Spherical Cells Promise To Expand
Applications for Solar Power
By harnessing more sunlight, spherical silicon micro solar cells developed by Kyoto
Semiconductor Corp. offer a new direction for solar power generation.

Because it is a clean and renewable, solar power is an attractive source of energy. However,for solar cells to proliferate, twochallenges need to be addressed. First, thecost of solar cells must be further reduced.
Second, applications for solar cells must
be expanded.
In Japan, the New Energy Development
Organization (NEDO) leads efforts in thisarea. Through NEDO’s efforts and collaboration among Japan’s industrial, academic and government sectors, Japan has
grown into the leading producer of solar
cells in the world.
Current residential solar power generation systems generate power at a cost of¥70 to ¥100 per kilowatt-hour. This isroughly three times the cost of conventional residential electrical power and six
times that of commercial electric power.
This high cost is an important factor inholding back the large-scale proliferationof these systems.
In addition, solar power generation systems are being used only in residential applications as supplemental power to existing power supplies.
With conventional energy sources becoming scarce and with the looming problem of global warming, solar power should
be more widely applied.
One way to make better use of solar
power is to develop it into a primary powersource. To make this practical, power generation costs must be reduced. Also new
power generation systems must be developed that would, for example, electrolyze water using surplus electrical
power generated during the day to produce hydrogen and oxygen gas. That hydrogen and oxygen would be stored to belater supplied to fuel cells and gas turbines
when required to generate electrical power.
Kyoto Semiconductor Corp. has developed spherical silicon micro solar cells
designed to play a central role in these so44

AEI October 2001

lar power generation systems. They alsooffer the promise of reducing the cost ofsolar power generation equipment andmaking such equipment easier to use.
Greater Efficiency
Conventional solar cells employ a flat
light-receiving surface regardless of thekind of semiconductor, the shape of thebulk crystals or the structure of the amorphous crystals employed in the cells. Modules consisting of rows of these cells also
are typically flat.
Solar cells vary in weight dependingon the installation site of the module. Sunlight enters each cell from all directionsin the form of direct radiation, reflection
and scattering. However, in conventional
modules with flat light-receiving surfaces
on one side, the light that enters the solar
cells is limited to the two-dimensional sizeof the light-receiving surface. This meansthat very little of the light shining onto
the sides and back of the cells can makeany contribution to power generation.
To make sure the modules capture as
much sunlight as possible, installers ofconventional solar cell modules select thebest possible azimuth and angle of inclination for the light-receiving surface.
However, when the incident angle changes
with the movement of the sun, reflection
on the light-receiving surface increases,
decreasing power output.
To be more effective, a solar cell must
make use of sunlight entering from all directions. This increases optical input,
which in turn increases output.
Spherical micro solar cells are suchcells. They have a light-receiving surface
covering the entire sphere, enabling more
light to enter the cells for greater electrical power generation.
Spherical Silicon Crystals
The production of spherical silicon cells
differs significantly from the productionof conventional solar cells. Bulk solar cells
now in use are made from recycled scraps
and other remnants of high-purity silicon
for semiconductor devices. This silicon isre-melted, formed into ingots and then cut
into wafers. They are then formed into
square cells measuring 15cm on a sidewith a thickness of about 0.3mm.
The wafers could be made thinner,which would reduce waste and costs.
However, thinner wafers are more likely
to break during handling. Thus they arethicker than otherwise necessary only tomaintain strength, adding to material costs.
One way to address this problem isthrough the use of amorphous and microcrystalline structures formed when asilicon thin film is generated from a vapor phase. However, solar cells made inthis way have a lower photoelectric conversion efficiency than bulk varieties.
Also, they deteriorate more readily thanother models, require expensive vapor deposition equipment and need costly highpurity glass as a substrate.
Better Solution
Spherical silicon solar cells offer a better solution. First, the light-receiving surface receives light from all directions. It
is made up of silicon formed into sphereswith p-n junctions over most surfaces.
Second, spherical cells, formed as microspheres, have a diameter of about 1mm
which raises the ratio of surface area tovolume. This means a small amount ofsilicon forms a large light-receiving surface, saving on the cost of materials.
Third, single crystals are produced bymelting granular silicon polycrystals andthen allowing them to solidify into spheres
using surface tension in a microgravity
environment. This eliminates the processing loss associated with conventional
wafer production, ensuring a high photoelectric conversion efficiency and reducing problems related to deterioration.

SPECIAL REPORTThe production process begins when
granular silicon is fed into the upper melting unit where it melts and then falls (Fig.
1). During the course of falling about 14m
in 1.5 seconds, the silicon solidifies into
spheres to produce single crystals. During melting, the volume of the silicon
shrinks and impurities evaporate from themolten liquid surface. After the silicon solidifies, the purity is further increased bysubsequent etching processes to remove
impurities on surfaces.
With this method, less silicon is requiredfor producing a cell and processing costs
are low. Additionally, this method produces strong, high-quality spherical crystals while conserving resources, savingenergy and saving time.
Texas Instruments Inc. (TI) has conducted research on similar methods for
producing solar cells using spherical silicon. TI researchers propose spherical solar cells made by affixing silicon spheresthat form a p-n junction so as to allow direct contact by two isolated aluminum foil
layers on a flat substrate and each p andn surface. Instead of each cell having an

1

P-type silicon
3Spheres form by surface
tension in a weightlessstate, after which thespheres become solid single spherical crystals. Etching removes impurities appearing on the surface dueto segregation.

2A spherical p-n junction isformed by diffusing phosphorous at high temperatures to provide an n-typelayer. At the same time, ananti-reflection film is formedon the surface.

4A thin film containing p
in the form of n-type impurities is formed on thesurface of p-type silicon.

Impurity diffusion source
Ag as the n-type (negative) electrode and Al as
the p-type (positive)
electrode are printed inopposition.

Ag (negative)

Al (positive)

p-n junction boundary
P-type sphericalsilicon crystals

5

Negative electrode

Heat treatment
p-n junction

n+diffusion layer

Anti-reflective film
Positive electrodeFig. 2: Structure and production process of spherical micro solar cells with anti-reflection film

independent set of electrodes, the cells are
all connected in a row in contact with thealuminum foil while sharing
electrodes. Consequently, thepositive and negative electrodes
Semiconductor material supply unitof the cells are biasly formed
on a common substrate, and theGas inlet
substrate blocks out light on
Melting unit
the back side, restricting thecapture of surrounding light. InTovacuumpump
Camera unit
or gas outlet
addition, the cells can only beconnected in parallel.
Liquid molten spheres
4F bedIn contrast, spherical microsolar cells each have their own
independent electrodes, and theSolidification
cells themselves can each be
Camera unit
used alone as minute solarcells. Because the electrodes
3F bedare provided in opposition atsymmetrical locations relative
to the center of the sphere, theelectrodes of adjacent cells canGas inlet
To vacuum pump
be directly coupled in series usor gas outlet
Camera unit
ing an adhesive. Because this
2F bedSolidification
allows the effective use of lightin all directions in addition to
the axial direction when theSolidification
cells are integrated into a modsphereule, output is improved andseparation results in its non-diTo vacuum pump
Camera unit
or gas outlet
rectivity (Fig. 2).
1F bedSpherical single crystal
recovery liquid tankCompared to conventional
processes for making solar
Fig. 1: Method and production equipment for producing cells, the process for making
spherical single-crystal siliconspherical micro solar cells issimple. According to measurements using the light of a solar simulator from asingle direction (Fig. 3), output increases
by a factor of 1.5 when a reflecting plateis placed in back of the cell. Furthermore,when laterally scattered light is incorporated, output nearly doubles. Therefore,
through the effective use of surrounding
light, output is significantly higher thanthat produced by flat solar cells.
Solar Cell Modules
In a conventional residential solar cell
module using bulk crystals, a filler sheet
and square solar cells measuring 10 to15cm on a side are arranged on front protective transparent glass. The cells are thenconnected in series with an inner lead. A
laminated sheet is affixed to protect theback of the cells and the entire structure
is contained in an aluminum frame. However, because of the size of the wafers, thecells are especially fragile. Also, the modules are troublesome to produce becauseof all the work of arranging and connecting the cells.
Modules using spherical micro solar
cells are much easier to manufacture.
These can be produced first by coupling
the required number of cells in series andin parallel, and then molding the entire
structure, excluding the external terminals, into a single unit with transparent
resin, thereby forming a package.

AEI October 2001

45

SPECIAL REPORT

Through the use of curved surfaces,
light convergence and reflection are prevented. Also, the package has a coatingof anti-reflective film. These features serve
to increase the amount of external lightreaching the module.
Moreover, the structure uses an opticaldesign that guides as much light as possible to the cell surface inside the resin.
That is, the structure is nearly free of components that block light. Light is only
blocked in the axial direction, the direction in which the cells are coupled.
A basic module using spherical microsolar cells would typically have a whitereflector added to the back side (Fig. 4).
Each cell string would have a spherical
reverse current preventing diode. The cells
would be molded together with transparent resin. The reflector would reflect lightthat passes between each cell string backonto the backs of the cells, thus increasing the amount of light reaching each cell.
The simplicity of this structure makes
it suitable for mass production. Throughthe use of standard sizes and other means,makers can reduce costs and facilitate efficient coupling.
Solar cell panels using spherical microsolar cells can be made in various sizes,
curvatures and colors to match diverse
needs (Fig. 5). The modules can also bereplaced easily.
These panels can be installed on thesides of buildings and vehicles to take advantage of their low-directivity characteristics. Other uses also will tap the enhanced design.
1.0
0.90.8Current (mA)

0.70.6

While solar power has
many obvious benefits,
many problems still remain. For example, solar
power generators cannot
be counted on during
darkness, and their output fluctuates a greatdeal. Furthermore, solar
power generators take up
considerable amount of
space and are costly.
How these problems areaddressed will determine
the role solar power generation will play in this Fig. 4: Basic module using spherical micro solar cells
century.
Spherical micro solar
cells may provide someof the answers. Such cells
are more efficient thanconventional cells at converting light to power.
They also should reducematerial and production
process costs. In addition,
due to the low level of directivity, they allow
space to be used three-dimensionally, enabling Fig. 5: Prototype of spherical micro solar cell module
panels to be installed vertically, thereby saving space.
light entering the tank and provide energy
Because spherical micro solar cells canfor hydrolysis. The hydrogen and oxygen
also be protected with transparent glass,generated in this way can be stored,
they are suited to methods for generating
thereby enabling unused electrical energyhydrogen and oxygen. They can, for exto be accumulated in the form of chemiample, be immersed in an electrolysis tank.
cal energy. This gas can be used for power
In such a tank, the cells could capture sungeneration by fuel cells. A power generation system combining solar cells andfuel cells would be able to generate elec500
tricity even in the absence of sunlight.
Many have high hopes for such systems
400
to stimulate demand for solar cells whileat the same time providing a more benign
alternative to the use of fossil fuels.
300

0.5
0.4

2000.30.2

1000.1
0.0
0.0

00.2
0.3 0.4
0.5
0.6
Voltage (V)Characteristics of spherical micro solar cells(diameter: 1.8mm)
0.1

Output increases roughly 1.5 times when a reflective
plate is provided on the back side. When laterally
scattered light is incorporated, output nearly doubles.

Ambient temperature : 24¡C
Open circuit voltage Voc = 0.546V
Short circuit Isc = 0.83mA
Current density Jsc = 33.2 mA/cm2
Optimum operating voltage Vpm = 0.42 V
Optimum operating current Ipm = 0.76 mA
Output power P = 0.319mW
Curve factor FF = 0.7
Conversion efficiency = 12.7%(AM 1.5, 100mW/cm2 , no anti-reflective film,irradiated on one side)

Fig. 3: Basic characteristics of spherical micro solar cells

46

AEI October 2001

Reference
J.D. Leine, G.B. Hotchkizz and M.D.
Hammer-Bacher: Proc. 15th IEEE Photovoltaic Specialist Conference, Las Vegas, IEEE, 141, 1991.

About This Article
The author, Josuke Nakata, is President and Chief Executive Officer of Kyoto Semiconductor Corp.