Boron Tribromide Sourced Boron Diffusions for Solarcell

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Boron Tribromide Sourced Boron Diffusions for

Silicon Solar Cells

by

Alexander Slade

Submitted for the degree of

Doctor of Philosophy

2003


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Acknowledgements

The work in this thesis was made possible by the unheralded efforts of the laboratory

maintenance staff who kept the equipment (especially the lasers) running so the

focus of this work could be directed toward the experiments themselves.

Thanks Jules, Martin and Tim.

A big thanks to everyone at the centre for making it a vibrant, enjoyable place to

work.

Thanks to Alistair Sproul and Martin Green.


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Table of Contents

Chapter 1

Introduction

1.1 Electricity generation 1

1.2 Photovoltaics 2

1.3 Solar cell research 3

1.4 Silicon solar cells 5

1.5 Aim of the thesis 7

Chapter 2

Recombination and other efficiency limiting aspects of silicon

2.1 Overview 10

2.2 Recombination in silicon 11

2.2.1 Auger recombination 12

2.2.2 Shockley Reed Hall recombination 17

2.3 Mobility 20

2.4 Diffusion length 23

2.5 Bulk defects and contamination 24

2.6 Surface recombination 26

2.6.1 The Si/SiO2 interface 29

2.6.1.1 Texturing the front surface 31

2.6.1.2 Anti-Reflection coating 32

2.6.2 Passivation of diffused regions 33

2.6.3 Emitter saturation current and the surface recombination velocity 36

Chapter 3

High efficiency designs an overview of device development

3.1 Summary 40

3.2 A brief history of silicon solar cell development 41

3.3 Design principles of high efficiency solar cells 46

3.3.1 Emitter passivation 47

3.3.2 Contact passivation 49


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Abstract

This thesis undertakes the development, characterization and optimization

of boron diffusion for silicon solar cells. Heavy diffusions (sheet < 40 /) to

form a back surface field, and light diffusions (sheet > 100 /) to form oxide-

passivated emitters were developed. Test structures and solar cells were

fabricated to assess uniformity, lifetime and recombination effects due to the

light and heavy boron diffusions.

It was found that the growth of a thin ~200 , thermal oxide, during

stabilization immediately prior to the boron diffusion - was required to

maintain high lifetime and reduce surface recombination (reducing the emitter

saturation current) for all boron diffusions.

The heavy boron diffusion process was incorporated into the single side

buried contact solar cell processing sequence. The solar cells fabricated had

both boron diffused and Al/Si alloyed P+ regions for comparison. This research

conclusively showed that boron diffused solar cells had significantly higher

open circuit voltage compared to Al/Si alloyed devices. Fabrication of n-type

solar cells, and their subsequent characterization by overlayed secondary

electron image and the electron beam induced current map showed that the Al/Si

alloy varied in depth from 5 to 25 m deep.

Methodology and characterization for light, oxide-passivated boron

diffusions are also presented. This study yielded boron diffused emitters (sheet

> 100 /) with low emitter saturation current. It was observed that this was

possible only when the thermal oxidation after the boron diffusion was minimal,


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less than 1,000 . This was due to the segregation effect of boron with oxide,

decreasing the surface concentration that in turn decreased the electric field

repulsion of electrons from the surface.

Device modelling of n-type solar cells is presented where the parameters

of the modelling include the results of the light, oxide-passivated boron

diffusions. This modelling shows n-type-base material with light oxide-

passivated boron diffusion has higher potential conversion efficiency than

forming a solar cell from phosphorous diffused p-type material.


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Chapter 1

Introduction1.1 ELECTRICITY GENERATION

Standard means of terrestrial power generation are from nuclear fission, coal-

fire, gas turbines and diesel. All of these power generating methods produce either

green house gases or radioactive by-products. Green house gases act to alter the

make-up of compounds present in the upper atmosphere and in turn increase the

internal reflection of heat-radiation by the upper atmosphere thus warming the

planets climate. It may not have been unequivocally accepted that the warming of

the worlds temperature is directly related to the increased CO2 levels, however, a

30% rise in CO2 since 1940 compared to a < 1% variation over the last few hundred

years, and, that 7 of the 10 warmest years recorded have occurred in the last 10 years

gives a strong cause for concern that these changes to the worlds climate have been

the result of the modern way of life [Holper 1999]. Also, there is still no means by

which high-level nuclear waste can by disposed of in an acceptable way.

Increased public awareness of the detrimental effects of these green house

gases has motivated 1% of consumers in Australia to buy green electricity [see

www.seda.nsw.gov.au and links therein]. Green electricity is sold as having been

generated by renewable sources: photovoltaic, biomass, hydro, or wind. Counter-

intuitive as it may be, methods of generating electricity in a way that produces no by-

products, requires no maintenance and which have no fuel costs remain more


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expensive than the tradition (polluting) alternatives. Therefore, green energy as it

is called has a higher price to the consumer. It is the initial high cost of the

generating system that causes the cost to the consumer to be higher than coal-fire

electricity. This lower price for coal-fire generated power is evaluated on the basis

that the cost to the environment is not a monetary one. That is, no financial value is

placed on the detriment to the worlds environment caused through the standard

methods of electricity generation. However, as attitudes change, whether they are

governmental or personal, the use of renewal sources of electricity is increasing.

1.2 PHOTOVOLTAICS

Photovoltaics is defined as the branch of science or technology concerned with

the study of utilising the generation of an e.m.f. by light incident on an interface

between certain pairs of substances [Oxford Dictionary]. A simpler expression could

be: the means of converting light (photo) into electricity (volt). Photovoltaic cells,

also called solar cells, produce no by-products, are fuelled entirely from light and

have a very long lifespan (>20 years). For these reasons and that installation can

take up a small area, makes photovoltaics a suitable means of producing electricity

cleanly in an urban environment. This advantageous because having the solar cells

in a city reduces distribution costs and concerns.

The challenge remains that although solar cells generate electricity cleanly it

can rarely be supplied at a competitive price, remote area applications and space

being the common exceptions. The financial cost of that electricity to the consumer

is dependent on the solar cells price and their efficiency of energy conversion.


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Reducing cost while improving conversion efficiency are dominant areas in the field

of solar cell research.

There is a wide range of materials and processes that are in use by industry and

are under research throughout the world for generating electricity from sunlight [for

example see Solar Energy Materials and Solar Cells, Elsevier]. Apart from the

Israeli liquid solar cell all of these are solid-state devices.

1.3SOLAR CELL RESEARCH

Silicon solar cell research began at Bell Labs. in the early 1950s. As

semiconductor processing advanced so did the efficiency of solar cells.

Improvements in energy conversion saw the use of silicon solar cells in space in

1958s. Cost prohibited terrestrial use until the 1970s. Over the last ten years the

solar cell industry has grown 3 fold. Silicon dominates the market although CdTe

and CIGS are in large-scale production. Both CdTe and CIGS have shown excellent

results from minimal research. The ability to continue research is limited because

the support industries are not established, as is the situation for silicon

semiconductors. Other issues such as material availability and toxicidicy further

hinder the development of CdTe and CIGS. Exotic materials such as GaAs and its

alloys are used for the highest efficiency cells yet their price is many times that of the

most expensive Si devices and relegates their use to space applications only.

Research into silicon solar cells has two basic aims:

Increase the understanding of how a solar cell functions;

Reduce the manufacturing cost while maintaining their efficiency.


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There are many aspects to their operation that remain unaccounted for or not

completely understood. Although not all research is concerned with the

fundamentals of solar cells, it is often that major improvements to their efficiency

follow new insights into their operation (see figure 1.1). Such as:

Improving the optical performance by texturing of the surface [Haynos et al

1974].

Electrical improvements include passivation of the surface by a thermal oxide

[Godfrey and Green 1979] and reducing recombination at the metal contacts by

a heavy diffusion [Fossum and Burgess 1978].

Figure 1.1. The highest efficiency silicon solar cells throughout history are plotted

with notes regarding the means by which developments were made. A detailed

review of these developments is given in Chapter 3. Figure original from Green

[1995].

0

10

20

30

1940 1950 1960 1970 1980 1990 2000

Efficiency (%)

UNSW

Surface & ContactPassivation

Basic SiProcessingAdvancement

Front grid &AR coating

RandomPyramids


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Although solar cells offer a clean and maintenance free method of generating

electricity there is an issue of cost that determines their application. Ongoing

research has seen developments in not only their efficiency but design and

processing. That is, an improvement in efficiency may not readily translate into an

improvement in commercially produced solar cells. The challenge is thus, to

incorporate high efficiency design aspects into a device that could be produced on a

commercial scale without incurring excessive cost. Such developments in design

have lead to the most efficient cells available [SunPower, Amonix, Sanyo, and

BPSolar]. Unfortunately this is the exception and not the rule. Currently most

silicon solar cells are made from processes that were developed over 25 years ago.

Applications such as the Mars Pathfinder use extremely expensive

GaInP/GaAs solar cells as the energy density (Watt/m2) is much more important than

the cost of the electricity ($/Watt). Generally though, the effectiveness of a solar cell

is based on both their efficiency and their manufacturing cost. In this realm silicon

exceeds. Based on recent study [Jones et al. 2000] the cost to produce a Watt of

electricity from a solar panel was the lowest for silicon-based devices, $2.10 for

multi-crystalline Si, compared to $2.30 for CdTe and $2.25 for CIGS. This thesis

will only look at Si solar cells.

1.4 SILICON SOLAR CELLS

There are three types of Si solar cells: Thin film, multi-crystalline and mono-

crystalline. Thin film solar cells are made by depositing Si, often in amorphous


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form, onto a substrate at low temperature. As their name suggests they are very thin

< 50 m and are potentially very cheap to make.

Multi-crystalline and mono-crystalline Si solar cells are made by first starting

with a wafer, a slice of ~ 160 525 m from an ingot (mono-crystalline) or cast

(multi-crystalline). An ingot is one large crystal whereas a cast is a block of many

small crystals directly adjacent. Once a wafer is made the processing required to

make a solar cell is similar for both of multi and mono- crystalline wafers. Solar

cells made from wafers are known as bulk devices.

Multi-crystalline is cheaper to form but the quality is below that of mono-

crystalline. However, as was found by Jones et al. [2000] multi-crystalline solar

cells produce electricity for the least expense. This analysis was based on various

assumptions, two of which were that the efficiency, , for multi- and mono-

crystalline were multi = 14% and mono = 15%, respectively. These efficiencies

indicate that although the mono-crystalline material is of higher quality, its potential

is not being fully realised as it has been demonstrated that the efficiency can reach

18% on Cz (Cz is an abbreviation for a single crystal grown by the Czochralski

method.) from a commercially produced buried contact solar cell [Burton 1992].

This gap between performance and potential can be explained by two reasons. The

first of which is that when mono-crystalline material is boron doped, with a boron

concentration above 1016 a boron/oxygen complex forms that degrades the lifetime of

minority carriers by up to 10 fold [Glunz et al. 1999, Schmidt et al. 1997]; the

oxygen is present in all Czochraski grown crystals and cannot be avoided. Using the

float zone method to grow crystals avoids high oxygen content but due to lower

demand, the price of them is significantly more than for Cz. Unfortunately most


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mono-crystalline solar cells are made from oxygen-contaminated boron-doped Cz.

The second reason that solar cells do not perform as well as expected is that the

designs of the mono-crystalline solar cells do not differ from those of multi-

crystalline devices. That is, if the lifetime of Cz material were improved, little

change in the efficiency would result. As such, issues in device design must be

addressed to bridge the gap between high efficiency devices and standard

commercial product.

It has been identified that for Cz grown crystals increasing the material

resistivity (adding less boron) or by using phosphorus doping (to form n-type wafers)

produces high lifetime, inexpensive material [Glunz et al. 1999, Schmidt et al. 1997].

However, alternative designs are needed to take full advantage of these types of

wafers.

1.5 AIM OF THE THESIS

This thesis undertakes an investigation into the development and application of

boron diffusion for surface passivation. This includes the passivation of metal

contacts (a.k.a. a back surface field) and oxidised surfaces for silicon solar cells.

The means to investigate this was to undertake the development and

characterization of boron diffusions using a boron tribromide (BBr3) liquid source

and a standard quartz diffusion furnace. Following the development of the test

structures, the heavy boron diffusion, in particular was applied to single sided buried

contact solar cells to assess their performance in a well-characterized device

structure.


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The light boron diffusions were characterized by fabricating test structures.

The data from the test structures was used to perform device modelling that predicts

that solar cells made from standard phosphorus doped Cz grown material could reach

over 20% for a single sided buried contact solar cell.

The structure of the thesis is as such:

Chapter 2: A chapter outlining the physical explanation of the efficiency loss

mechanisms in silicon solar cells is included, focusing on recombination and

including the latest research and its implications to silicon solar cell modelling.

Chapter 3: Well-developed high efficiency designs are analyzed to pinpoint

the key areas that are required to minimize recombination losses. The potential to

use such processing methods in commercially compatible designs and previous

attempts of simplifying high efficiency designs are reviewed. It is concluded that for

the development of monocrystalline silicon solar cells a method to perform boron

diffusions, an open discussion of the method and the characterization of optimized

boron diffusions would provide the foundation to the further development of

commercially produced bulk silicon solar cell devices.

Chapter 4: The development of the boron diffusion method is discussed and

the characterization of test structures is presented. The results clearly show that,

given the correct diffusion conditions, the use of boron for heavy p-type diffusions

will maintain high bulk lifetime and have a low saturation current (at least in the

boron diffused region) even when the deposition and drive-in is performed at

temperatures not exceeding 950oC for only 30 minutes. These results dispel the long

held opinion that high quality boron diffusions, if they were possible, had to be

performed at high temperatures for long times.


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Chapter 5: Application of the heavy boron diffusions to commercially

compatible silicon solar cell devices is investigated. A range of resistivities will be

used that accentuate the quality of the rear surface passivation. Single sided buried

contact solar cells that have a standard Al/Si alloy and a heavily boron diffused rear

contact region were compared. These results clearly favor the boron diffused solar

cells by up to 50 milliVolt (mV). Furthermore, the boron diffused solar cells

displayed the highest open circuit voltages for single sided buried contact solar cells

for their respective materials, 3 cm, 10 cm & 100 cm float zone grown wafers.

Chapter 6: Light boron diffusions are applied to test structures for the

extraction of the emitter saturation current (see chapter 4). The results are discussed

in comparison to the fundament recombination of both boron and phosphorus

diffusions (the Auger component). Lastly, device modelling of n-type front junction

buried contact solar cells is performed.

Chapter 7: Concludes the thesis with a summary of the main results and the

original contributions made by the author.


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Chapter 2

Recombination and Other Efficiency Limiting

Aspects of Silicon

2.1 OVERVIEW

Due to the vast knowledge base on silicon and silicon semiconductor device

processing, research into silicon solar cells has a major advantage over solar cells

made from other materials [Ghandi 1994]. The ability to easily grow a dielectric

film on the surface, which acts as a diffusion barrier and serves as a means to reduce

interface states at the surface is unique to silicon. However, some aspects of

recombination in silicon have remained somewhat less than fully understood. For

example, the recombination velocity at a boron doped surface has been called one of

the more elusive aspects of silicon research - Cuevas et al. [1997].

A broad overview of the basic recombinative processes and their related issues

is presented in this chapter. The aim being to familiarize the reader with the more

basic concepts and issues of silicon solar cells that will be discussed in detail in the

subsequent chapters.

For further reading see Silicon Solar Cells [Green 1995] and Crystalline

Silicon Solar Cells [Aberle 1999].


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2.2 RECOMBINATION IN SILICON

An important intrinsic mechanism that limits efficiency of silicon solar cells is

recombination. Recombination is the process in which free carriers, either electrons

or holes, combine so that each particle returns to its non-conducting energy state.

The rate of recombination that occurs, per unit volume, is known as the

recombination rate, U(cm-3s-1) and is caused by three separate processes:

Radiative recombination. This plays no significant role as silicon is an indirect

band gap semiconductor, Uradiative;

Shockley Read Hall (SRH) recombination. This determines the minority carrier

lifetime in the bulk regions (three dimensional) of solar cells up to doping levels

of 1017 cm-3, USRH. A particular type of SRH recombination occurs at the

surfaces, Us, of the material where the theory is adjusted to account for the two

dimensional nature of a surface.

Auger recombination. This limits the carrier lifetime in silicon that has a doping

level in excess of 1017 cm-3, UAuger.

The relationship between the total recombination of each of the mechanisms is

given by,

AugeredHallSchokeyRadiativeTotal UUUU ++= Re (2.1)


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Either Auger or SRH recombination may dominate the total recombination.

Domination depends on the region of the solar cell and is discussed in detail in

sections 2.5 and 2.6.

Although the recombination rate itself is rarely measured, insights into the

quality of a crystal lattice can be obtained by measuring the excess minority carrier

lifetime, (s). This quantity is the inverse of the recombination rate, U (cm-3s-1),

multiplied by the excess carrier concentration n (cm-3),

Un= (2.2)

2.2.1 AUGER RECOMBINATION

The intrinsic electrical efficiency limit (not considering the optical limits) for

silicon solar cells is determined by the Auger recombination rate. This form of

recombination does not depend on the structural or electronic properties but on the

presence of dopant atoms within the lattice. This is due to the mechanism by which

Auger recombination occurs as is shown in figure 2.1.


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Figure 2.1 Representation of Auger recombination. Auger recombination requires

three free carriers (T = t0). This figure represents the electron-electron case. A free

carrier gives its energy to another free carrier (T = t1), due to conservation of energy

one carrier returns to the ground state and recombines with a hole while the other has

twice its original energy (T = t2), the highly energetic electron returns to the

conduction band energy state by phonon emission T = t3, so at the completion of the

Auger recombination event there is one electron with ~1.1 eV and the lattice has

absorbed the other ~1.1 eV (T = t4). The reverse is true for holes.

The coefficients that determine the rate of Auger recombination are under

investigation since it has recently been shown that this form of recombination plays a

more significant role at low doping levels than was previously recognized

T = t0 T = t1 T = t2 T = t3 T = t4


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[Hangleiter and Hcker 1990, 1994]. Consequently, previous calculation of the

efficiency limits of silicon solar cells must been viewed with caution. The equations

that describe the Auger limited lifetime for the case of having no interacting particles

are as follows:

1

Auger= Cnnp + Cpp2

1

Auger= Cpnp + Cnn2 (2.3)

The carrier lifetime in a region where the doping is in excess of 5 1018 cm-3 is given

by the equations above (2.3) where the coefficients are Cn = 2.8 10-31 cm6/s and Cp

= 0.99 10-31 cm6/s for an n-type and p-type region at 300 K, respectively.

The determination of the coefficients, Cn and Cp, was first made by Beck and

Conradt [1973] then later by Dzierwior and Schmid [1977]; the coefficients in this

work are those of the latter. The coefficients were determined empirically from the

trend of the data from log(lifetime) against log(doping concentration) for p- and n-

type wafers in low injection (see figure 2.2). The trend was found to depend on the

square of the majority carriers concentration. The work of Dzierwior and Schmid

[1977] remains well accepted for doping levels in excess of 5 1018

cm-3

.


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Figure 2.2 Minority carrier lifetime as a function of dopant concentration for heavily

doped Silicon for a) boron and b) phosphorus doped materials. Figure from

Dziewior & Schmid [1977].

As can be seen from equation 2.3 the majority carriers concentration and the

type of dopant both dominate the lifetime such that the recombination rate in an n-

type region is nearly a factor of 3 higher for an equivalently doped p-type region.

This higher recombination rate is balanced somewhat by needing more dopant atoms

in a p-type region to obtain the same conductance and is due to the lower mobility of

holes than electrons (see section 2.6 and 2.7). Therefore, the Auger recombination

rate for either a boron or phosphorus diffused emitter, that is of a given sheet

resistivity, is theoretically very similar.

There is a lower lifetime in material having a doping level below 5 1018 cm-3

than equation 2.3 predicts. In order to resolve the difference in measured and


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predicted lifetimes, Hangleiter and Hcker [1990, 1994] extended the theory of

Auger recombination to account for the electronic charge, and the resultant

interaction, of free particles; including electron-electron, electron-hole and hole-hole

interaction. Their extension did resolve the discrepancy between the observed results

and the lifetimes predicted by the previous theoretical model. The explanation

accounts for the electrical attraction between the particles and is known as Coulomb-

Enhanced Auger recombination. The intrinsic lifetime at low doping levels, CE-Auger,

is determined (approximately) by the equation in (2.4) below:

CE-Auger= 2.374 1024Ndop

-1.67 (seconds) (2.4)

The doping level, Ndop, solely determines the low level doping Auger lifetime for

samples in low injection.


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Figure 2.3. The theoretical lifetime for Standard (p-type) and Coulomb-Enhanced

Auger recombination is plotted against doping density. Included are some results

for gallium doped FZ from Ciszeket al [1989] and from samples boron doped FZmeasured for this work.

As can be seen from figure 2.3 the Auger process plays a minor role for bulk

recombination in one-sun solar cells. The following sections constitute a detailed

discussion of the dominant process, SRH recombination. However, Auger can be the

dominating recombination process for heavy diffusions (see chapter 5 and 6).

2.2.2 SHOCKLEY READ HALL RECOMBINATION

Shockley and Read [1952] and Hall [1952] independently developed a theory

to account for the recombination of holes and electrons that is due to energy states

1.E-09

1.E-07

1.E-05

1.E-03

1.E-01

1.E+01

1.E+14 1.E+15 1.E+16 1.E+17 1.E+18 1.E+19 1.E+20

Doping (cm-3

)

Lifetime(seconds)

CE Auger Auger

"This Work" Ga Doped FZ


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within the band gap (see figure 2.4). These energy states are present due to various

reasons, such as:

Unwanted impurities (contamination);

Faults within the crystal (stacking faults, lattice mismatch, dislocations, etc);

Boundaries of the lattice (surfaces).

Figure 2.4 A representation of SRH recombination via a single energy level within

the band gap. Solid circles represent an electron. The empty circle represents a hole.

Figure based on Green [1995].

Conduction Band

Valence Band


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The relationship of the recombination rate Uto these factors is:

( )( ) ( )1010

2

ppnnnnpU

np

i

+++=

,

where

=

kT

EEnn iti exp1 ,

=

kT

EEnp tii exp1 ,

211 inpn = ,

tthpp Nv =1

0 and tthnn Nv =10 (2.5)

The SRH recombination rate for a given mid gap energy level, where k is

Boltzmanns constant, Tis temperature in Kelvin,Et is the energy level of the defect

level, Ei is the thermal equilibrium concentration. The latter two equations are

known as the capture time constants of holes p0 and electrons n0, which are both

dependant on the thermal velocity of the free carriers, vth (a constant of ~ 107

cm/s in

Si at 300 K), the defect concentration, Nt (the subscript t refers to trap, an often

used synonym for defect), and p and n which are the capture cross sections for

holes and electrons, respectively.

The terms in equation 2.5 of particular note are the capture cross section n,p,

the defect concentration Nt and the product of the majority and minority carrier

concentrations np. The capture cross section is a quantity that describes the

probability of recombination. It has been found that electrons have a higher capture

cross section under certain conditions [Robinson et al. 1994], however such results

tend to reflect the technological aspect of a sample rather than the fundamental

aspects of recombination [Green 1995]. The defect density, Nt, is also a


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technological quantity dependant on the process conditions of a finished sample.

Importantly, it is the product of these two quantities that determines the capture time

constants so if one is appreciably minimized the effect of the other is suppressed.

Furthermore, if one carrier concentration can be engineered to favor a reduced

recombination mechanism then there is even less dependence on the defect density;

the latter is the most demanding (expensive) of the three quantities to minimize.

These three forms of SRH are significant issues in silicon solar cell research as

they can adversely affect the efficiency of such a device. Each of these topics will be

discussed separately in sections 2.5, 2.6 and 2.7. However, in the next two sections,

two physical concepts (the mobility and the diffusion length) that are used to

characterize the quality of silicon are introduced. Both concepts are closely related

to solar cell efficiency.

2.3 MOBILITY

Another important quality is the mobility of free charge carriers, (cm2/Vs).

This quality describes the ability of free carriers to move within a lattice given an

applied electric field per unit time (cm2/Vs). It is determined empirically and is

found to depend on the doping level, the doping type and injection level. For carriers

in a quasi-neutral region transport is through diffusion, where both electrons and

holes have different diffusion coefficients, D. These quantities are not independent

and are related as follows:


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q

TkD B= (2.6)

Mobility (or diffusivity) is not a measure of lattice perfection (within a crystal)

per say and is independent of recombination. Therefore, the mobility is an intrinsic

limit of the material (silicon and GaAs have different mobilities) and thus is altered

by the scattering of the atoms (and free carriers) incorporated in the lattice. As this

model assumes that the lattice is of high quality it is applicable primarily to float

zone silicon, though measurements for Cz silicon are near that of FZ and thus

mobility data can also be used for Cz (see figure 2.5). Furthermore, as holes are

merely vacancies of electrons, their mobility is different to actual electron flow (see

figure 2.5 (a)).


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(a)

(b)

Figure 2.5 The mobility of (a) holes and (b) electrons are shown where the solid line

is the majority carrier mobility and the dashed line is the minority carrier mobility.

The figures are taken from Green [1995]. The data for (a) was taken from Sproul et

al. [1992] (), Dzierwior and Silber [1979] (r), and Swirhun et al. [1986] (S) and

a single data point from Stephens and Green [1993] (+). For (b) the data was from


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Dzierwior and Silber [1979] (S), Swirhun et al. [1986] (U), Wang and Neugroschel

[1990] (), del Alamo and Swanson [1987] (+) and Sproul et al. [1992] and

Stephens and Green [1993] () for FZ and (r) for Cz.

2.4 DIFFUSION LENGTH

The diffusion length is a product of the mobility and the lifetime. The lifetime

is sensitive to the method of the crystal growth and the subsequent processing of the

wafers and not to the mobility. However, for material used to make solar cells the

minority carrier mobility can span a five fold difference, from 1500 to 300 cm2/Vs

depending on the type (p or n) and the resistivity of the material. Therefore a single

expression that combines both mobility (or diffusivity) and lifetime is used the

diffusion length.

The diffusion length, L, is the square root of the product of the lifetime and the

mobility (see equation 2.7). This expression gives the average distance an electron

or hole may travel prior to a recombination event. This quality, the diffusion length,

will be discussed throughout this work, as it is a vital aspect to high efficiency device

design.

= DL (2.7)

The relationship in equation (2.7) for a given wafer resistivity would simplify to

L 1/2. Because of this relationship it is common to assess a process (or material

quality) by stating only the lifetime.


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2.5 BULK DEFECTS AND CONTAMINATION

For current technology the most significant cause of contamination is the

subsequent processing of the wafers. For example, the incorporation of as little as

10-5 parts per million of Molybdenum into the silicon lattice begins to decrease the

minority carrier lifetime via increased SRH recombination (see figure 2.6).

Unfortunately, wanted impurities such as phosphorus and boron diffuse in silicon

slower than nearly all other elements. This fact heightens the need of cleanliness

prior to the exposure of silicon to high temperature processing, as the removal of

metal ion contamination through gettering cannot be performed once a solar cell has

been fabricated.


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Figure 2.6 The concentration of impurities that effect the solar cell efficiency in n-

type 1.5 cm (top) and p-type 4 cm (bottom) Silicon for various elements is

shown. Figure from Davis et al. [1980].


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Atoms such as phosphorus and boron do not exactly fit into the lattice. The

extent to which they do not match a silicon atom is known as a mismatch factor.

This mismatch locally deforms the lattice allowing energy levels to arise in the band

gap, increasing the bulk SRH recombination and thus lowering the lifetime.

Furthermore, when dopant atoms are at interstitial sites they serve no purpose except

to create more SRH recombination due to local deformation of the lattice.

2.6 SURFACE RECOMBINATION

A surface is a discontinuity of the crystal axis. As such, many dangling bonds

exist there. These dangling bonds make excellent SRH recombination sites because

they allow energy states within the band gap (see figure 2.4). The number of the

states for a given energy level is Dit (unit cm-2eV-1). A measure of the number of

allowed energy levels in the band gap is known as the interface state density, Nit

(= C

V

E

E

itdED ). As can be seen from equation 2.5 the recombination rate is at a

maximum when holes and electrons are in equal concentrations. Therefore, the

reduction of surface recombination, herein called surface passivation, has two aims:

Improve the quality of the interface, such that the surface state density, Nit, isreduced and, or

Create a population imbalance so as to limit the recombination due to the relative

scarcity of either holes or electrons for an n-type or p-type region, respectively.


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The methods of reducing the SRH at the interface will be addressed in the

following section, but firstly a brief description of the theory will be given.

The formulation of the recombination rate for the surface SRH recombination

is similar to that for Eq. 2.5 except that the density of traps, Nt, is now measured in

cm-2 not cm-3 due to the 2 dimensional nature of a surface as apposed to the bulk that

is 3 D. This results in the quantity vtNit no longer having the units of seconds as it

does for the capture time constant. The quantity vtNit at a surface is expressed in

s/cm and is represented by S. Therefore, analogous to the lifetime for bulk

recombination is the surface recombination velocity S(see equation 2.8).

S = Us/n where

( )( ) ( )

np

is

S

pp

S

nn

npnU

0

1

0

1

2

++

+

= (2.8)

This concept of a surface recombination velocity can be approached by

considering two physical mechanisms:

There is recombination at the surface that is dependent on the density of states,

Nit(assuming vtand are constant);

A free electron (or hole) requires a free hole (or electron) to recombine with.

Thus the availability of a free hole (or electron) limits the recombination rate

(np-ni2) (see figure 2.7).

When there are equal populations of both holes and electrons, at flat band conditions,

one would expect the recombination to be a maximum. However, recombination

between holes and electrons is complicated by asymmetric capture cross sections.


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Also, the capture cross sections of holes h and electrons e depend on the particular

mid-gap energy levels, which is dependent on the surface conditions (metal coated,

bare or passivated).

This work will be concerned with creating a significant population imbalance

to reduce the recombination at a surface (oxidized or metal plated/coated) while

minimizing surface damage. At the high doping levels required to achieve this,

information on the capture cross sections and the location of traps within the band

gap in not measurable, nor would the information be of any foreseeable consequence

to the development of a heavy boron diffusion process. As such a discussion of the

characteristics of phosphorus and boron in silicon will be given and followed by a

brief description and a topical review of the emitter saturation current and the surface

recombination velocity. Preceding this a brief description of the Si/SiO2 interface

will be given as the work in this thesis extends to include oxide-passivated light

boron diffusions.


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Figure 2.7 A PC1D simulation of the carrier concentrations (and the square root of

their product) at the rear of a solar cell. The minimum in the product of the carrier

concentrations (n p) is at the surface (1.25E-4 cm) due the high concentration of

holes created by a diffusion. The square root of np is shown so that it can be

plotted on the same scale.

2.6.1 THE Si/SiO2 INTERFACE

For a given electron-hole population ratio, to decrease the surface

recombination velocity a reduction ofDit must be achieved. Various authors have

found that a minimum inDit is obtained by:

Oxidation of Si (Si + O2 -> SiO2 or Si + H2O -> SiO2 + H2) at high temperature

(T > 1000o

C);

The carrier concentrations at the rear of a solar cell

1.E+11

1.E+12

1.E+13

1.E+14

1.E+15

1.E+16

1.E+17

1.E+18

1.E+19

1.E+20

1.22E-04 1.23E-04 1.24E-04 1.25E-04

Distance (cm)

C

arrierconcentration(cm-3)

Electron Density

Hole Density

Sqaure root of the pn-product


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Incorporation of hydrogen at the Si/SiO2 interface through a forming gas (95%

N2, 5% H2) anneal or an alneal reduceDit to a minimum.

The first point, oxidation of silicon, was widely research by the

microelectronics industry from the 1960s onwards, and has lead to the rapid

development of digital electronics.

Factors that decrease the passivating qualities of a thermal oxidation are:

Reducing the temperature below 1100oC [Sze 1981];

Reducing the thickness of the oxide below 100 nm [Wang 1992];

Impurities being incorporated at the interface due to poor preparation of the Si

surface (cleaning) prior to SiO2 growth or through impurities in the furnace or in

the O2, H2 or H2O;

Increasing the dopant concentration in the Si wafer above 1016 for boron or 1017

for phosphorus due to the generation of extrinsic interface [Snel 1980];

Altering the crystal plane of the Si from (100). That is, Dit(100) < Dit(111).

The mechanisms through which degradation or improvement in SiO2

passivation need to be understood such that the process design is as simple as

possible while maintaining the desired electronic properties of the device.

Subsequent sections deal with the various issues that arise in the processing of solar

cells that are affected by the five aforementioned factors.


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2.6.1.1 TEXTURING THE FRONT SURFACE

It has been reported that the growth of over 350 nm of oxide on a random-

pyramid textured surface causes an increase in the dislocation density (density of

dislocations to the lattice) [Wenham 1993]. An increase in the dislocation density

would then increase the SRH recombination due to the loss of lattice perfection

creating mid-band gap states. These dislocations are believed to arise from the

cooling of the oxidised wafers because the thermal expansion coefficients of Si and

SiO2 are different. The different rates of contraction are said to cause dislocations at

the tips and bases of the pyramids [Chong 1989]. However, Chan [1993] refutes

these claims based on the minimal difference in the thermal expansion coefficients

and explains Chongs [1989] (also reported by Wenham [1993]) results as simply the

consequence of growing the oxide in a metal-ion-contaminated furnace. The

interpretation of Chan [1993] may be valid though there is no evidence to support the

assertion that thick oxides can be grown on textured surface without increasing the

dislocation density. These issues are of concern to researchers who are developing

more complex processing sequences because the oxide must be thick enough to mask

the diffusions and the metal plating though thin enough that excess SRH

recombination sites are not generated by the growth of too much oxide. This point

strong encourages researchers to work with planar wafers in the initial development

phases of silicon solar cells research. This author has used untextured wafers as

there is much to be gained by the research of the operation of planar devices prior to

the demonstration of high efficiency solar cells with textured front surfaces.


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2.6.1.2 ANTI-REFLECTION COATING

The use of thin passivating oxides underneath an AR coating reduces the

surface recombination velocity compared to the absence of the SiO2 [Fossum and

Burgess 1978]. However, these oxides need to be as thin as possible so that the AR

properties of MgF2/ZnS, SiN, or TiO2 remain beneficial. However, reducing the

thickness of the oxide below 20 nm reduces the effectiveness of its passivating

property [Wang 1992]. Fortunately, a post-processing anneal can be used to restore

and even improve the surface passivation of SiO2 even when the thickness is below

20nm. This is achieved by coating the SiO2 with Al and sintering in either N2 or

N2(95%)/H2(5%), also known as forming gas. This annealing process with an

aluminum coating has come to be known as an "alneal".

An alneal of oxides as thin as 20 nm will decrease Nit compared to that of a

thick high quality SiO2 that is not alnealed (see figure 2.8). The process at work in

the alneal treatment is assumed to be the reaction of aluminum and the water vapour

the silicon dioxide absorbs (a wet oxidation need not be grown) which releases

atomic hydrogen that easily diffuses through the SiO2 to the silicon surface and

passivates the dangling bonds (see Balk [1965] for a description of the hydrogen

transport and Reed and Plummer [1988] for an explanation of what the hydrogen

does at the interface). Annealing in forming gas without aluminum has a reduced

benefit. It is believed that the forming gas anneal releases molecular H2 that less

effective in passivating the dangling bonds at the surface than atomic hydrogen.


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diffuse into the SiO2 or to be pushed deeper into the silicon. These two responses

to oxidation are categorized by a segregation coefficient m. If an atom prefers

silicon, the segregation coefficient is k>1 or if atom will prefer to diffuse into the

SiO2 then k


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Figure 2.9. The redistribution of dopant atoms due to the thermal oxidation of Si for

two different cases (carrier concentrations ND & NA are normalized). Boron is a

slow diffusant through oxide (k < 1), whereas phosphorus in known as a fast

diffusant through SiO2 (k > 1). Figure base on Gandhi [1994].

1.0

D&NA

SiO2 Si

k > 1

k < 1


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2.6.3 EMITTER SATURATION CURRENT AND THE SURFACE

RECOMBINATION VELOCITY

In a selective emitter solar cell the emitter doping is commensurate with the

need to reduce lateral resistance losses; an emitter diffusion between 100 and 300

/ is generally used. At these doping levels the surface conditions (extent of

passivation) are influential on the surface recombination velocity and is known as a

transparent emitter. As light diffusions are acceptable for lateral carrier transport, the

issue of surface recombination dominates.

An emitters quality can be quantified by the property known as the emitter

saturation current Joe which is the current that flows from the bulk to the emitter at

open circuit. Its magnitude depends on both the Auger recombination in the emitter

and on the SRH recombination at the surface. Although Auger recombination

servery reduces the lifetime in heavily doped regions it is not necessarily dominant

(see chapter 6).

As previously mentioned, a means to passivate a surface is by population

imbalance. Hence the emitter has a dual purpose: to provide sufficient conductance

and passivate the surface. Passivation is especially important at the metal contacts

as these have an infinite recombination velocity, limited by the velocity of a free

carrier. This means that minority carriers that reach the metal/Si interface will

recombine. In effect the surface itself is not passivated at all. As such, a population

imbalance is the only means to reduce the recombination rate at metal contacts. The

population imbalance in this case is caused by the resultant electric field of a heavily

doped region adjacent to a lower doped region. For metal contacts with the same


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polarity of the base, these regions are called high-low fields, or more commonly back

surface fields. A measure of the electric fields effectiveness can be represented by

the effective surface recombination velocity Seff. This quantity is the recombination

velocity at the interface between the base and the heavy diffusion. The lower the

value the more effective the diffused region is in repelling minority carriers from the

actual surface.

Due to the two mutually exclusive requirements of an emitter and a metal-

contacted region, all high efficiency designs use a two-step process. Both the emitter

and metal contact diffusions are made at separate times. These types of solar cells

are called selective emitter solar cells. This is a structure with a light surface

diffusion (generally SiO2 passivated) and a heavy contact-passivating diffusion.

Although this adds to the complexity to the processing (and thus is rarely used on a

large commercial scale), homogeneous (single step) emitters are inherently limited to

a lower efficiency. The emitter diffusions are generally around 40 / for

homogeneous emitter cells causing a loss in current due to the reduction of the

diffusion length in the emitter, and a reduction to the voltage because of the

increased recombination in the emitter.

For light phosphorus diffusions that are oxide passivated both Cuevas et al.

[1996] and King et al. [1990] have found that the surface recombination velocity

increases linearly with the surface concentration,Ns, as follows:


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SNs10-16cm4/s, for Ns>10

18 cm-3 (2.9)

The increase in Sappears to be due to the increase in extrinsic surface states

generated by the increase in the doping concentration (as unlikely as it may seem the

concentration of dopant atoms is 1 part in 5 million for NA,D ~ 1016 cm-3).

Irrespective of the mechanisms at work, the empirical conclusions of Cuevas et al.

[1996] and King et al. [1990] are in good agreement with Snels [1980] findings

(see figure 2.10). Therefore, it can be concluded that in order to reach the highest of

efficiencies a passivated emitter ought to have a low surface concentration because at

these doping levels it is S that is dominant (technologically, not intrinsically) not

Auger recombination. Wang [1992] has also concluded that a low NAs was

appropriate for high efficiency solar cells. This is in contrast to the model used by

PC1D where, for the same sheet resistance, a lower Joe is calculated for a higher

surface concentration. This disagreement is reached through the competing

considerations of creating a population imbalance and yet not increasing the interface

state density. A minimum in the emitter recombination must be reached such that

there is sufficient masking of the surface through electric field repulsion (population

imbalance) combined with minimal generation ofNit, which increase U for carriers

that are at the surface.


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Figure 2.10 The concentration of surface states as a function of surface dopant

concentration. The data shown was measured before and after an alneal for

phosphorus (a) and boron (b). Figure taken from Snel [1980].

The work by King et al. [1990] showed that a minimum in the emitter

saturation current is reached when the surface concentrationNs = 5 1018 cm-3. As

the surface concentration increases so doesJ0e due Sand UAuger both increasing. As

Ns decreases the recombination becomes dominated by the gain in Us as np-ni2

approaches a maximum. This analysis must be combined with the restrictions of the

need to provide a low resistance path in the emitter. As such the emitter design is

limited by the means of contacting the solar cell.

From these previous discussions, it can be seen that the method of making the

ohmic contact to a solar cell is a vital aspect of solar cell design.


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Chapter 3

High Efficiency Designs An Overview of

Device Development

3.1 SUMMARY

This chapter reviews the development of silicon solar cells in two parts. First,

a review is given of high efficiency silicon solar cells from the 1950s to present day

technology. This enables the identification of the processes that reduce

recombination in currently produced commercial solar cells and those of the highest

efficiency. The review summarizes that which can be found in Green [1995].

Second, buried contact solar cells are introduced, which are the highest efficiency

silicon solar cells in mass production. This review aims to elucidate the current state

of development for the buried contact structure.

As a result of this exposition the current obstacle facing development of the

buried contact structure is identified as the heavy boron diffusion. Furthermore, it is

recognized that this process has the potential to increase the efficiency of most

crystalline silicon commercial solar cells.


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3.2 A BRIEF HISTORY OF SILICON SOLAR CELL DEVELOPMENT

Bell Labs were the pioneers in silicon solar cell development in the 1940s. During

this early developmental time processing technology for silicon was advancing

rapidly and researchers took advantage of such technology. By using a boron

diffusion on an n-type wafer the efficiency jumped to ~ 6% [Chapin et al. 1954]

from ~ 1% recorded two years previously (see figure 3.1) [Kingsbury and Ohl

1952]. The processes were further optimised such that the same structure (see figure

3.2) reached 11% by 1955 [Bell Labs, Record, page 166, 1955] (a gain in efficiency

of 0.3% absolute per month for 16 months). Due to low material quality (and low L)

the boron diffusion had to be applied to the perimeter of the solar cell for current

collection; a rear junction cell would be unrealistic at that time. By moving the p-

type contact to the front of the cell the efficiency became 14% by 1959 [Green

1995]; the average of todays commercially produced mono-crystalline solar cells.

Even though an increase in shading would result, the efficiency improved

presumably due to the gains in the fill factor because previously the current would

have to travel the perimeter of the cell before collection at the p-type rear contact,

increasing the series resistance.


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Figure 3.1 The first solar cell to have an efficiency over 1%. The junction was

formed by a melt of n and p type silicon. Figure from Green [1995].

Figure 3.2 The Wraparound silicon solar cell of Chapin et al. [1954]. Figure from

Green [1995].

The next advance in efficiency came through closer attention to the front

surface. The cells were now made of p-type wafers with a phosphorus diffused

emitter. These phosphorus diffusions were extremely heavy by todays standards.

The concentration at the surface was higher than the solid solubility of phosphorus in


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Si, causing major damage to the surface area. This fact, combined with the loss in

diffusion length at such high doping concentrations, meant that the light absorbed

near the front surface (UV) was not contributing to the current and the surface

recombination was very high. Consequently, by performing shallower and lighter

diffusions the current collection and the front surface recombination were improved,

each in turn improving the efficiency. Also, by using photolithography to define

finer yet more closely spaced fingers the fill factor was not sacrificed. This new

structure also had an Al/Si alloy at the rear [Mandelkorn and Lamneck 1972]. These

new processes saw the efficiency increase from 14.5% in 1961 (see figure 3.3) to

15.2% in 1973 (see figure 3.4) [Lindmayer and Allison 1973].

Figure 3.3 The 14.5% efficient solar cell first reported in 1961. Figure from Green

[1995].


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Figure 3.4 The violet solar cell, so called due to the increase short wavelength

response due to a lighter phosphorus diffusion to form the n-type layer on the top

surface. Figure from Green [1995].

Soon after these alterations were made to the overall design, front surface

texturing, known as random pyramids was discovered [Haynos et al. 1974]. This

solar cell was called the black cell due to the low reflection from the front surface

texture (see figure 3.5). The black cell became the new benchmark for high

efficiency solar cells as it converted 17.2% of incident light energy into electrical

power [ibid]. This record in efficiency was not surpassed until 1983 when both the

Metal-Insulator-NP junction (MINP) and the Passivated Emitter Solar Cell (PESC)

structures were measured at > 18% [Green 1991]. Over the next 16 years many

minor improvements were made to the PESC that now has developed into the PERL

(Passivated Emitter Rear Local diffusion) solar cell (see figure 3.6). These minor

improvements will be discussed in the following section as it is these areas of solar

cell technology that are of most relevance at this stage of device design and

development, due to improved Si growth technology, higher quality diffusion


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sources and process, and superior oxidation and cleaning methods. The PERL cell

currently holds the record for the highest 1-sun efficiency that is 24.7% [Zhao et al.

1999].

Figure 3.5 The black cell held the efficiency record for nearly a decade. This was

the first solar cell to have a random pyramid front surface texture which significantly

reduced reflection. Figure from Green [1995].

Figure 3.6 The Passivated-Emitter Rear-Local (PERL) contact solar cell. This solar

cell currently has the high conversion efficiency at 100 mW/cm2 (1-sun) of 24.7 %

(see text). Figure from Green [1995].


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3.3 DESIGN PRINCIPLES OF HIGH EFFICIENCY SOLAR CELLS

Structures that have reached the highest efficiencies will be reviewed in order

to identify common advanced design principles. These principles, once identified,

will be reviewed for their applicability to simpler structures that require reduced

processing steps.

Fortunately, there are two solar cell designs that have been developed over the

last 15 to 20 years providing an excellent resource base for this analysis. The

designs are the rear-junction point-contact solar cell (see figure 3.7) and the PERL

structure. Each of these designs incorporates a significant number of non-

commercial processes. However, understanding what makes a device more efficient

is of particular interest, as it will enable the identification of:

Loss mechanisms and their effect on overall performance;

The applicability of the processes to commercially constrained devices.

This analysis will be separated into three sections. Firstly, emitter passivation will be

reviewed below and, secondly, the passivation of the metal contact regions will

follow in section 3.3.2, finally a review of rear surface structures will be made,

including a separate section on the simplified point-contact solar cell. The last of

these sections will enable a broader appreciation of how and the buried contact solar

cells may be improved in their design and the importance of the subsequent

processing.


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Figure 3.7 The point-contact solar cell. Each diffusion is made through small

squares (a point) in the masking oxide that are connected together by strips of metal.

Figure from Swanson and Sinton [1990].

3.3.1 EMITTER PASSIVATION

PERL is an acronym for Passivated Emitter Rear Local contact solar cell. As

previously mentioned, this structure is based originally on the PESC (Passivated

Emitter Solar Cell). The PESC used thin, photolithographically defined contacts to

minimise the effect of the recombination at the metal/Si interface and provide a well-

passivated front surface, hence the term Passivated Emitter.

The first implementation of the PESC used a homogeneous emitter. The

doping of the emitter was ~100 /. This structure was the first solar cell that

demonstrated Voc > 650mV. The major benefits were due to a high quality SiO2


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layer grown on the front surface that provided a substantial improvement to the

surface passivation. The use of the high quality oxide was adapted from

developments in the microelectronics industry although there it was used for

different reasons.

The work of Fossum and Burgess [1978] further supported the connection

between front surface passivation and the use of high quality SiO2. They showed

that by growing only 5 nm of oxide underneath a non-passivating SiN film, the

efficiency improved by 1% absolute compared to the identical solar cells without the

SiO2. Thereafter, thermally grown SiO2 front surface passivation has been an

integral part of high efficiency solar cell design.

Further developments lead to the use of heavy diffusion under the front

contacts; as previously mentioned this passivates the Metal/Si surface. Consequently

the emitter can be redesigned, as it longer needs to provide contact passivation but

only surface passivation and current transport. Also, due to ongoing developments in

photolithography, the width of the fingers could be reduced so that they could be

closer together without increasing their contribution to reflection. As a result of

these new techniques the doping level of the emitter could be further reduced. The

lighter emitter would allow the surface concentration of the emitter diffusion to be

lower, increasing the short wavelength response and the voltage due to a lower front

surface effective recombination velocity, SFeff(see equation 2.5).


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3.3.2 CONTACT PASSIVATION

The PESC and BCSC both have an Al/Si alloy at the rear of the device (the p+

region in figure 3.5). The Al/Si alloy is formed by first depositing Al on the surface

then sintering at temperatures above the eutectic temperature, 577 oC. When this

alloy of Si and Al cools it recrystallises to form a heavy Al doped region of Si. The

alloy was first used in the early 1970s and an immediate gain in the efficiency was

observed due to high open circuit voltage and high current. The improvement in the

efficiency was attributed to the gettering (reduction of SRH recombination) effect of

the Al. Mandelkorn and Lamneck [1973] took a theoretical view and explained the

improvements in efficiency by the leakage of majority carriers from the heavily

doped Al/Si region into the bulk, which increased the effective bulk concentration

and increased the voltage accordingly; the correct explanation was given by

Golglewski et al [1973]. This latter work showed that the voltage and current

benefited when the effective rear surface recombination was reduced. The rear

surface recombination velocity is reduced by the heavy doping of the Al in the Al/Si

alloy that repels minority carriers from the metal contact, technically called a high

low field as the polarity of the regions are the same. This region, the Al/Si alloy, has

been identified as the efficiency-limiting factor for both the PESC and BCSC designs

[Wenham et al. 1994].

A solution to the high rear surface recombination velocity came through the

Point Contact solar cell that was originally developed for concentrated-light solar

cells [Schwartz 1982]. Metal contacts are made via openings in an otherwise oxide

passivated, non-diffused surface. The exposed silicon is heavily diffused with


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phosphorus or boron, depending on the intended polarity of the contact. Strips of

aluminum join the point contacts together so that most of the aluminum lies on the

SiO2 surface.

These types of contacts were transferred to the PESC such that heavy boron

diffusion acted as a back surface field for the p-type contacts on the rear surface.

This diffusion and subsequent metal contacts were over a small area (point contacts).

The reduction to the area of the metal contacted silicon surface could be made for the

Al/Si alloy of the PESC also. However, significantly, the saturation current from p-

type high-low regions has been showed to be far higher for Al/Si alloy than for heavy

boron diffusion (see Kane and Swanson [1985] for boron and King et al. [1991] for

Al/Si alloy). Hence, the point contact solar cell offered a dual benefit over the Al/Si

alloy rear surface in that, the contact area was reduced from 100% to 1%, and, the

dark saturation current from the contact area was approximately a factor of 3 lower

for boron than for the Al/Si alloy (per unit area of P+).

The advantage of the lower saturation current of boron compared to Al/Si alloy

was not utilised in BCSC processing due to the wide spread belief that a heavy boron

diffusion over a large area (the entire rear surface) would cause the lifetime of the

wafer to be substantially reduced [Aberle 1999], although there was evidence to the

contry as early as 1978 [Fossum et al. 1978].

Adapting the PESC to these sorts of p-type contacts produced the PERL cell.

The open circuit voltage rose from 660 mV to over 710 mV. Efficiency for the

newly created PERL cell was measured at 23%, 0.7% higher than the point contact

solar cell. Since this time however, the point contact solar cell has been the most

efficient solar cell under concentrated light. When the same rear contacts were


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applied to BCSCs, the open circuit voltage jumped from 645 mV to 695 mV. Not

surprisingly rear surface (p-type) point-contacts are used for all silicon solar cells

that have been measured at > 22%. Approaching this value though without the use

of point contacts is the simplified point contact solar cell. This solar cell will be

reviewed in section 3.3.4.

3.3.3 REAR SURFACE PASSIVATION

The limiting aspect of the PERL solar cell was that the fill factor was well

below its theoretical limit for such high open circuit voltages. Increasing the amount

of metal for the front fingers saw the fill factor rise, though still below expectations.

After careful analysis of the dark current-voltage characteristics it was found that the

recombination at the rear oxide (99% of the rear surface) was only low once the

voltage was very near the open circuit voltage [Robinson et al. 1994]. This meant

that the effective rear surface recombination velocity was very high at the maximum

power point. The velocity would change from 104 cm/s at low voltages to ~30 cm/s

at open circuit. Improvement to the quality of the rear oxide was made and a fill

factor of 83.6% has been demonstrated.

In an effort to further improve the rear surface, a phosphorus diffusion was

performed on the non-contacted region of the rear surface resulting in the Passivated-

Emitter Rear-Floating junction solar cell (PERF). The rear phosphorus diffusion

creates a floating-collector that passivates the rear surface by repelling minority

carriers from the surface (see section 3.4).


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The PERF cell has demonstrated the highest open circuit voltages for silicon

solar cells: 720 mV with planar surfaces where the PERL cell appears limited to 716

mV. The same means of connecting the p-type contacts was used: entire rear side

coated with Al. This method of contact appeared to be inappropriate, as the Al

would contact the p+ regions andthe floating collector through pinholes in the rear

oxide and low fill factors resulted. Though using a much lighter diffusion (sheet

resistivity of 1000-4000 / to form the rear floating collector) alleviated the

problem to some extent. This structure has not been developed since these initial

trials.

The PERT (Passivated Emitter, Rear Totally diffused) solar cell has re-

emerged as a high efficiency solar cell. This cell structure has a light boron diffusion

over the entire rear surface and the same heavily diffused point contacts of a PERL

cell. This structure is particularly suited to high resistivity wafers where the rear

surface diffusion aids current transport, increasing the fill factor. Interestingly, the

open circuit voltage of these cells is as high as a PERL cell [Zhao et al1999]. As

such, it can be deduced that a light boron diffusion can be performed that retains a

low effective rear surface recombination velocity.

3.4 BURIED CONTACT SOLAR CELLS

The selective emitter concept was elegantly incorporated into a low cost design

by the laser scribing of grooves into the front surface [Wenham 1985]. These types

of solar cells are called buried contact solar cells (BCSC) (see figure 3.8). In this

design the emitter diffusion precedes the laser cutting of grooves deep into the wafer


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(30-60 m). The grooves are cleaned of excess Si and diffused far more heavily than

the initial emitter diffusion. The surface is masked from the heavy diffusion by an

oxide that is grown between the emitter diffusion and the laser grooving. Therefore,

the grooves can be deglazed and plated while the front surface remains free of heavy

phosphorus diffusion and metal. The advantage of such a design is that lighter

emitter diffusion is required because the grooves can be closer together as most of

the metal is contained below the surface, significantly reducing the shading.

Furthermore, the Metal/Si interface is also passivated by the heavy diffusion

reducing the recombination at the front metal contacts. The rear side is unaltered

from standard bulk silicon solar cells where an Al/Si alloy is formed to provide a

high-low electric field at the rear metal/Si interface, which covers the entire rear

surface. The efficiency limiting aspect of the design has been identified as the rear

surface [Wenham et al. 1994].

plated metalp-type

n++

n+oxide

p+

metal

Figure 3.8 A schematic of a single sided buried contact solar cell. Figure from

Green [1995].


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This design has been utilized in high-efficiency hybrid solar cells. These

hybrid cells have the front surface of a buried contact solar cell but a PERL-cell rear

surface. The efficiency reached 22 % and the cells were used to make the worlds

first 20% efficient silicon module. However, the processing of the rear side was

applicable only to high-end solar cells and thus would never be considered a

commercially compatible process. Nevertheless, these cells clearly showed the

potential of the front surface of the buried contact design. Thereafter, efforts to

increase the efficiency of the BCSC have focused on improving the rear surface

passivation by a commercially compatible process.

Knowing that the efficiency limiting aspect of the BCSCs is the high effective

rear surface recombination velocity, SReff, the double-sided laser groove (DSLG)

solar cell [Ebong 1994] was proposed. These cells confine the p-type metal contact

to only 5-10% of the rear surface area while the other 90-95% is phosphorus diffused

and oxide passivated (see figure 3.9). These cells not only utilised the better metal

contact passivating properties of boron but also reduced the area of the contact to

around one tenth of its previous value. The DSLG solar cell has many aspects of a

high efficiency solar cell but requires a fraction of the processing. However, a

problem arose early on in the development of the DSLG cell that remains

unexplained: a lower than expected fill factor, seemingly due to a low shunt

resistance across the rear floating junction. This effect of a poor shunt resistance at a

junction is commonly called shunting.


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Figure 3.9 A schematic of a double sided laser groove (DSLG) buried contact solar

cell. Figure from Gohazi [1996].

A solution to the apparent shunting of the DSLG was presented by Honsberg et

al. [1996] though it required many additional high temperature steps. These

additional steps were performed so that the rear phosphorus diffusion was very light,

~1,000 /. Albeit this new design did have a high fill factor, the extra processing

that was required rendered the new design commercially incompatible. Nonetheless,

the new design did show that it was possible to overcome the low fill factor and as

such it is reviewed to better understand the design so that the mechanism responsible

for the shunting can be clearly identified.

The motivation to increase the sheet resistivity of the rear diffusion may have

arisen due to a similar outcome for the Passivated Emitter Rear Floating junction

solar cell, the PERF. The PERF cell is an extension of the PERL cell except that the

plated metalp-type

n++n +oxide

n+

oxide

n+

p+


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non-contacted area of the rear surface is phosphorus diffused rather than non-

diffused. Calculations showed this structure had, potentially, a higher efficiency than

a PERL cell and development was undertaken to access the actual operation of such

a device. Early results were encouraging with the PERF cell having the highest

recorded open circuit voltage for a 1-sun silicon solar cell of 720 mV. However, as

the PERF cell had only a 0.6% relative gain in the open circuit voltage this was not

enough to compensate for the much lower fill factor than that of the PERL. The fill

factor of the PERF cell was particularly low for rear sheet resistivities up to 250

/. As the rear phosphorus diffusion became as light as 1,000 to 4,000 / the

shunting the reason the fill factor was low - was greatly reduced, though not

sufficiently to receive further development. Nevertheless, some effort was given to

explain the rise in the fill factor as the sheet resistance of the rear phosphorus

diffusion increased [Altermann et al. 1996]. The outcome of this work was that the

metal that was evaporated over the entire rear surface was contacting the light

phosphorus diffusion through pinholes in the oxide as well as the p-type contacts,

causing a loss in the shunt resistance. This was explained as follows. A short

between the base and the floating collecting was created due to the pin holes and by

increasing the sheet resistance the flow of carriers through this circuit becomes

inhibited, due to a higher lateral resistance, reducing the effect of the short [ibid]. As

elegant an explanation that this is, the same analysis does not apply to the DSLG

solar cell as there is no metal over the rear surface to provide a contact to the

phosphorus diffused surface and the metal plated p-type grooves. Therefore, raising

the sheet resistance of the rear floating collector in the DSLG cells and observing a


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rise in the fill factor does not mean that the same parasitic effect present in the PERF

cell has been overcome.

Preceding a discussion of other possible effects that cause the reduction of the

fill factor in the DSLG will be a review of other DSLG-type devices that have

slightly different methods of rear surface passivation. These devices were fabricated

in order to investigate other means of passivating the rear surface, though neither

would be commercially viable. However, insights into the overall problem in the

development of buried contact solar cells can be gained.

One of the designs had no phosphorus diffusion on the rear surface but used a

thermally grown alnealed-oxide for surface passivation, similar to the PERL cell rear

surface [Tang et al. 1997]. The other design had laser ablated wells or dots in place

of grooves and retained the rear phosphorus diffusion [Honsberg et al1994]. This

dot-contact cell demonstrated an open circuit voltage of 685 mV, approximately 15

mV high than any previous DSLG solar cell. Once again the fill factor was low

(0.76) and on this occasion it was attributed to the small area of the contacts

supposedly increasing the contact resistance. Nonetheless, the open circuit voltage

of this cell was used to make a calculation of the dark saturation current caused by

the boron diffused grooves in the older design. The analysis concluded that the

saturation current from the heavy boron diffused grooves was 9.5 10

-13

A/cm

2

,

attributing the 15 mV difference to the decreased area of the p-type grooves, from

10% to 1% of the rear surface area. Logical as this may well have appeared, this

earlier analysis come into question when Tang et al. [1997] fabricated a 687 mV

solar cell that had rear grooves that were boron diffused and that covered 10% of the

area of the rear surface. An analysis of earlier results augmented with the result by


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Tang et al. [1997] provides some insight into the development that is required for the

DSLG solar cell. It may be that the increase in the saturation current that is observed

from a standard DSLG cell, compared to the cell by Tang et al. [1997], may be a

consequence of the poor junction between the heavy boron diffusion and the rear

phosphorus diffusion. This analysis is consistent with the characteristics of the dot

contact solar cell as that device also had a rear phosphorus diffusion. Alternatively

the boron diffusion could have been of low quality giving a dark saturation current

almost 5 times greater than had been published by a different diffusion method [Kane

and Swanson 1985]. Therefore, conclusions cannot be draw as there are more than a

single variable between the two experiments.

The fill factor of the dot-contact solar cell was low (0.76) and as previously

stated, was thought to be a consequence of high contact resistance. However, the

area of the ohmic contact was at least as large as it is for a PERL cell that does not

suffer from high contact resistance. As such the low fill factor for the dot-contact

solar cell could be due to the same mechanism that reduces the fill factor for the

standard DSLG solar cells, although as the series and shunt resistance values were

not given for the dot-contact solar cell the last point must remain speculation.

Furthermore, the dot-contact solar cell had a similar rear metallisation scheme as that

used for the PERF cell. The dots are nickel and copper plated then connected

together by evaporated aluminum that covers the entire rear surface. Any pinholes in

the rear oxide could cause the same shunting effect observed for the PERF cell.

Also, the research was not accompanied by any contact resistance values. Once

again there are two many variables in the experiments to draw conclusions.


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Returning to the possible causes that reduced the fill factor for the floating

junction passivated DSLG solar cell, a parallel is draw with the simplified point

contact (SPC) solar cell (see section 3.3.4). This comparison is formed due to 1)

both structures possessing adjacent diffused regions and 2) the observation of a low

shunt resistance [Sinton et al. 1988]. When the intensity of light increased to 500

mW/cm2 the fill factor of the SPC increased, presumably due to the higher bias

across the junction in question. Such an effect has also been measured in buried

contact solar cells where the spectral response from the rear side of a DSLG cell

dramatically increased by the application of a bias light [McIntosh et al. 1998]. The

low fill factor in the simplified point contact solar cell was attributed to a high

tunneling current across the rear junction. Careful adjustment of the boron and

phosphorus diffusions was need to alleviate the tunneling and fill factors over 0.81

were achieved. Therefore, addressing the qualities of the rear junction for the DSLG

solar cell may prove beneficial also. The work by Gohazi [1996] can also be seen to

support the suspicion that the rear n+/p++ junction reduces the DSLG cells

performance, as a very light rear phosphorus diffusion (Rsheet > 1,000 /) was

appropriate to minimize the shunting problem. The diffusion processes Gohazi

[1996] used to fabricate the cells that did not suffer from low shunt resistance were

not characterized (such as measuring the diffusion profiles), retarding the ability to

pinpoint the mechanism that eliminated the shunting. The light diffusion was

performed at the beginning of the processing sequence so the diffusion may well

have been relatively deep, with a low surface concentration; hence it cannot be

dismissed that this aspect of the diffusion alone eradicated the shunting.


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Lastly, the effect of the boron groove diffusion has been investigated [Ebong et

al. 1996] though the findings indicate that the boron diffusion process that was used

was not optimal. This conclusion is arrived at due to the very low open circuit

voltages obtained in that experiment less than 620 mV. This compares to the

demonstration of 670 mV for the same structure with a lighter boron groove

diffusion [Ebong et al. 1996]. Experiments have shown (see chapter 4) a heavy

diffusion is desired to passivate a metal/Si region, the means of obtaining a heavy

diffusion by Ebong et al. [1996] may have been underdeveloped. Therefore,

developing the diffusion conditions that enable a boron diffusion to be performed

without harming the bulk lifetime or incurring a high saturation current is necessary

for further development of the DSLG solar cell. Furthermore, the work of Gohazi

[1996] is brought into question as the boron diffusion

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