Thick Film Metallisation of Crystalline Silicon Solar Cells : Mechanisms, Models and Applications

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Gunnar Schubert

Thick Film Metallisation of Crystalline Silicon Solar Cells

Mechanisms, Models and Applications

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Thick Film Metallisation of Crystalline Silicon Solar Cells

Mechanisms, Models and Applications

Dissertation

zur Erlangung des akademischen Grades des Doktors der Naturwissenschaften

(Dr. rer. nat.)

an der Universität Konstanz Fachbereich Physik

vorgelegt von

Gunnar Schubert

Tag der mündlichen Prüfung: 9. August 2006 Referent: Prof. Dr. Ernst Bucher

Referent: Prof. Dr. Wim Sinke

Konstanzer Online-Publikations-System (KOPS) URL: http://www.ub.uni-konstanz.de/kops/volltexte/2007/2559/

URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-25592

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Introduction

Thick film metallisation is the predominant technology in photovoltaic industry for contacting solar cells. More than 85% of the solar cells produced world-wide in 2005 were fabricated using silver thick film contacts on the front side and aluminium thick film contacts on the rear side of the solar cell [1]. The advantages of this technology are the high throughput rate, the limited number of process steps, and the possibility to benefit from the experience in the microelectronic so that this technology is very cost-effective. Efficiencies of at least η=15.8%

on multicrystalline silicon and of at leastη=17.0% on mono crystalline silicon solar cells are currently reached in photovoltaic industry¹. One way for further reduction of the production costs per watt peak is to increase the efficiency of industrial solar cells. Beside material induced losses, the major loss mechanisms are related to the thick film metallisation.

Today, commercially available thick film pastes contain lead most often in form of lead oxide incorporated in a glass frit. However, from July 2006 on, the EC “Directive on the restriction of the use of certain hazardous substances” (RoHS) regulates that electrical and electronic equip- ment put on the EU-market must not contain lead and other hazardous substances [2]. Only a maximum concentration value of 0.1 wt.% lead shall be tolerated in homogeneous materials.

Although this directive is currently not applied on solar modules or solar cells, this situation might change in future [3].

Two main questions arise regarding the development of improved thick film pastes:

1. What are the reasons for the losses in industrial solar cells related to thick film contacts and is it possible to modify the thick film metallisation process to reduce these losses?

2. Is it possible to develop lead free thick film pastes that perform at least equally well as lead containing reference pastes?

This thesis deals with the fundamental understanding of thick film contacts to crystalline silicon solar cells. The main focus is on the silver thick film contact. Silver thick film metallisation is applied for contacting n-type emitters of silicon solar cells since the 70s [4]. However, at the beginning of this study only a limited number of investigations were published dealing with the formation and nature of silver thick film contacts. The knowledge of these contacts to the emitter of a solar cell was therefore limited. In the past years progress in silver thick film metallisation of solar cells was mainly obtained by optimising the process parameters using a try-and-error approach. In this work a different approach is chosen. The formation and nature of silver thick film contacts is investigated by separating competing processes. Models for contact formation and electrical conduction are built. These models serve as basis for the development of highly efficient and lead free silver thick film pastes. The aluminium thick film contact and its formation was already subject to several investigations (see e. g. [5]). In this work existing models are used to develop a lead free aluminium thick film paste.

This thesis is divided into three parts. Part Ideals with the industrial solar cell in general. In chapter 1 this type of solar cell and its fabrication process is introduced. The main process steps

1Result of a data sheet survey of main European solar cell producers, status: March 2006.

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will be presented and discussed. An overview of thick film deposition technologies and basic concepts is given. In chapter 2 the major loss mechanisms in typical industrial solar cells are identified and investigated. Typical characterisation methods, used throughout this work, are introduced and applied. Finally, the impact of the contact firing process on the cell performance will be investigated.

Part IIdeals with the investigation of the silver thick film metallisation applied to solar cells. In chapter 3 the formation of low line resistances due to silver particle sintering in fast firing processes will be investigated. A new developed measurement method, the in-situ line resistance measurement, will be introduced. This measurement tool will be useful to study the kinetics of sintering processes in fast contact firing sequences. The microstructure of the contact interface of silver thick film contacts to silicon will be investigated by scanning electron microscopy and energy dispersive x-ray spectroscopy in chapter 4. The electrical contact formation of silver thick films on phosphorous doped silicon is topic of chapter 5. Suitable test structures will be prepared and investigated to separate the processes occurring during contact firing. The focus is thereby on the role of lead oxide contained in the glass frit commonly used in silver thick film pastes. The impact of the phosphorous surface concentration as well as the surface texture on the contact formation will be studied. The results of these investigations will lead to a model of contact formation. In chapter 6 the electrical properties of the formed silver thick film contact will be examined. After a review of existing hypotheses the basic metal - semiconductor fundamentals are used to simulate the properties of a silver - silicon thick film contact. These results will be compared to experimental measurements. The kinetics of the electrical contact formation are studied using the new developed in-situ contact resistance measurement. To get a deeper insight in the current transport mechanisms in the silver thick film contact, the beneficial effect of a forming gas anneal on the contact resistivity of silver thick film contacts to silicon is investigated. The results of these investigations will lead to the successful fabrication of silver thick film contacts on moderately phosphorous doped silicon and to the development of a current transport model.

Part IIIsummarises the development of lead free thick film pastes. After a brief introduction in glass theory, fabrication, and characterisation in chapter 7 the model of silver thick film contact formation developed in chapter 5 will be applied to the development of lead free silver thick film pastes. The model will be used to predict suitable substitutes for lead. Experimentally, the properties of the lead free glass frits are tested. These investigations will lead to the development of a successful lead free silver thick film paste. In chapter 8 experiments on lead free, glass frit containing aluminium pastes on thin multicrystalline silicon substrates are presented.

The approach is to apply existing models to develop lead free aluminium thick film pastes. The experiments will be the basis for the development of a successful aluminium thick film paste.

Most parts of this work were performed within the framework of the R&D-project EC2Contact, funded by the European Commission in the 5th Framework Programme [6], in cooperation with the research institute ECN Solar Energy, the powder and precious metal supplier Metalor, and the wafer, solar cell and module producer RWE SCHOTT Solar.

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Part I.

Industrial Solar Cells

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1. Basic principles

In this chapter the industrial silicon solar cell is introduced. Throughout this work “industrial silicon solar cell” denotes solar cells with a multi- or monocrystalline p-type silicon substrate with or without surface texture, a phosphorous emitter on the front side, and a silicon nitride layer as antireflection coating. Silver and aluminium thick film pastes are used for front and rear side metallisation, respectively. The typical fabrication process of industrial solar cells is presented. Subsequently, basics about the thick film process are discussed and an overview of thick film deposition technologies and basic concepts is given.

1.1. Fabrication of industrial solar cells

The predominant substrates used to fabricate industrial solar cells are multicrystalline (mc) and Czochralski (Cz) grown silicon wafers. The advantage of the former are the cheaper production costs, the advantage of the latter the potential for higher efficiencies. Both wafer types are commonly doped with the p-type dopant boron. In the last years the wafer size for mc-Si wafers increased to 21×21 cm². However, the most common size used in production is currently 15.6×15.6 cm². The maximum size of Cz solar cells depends on the diameter of the grown Cz crystal, currently around 20 cm, resulting in semi-square wafers withA≈240 cm²[7]. The production process of crystalline silicon solar cells, commonly used in solar cell industry, is characterised by a limited number of process steps (Figure 1.1).

Figure 1.1.:Standard industrial solar cell process (from: [8])

Most methods used are well-known and have been established in the microelectronic industry for many years. In the following a short description of the five main process steps is presented.

1. Saw damage removal / Surface texturisation

Firstly, the surface damage, resulting from cutting blocks into the silicon wafers using a wire saw, has to be removed. Alkali etchants like NaOH or KOH most often mixed with iso- propanol are predominantly used for monocrystalline wafers to benefit from the anisotropic

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1.1 Fabrication of industrial solar cells 5

etching behaviour. As [100] silicon planes are etched faster than [111] oriented planes, pyramids remain on <100> oriented silicon surfaces, improving the optical properties of the device (see chapter 2.3). Although an effective surface texturing is not achieved with alkali etchants on mc silicon substrates, NaOH or KOH are used to remove the saw damage in solar cell industry. Wet acidic etching (isotexturing) using a solution of HNO₃, HF and water becomes more and more popular. Acidic etching can lead to textured surfaces regardless of the crystal grain orientation decreasing the substrate’s reflection [9]. The saw damage removal is followed by successive wet chemical cleaning steps with HCL, to remove metal precipitates, and with HF, to remove the thin SiO2layer on the surfaces.

2. P-N junction formation

In the next step the p-n junction is predominantly formed by diffusion of the n-type dopant phosphorous. In industry two main systems are used. In batch mode facilities phosphorous diffusion is performed in a tube furnace. POCL₃ is transported to the silicon wafers via a carrier gas (usually N₂) . At the silicon surfaces phosphorous-silicate glasses, acting as quasi infinitive diffusion sources, are formed. In in-line facilities the diffusion source is sprayed or printed on the wafer. After drying, the wafer is exposed to temperature in an IR heated conveyor belt furnace.

Temperature and time determine the diffusion profile. Today, sheet resistances R_sheet <

65Ω/sq are used (see chapter 2.2.2 for a detailed discussion). At the University of Konstanz both diffusion technologies are available. However, for standard diffusions the batch method (POCL3diffusion) is used.

3. Edge isolation

Despite of the possibility to form the emitter only on the front side of the wafer¹, it has been shown that it is necessary to isolate parasitic p-n junctions at the wafer’s edges to increase the electrical performance of the solar cell. In industry the edges are usually isolated by laser-cutting, plasma-etching or wet chemical etching of the back side. Edge isolating by sawing is not frequently applied although the electrical quality is the best, i.e. the shunt resistance is highest [10].

4. Anti-reflection coating

Today, the most common anti-reflection coating is silicon nitride deposited by PECVD (Plasma Enhanced Chemical Vapour Deposition). Neither LPCVD (Low Pressure Chemical Vapour Deposition) nor SiN_xsputtering are frequently used in PV industry. These methods are still in development stage for industrial use [11,12]. Properties and benefits of SiNx

anti-reflection coatings are discussed in more detail in chapter 2.3.

5. Metallisation

In the last step metal contacts are applied to the device. As a characteristic of industrial solar cells in the context of this work, thick film pastes are used for metallisation. Contacts to the n-type emitter are made using silver pastes. For base contacts aluminium thick film pastes are used. To allow for solderable cell interconnections in a solar module, additionally Ag/Al thick film pads are applied to the base. The pastes are deposited successively on the device commonly by screen-printing. Techniques like stencil-printing, syringe-printing, roller-printing or pad-printing are not widely spread. In the next section an overview of these techniques is given.

After each deposition step the pastes are dried in an IR conveyor belt furnace. The order of printing does not play a major role. In general, the Al/Ag pads are deposited prior to

1In case of single-side diffusion (e.g. back-to-back diffusion in tube furnaces or applying the spray-on dopant only on one side) phosphorous diffuses around the edges and forms a (weakly) doped emitter on parts of the rear side.

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6 Chapter 1: Basic principles

the deposition of the Al paste. After the last deposition and drying step both, front and rear contacts, are formed simultaneously in a high temperature step for several seconds, usually performed in an IR heated conveyor belt furnace. In Figure 1.2 a typical temperature profile measured with thermocouples attached to a completely processed solar cell is shown.

Figure 1.2.:Temperature profile of a completely processed solar cell in an IR conveyor belt furnace.

Typical heating up rates are between 40 K/s and 90 K/s. The total time at a temperature above 500^◦C is∆t_T>500^◦_C<26s. The plateau during heating up at 577^◦C is due to the endothermic Al-Si eutectic melting at the back side. Its recrystallisation is visible as a temperature plateau on cooling down.

With this process average efficiencies at leastη =15.8% on mc silicon and at leastη=17.0%

on mono crystalline silicon solar cells are reached in photovoltaic industry². At the University of Konstanz industrial-like fabricated mc solar cells show efficiencies up toη=16.2%. At the Energy research Centre of the Netherlands (ECN) efficiencies up to 17.0% on mc silicon were reached [13]. Beside increasing material quality, especially increasing mc silicon quality, in the past years efficiency increase was mainly obtained by substituting TiO₂ by SiN_x as the ARC layer, by co-firing of front and back contact and by optimising process steps using statistical methods like DOE (Design of Experiments) (see e.g. [8]). Further efficiency increase is expected to be obtained by gaining inside in the complex processes during each manufacturing step. In this work the main focus is on the investigation of step 5, the thick film metallisation.

During metallisation and especially during contact formation in the firing sequence a number of processes occur simultaneously. In this work the approach is to separate competing processes in order to obtain a detailed understanding of the metallisation process.

1.2. Thick film metallisation

Paste deposition on substrates is a very old technique having its origin around 1000 B.C in China. Between 5 µm and 10 µm wide coloured lines were printed using silk-screens to form patterns on walls and cloth [14]. Since the 1950’s this technology was further developed to deposit electric circuitries including resistors, conductors and capacitors on (ceramic) substrates.

2Result of a datasheet survey of main European solar cell producers, status: March 2006

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1.2 Thick film metallisation 7

In 1975 E.L. Ralph was the first who presented the successful transfer of thick film conductors to the metallisation of solar cells [15].

1.2.1. Deposition techniques

Thick film pastes can be deposited in several ways. In this subsection a short survey of most common methods is presented.

Screen-printing

In photovoltaic industry screen-printing is the most commonly used method to deposit thick film pastes onto silicon. Therefore, all solar cells and most of the test structures in this work were metallised using this technology.

The basic principal is quite simple. A squeegee forces paste through openings of a screen.

The screen is a wire netting, typically consisting of stainless steel, coated by an emulsion. This emulsion is photosensitive which allows to define pattern by photolithography. The print quality depends to a great extend on the emulsion, the mesh opening, the wire diameter and the mesh thickness (see e.g. [16]). Typical quantities are the number of wires per inch (mesh count), the open area and the theoretical wet print thickness. For printing fine and thick lines both, a high mesh count and a large open area would be desirable. Therefore, it is necessary to use as thin wires as possible. A general rule is that the screen wire is 1/3 of the minimum line width and the mesh opening about three times the particle size of the paste [14,17,18]. As screens with thin wires show a shorter lifetime, an optimum for screens, used for industrial mass production of solar cells, has to be found.

Syringe printing

Although applied in industry, dispensing of thick film paste via syringes is not a very common technology for line printing in photovoltaic industry. Huster et al. used this technology to deposit busbars on mechanically textured silicon surfaces [19,20]. Dispensing fine lines is a challenge because the paste particles have to be pressed through the thin syringe tip without blocking it. Additionally, the fingers should be deposited without interruption, which requires exact rheology control. One advantage is the possibility to deposit fingers on very rough surfaces provided that the distance between syringe tip and substrate is controlled. The deposited fingers can show high aspect ratios. In this work syringe printing was used to fabricate special test structures.

Stencil printing

Stencil printing is well established in metallisation of printed circuit boards. The transfer to the front grid metallisation of solar cells is still in development stage. Stencil printing can provide printing of finer and higher lines compared to screen-printing [21]. The main problem is the stability of the stencils whilst providing the good fine line printing properties especially on textured surfaces [22]. Currently, a new method for the production of laser-formed stencils is developed [23].

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Pad printing

Pad printing is commonly used to decorate objects with variable surfaces. Thick film metallisation using pad printing as the deposition method should therefore be suitable especially for rough surfaces, which was shown by Hahne et al. [24]. To obtain best results, the organic system of the paste has to be modified [25,26]. Although promising, the technology is presently not commonly applied in photovoltaic industry.

Roller printing

Roller printing can be applied to both, the full area metallisation on the back side and the finger grid on the front side. The latter is realised e.g. by defining the printing areas by mechanical texturisation of the silicon surface [19,20]. The technology is still in development stage and is, to the author’s knowledge, currently not used in photovoltaic industry.

1.2.2. Pastes requirements

Regardless of the application a conductor thick film paste has to fulfil five main requirements.

A paste should

1. be printable and guarantee a high aspect ratio³, 2. provide high lateral conductivity,

3. establish sufficient mechanical contact to the substrate, 4. be solderable and

5. long term stable.

Consequently, a thick film paste contains two main groups of constituents - the first determining the printing properties, the second determining the end product properties. The latter consists of fine metal particles that guarantee high lateral conductivity. In general, a second material, e.g.

glass powder is added to act as a binder in the solid state. The first group consists of an organic binder and a low vapour pressure solvent, typically terpineol. For firing in air atmosphere often ethyl cellulose acts as the binder [18]. The rheology of the final paste is determined by both groups, the solid particles and the organics, and has to be adapted to the deposition method. In general, the surface tension of the paste has to be adjusted so that it wets the substrate more effectively than the deposition tool. It is also necessary that the paste shows pseudoplastic properties. The viscosity of the paste should decrease rapidly while exposed to increasing shear rates and increase its viscosity after the deposition cycle [27]. Most pastes show additionally thixotropic behaviour. Therefore, the proper rheology is reached after some deposition cycles [18].

The paste composition determines the typical processing scheme of thick film applications.

After deposition a drying step is necessary to evaporate the solvent carefully . Metal and glass particles embedded in the organic binder are left behind. Ideally, during the subsequent firing cycle the organics burn off completely, the metal particles sinter to form a compact structure and the glass, fluid at elevated temperatures, establishes the mechanical contact to the substrates.

Being in liquid phase, the glass can also support sintering of the metal particles [28,29].

The main difference between conductor pastes for photovoltaic devices and conductor pastes for electric circuits is the necessity for solar pastes to achieve good conductivity not only laterally but also vertically. The substrate, here silicon, has to be electrically contacted by the thick film paste. Furthermore, contamination of the substrate due to in-diffusion of unwanted impurities

3Aspect ratio: line height line width

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1.2 Thick film metallisation 9

at high temperature steps must be avoided. In the following basic requirements of thick film pastes for the metallisation of photovoltaic devices are presented. As a typical industrial solar cell process includes thick film metallisation of both, the phosphorous-doped emitter on the front side and the boron-doped base, two paste types have to be distinguished.

Thick film pastes for the metallisation of phosphorous doped emitters

In a standard industrial solar cell the phosphorous doped emitter is exposed to the sun, thus the total contact area should be as small as possible to allow sunlight to penetrate into the solar cell.

Most often an H-pattern finger grid with two busbars perpendicular to fine fingers, necessary for interconnecting solar cells in a module, is chosen. The necessity of a small metallisation fraction determines the two main requirements of front side pastes: 1. A high aspect ratio to provide both, high lateral conductivity and small finger width. 2. A small contact resistivity to ensure low contact resistances despite of small contact areas. As the SiN_x layer is deposited prior to metallisation, the paste has to be able to fire through the insulating layer. Additionally, at least the busbar adhesion must be strong enough to withstand the interconnection process of the cells during encapsulating in a module. These requirements determine the three main components of a standard front side paste:

• Binder and solvent (15-30wt.%)

• Silver powder (70-80 wt.%)

• Glass powder (1-10 wt.%)

Silver is used because it is the best electrical conductor (ρ_Ag=1.61 µΩcm). Copper being the second best conductor is known to be fast diffuser in silicon and can therefore not be used for high temperature processes. Surfactants are sometimes used for example to minimise particle agglomeration in the paste or to optimise the sintering of the silver particles [30]. At temperatures below 840^◦C silver does not react with SiNx and Si (chapter 5). Therefore, glass powder, typically a lead-borosilicate glass, is added. Its main task is to etch through the SiN_xlayer and to establish mechanical contact to the silicon. The binder and solvents are used to tune and guarantee good printing properties and a high shelf life. Like in silver pastes for the microelectronic industry the most common binder and solvent is ethyl cellulose and terpineol. In general, for printing fine lines with a high aspect ratio the paste viscosity is increased.

Thick film pastes for the metallisation of boron doped bases

The back contact can be applied to the full cell area. Therefore, neither a low specific contact resistance nor a high aspect ratio is required. In case of this type of thick film pastes the challenge is rather to reduce recombination at the rear (see section 2 for details). One possibility is to introduce a highly doped p region below the metal contact, a back surface field [31]. This can be realised by using aluminium as the active metal in the rear paste because it acts as a p-type dopant when incorporated in silicon. Thus the p⁺-region can be formed during the firing process (chapter 8). The three main components of a standard rear side paste are therefore:

• Binder and solvent (15-30wt.%)

• Aluminium powder (70-80 wt.%)

• Glass powder (0-10 wt.%)

The rheology of an Al paste for full area metallisation differs from that of a front side paste.

Binder and solvent have to guarantee that the substrates do not stick to the screen after a printing cycle. The paste should be deposited over the whole area homogeneous in thickness. Therefore, the viscosity of the paste compared with front side pastes is often decreased. Glass is not

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necessarily added to a rear side paste. The role of the glass frit for rear contact formation will be further investigated and discussed in chapter 8. As aluminium is oxidised in air even at room temperature, the Al particles in the paste are usually covered by an oxide layer. Additionally, surfactants can be used to optimise the contact formation process.

The front and rear contact and their formation are investigated in detail in this thesis.

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2. Losses in industrial solar cells

To increase the efficiency of industrial solar cells, it is primarily necessary to analyse the loss mechanism in state-of-the-art cells. The following results are obtained on solar cells that were completely processed at the University of Konstanz, using the processing scheme presented in section 1.1. These cells are representative and show the major losses that are present in today’s solar industry¹.

The main parameter of a solar cell is its efficiencyη. It is defined as the ratio of the maximum power output P_max and the power of the incident lightP_in. The efficiency is obtained from the J-V characteristic of the solar cell at a defined temperature and illumination²by equation 2.1.

η =P_max

P_in = FF×J_sc×V_oc

φ (2.1)

withφ, the power of incident light per area, the short-circuit currentJ_sc, the open circuit voltage V_ocand the fill factorFF, defined as the ratio betweenP_max and the product ofJ_sc×V_oc.

The J-V characteristic of the solar cell can be described as an ideal diode in parallel with a constant current source (see e.g. [32,33]). Ohmic losses are introduced by a series resistanceR_s and leakage currents are described by a shunt resistanceR_sh(Figure 2.1 and equation 2.2).

J(V) =J₀₁

exp

qV−JR_s

n₁kT

−1

+qV−JR_s

R_sh −J_l (2.2)

with the photo currentJ_l, the elemental chargeq, the Boltzmann constantk, and the temperature T in Kelvin.

Figure 2.1.:Equivalent circuit of solar cell

1Loss analyses were routinely performed on solar cells of major industrial companies to determine processing steps with potential for further improvement.

2Standard reporting conditions are: Intensity: 100 mW/cm², Spectrum: AM1.5, Temperature: 298.15 K.

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12 Chapter 2: Losses in industrial solar cells

J₀₁is the saturation current density of the ideal diode and is composed by the sum of the recombination currents in emitter and base (equation 2.3)³.

J₀₁=J_0E+J_0B (2.3)

Radiative, Auger, Shockley-Read-Hall and the surface recombination in emitter and base lead to an ideality factor of n₁ =1. Further recombination mechanisms can be introduced by an ideality factorn₁>1 or by a second diode with ideality factorn₂in parallel to the first. The J-V characteristic in the two diode model is described by equation 2.4.

J(V) =J₀₁

exp

qV−JR_s

n₁kT

−1

+J₀₂

exp

qV−JR_s

n₂kT

−1

+qV−JR_s

R_sh −J_l (2.4) Recombination in the space charge region via defects in the middle of the bandgap leads to n₂=2. Industrial solar cells, however, often show deviations having several physical origins.

Thus interpretation is sometimes difficult [34]. Typical origins occurring in industrial solar cells are:

• a distributed series resistance leading to different working points of the solar cell at an applied voltage [35]. Typical origins are the line resistance in the contact fingers [35] and a non- homogeneous contact resistance distribution [36].

• diode like shunts [37].

Injection dependent device parameters also lead to discrepancies, but they are not dominant in industrial solar cells.

From equation 2.4 it is obvious that both recombination currents,J₀₁andJ₀₂, determineV_oc. The influence ofJ₀₁ is greater. The contribution of the base toJ₀₁,J_OB, can be derived analytically for finite cell dimensions [38].

J_0B= qDn²_i

L_{e f f}N_A (2.5)

withD, the diffusion coefficient of minority carriers in the base,N_A, the acceptor concentration and n_i, the intrinsic carrier concentration. L_{e f f} is the effective diffusion length, dependent on the recombination velocity at the rear,S_r, the bulk diffusion length,L_b, the diffusion coefficient Dand the cell thicknessW.

L_{e f f} =L_b×1+^S^r_D^L^btanh(_L^W

b)

SrL_b

D +tanh(_L^W

b) (2.6)

The generated photo current, and thus J_sc, is determined by both, the recombination and the optical properties of the device. Being an indirect semiconductor, the absorption in silicon is weak compared with direct semiconductors like GaAs. In Figure 2.2 the absorption length in silicon is plotted versus the wavelength indicating that the solar cell device should be thicker than 300 µm if no light trapping design is applied.

As the reflection of bare silicon after an NaOH-saw damage step is high (average weighted reflectance for 300nm < λ <1200 nm: ≈36% [9]), antireflection designs are necessary to increase the number of photons entering the cell. Another main loss mechanism is due to series

3Throughout this work the recombination currents at the front and rear surface are included in J_0E and J_0B, respectively.

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2.1 J-V characteristic 13

Figure 2.2.:Absorption length of silicon vs. wavelength [38]

and shunt resistances.

In the following section these losses are exemplified and discussed. Finally, the impact of the firing process on the cell performance is investigated.

2.1. J-V characteristic

The diode parameters of the analysed solar cells were extracted from the illuminated, dark and the series resistance free, intensity dependent J_sc-V_oc characteristics using the procedure described in [35]. In Table 2.1 the measurement results and extracted diode parameters of a typical alkaline etched mc industrial silicon solar cell are shown.

Area: 156 cm² J_sc: 31.8 mA/cm² J₀₁: 1.1 pA/cm² Thickness: 200 µm V_oc: 618 mV J₀₂: 21 nA/cm²

R_sheet: 50Ω/sq FF: 79.1 % R_sh: 5000Ωcm²

ρ_B: 0.7Ωcm η: 15.5 % R_s: 0.48Ωcm²

Table 2.1.:Diode parameters of typical alkaline etched mc industrial silicon solar cell

The two diode model fits well. The cell shows a high fill factor, low series resistance and high shunt resistance values. An acidic texturing would lead to an increase inJ_sc between 0.5 and 1.3 mA/cm²[39,40]. However, the enlarged surface area would lead to losses inV_ocof about 2 mV [39]. The fill factor was often found to be higher so that isotextured solar cells can show an efficiency increase of up to 0.4 % absolut in an industrial solar cell production [39].

Alkaline textured Cz solar cells show a higher current (J_sc= 35.5 to 36.0 mA/cm²) and a higher voltage (V_oc≈615 mV) depending on the bulk doping.

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2.2. Recombination losses

2.2.1. Bulk and rear surface losses

Typical boron concentrations for mc silicon wafers are in the range ofN_A=3.3×10¹⁶cm⁻³to N_A=7.2×10¹⁵ cm⁻³corresponding to specific resistivitiesρ between 0.5Ωcm and 2Ωcm.

Cz silicon wafers are typically doped with N_A =7.2×10¹⁵cm⁻³ to N_A =2.3×10¹⁵ cm⁻³ (ρ =2..6Ωcm). The three most prominent recombination processes in these solar cells are radiative recombination, Auger recombination and recombination via defect states in the bandgap, the Shockley Read Hall (SRH) recombination⁴. The latter is the most dominant in industrial solar cells in low injection conditions. Auger recombination gets relevant for doping levels higher than 1×10¹⁷cm⁻³in good quality silicon [41].

In mc silicon the lifetime is small due to impurities like Fe and the existence of highly recom- binative grain boundaries. As-cut wafers show averaged lifetimes of 4 µs to 30 µs [43,44]. The process sequence to fabricate solar cells usually leads to an increase in lifetime. The phosphorous diffusion was identified as a gettering step of impurities [43,45]. During the firing process hydrogen, incorporated in PECVD-SiN_x, diffuses into the bulk and passivates defects [46].

The lifetime of Cz silicon is generally higher than that of mc silicon. Nevertheless, in industrial production of Cz solar cells the substrates show lower boron concentrations than one would choose for single crystalline silicon to obtain a maximalV_oc. The reason is the carrier induced degradation of lifetime due to the formation of boron-oxygen related defect. On 1Ωcm material a drop in efficiency of up to 9% relative is observed after illumination for about 50 hours. The degradation can be completely reversed by annealing the cells at elevated temperatures⁵. As degradation of silicon is related to boron and oxygen, the common ways of avoidance aim at the reduction of the oxygen and/or the boron content. Therefore, Cz silicon with a boron doping ofN_A<4.7×10¹⁵ cm⁻³ (ρ>3Ωcm) is frequently used in photovoltaic industry. The use of other dopants like gallium or aluminium is also possible. The disadvantage is, however, the inhomogeneous dopant distribution in the ingot due to the low segregation coefficient. Another approach is to reduce the oxygen content in the substrates. Common crucibles of Cz puller consists of quartz glass so that oxygen diffusion into the silicon melt is inevitable. Magnetic fields are therefore used to confine the silicon melt without contact to the crucible.

In the frame of a diploma thesis [49] a completely new approach was developed and investigated. In a simplified model the degradation - annealing process can be described by a reaction of microscopic systems from a less recombination active but instable state A to a highly recombination active state B. This reaction was subject to several investigations (e.g. [47,48] and others). It was found that it is possible to introduce a third state C. This state is characterised by the following:

• not lifetime limiting similar to state A

• stable regarding a reaction from C to B under working conditions of a solar cell

• stable regarding a reaction from C to A under working conditions of a solar cell

It was found that an industrial Cz silicon solar cell can be transferred from state B to state C by applying a new developed process, the regeneration process. As this process is patent pend- ing, the interested reader may be addressed to references [49,50]. The process was found to be

4Details about the recombination mechanism can be found in the appropriate literature e.g. [41,32,33,42].

5A good overview about carrier induced degradation and annealing of Cz silicon is given in [47,48] and references therein.

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2.2 Recombination losses 15

temperature dependent. The temperature defines the time constant of the regeneration process.

Regenerated cells have been proven to be stable for 140 h under one sun illumination at 25^◦C [49,50].

The recombination at the rear side of industrial solar cells is determined by the back surface field. From equation 2.5 and 2.6 it is obvious that the smaller the ratioW/L_b the greater the influence of the rear surface recombination. The effective diffusion length can be extracted from the internal quantum efficiency (IQE). To demonstrate the potential of an Al-BSF, the IQE and reflectance of a FZ industrial solar cell is shown in Figure 2.3. The open circuit voltage of this cell wasV_oc= 635.7 mV (substrate: 250 µm,ρ = 0.5 Ωcm). The spectral response was

Figure 2.3.:Internal quantum efficiency and reflection of industrial FZ silicon solar cell

measured using the setup of Fischer [34]. Under the assumption of incident light penetrating the cell under the same angle, no parasitic absorption like free carrier absorption and an absorption length, small compared with the cell thickness, the IQE in dependence of the wavelength is

IQE(λ) = 1

1+L_α/L_{e f f} (2.7)

Plotting 1/IQE againstL_α leads to L_{e f f}⁶ [51]. The suitable wavelength range for the Basore fit is typically between 700 nm and 940 nm. The lower wavelength boundary is chosen to minimise the influence of the emitter on the IQE⁷. The assumption of L_α <<W leads to the upper wavelength boundary. The effective diffusion length for the FZ cell with a full aluminium back surface field was determined to beL_{e f f} =490 µm leading toJ_0B= 230 fA/cm². The current loss due to recombination in the bulk and at the rear is 0.5 mA/cm²and 1 mA/cm², respectively.

In case of mc industrial solar cellsL_{e f f} is typically between 300 nm and 500 nm, for Cz cells between 400 nm and 800 nm. The current loss in the bulk is higher due to the lower lifetimes, typically in the range of 1 mA/cm²to 1.5 mA/cm².

6If the incident light penetrates the cell under an angleγ(e.g. due to surface texturing) the absorption length has to be divided by cos(γ).

7The “dead layer model” proposed by Fischer et al. [52] is used to consider the emitter effect.

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Figure 2.4.: Profile of the electrical active phosphorous concentration in a typical, POCl₃ diffused 50 Ω/sq. industrial emitter measured with ECV. The dotted line is the (estimated) chemical phosphorous concentration, which can be measured e.g. with SIMS.

2.2.2. Emitter and front surface losses

Further origins for voltage and current losses in industrial solar cells due to recombination are the emitter and the front surface. Typically, emitters with low sheet resistances in the range of 40Ω/sq to 70Ω/sq and high electrical active doping concentrations at the surface (>1× 10²⁰cm⁻³) are used. These emitters are currently necessary to allow for thick film contacts with low contact resistances on the front side and a wide process window. In Figure 2.4 the electrical active phosphorous concentration of a POCl₃-diffused 50Ω/sq emitter, measured with the Electrochemical Capacitance Voltage (ECV) method, is shown.

A characteristic of industrial type emitters is the plateau in the highly doped region and the kink and tail shape originating from concentration dependent activation energies for the diffusion process [53,54]. The chemical phosphorous concentration⁸in the plateau-region exceeds 1×10²¹cm⁻³. This high doping concentration leads to a decreased mobility and to Auger and SRH recombination losses so that the lifetime in such an emitter is typically very low.

Therefore, the probability of carriers generated in the emitter to be collected by the junction is reduced. Consequently, the internal quantum efficiency is reduced in the blue wavelength region (Figure 2.3). The current loss on industrial type emitters with 50Ω/sq is between 0.7 mA/cm² and 1.1 mA/cm². This loss was estimated using ”dead layer model” proposed by Fischer et al.

[52].

The highly doped emitter causes a high emitter saturation currentJ_0E and is therefore an origin for losses in the open circuit voltage. Due to the doping gradient most often the differential equations describing J_0E are solved numerically, e.g. with one-dimensional numerical simulation program PC1D [55]. Carrier loss in the plateau region of the emitter is, however, often underestimated because in the surface near regions the number of silicon atoms per cubic cen- timetre is about five times higher than the number of phosphorous atoms. This layer is rather

8The chemical impurity concentration can be measured with secondary ion mass spectroscopy (SIMS).

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2.2 Recombination losses 17

(a) (b)

Figure 2.5.: (a)J_0E determined on Cz silicon with different surface textures and a 50Ω/sq emitter covered by a d-PECVD SiNxlayer before and after firing. (b)J_0E determined on polished FZ silicon with different emitters and differently deposited SiNx.

a Si-P alloy than a doped silicon crystal ⁹. Furthermore, PC1D uses a simplified band gap narrowing model, making it necessary for realistic simulations to adapt the parameters to get a self-consistent data set [56]. From equation 2.5, it can be qualitatively concluded thatJ_0E is decreased with increasing effective diffusion length. Lower doping concentrations would lead to a higher lifetime and mobility and thus diffusion length in the emitter. Another way to reduce loss in the emitter is to reduce its depth. A good measure for the emitter quality is therefore the sheet resistance combining doping concentration and emitter depth. However, an enlarged diffusion length in the emitter and/or reduced emitter thickness need not necessarily lead to a reducedJ_0E due to the influence of the surface. The smaller the ratio of emitter depth and emitter diffusion length, ^x_L^E

p, the stronger the influence of the recombination at the surface. In industrial solar cells PECVD-SiN_x, typically used as the antireflection coating, has the beneficial effect to act as a passivation layer for a phosphorous doped emitter. In Figure 2.5(a) J_0E currents of industrial emitters with SiN_x surface passivation before and after firing are shown. In consistence with other authors (e. g. [34]), it can be deduced that the performance of the SiN_xsurface passivation is increased during firing. As expected, the emitter saturation current is higher on textured sam- ples due to the enlarged surface. In Figure 2.5(b) the emitter saturation current passivated with differently deposited SiN_x in dependence of the sheet resistance is shown. It is clearly shown that the beneficial effect of surface passivation is greater, the higher the sheet resistance. In contrast,J₀₁of emitters without surface passivation increases with increasing sheet resistance.

Below metal contacts the surface recombination velocity can be estimated to be S_met =1× 10⁶cm/s[42]. The saturation current J_0,met depends on the emitter profile. In general, deep and moderately doped emitter profiles result in lowest saturation currents [42]. J_0,met on the 50 Ω/sq emitter is simulated to be ≈1×10⁻¹² A/cm² using PC1D. The total emitter saturation current including metallisation is calculated with

J_0E =F_mJ_0,met+ (1−F_m)J_0,pass (2.8) with F_m, the fraction of the metallised area, and J_0,pass, the emitter saturation current of the passivated emitter, resulting inJ_0E in the range of 0.4−0.5×10⁻¹²A/cm²for typical industrial

9The highly doped plateau is often called “dead layer”.

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solar cells.

2.3. Optical losses

To maximise the generation of electron hole pairs and thusJ_sc, the light absorption in the device has to be optimised. As described in section 1.1, an anti-reflection coating is deposited to reduce the reflection. Additionally, the surface of the solar cells is often textured - in case of mc solar cells using an acidic etching, in case of Cz cells with an alkali etching. The reflection weighted with the AM1.5 spectrum is reduced from 36% for alkaline etched mc silicon to≈10% for mc solar cells without additional texturing but SiN_x-ARC and to 7% for isotextured mc silicon solar cells with SiN_x-ARC. The current loss due to reflection is between 4 mA/cm²(NaOH) and 2.5 mA/cm²(acidic).

The area, covered by the metal grid on the front, should be as small as possible. Thus it is necessary to print fine lines. A typical industrial solar cell has 130 µm wide fingers with a spacing of 2.6 mm and two 2 mm wide busbars, covering about 7.4% of the cell area. The current loss due to the metal coverage is 2.5 mA/cm².

Parasitic absorption also leads to current loss. The SiN_x-ARC absorbs light, leading to a current loss of about 0.1 mA/cm², depending on the refractive index of SiN_x. Generally, the higher the refraction index, the higher the silicon fraction in SiN_x and the higher the absorption. As a SiN_x layer with a higher refractive index passivates silicon surfaces best, an optimum has to be found (see e.g. [57]). Another source for parasitic absorption is the free carrier absorption in the highly doped regions of the cell, in the emitter and in the back surface field. In a typical industrial solar cell≈0.5 mA/cm²is lost.

The thinner the cell the more important the light trapping design of the solar cell. As shown in Figure 2.2 the rear side reflection should be as high and as diffuse as possible. Additionally, it should be taken care of that the internal reflection at the front side is maximised. A typical industrial solar cell with a full area aluminium back contact has a back side reflection of 60% to 70% and a Lambertian factor¹⁰ of 70%-90% leading to absorption losses at the rear of

≈2.5 mA/cm².

2.4. Series resistance losses

The fill factor of the solar cell is primarily affected by the series resistance, described by the approximate expression [38]

FF =FF₀

1−R_sJ_sc V_oc

(2.9) with FF₀ being the ideal fill factor without parasitic resistances. FF₀ can be determined by simulating the measured J-V curve using an appropriate diode model, e.g. equation 2.4, and by re-calculating the characteristics with an eliminated series resistance.FF₀is then limited by recombination and shunt losses only¹¹. From equation 2.2 or 2.4 it can be deduced that for very high series resistances in short circuit conditions terms with the product J×R_s are no longer

10For details about the optical models used in this work see [51,?]

11Green [38] gave an empirical expression to calculateFF₀by using the normalized voltagev_oc=V_oc/(nkT/q).

However, as industrial solar cells do not always follow the one or two diode model exactly, the numerical method was applied to determineFF₀throughout this work using the simulation program IV-CC [58].

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2.4 Series resistance losses 19

Figure 2.6.: Schematic drawing of series resistance contribution in an industrial solar cell. The busbar resistance and the lateral resistance in the back contact layer is not displayed.

negligible, so thatJ_scis lowered.

The total series resistance of the studied cell, extracted form the J-V characteristics, is 0.48Ωcm² leading to a fill factor of 79.1%. Without series resistance losses a fill factor of FF₀=81.3%

would be possible. In the following the individual contributions are discussed.

The total series resistance in a solar cell can be described as the sum of seven resistances (Figure 2.6):

1. R₁: lateral resistance of aluminium layer

2. R₂: contact resistance of aluminium contact to base 3. R₃: resistance of base

4. R₄: resistance of emitter

5. R₅: contact resistance of silver contact to emitter 6. R₆: lateral resistance of silver finger (line resistance) 7. R₇: lateral resistance of busbar

As in industrial solar cells studied in this work, the back contact is applied to the full cell area, the contact resistance of the aluminium paste to the BSF is in general negligible as well as the contribution of the lateral resistance in the aluminium layer. In this work the busbars and the Ag/Al pads on the rear side were contacted with an array of current probes during J-V measurements. Therefore, in the following the series resistance contributions of the busbar and the Ag/Al pads are not considered.

2.4.1. Bulk resistance

The specific resistance of industrial solar cells is in the range of 0.5Ωcm for mc solar cells up to 6Ωcm for Cz cells. With a thickness smaller than 300 µm the series resistance contribution of the bulk (R_bulk=ρ×W) is between negligible 0.02Ωcm² and 0.18Ωcm². This is one reason why lowly doped material has a lower efficiency potential. The analysed cell has a specific resistance of 0.5Ωcm and a thickness of 250µm, leading toR_s,bulk=0.01Ωcm².

2.4.2. Emitter resistance

The contribution of the emitter to the total series resistance is calculated by

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R_s,emitter= R_sheet(d−W_f)²

12 (2.10)

with d, the finger distance, andW_f, the finger width. Accordingly, on high sheet resistance emitters the finger distance should be reduced to minimise the series resistance contribution.

However, it has to be taken into account that an increasing number of fingers increases the metal shadowing unless the finger width is reduced. Therefore, an optimum exists between current and resistance losses. The typical cell has an 50Ω/sq emitter and an optimised finger spacing of 2.6 cm leading to a series resistance contribution of 0.23Ωcm².

Figure 2.7.: Typical front side grid of an industrial solar cell. The red lines indicate the stripes for the TLM contact resistance measurement.

2.4.3. Contact resistance

The contribution of the contact resistance to the total series resistance is calculated by dividing the specific contact resistance ρ_C by the finger fraction. In this work ρ_C is determined by the Transfer Length Method (TLM) (see [59] for details) by cutting 1 cm wide stripes out of different cell areas (see Figure 2.7). Typically, standard industrial solar cells on emitters <60 Ω/sq with silver thick film fingers show contact resistivities <10 mΩcm². With a typical finger width of >80 µm for screen-printed silver fingers the contact quality on highly doped emitters is sufficient to enable fill factors >77.5% (Figure 2.8). Reducing the finger width to obtain a higher short circuit current due to reduced shadowing losses would increase the contact resistance contribution. Assuming a contact resistivity of ≈3 mΩcm², the series resistance contribution is 0.06Ωcm²with a finger fraction of 4.9%¹².

Contacting emitters with a low total phosphorous concentration (i.e. R_sheet >80 Ω/sq ) with thick film technologies, leads up to now to very high contact resistances. With other, costly metallisation technologies like evaporation of metals¹³, very low contact resistivities on high sheet resistance emitters are reached (ρ_C≈100−200µΩcm²on 100Ω/sq [60]).

1248 fingers, finger length: 122 mm, distance: 2.6 mm, width: 130 µm.

13Typically titanium, palladium and silver are evaporated subsequently using photolithography to define fine finger structures. After the cell process the line resistance is reduced by plating techniques.

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2.5 Shunt losses 21

Figure 2.8.: Simulation of influence ofρCon fill factor (simulated with IV-CC [58]. Cell parameters:A=125x125 mm², 48 fingers, Rsheet=50Ω/sq ,ρ=1Ωcm,Rline=260 mΩ/cm. Contribution toRswithoutRC: 0.46Ωcm². R_shunt=5000Ωcm²,J₀₁=1×10⁻¹²A/cm²,J₀₂=2×10⁻⁸A/cm², n_1,2=1,2,J_ph=32.5 mA/cm²

2.4.4. Finger resistance

The contribution of the finger resistance to the total series resistance depends on R_F, the line resistance inΩ/cm, the distanced and the lengthlof the finger.

R_{f inger}=R_F×d×(l/4)²

3 (2.11)

A typical value for industrial solar cells isR_line≈260 mΩ/cm resulting inR_{f inger}=0.19Ωcm². Assuming the screen-printed silver finger to consist of bulk silver without any pores, the finger contribution would be 2 to 3 times less. It has to be considered that minimising the metallised area by reducing the finger width should be accompanied by an increasing finger height to keep the finger’s cross-section area as large as possible and thus series resistance losses as small as possible.

In Figure 2.9 the individual contributions to the total series resistance are visualised. The front metallisation determines the three most important contributions (emitter, contact and line resistance), indicating the need for a better understanding of the metallisation process for further optimisation of the industrial solar cell.

2.5. Shunt losses

Another source for efficiency losses are leakage currents, ohmic and/or rectifying in nature. The analysed cell shows no diodic shunt behaviour and an ohmic shunt resistance of 5000 Ωcm², high enough to avoid sever efficiency losses. Usually a shunt resistance of >1000 Ωcm² is regarded to be tolerable, leading to a relative efficiency loss of 1.6% for a simulated cell (Figure 2.10). Lower shunt resistances mainly lead to fill factor losses that are the major origin for efficiency decrease. J_scandV_oclosses due to low shunt resistances only play a minor role.

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Figure 2.9.: Series resistance distribution of a typical industrial solar cell.

The most common sources for reduced ohmic shunt resistances are leakage currents at the edges of the solar cell due to incomplete edge isolation, crystal defects like grain boundaries or impurities in the space charge region [38] or microcracks through which metal paste is pressed during screen-printing. The ohmic shunt resistance limits the reverse current thus it can be extracted from the J-V characteristic. To localise shunts, voltage and light modulated lock-in thermography is used (for details about the measurement methods see [61] and [62]) to map local heating induced by current flow. In case of light modulated lock-in thermography LBIC measurements are performed because heating due to recombination currents especially in bad regions of a mc silicon solar cell has to be distinguished from heating due to leakage currents.

Solar cells can show deviations from the two diode model with ideality factorn₂=2 that can be explained by leakage currents with a diodic characteristic [37]. These shunts are only sig- nificant in forward bias and show a characteristic hump in the maximum power point region, reducing FF andV_oc. Typical origins of diodic shunts in industrial solar cells are point-like

Figure 2.10.: Relative losses inFF,V_oc,J_sc and efficiency in dependence ofR_shunt of a simulated industrial solar cell with typical parameters (Rs=0.8 Ωcm², J₀₁=1×10⁻¹² A/cm², J₀₂=2× 10⁻⁸A/cm²,J_ph=32.5 mA/cm²andn_1,2=1,2).

Thick Film Metallisation of Crystalline Silicon Solar Cells : Mechanisms, Models and Applications

Gunnar Schubert

Thick Film Metallisation of Crystalline Silicon Solar Cells

Mechanisms, Models and Applications

Thick Film Metallisation of Crystalline Silicon Solar Cells

Mechanisms, Models and Applications

Dissertation

zur Erlangung des akademischen Grades des Doktors der Naturwissenschaften

(Dr. rer. nat.)

an der Universität Konstanz Fachbereich Physik

vorgelegt von

Gunnar Schubert

Tag der mündlichen Prüfung: 9. August 2006 Referent: Prof. Dr. Ernst Bucher

Referent: Prof. Dr. Wim Sinke

Contents

Introduction

Part I.

Industrial Solar Cells

1. Basic principles

1.1. Fabrication of industrial solar cells

1.2. Thick film metallisation

2. Losses in industrial solar cells

2.1. J-V characteristic

2.2. Recombination losses

2.3. Optical losses

2.4. Series resistance losses

2.5. Shunt losses