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Solar cells have come to be in-creasingly utilized. The cumula-tive capacity of solar cell powergeneration systems installed in

2000 reached 711MW worldwide and317MW in Japan. And in the future, theinstallation of solar power systems is ex-pected to grow rapidly.

Favorable conditions are speeding up theinstallation of these systems. The im-provement in manufacturing technologyhas made it possible to lower productioncosts, and the governments of various coun-tries are taking measures for the introduc-tion 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.

Working With the SourceNeedless to say, sunlight is the energy

source of solar cells, which are designed

to capture sunlight and convert it to elec-tricity. As such, a solar cell power gener-ation system is very simple. Unlike fos-sil energy resources, which are distributedunfairly on the earth, a solar cell power

generation system can produceelectricity directly anywherethere is light, so that there is noneed to worry about the deple-tion of resources or an adverseeffect of waste on the environ-ment. Because of these advan-tages, the gradual spread of itsutilization is a natural course ofthings.

However, there are not a fewproblems to be addressed for afurther spread of this power gen-eration system. Basically, it can-not produce electricity when andwhere there is no light, thus itmust be supplemented by otherenergy sources. Therefore, itmust be developed in combina-tion with other energy storageand power generation systemssuch as batteries and fuel cells.

Overcoming ObstaclesAs for the problems with the

solar cell itself as a power gen-eration device, a further reduc-

tion in the production cost ofsolar cell systems is needed. As

the matter stands at present, the powergeneration cost by solar cells is about twoor three times higher than that of utilitypower supplied to ordinary households.Furthermore, since a solar cell system is

exposed to sunlight, it is important that itshould be improved in weatherability anddurability so that it may be sufficientlyreliable.

Coming Up With SolutionIt is more than half a century since the

solar cell power system was invented. Ithas been a product rather in the shade com-pared with semiconductor ICs and othersemiconductor products whose progresshas been spectacular.

So far, various materials and manufac-turing methods, together with a variety ofsolar cell types, have been developed andintroduced to the market. However, theyhave been developed based on the typesthat are designed to capture sunlight witha flat surface. Kyoto Semiconductor Corp.however has developed a spherical microsolar cell that captures sunlight three-di-mensionally (3-D) to improve its powergeneration capacity.

Basic Concept of Spherical MicroCell

Sunlight is captured by a solar cell not

only as direct sunlight but also as lightdiffused by clouds and as light reflected

Spherical Solar Cells Solve Issue of3-D Sunlight Reception

45AEI February 2003

Fig. 1: Equipment and method for manufacturing spherical

micro solar cells (model)

Semiconductor material supply unit

Gas inlet

Gas inlet

Camera window

Camera window

Camera window

Camera window

1F bed

2F bed

3F bed

4F bed

To vacuum pumpor gas outlet

To vacuum pumpor gas outlet

Molten sphere

Solidification

Solidification

Solidified sphere

Heating and melting unit

To vacuum pump or gas outlet

Liquid tank for recovery of spherical single crystals

Fig. 2: Cross sectional structure of spherical Si micro solar cell device

p-type spherical Si crystal

(0.5-2mm in diameter)

Negative electrode (Ag)

n+ diffusion layer

Positive electrode (Al)

pn junction

Surface protection, reflection prevention film

Surface withanti-refelection film

n+ diffusion layer

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from buildings. The quantity and energy

of sunlight as incident light to a solar cellvary according to the positions of the sun,weather conditions and the conditions ofthe 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 elec-

tricity. Viewed from this standpoint, it can-not be said that solar cells with a flat lightreception surface, which are now in prac-tical use, can sufficiently meet these di-verse conditions.

In contrast, the basic concept of a spher-

ical solar cell is that a spherical sunlightreception surface can capture light in alldirections and increase its power genera-tion capacity and that it can minimize out-put fluctuations even under direct sunlight

and even when the angle of reflected in-cident light changes. Furthermore, the di-ameter of a spherical solar cell should bereduced in order to increase the propor-tion of the light reception surface area ofa semiconductor crystal to its volume soas to raise the efficiency of the material.

Manufacture of Spherical SiliconSingle Crystals

Since silicon exists in abundance as anatural material and can be obtained at arelatively low cost, almost all solar cellsuse this material. Compared with amor-phous and polycrystalline silicon, singlecrystal silicon has higher conversion ef-ficiency of light to electricity. It also ex-hibits higher reliability as it suffers dete-rioration 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. Thesphere is obtained in the following man-ner: A certain amount of material siliconis fed to the upper part of a drop tube, 14mhigh, and is melted and made to fall in a

free state through the tube. The moltenmaterial silicon becomes spherical andsolidifies in the course of the fall.

Impurities contained in the material sil-icon are also removed in this process byevaporation and segregation. This process-- the manufacture of spherical single crys-tals close in size to finished cells in thecourse of crystal manufacture -- elimi-nates conventional production processessuch as the pulling of single crystal ingotsand their slicing, thus reducing produc-tion costs by saving power. (Fig. 1)

Cell Structure and Its CharacteristicsDiffused on the surface of a p-type

spherical silicon single crystal without ex-cepting for a part of its surface are n-typeimpurities, and a thin n-type layer is pro-vided to form a spherical pn junction par-allel to the surface of the sphere. Afterthat, a positive and a negative electrode,opposed to each other, are formed in thecenters of the p-type and the n-type sur-face, respectively. Covering the other partsof the surface with anti-reflection filmcompletes a solar cell (Fig. 2). This elec-

trode arrangement makes the cellnon-directive and can realize an even

46 AEI February 2003

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

Flat type

Sphericaltype

Angle (deg) - Relative sensitivity (%)

-40

-50

-60

-70

-80

-90

-100

-110

-120

-130

+50

+60

+70

+80

+90

+100

+110

+120

+130

-30 -20 -10 0 +10 +20 +30 +40

-140 -150 -160 -170 180 +170 +160 +150 +140

100

1.2

1.0

0.8

0.6

0.4

0.2

0.0

0.8

0.6

0.4

0.2

0.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6

SC

Fig. 4: IV characteristics of spherical micro solar cells

Current/mA

Output/mW

Voltage/V

When one side is irradiated

Measurement temperature: 25COpen-circuit voltage Voc: 0.59VShort-circuit current Isc: 1.08mAOptimum operating voltage Vpm: 0.48VOptimum operating current Ipm: 1.01mAOutput power Pmax: 0.48mW

Fill factor FF: 0.75

Solar simulator light source

AM 1.5 (100mW/cm2)

Reflection plate (diffusion plate)

Reflection plate Output increased 1.5 times

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dist ribution of generated current, facili-tating a series and parallel connection ofthe cell to other cells.

Conventional solar cells invariably havea flat light reception surface and pn junc-tions parallel to the light reception sur-face, irrespective of the kinds of semi-conductors employed, and no matterwhether they are bulk, thin film or amor-phous crystals. Therefore, their outputpower is the highest when incident lightis vertical relative to it and the reflectionincreases and the output declines, as theangle of incident light becomes larger.Furthermore, since the light reception sur-face is generally on one side, no electric-

ity is produced on the other side even iflight is shed on it.

In the case of a spherical micro solarcell sphere, a larger part of its surface canreceive light excepting for its positive andnegative electrodes. It has an extremelylow degree of directivity and can capturelight in almost all directions to produceelectricity (Fig. 3).

Neither the output power of a spheri-cal micro cell can be correctly measuredby a conventional method, nor has amethod yet been decided for its mea-surement. Basically, a solar cell shouldcapture direct, reflected and diffused lightfor power generation, thus it cannot becalled an insufficient method to measurethe output power of a solar cell by show-

ering light on it vertically from a singlelight source as is done for the output mea-

surement of flat-type light reception so-lar cells.

A current-voltage (IV) measurementsystem using a solar simulator (artificialsunlight source) is employed to measure

the IV characteristics of a spherical mi-cro cell, a method adopted for conven-tional flat-type single junction solar cells(Fig. 4). The output of a solar cell increases50 percent if a reflection plate is providedon the opposite side of a light source com-pared with its output measured without areflection plate. This means that there isample room for a spherical micro solarcell to improve its output by devising ef-fective ways of collecting reflected light.

The light-electricity conversion effi-ciency of a test-manufactured sphericalmicro solar cell was calculated by usingthe conventional method of output mea-surement. In case the light of a solar sim-ulator was radiated at AM 1.5 conditionin a vertical direction relative to the axisof the electrodes of a solar cell, its light-electricity conversion efficiency -- a pro-portion of its output power to the radiatedintensity of illumination per projected area-- was 18.9 percent without a reflectionplate. There is no doubt that this valuewill 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 sin-gle spherical pn junction. The cell is verysmall, but its maximum open voltage isthe same as that of a larger flat junctiontype cell.

If spherical micro solar cells are com-bined, they respond to the need for a largecapacity power source as well as for asmall capacity power source. The cellsare spherical and excellent in mechanicalstrength, and a necessary number of thesecells can be connected for series or par-allel connection. Asmall current and highvoltage module of spherical micro solarcells is economical, as it is small as awhole because of its low current, thoughthe number of cells connected in series islarge.

The test-manufactured module of spher-ical silicon micro solar cells is simple instructure (Fig. 5). The module is connectedin series and parallel with fine copper wire,and is mounted on a white resin reflec-tion plate, with its surface covered with

transparent resin. Measuring 130 130 10mm, it has 57 cells connected in se-

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No. of cells: 1760Aperture: 60 percent (projected cell area - 40 percent)

Fig. 5: Test-manufactured module (example) -- flat type module for collection of diffused solar radiation

Current density [A/cm- 2 ] Electric power [W]

I-V curve

Voltage [V]

(Reference: IV characteristic curve)

Temperature=25.2C

10.13m

I-V

Power

Pmax

-0.4822m

31.2m

-1.486m3.081-0.1467

Voc = 2.984VVpm = 2.338 VPmax = 1053mW

Isc = 0.4822AIpm = 0.4504AFF = 0.744

AM1.5 (100mW/cm2 )

20 40 60 80 100 120 140 160 20 40 60 80 100 120 140 160

1,400

1,200

1,000

800

600

400

200

0

1.2

1

0.8

0.6

0.4

0.2

0

Dependence on angles of incident light Dependence on angles of incident light of normalized value

Output converted to that of 100cm2 module

Normalizedvalues

Angle of incident light (deg.) Angle of incident light (deg.)

Light source at 45

Light source at 90

Spherical micro solar cell mini-module

Flat type polycrystal solar cells (Sales of this type are the largest)

two layers - light source at 90two layers - light source at 45

flat type

two layers - light source at 90two layers - light source at 45

flat type

Pmax[mW]

Fig. 6: Spherical type compared with conventional type

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ries and 30 cells connected in parallel. Un-der direct daylight in the daytime on aclear day, it develops a voltage of about25V and a power of 1W (max.).

An illustration (Fig. 6) shows the data

on the dependence on the angles of inci-dent 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 cellmodule with the same light reception sur-face, 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 re-flection place underneath.

In case a solar cell is used outdoor forpower generation, the angle of incidentlight changes as time passes. Thus it is notproper to compare the efficiencies of so-lar cells in terms of light-electricity con-version efficiencies measured only whena solar simulator is in a vertical directionrelative to the module. Rather, compari-son should be based on the quantity ofelectricity generated by a days sunlight.A spherical micro solar module is lowerin output decline and produces more elec-tricity than the flat-type solar module even

when the angle of incident light is small.The photo above shows a test-manu-

factured flexible module with sphericalsolar cells arranged on a flat surface andconnected inside a thin transparent resinsheet. This is thin and transparent and canbe used in various forms as a solar powergeneration cell sheet, which partially re-ceives light or partially shields light. Thephoto shows the module as its four diodesemit light under sunlight.

Featuring Spherical Micro SolarCells

Compared with conventional solar cells,spherical micro solar cells have simplestructure and require a simple production

process. Spherical micro solar cells canbe manufactured easily into standard mod-ules, and with mass production, manu-facturing costs will be further reduced.

Spherical micro solar cell modules re-spond to a variety of power source needsranging from an extremely small to alarge power source through free con-nection of cells. Spherical micro solarcell modules can respond in size andshape to diverse design and other appli-cation needs. Practically non-directive,they can increase their output powerthrough improvement of the efficiencyof light utilization.

With a broad-ranging directivity, thereare few restrictions on their mounting, andas they can be shaped freely, they will beused on structures and cars in which im-portance is attached to design.

About This Article

The author, Josuke Nakata, is Presi-dent and Chief Executive Officer of Ky-oto Semiconductor Corp.

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

directions

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48 AEI February 2003