3d-inorganic solid state solar cells

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Inorganic

Solid State

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3D-Inorganic Solid State Solar Cells

Proefschrift

ter verkrijging van de graad van doctor

aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus Prof dr.ir. J.T. Fokkema,

voorzitter van het College voor Promoties,

in het openbaar te verdedigen

op dinsdag 11 april 2006 om 13:00 uur

door

Marian NANU

Master of Science, Universitatea TRANSILVANIA Brasov

geboren te Brasov, Roemenie

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Dit proefschrift is goedgekeurd door de promotor Prof dr. J. Schoonman

Toegevoegd promotor: Dr. A. Goossens

Samenstelling promotiecommissie:

Rector Magnificus,

Prof.dr. J. Schoonman, Dr. A. Goossens,

Prof.dr. L.D.A. Siebbeles, Prof.dr. M. Burgelman, Prof.dr. M. Gratzel, Prof.dr. D. Blank, Prof.dr. D. Lincot, Prof.dr. A. Schmidtt-Ott, Voorzitter

Technische Universiteit Delfl, promotor

Technische Universiteit Delft, toegevoegd promotor Technische Universiteit Delft

Universiteit Gent

EPFL Lausanne, Switzerland Twente Universiteit

École Nationale Supérieure de Chimie de Paris France Technische Universiteit Delft, resen.'elid

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4

Contents

5. CuInS2-based solar cells obtained by Atomic Layer Deposition

5.1 Introduction

5.2 Experimental aspects 5.3 ResLilts and discussion

5.3.1 CulnS: growth

5.3.2 Post-deposition Ireatments

5.3.3 Cu-rich films post deposition treatments 5.3.4 Cu-poor films post deposition treatments 5.5 Conclusions and fiitiire work

55 55 56 57 57 59 60 63 64 7.

6. The influence of an interfacial In^Sj buffer layer in Ti02/CuTnS2 heterojunctions on the formation and recombination of charge carriers 67

6.1 Introduction 67 6.2 Experimental procedure and data analysis

6.3 Results 6.4 Discussion 6.4.1 Single layers 6.4.2 Bi-layers 6.4.3 Triple layers 6.5 Conclusions

Solar energy conversion in Ti02 /CuInSj nanocomposites

7.1 Introduction

7.2 Experimental aspects 7.3 Results and discussion 7.4 Conclusions

Deep Level Transient Spectroscopy of Ti02/CuInS2 heterojunctions

8.1 Introduction

8.2 Experimental procedure and data analysis 8.3 Results and discussion

8.4 Conclusions

Study of charge-carrier dynamics in 3D inorganic solar cells

9.1 Introduction

9.2 Experimental aspects , 9.3 Results and discussion

9.4 Conclusions

8.

9.

Contents

10. Nanocomposite 3D solar cclls obtained by chemical spray deposition

IU.1 Introduction

10.2 Experimental aspects 10.3 Results and discussion 10.4 Conclusions 125 125 126 127 134

11, Summary and Conclusions

Summary and Conclusions in Dutch

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1 General Introduction

T/lis chapter gives the motivatïon of the present wovk, the objectives, and the ahernatives one coiddfollow to hitild iip a soJar cell device.

1.1 Introduction

Since the industrial revolution in the 19 century, the demand for energy has increased exponentially. lts presence and availability, which have increased concun*ently with the development of technology, have been taken for granted,

except for shortages that arose during local crises. Tliis was not recognized until the 1970's when the oil crisis struck the world and almost over night the price for

crude oil, one of the world's major energy resources, has tripled [1]. Although it was more a poHtical decision rather than a natural catastrophe that caused this

incident, it stimulatedpeople's thoughts about energy resources. Nevertheless the decision to find solutions to the problem of energy resources was also partly driven by govemments. Considerable effort was undertaken to identify the limits of current energy resources. At that stage, the availability of crude oil and other fossil energy carriers was estimated to last only shortly bevond the turn of the millemiium. Recent advances in exploration teclinology have extended this threshold [2]. Nevertheless, the oil crisis was the first time the public was

confronted with a possible step back in comfort, the natural driving force of technology and science. Since the development of mankind has been, and it will

always be related to the availability of energy, and due to an ever-increasing energy demand, to date one of the major challenges is to fmd a sustainable supply of electrical energy.

Most of tlie present global energy production is accomplished by buming fossil fLiels. However, the inherent problems associated wïth the use of fossil fuels (e.g., enviromnental poUution) force mankmd to search for new and sustainable long-term solutions to warrant the availabilit>' of energy in the future. Besides the negative enviromnental impact, the energy demand often induces political instability, like the control over oil and gas reserves and the use of nuclear power.

It is, therefore, desirable to find altematives to create a sustamable and robust energy infrastmcture. Solar radiation, wind power, and biomass could be used as

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1. General introduction _{1. General introduction}

3

based on the latter two is difficult. In order to be robust the energy mfrastnicture should be distributed and close to the source. The photovoltaic source of energy (solar radiation) has the advantage of being widely distributed over the world, although the largest demand does not always correlate with the supply. The solar radiation impinging on the eartli's surface is not a Umiting factor and supersedes our needs by far. Moreover, solar energy is reliable and stable, and hence it offers an attractive opportunity towards sustainable energy systems. Photovoltaic Systems can be installed on rooftops and facades of buildings, and they can be combined with solar water heating systems. The power generated by rooftop solar cells can be used locally, and the surplus can be exported to the commercial grid, if there is one in the region [3]. The possibility for local electricity production offers consumers more freedom by reducing their dependence on the availability and price of commercial electricity. This is a crucial feature especially in remote areas that lack the infrastructure of electrification. It is actually more cost-effective to Install a photovoltaic system than to extend the grid if the power requirement lies more than about half a kilometre away from the electrical line [4]. Rooftop photovoltaic installations, both by public institutions and by individual citizens, are becoming more and more common woHdwide [5].

One of the main obstacles for photovoltaics to become more popular in short term is the fact that the price of the electricity (cost per watt) produced by photovoltaics is, in most cases, not yet competitive with that produced by the

conventional methods. Either improving the efficiencies or reducing the production costs of photovoltaic modules can change this situation. The price of this sustainable energy resource must be m level with today's cost of energy.

Furthermore, we must have a technology to scale up and produce this system. Solar cell technology is about to meet all of these requirements.

1.2 Solar cells

Although today's photovoltaic (PV) mdustry is actually a product of the twentieth century solid-state physics, its beginnings can be traced back to 1839. In that year, Becquerel observed that light falling on certain materials created a difference in electric response, i.e., it created electricity. He discovered ftarther that the amount of electricity produced varied with the amount of light and its intensity. Tliis process is referred to as the photovoltaic effect - "photo" for light and "voltaic"

for voltage.

During the past few years, the efficiency of PV cells has increased tremendously, and laboratoiy experiments have shown conversion efficiencies of sunlight to electrical energy of more than 24% for silicon and 30% for GaAs [6]. New PV cel! materials are more reliable and efficiënt, and offer new opportunities for

further improvements of the conversion.

Solar cells are based on p-n or p-i-n junctions made of semiconductors. When two layers of p- and n-type semiconductors arejoined, the positive holes and electrons

at the junction immediately (within milliseconds) cross over, forming an electrostatic field. Hence, there is a thin region of negative charge on the p-side and a thin region of positive charge on the n-side. The p-side overall has too few

electrons, and therefore has more positive holes; the n-side is the one with too many electrons and thus it has more freely roaming electrons. The electrostatic field immediately fonned at the junction prevents the electrons fi'om the n-side (where the electrons are the majority carrier) fi*om crossing over easily, though it does accelerate the free electrons fi'om the p-side if tliey happen to be near the junction. This is the most important aspect of the conduction of electricity in

semiconductors.

1.2.1 Organic solar cells

In recent years organic semiconductors are studied extensively because their use in (opto-)electronic devices, like solar cells, seems to be attractive. Organic solar cells involve an organic electron and hole conductor as semiconductor, used as and/or organic visible-light absorbing material or both. The jimction is made with a liquid or with another organic or inorganic solid. The use of a visible-light absorbing dye to sensitize a wide bandgap semiconductor has been known for many years [7]. With the aid of suitable sensitive chromophores, a liquid-junction

M

photovoltaic device can be constructed.

A major step foi-ward was made, when Gratzel and co-workers used an organic dye attached to the surface of highly porous nanostructured anatase TiOi- In their

cell only the dye absorbs visible light (Figure l.l). Upon absorption of a photon with suitable energy, the dye is excited and mjects an electron into the conduction band of anatase Ti02. The electron is transported to the back contact and looses

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1. General introduction

redox couple. This redox couple reduces the oxidized state of dye and closes the electrical circuit. This type of solar cells (the so-called the Gratzel-type) has an efficiency for white light of about 11% on a laboratory scale [8]. Li order to achieve such liigh efficiency it is important to use naiiostructured electrodes. The surface area of these electrodes can be 800 times larger than the projected area.

Wolk

li

\ ^ J^ CuiTCIll Nanoporous TiO \ •'i-Thi

y

r'

-'rr-^^- '•*_^'r - i ^ j _ • r ' J i EItxiroiytc Soluiion a^-^ >

- ^ <4

# . -Ncguli\c

Elcctrodc _ElcclrodeI'o,sili\c Figure 1.1 Dye-Gratzel sensitized solar cell

In parallel to the development of dye-seiisitized solar cells, similar ideas evolved from the field of polymer physics. Soon after semiconducting polymers were available, it was realized that it is possible to mix p- and n- type polymers or to mix a p-type polymer with soluble n-type molecules like C60 fullerenes. In such heteroj unctions, electron donating domains and electron accepting domains with nanometer dimensions are present. Also these polymer 3D structures show remarkable efficiënt photovoltaic activity. In the case of polymer solar cells, a

serious banier that hinders advancement towards practical utilization is the inherent poor chemical stability of (semi conducting) polymers against the simultaneous presence of oxygen and light.

1. General introduction ₅

7.2.2 tnorganic solar cells

A variety of semiconductor matenals are used in inorganic photovoltaic cells. In this type of solar cells single crystalline and polycrystalline silicon, thin-film amorphous silicon, and other materials are used. Thin-film technology benefits

from low material requirement and low production cost compared to crystalline silicon cells. The up scaling of the thin film technology from a single cell to a large area module is straight forward, since many cells can be interconnected. Compared to the Si crystalline material, thin-film solar cells can be manufactured witli less input of energy. This will shotten the energy pay back time, i.e., the time in which the photo-generated energy output equals the energy consumed to produce the device.

Silicon is used to construct junctions in a single material (homojunction), whereas solar cell developments have also led to p-n junctions between two different semiconductors (heterojunctions). In addition to having the proper physical attributes needed for the process, silicon is an excellent choice for PV cells, because it is abundant, i.e., it composes 28% of the earth's crust [9]. However, the costs of energy produced using devices based on polycrystalline silicon are high, when compared to the use of fossil fiaels.

Altemative semiconductor materials from the so-called III-V family, like GaAs, InAs, InP, along with temary and quatemary compounds are also used in inorganic solar cells. The efficiency of these cells is even higher than that of the

silicon based ones, and nowadays approaches 30% [10]. Yet, since the production of these cells is extremely expensive, they are only applied in space, where weight

and volume are more important than price. Finally, sulphur-, selenium-, and tellurium-based semiconductors receive wide attention [11]. Compounds of

copper, indium, gallium, and sulphur or selenium are semiconductors with suitable bandgaps for visible solar light conversion. The present status of these materials shows theü- great potential. Photovoltaic cells from thin films of Cu(InGa)Se2 reveal a conversion efficiency of almost 19% [12]. Solar cells of

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6 1. General introduction

1.2.3 New concepts

A new type of solar cell refeired to as the Extremely Tliin Absorber (ETA) solar

cell, appears to be a new challenge for the scientific community. A 3D geometry for a soHd-state solar cell offers an attractive altemative and nowadays represents a new field of research. It is the aim of this thesis to explore the challenges of developing a 3D solid-state thin-film inorganic solar cell based on a nanostructured Ti02/CuInS2 heterojunction.

1.3 Deposition techniques

In order to obtain thin films with certain characteristics, many techniques have been developed. Among these, physical and chemical vapour deposition techniques (PVD, CVD) yield the highest quality films in teniis of purity and homogeneity [14]. Therefore, these are the most used and advanced techniques. Nevertheless, infütration of one or two semiconductors into a nanostructured

matrix is a challenge, since it is difficult to obtain a homogeneous composition of the materials. Chemical vapour deposition or otlier vacuüm techniques can be used to prepare nanostructured materials, but traditional CVD is not able to infiltrate sufficiently into pores in nanostructured TiO?. The Atomic Layer Deposition (AL-CVD) technique, the latest development of PVD-CVD techniques, is very well suited for this purpose.

1.3.1 Atomic Layer Deposition

The concept of Atomic Layer Deposition (AL-CVD) was first introduced [15] as a variant of Physical Vapour Deposition (PVD). It was originally implemented for epitaxial film growth and it was therefore referred to as Atomic Layer Epitaxy (ALE). Conceptually, the AL-CVD (ALE) process should follow the basic idea that precursors are led into the reaction chamber one by one, with a purging step between the reactants supplied. The films are deposited by repetitive application of a single layer or less. Each sequence is composed of several gas surface interactions (i.e., adsorption/desorption or chemical reaction) that are all self-limiting. The self-limiting characteristic of the steps of the process is the most important condition for an AL-CVD process.

\

n

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Propositions

part of the fliesis

3D-Inorganic Solid State Solar Cells

The self limiting growth in an AL-CVD process is the most important condition to achieve infiltration of a nanoporous film. (This thesis)

In the new concept (i.e., nanocomposite 3D inorganic solar cell), bulk recombination is suppressed due to the reduced migration distance of the minority carriers down to a few tens of nanometers. (This thesis)

The proposed 3D solar cell, based on nanocomposite TiOilCuInSi, opens a new pathway towards cheap and efficiënt PV devices. (This thesis)

The energy demand often induces political instability, like the control over oil and gas reserves and the use of nuclear power.

The new scientific quest in the field of plasma deposition related techniques should be the introduction of environmental friendly gas chemistries.

Despite the rapid development of computer power and computational methods it is questionable whether experimental measurements will ever be replaced by computer

simulations.

Since for the progress of science and technology the negative ('no go') results could be equally important as the successful ones, the scientific joumals should also

publish papers reporting unsuccessful research activities.

Fundamental science without technological relevance will neverbe considered useful by the (scientific) community.

A very clean chemistry laboratory expresses, most of the time, the enthusiasm of the people working in it,

Understanding the culture of a foreign country is not necessarily related with understanding the language of that country.

Marian Nanu

These propositions are regarded as defendable, and have been approved as such by the supervisor Prof.dr. J. Schoonman.

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1. General introduction 7

Stellingen

behorende bij het proefschrift

3D-Inorganic Solid State Solar Cells

1. Het zelf-limiterende karakter van de stappen in een AL-CVD proces is de belangrijkste voorwaarde voor het infiltreren van een nanoporcuze film. {Dit

proefschrift)

2. In het nieuwe concept (d.w.z. nano-composiet 3D anorganische zonnecel), wordt

bulk rccombinatie onderdrukt door de tot een tiental nanometers gereduceerde migratielengte van de minderheidsladingdragers. {Dit proefschrift)

3. De voorgestelde 3D zonnecel, gebaseerd op een nano-composiet van Ti02|CuInS2, opent een nieuwe route naar goedkope en efficiënte conversie van zonlicht. {Dit

proefschrift)

4. De vraag naar energie geeft vaak aanleiding tot politieke instabiliteit, zoals de controle over olie en gas reserves en het gebruik van nucleaire energie.

5. Het nieuwe wetenschappelijke onderzoek in het veld van plasma-gerelateerde depositie technieken zou de introductie van milieuvriendelijke gaschemie moeten zijn.

6. Ondanks de snelle ontwikkeling van computerkracht en rekenmethodes is het nog steeds de vraag of experimentele metingen ooit compleet door computersimulaties vervangen kunnen worden.

7. Omdat voor de voomitgang van wetenschap en technologie negatieve resultaten even belangrijk kunnen zijn als positieve, zouden wetenschappelijke tijdschriften ook

artikelen met minder succesvolle resultaten moeten publiceren

8. Fundamentele wetenschap zonder technologische relevantie zal door de (wetenschappelijke) gemeenschap nooit als nuttig onderzoek beschouwd worden.

9. Een opgeruimd laboratorium drukt meestal het enthousiasme uit van de mensen die er in werken.

10. Het begrijpen van de cultuur van een vreemd land is niet noodzakelijkerwijs gekoppeld aan het begrijpen van de taal van dat land.

Marian Nanu

The implemeutation of the basic AL-CVD concepts using chemically reactive molecular precursors has become the dominant path to grow high-quality

thin-films with a confomial coverage of the substrate. In this case, the basic components are delivered to the substrate in the fon-n of volatile reactive molecular precursors. These precursors contain the desired clements. The reactive

precursor molecules react at the substrate to incorporate the clements and eliminate the organo-metallic ligands as volatile by-products. Two fundamental

self-limiting process sequences are identified, i.e., a chemisorption saturation process foliowed by exchange reaction, and sequential surface chemical reactions.

The first precursor is chemically or physically adsorbed at the surface. After this first step a purging step with an inert gas is required. The second precursor is adsorbed at the active surface and fonns a solid product. In addition, because of the altemate supply of the precursors no detrimental gas-phase reaction can take place. The purity of the deposited compound is very high. Workmg with different process parameters it is possible to control the quality of the tliin fihns. With the number of deposhion steps, one can control the thickness of the film.

AL-CVD has been used m the present investigations to deposit CulnS2 thin fihns to form dense and nanostructured Ti02/CuhiS2 heterojunctions. The major

disadvantage of this technique is a low deposition rate and the high production costs. The typical growtii rate for atomic layer deposition is in the range of

100-300 nni/h. AL-CVD is a slow process and infiltration of a 2-micrometer nanocrystalline TiO: film takes about 3 hours. Tlierefore, it is questionable as to whether AL-CVD can be employed to produce 3D solar cells in a large-scale

industrial process. However, as we show later in this thesis, a much simpler spray deposition process can be used to reach the same, or an even somewhat higher energy conversion efficiency.

1.3.2 Chemical spray pyrolysis technique

Deze stellingen worden verdedigbaar geacht en zijn als zodanig goedgekeurd door de promotor Prof dr. J. Schoonman.

The simple and cheap spray pyrolysis method is widely used for tlie production of large-area metal-oxide films. It can also be used to deposit thm films of

semiconductors for photovoltaic solar cells, although the reported conversion efficiencies are still very low [16].

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8 1. General introduction

The technique consists of spraying a fmely atomized solution, called the precursor solution, onto a hot substrate. The precursor solution has to meet the following requirements:

The chemicals in the solution must provide species or complexes that undergo a themially activated chemical reaction to yield the desired thin fihn material.

The remainder of the constituents, including the camer liquid, should be volatile at tlie substrate temperature.

hl order to yield a uniform film for large-area application, the nozzle and/or substrate must be scanned or rotated to ensure unifomi deposition. hi spray pyrolysis the unifomiity of the droplet size is one of the prime requirements. After

atomization, the carrier gas transports the fine droplets towards the heated substrate. Upon arrival the solvent must vaporize entirely, leading to deposition of the solid components. The droplet size influences the deposition process in the

following ways:

• very large droplet size - The themial energy absorbed by a droplet during transport to the substrate is msufficient to vaporize the solvent entirely before arrival. The residual solvent is then evaporated fi-om tlie substrate, resulting in localized cold spots and poor film quality.

• medium droplet size - All tlie solvent in the droplet vaporizes before reaching the substrate, leaving only precipitated particles of the reactants.

These are deposited on the substrate, where they nielt or sublhne, again causing localized cooling and resulting in films of inferior quahty.

• ideal droplet size - The solvent vaporizes completely before reachmg the substrate. The precipitated particles then melt, vaporize and gas-phase reactants are transported to the substrate surface. The reactant molecules undergo processes of adsorption, surface diffusion, and reaction, which lead to nucleation and layer growth. Volatile waste products evaporate

and diffuse away fi'om the surface. These processes constitute genuine Chemical Vapour Deposition.

• very small droplet size - The whole reaction may be completed during transport fi-om the spray nozzle to the substrate. The product molecules

condense as micro-crystallites, which form a powdery deposit on the

substrate. '

revolutionair reduction of the producfion costs. In addition, the energy payback time is reduced as well.

The spray pyrolysis teclinique finds widespread use in industrial large-area coating processes and its introduction into the photovoltaic industry allows a

1.4 Aim and outline of this thesis

The aim of the work reported m this thesis was to explore the challenge of developing a thin-film inorganic solid-state solar cell based on nanostructured TiOi/CuInSi heterojunctions.

A new concept of an all-solid-state inorganic solar cell based on a thiee dimensional (3D) architecture is proposed in this thesis. In the new concept, the solar cell consists of CuInS^ as visible-light absorber applied inside tlie pores of a porous nanostructured anatase Ti02 thin film (2 micron thick). The nanoporous

Ti02 matrix, which comprises particles with 10 to 100 rmi diameter, is applied onto a 100 nni dense anatase Ti02 film. The 3D structure is made by growing p-type CuInS^ inside the nanoporous Ti02, by means of AL-CVD or spray pyrolysis

techniques. The research reported in this thesis is focussed on the deposition and optimisation of the individual components of the proposed solar cell, as well on the optimisation of the overall structure and properties of the device aiming at improving its energy conversion efficiency. Tlie studies performed with this

purpose revealed important aspects that contribute to the fiindamental understanding of the processes taking place during the working regime of this

innovative solid-state solar cell.

The research reported in this thesis has been published rn a number of papers in international scientific joumals. This thesis gives an overview of the main results and it is organised as foUows.

The short introduction to the field of photovoltaic and deposition techniques given in this chapter is foliowed in Chapter 2 by the description of the concept of the 3D solid-state solar cell put forward in this work. All tlie requirements that need to be fuU-fiUed in order to obtain an efficiënt device are presented. Then, the materials

chosen for constmcfing the device are introduced in Chapter 3. Some aspects regarding the choice of Ti02 as electron conductor and some issues regarding the

optical properties of the CuInS2 absorber p-type semiconductor are presented

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-^

10 1. General introductjon 1. General Introduction U

Fiirther, Chapters 4 to 6 deal with properties of Ti02/CuInSi helerojunctions. The AL-CVD deposition of CulnS^ on flat substrates and into nanoporous TiOi matrix is the subject of Chapter 4. The fonnation of a flat TiOi/CuinS: heterojunction and the influence of the concentration of sulphide-ion vacancies on tbc photo-current output are discussed in Chapler 5. A study of tlie influence of an interfacia! In^Sj buffer Ia}er in Ti02/CuhiS2 heterojunctions on the fonnation and recombination of charge carriers by means of Time Resolved Microwave

Conductivit>' (TRMC) is given in Chapter 6.

A detailed analysis on how to construct a 3D TiOi/CuInS: nanocomposite using AL-CVD is presented in Chapter 7.

More fundaniental issues regarding the energetic localization and the quantitative infomiation of the intrinsic defects in CIS and tJieir contribution to the dynamics of the charge carriers are discussed in Chapters 8 and 9, rcspectively.

The possibilities for practical industrial application of the proposed 3D solar cell were also investigated in this work. A low-cost method (chemical spray pyroKsis) was successfully used to build up the proposed TiO: CuInS^ nanostructured solar

cell. Some results are presented in Chapter 10. The results are promising and might open the pathway towards the production of a ncw, efficiënt and cheap solar cell.

7. Skotheini, T., Dye Proceecfings of the Awwal Meeting - American Section

of the Intcmationaï Solar Energ)- Society; l(Sect. 14-25), 23, 16 (1977).

8. Gratzel, M., MRS BiiUetin, 30( 1), 23 (2005).

9. Wedepohl, K.., Met. Their Compd. Environ, 3 (1991).

10. Fatemi, N. S., Hou, H.Q., Sharps, P.R., Martin, P. M., Hammons, B. E.. and Spadafora, F., NASA Conference Pnblication 2001-210747 (REVJ,

16th Spacc Photovoitaic Research and Technohg)- Conference, 1999), 94

(2005).

1 1. Johnson, P. K.. Heath. J. T., Cohen, J. D., Ramanathan, K., and Sites, J. R., Progress in PhotovoJtaics, 13(7), 579 (2005).

12. Shafamian, W. N., Stolt, L., Handbook of Photovoitaic Science and

Engineering, 567 (2003).

13. Scheer, R., Klenk. R., Klaer, J., and Luck, L, Solar Energ); 77(6), 777 (2004).

14. Schroff. A. M., Traitements de Surface, 13(111), 13-15, 17, 19-21, 24 (1972).

15. Suntola. T., Antson, J., US Patent 4 058 430, (1977).

16. Biswas, D. R, Journal of Materials Science. 21(7), 2217 (1986).

The main conclusions of the present work, as well as a few suggestions for further development of this type of solar cell, are presented in Chapter 11.

References

1. Dias, M., and Guimaraes, A., Juiunal of Petroleum Science &

Engineering, 44(1-2), 93 (2004).

2. Shuang-Qing, W., and Sun, W.-L., ïankuang Ceshi. 24(4), 271 (2005). 3. Zahedi, A., Renewable Energy; 31 (5), 711 (2005).

4. Faiiiiey, A. H., Weise, E. R., and Henderson, K.R., Journal of Solar

Energ\>Engineering, 125(3), 245 (2003).

5. Tawada, Y., Yamagislii, H., and Yamamoto, K., Springer Series in

Photonics, 13 (Thin-Film Solar Cells), 163 (2004).

6. Green, M. A., Emer}', K., King, D.L., Igari, S., and Warta, W., Progress

in Photovoltaics. 13(5), 387 (2005).

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2 Inorganic nanocomposites of n- and

p-type semiconductors: a new p-type of 3D

solar cell

Here ^ve report a ne^v approach to-wards whot ne re/er to as the 3D solar cell concept. Alonüc-Layer Chemical Vapoitr Deposition (AL-CVD) is eniployedfor infiltratimi ofCitlnS: inside the pores ofnanostructured TiOj. In ihis way, it is possible to obtain a nanometre-scale interpenetrating

network behveen n-type TiO: and p-t}pe CulnS:- Siich cells show overall

energ}' conversion efficiencies of 4%.

2.1 Introduction

The major drawback of the present generation of photovoltaic solar cells is their laborious. energy consuniing, and costly production. Tlierefore, a completeiy new approach is desired. With the advent of dye-sensitized (Gratzel-type) solar cells [1], CóO/polymer [2], and CdSe/polymer bulk heterojunctions [3] challenging altematives are ofTered. A major concern in these altematives is their poor stability when operating \n full sunlight. All-solid, completeiy inorganic, bulk heterojunctions do not require expensive sealing. Here, we report a new approach towards what is reportcd to as the 3D solar cell concept. Atomic-Layer Chemical

Vapour Deposition (AL-CVD) is employed for infiltration of CuInSi inside the

pores of nanostructured Ti02. Tn this way it is possible to obtain a

nanometre-scale interpenetrating network between n-type TiOi and p-type CuInSi. Such cells show photovoltaic activity with a maximum monochromatic incident photon-to cunent conversion efficiency of 80%. If AM 1.5 irradiation is applied, the open-circuit voltage is 0.5 V, the short-open-circuit current 18 mA cm"", and the fill factor is 0.45. The overall energy conversion efficiency of 4% doubles tlie perfomiance of the best inorganic nanostructured solar cell reported so far.

2.2 Deposition methods

The 100 mn dense anatase TiO^ films are obtained by spray pyrolysis. A mixture with the following composition is used: 54 ml pure ethanol (99.99%); 3.6 ml

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\

14 _{2. A new type of 3D solar ce}

2. A new type of 3D solar cell ₁₅

acetyl acetonate; 2.4 ml titanium tetra isopropoxil (TTIP, 97%). The precursors are mixed and sprayed on a heated TCO-coated glass substrate. The substrate surface is maintained at a constant temperature of 350 "C. The 2 pm nanoporous Ti02 films are obtained using doctor blading [5]. AI2O3 and IniSj are deposited by AL-CVD. AICI3 (99.999%) and O. are the precursors for the AI2O3 deposition, which takes place a temperature is 400 °C and a reactor pressure of 2 mbar. hiCl3

(99.999%) and H2S are the precursors for the deposition of 111283. hi this case the

temperature deposition is 450 ^C and the reactor pi'essure is 7 mbar. The deposition of CuInS2 (CIS) flms is described elsewhere [6]. CuCl (99.999%) and InCl3 (99.999%) are the precursors. The reactor temperature and pressure are 400 °C and 7 mbar, respectively. A 50 nm gold film is evaporated as ohmic back contact.

2.3 Characterization

Current-voltage (I-V) curves are recorded with a DC Source Meter (Keithley, Model 2400) in the daik and under illummation. A calibrated solar simulator^ SolarConstant 1200 (K.H. Steuemagel Lichttechnik GmbH), is used as visible light source. Transmission Electron Microscopy (TEM) is perfomied using a Philips CM30T electron microscope with a LaB6 filament operating at 300 kV.

Samples are mounted on Quantifoil® carbon polymer supported on a copper grid by applying a few droplets of a suspension of ground sample in ethanol, foliowed by drying in air.

2.4 The 3D concept

When pursuing the construction of nanometer-scale interpenetrating networks, one faces the question of how to obtain such a nanocomposite. This question is even more relevant when n-type and p-type semiconductors are to bc blended, since in that case the concentrafion of impurities and defects must be kept low and the interfaces need to be passivated. Wet-chemical deposition of Cul [4] and CuSCN [7] wide-bandgap hole conductors has been applied to replace the organlc electrolyte in dye-sensitised cells. In this approach, dye-molecules take care of light absorption. hi the extremely-thin absorber (ETA-cell) concept [8], light absoiption takes place in a very thin semiconductor film, sandwiched between optically transparent, wide-bandgap nanoporous electron and hole conductors.

The best ETA-cells are obtained by sequential dipping of microporous anatase Ti02 (partiele size of 0.1 micron) in liquid precursors of Cd and Te, foliowed by an annealing process. If the CdTe absorber film is fonned, a precursor solution of Cu+ and SCN- is applied to deposit the CuSCN p-type conductor. To date, ETA-cells exhibit an energy conversion efficiency of around 2% [9].

Stimulated by these studies, we have applied Atomic Layer Chemical Vapour Deposition (AL-CVD) to deposit CuInS2 inside the pores of nanostiiactured anatase Ti02 (nc-TiO:). CuInS? is a well-known semiconductor with a direct bandgap of 1.5 eV. It shows strong light absorption, matched to the solar

spectrum. The usual way to prepare CuInS2 is to use co-evaporation of CuS and In2S3 in high vacuüm (10"^ mbar) and subsequent annealing in sulphur ambient at 600 °C. Since vacuüm evaporation is a so-called line of sight technique, it cannot be used to infiltrate nanoporous matrices. AL-CVD is renowned for its ability to

confonnal deposit extremely thin films on textured substrates [10]. Recently, AL-CVD has been introduced in the microelectronics industry for deposition of gate dielectric thin films in the new generation MOS-FET's. However, infiltration of nanoporous matrices by AL-CVD has not yet been reported.

The structure of an inorganic nanocomposite solar cell is presented in Figure 2.1. The solar cell constnaction starts witli commercial F-doped SnO^ glass (TEC-20) onto which a dense 100 mn thin film of anatase TiOi is deposited using spray pyrolysis. Subsequently, an anatase Ti02 paste with 50 mn particles (courtesy of

ECN) is applied which, after calcinations in air at 450 °C, yields a 2 micron nanoporous film witli a void fi^action of around 50%). The intemal surface is about

500 times the geometrical area. These substrates, 50 x 50 rmn in size, are introduced into the AL-CVD reactor. InCh, CuCl, and H2S gas pulses are subsequently applied at 400 °C until the pores are completely filled with CuInS2. Additional annealing m sulphur vapour and oxygen is required to suppress the concentration of ionic defects. Finally, Au dots are evaporated as back electrode.

AL-CVD of a temary semiconductor, such as CuInS2, inside a nanoporous matrix

^

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>

16 2. A new type of 3D solar cel

ranging between 10 and 50 nm and a füm thickness of up to 5 micrometer. The nanocomposites have been studied witli Rutlierford backscattering (RBS),

grazing-incidence X-ray diffraction (GI-XRD), and Transmission Electron Microscopy (TEM). In Figure 2.2 a TEM micrograph of a 50 mn Ti02 matrix completely filled with CuInS^ is shown. Tiie lattices of TiOi and CuInS: have close contact on atomic scale, which is a prerequisite for fast electron transfer across the interface.

CuInS

TCO glass

nc-Ti02

Solar irradiation

dense TiO

(100 nm)

Figure 2.1 Structure of an inorganic 3D solar cell.

The as-deposited nanocomposites, however, show a veiy moderate photo response. This can be ascribed to the quality of tlie deposited CuhiS2, which has a high concentration of sulphur vacancies. A themial anneal in sulphur vapour at

500 °C foliowed by anneal in oxygen at 200 "C reduces the presence of these defects. A p-type material with an effective acceptor density of lO'^ cm"^, as determined with Mott-Schottky analysis, is obtained. Annealing in oxygen also improves the stoichiometry of the nanoporous Ti02 matrix.

2. A new type of 3D solar ce

17 w^- -^

:t^,m ' 1 -^0f^ 'i n .r-i^ '^ -. -W! - ^ . M v:« I b y yr- ••: '1 20 um

Figure 2.2 TEM micrograph of the TiO^ICuInS:; nanocomposite. The lattices of

I1O2 and CuLiSi are m close contact, as is required for fast electron transfer.

These treatments reduce the concentration of bulk recombination centres, but due to the large interna! surface area, also surface recombination must be suppressed.

This can be accomplished by applying a buffer layer between Ti02 and CuInS^. Obviously, the buffer layer must also be deposited confonnal inside the pores. Recently, AL-CVD has been used to apply In2S3 buffer layers in CIS solar cells [11]. The choice for an In^Ss buffer layer is convenient because the In^Ss buffer and CuInS2 photoactive material can be deposited in the same AL-CVD reactor. IniSa is a semiconductor with a bandgap of 2.1 eV (Figure 2.3) [12]. For thin films of luiSa with small grains contaminated with oxygen, the bandgap increases up to 2.8 eV [13]. The conduction bands of In2S3 and CuInS2 are close together and have an offset of about 1 eV when compared to the conduction band value of anatase Ti02 [14]. Therefore, conduction-band electrons can easily cross the interface between CuInS^ and In2S3, from where they are injected into the

conduction band of TiOi, which is located close to 4.2 eV [15]. Once they are inside Ti02 their back flow is inhibited. Retuming into the conduction band of CuInS2 is impossible because of the 1 eV energy gap. Recombiaation between conduction band electrons of Ti02 witli valence band holes of CuInS2 can also be

disregarded, since tliese materials are physically separated by the 10 nm thin In2S3 buffer layer, through which tunnelling cannot occur. Therefore, one expects a

significant increase of both the photocurrent and the photovoltage if such a buffer

r

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18 2. A new t y p e of 3D solar ce

layer is appUed, as is obsei-ved indeed. The perfomiance improves about two orders of magnitude.

In the band diagram presented in Figure 2.3, a tunnel barrier of AI2O3 is included. The presence of such a layer is related to aiiotlier problem that has to be tackled. Although clear evidence is lacking, we have indications that the surface of TiO? is modified chemically dui'ing the AL-CVD and anneal processes. Although Ti02 is

an inert material, oxide-ion vacancies can be generated under the AL-CVD conditions. In addition, copper can migrate from CuInS2 thi-ough the In2S3 buffer into the TiOo lattice. We have found that the application of an AI2O3 tunnel barrier, less than 2 mn thick, between Ti02 and In2S3 improves the cell performance significantly. eV _O -3 -4 -5 -6 r -7 -8 V

Figure 2.3 Band diagram of a 3D solar cell based on CUL1S2 as light absorber. Buffer layers of In2S3 and AI2O3 are applied to suppress surface recombination and to protect the Ti02 against contamination.

Hence, the best inorganic nanocomposite 3D solar cells, obtained with AL-CVD in the present study, have a band diagram as presented in Figure 23. The energy conversion efficiency of a cell with a geometrical area of 3.14 10"" cm" is a little over 4%, which doubles the best performing inorganic ETA cells (Figure 2.4).

2. A new type of 3D solar ce 19

IN E u E in c 0) c (U 3 u -1.0 1.0 0.8 0.6 UJ o o. 0.4 0.2 -0.0 500 0.0 Voltage V I \ 1 1 X 600 700 800 Wavelength (nm) 900

Figure 2.4 Current-voltage response of a 3D solar cell comprising a Ti02 and CuInS2 nanocomposite. (A) Solid symbols: current in darkness; open

symbols: current with AM 1.5 irradiation. (B) The incident photon-to-current conversion efficiency of this cell.

Under AM 1.5 irradiation (1000 Watt m"^) the cells have an open circuit voltage

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20 2. A new type of 3D solar ce

The perfomiance of this new type of solar cell is limited by bulk and surface recombination, as well as the design of the electrical contacts. However, in view of the novel approach to use AL-CVD for synthesis of nanocomposite 3D solar cells, our findings pro vide the evidence that the 3D solar cell concept is a step forward towards realization of large-scale application of solar energy conversion.

2.5 Conclusions

A promising result of around 4% overall energy efficiency is reported for 3D nanocomposite solar cell based on TiOi/CuInSz. Due to the back flow of cuirent rather poor fill factors are obtained. High concentration of the interface states

detennine a low Voc- Significant increases are expected by reducing surface recombination and improve tlie quality of the buffer and absorber fihiis. Thus a

comparable efficiency of an optimised device based on the concept presented here witli tlie common thin layers solar cells could be obtained.

2. A new type of 3D solar cell 21

10. Puurunen, R.L., Root, A., Sarv, P., Viitanen, M.M., Brongersma, H.H., Lmdblad, M., and Krause, A.O.I., Chem. Mater., 14(2), 720 (2002).

11. Yousfi, E. B., Weinberger, B., Donsanti, F., Cowache, P., and Lincot, D.

Thin SolidFilms, 387 (1-2), 29 (2001).

12. Rehwald, W., and Harbeke, G., J. Phys. Chem. Solids, 26, 1309 (1965).

13. Baireau, N., Bemède, J. C , El Maliki, H., Marsillac, S., Gastel, X., and Pinel, J-, Solid State Communications, 122, 445 (2002).

14. Siebentritt, S., Thin SolidFihns, 403, 1 (2002). 15. Gratzel, M., Nature, 414, 338 (1999).

References

1. O^Reagan, B. and Gratzel, M., Nature, 353, 737 (1991).

2. Yu, G., Gao, J., Hunimelen, J.C., Wudl, F., and Heeger, A.J., Science, 270(1995).

3. Sun, B., Mai-x, E., and Greenham, N. C., Nano Letters, 3(7), 961 (2003).

4. Tennakone, K., Kumara, G.R.R.A., Kottegoda, I.R.M., Wijayantha, K.G.U., and Perera, V.P.S., J. Phys. D,3l, 1492 (1998).

5. Nazeeruddin, M.K., Kay, A., Rodicio, L, Humphry-Baker, R., Muller, E., Liska, P., Vlachopoulos, N., and Gratzel, M., J. Am. Chem. Soc, 115, 6382(1993).

6. Nanu, M., Reijnen, L., Meester,B., Schoomnaii, J., and Goossens, A.,

Chem. Vap. Dep., 16(1), 14(2004).

7. 0'Regan, B., Lenzmann, F., Muis, R., and Wienke, J., Chem. Mater., 14(12), 5023(2002).

8. Tennakone, K., Kumara, G.R.R.A., Kottegoda, I.R.M., Perera, V.P.S., and Aponsu, G.M.L.P., Journal of Photochemistry and Photohiology A:

Chemistiy, 108, 175-177(1997).

9. Ernst, K., Belaidi, A., and Koenenkamp, R., Semicondiictor Science and

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Il II.

r

3 Semiconductor materials for solar cells

applications

In all solar cells (photovoitaic devices) a photo-voltage is developed across a p-n jiinction. A p-n jimction is the comiection between a p-t)^pe and n-type semiconductor. The properties of n-t)pe Ti02 and p-type CiiïnSj thin films semiconductors, which are used in the device proposed

in this thesis, are discussed in this chapter.

3.1 Introduction

The photovoitaic effect is observed when an electric voltage emerges between two electrodes attached to a solid or a liquid system upon shining light onto this

system. Usually, in photovoitaic devices, also referred to as solar cells, a photo-voltage is developed across a p-n junction. A p-n junction is the connection between a p-type and n-type semiconductor. Thin fihns of metal oxides are flnding increasing applications as semiconductor materials in electronic devices and components. In contrast to the conventional semiconductor materials (e.g. silicon), metal oxides are cheap and easy to produce. Most metal oxides are chemically stable in aqueous electrolytes in a large pH-range. However, a high chemical stability is often accompanied by a large band gap and a simultaneous absence of light absorption in the visible region of the electromagnetic spectrum. Attempts to shift the absoiption spectmm from the ultraviolet towards the visible part of the spectrum by doping with other metals have been discouraging and have led to a declining mterest in this subject in the past decades. The (photo)-electro-chemistry of transition metal oxide semiconductors has received

considerabie attention since 1972, when Fujishima and Honda discovered that water could be split into H2 and O2 by irradiating rutile (Ti02) with ultraviolet light. After that, Ti02 was used for decomposition of organic compounds under ultraviolet irradiation. However Ti02 has also been used with success as a transparent n-type semiconductor for solar cell devices. In last years, Ti02 became more and more a useful semiconductor in different types of solar cells

(e.g., dye-Gratzel solar cells, solid-state solar cells, and organic solar cells). In the

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^

24 3. Semiconductors materials for solar cells applications _{3. Semiconductors materials for solar cells applications} ₂₅

work discussed in this thesis TiOi is used as electrons conductor in heterojunction with CuInS:- A short overview about Ti02, and its properties and applications is given in Section 3.2.

Some aspects regarding the defect chemistry of the absorber material used in the device proposed in this thesis are discussed in Section 3.3. The study of ionic defects in CUIHST (CIS) thln films is required in order to optimize photovoltaic

solar cells based on this material. Tliese defects mtroduce energy levels in the band gap, which detemiine the conductivity type and the minority carrier lifetime. In this study different characterization methods are applied to elucidate the defect chemistry of CIS. Thin fihiis of CIS are obtained by sulphurization of a CuIn metallic alloy. The process conditions are: reactor pressure 1 bar, substrate temperature between 450 and 500 °C, sulphurization time between 2 and 15 min. The films are investigated with X-ray diffraction, Ranian spectroscopy, and Photoluminescence spectroscopy. It is found that the deposition parameters, such as the sulphur pressure and the reactor temperature, determine the concentration and type of defects. In the present investigations we relate the process conditions to the defect chemistry and show how the quality of CIS thin füms can be iinproved.

3.2 TiOa

3.2.1 Natura! forms and stabiüty

Titanium is the 9* most abundant element on earth and comprises 0.63% of the earth crust. Gregor first discovered it in 1789 [1]. One of its oxides, titanium dioxide (TiO^), occurs naturally as rutile in some acid igneous and metamoiphic rocks, and also in sedimentary rocks and beach sands. In heavy mineral sand

deposits, rutile is commonly associated with another titanium mineral, ilmenite, together with zircon, monazite and magnetite. Beach sand deposits have provided long-term mining projects on the eastem and western coasts of Australia, and have recently been discovered also in the Murray Basin of NSW and Victoria.

Ilmenite has the chemical formula FeTiOa and nominally contains 53% pure TiOi-The term 'nominally' is employed because natural ilmenite has variable amounts of iron (Fe), chromium (Cr), silicon (Si), manganese (Mn) and phosphorus (P). Thus some ilmenite ores are richer in titanium dioxide than others. The production of titanium dioxide from ilmenite is complex and various processes

have been employed. The process upgrades any ilmenite to a high-grade titanium dioxide feedstock.

Approximate 93% of the worldwide use of titanium is in the oxide fonn - titanium dioxide is therefore in high demand. The paint industry is the largest user of titanium dioxide, consuming 51% of the world TiO: production, while the plastics

industiy consumes 19% and the paper mdustry 17%. The volume of the market is currently US$8 billion per year and increases at an average rate of 3% per year. Approximately 33% of global TiOi consumption is in the United States, foliowed by Europe (24%) and Japan (8%). A substantial growth in consumption is

expected in rising economies in the more highly populated countries (e.g., India).

The TiOi production is achieved through one of two methods, which are referred to as the sulfate process and the chloride process. The chloride process is taking

over as the preferred procedure; in 2000 the chloride process performs approximately 60% of world production of TiO^, and this is forecast to increase to

7 0 % b y 2 0 1 0 [ l ] .

At room temperature, TiO^ occurs in three natural forms, [2]: rutile (tetrahedral), anatase (tetrahedral), brookite (rhombic). Additionally, other polymoiphs can be obtained synthetically. Practical applications are limited to the anatase and the rutile fonns, (Figure 3.1) which are most easily produced and seeni to have the most mteresting properties. The major difference between both structures is the arrangement of the oxygen ions.

RUTILE

ANATASE

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26 3. Semiconductors materials for solar cells applications _{3. Semiconductors materials for solar cells applications} ₂₇

Rutile is a thennodynamically stable structure at all temperatures and can be obtained by heating otlier polymorphs to temperatures higher than 700 °C. Anatase is generally fonned by chemical reactions at temperatures below 500 °C.

3.2.2 The electronic structure of TiOz

Titanium is a transition metal and TrOj belongs to the family of transition metal oxides. The oxidation state of the titanium in TiO: is IV+ obtained by donating lts 3d and 4s valence electrons to the oxygen 2p orbits and leaving an empty "d" band. The empty metal band and the filled oxygen 2p valence band are separated by a gap of about 3 eV. In reality, the electronic structure is much more

comphcated, because the electron transfer from titanium to oxygen is not complete and the resulting band has an intennediate ionic-covalent character. These complications are responsible for the reduced accuracy in the calculations of the electronic band structure of Ti02. Wlien calculating tlie values of the band gap, it is usually found that the results are significantly lower than the experimentally determined values.

3.2.3 Domain of utillty

TiOi has many properties that make it very useful in different domains. As a result of the low position of the valence band due to tlie large affmity for the electrons (i.e., an energy gap between valence band (BV) and conduction band (BC) of about 3.2 eV), has an excellent chemical stability. This makes it usefnl in the applications in chemically aggressive enviromnents as a native oxide layer, which can protect the metal against corrosion.

'i

I

However, the well-known application of TiOi is as a white pigment m paints. The combination of its refractive index with sufficiently small partiele size results in a very efficiënt light scattering over a large wavelength range. Other important optical applications are its use as UV absorber in cosmetic products (such a sun cream) and as a transparent UV barrier in the food packaging industry. Since rutile has a larger refractive index than anatase it is generally prefen*ed in optical applications (anti-reflection coatings) [3].

ï

i

Most of anatase-based applications are relatively new and are ofïen based on a combination of optical, chemical, and electrical properties. Anatase has a higher electron mobility, and (photo) catalytic activity due to a more open structure than rutile. For these reasons anatase is used in dye-sensitized solar cells, wastewater purification reactors, lithium ions batteries, and electro- chromic wmdows [3].

TiO: is presently widely used as a transparent n-type semiconductor in solar cells. The photo-response of Ti02 in combination with different p-type materials is the

subject of many investigations.

3.3 CuInSz

3.3.7 Introduction

Among the group of chalcopyrite semiconductors, CuInS2 (CIS) has attracted interest for use in photovoltaic solar cells [4]. CuInS2 has a direct band gap of Eg^l.5 eV, and a large absorption coëfficiënt. Photovoltaic (PV) devices based on CIS now reach efficiencies of more than 12% [5]. Depending on the growth

condition, In-rich and Cu-rich materials can be obtained [6]. To date, the highest solar cell efficiencies are obtained for Cu-rich materials. Fuilher development of PV technology based on CuInS: requires fundamental understanding of the defect physics and chemistry of this material, which is the topic of the present

investi gation.

Different techniques, such as co-evaporation of Cu, In and SQ [7], sequential evaporation of CuS and In^Ss [8], sulphurization of Cu-In layers in Sn-vapour [9] or H2S [10], chemical vapour deposition (CVD) [11], and atomic layer deposition (ALD) [12] are used to obtam CuInSi. Depending on the deposition technique and the process conditions, CuInSi can be n- or p-type and can show a large variety in PV activity.

The lattice defects play an important role in the conductivity of the I-IIÏ-VI2 compounds. It is the lattice defects in the material that are responsible for the shallow impurity levels. Annealing in a Vl-element (i.e., sulphur, oxygen) influences the lattice defects present, the conductivity type of the material can change after annealing. The simple defects that can appear are of several types, based on an ionic model, and are sumraarized in Table 3.1.

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f-r

28 _{3. Semiconductors materials for solar cells applications}

Table 3.1 Simple defects responsible for the shallow impurity levels in the I-UT-VIj compounds.

Defect _Element Vacancy Interstitial I III VI Level type Acceptor Acceptor Donor I _Donor III VI Donor Acceptor

Some lattice defects are more stable than others. The Vl-vacancies have low fonnation energy and are important in n-type conducting materials. Cu vacancies are more easily fonned than vacancies of the elements fi-om the Ill-group, wliich is caused by the fact than Cu only sliglitly participates in the interatomic bonds. So, annealing in the VI element might result in a large number of Cu vacancies. It has also been observed that from the I-III-VI2 chalcopyrite compounds only those with copper show good conductivity. The Cu vacancies might be responsible for the p-type conductivity.

In undoped CuInS2 the intrinsic point defects that we can expect are cation and anion vacancies (V^^^,Vj^^ , V^), interstitials {Cn'.,In'.'), and cation-anion anti-site disorder. The latter occurs when the cations swap places in the lattice, i.e.

Cujj^ and In^^^. The anti-site defects form an ordered arrangement, which is

referred to as the Cu-Au-ordering. It has been proposed that the presence of this type of disorder is related to the concentration of the S vacancies [13]. The mtrinsic defects are single, doublé, or triple, and give rise to one, two, or tlii*ee different energy levels. Table 3.2 summarizes the intrinsic defects that can occur in CuInS?. their electrïral nrtivit^/ ^nA tïi^iV ,-ol^i.i^fo^ „^+:,,„+: ^-- ri A-I

The intrmsic defect structure of CuInS2 can also be represented in the form of defect lattice reactions. Three types are generally found in this material. The first is normally referred to as Schottky disorder and it is defined as the simultaneous formation of cation and anion vacancies. In defect lattice reaction this is

represented as: ,

« W / / < = > F : + K : + 2 F ; ' _Oi _In

3. Semiconductors materials for soiar cells applications 29

Table 3.2 Intrinsic defects in CuInS^, their electrical activity and calculated activation energies. Intrinsic point defects Electrical activity Ctr K5 and In;. _Cu In: Single donor Activation energy [meV] Reference 20 Doublé donor Triple donor 34,80 19 19 50, 70, 120

K _Cii Single acceptor

Cu _InII K / / / In Doublé acceptor 100 80, 100, 150 20 19 20 Triple acceptor 50, 110,178 21

From the lattice reaction follows the electroneurality condition:

[VcJ^W:] = 2[Vs]

The second type of disorder is called Frenkel disorder and it is defmed as the fonnation of interstitial cations and cation vacancies:

mdl <^ Cu' + K _Cu

with the electroneutrality condition

[CM; ] = K, ]

and

null o In" + VZ

_In

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30 _{3. Semiconductors materials for solar cells appijcations}

The formation of interstitial anions and anion vacancies is called anti-Frenkel disorder, and in CuhiS2 this type of disorder can be found in the form of interstitial sulphur atoms and sulphide-ion vacancies. The defect lattice reaction

t

is:

miU <^S^-V V^

3. Semiconductors materials for solar cells applicatlons

w:A=wn

BI Cii.S -£!!!ü^2^ Cu'^^^ + Cu] + Vl + SI + V^

31

witli the electroneutrality condition with the electroneutrality condition:

[s"A

=

[v,]

Besides the addition of dopants changing the stoichiometiy of the material can also influence the defect chemistry of a material. By examining Cu and In rich CUI11S2 samples several defects and their corresponding energies have been identified (Table 3.3). The acceptor at OTO eV may coixespond to V^^,, because it occurs only in In-rich materials. The acceptor at 0.15 eV is only observed in

Cu-rich material, and it is probably due to F^f or Ciii. The electrically active centres in CuInSi are fonned by intrinsic defects [15].

The extrinsic impurities in CuInS. are compensated by intrmsic defects, which are produced by deviations from stoichiometry and molecularity. The extrinsic impurities associate with intrinsic defects to form complexes and precipitates. At low doping concentration the single defect wiU be dominant. At high doping concentrations the neutral or charged associates become dominant, which is responsible for the large degree of self-compensation. Therefore, the canier concentration and conductivity type depend on the degree of self-compensation between the dominant defects and the extrinsic impurities [16].

To clarify what happens in Cu-rich or In-rich CuInS. we examine the possible defect lattice reactions. In a solid material the defect mechanism is the one with the lowest number of defects, because they are energetically most favourable. The preferred defects are the ones with the lowest charge, because they are the easiest to compensate. In the case of Cu-rich CuInS., the deviation from molecularity is

due to excess of CU2S. The possible defect mechanisms are:

A l _Cu^S CtiInS^ _{> 2Cu' + 2V... + SI + W:}

Cu _hl

ft

l

\

[c«; ]+2[K; ] = wl ]

Cl Cii^S CulnS ^ Cii^,, + Cu,„ + S^ + V,

[Ci>:, i^iVs]

Table 3.3 Donor and acceptor levels observed in CulnSi. Level Acceptor Donor Acceptor Donor Acceptor Acceptor Donor Sample Normal In-rich Nomial, S-anneal All samples Cu-rich, S-armeal In-rich, S-anneal Cu/hi-rich, S-aimeal lonization energy EA/ED [eV] 0T5 0.07 0.15 0.019 0.15 0.10 0.035 Defect

C or

Cu'L

Vs or In-^,,

v;: or

c»;;,

V,

or

Mc,

^7» or CUj„

V'

V^ or /Wc„

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Jt'

\

32 3. Semiconductors materials for solar cells applications

hl the case of In-rich CuhiS2 the deviation from molecularity is due to excess hiiSa and the possible defect mechanisms are:

A2

In.S, '"•'"'^- > 2K + Ku + 35; + V,

with the electroneutraUty condition:

WcA-wn

B2

In-^S

_{^ ' ^ a} CuInS.

> K , + K + Ven + 25; + 5,

with the electroneutraUty condition

^\ïn:'^=A^>Wc^

C2 In-^S-^ CiihiS-,

> K + / « ö , + 2 5 ; + 5;

Wc,. ]=[s: ]

Following the same line of reasoning as in the case of Cu-rich CuInS?, the preferred defect fonnation mechanism is described by A2. In this mechanism the Cu-Au ordering is not involved.

In both cases, Cu- and In-rich, the properties of the material are detennined by the ionic defects.

The deviation from stoichiometry in CuInS? can be influenced by variation in the sulphur content. The mixed ionic and electronic conductivity of the material is explained by sulphur excess or deficiency. The defect lattice reaction is:

Ss'«KS,(„+^,+2e'

h

H

3. Semicon ductors materials for solar cells applications 33

2[V,] = [e^]

The incorporation of sulphur into the lattice can be described W ^ . ^ ^ ^ ^ ^ ^ j ^ " ^ Ï c h c l b i i ^ e the deviation from molecularity ^vith a deviation from

stiochiometry;

and

2Cul,, + 2S; o K S,(„ + 2F, + 2V', + Cu,S + 2 . '

2Inl, + 4S; « K S,„, + ^Vs + 2^: + '"A + 2e'

These are the possible equations of fundamental defect generation reactions in CuïnS2. Since it is a temary compound, CulnSa may exhibit both deviation from

molecularity and stoichiometry, and extrmsic and intrinsic point defects are generated [17].

Raman [18] and photolurainescence (PL) [19] spectroscopy have been identified as among the most important characterization methods for the study of defects in

CuInS2 thin films. Despite of the potential offered by these nondestructive techniques, the defect chemisti'y of CuInS2 is still not well understood. In this

paper Raman and PL spectra are studied as a frmction of the preparation conditions of CulnS2 (Cu-rich) tliin films, obtained by sulphurization of Cuin

(atomic ratio Cu/In^l.8) metal aUoy layers.

3.3.2 Experimental aspects

CUIUST thin films are obtained by sulphurization of a metallic CuIn alloy on

molybdenum coated glass. The metallic precursor layers are obtained with a galvanostatic deposition process. Sulphurization is carried out in a quartz tube hot-wall reactor at 1 atmosphere pressure. The reaction temperature is maintained

at 450 °C. Argon (99.99% Air Products) is used as carrier gas and H2S (4.8, Hoekloos) and Sn (99.99%, Merk) as sulphur precursors. Oxygen (99.99% Air

Products) can be injected in concentrations up to 0.05%. After sulphurization all the samples are treated with 5% KCN solution for 10 minutes to remove the

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34 3. Semiconductors materials for solar cells applications

The crystal structure is detennined with X-ray Diffraction (Bruker D8 Advance Diffractometer) and the morphology is studied with a Scanning Electron Microscope (SEM. Jeol JSM-5800LV).

Photoluminescence measurements are perfomied using a home-built set-up. The excitation source is a Nd;YV04 laser operating at a wavelength of 532 nm (SpectraPhysics Mïllemiia). Neutral density filters are used to adjust the power of the laser. Light detection occurs with a liquid-nitrogen cooled CCD camera

(Princeton Instruments LN/CCD-1 lOOPB) connected to a Spex 340E monochromator equipped with a 100 grooves/mm grating. The sensitivity of the

CCD camera and the monocliromator is calibrated. For low-temperature measurements the samples are mounted in a closed-cycle heh'um cryostat {APD

Cryogenics CSW-204sl). Raman spectra are recorded with the same set-up using the backscattering mode. Raleigh scattering is removed effectively with two notch

filters (Kaiser). For Raman spectroscopy, the Spex 340E monoclü'omator is equipped witli an 1800 grooves/mm grating.

3.3.3 Results and discussions

In order to obtain Cu-rich films the Culn metal alloy layers contain an excess of Cu. During the growth process, the alloy is transformed into phases of CuInSi and CuxS. Due to its surface energy, Cu^S is segregating at tlie surface as can easily be

observed in SEM images obtained before and after KCN treatment (Figure 3.2).

X-ray diffraction pattems recorded after the KCN (sol 5%) etch shows only the presence of CuInS2 and Mo (Figure 3.3).

Recently, Alvarez-Garcia et al. [18] have shown that in Cu-rich samples, which are growth at 450 °C, the so-called Cu-Au (CA) ordering is present. The relative intensity of the peak at 32.3° 2-theta can be used to monitor the fraction of tlie CA phase present 'm the films. A relative high mtensity of this peak is related to a low

concentration of the CA phase. i

3. Semiconductors materials for solar cells applications ₃₅

t

t * ' ^ JT

C

"W

' _ i ^ ' ^ .r. •i ^^

M

\ F J * -J u

fUi

k y^.

m

_<; (A) _(B)

Figure 3.2 SEM graph of CIS films before (A) and after treatment with KCN (B).

ti

è

Chalcopyriie sfructure (a)

Cu-Au structure (b)

i

20 ₃₀ ₄₀ 2-Theta-Scale 50

ï

Figure 3.3 X-ray pattems of CuInS2 thin films. Tlie relative intensit>' of [200] peak. at 2 0 = 32.3, is related to Cu- Au ordering concentration

The Raman spectra of CuInS, fihns obtamed with various sulphur partial pressures are shown in Figure 3.4. The dominant mode, i.e., the Al mode, which

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e

>

36 3. Semiconductors materials for soiar cells applications

-1

(space group P4m2}. Wlien the relative intensity of the 290 and 305 cm" is measured as a function of the sulphur partial pressure we find that by increasing the sulphur partial pressure a relative decrease of the intensity of the 305 cm' peak occurs. Furthennore, when O: is applied in low concentration (0.05%) the

305 cm"^ peak can be removed completely. It is repoited [20] that oxygen can passivate sulphur vacancies and so we propose that the concentration of Cu-Au

ordering is associated to the concentration of sulphide-ion vacancies.

When comparing Raman spectra fi*om CIS films obtained with H2S compared to Sn we fmd that in the former case the 305 cm' peak is more prominent, indicating a higher fraction of Cu-Au ordering. Since sulfurization with HiS creates more sulphur vacancies than when Sn is used, this obsei"vation is in line with our proposition that sulphur vacancies and Cu-Au order are con-elated.

ir (/) C 0) A »^*

uu ' hVi' J4U ' JbU' JBU ' JUU ' JjU' J4U ' JbU ' JHU

J — 1 1 L 1 1 1 O 220 240 260 280 300 320 340 360 380 00 4 O 4 O Raman shift cm

Figure 3.4. Raman spectra of CIS films, which contain various concentrations of Cu-Au ordering, evaluated with the Raman spectroscopy

Photoluminescence (PL) spectra have been recorded to connect the presence of Cu-Au ordering to the presence of sub-bandgap electronic states. Li all cases, a broad emission rouglily between 1.25 and 1.55 eV is observed. This emission

shows a fine structure, which is related to the presence of electronic states in tlie

3. Semiconductors materials for soiar ceils appiications 37

forbidden band. The involved electronic states are located on native point defects, as will be discussed below. The PL spectra presented in Figure 3.5 show that the used sulphur source, H^S or Su influences the relative intensities of the PL

emission peaks. Tt should be noted that the emission spectmm of HiS treated samples is scaled douai by a factor of 8.4, which implies that PL from these samples is much more intense than from S^ treated samples.

5000 4000 -co O Ü 3000 (A O 2000 1000 -O 1.2 1.3 1.4 1.5 1.6 1.7 Energy eV

Figure 3.5 PL spectra of CIS films obtained by sulphurization with H2S (A) and S^ (B). Different intensitj' of the emissions is observed.

Photoluminescence spectra of samples obtained in H2S show peaks at 1.31, 1.4, 1.45, and 1.525 eV (Figures 3.5 and 3.6). The temperature dependence of PL spectra of samples obtained with H2S are shown in Figure 3.6.

The area below the PL peak at 1.4 eV is plotted as a flinction of temperature in the insert of Figure 3.6. From this plot we conclude that the intensity (7) follows the Curie equation for the probability for non-radiative recombination, equation (3.1).

/ =