SolarCells_Varios

Bibliography

226 Silicon Solar Cells, CrystallineGlossary

Antireflection coating layer (ARC) Layer deposited

on the illuminated surface of a cell which reduces

the reflection of the light.

Dislocations Lack or additional line of atoms in the

crystal.

Gettering process Extraction of unwanted impurities

and trapping in a region of a wafer which does not

contribute to the photocurrent.

Grain boundary (GB) Border zone of two adjacent

crystalline grains.

Homojunction (heterojunction) Part of a semicon-

ductor diode which separates a p-type region

from an n-type one made with the same material

(in a heterojunction, the materials are different).

C. Richter et al. (eds.), Solar Energy, DOI 10.1007/978-1-4614-5806-7,Silicon Solar Cells, Crystalline

SANTO MARTINUZZI1, ABDELILLAH SLAOUI2, JEAN-PAUL

KLEIDER3, MUSTAPHA LEMITI4, CHRISTIAN TRASSY5, CLAUDE

LEVY-CLEMENT6, SEBASTIEN DUBOIS7, REMI MONNA7, YVES

VESCHETTI7, ISABELLE PERICHAUD8, NAM LE QUANG9, JED

KRAIEM10

1Formerly University of Marseilles, Marseilles, France2CNRS-INESS, Strasbourg, France3LGEP, Paris, France4INL-INSA, Lyon, France5CNRS, Grenoble, France6CNRS, Thiais, France7INES-CEA, Le Bourget du Lac, France8University of Marseilles, Marseilles, France9Photowatt International S.A, Bourgoin Jailleu, France10Apollon Solar, Lyon, France

Article Outline

Glossary

Definition

Introduction

The Silicon Feedstock (Polysilicon)

Silicon Single Crystals, Multicrystals, Ribbons, and

Sheet Growth Techniques

Future Directions

Abbreviations# Springer Science+Business Media New York 2013

Originally published in

Robert A. Meyers (ed.) Encyclopedia of Sustainability Science and Technolospace applications, solar cells were progressively taken

into account for terrestrial applications. The main

problems to solve are, today, the photovoltaic energy

cost which is too high and the conversion efficiency

which is limited. Most of the cells are based on a pn

junction made with a p-type semiconductor and an

n-type semiconductor. When both materials are the

same, the cell is based on a homojunction. When

the materials are different, the cell is based on

a heterojunction. One silicon cell, 15.6 15.6 cm2,can deliver 79 A under 0.6 V only, and for thisreason, the cells are connected in modules in order to

provide a substantial electrical power (about

100250 W). The semiconductor silicon is known as

an extremely pure material; 9 N purity level is required

for electronic components. However, for solar cells,

a 6 N purity level could suffice, and the use of

polycrystalline materials is acceptable. Wafers were

initially cut out from monocrystalline and also of

multicrystalline ingots grown from electronic-gradeMinority carrier lifetime t and diffusion length L Dur-

ation and distance run by an electron in excess

(generated by the sunlight) in p-type silicon or by

a hole in excess in n-type silicon.

Minority carrier diffusion length L Distance run by

carriers in excess.

Multicrystalline silicon Large grained polycrystalline

material, a few mm to 1 cm in size.

Passivation Mechanism which reduces the electrical

activity of crystallographic defects, of surfaces, of

interfaces, and of unwanted impurities.

Precipitates Agglomeration of impurity atoms within

the crystal.

Solar cell conversion efficiency h Ratio of the

electrical power output to the sunlight power input.

Texturization Chemical or physical technique which

increases the roughness of the surface in order to

reduce the light reflection and the diffusion.

Wafer Trench of silicon cut out from an ingot,

200 mm in thickness.

Definition

Solar cells are sources of electrical energy when they are

illuminated by solar radiations. They deliver to a load

a photocurrent and a photovoltage. First used forgy, # 2012, DOI 10.1007/978-1-4419-0851-3

227Silicon Solar Cells, Crystallinethe lack of toxicity of this element, and, last but not the

least, to its very high abundance in the Earths crust.

Moreover, the huge knowledge accumulated in the

electronic device research and industrial production is

a source of continuous progress.

However, the cost is always a problem, and a lot of

efforts in the field of research and engineering

are devoted to reduce the cost of the peak watt Wp,

i.e., the cost of the electrical power produced when the

cells are illuminated by the sun under optimal

conditions. Such a cost depends on the cost of mate-

rials, of the cell processing steps, and of the

module fabrication, but it depends also markedlyfeedstock, but progressively, solar-grade feedstocks

could be used. The physical, the chemical, and the

electrical properties of such wafers must be investigated

in order to be qualified for cell production. The struc-

ture of the solar cells can be very basic, i.e., one pn

junction with simple metallization and antireflection

coating. However, in order to increase the conversion

efficiency, which tend to reduce the cost of the

produced electrical energy, advanced concepts must

be used in industrial production like selective emitters,

new metallization techniques, and new cell structures

like interdigitated back contact and back junction

cells and heterojunction cells.

Introduction

Silicon solar cells were used, for the first time, as

electrical energy sources for satellites. The first one

was Vanguard launched 52 years ago. The technical suc-

cess of the photovoltaic panels was so clear that their use

in terrestrial applications was proposed at the end of the

1960s. Several solutions have been considered to decrease

the cost of the photovoltaic systems and energy. Thin film

cells, like CdSCu2S or CdTeCu2 Te structures, were

investigated at the end of the 1960s, but they were not

convincing. Silicon solar cells based on monocrystal-

line, on multicrystalline wafers, and on hydrogenated

amorphous silicon thin films appear, during the 1980s,

as a possible solution. Indeed, although new thin films

cells have been developed during the last 20 years, at the

end of 2010, crystalline silicon solar cells share87% ofthe worldwide photovoltaic market. This success is due

to the stability of the photovoltaic silicon structures, toon the cell conversion efficiency. Therefore, there

was a competition between monocrystalline and

multicrystalline solar cells during the last 20 years,

and today, both type of cells share approximately the

same part of the worldwide market. The conversion

efficiency of commercial monocrystalline cells achieve

1824%, while that of multicrystalline cells, whichincreases continuously, is by 23% absolute less higher.

Although multicrystalline wafers contain large

densities of imperfections like grain boundaries, dislo-

cations, and impurities, they lead to acceptable solar

cells because the processing steps needed to make the

devices (phosphorus diffusion, aluminumsilicon

alloying, deposition of a hydrogen-rich antireflection

coating) improve the electrical properties of the wafers,

enhancing the minority carrier lifetime and diffusion

length, unless the cost of the cells be increased.

An intense research activity was developed in the

field of impurity gettering and passivation, which

contributes to a clear understanding of the influence

of metallic elements, distinguishing the role of fast

and slow diffusers in silicon. It was clearly shown

that the extended crystallographic defects, by

themselves, are not very detrimental to the electrical

properties of the wafers, and it is the interaction

between impurities and crystallographic defects which

impair the electrical properties.

From a material point of view, R&D efforts must

lead to a very abundant production of a low-cost

solar-grade silicon feedstock which can be used in the

monocrystalline ingots pullers, as well as in the casting

of multicrystalline ingots. The crystalline and the

electrical quality of the ingots, as well as that of

the wafers, must be improved and controlled. In order

to save a pure and crystallized material, the thickness of

the wafersmust be reduced to less than 120 mm.To avoidthe sawing of ingots, maybe ribbons or sheets could

replace the wafers cut out from a crystalline ingot.

Solar cell structures are still improving at an

industrial production level because well-investigated

concepts are included in the processing steps. Such con-

cepts like the selective emitter, the back contact, and

back junction cells, the use of heterojunctions in place

of homojunctions, present definite advantages which

contribute to reach conversion efficiencies higher than

20% at an industrial level and 24% at a laboratory scale.

The Purification of MG-Si

reaction with HCl at 575 K, as indicated by the overall

reaction:

Si s 3HCl g ! SiHCl3 g H2 g 2Obviously, other chlorides are also produced, like

SiCl4.

Trichlorosilane is generally transported to

polysilicon production plants where it can be purified

to a very high level by fractional distillation because

its boiling point is higher than those of other

chlorides [1]. Boron, phosphorus, and metallic atom

concentrations are reduced to less than 1 ppba (9 N

level).

(Notice that a large par of SiCl4 can be converted

into SiHCl3 by simple processes.)

A lot of extraction techniques have been proposed

in the past [2]; however, today, the Siemens process is

dominant [3]. It is based on the bell-jar reactor system

and on elaborated techniques to recover the chemical

by-products from the reactors. Inside the reactor

228 Silicon Solar Cells, CrystallineThe required purification results from two different

ways: the chemical or liquid routes and the direct

metallurgical routes.

The Chemical Route The requirements of the semi-

conductor industry appear extremely hard to be

satisfied but, fortunately, chlorine when purchased as

HCl and MG-Si are inexpensive, and that open the

way for the use of trichlorosilane SiHCl3 (TCS).

TCS is a very pure precursor for the deposition of

EG polysilicon by the chemical vapor deposition tech-

nique (CVD). Indeed, MG-Si is transformed mainly

(90%) into trichlorosilane SiHCl3 (TCS) by theSuch results are close to the theoretical limits of

silicon solar cells based on single junction, and the way

is open for new cell structures, like tandem cells and new

silicon-based materials (nanomaterials, intermediate

band materials, nanowires, quantum dots).

Crystalline silicon solar cells are made with wafers

that are cut out frommonocrystalline ormulticrystalline

ingots after some processing steps. Ingot growth requires

very pure silicon feedstock, although the purity level is

lower than that needed for electronic devices. The fol-

lowing chapters describe successively the feedstock pro-

duction, the ingot growth and properties, the

conventional and advanced cell structures.

The Silicon Feedstock (Polysilicon)

The Metallurgical Silicon

Metallurgical-grade silicon (MG-Si) is produced by the

carbothermic reduction of pure quartz and quartzite

rocks (silica), as indicated by the overall reaction:

SiO2 s 2C s ! Si l 2CO g 1The reaction occurs in an electric arc furnace and

requires a huge amount of electrical energy.

MG-Si is99% pure (2N), andmajor impurities areAl, Fe, and C. MG-Si is mainly used in the aluminium

and iron industries (60%) and for the production ofsilicones (25%). Only a few part (15%) is highlypurified in order to reach the electronic-grade (EG-Si),

at least 9 N pure, required by the semiconductor indus-

try, or 67 N needed for solar cells.chamber, gaseous trichlorosilane is dissociated at

575C in elemental silicon thanks to resistance-heatedfilaments. Several hundreds of reactors are located in

one silicon plant in order to produce several thousands

of metric tons a year. A schema of the Siemens-type

bell-jar reactor is given by Fig. 1. TCS is carried by

preheater

Quartzbell jar

Siliconcore

polysilicon

TCS + H2exhaust Electric energy

Silicon Solar Cells, Crystalline. Figure 1

The Siemens bell-jar reactor

instance, Wacker Chemie is contributing to polysilicon

supply by expanding its annual capacity to more than

25,000 metric tons [6]. Probably, the worldwide

amount will be larger than 150,000 metric tons in 2010.

TheMetallurgical Route For solar cells, a solar-grade

silicon (SoG-Si), less pure than EG-Si, could be accept-

able at a 67 N level. Downgrading the Siemens process

to make it less energy consuming was proposed, but

such a technique does not enable to reach the capacity

production for GWp installation of solar cells.

Another approach is to upgrade MG-Si more

cheaply with metallurgical methods, e.g., slagging,

leaching, or impurity segregation combined with

directional solidification, as proposed by Crystal

Systems Inc. [7], leading to the so-called upgraded

metallurgical silicon (UMG-Si). Investment cost and

energy consumption are drastically reduced; the mate-

rial could be very abundant, but the degree of impurity

is higher, and refining processes are needed, especially

to reduce the concentrations of phosphorous and


Schema of the fluidized bed reactor

229Silicon Solar Cells, Crystallinehydrogen, comes in contact with filaments heated at

1,100C, and then, silicon is deposited in form of rods.The sizes of a 40-metric tons/year capacity reactor are

1.2 m in diameter and2.5 m in height. The reactorscan dissipate thousands of kilowatt of heat, accept and

exhaust thousands of cubic meters of hydrogen per

hour, use corrosive HCl, and may produce 1,000 kgof silicon in a single run. The electrical energy

consumption is 120160 kWh/kg.

The Siemens process is very efficient; however, the

plants are very expensive and need huge investment as

a 1,000 t/y production plant requires 100 M.Moreover, the chemical products are hazardous

(SiHCl3; SiH4) or explosive (SiH2Cl2), and a lot of

cautions must be taken. These plants are in fact more

suited to the silicon production for the electronic

device industry than for a mass production devoted

to solar cells.

Other techniques to produce polysilicon have been

investigated like the fluidized bed reactor (FBR), the

vapor to liquid deposition process, or the free-space

reactor. The more efficient is FBR, investigated by REC

and Wacker Chemie [4]. In this technique, silicon

fluoride instead of MG-Si is converted to monosilane

SiH4, and then, a silicon seed is dropped into the

reactor in a flow of monosilane and hydrogen gases,

as shown by Fig. 2. At the heated zones of the

reactor, the gaseous flow dissociates to solid silicon in

form of pellets. The theoretical advantages of this

process are the possible continuous operation as well

as lower capital and electricity costs. The produced

polysilicon is less pure than that given by the Siemens

process and could be a good candidate for photovoltaic

devices.

Polysilicon could also be obtained through decom-

position of silane (SiH4), as indicated by the overall

reaction:

SiH4 g ) Si l 2H2 g 3The advantages are that the temperature needed for

decomposition is lower than for TCS, the conversion

efficiency is higher, and corrosive compounds are not

formed [5].

It is difficult to give the amount of polysilicon

produced in 2010 because all the production plants

have increased their capacity in order to satisfy the

requirements of the photovoltaic industry. ForSilicon seedintroduction

heater

Effluent gas

hydrogen

SiCreactorwall

Reactor gaz inlet +Granules removal

Promising results have been obtained with the Elkem

Solar Silicon (ESS) [18, 19] as well as with the Sintef

Solsilc silicon [8] when the material is blended with EG

polysilicon (ESS results from the reaction between

highly pure raw materials like carbon black and high-

purity quartz).

Several other techniques of refining MG-Si have

been proposed, such as slagging or segregation steps.

However, the dopant concentrations, especially

boron, are generally too high (above 5 1017 cm3).The mixing of EG and UMG feedstock was used

successfully and more recently by IBM [20]. Details of

the processes are generally missing, and information

may only come from the laboratory scale about plasma

torch process [16] used by FerroPem. In such

a process, the feedstock is introduced and melted in a

crucible, and an argon plasma gas, containing other

230 Silicon Solar Cells, Crystallineboron (n- and p-type dopants) which are difficult to

eliminate, in order to avoid a compensation of silicon

and a reduction of minority carrier lifetime.

The metallurgical refining techniques include

raw material selection, segregation, leaching, and slag

treatment. However, the most critical point remains the

dopant concentration.

High-purity precursors have been used in the frame

of the Sintef Solsilc project [8]. Ultrapure silica is

obtained by selection of high-purity quartz or

by oxidation of gaseous silicon chloride. This silica is

reduced by black carbon obtained by gaseous route in

an arc furnace. However, the high-purity silicon

obtained is polluted by SiC, which has to be removed

by several solidification steps.

The choice of pure quartz and pure reducer,

generally charcoal, is the first step to obtain MG-Si

with a low content of boron and phosphorus. At this

stage, metallic pollutants are not the most critical

because they can be reduced afterward by segregation

or acid leaching, what is not the case of boron and,

to a lower extent, of phosphorus. Slag treatment can be

used to reduce boron [9]; it is the route developed by

Elkem.

Another way to remove boron or phosphorus is to

use plasma treatment or electron beam evaporation

under low pressure. The first studies on plasma

treatment have been carried out at laboratory scale by

a French research group [10]. But it is the Kawasaki

research group, in the frame of a NEDO project, which

has developed a four-stage process. This process used

electron beam for phosphorus removal, arc plasma

with steam added argon for boron removal, and two

segregation steps [1113]. The boron and phosphorus

concentrations were reduced to 0.2 ppma.

Alemany et al. have developed, at laboratory scale,

a process involving an inductive plasma torch with the

electromagnetic stirring of the molten silicon [14, 15].

The reactive gas was hydrogen and oxygen.

This process has been implemented by FerroPem in

the frame of the Photosil project [16].

Several companies and academic laboratories

[6, 17] are currently involved with the development of

solar-grade silicon by different approaches. For

instances, Elkem Solar uses in house production of

MG-Si followed by three sequential purification

steps to reduce impurity level of critical elements.reactive species, is created by induction. The plasma is

blown onto the surface of liquid silicon, which is

continuously renewed by electromagnetic stirring.

Impurities like B, C, Al, and Ca are partly removed

and volatilized. It is less easy to remove phosphorus

atoms, even by evaporation, and the best solution is to

lower the phosphorus content at the beginning of the

entire process. After the plasma treatment, the

concentration of boron is reduced by a factor 23.

Following such treatment, the boron concentration

can be adjusted to less than 2 ppma. Figure 3 shows

a picture of the plasma torch in operation.


Purification of melted UMG-Si by the plasma torch

the top of the rod, and the rod is rotated in order to

reduce the inhomogeneities. Figure 4 shows a diagram

of the pulling process.

During the growth process, two basic effects are

operating, crystallization and purification. Indeed,

impurities, with higher solubility in the liquid

compared to that in the solid phase, are transported

with the molten zone and can be accumulated at the

extremities of the grown crystal, as a consequence of

the impurity segregation phenomena, resulting of their

distribution coefficients between the solid and the

liquid silicon phases, given in Table 1.

The repeatability of the process, the lack of crucible,

and the use of a neutral atmosphere lead to a quasi-

perfect single crystal (practically without dislocations)

of extreme high purity, provided the silicon rod is itself

of high purity. These basic steps have not changed since

they were first established in the 1950s [2224], and

if FZ silicon crystal sizes have considerably increased,

such crystals are always the best from a crystallographic

as well as from a purity level point of view. Indeed,

today, commercial FZ silicon ingots achieve15 cm in

231Silicon Solar Cells, Crystallinedescribed in Ref. [1]), while the properties of

multicrystalline silicon will be detailed in section

Physical, Chemical and Electrical Properties of mc-Si

Wafers.

Float-Zone-Grown Monocrystals (FZ)

The FZ pulling method, described in details by Dietze

et al. [21], has the great advantage to be crucible free.

A rod of polysilicon, containing a monocrystalline seed

at its bottom, experiences a local fusion by means of an

RF coil. The molten zone is moved from the bottom toNotice that a marked advantage for refined UMG-Si

is that the processes for producing solar cells are similar

to those used for current silicon material but with

different parameters to be applied, as will be shown later.

Silicon Single Crystals, Multicrystals, Ribbons,

and Sheet Growth Techniques

Crystalline siliconwafers used for solar cells are cut from

monocrystalline float zone (FZ), from Czochralski

grown (Cz), from multicrystalline (mc-Si) ingots, or

from ribbons. Such crystals are grown from a melt

contained in various crucibles, except for FZ crystals.

Cz silicon is mainly used for manufacturing highly

integrated low-power devices, especially memories,

whereas FZ silicon is mainly used for power devices

and photo detectors. Both crystals can be used

for high-efficiency solar cells, especially FZ silicon.

In 2009, 37% of the photovoltaic market wasshared by single-crystalline cell modules, 45% bymulticrystalline cell modules, and 2% by ribbon-based cell modules.

The crystalline ingots are cut into thin wafers

(180 mm) by means of a diamond blade or a wiresaw, while from ribbons wafers, crystalline ingots are

cut out by means of a laser.

In the following paragraphs, a few basic properties

of single crystals are given (they are well known andAdditional problems in the material come from

light elements like C and O, as well as from slow

diffusers like Ti or Al.

Researches have been carried out recently in Europe

and Japan to develop cleaning and crystallization pro-

cesses for metallurgical silicon feedstock and wafers. Due

to confidential restrictions, only few results are known.Polycrystallineingot

Molten silicon

RF coil

Grown singlecrystalline material

Single crystalline seed


Diagram of the FZ growth process (Courtesy of UNSW)

ie

ib

04 Bismuth 7 104

104 Oxygen 1.25

02 Sulfur 105

101 Manganese 10 5

102 Iron 8 106

04 Cobalt 8 106

101 Nickel 3 105

01 Tantalum 107

102

Argon

Si melt

SiO

Single crystal

Seed

Quartz crucible

Susceptor

Bottle neck

232 Silicon Solar Cells, Crystallinediameter, 60 cm in height, and the purity level ishigher than 10 N (less than 1 ppba, except for oxygen

and carbon). In FZ single crystals, the minority carrier

diffusion length and lifetime can reach a few mm and

a few ms, respectively.

Czochralski-Grown Monocrystals (Cz-Si)

Invented in 1918, the crucible pulling of single crystals

according to the Czochralski (Cz) technique [25] was

applied first to germanium crystals and later to silicon

[26]. As shown in Fig. 5, polysilicon is melted in a pure

quartz crucible, and a seed crystal just touches the top

Silicon Solar Cells, Crystalline. Table 1 Distribution coeffic

silicon (From Ref. [1])

Impurity Distribution coefficient Impurity Distr

Lithium 102 Indium 4 1Copper 4 104 Thallium 1.7 Silver 106 Carbon 6 1Gold 2.5 105 Germanium 3.3 Zinc 105 Tin 1.6 Cadmium 106 Nitrogen 7 1Boron 8 103 Phosphorus 3.5 Aluminium 2 103 Arsenic 3 1Gallium 8 103 Antimony 2.3 of the liquid before it is then slowly retired in order that

liquid silicon solidifies close to the seed. The slow

vertical ascent of the solidified silicon gives rise to

a single-crystalline ingot, which emerge from the

melt. To obtain a dislocation-free material, a crystal

neck must be grown which becomes dislocation-free

after few cm because dislocations grow laterally out

of the crystal. Then, the diameter is enlarged, and the

crystal growth is finished without dislocations by

reducing the diameter to zero at the bottom of the

crystal.

Like for FZ pulling, the Cz process is

a crystallization and a purification step, and a large

part of the impurities contained in the polysilicon

and coming from the silica crucible remains in the

melt, except oxygen which the distribution coefficient,

given in Table 1, is higher than 1.nts k between liquid and solid phases of some impurities in

ution coefficient Impurity Distribution coefficientFortunately, silicon monoxide evaporates easily

from the melt. Nevertheless, oxygen concentration

can achieve up to 20 ppma in the crystals, which, in

Heater

Argon + SiO +CO


Diagram of a Cz single-crystal growth puller. Notice the

counter rotation of the pulled single crystal and of the

crucible

233Silicon Solar Cells, Crystallinethe reader could consult the chapters 68 in Handbook

of Semiconductor Silicon Technology [1].

Albeit they are close to perfection, FZ and Cz

crystals contain imperfections like swirl defects. Such

defects are more concentrated in Cz crystals due to

oxygen-related defects coming from an oxygen super

saturation and a slightly higher metallic impurity level.

Nevertheless, such crystals are used to make integrated

circuits for electronic device thanks to internal

gettering effects [27] and to the formation of

a precipitate-denuded zone in the region where the

components are realized. Large-area efficient solar

cells can also be obtained, provided that the material

does not experience high-temperature anneals

(above 900C) which could lead to the formation ofoxygen-related precipitates and stacking faults.

In Cz single crystals, minority carrier diffusion

lengths and lifetime are higher than 400 and 200 ms,respectively.

However, in p-type Cz wafers, the high concentra-

tion of oxygen led to the formation of boronoxygen

complexes under sunlight, and this will be a drawback

for the high-efficiency cells as will be shown later.

Topsil in Denmark, MEMC in USA and in Italy, as

well as Siltronic in Germany and France are the more

important producers.

Multicrystalline Silicon (mc-Si) Ingot Growth

For a massive production of large-size cells, mc-Si cast

ingots are the material of choice. In 2007, more than 4

GWp of mc-Si modules have been installed. In 2010,

mc-Si modules shared more than 45% of the 24 GWp

produced.

If single-crystalline silicon wafers are a very mature

product, which the properties are well controlled,

mc-Si ingots suffer from a large variation of basicfact, are supersaturated at room temperature.

To enhance the removal of SiO and to prevent

a contamination by CO, the furnace is purged by a

strong argon stream. Today very large commercial

single-crystalline ingots are produced by the Cz

techniques up to 30 cm in diameter and 1.52.5 m in

height. Ingots, 45 cm in diameter, are announced. The

properties of these single crystals have been widely

investigated, and it is impossible to give an exhaustive

bibliography in this chapter. For detailed information,properties like crystalline defect and impurity concen-

trations. Each ingot may have various properties,

depending on the growth techniques, and in a given

ingot, the basic properties of the wafers are depending

on their position in the ingot. This is why the crystal-

line, the chemical, and the electrical properties of mc-Si

wafers will be detailed in section Physical, Chemical

and Electrical Properties of mc-Si Wafers.

In order to decrease the cost of the material and to

increase the size of solar cells, a lot of researches were

devoted to the production of large grain size (few mm

to few cm) crystalline silicon in which the concentra-

tion of impurities could be below the 0.01 ppma.

Square cells, up to 21 21 cm2, could be realized.Such a material was frequently labeled solar-grade

silicon because its utilization is restricted to the

production of solar cells. In addition to metallic impu-

rities of carbon and, at a lesser extent, of oxygen and

nitrogen, mc-Si wafers contain extended crystallo-

graphic defects like grain boundaries (GBs), disloca-

tions, and twin boundaries. Moreover, metallic

precipitates are frequently formed at extended defects

and within the grains. When the carbon and oxygen

concentrations are both higher than 10 ppma, a severe

crystallographic degradation occurs with the formation

of grit structures. In these grit structures, it is found

with high concentrations of C, O, and SiC, and the

grain size becomes very small. Such regions cannot

contribute to the photocurrent of solar cells, but

today, such bad regions are practically eliminated by

a better control of the growth conditions.

The first material produced at an industrial level

was the so-called Silso, developed by Wacker/

Heliotronics [28] in Germany at the end of the 1970s.

It was followed by other companies like Photowatt in

France with the Polix in USA and in Japan. More

than 220 bibliographic references of papers and patents

can be found in Ref. [30], for a period going from 1970

to 1980 only. Today, mc-Si is a quasi-standard product

produced worldwide, especially in China and Taiwan,

which the growth is based on the directional solidifica-

tion, and most of the research and development

activities are concentrated in Germany, Japan, and

more recently in China.

The basic principle is the following. In a quartz

crucible, heated by graphite heaters and coated with

Si3N4, silicon (EG-Si or silicon waste coming from the

is the quasi-monocrystal or the monocast silicon. This

point will be developed at the end of the chapter in

Future Directions.

Wafer Sawing Wafers are commonly cut out of the

ingots or of the bricks bymeans of wire saws using a SiC

loose abrasive, which enables the production of

200 mm thick slices. This technique possesses a highthroughput, but up to 50% of the crystallized and

purified silicon is lost during process. In order to

reduce this waste, 100 mm diameter wires can be usedwith fine SiC particles in the slurry [35]. The sawing

can also be improved, employing diamond grains fixed

onto the surface of the wires by means of a resin [36],

because the sawing speed is increased by 2.5 while

the thickness variation is decreased by 3. Moreover,

after sawing, the slurry could be reused because it

does not contain SiC particles like in the conventional

process. Obviously, kerf-loss-free techniques are also of

a great interest, as that proposed recently by IMEC

group [37].

Directional solidification of multicrystalline silicon ingot.

A temperature gradient results from heat extraction

through the crucible bottom

234 Silicon Solar Cells, Crystallinemicroelectronic industry) is melted. Then, a vertical

temperature gradient between the bottom and the top

of the melt is established in order that the bottom

solidifies first and the top solidifies last. Crystallization

begins at the contact of the crucible bottom when its

temperature decreases below the fusion point. After

a few cm from the bottom of the crucible, quasi-

columnar grains grow more or less vertically. Larger

blocks can be obtained up to 650 kg in weight and up to

35 cm in height [29, 30].

The temperature gradient results from a mobile

heating RF core or, more simply, by the so-called heat

exchange method [29], in which the bottom of the

crucible is cooled by a gas flow. Other methods of

cooling use a natural cooling down due to the

removing of the thermal insulation of the crucible

bottom or, more originally, the action of an infrared

radiation transparent bottom [31]. The process time

for one cycle, from loading the crucible to unloading,

is usually around 48 h. Figure 6 summarizes the

directional solidification process.

The dominant (in concentration) impurities in

mc-Si are carbon, oxygen, and nitrogen due to the

contact with the crucible followed, at a lesser extent,

by metallic atoms.

Notice that larger ingots can be produced by the

electromagnetic continuous casting into a cooled

crucible. This technique was first developed by Sumco

in Japan, formerly Sumitomo-Sitix [32], and then in

France by EMIX [33, 34]. The principle of the electro-

magnetic continuous casting is to feed continuously

a copper cold crucible in which alternative

electric current is flowing. This current creates an

induced current in the silicon, and the Joule effect

heats the material up to the melting point. Simulta-

neously, the induced magnetic field and current create

a Lorentz force close to the surface of the melt that

prevents contact with the cold crucible wall (a gap of

some hundreds of mm exists). The process begins byheating a graphite piece which is pulled downwhen the

Si melt is large enough. The pulling rate is typically

50 mm s1.The blocks are cut into columnar bricks, which are

wire-sawed into thin wafers (200 mm), then cleanedand texturized.

Notice that a new product will be proposed which

combines the advantage of mono- and multicrystals: itheat

crucible

Solid silicon

Liquid silicon


235Silicon Solar Cells, CrystallineSi melt

Ribbon

Die


Schema of the growth of an edge-defined film-fed (EFG)Ribbons The sawing of the ingots wastes at least 40%

of an expensive, pure, and crystalline material

(the subproduct powder is too much contaminated to

be reused). To avoid this wasting, the growth of shaped

crystalline ribbons from silicon melt, which has been

already investigated in the 1970s, became of interest

during the last 20 years [3840].

Ribbons have been prepared mainly by various

techniques such as edge-defined film-fed growth

(EFG), edge-supported pulling called also String Rib-

bon (SRT), ribbon on graphite substrate (RGS), and

ribbon on sacrificial carbon template (RST). EFG and

SRT techniques have, today, reached the production

level. Such techniques are recognized to enable the

direct production of flat wafers after laser cutting.

The EFG ribbons, which the growth technique is

described in Fig. 7, can be pulled in a multiple ribbon

growth furnace, for instance, in shape of nonagon tubes

with a side width of 156 mm or in form of 12-face

tubes with a side width of 125 mm. The length of the

tube can be as long as 6.5 m [41, 42]. The EFG ribbon is

developed by RWE Schott Solar.

ribbonIn the STR ribbon [4346], two high-temperature

Ribbon

Silicon melt

Seed

Filament orString


Schema of the growth of a string (STR) ribbonresistant wires (strings) are pulled vertically through

a shallow siliconmelt, and the molten silicon spans and

freezes between the strings, as shown by Fig. 8.

The process is continuous: long strings are used, the

melt is replenished, and the silicon ribbon is cut to

length for further processing, without interrupting

growth. The STR ribbon was developed by Evergreen

Solar.

RGS ribbons that consist mainly in melting of

silicon powders possess the advantage of a higher

throughput production, but it remains to a pilot line

production due to contamination problems [47].

These horizontal growth processes, described by Fig. 9

which was developed at ECN, the Netherlands, have the

highest throughput rate. However, these processes

appear to be bound to the production of thick wafers,

typically over 400 mm, with a poor crystalline texture,and, in turn, lower cell conversion efficiencies.

Moreover, extended chemical treatments are necessary

to eliminate surface corrugations and backside

contamination of the sheets.

The RST ribbon growth process [48] is based upon

the crystallization of two opposite silicon films drawn

of carbon and also of metallic impurities coming from

the dies. There are also large densities of intragrain

defects like dislocations and twins. Dislocations are

detrimental, but the minority carrier lifetime is higher

in twin lamellae lying parallel to the growth direction.

The main impurity is carbon, with a substitutional

concentration of 1018 cm3, exceeding the solubility

Throughput (cm2/min) >300 165 55 (4 ribbons)

Thickness (mm) 80 300 280

236 Silicon Solar Cells, Crystallinealong by a carbon ribbon substrate, which is pulled

upward through a siliconmelt (see Fig. 10). The carbon

ribbon is passed vertically through a slot at the bottom

of a crucible (silica or carbon), which contains the melt.

As it emerges from the melt upper surface, this ribbon

shapes the freestanding silicon meniscus and is coated

on both sides with silicon layers.

After growth, the carbon ribbon is burnt in an

oxygen-containing gas at high temperature, upon

which the self-supported silicon sheets are released

and ready for the fabrication of the solar cells. The

substrate participates in the elimination of the latent

heat of crystallization, which allows relatively high pull

rates. It shapes the freestanding freezing meniscus,

which ultimately yields essentially flat silicon sheets

free of grooves. However, thermoelastic stresses, gener-

ated by the difference in thermal expansion coefficients

between the silicon films and the carbon substrate, set

a limit to the minimal thickness of the silicon films at

Si melt

Ribbon

Substrate


Schema of the growth of a ribbon on graphite substrate

(RGS)around 4050 mm.Table 2 gives the throughput rate (cm2/min) and

compares the actual thickness for vertical ribbon

growth. Although not in production yet, the RST pro-

cess, developed by Solar Force in France, is the only

ribbon process which can simultaneously achieve thin

silicon films with relatively flat and smooth surfaces

and a high throughput rate. Typically, at a pull rate of

10 cm/min, the thickness of the silicon films is 80 mm(or below).

In all ribbon technologies, there is a very high-

temperature gradient at the solidliquid interface

related to the high growth speed. Wafers cut out of

ribbons are multicrystalline and contain large amountsPull direction

High pull

Extendedgrowth front


Schema of the RST ribbon growth process

Silicon Solar Cells, Crystalline. Table 2 Throughput rate

and actual thickness for vertical ribbon growth processes

Process RST EFG STRlimit at the melting temperature.

In addition to the material saving (there is no

kerf-loss), ribbons posses another advantage: the

energy pay back of the cells is lower than that of other

crystalline cells because the pulling processes are less

energy consuming.

Silicon Sheets Another technique to produce directly

large-size flat wafers proposed by the Sharp group in

1997 is the crystallization on dipped substrate (CDS).

The basic principle of CDS technology consists of to

dip a cold refractory substrate into molten silicon. The

silicon crystallizes uniformly; then, the substrate is

pulled out of the molten silicon; a multicrystalline

crystals, multicrystalline silicon wafers contain

grain boundaries (GBs) and intragrain defects like

subgrain boundaries, dislocations, and twin bound-

aries, as shown by Fig. 12. Due to a higher impurity

concentration, precipitates could be formed within the

grains or at extended defects.

Notice that the grain growth is columnar, and this is

a great advantage because in the wafers cut perpendic-

ularly to the ingot growth direction, there is only one

High porosity Substrate


Microphotography of an epitaxial layer before detachment


Photography of a polix mc-Si wafer after alkaline etching

237Silicon Solar Cells, Crystallinesilicon sheet is detached from the substrate and cut by

a laser to 15.6 15.6 cm2 size. The main advantage ofthe CDS is a high production throughput which

achieves 1,825 cm2/min [49].

The wafer molding has also been studied by some

laboratories, and the molded wafers have been realized

on graphite mold coated with SiC or Si3N4.

The research efforts are focused today on the

improvement of molded wafers photovoltaic quality

and on the reduction of their thickness. Moreover,

directional crystallization is obtained by means of

a reusable oriented seed.

Transfer Layer To produce thin substrates of mono-

crystalline silicon, less than 50 mm, a new technologicalroute has been considered: the transfer of thin film

[5053]. It is based on the use of a sacrificial layer of

porous silicon on which grows by epitaxy, the active

layer of silicon, as described by Fig. 11. Then, the thin

film is transferred on a low-cost substrate and depends

on the architecture of the cell considered. The starting

silicon substrate can then be reused after cleaning its

surface. This avoids the sawing step.

Porous silicon (PS) is formed using a specific

anodization cell. Indeed, in order to simplify the

further transfer step, porous silicon is formed on

the whole surface. Electrochemical anodization of

(100) silicon wafers in HF solution is realized on the

entire surface. A double-phase etching process generates

two superposed PS layers: a top layer with low porosity

(2023%) allowing high-quality crystal growth and

a buried layer with high porosity (6570%) for the

further detachment of the epilayer [54, 55].

Physical, Chemical, and Electrical Properties of

mc-Si Wafers

The huge number of publications and conferences

dedicated to the properties of mc-Si wafers is so high

that it is impossible to respect an exhaustive list in the

references. Moreover, progresses are so fast that only

some historical and recent results will be given (233

references can be found in Ref. [30] covering the results

up to 1980).

Extended Crystallographic Defects and Impurities

In addition to the whole defects found in singleEpitaxial layer = 40 m

Low porosity restructured

a polix ingot [69]. As shown by Fig. 13, there is an

increase of t at the vicinity of GBs which could beexplained by the trapping of impurities (metal; oxygen)

by these defects.

The electrical activity of extended crystallo-

graphic defects is related to the presence of shallow

or deep energy levels in the silicon band gap.

A simple distinction could be made by cooling the

wafer and investigating their electrical activity by elec-

tron or light beam induced current (LBIC) contrast. If

the contrast increases when T decreases, deep energy

levels have to be considered, generally related to

metallic impurities. If the contrast is irrespective of

the variations of T, shallow levels are dominant

[7072]. Figure 14a, b gives an example of such vari-

ation. Low-temperature scan maps reveal the pres-

ence of extended defects in raw samples which are

poorly recombining in the initial material. Such

defects are sleeping defects which are activated

238 Silicon Solar Cells, Crystallinegrain between the front and the back surface.

Consequently, the photogenerated carriers do not

have to cross grain boundaries before to be collected

by a junction.

The large grains have various orientations; the

dominant one is (111). GB types vary from one grain

to another, and one can distinguish small-angle GBs

with a misorientation level from 0 to 10 fromlarge-angle GBs with larger misorientation angles

[56, 57]. In these large grained materials, dislocations

are the more harmful intragrain defects. Dislocations

aremore or less homogeneously distributed or agglom-

erated in clusters, which frequently induce metallic

atom precipitation [58].

Crystallographic defects are harmful to the electrical

properties of the material, mainly in terms of minority

carrier lifetime t or diffusion length L. However, bythemselves (without any decoration by impurities),

their influence is weak, and it is the interaction between

the crystallographic defects and segregated or precipi-

tated impurities (light elements; metallic atoms), which

generates the more harmful recombination centers.

The effect of twin boundaries has also been

extensively studied. Exact twinning (the S3 twin) hasno electrical effect because all bonds are saturated and

there is no distortion of the bond angles across the twin

boundary. More deviated twins show distorted

reconstructed bonds and a dislocation structure,

and they have an electrical effect, as was shown for S9twins. Subgrain boundaries formed essentially by

dislocations [59] are very detrimental in terms of

minority carrier lifetime reduction.

From an electrical point of view, rain boundaries

(GBs) are characterized both by an electrical potential

barrier which reduces the mobility of the majority

carriers and energy levels in the band gap which

reduce the lifetime t (or diffusion length L) of theminority carriers. However, electron beam induced

current (EBIC), as well as light beam induced current

(LBIC), scan maps indicate that this electrical activity

is inhomogeneous from 1 GB to another one and also

along a given GB [6062]. In fact, this activity

depends strongly on the segregation of metallic

atoms or of light elements. In large grained wafers,

the electrical influence of GBs is limited, while that

of subgrain boundaries and of dislocations is dom-

inant, especially when they are decorated byimpurities. This was very well demonstrated first

by Sopori [63] by means of diffusion length measure-

ments as a function of dislocation density determined

by etch pit counting, later confirmed by El Ghitani

[64, 65], and more recently by Warta [66, 67]. Never-

theless, the role of transition metallic impurities is

dominant [68]. An illustration of the impurity defect

interaction at GBs is given by the lifetime scan map

around GBs in a raw wafer cut out from the top of

tau (s)11

9.7

8.4

7.1

5.8

4.5

3.2

2

1000 m


Minority carrier lifetime scan map around a GB in a wafer

cut out from the top of a large polix ingot: segregation of

impurities improves the grains near the grain boundary

239Silicon Solar Cells, Crystalline.70

.52

.35after an annealing at T > 500C and becomerecombining after processing steps.

Like in the FZ and Cz growth processes, liquid and

solid phases are present during the solidification of the

multicrystalline ingots, and in addition to the crystal-

lization, a marked purification by impurity segregation

occurs. Impurities which are contained in the silicon

melt or which have been introduced by contact with the

.17

1 mm

.70

.52

.35

.17

1 mm

a

b


(a) Light beam induced current contrast scan map at 300 K

of a typical raw polix sample. Few intragrain defects appear

in the picture because the contrast is weak. (b) Same

picture at 100 K of the sample scanned in a. Due to the

presence of deep levels in the gap, probably associated to

metallic impurities, both grains in the right of the picture

appear in white. Due to the presence of shallow levels,

a marked contrast appears at the extended defects in the

left part of the picturecrucible walls are progressively accumulated at the very

top of the future ingot. The segregation efficiency is

directly related to the distribution coefficient k0 given

in Table 1. Metallic impurities which the k0 value is

below 103 tend to remain in the liquid phase, whileoxygen accumulates preferentially in the solid phase

which solidifies first. Nevertheless, metallic impurities

and light elements are present in the crystallized ingot

in which they can be dissolved and precipitated in the

homogeneous regions of the grains like at extended

defects. As a consequence of all these imperfections,

mc-Si ingots and wafers are characterized by

a pronounced in homogeneity, and along a given

ingot, the macroscopic and microscopic electrical

properties of the wafers are not the same. It was

found that the very bottom and the very top of the

ingots must be discarded; the mean values of lifetime

t and diffusion length L increase from the bottom tothe middle of the ingot and then decrease to the top

[73]. Figure 15 shows the variation of L as a function of

the height in a typical brick cut out of a large polix

ingot. In virgin p-type wafers, the minority carrier

properties vary very widely, diffusion lengths are

found between 50 and 200 mm, and the lifetimes arein the range 320 ms. All materials need improvementsbefore to be used to make efficient solar cells;

fortunately, improvements come from the solar cell

processing steps.

Material Improvement Techniques The electrical

quality of the wafers, evaluated by the measurement of

lifetime t or diffusion length L, could be improved by twobasic treatments: hydrogenation and impurity gettering.

Hydrogen is able to in-diffuse rapidly as an atomic

species H+, at T 450C, and can interact with dan-gling bonds and impurities [7476]. It was verified that

deep-level transient spectroscopy (DLTS) signal of

some impurities disappears after a hydrogen

in-diffusion and reappears after a short anneal at

T 500C. However, H+ ions tend to recombine toform hydrogen molecules which diffuse slowly, and the

passivation depth is limited to a few tens of micrometer

below the surface.

Hydrogenation can be carried out by different

techniques such as the immersion in direct or remote

plasma, or by low-energy ion implantation, using

a Kaufmann source. All these techniques have been

lon

eig

tte

240 Silicon Solar Cells, Crystallinedescribed by M. Stavola [76]. However, the best

0 5 10

30405060708090

100110120130140150160170

Diff

usio

n Le

ngth

(m

)

Bottom 1022 cm3), and orthorhombicprecipitates of SiP are formed near the surface.Dislocations are formed at the interface between the

15 20 25g the ingot=> Top

RawP diff. 850C 30 mnP diff. 900C 2h

ht (in cm) in a polix ingot for raw and phosphorus diffused

ring treatment)precipitates and the cubic silicon matrix, leading to

a relaxation gettering in the n+ layer. Moreover, due

to the molar volume expansion developed by these

precipitates, self-interstitials are injected in the bulk

in which they interact with dissolved and precipitated

impurities. Substitutional metallic impurities are trans-

ferred in interstitials sites, oxygen-related precipitates

are shrunken, and, provided they are fast diffusers,

metallic atoms diffuse through the wafer and can be

trapped in the n+ layer.

On the other hand, a thermal treatment at

T 850C for at least 30 min of a mc-Si wafer coveredwith a thick Al layer (1 mm thick) can lead to anefficient gettering as well. Indeed, at T > 570Ca liquid eutectic alloy is formed and vacancies are

injected in the bulk. In the liquid alloy, the solubility

of most metallic impurities is ten to thousand times

higher than in the solid silicon [81], and fast metallic

diffusers can be trapped easily. This is a pure segrega-

tion gettering with an efficiency which depends on the

diffusion coefficient of impurities at the annealing

temperature. The best results are obtained for T in the

range 700900C.

dislocation cores, at dislocation clusters, and at GBs.

These precipitates can, in turn, trap metallic impurities

and become recombining. This is probably why

the wafer properties are degraded after anneals at tem-

peratures higher than 900C.The improvement of the wafers is enhanced when

phosphorus gettering, aluminium gettering, and

hydrogenation are applied successively or combined,

provided that phosphorus diffusion is applied first

[8288]. Certainly, they induce complementary

effects on the different defects and impurities

contained in mc-Si wafers. For instance, the diffusion

length scan map of a multicrystalline sample which

experienced long phosphorus diffusion and then long

aluminium gettering is given by Fig. 17. Very high

values are found, indicating that the material has been

strongly improved due to the extraction of impurities.

241Silicon Solar Cells, CrystallineAfter each preceding treatments, t and L substan-tially increase, and such improvements are observed,

irrespectively of the position of the wafers along the

ingots, as shown by Fig. 15.

If the gettering treatment can extract fast diffusers,

it is practically ineffective toward slow diffusers which

remain in the wafers and limit the increase of L. From

the values of t0 (initial bulk lifetime) and of tdif(after phosphorus diffusion at 850C for 30 min), it ispossible to deduce a lifetime tMi related to the intersti-tial metallic atoms (Mi) which are extracted by

gettering during the phosphorus diffusion, according

to the expression:

1

tMi 1

t0 1

td i f5

Assuming that the captured cross-section of the

recombination centers related to these interstitial

metal atoms is 1015 cm2 and that SRH statisticscould be used, it is possible to evaluate approximately

their concentration [Mi]. The variation of [Mi] along

the height of the ingot is given by Fig. 16. In wafers cut

out of large (310 kg) polix ingot [73], it is found, after

phosphorus diffusion, that [Mi] 1013 cm3 in thebottom, 1012 cm3 in the central part of ingots,and 5 1012 cm3 in the top. Half of these impurityamounts are due to iron, which the identification and

the evaluation of the concentration as interstitial atoms

[Fei] in p-type mc-Si are possible by means of the

dissociation of the FeB complexes.

It is important to evaluate also the concentration

variations of recombination centers due to impurities

precipitated or segregated by extended defects and

also to slow diffusers. This variation could be given

by that of the reverse of tget (after phosphorus diffu-sion at 900C for 2 h) because it could be reasonablyassumed that most of fast diffusers are removed what-

ever is their position in the wafer (interstitial atoms,

easily dissolved precipitates in grains and at GBs). It is

observed that the concentration of such recombina-

tion centers is more marked in the bottom and in the

top than in the central part of the ingot. This is

a consequence of the precipitation and segregation at

extended defects of impurities coming from the cru-

cible floor and of the accumulation of impurities in

the top, linked to the duration of the solidification

process.Figure 15 illustrates also the increases of the

diffusion length after a long phosphorus diffusion,

which is a gettering step, and shows that the gettering

is very efficient in the lower middle of the brick because

this part is protected from the back diffusion of impu-

rities coming from the top and sufficiently far from the

crucible floor.

Due to the growth conditions of the mc-Si ingots,

oxygen atoms are also present at concentration levels

larger than those deduced from infrared spectroscopy.

Indeed, with such a technique, interstitial oxygen

atoms are detected while precipitated or aggregated

ones are not. Oxygen precipitation occurs certainly at

00.20.40.60.8

11.21.41.61.8

5 10 15 20 25h (cm)

(Mi)

(1013

cm

3 )


Computed variation of the concentration of interstitial

metallic atoms [Mi] along a polix ingot

hic

0

242 Silicon Solar Cells, CrystallineFortunately, the preceding treatments are

processing steps which are included anyway in most

of the preparation techniques leading to industrial

mc-Si solar cells. For instance, for cells made with

wafers of Fig. 12, diffusion length Ln values reached


Minority carrier diffusion length Ln scan map of a sample w

aluminium gettering; both treatments were carried out at 90

at the grain and twin boundariesup to 300 mm [73]. These strong improvements explainwhile conversion efficiencies higher than 16% can be

achieved, despite that the initial material properties

were very poor compared to those of single crystals.

Probably, it seems today that the major part of the

improvement is due, after the phosphorus diffusion,

to the aluminium gettering developed during the rapid

thermal anneal at700Cwhich gives rise to the ohmicback contact [89] and to the back surface field effect, as

will be shown in section Single Junction p-type Silicon

Based Solar Cells (n+-p-p+ Structure).

n-Typemc-Si Although the first solar cells have been

made with n-type single-crystalline silicon, most of

the industrial production is based on p-type material,

essentially because the processing steps are simple and

relatively not expensive. However, n-type material

possesses some remarkable advantages. First, there

are little or no boronoxygen complexes and no

light-induced degradation. Secondly, the minority

carrier capture cross-sections of metallic impurities,frequently found in processed silicon, are markedly

smaller for holes than for electrons. This property of

n-type siliconwas clearly explained [90] on the basis of

the formation of donor trap levels in the gap which

are positively charged or neutral and can be strongly

400m

302m

205m

107m

100 m

h experienced first a phosphorus gettering and then an

C for 2 h. Notice the high values of Ln in the grains and alsoattractive for electrons or poorly attractive for holes.

Experimental results have reported exceptional

values of lifetime in raw n-type wafers cut from

ingots made by Italsolar and then by Deutsche

Solar or from ribbons. Indeed, Cuevas et al. [91]

and Libal et al. [92] have found lifetime values of

several hundreds of ms in raw materials and values inthe range of ms in wafers gettered by weak phosphorus

diffusion. A lot of papers have investigated the role of

impurities and have confirmed the interest of n-type

material [9395]. For example, interstitial ironwhich is

one of the more harmful contaminant in mc-Si wafers

is a good example because the capture cross-sections

for electrons and for holes, reported by Istratov [96],

are sn = 5 1014 cm2 and sp = 7 1017 cm2,respectively. Assuming that Schockley-Read-Hall

(SRH) recombinations occur and that interstitial

iron is the dominant impurity, high lifetime values

are expected for holes in n-type iron-contaminated

silicon. The same tendency is observed for several

other metallic impurities except for chromium for

on where UMG feedstock was obtained and on its

impurity concentrations as well as on the nature of

these impurities. This is why we give more details on

the results obtained for ingots made with plasma torch

refined UMG-Si [103, 104].

The wafers cut out from the bottom and the

middle of the ingots are p-type and contain high con-

centrations of Al, Cu, and Zn (other impurities are

below the detection limit of the high resolution glow

discharge mass spectroscopy). Interstitial oxygen and

substitutional carbon atom concentrations detected by

Fourier transformed infrared spectroscopy (FTIR) are

about 5 1017 cm3 and 7 1017 cm3, respectively.In these wafers, electron diffusion length Ln values are

in the range 3060 mm because, probably, B, P, and alsoAl concentrations [105, 106] are too high. Obviously,

other impurities, like Ti, are present.

In the top of the ingot, wafers are n-type. The

advantage of n-type silicon is again very clear: the

243Silicon Solar Cells, Crystallinewhich the capture cross-sections are closed in both n-

and p-type silicon, but this impurity cannot be present

in sufficiently large concentrations in the wafers after

the end of the crystallization and after a gettering treat-

ment [97]. Although the diffusion coefficient of elec-

trons is three times higher than that of holes, it is

expected that diffusion lengths of minority carriers, in

such a simple configuration, are neatly higher in n-type

than in p-type silicon (provided the nature and the

concentration of impurities is the same in both types

of wafers). Oxygen precipitates are also less harmful in

n-type silicon because their surfaces are expected to

have a hole repelling positive charge [98]. Obviously,

there are other sources of recombination centers, like

precipitates, and the reality is more complex.

Another remarkable advantage of n-type silicon

results from the lifetime of minority carrier variation

with the injection level: it does not decrease too much

with this level, suggesting that n-type base cells will be

more efficient at low illumination levels than p-type

base cells.

As an example of the typical high electrical quality

of the n-type wafers, Fig. 17 shows a light beam induced

current scan map (LBIC) at l = 960 nm, with localvalues of the hole diffusion length Lp, of a region which

contains GBs and dislocations, especially in the center

of the picture (Fig. 18).

Such interesting properties can be useful for solar

cells, as will be described later.

Refined UMG-mc-Si To open the way to the huge

production of silicon solar cells which could

be predicted for 2013 [99], another approach is

to cast refined upgraded metallurgical silicon

feedstock (UMG-Si). After crystallization by direc-

tional solidification, the wafers look like the conven-

tional ones (large grains, same intragrain defects).

Problems come from the high concentrations of dop-

ant and of light elements, mainly carbon. The dopant

concentrations are close to 2 ppma, and due to the

distinct distribution coefficients of boron and phos-

phorus atoms, the material is compensated, and in

the top of the ingot, the conduction type changes

from p- to n-type. Concentrations of carbon are close

to 20 ppma in the top of the ingots, as a consequence of

the carbothermic reduction and process contamina-

tion. Precipitation of SiC filaments occurs frequentlyin the upper part of the silicon block [100] as shown by

Fig. 19.

These filaments can be detrimental because they

cause damage during the sawing and can form ohmic

shunts [101, 102]. Blends of electronic grade (EG) and

of upgrade metallurgical (UMG) feedstock have given

unexpected good results because L values are found

higher than 100 mm in raw wafers [1821]. However,such results cannot be generalized; they are depending

350

275

120

1.0

.75

.50

.25

100 m


Light beam induced current contrast scan map of a raw

n-type silicon wafer. Some local values (in mm) of minority

carrier diffusion length Lp are indicated. Notice the high

values of Lp, even in the central zone where the density of

extended defects are high

244 Silicon Solar Cells, Crystallinelight beam induced current (LBIC) contrast is more

marked than for p-type wafers, and the mean values of

electron diffusion length Lp reach 100 mm in the rawwafers, as shown by the scan map of Fig. 20.

When n-type wafers experimented phosphorus

diffusion and aluminium alloying, the mean values

of Lp achieve 180 mm. Light beam induced current(LBIC) scan maps of n-type wafers look like those of

conventional p-type mc-Si wafers; the grain boundary

0 100 m


SiC filaments observed in UMG-Si wafers (Courtesy of Dr

Moller-TU-Freiberg)contrast is marked while intragrain defects are

weakly active.

However, a difference with conventional mc-Si

could be noted: there is a weak improvement of Lpafter short phosphorus diffusion. Probably, that is due

to the presence of slow diffusers like Al and Zn and

other ones which have not been detected by HR-GDMS

but which are certainly contained in the material.

Notice that the preceding results could be impaired

when the concentrations of oxygen and carbon are higher

than 8 1017 cm3, as frequently observed in the top ofthe ingots. As reported by Pizzini [107], a detrimental

coprecipitation of these elements could occur.

Single-Junction p-Type Silicon-Based Solar Cells

(n+-p-p+ Structure)

Solar cells made with crystalline silicon wafers have

been investigated for a long time, and in 2010, theyshare at least 83% of the total photovoltaic market

(45% for mc-Si cells), although the part of thin filmcells is increasing. This success is explained by

the relative low cost and/or by the high conversion

efficiency of these devices.

The conversion efficiency of solar cells depends on

the quality of silicon wafer, i.e., on the values of the

minority carrier lifetime t and diffusion length L, andon the cell structure design. Particularly, the passiv-

ation of the surface, the surface texturization, the

antireflection coatings, as well as the metallization

techniques are also of a paramount importance because

they strongly influence the short circuit photocurrent

density Jsc, the open circuit photovoltage Voc, and the

350m

262m

175m

87m

1 mm


Hole diffusion length scan map Lp in a raw n-type refined

UMG-Si waferfill factor FF.

Notice that the conversion efficiencyZ of a solar cellis given by:

Jsc Voc FFP

6

where P is the incident sunlight power per cm2 (W/cm2).

The Basic Structure Figure 21 shows the schema of

a very basic solar cell structure made on p-type

silicon. Most of such conventional cells used an n++-p

or an n+-p junction which results most often from the

in-diffusion of phosphorus at 850C from POCl3through the front surface. The depth of the n+ layer

is typically 0.30.5 mm; its sheet resistance is inthe range 4090 O/square; phosphorus surface

245Silicon Solar Cells, Crystallineconcentration can exceed 1021 cm3 in the case of ann++-p junction. A dead layer is thus formed at the

emitter surface, which does not contribute to the

photocurrent. Recombination in this heavily doped

region causes a high dark current which impairs the

photovoltage. This very high doping level makes easier

the formation of a good ohmic contact with the metal-

lic front grid used as top electrode; however, it is not

easy to reduce the carrier recombination at such

a surface although a silicon oxide or a silicon nitride

layer is deposited or grown on it.

The back surface is also a source of recombination.

To reduce this recombination activity, the back surface

doping level is enhanced (p+-type), totally or locally,

by means of aluminiumsilicon alloying or boron

diffusion in order to develop a repulsive back surface

field (BSF) for electrons. The edges of the cell must be

discarded in order to eliminate parasitic shunting

paths, and a single or a double antireflection coating

(ARC) layer is deposited on the front surface before or

ARC (oxyde +SiN) n++ or n+

p- type -Si BSF

p+ zone


Diagram of the very basic structure of a p-type silicon cell.

Metallization are drawn in black (Figure is not drawn to

scale)after the metallization step.

Notice that the processing steps used to make the

cells, e.g., phosphorus diffusion, aluminiumsilicon

alloying, deposition of a hydrogen-rich silicon nitride

layer always improve the performances of the cells,

especially when the base is made with multicrystalline

silicon.

Antireflection Coating and Texturization The opti-

cal properties of silicon play a major role in the design

and operation of solar cells; the light absorption and

the reflectance of the front surface must be taken

into account. The reflectance is 30% for wavelengthsl < 1.1 mm and increases to 60% for l 0.4 mm[108]. Thus, to minimize the optical losses, deposition

of an antireflective coating or/and texturization of

the silicon surface are essential.

Antireflection Coatings Reflectance is reduced by

employing an antireflective coating (ARC) on the

silicon front surface. Predominantly, single-layered

ARCs are generally used industrially. The optical

thickness of the ARC should be equal to a quarter of

the wavelength lmin at which zero reflectanceoccurs (i.e., reflected waves must be out of phase

by p) [109]:

nARCdARC lmin4

7

The refractive index of the layer, nARC, has to be

equal toncSi

pfor the crystalline silicon (c-Si) cell in

air. As nc-Si varies from 4.28 to 3.76 between 0.5

and 0.7 mm, the refractive index of an optimizedARC should be, for instance, equal to 1.9 at 600 nm.

For a glass-encapsulated cell, the refractive index of

the glass being nglass = 1.5 instead of 1 for air; the

optimal index value for the ARC will be 2.3. Such

ARC reduces the sunlight lost by reflectance on

polished silicon from 30% down to 10% and from

20% to 5% in air and under a protecting glass,

respectively.

Antireflection coatings used in the industry are

dielectric materials such as TiO2 (n = 2.3) [110] and

SiNx:H (whose refractive index varies between 1.9 and

2.3) [111]. For laboratory cells, two complementary

dielectric layers are sometimes applied to broaden the

spectral band reduction of the reflectance.

TiO2 was first introduced in the early 1970s

[112, 113] and presents numerous advantages such

as high chemical stability, insulating properties when

stoichiometric, good mechanical properties. TiO2thin films can be deposited at low temperatures

(400C) by atmospheric pressure chemical vapordeposition (APCVD) or by spray pyrolysis [114].

Nontoxic and noncorrosive liquid precursors, such as

tetraisopropyl titanate (TPT), are used as a deposition

source. At such temperatures, the metastable anatase

crystalline phase is formed which exhibits an optimal

refractive index for glass-encapsulated silicon solar

cells, as well as excellent transmittance. A thin silicon

less than the cell thickness. Thirdly, long-wavelength

photons which are reflected from the rear surface will

encounter an angled silicon front surface, improving

their chance of being internally reflected [119].

This process is referred to as light trapping and gives

an improved response to infrared light, especially for

thin Si substrates.

There are two types of texturization. When the

etching rate varies with crystallographic direction,

the texturing is anisotropic, while when the material

etches in all directions at same rate, the texturing is

isotropic.

Anisotropic Alkaline Etching A geometrical texture

(V-shaped grooves texture, random pyramidal struc-

tures) at the front surface of silicon can be done

mechanically, for instance using specially developed

dicing saws or (electro)chemically, by etching the

silicon in an alkaline or acidic solution [120].

60

246 Silicon Solar Cells, Crystallinedioxide (SiO2) layer could be grown before deposition

of TiO2 in order to provide passivation of the front

silicon surface.

SiNx:H deposition by PECVD was invented in 1965

[115]. The amorphous a-SiNx:H films are typically

deposited using plasma-enhanced chemical vapor

deposition (PECVD), at temperatures of less than

450C with deposition rates as high as 1.7 nm/s.The properties of a-SiNx:H films are extremely

dependent on the deposition system, process condi-

tions, and gas composition. Thus, when using dilute

silane and ammonia reactant gases, the best surface

passivation results (see Surface Passivation. The Selec-

tive Emitter Concept) from stoichiometric a-SiNx:H

films that have a refractive index of 1.95, while the

Si-rich films deposited from pure silane and ammonia

possess a refractive index of 2.3 [114]. Effective

reflectance of 11% is observed on a planar siliconsurface, as shown by Fig. 22.

SiNx:H coating is presently widely used in the

industry as the efficiency of the solar cells is strongly

increased.

Notice that porous silicon exhibiting a refractive

index intermediate between those of silicon and air

has been widely investigated as an AR coating in the

1990s [116, 117].

Silicon Surface Texturization Light incident normally

to the silicon is transmitted to an amount of70% intosilicon while 30% is reflected away from theflat surface as shown by Fig. 23a. Texturing the front

surface of a solar cell increases the short-circuit current

due to three distinct mechanisms. All of which are

related to the fact that the incident photons strike the

cell surface at a specific angle [108, 118]. Firstly, some

light rays will be reflected from one angled surface

merely to strike another as shown by Fig. 23b,

resulting in an increased probability of absorption,

and therefore reduced reflection. Minimal facet angles

of 30 and 54 are required for double- andtriple-bounce reflectance, respectively. For the double

bounce case shown in Fig. 23b, the light reflected away

from the silicon is lost. However, if the silicon is

encapsulated under glass, it is possible to totally

reflect the light arriving at the glassair interface,

above the critical angle of 42, whereby it willbe redirected toward silicon yet again for possible

re-incidence. Critical angles for total internal

reflectance at siliconair interface are 1617.Secondly, photons refracted into the silicon will

propagate at an angle causing them to be absorbed

closer to the junction than would occur with a planar

surface. This is especially relevant in material with

minority carrier diffusion lengths comparable to or

50

40

30

20

10

0

Ref

lect

ance

, R (%

)

400 500 600 700Wavelength, (nm)

800 900 1000 1100

air/TiO2/planar Si (Rw =8.5%) air/glass/TiO2/planar Si (Rw =11.7%) air/glass/TiO2/textured Si (Rw =5.8%)


Effective reflectance of planar and textured emitter

surfaces with TiO2 single-layer antireflection coating (From

[114], courtesy of Dr. BS Richards)

h

ar

247Silicon Solar Cells, CrystallineSi surface

h

(i)

(t)

(r)

Si bulk

a b


The influence of surface texture on light reflection: (a) plan

reflected lights, (t1) and (t2) absorbed lightsThe first solar cell using the alkaline etching process

was reported in 1974 [121]. It is a well-established

texturing method for single-crystal silicon solar cell.

The wet etching process can be globally represented

by the overall reaction:

Si 2H2O 2OH ! Si OH 2O2 2 2H2 8

Etching rates in NaOH or KOH depend on the

crystallographic orientations; the (100) planes being

etched more rapidly than (111) planes, which are thus

revealed. (111)-oriented surfaces are flat etched, with

some triangular features due to the presence of terraces,

while (100)-oriented surfaces exhibit random square-

shaped pyramids [122] made of (111) facets, as shown

by Fig. 24.

An anisotropic etched on (100) plane forms a facet

angle of 54.7 on a (100)-oriented surface [123].

a


SEM images in plane view of NaOH-textured silicon: (a) (100)-(i)

(r1)

(r2)

(t1)(t2)

surface; (b) textured surface. (1) incident light, (r1) and (r2)This angle satisfies the requirements for both external

reflection control and total internal reflections.

The reflectivity decreases from 34% for a mirror-

polished silicon surface to 11% for an alkaline-textured

silicon wafer [126], as shown in Fig. 24.

A promising alternative solution is quaternary

ammonium hydroxide etchings, typically tetramethy-

lammonium hydroxide (TMAH) [124, 125]. This solu-

tion is clean (room compatible, nontoxic, and easy to

handle). A low weighted reflectance of 2.7% can be

obtained with a combination of TMAH pyramidal

texture and SiNx:H AR coating, as shown in Figs. 25

and 26 [126].

Anisotropic alkaline texturing does not work very

well on multicrystalline substrates due to the different

crystallographic grain orientations. A number of

different surface geometries will determine the

b

and (b) (111)-oriented crystals

interaction of incident light with the surface and will

contribute to the overall reflectance of the

multicrystalline wafer. The bare saw etched wafers

have reflectance in air equivalent to polished silicon

because the tilt angles of the dominant etch facets

exposed are lower than the 30 required for double-bounce reflection. The (111)-oriented surfaces are flat

etched, with triangular plateaus as predicted by Sopori

[109], while (100)-oriented surfaces exhibit the upright

random pyramids. By measuring the reflectivity of

individual grains, it was found that (100) and

(111) grains exhibit minimum and maximum effective

reflectivity between 400 and 1,100 nm of 11.7%

and 34.2%, respectively, while grains with other

orientations exhibit intermediate values [127]. How-

ever, the anisotropic texturization is still currently used

in multicrystalline silicon industry because under

encapsulation, both saw damage and textured etched

multicrystalline wafers couple light more effectively

into the silicon.

Isotropic Texturization Numerous methods such as

chemical, electrochemical, defect etching, or based on

more sophisticated techniques (e.g., reactive ion etch-

ing (RIE), lithography, laser scribing, and mechanical

grooving) have been investigated.


SEM picture of a typical texturized silicon surface using

TMAH texturization

50

45

40

35

30

25

20

15

10

5

0

Polished +

Polished (W

TMAH Tex

TMAH Tex

vel

Hem

isph

eric

al re

flect

ance

(%)

ie

248 Silicon Solar Cells, Crystalline350 450 550 650Wa


Weighted reflectance of different silicon surface morpholog SiN ARC (WR 10,2%)R 41%)

turing (WR 13%)turing + SiN ARC (WR 2,7%)

ength (nm)750 850 950 1050

s, with or without ARC and TMAH texturing

Plasma-enhanced dry chemical etching at atmo-

spheric pressure is a promising alternative to isotropic

wet etching, especially when combined with similar pro-

cess technologies, to provide a continuous in-line

processing steps. Plasma texturing is particularly appro-

priate for advanced solar cell structures, for wafers pro-

duced without surface damage such as Si ribbons and

epitaxial layers on low-cost Si substrates. It allows

decoupling the properties of the front and rear surfaces


SEM plan view image of multicrystalline silicon surface

after isotropic acid etching (From [130], Courtesy of Dr.

R Einhaus)

oxide

p-type

rear metal contact

platedmetal

n++n+

p+


Passivated emitter and buried front contact solar cell

(Courtesy of UNSW)

249Silicon Solar Cells, CrystallineAmong these methods, the acid texturization

method has been the most developed and is currently

used in industry. The RIE method is very attractive

because it allows a better automatized solar cell

processing, avoiding the use of wet techniques.

In search of a simple method with high throughput,

low-cost investment, and compatible with thin wafers,

Sarti et al. proposed the wet acidic texturing that tends

to etch isotropically the multicrystalline silicon wafers,

resulting in features with rounded surfaces [128].

The acidic water solution consists of a mixture of nitric

acid, hydrofluoric acid (HFHNO3), water, and

eventually some additives. The overall reaction is the

following:

3Si 4HNO3 18HF! 3H2SiF6 8H2O 4NO9

The etching rate as well as the surface morphology

depends strongly on the chemical composition of the

texturization bath but not on the crystallographic

orientation. Etch textures vary between rough, concave-

shaped tub structures, and smooth surfaces. The rough

texture is of interest for solar cell industry [129, 130].

The texturing process starts on as-cut multi-Si

wafers. The saw damages trigger the etching process.

Thus, the removal of saw damage and the surface

texturing are being done simultaneously in one short

single chemical step. The resulting etch pits of 110 mmin diameter are uniformly distributed over the wafer

surface irrespective of crystallographic orientation,

giving a homogenous reflectance over the surface of

the wafer and the absence of steps between grains, as

shown in Fig. 27.

The acidic isotexturing results in lower reflection than

traditional anisotropic etching on multicrystalline mate-

rial, as shown in Fig. 28, and better conversion efficiency

[130132]. Short-circuit current density increases of

up to 1.5 mA/cm2 have been measured.

In laboratory, the masked acidic etching has been

developed to produce hexagonally symmetric honey-

comb surface texture. This texture reduces reflection

loss as well as substantially increasing the cell effective

optical thickness by causing light to be trapped within

the cell by total internal reflection. Multicrystalline

solar cells with record efficiency of 19.8% have been

obtained [130, 131].

250 Silicon Solar Cells, Crystallinewhich are both texturized with the wet etching.

A nontextured rear surface appears to be needed to reach

low effective recombination in advanced solar cells.

First report on reactive ion etching (RIE) texturization

of Si in c

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