Photovoltaic modules of dye solar cells

Transcription

1 Photovoltaic modules of dye solar cells Dissertation zur Erlangung des Doktorgrades der Fakultät für Mathematik und Physik der Albert-Ludwigs-Universität Freiburg im Breisgau Ronald Sastrawan Freiburg im Breisgau Juni 26

2 Dekan: Leiter der Arbeit: Referent: Korreferent: Tag der Verkündung des Prüfungsergebnisses: Prof. Dr. Josef Honerkamp Prof. Dr. Joachim Luther Prof. Dr. Joachim Luther Prof. Dr. Matthias Weidemüller 13. September 26

3 Contents 1. Introduction The gap between research on the dye solar cell and large area applications Module-related physics of the dye solar cell Partial shading or electrical mismatching Leak in the internal sealing or photoinduced electrophoresis Optimization of module design Production technology for large area dye solar modules Outline of this thesis Physics of the dye solar cell The architecture of the dye solar cell Energy diagram and working principle of a dye solar cell Recombination losses in a dye solar cell Electrochemical potential Mass transport in an electrochemical system Gradient in the electrochemical potential Fick s diffusion laws Electrode-electrolyte interface Helmholtz double layer Charge transfer over an electrode-electrolyte interface Potential controlled reaction: Butler Vollmer equation Diffusion controlled reaction General case Nernst diffusion layer Diffusion-limited current I-V characteristics of a solar cell One-diode model... 25

4 3. Module-relevant physics of solid-state solar cells From cell to module: methods of interconnecting solar cells Methods of interconnecting dye solar cells Electrical mismatching of solar cells or partial shading of a module Physics of reverse biased solar cells I-V curve of a solar module under partial shading Electrical mismatching of a dye solar module Electrical shunt between solar cells in a module Shunts in a dye solar module Optimization of module design Distributed series resistances Approximation of the distributed series resistances by a lumped series resistance (one-third rule) Distributed series resistances in dye solar modules Encapsulation of the module Encapsulation of dye solar modules Conclusions of Chapter Integrated series connection in dye solar modules Z-connection W-connection Monolithic connection Conclusions of Chapter Charge transfer under reverse bias potential Measured I-V characteristic of a dye solar cell under forward and reverse bias potential Electrical mismatching of dye solar cells in series connection Investigation of the charge transfer in reverse-bias I-V characteristics of electrode set-ups to determine the dominating charge transfer route in reverse bias Interfaces studied by electrical impedance spectroscopy Model for the I-V characteristics of a dye solar cell under forward and reverse bias potential One-diode model for the dye solar cell under forward bias potential Butler-Vollmer model for the dye solar cell under reverse bias potential Complete model for the I-V characteristic of a dye solar cell over the full voltage range Model for the electrical mismatching of dye solar cell in series connection Long term stability of reverse biased DSC Charge transfer under reverse bias suppressed by a dense TiO 2 blocking layer Pattern in the scattering layer of dye solar modules Applications of decorative solar modules...69

5 5.6 Spatially resolved photocurrent imaging technique for large area, series interconnected dye solar modules Conclusions of chapter Photoinduced electrophoresis in the electrolyte of dye solar modules Model of the photoinduced electrophoresis in dye solar modules Energy levels in a series connection of DSC Model system for the theory of photoinduced electrophoresis Potential gradients in a Pt/electrolyte/Pt cell with applied voltage and electrolyte barrier Modelling the ion concentration profiles under photoinduced electrophoresis in open-circuit Modelling the regeneration process through diffusion Modelling the short-circuit current densities under photoinduced electrophoresis Set of parameters Experimental Apparatus Measurement and simulation of the photoinduced electrophoresis Photoinduced electrophoresis in a low viscous electrolyte Photoinduced electrophoresis in an undiluted, high viscous ionic liquid Charge transport in highly concentrated ionic liquids Regeneration in the dark Regeneration with an externally applied voltage Diffusion current Influence of parameters on the theoretical model of the photoinduced electrophoresis Requirements on the barrier properties of the internal sealing material Conclusions of Chapter Optimization of module design Modelling the I-V curve of a dye solar module Modelling distributed series resistances Modelling discrete series resistances Modelling the efficiency of the module Influence of parameter variations on the I-V curve of a dye solar module Standard set of parameters Variation of the width of photoactive area Variation of the width of photoinactive area Variation of TCO sheet resistance Variation of contact resistance Variation of illumination intensity Module design I: Strip module Electrolyte canal Experimental

6 7.4 Module design II: Interdigital meander module Experimental Conclusions of chapter Long term stability Glass frit sealing Thermal stability of glass frit Electrode distance in glass frit dye solar modules Resistance of the series interconnection in glass frit dye solar modules Up-scalability of glass frit sealing technology Colourful glass frit Accelerated ageing of small test cells Long term stability under visible light soaking Estimating the number of stable turnovers for the Ruthenium dye Long term stability under 85 C in the dark Combined ageing under 85 C in the dark and subsequent ageing under visible light Thermal cycling test Model for the degradation under thermal ageing TCO layer TiO 2 layer Platinum layer Dye Electrolyte Modelling the degradation of the short-circuit current density under thermal ageing Conclusions of Chapter State of the dye solar module technology Research and development on dye solar module technology Sharp Corporation, Ecological Technology Development Centre Toyota Central R&D Laboratory and Aisin, Seiki Sony, Materials Science Laboratories Fujikura Ltd., Material Technology Laboratory Dyesol Solaronix Hitachi Electronic and Telecommunications Research Institute Energy Centre Netherlands Peccell Institute of Plasma Physics IVF, Industrial Research and Development Corporation Industrial Technology Research Institute Fraunhofer Institute for Solar Energy Systems Conclusions of chapter

7 1. Summary and Conclusions Module-related physics of the dye solar cell Partial shading or electrical mismatching (Chapter 5) Spatially resolved photocurrent imaging technique (Chapter 5) Photoinduced electrophoresis (Chapter 6) Optimization of module design (Chapter 7) Production technology of dye solar modules Hermetic sealing material (Chapter 8) Long term stability (Chapter 8) Interdigital meander design (Chapter 7) Applications of results of this thesis Appendix A1. Experimental A1.1. Module manufacturing A1.2. Test cells: masterplates A1.3. Accelerating ageing procedures A1.4. Electrolyte cells for the characterisation of the platinum electrode A2. Approximation of the transient current in photoinduced electrophoresis171 A3. List of symbols, physical constants and abbreviations A4. References A5. Publications A5.1. Publications in reviewed journals A5.2. Conference proceedings, oral presentations, poster presentations A5.3. Patents A6. Acknowledgements

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9 1. Introduction 1. Introduction Photovoltaic (PV) research, development and industry have been expanding rapidly for a number of years now. Especially in Germany, the Renewable Energy Sources Act (EEG) has triggered an average annual economic growth of 9 % in the PV industry sector for the last 5 years. In spite of this solar boom, to date, merely.2 % of electricity in Germany is produced by PV. On the other hand, this technology has an enormous available potential, since PV is expected to play a major role in future energy supply. [Luther '5] The pillar of today s PV technology is the crystalline silicon solar cell. Crystalline silicon solar cells will continue to dominate the PV sector in the foreseeable future. However, new solar technologies are expected to gain market shares in the coming years. And in a long term perspective, the solar age [Scheer '99] will be founded on one or several of these new technologies. New PV technologies have the potential of becoming a true mass product with significant cost-reduction potential and new applications. New PV technologies competing for their market share today, are highly efficient concentrator solar cells (based on III/V compounds, such as GaAs), and thin film solar cells such as amorphous silicon (a-si), copper indium diselenide (CIS), cadmium telluride (CdTe) and crystalline silicon thin film (CSiTF) solar cells. New concepts, such as the organic solar cell (OSC) and the dye solar cell (DSC) are still under fundamental investigation. However, the DSC technology has meanwhile progressed to a stage very close to a possible commercialization. This work focuses on the module-relevant physics of the DSC technology, with special attention to the producibility of large area applications. 1

10 1. Introduction 1.1 The gap between research on the dye solar cell and large area applications Figure 1-1: Architecture of a dye solar cell. The nanoporous titanium dioxide and the liquid electrolyte are located between two glass plates, coated with transparent conducting oxide. The dye solar cell (DSC) technology has attracted wide attention since its invention in 1991 by M. Grätzel [O'Regan '91]. Soon laboratory scale efficiencies of over 1 % were reported [Grätzel ']. Due to the potential for low production costs and attractive colour and design, considerable efforts are being increasingly undertaken to enable a commercial up-scaling of this new type of solar cell. The structure and working principle of a DSC differs fundamentally to solid-state solar cells. The DSC is an electrochemical solar cell. Between two transparent conducting electrodes a dye-covered, nanocrystalline titanium dioxide (TiO 2 ) layer and a liquid electrolyte is encapsulated (Figure 1-1). In order to understand a DSC, many fields of science must come together. Nanoparticles, electrochemistry of redox electrolytes, catalysts and dye synthesis all find their application in a DSC. Thus, the DSC has fascinated many scientists around the world and extensive research has been invested in understanding and optimizing the DSC. Long term stability a crucial requirement for solar cells has shown good improvement [Hinsch '1]. Recently, Wang et al. reported a stable 8 % efficient DSC based on a low volatile electrolyte under thermal stress of 8 C for 1 hours [Wang '5]. It may be stated, that the fundamentals of the single cell are understood reasonably well and that a single DSC is a controllable system. However, the initial enthusiasm about this new type of solar cell weakened when attempts were made to bring this technology from laboratory scale to industrial, large area applications. The electrochemical nature of the DSC, in particular the liquid electrolyte imposed problems on the scale-up process. A module technology could not be adopted from solid-state solar technologies, but new solutions had to be found. Apart 2

11 1. Introduction from technological problems, also new, module-related problems appeared. For example, first attempts of interconnecting DSC in series failed, because the photoinduced electrophoresis of the electrolyte had not been anticipated (see section below). Even today, 15 years after the discovery of the DSC, a gap exists between the laboratory research on single DSCs and the technological development of large area DSC modules. This work is intended to close this gap. Mainly, this work focuses on two points: The investigation of the module-related physics of the DSC The development of a producible technology for large area DSC modules A central issue of large area DSC modules is that although stable single cells have been demonstrated new module-related degradation mechanisms occur in a series connection of DSCs. 1.2 Module-related physics of the dye solar cell This research is not primarily concerned with the investigation and optimization of single DSCs. Rather, single DSCs are interconnected in series, to form large area modules. In this work, the term module-related physics refers to all process that are significant in a series connections of large area DSCs. A schematic cross section of a series connection of DSC is shown in Figure 1-2. single DSC single DSC single DSC series interconnect series interconnect electron flow conductor (silver) sealing material glas transparent conducting oxide titanium dioxide electrolyte platinum transparent conducting oxide glas break in transparent conducting oxide Figure 1-2: Schematic cross section of a series connection of DSC. The architecture of the series interconnect is shown in the enlargement. 3

12 1. Introduction The architecture of the cells and the series interconnect is shown in the enlargement in Figure 1-2. The exact architecture is not important in this chapter and will be discussed in detail in the course of this work. Here, the series connection of DSCs may be viewed as a series connection of direct current (DC) voltage sources in parallel with a diode. Each cell produces a voltage and current. In series connection, the voltages are added, while an equal current passes through each cell. The 3 major module-related aspects, which are investigated in this work are partial shading leak in the internal sealing optimization of module design Partial shading or electrical mismatching In Figure 1-3 it can be seen, that if one cell in the series connection is shaded (or electrically mismatched), the current still has to pass this cell. The simplest equivalent circuit of a conventional pn-junction solar cell is a DC source with a diode in parallel (see Figure 1-3 bottom). Then the current passes the shaded cell in reverse bias direction through the diode. This might pose a problem. In order to clarify the behaviour of a DSC module under partial shading, the physics of reverse biased DSCs is investigated. These results are presented in detail in Chapter 5: Charge transfer under reverse bias potential. shaded cell Figure 1-3: When one cell in a series connection is shaded, the current has to pass this cell in reverse bias. The simplest equivalent circuit of a solar cell is a DC source in parallel with a diode. 4

13 1. Introduction Figure 1-4: A leak in the internal sealing will cause electrophoresis of the electrolyte under illumination. electrolyte leak in internal sealing Figure 1-5: The width of the individual cells must be optimized, because of series resistances. distributed series resistance optimization of cell width discrete series resistance Leak in the internal sealing or photoinduced electrophoresis Another potential module-related degradation mechanism is shown in Figure 1-4: If there were a possibility of mass transport between the electrolyte of neighbouring cells, e.g. by a leak in the sealing material, an electrolytical shunt would occur. An electrolytical shunt in a DSC module causes electrophoresis of the electrolyte under illumination. During this photoinduced electrophoresis the redox couple (here: triiodide and iodide) is separated from each other. This research investigates the physics of the photoinduced electrophoresis in Chapter 6: Photoinduced electrophoresis in the electrolyte of dye solar modules Optimization of module design For large area solar modules, the design of the module is crucial for optimal photovoltaic performance. Essentially, the DSC does not differ here from conventional solid-state solar technologies. Upon increasing the individual cells, both a higher current is produced and the series resistance rises (Figure 1-5). In Chapter 7: Optimization of module design, the aim is to numerically investigate the influence of module parameters on overall efficiency. 1.3 Production technology for large area dye solar modules Based on the investigation of the module-related physics of DSCs, a producible technology for large area DSC modules can be developed. In particular, this includes the development of a hermetic sealing material and a (industrially) producible module design based on screen printing technology. 5

14 1. Introduction 1.4 Outline of this thesis In Chapter 2, the physics of the single DSC are discussed. The focus is placed on the charge and mass transport in a liquid redox electrolyte and over an electrolyte-electrode interface. In Chapter 3, the module-relevant physics of solid-state solar cells are studied in terms of their relevance to DSC. Module-relevant aspects such as partial shading and optimization of module design have already been investigated thoroughly for conventional solar technologies. New solar technologies like the DSC technology can benefit from these experiences. In this chapter, the solutions from solid-state solar technologies are discussed and evaluated in terms of their applicability to DSC. In Chapter 4, the loss factors of three types of integrated series connection in DSC modules are discussed. For the further development of the module concept in this work, the best type of integrated series connection will be chosen. Chapter 2, 3 and 4 describe knowledge from literature and up-to-date publications. My own research is presented in the following chapters. The partial shading (or electrical mismatching) of a DSC module is studied in Chapter 5. As the underlying process in partial shading, the charge transfer under reverse bias potential is investigated experimentally and theoretically. Based on these results, the partial shading as a module-relevant degradation mechanism is examined. Furthermore, DSC modules are presented, which are deliberately electrically mismatched. The electrical mismatching is achieved by a pattern in the photoactive area. And additionally, a method is presented, which allows the measurement of the spatially resolved photocurrent image of a large, series interconnected DSC module. This method is closely related to the operation of a DSC under reverse bias potential. The photoinduced electrophoresis of the electrolyte as a module-related degradation mechanism is investigated in Chapter 6. Here, the photoinduced electrophoresis in the electrolyte of DSC modules through an internal sealing is studied in a theoretical diffusion model and in experiments. The model allows the estimation of the required barrier properties of the internal sealing material, with respect to the diffusion constant of triiodide. In Chapter 7, the theoretical optimization of module design is carried out. In a numerical model of distributed series resistances, the influence of module and material parameters on the overall efficiency of the module is studied. In the model, the ratio of photoactive and photo-inactive area is varied and the series resistance of the interconnects and the contact material. Using these results, a module design is developed, which is very similar to solid-state thin film solar modules: the individual cells are strip-shaped. Additionally, in order to improve the industrial producibility, a DSC-specific design is 6

15 1. Introduction developed. This new design features meander shaped cells and a significantly smaller number of filling holes. In Chapter 8, the long term stability of single DSCs is tested with a newly developed glass frit sealing and a low-volatile electrolyte based on ionic liquids. The properties of the glass frit are investigated, both in terms of stability and up-scalability. In particular, electrode distance, resistance of interconnect, thermal stability, lead oxide (PbO) content and colour of the glass frit is examined. The glass frit sealing is applied to 3 x 3 cm² modules. Chapter 9 gives a brief overview on the state of the DSC module technology. Here the research and development of DSC modules in institutes and companies around the world is presented. 7

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17 2. Physics of the dye solar cell 2. Physics of the dye solar cell In this chapter, the physics are summarized, which are required for an understanding of the dye solar cell (DSC). In section 2.1 the structure and working principle of the dye solar cell is introduced. The DSC is an electrochemical solar cell, in particular it utilizes a liquid electrolyte. Therefore, the interface between a metal (or semiconductor) and an electrolyte must be very well understood in order familiarize oneself with the processes in a DSC. The central quantity here is the electrochemical potential, which is a general concept, that can be applied to liquid electrolytes, as well as solid state bodies. It is introduced in section 2.2. Mass transport in an electrochemical system occurs down the gradient of the electrochemical potential. In a redox electrolyte, however, the dominant form of mass transport is diffusion, i.e. the field driven current (migration) can be neglected. This is discussed in section 2.3. The interface between a solid-state electrode and a liquid electrolyte is further addressed in 2.4 and 2.5. In particular, the charge transfer over such an interface is of importance. And finally, in section 2.6 the I-V curve of a DSC (and a solar cell in general) is discussed. Important electrical parameters of solar cells are introduced and a model for the I-V curve is presented. 9

18 2. Physics of the dye solar cell 2.1 The architecture of the dye solar cell The architecture of a dye solar cell (DSC) is shown in Figure 2-1. The DSC consists of a dye-covered, nanoporous TiO 2 (titanium dioxide) layer and an electrolyte encapsulated between two glass plates. Figure 2-1: Architecture of a dye solar cell. The nanoporous semiconductor (TiO 2 ) and the redox-electrolyte are located between two glass plates, coated with transparent conducting oxide (TCO). The TiO 2 is covered with a monolayer of dye and the counter electrode is coated with a thin platinum layer. Front and counter substrates are coated with a transparent conducting oxide (TCO). Fluorine doped tin oxide (SnO 2 :F), FTO is most commonly used. The FTO at the counter electrode is coated with few atomic layers of platinum (Pt), in order to catalyze the redox reaction with the electrolyte. The front electrode is coated with a nanocrystalline TiO 2 layer with average particle sizes of 5-2 nm. Assuming a layer thickness of 1 μm, the resulting effective surface is about 1 times larger as compared to a dense, compact TiO 2 layer. Three modification of TiO 2 exist: rutile, anatase and brookit. In the DSC preferably only the anatase modification is used. On the surface of the TiO 2, a monolayer of dye molecules is adsorbed. The huge nanoporous surface allows for an adsorption of a sufficiently large number of dye molecules for efficient light harvesting. The employed dye molecule is usually a ruthenium (Ru) metal-organic complex. The spectral absorption of the dye lies between 3 nm and 8 nm. The chemical structure of the most widespread dye molecule in DSC, the socalled N719 [Nazeeruddin '93] is shown in Figure 2-2. Good adsorption of the dye to the TiO 2 is important and is achieved over the two carboxylic groups of the ligand (L=2,2'- bipyridyl-4,4'-dicarboxylic acid) of the RuL 2 (NCS) 2 Between the two glass substrates, a liquid redox electrolyte is encapsulated. In particular, the liquid electrolyte is able to penetrate the nanopores of the TiO 2. The redox couple iodide/triiodide (I - /I 3 - ) is commonly used. Iodide is usually applied as a room temperature molten salt (ionic liquid), e.g. an imidazolium iodide (Figure 2-3). The ionic iodide liquid acts as a solvent for iodine (I 2 ), which reacts with iodide to form triiodide (I 3- ): 1

19 2. Physics of the dye solar cell I I I3 During cell operation the following redox reaction takes place: I e 3I 2-2 In general, best efficiencies are obtained upon illumination from the TiO 2 side. However, the DSC is usually semi-transparent and may be illuminated from the platinum side as well. Figure 2-2: Chemical structure of the N719 dye molecule: RuL 2 (NCS) 2 with L = 2,2'- bipyridyl-4,4'-dicarboxylic acid Figure 2-3: Chemical structure of propyl-methyl-immidazolium iodide (PMII): a room temperature molten salt (ionic liquid) Energy diagram and working principle of a dye solar cell The energy diagram and electron transfer paths of a DSC are shown in Figure 2-4. The working principle of a DSC is based on the kinetics of the shown electron transfer reactions. Electrons are injected from the dye into the TiO 2 and the hole is injected into the electrolyte. Therefore, charge separation and charge transport occurs in different media and is spatially separated. By absorption of a photon, the dye molecule is excited. An electron is excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The excited electron is rapidly injected into the conduction band of the TiO 2. 11

20 2. Physics of the dye solar cell TCO TiO 2 Dye Electrolyte Pt TCO E CB e - e - D * /D + E F n light e V e - I - Energy D/D + E REDOX I 3 - E VB front electrode counter electrode e - e - external load Figure 2-4: Energy scheme and electron transfer paths of a dye solar cell. D is the ground state of the dye and D * the exited state. E CB and E VB are the energy of the conduction band and valence band of the TiO 2, respectively. E n F is the quasi-fermi level of the electrons in the conduction band of TiO 2. E REDOX is the redox energy of the redox electrolyte, with redox couple I - /I - 3. It should be noted, that TiO 2 is a semi conductor with a large band gap of 3.2 ev, corresponding to a wavelength of λ=39 nm. Accordingly, visible light is not absorbed by the TiO 2. Direct absorption by UV-light is unwanted, since the created holes in the valence band of the TiO 2 are highly reactive and produce side reactions in the electrolyte, which are destructive for the cell in long term operation [Kern '1]. Charge transport occurs in the conduction band of the TiO 2 by pure diffusion of electrons to the FTO electrode. Electric fields in the TiO 2 are screened by the cations in the electrolyte, which penetrates the pores of the TiO 2 on a nano-scale [Würfel '6]. Upon reaching the TCO electrode, the electrons are conducted to the counter electrode via the external circuit. Catalyzed by the platinum on the counter electrode, the electrons are accepted by the electrolyte. This means, that the holes in the electrolyte (the - I 3- ) recombine with electrons to form the negative charge carriers, I + 2e 3I 3. By diffusion, the negative charge (I - ) is transported back and reduces the oxidized dye molecule (D + ). Triiodide (I 3 - ) is formed and the electrical circuit is completed: + 2D + 3I I + 3 2D 2-3 From Figure 2-4 it is clear, that the negative pole of the DSC is at the TiO 2 coated TCO substrate and the positive pole is at the platinum-coated TCO substrate. 12

21 2. Physics of the dye solar cell Recombination losses in a dye solar cell Apart from these current-producing processes, loss processes occur in the DSC. An excited dye molecule may directly relax into its ground state, without injection of an electron into the TiO 2. This process is negligible, as injection is about 1 times faster [Nazeeruddin '93]. Also, electrons from the conduction band of the TiO 2 may recombine with the oxidized dye molecule, before the dye is reduced by the electrolyte. However, reduction by the electrolyte is about 1 times faster [Hagfeldt '95]. The most significant loss mechanism in the DSC is the recombination of TiO 2 conduction band electrons with the holes in the electrolyte, i.e. I 3 -. The electron transport by diffusion in the TiO 2, and their recombination with the electrolyte are the two competing processes in the DSC [Peter ']. It is important to realize, that for this reason the triiodide concentration in a DSC must be small. On the other hand, the triiodide concentration must be high enough as to provide enough recombination partners for the electrons at the platinum counter electrode. If this is not the case, the maximum current of the DSC will be diffusion-limited, i.e. limited by the diffusion of triiodide. Additionally, it should be noted, that the electrolyte penetrates the TiO 2 and is also in contact with the front FTO electrode. If the charge transfer resistance at the front FTO electrode would be the same as at the counter electrode, the DSC would indeed not operate properly. However, the charge transfer resistance at the counter electrode is reduced by many orders of magnitude by the platinum catalyst. Whereas the charge transfer resistance between pure, uncoated FTO and the iodide/triiodide redox couple is sufficiently high. This cannot be taken for granted, especially with redox couples other than iodide/triiodide. As optional layer in the architecture of a DSC, a very thin (<1 nm), dense TiO 2 layer may applied beneath the TiO 2. This layer is aimed to further suppress charge transfer from the front FTO to the electrolyte [Cameron '5]. Additionally, an optional light scattering layer may be applied on top of the TiO 2 in order to increase light harvesting [Hore '6]. Thus, transmitted light will be reflected back into the TiO 2 layer. This layer must be an insulator and porous, i.e. penetrable by the electrolyte. Such a scattering layer (e.g. ZrO 2 ) leads to an opaque (not transparent) DSC. 13