Spherical Solar Cells Solve Issue of3-D Sunlight Reception

Solar cells have come to be increasingly utilized. The cumulative capacity of solar cell power
generation systems installed in2000 reached 711MW worldwide and317MW in Japan. And in the future, theinstallation of solar power systems is expected to grow rapidly.
Favorable conditions are speeding up theinstallation of these systems. The improvement in manufacturing technology
has made it possible to lower production
costs, and the governments of various countries are taking measures for the introduction of solar cell systems. The public on theother hand, is getting more and more awareof the issue of the global environment andthe problems of future energy resources.

Negativep-type spherical electrode (Ag)Si crystal(0.5-2mm in diameter)n+ diffusion
diffusion layer
layer
n+

pn junction

Surface
withSurface
protection,
anti-refelection
filmfilm
reflection
prevention
Positive
electrode (Al)Fig. 2: Cross sectional structure of spherical Si micro solar cell device

to capture sunlight and convert it to electricity. As such, a solar cell power generWorking With the Sourceation system is very simple. Unlike fosNeedless to say, sunlight is the energy
sil energy resources, which are distributed
source of solar cells, which are designed
unfairly on the earth, a solar cell power
generation system can produce
Semiconductor material supply unit
electricity directly anywhere
there is light, so that there is noGas inlet
need to worry about the depleHeating and melting unit
tion of resources or an adverse
effect of waste on the environTo
vacuumpump
Camera window
or gas outlet
ment. Because of these advanMolten sphere
4F bedtages, the gradual spread of its
utilization is a natural course ofthings.
Solidification
However, there are not a few
problems to be addressed for aCamera window
further spread of this power gen3F bed
eration system. Basically, it cannot produce electricity when andwhere there is no light, thus it
must be supplemented by otherGas inlet
To vacuum pumpenergy sources. Therefore, itor gas outlet
Camera window
Solidificationmust be developed in combina2F bedtion with other energy storage
and power generation systems
such as batteries and fuel cells.
Solidified sphere
1F bedCamera window

To vacuum pumpor gas outlet
Liquid tank for recovery of
spherical single crystals

Fig. 1: Equipment and method for manufacturing sphericalmicro solar cells (model)

Overcoming Obstacles
As for the problems with thesolar cell itself as a power generation device, a further reduction in the production cost ofsolar cell systems is needed. As

the matter stands at present, the power
generation cost by solar cells is about twoor three times higher than that of utility
power supplied to ordinary households.
Furthermore, since a solar cell system isexposed to sunlight, it is important that itshould be improved in weatherability anddurability so that it may be sufficientlyreliable.
Coming Up With Solution
It is more than half a century since thesolar cell power system was invented. It
has been a product rather in the shade compared with semiconductor ICs and othersemiconductor products whose progresshas been spectacular.
So far, various materials and manufacturing methods, together with a variety ofsolar cell types, have been developed andintroduced to the market. However, they
have been developed based on the types
that are designed to capture sunlight witha flat surface. Kyoto Semiconductor Corp.
however has developed a spherical microsolar cell that captures sunlight three-dimensionally (3-D) to improve its power
generation capacity.
Basic Concept of Spherical Micro
Cell
Sunlight is captured by a solar cell not
only as direct sunlight but also as light
diffused by clouds and as light reflected
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45

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Angle (deg) - Relative sensitivity (%)-40-30-20-100+10+20+30+40

100-50+50-60+60-70+70-80+80-90+90-100+100-110+110-120+120-130+130

Flat type-140-150-160-170

180+170+160+150

Spherical
type+140Fig. 3: Directivity characteristics of spherical type compared with flat types

from buildings. The quantity and energyof sunlight as incident light to a solar cellvary according to the positions of the sun,weather conditions and the conditions of
the objects around that reflect sunlight.
Therefore, the main point of a solar cellis to capture as much sunlight as possible

under these conditions to convert it to electricity. Viewed from this standpoint, it cannot be said that solar cells with a flat light
reception surface, which are now in practical use, can sufficiently meet these diverse conditions.
In contrast, the basic concept of a spherSolar simulator light source
AM 1.5 (100mW/cm2)

0.8
1.2

0.4

0.6
0.4

µSC

0.2
0.2
0.0

0.0
0.0

0.1

0.2

0.3

0.4

0.5

0.6

Voltage/V
When one side is irradiatedMeasurement temperature: 25˚C
Open-circuit voltage Voc: 0.59V
Short-circuit current Isc: 1.08mA
Optimum operating voltage Vpm: 0.48V
Optimum operating current Ipm: 1.01mA
Output power Pmax: 0.48mW
Fill factor FF: 0.75

Reflection plate (diffusion plate)
Reflection plate

Fig. 4: IV characteristics of spherical micro solar cells

46

Manufacture of Spherical SiliconSingle Crystals
Since silicon exists in abundance as anatural material and can be obtained at a
relatively low cost, almost all solar cells
use this material. Compared with amorphous and polycrystalline silicon, single
crystal silicon has higher conversion efficiency of light to electricity. It also exhibits higher reliability as it suffers deterioration and other problems less thanother materials.
The spherical micro solar cell is madeby using a sphere (with a diameter of 0.5to 2mm) of single crystal silicon. The
sphere is obtained in the following manner: A certain amount of material siliconis fed to the upper part of a drop tube, 14m
high, and is melted and made to fall in afree state through the tube. The moltenmaterial silicon becomes spherical andsolidifies in the course of the fall.
Impurities contained in the material silicon are also removed in this process by
evaporation and segregation. This process-- the manufacture of spherical single crystals close in size to finished cells in thecourse of crystal manufacture -- eliminates conventional production processes
such as the pulling of single crystal ingots
and their slicing, thus reducing production costs by saving power. (Fig. 1)

0.6

0.8

Output/mW

Current/mA

1.0

ical solar cell is that a spherical sunlight
reception surface can capture light in all
directions and increase its power generation capacity and that it can minimize output fluctuations even under direct sunlightand even when the angle of reflected incident light changes. Furthermore, the diameter of a spherical solar cell should be
reduced in order to increase the proportion of the light reception surface area ofa semiconductor crystal to its volume soas to raise the efficiency of the material.

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Copyright © 2003 Dempa Publications Inc.

Output increased 1.5 times

Cell Structure and Its Characteristics
Diffused on the surface of a p-typespherical silicon single crystal without excepting for a part of its surface are n-type
impurities, and a thin n-type layer is provided to form a spherical pn junction parallel to the surface of the sphere. After
that, a positive and a negative electrode,
opposed to each other, are formed in thecenters of the p-type and the n-type surface, respectively. Covering the other partsof the surface with anti-reflection film
completes a solar cell (Fig. 2). This electrode arrangement makes the cell
non-directive and can realize an even

I-V curve
Current density [A/cm- 2 ]Electric power [W]
31.2m

10.13mI-VPowerPmax
No. of cells: 1760

-1.486m
3.081-0.4822m-0.1467

Aperture: 60 percent(projected cell area - 40 percent)

Voltage [V](Reference: IV characteristic curve)
AM1.5 (100mW/cm2 )Fig. 5: Test-manufactured module(example) -- flat type module forcollection of diffused solar radiation

Voc = 2.984V
Vpm = 2.338 V
Pmax = 1053mW

Isc = 0.4822A
Ipm = 0.4504A
FF = 0.744

Dependence on angles of incident light

Dependence on angles of incident light of normalized value

two layers - light source at 90˚
two layers - light source at 45˚flat type

two layers - light source at 90˚
two layers - light source at 45˚flat type1.2

Output converted to that of 100cm2 module
Normalized values

1,400
1,200
Pmax[mW]

Temperature=25.2˚C

1,000
800
600
400
200

10.8
0.6
0.4
0.2
00
20

4060
80
100120140
Angle of incident light (deg.)

160

20406080
100
120
140
Angle of incident light (deg.)

160

Light source at 45˚Light source at 90˚Flat type polycrystal solar cells(Sales of this type are the largest)

Spherical micro solar cell mini-module

Fig. 6: Spherical type compared with conventional type

distribution of generated current, facilitating a series and parallel connection of
the cell to other cells.
Conventional solar cells invariably havea flat light reception surface and pn junctions parallel to the light reception surface, irrespective of the kinds of semiconductors employed, and no matter
whether they are bulk, thin film or amorphous crystals. Therefore, their output
power is the highest when incident lightis vertical relative to it and the reflection
increases and the output declines, as theangle of incident light becomes larger.
Furthermore, since the light reception surface is generally on one side, no electricity is produced on the other side even if
light is shed on it.

In the case of a spherical micro solar
cell sphere, a larger part of its surface canreceive light excepting for its positive andnegative electrodes. It has an extremely
low degree of directivity and can capture
light in almost all directions to produce
electricity (Fig. 3).
Neither the output power of a spherical micro cell can be correctly measured
by a conventional method, nor has amethod yet been decided for its measurement. Basically, a solar cell shouldcapture direct, reflected and diffused lightfor power generation, thus it cannot becalled an insufficient method to measure
the output power of a solar cell by showering light on it vertically from a single
light source as is done for the output mea-surement of flat-type light reception solar cells.
A current-voltage (IV) measurement
system using a solar simulator (artificialsunlight source) is employed to measure
the IV characteristics of a spherical micro cell, a method adopted for conventional flat-type single junction solar cells(Fig. 4). The output of a solar cell increases
50 percent if a reflection plate is provided
on the opposite side of a light source compared with its output measured without areflection plate. This means that there isample room for a spherical micro solar
cell to improve its output by devising effective ways of collecting reflected light.
The light-electricity conversion efficiency of a test-manufactured sphericalmicro solar cell was calculated by using
the conventional method of output measurement. In case the light of a solar simulator was radiated at AM 1.5 condition
in a vertical direction relative to the axisof the electrodes of a solar cell, its lightelectricity conversion efficiency -- a proportion of its output power to the radiatedintensity of illumination per projected area-- was 18.9 percent without a reflection
plate. There is no doubt that this value
will be raised through the improvementof design and production technology inthe future.
Structure, Characteristics ofSpherical Micro Solar Cell Modules
The spherical micro solar cell has a single spherical pn junction. The cell is verysmall, but its maximum open voltage isthe same as that of a larger flat junction
type cell.
If spherical micro solar cells are combined, they respond to the need for a large
capacity power source as well as for asmall capacity power source. The cells
are spherical and excellent in mechanicalstrength, and a necessary number of thesecells can be connected for series or parallel connection. A small current and highvoltage module of spherical micro solar
cells is economical, as it is small as awhole because of its low current, though
the number of cells connected in series islarge.
The test-manufactured module of spherical silicon micro solar cells is simple instructure (Fig. 5). The module is connected
in series and parallel with fine copper wire,
and is mounted on a white resin reflection plate, with its surface covered withtransparent resin. Measuring 130 × 130
× 10mm, it has 57 cells connected in seAEI February 2003 47Copyright © 2003 Dempa Publications Inc.

ries and 30 cells connected in parallel. Under direct daylight in the daytime on aclear day, it develops a voltage of about
25V and a power of 1W (max.).
An illustration (Fig. 6) shows the dataon the dependence on the angles of incident light of the output of a module ofspherical micro solar cells arranged in twolayers (upper and lower layer) and that ofa flat-type silicon polycrystal solar cell
module with the same light reception surface, together with the outside views ofthese modules.
The micro solar cell module used forthe measurement has its cells arranged sothat the gaps between adjacent cells arefilled with other cells, with a white reflection place underneath.
In case a solar cell is used outdoor for
power generation, the angle of incident
light changes as time passes. Thus it is notproper to compare the efficiencies of solar cells in terms of light-electricity conversion efficiencies measured only whena solar simulator is in a vertical direction
relative to the module. Rather, comparison should be based on the quantity ofelectricity generated by a day’s sunlight.
A spherical micro solar module is lower
in output decline and produces more electricity than the flat-type solar module even

48

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A new spherical solar cell module whichis flexible and can utilize light in all
directions

when the angle of incident light is small.
The photo above shows a test-manufactured flexible module with sphericalsolar cells arranged on a flat surface andconnected inside a thin transparent resin
sheet. This is thin and transparent and canbe used in various forms as a solar power
generation cell sheet, which partially receives light or partially shields light. The
photo shows the module as its four diodes
emit light under sunlight.

Featuring Spherical Micro Solar
Cells
Compared with conventional solar cells,
spherical micro solar cells have simple
structure and require a simple production
process. Spherical micro solar cells canbe manufactured easily into standard modules, and with mass production, manufacturing costs will be further reduced.
Spherical micro solar cell modules respond to a variety of power source needs
ranging from an extremely small to alarge power source through free connection of cells. Spherical micro solar
cell modules can respond in size andshape to diverse design and other application needs. Practically non-directive,
they can increase their output power
through improvement of the efficiencyof light utilization.
With a broad-ranging directivity, there
are few restrictions on their mounting, andas they can be shaped freely, they will beused on structures and cars in which importance is attached to design.
About This Article
The author, Josuke Nakata, is President and Chief Executive Officer of Kyoto Semiconductor Corp.