semiconductors.
QD is nanometer-scale structure, so thatelectrons are confined in all three dimensions. For fabrication of QDs, we utilize“droplet epitaxy”, which is an originaltechnique developed in our Institute.
In this method, a Ga flux is supplied onan AlGaAs surface without an As flux,
resulting in the formation of Ga droplets of approximately 10 nanometers indiameter. Then, an As flux is irradiatedonto these Ga droplets, which causes
reactions between Ga and As, leading
to the formation of GaAs quantum dots.

MANA Independent Scientist,
International Center for Materials
Nanoarchitectonics (MANA)

Lattice-matched QDs have advantageous
for staking of a large number of QD
layers with close proximity. “Since solar
cells that are designed under the concept
of intermediate bands have relatively
simple structures, we think they can be
manufactured at low cost.” says Noda.
Noda is also working on quantum well
(QW) solar cells, in which coupled QWs
are used to explore the influences ofcoupling of electronic states or a miniband on solar cell properties. Quantum
wells are structures capable of confining
electrons in one dimension, and they canTakeshi Noda

Leader of the Photovoltaic Materials Group,Center for Green Research on Energy andEnvironmental Materials

easily be fabricated using the methods
for growing ultrathin-films such as molecular beam epitaxy.
“If solar cells that meet the concept ofintermediate bands are successfully created, a conversion efficiency of 60% isachievable in theory. At this moment,
there are still many issues to address
such as a decrease in open-circuit voltage. We would like to overcome theseissues by reviewing current solar cellstructures and developing new materials,” says Noda.
(by Kumi Yamada)

Aiming to high efficient solar cells
using quantum dots
On the other hand, Takeshi Noda of the
Photovoltaic Materials Group is working on high efficient solar cells using
quantum dots (QDs), where QDs are expected to serve as an intermediate band.
We use GaAs and AlGaAs (aluminumgallium arsenide)—other types of III–V compound semiconductors than the

III-V nitride semiconductor thin film growing device

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III–V nitride semiconductor, that haslarger band gap, on top of a GaP-based
concentrating solar cell, which has recorded the highest conversion efficiency.
It is conceivable to further increase theconversion efficiency by deriving electricity from each cells individually. “Bycombining this crystal with concentrating solar cells, we hope to achieve theworld’s highest conversion efficiency,”
says Sumiya.
Solar cells are susceptible to defects
formed in cell materials and interfaces.
Accordingly, upgrading of solar cellmaterials, interface control and device
fabrication processes leads to an increasein the conversion efficiencies of III–V
nitride solar cells. Concurrent with thisapproach, Sumiya and Sang’s group isalso planning to conduct R&D of power
conductor devices using III–V nitrides,
taking advantage of techniques andknow-how they have acquired throughsolar cell research and development.

Chief Researcher,Wide Bandgap Materials Group,
Electric and Electronic Materials Field,
Research Center for Functional Materials

Sang Liwen

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The theoretical conversion efficiency
limit of ordinary crystalline silicon solar
cells is around 30%. To further increase
conversion efficiencies beyond this theoretical limit, a creative approach must be
taken to find the way to convert sunlightat more wide range of wavelengths into
electrical energy. To this end, NIMS is
actively engaging in R&D of solar cells
using compound semiconductors.
In particular, III–V compound semiconductors consisting of elements in group
13 (group III) and group 15 (group V) of
the periodic table, namely gallium arsenide (GaAs) and indium gallium phosphide (InGaP), have bandgap energy
close to 1.5 eV (electron volts), which is
the optimum energy level for maximum
conversion efficiency. Also, as thesecompounds are resistant to radiation,
they already have been put to practicaluse in space. In addition, III–V compound semiconductors efficiently absorb sunlight, and non-absorbable lightefficiently passes through them. Becauseof these properties, these semiconductors
are capable of dramatically increasing
conversion efficiencies when they arearranged in a tandem or multijunction
structure, comprising a lamination ofseveral types of compound semiconductors with different bandgaps. However,the structure of the tandem structure isso complex that it is difficult to manufacture technologically.
While striving to address this issue, a
research group consisting of Masatomo
Sumiya of the Wide Bandgap Materials Group and MANA Independent
Scientist, Sang Liwen, focused on gallium nitride (GaN) and indium nitride(InN)—two compounds that were widely recognized as the materials for blue
light-emitting diodes on the occasion of
the Nobel Prize announcements.
A III–V nitride semiconductor, GaN,
has a large bandgap (i.e., wide gap, 3.42eV), and is mixed with InN (0.7 eV) to
create InGaN crystal, which enables easier control of the bandgap. If a mini band(i.e., intermediate band) can be formedin the bandgap using wide-gap semiconductor materials, it is feasible to utilize awide range of sunlight spectrum without
using a tandem structure. To achievethis, the research group laminated 30
layers of InGaN thin films with different
In compositions over an n-type InGaN
semiconductor layer.
The resulting solar cell was able toconvert not only a part of sunlightequivalent to intrinsic InGaN bandgap
energy but also visible light at longer
wavelengths into electricity. This was
the first study demonstrating that a III–V
nitride semiconductor material is capable of forming an intermediate band. The
research group is also trying to laminate

Masatomo Sumiya

“NIMS should serve as
materials research hub forthe development ofinnovative solar cell materials

12

Will all energy problems be solved some day?

”Written by Akio Etori

Title lettering and illustration by
Shinsuke Yoshitake

Photovoltaic Power Generation Technology Research
Association (PVTEC) promotes the advancement of photovoltaic
industries in Japan. As of November 2015, 51 organizations,mostly industries, are serving as PVTEC members. We asked
Dr. Hiroshi Morimoto, PVTEC President, about his view as
an industrial representative on the current status of solar cell
development in Japan and his expectations for NIMS.

PVTEC President

Dr. Hiroshi Morimoto

HIROSHI Morimoto

Dr. Morimoto With regard to thedevelopment of silicon solar cells, there
are two major objectives from theindustrial perspective: first, to develop
low-cost devices with high conversion
efficiencies, and second, to develop
technologies enabling the integration ofphotovoltaic power generation systems
into the existing electric power system.
As for increasing conversion efficiency,
national projects are underway targetinga module efficiency of 20% or higher.
From the aspect of reducing production
costs, a goal has been set to develop
devices with costs less than the runningcosts of a conventional thermal power
plant: 7 yen/kWh.
Concerning the issue of integrating
photovoltaic power generation systems
into the existing electric power system,
efforts to develop technologies have been
intensifying in relation to the upcoming
retail electricity deregulation. Also, theadoption of the Paris Agreement at theParis climate conference (COP21) inDecember 2015 is expected to promote—What is your vision on the development of new materials?
Dr. Morimoto Compound semiconductors, including perovskite and III–V
compounds, are promising, new, alternative materials to silicon. In particular,
perovskite is drawing much attention
given that its conversion efficiency has
dramatically increased over the past few
years. On the other hand, solar cells
consisting of III–V compounds also
demonstrate high conversion efficiencies, but are expensive to manufacture.
For this reason, their applications havebeen limited only to certain areas suchas outer space. Once these two types ofmaterials have achieved higher conversion efficiencies and lower production
costs than conventional solar cells, I
believe that the demand will rise rapidly.

—Chinese industries are ahead ofJapanese industries in terms ofsolar cell market share.

Dr. Morimoto The reasons for China’s
recent dominance over Japan in the solar
cell market are due to the fact that German industries developed the systems
capable of easily producing inexpensive
and higher-performance silicon solar
cells, and that China has acquired thesystems. For Japanese industries tocounter this situation, we need to develop
new materials and also systems capableof manufacturing inexpensive andhighest-performance devices using the
new materials.

—What are your expectations of
NIMS regarding these efforts?
Dr. Morimoto The global community
has strong interests in developing lowcost solar cells with high conversion
efficiencies. Given that materials development tends to be an extremely longterm endeavor, it would be too much burden for industries to carry out this task bythemselves. So, I really hope that NIMS
will take the lead in the development ofinnovative materials. Moreover, I want
NIMS to firmly promote collaboration
among industries, governments andacademia by serving as Japan’s materials
research hub.
(by Kumi Yamada)cells with a potential conversion efficiency of 40% or greater.
Furthermore, the same goes for algae
capable of producing heavy fuel oil
using CO 2 and water. To efficiently
produce heavy-oil-producing algaethat work in a real world situation,ideas have been brought up to create
artificial algae using such information
technology as genetic analysis/engineering. In addition, the availabilityof arithmetic processing technology
capable of performing huge amountsof calculations instantaneously is also
critical to achieve practical use of thermonuclear fusion which is believed tobe very difficult.
Rapid advancement of information
technology can make all of thesethings possible.
When I visited the nuclear plant in
Kashiwazaki City, I met Nobel Prizewinner Prof. Susumu Tonegawa. He
said that if humankind happens toperish, energy issues would be themost probable cause (I agreed withhim at that time). But after all, thatfear appears to be unnecessary.

Akio Etori: Born in 1934. Science journalist. After graduating from College of Arts and Sciences, the University of Tokyo, heproduced mainly science programs as a television producer and director at Nihon Educational Television (current TV Asahi) andTV Tokyo, after which he became the editor in chief of the science magazine Nikkei Science. Successively he held posts including
director of Nikkei Science Inc., executive director of Mita Press Inc., visiting professor of the Research Center for Advanced Scienceand Technology, the University of Tokyo, and director of the Japan Science Foundation.

15
2016 No.3—Would you tell us about the current status of solar cell development by Japanese industries?

the use of renewable energies worldwide. In harmony with these trends,
we need to consider how to incorporate
solar cell technology into Japan’s social
system, taking account of increasing
public interest in the concept of a “Smart
City.”

issues while overcoming the urgent
challenges of global warming and air
pollution?
The answer to the question might lie
in the latest information technology.
For example, while the performanceof lithium ion batteries has dramatically
improved over years, the batteries arestill expensive, large and heavy. Also,
they are susceptible to deteriorationand take a long time to fully charge. Iflithium ion batteries can be replacedwith “conductive polymer batteries,”
which are much lighter and smaller,
it may be feasible to make the 75
million automobiles existing in Japan
play a role of “mobile, high capacity
storage batteries.” However, because
conductive polymer batteries have athree-dimensional network structure, it
is much more complex to analyze theirmaterial structure than that of current
secondary batteries. For this reason,super-fast computing capability is necessary for the development of conductive polymer batteries. Similar computing resources are also essential forthe development of quantum dot solar
cells, promising next-generation solar

NIMS NOW

14
2016 No.3

NIMS NOWHigh expectations for perovskite
and III–V compounds

In this day and age of mass energy
consumption, meeting ever increasing energy demand is a tough issue aroundthe world. International community
leaders and experts often discuss andshare ideas to resolve this issue. However, concrete solutions have not yet
been established.
Currently available energy resources
include fossil fuels—such as petroleum, coal and natural gas; naturalenergy—such as sunlight, solar heat,wind power, hydropower and geothermal heat; nuclear power and biomass.
Each type of energy has its own
advantages and disadvantages.
Fossil energy is a small in cost andeasily accessible resource, but its
availability is limited and its utilization
results in emission of a large amount of
CO2 into the environment. In contrast,
natural energy is an unlimited andenvironment-friendly resource, but it
is difficult to utilize efficiently and economically based on current technology.
Nuclear power is an excellent means
to generate electricity as it can be acquired in large amount at low cost, andit can be used to generate electricity
without emitting CO2. However, utilization of nuclear power in places like
Japan requires meticulous risk management in light of the consequencesof the Great East Japan Earthquake.
Biomass and geothermal heat have
limited availability at a time, so theyalso are unsatisfactory as major energy sources.
How can humankind solve energy