Semi transparent, organic tandem solar cells for window applications

Transcription

1 Semi transparent, organic tandem solar cells for window applications Master Thesis by Nicole Christine Klindtworth Supervisors: Morten Madsen Roana Melina de Oliveira Hansen In cooperation with Mads Clausen Institute SDU Sønderborg and NanoSYD

2 Acknowledgment I owe many people for their assistance, contributions and most importantly, encouragement. I am deeply grateful to all of them and the time that they have imparted, as their help is the major reason for the completion of this master thesis work. I will especially thank my supervisors Morten Madsen and Roana Melina de Oliveira Hansen, much appreciate to their theoretical instruction and practical guidance. Owing to their help in many aspects did I learn a lot, I am so delighted to have those profound mentors. I am also indebted to Yinghui Liu, as advice guide of the many experiments and theory in this work. I would like to thank André Luis Fernandes Cauduro, for his help as a guide in theory and experiment in the area of the material Molybdenum dioxide. Finally thanks, to my boyfriend Clemens Rudolf Schneidewind. Your influence can be found on every page of this master thesis. If it were not for your constant understanding and support, and belief in my abilities and potential, I would not be obtaining this degree. You remain my inspiration of my life and work, and I love you. Nicole Christine Klindtworth 1

3 Abstract Organic solar cells have the potential to provide low cost photovoltaic devices as a clean and renewable energy resource, offering flexibility and easy processing. In addition, by choosing different materials for the active layer, the solar cells can be tuned to colourful aesthetic semi transparent devices, which designers and architects can explore, e.g., for window designs. In this master thesis, we will try to enhance absorption in the active layers and thus efficiency of semi transparent devices by using tandem bulk heterojunction solar cell architecture. We will have a look on two difference active layer as the first solution blend will be made of PCPDTBT (donor) and PCBM (acceptor), whose absorption peak is at the red infrared part of the solar spectrum, while the second active layer will be made from P3HT (donor) and PCBM (acceptor), whose absorption peak is at the blue green part of the spectrum. As the absorption ranges of these two active layers are complementary, the resulting absorption will cover a wider spectral range, enhancing the power conversion efficiency. At the same time, the semi transparency of the solar cells will be optimized by controlling the thicknesses of the layers. To be sure that the solar cells are getting semi transparent the electrodes must also be semitransparent, and this will be made from ITO and from commercially available conductive polymers. 2

4 Table of contents Acknowledgment... 1 Abstract... 2 Table of contents... 3 Abbreviation... 5 Chapter 1: Introduction to the solar cells world The Sun How does it work?... 7 Chapter 2: Organic solar cells Introduction Why organic solar cells? Chapter 3: Fabrication of the organic solar cells Architecture of organic solar cells Introduction to the materials Cathode fabrication (bottom electrode) Material deposition Electron beam RF sputtering Thermal evaporated Spin coating Characterization The blend of the organic material for active layer P3HT:PC 60 BM PCPDTBT:PC 70 BM Anode fabrication (top electrode) PEDOT:PSS Chapter 4: Results and discussion Single solar cells P3HT:PC 60 BM solar cell PCPDTBT:PC 70 BM solar cell Tandem solar cell MoO x layer ZnO as top cell cathode TiO x as top cell cathode Cs 2 CO 3 as top cell cathode Transparency of solar cells Single solar cells

5 4.3.1 Tandem solar cell Encapsulation of solar cells Chapter 5: Conclusions and outlook Conclusions Outlook Bibliography List of figures List of Tables Appendix I: Resistivity measurements Appendix II: The process recipe of photolithography The Metal deposition Appendix III: The material for bottom electrodes Appendix IV: The process recipe for commercial ITO coated glass Appendix V: The process recipe for spin coating in Glovebox Appendix VI: Solar Cell Fabrication A. P3HT:PC 60 BM B. PCPDTBT:PC 70 BM C. Sandwich D. Tandem Appendix VII: Research solar cell efficiencies Appendix VIII: PCPDTBT:PC 60 BM temperature optimization Appendix IX: PCPDTBT:PC 60 BM annealing times optimization Appendix X: PCPDTBT:PC 60 BM acceleration speed optimization Appendix XI: PEDOT:PSS optimization Appendix XII: PCPDTBT:PC 70 BM Appendix XIII: PCPDTBT:PC 70 BM + 10mL chlorobenzene Appendix XIV: MoO X and HTL Solar Appendix XV: MoO X and PH Appendix XVI: Tandem solar cells with TiO X Appendix XVII: Tandem solar cells with Cs 2 CO Appendix XVIII: Encapsulation for the tandem solar cells with Cs 2 CO

6 Abbreviation A- Accepteor AFM- Atomic Force Microscopy BHJ- Bulk-Heterojunction BK7- Borosilicate Crown Glass C- Celsius CPP- 3,3-(2 Carboxypiperazine-4-yl)-Propyl-1-Phosphonate Cs 2 CO 3 - Caesium Carbonate D- Donor DC- Direct Current E-beam- Electron-beam FF- Fill Factor HOMO- Highest Occupied Molecular Orbital HTL- Hole Transport Layer IPA- Isopropyl Alcohol ITO- Indium Tin Oxide KS- Karl Suss LED- Light Emitting Diode LUMO- Lowest Unoccupied Molecular Orbital MoOx- Molybdenum Oxide NREL -National Renewable Energy Laboratory OPV- Organic Photovoltaic P3HT- Regioregular 3-hexylthiophene-2, 5-diyl 5

7 PC 60 BM- Fullerene [6, 6] Phenyl C61 Butyric Acid Methyl Ester PC 70 BM- Fullerene [6, 6] Phenyl C71 Butyric Acid Methyl Ester PCE - Power Conversion Efficiency PCPDTBT- Poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)- 4H-cyclopenta[2,1-b:3,4-b']dithiophene-2,6-diyl]] PEDOT:PSS- Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) PMMA- Poly (methyl methacrylate) PV- Photovoltaic PVD- Physical Vapour Deposition RF-sputtering- Radio Frequency Sputtering Root Mean Square- Sq RPM- Revolutions per Minute Scanning Probe Image Processor- SPIP SEM- Scanning Electron Microscopy SDU-Syddansk Universitet (English: University of Southern Denmark) SiOx- Silicon Oxide STC- Standard Test Condition Stdev- Standard deviation Ti- Titanium TiOx- Titanium Oxide UV- Ultraviolet ZnO- Zinc Oxide 6

8 Chapter 1: Introduction to the solar cells world The Sun as an energy source and how we can benefit from this energy will be the focus of this chapter. 1.1 The Sun The Sun is the main energy source for our planet, energy which is free and unlimited, while 50% of this energy is infrared radiation or heat, 40% visible light and 10% ultraviolet light. It is positioned in the centre of our solar system, approximately 1,496* m or one astronomic unit (1 UA) from Earth [1, 2]. Sun light is travelling at speed of 299,792,458 m/s and it takes around 8 minutes to reach the Earth atmosphere. The average power density of solar radiation above the Earth s atmosphere is I = 1367 W/m 2 [1, 2, 3, 4]. I represent the solar averaged irradiance and it is a constant, even though the real intensity changes because of the Earth s elliptical orbit around the Sun and due the solar activity. The amount of energy reaching the Earth s surface is a function of geographical latitude, meteorological conditions, time of the day and season [2, 4]. The energy released by the Sun is named solar radiation. The atmosphere is made by several chemical elements; it contains also water, dust, clouds vapors and the ozone layer [4]. After passing the atmosphere, the amount of sunlight reaching the Earth surface, at sea level, on a clear day is reduced to 1000 W/m 2, named irradiance [1, 5]. 1.2 How does it work? Solar cells are responsible to converse light from the Sun into electricity. Most of the inorganic solar cells that are used today are made from silicon. An inorganic solar cell consists of a positive (p type) and a negative (n type) layer of semiconductor materials. To achieve the different polarities, a process called doping is used. In order to get our p type silicon semiconductors diffusion of e.g. boron atoms into the silicon crystal lattice is performed. The Silicon atoms have four valence electrons whereas boron only has three valence electrons, so the boron doping results in an extra hole. For n type silicon semiconductors diffusion of e.g. phosphorus atoms into the silicon crystal lattice is made. And because the phosphorus atoms have five valence electrons, the results will be an extra electron (Figure 1). 7

9 Figure 1: Doped silicon crystal lattice e.g. phosphorus or boron [6] The p type and n type layers are placed together, forming a junction. Because of this electrons difference, there is an electric field across the junction between the layers [6]. The light received by the Sun is formed by photons. Each photon carries an amount of energy corresponding to it is wavelength [2, 3]. When a photon reaches a solar cell it can pass through, be reflected or absorbed by the silicon crystal. If the photons are absorbed, its energy is absorbed by an electron from the n type layer which travels to the p type layer. Because electrons are moving across the p n junction, the positive charge moves in the opposite direction around the load circuit [6] (Figure 2 and 3). Figure 2: PN junction [36] Figure 2, shows an n type and p type doped semiconductors there are connected and a depletion zone is created, as some electrons near the p n interface diffuse to the p type region and some holes 8

10 diffuse to the n type region. This depletion region creates an electric field opposing the diffusion process for both electrons and holes. Figure 3: Photons conversion process into electricity [6] 9

11 Chapter 2: Organic solar cells The aim of this master thesis is to create semi transparent tandem solar cell enhance absorption in the active layer and thus efficiency of semi transparent devices by using an bulk heterojunction solar cell architecture. For this project two different active layers will be used, P3HT:PCBM and PCPDTBT:PCBM, their absorption peak are in different part of the solar spectrum. The resulting absorption will cover a wider spectral range, enhancing the power conversion efficiency. To get a better understanding of the using of organic solar cells there will be a short introduction at the history of organic solar cells and their efficiencies will be the topics of this chapter. Also, how a bulk heterojunction solar cell works, which is a basic principle of organic solar cell, as well the main characteristics of a solar cell. 2.1 Introduction The organic solar cells have been playing more and more important roles in our era. Due to the progress in the past decades, it is well reasonable to believe that its bloom is forthcoming. The organic solar cells have are commonly used in plenty of fields, from windows to roofs, through external and internal walls [7]. Figure 4: New flexible solar modules are integrated into existing or new buildings [7] The rapidly growing high consumption of traditional fossil fuels results in inevitable global energy shortage, plus environmental pollution, which is also accompanied for global warming and it will 10

12 become more and more over time. People are eager to find renewable, clean and safe energy with high efficiency, instead of the current unclean energy, than any time in history. Since the Sun is the main energy source for our planet, providing free and unlimited energy, is its one of the best solution to use for clean and safe energy. Figure 5 shows the solar irradiance spectrum over the wavelength, for direct sunlight at the top of the atmosphere (the yellow part), and at the sea level (the red part) [7]. It shows nearly all solar electromagnetic radiation. This band of significant radiation power can be divided into five regions in increasing order of wavelengths: Ultraviolet C (100nm to 280nm), Ultraviolet B (280nm to315nm), Ultraviolet A (315nm to 400nm), Visible or light (380 to 780nm) it is also the strongest output range of the Suns total irradiance spectrum and infrared (700nm to 10 6 nm) [35]. Figure 5: Solar cell irradiance above Earth s atmosphere and at the sea level [7] Many researchers have in a long time investigated in the solar energy field; their results in this area can be seen in Figure 6. To get a better look of Figure 6 see Appendix VII. Figure 6: The Best Research Solar Cell Efficiencies from NREL [8] 11

13 Figure 6, from the National Renewable Energy Laboratory (NREL) (that focuses on creative answers to today's energy challenges) shows the research of the solar cell efficiencies from around 1975 to this year [8]. It shows the research of the solar cell efficiencies divided into different areas. 2.2 Why organic solar cells? The organic solar cell has the same idea of charge carrier generation as inorganic solar cells, but using organic semiconductors instead. Some advantages of organic solar cells over the inorganic solar cells are listed below. High flexibility it allows for the integration with elements of various shapes and sizes Customized and Integratable they can have various lengths and widths, thus rendering the technology very attractive for customized integrated solutions. Thin and Lightweight solar PV foils can be used for applications were weight is important. Unbreakable flexible solar PV cells are made of thin and flexible polymers, which are tough, durable and safe to use. Environmentally friendly the produced electricity is clean and the manufacturing process is based on abundant, recyclable materials [7]. Another significant difference between organic and inorganic solar cells is that organic semiconductors are amorphous and this makes a charge transport more difficult. Because of that organic solar cells have less efficiency than inorganic. This would be the main disadvantage for organic cells, but scientists are working hard to improve their efficiency. But how are these solar cells made? There are two main types of organic optoelectronic devices, namely light emitting diode devices (LED) and photovoltaic devices (PV) [7]. The organic layer is placed between two different electrodes. The most typical electrode materials are shown in Figure 7. Figure 7: Light emitting diodes or Photovoltaic device [7] 12

14 In the PV mode the electrons are collected at the metal electrode and holes are collected at the ITO electrode. The reverse happens in the LED mode, where the electrons are introduced at the metal electrode (cathode), which recombines with holes introduced at the ITO electrode (anode). Charge separation takes place in the organic phase, the anode and cathode being chosen to have largely different work functions, in order to enhance modest charge separation. However this configuration has a limiting factor. In order to fully absorb the incident light, the layer thickness of the absorbing material has to be of the order of the absorption length. In the 1990s the scientists introduced a new concept for organic cells called bulk heterojunction (BHJ). This approach has a distributed junction between the donor and acceptor materials and the interface between them is no longer planar but is spatially distributed. The Bulk heterojunction have the advantage of being able to dissociate excitons very efficiently over the whole solar cell, and thus can create polaron pairs anywhere in the film. However, it is more difficult to separate these polaron pairs due to the increased disorder; also it is more likely that trapped charge carriers will recombine with mobile ones [7]. Photons with the needed energy excite electrons from the valence band into the conduction band, starting to conduct electricity, as free charge carriers electrons move toward the n type layer, where a contact drives the electrons to an external circuit where they lose energy doing work for a load (e.g. light bulb). The electrons return to the valence band of the semiconductor material through a second contact which closes the circuit (Figure 8) [7]. Figure 8: Relation between the energy and the spatial boundaries [7] 13

15 Figure 9: Process of generation and recombination in BHJ solar cell [7] Figure 9 shows the most important processes in BHJ organic solar cell. The excitons are photogenerated, diffuse to a donor (polymer) acceptor (fullerene) junction and dissociate to a polaron pair (a) or recombine radiatively (b). If polaron pairs are generated, then they can also be separated with the help of an external electric field; the free polarons can then hop to the corresponding electrode and generate current (a) or recombine with other mobile or trapped charges (c) [7]. When solar radiation strikes a solar cell are not all the energy absorbed and converted into useful energy. This is because of some loss processes that take place in and on the solar cells [7], these loss processes are showing in Figure 10, (1) Non absorption of below band gap photons, (2) lattice thermalisation loss, (3) and (4) junction and contact voltage losses and (5) recombination loss. Figure 10: Loss processes in a standard solar cell [7] 14

16 For a solar cell these four quantities J SC, V OC, FF and are thus its key characteristics. With the help of solar cells I V curves and Figure 11 these key characteristics can be finding. Figure 11 shows the current density (black) and power density (gray) characteristics of an ideal solar cell. The solar cell delivers the largest output power at the point where the product of current and the voltage get maximized, J m and V m [7]. Figure 11: I V characteristic of a solar cell [7] The operating regime of the solar cell is in the range of bias, between V = 0 and V OC, in which the cell delivers power. And the cell power density P is: 1 P reaches a maximum (P max ) at the solar cell maximum power point. This occurs at the voltage V m with a corresponding current density J m as shown in Figure 11. The fill factor FF is thus defined as the ratio: 2 The maximum power density J m *V m = P max is given by the area of the inner rectangle (Figure 11) and the outer rectangle has area J SC *V OC. If the fill factor is equal to 1 or 100%, the inner rectangle follows the outer rectangle and then it is perfect solution. Efficiency ( ) is related to J SC and to V OC through the fill factor and the efficiency of a solar cell is the power density delivered at the operating point as a fraction of the power density of the incident light P in : 15

17 3 For effective comparison, all must thus be expressed under standard illumination conditions. The standard test condition (STC) for solar cells uses an incident power density of 1000 W/m 2, which is defined as the standard 1 Sun value, at an ambient temperature of 25 C. These give the power density of the incident light to be: 100 cm 2 In inorganic solar cells light is incident from the n type material side and if the energy of the photon is high enough it will free an electron, which in turn leaves a hole. The electron and the hole are later split by the internal electric field [16]. In organic solar cells the incident light excited electron is transferred from a p type hole conducting material onto the n type conducting material, therefore the notation of donor and acceptor with respect to the electron transfer has been introduced [16]. The bulk heterojunction concept was introduced by blending two polymers in a solution having donor property and acceptor property. There are two terms that have to be clarified: The first is the ionization energy and the second is the electron affinity. The ionization energy that comes from the highest occupied molecular orbital (HOMO), is the minimum energy required to remove an electron from an atom or molecule, while the electron affinity that can be added to the lowest unoccupied molecular orbital (LUMO), is the opposite where instead of removing an electron it add one [17]. The concept dictates that the HOMO level and the LUMO level of the donor material should be lower than that of the acceptor material otherwise it would not be able to donate an electron and accordingly the HOMO and LUMO of the acceptor material should be larger than the electron affinity of the donor otherwise the electron would recombine [17]. Figure 12, shows how the energy levels from a donor and an acceptor interface with their HOMO layer and LUMO layer can look out and the arrows denoting the movement of an electron through the two layers. Figure 12: The energy level in a donor and an acceptor interface (Own design) 16

18 Figure 13: Zoom in on the emerging PV efficiencies from NREL [8] Figure 13, shows a zoom in from Figure 6 on the emerging PV part; this shows the efficiencies over the years. For the area which are important for this project are the organic cells (the red point) and the organic tandem cells (the red triangle), can been seen that until now it has an efficiencies of respectively 11.1% for the organic cells and 12% for the tandem organic cells. 17

19 Chapter 3: Fabrication of the organic solar cells In order to produce organic solar cells, it is important to know how the organic materials relate and find the best way to mix them together to get the best result for the fabrication of organic solar cells. 3.1 Architecture of organic solar cells The single organic solar cell was based on single layer structures. In this case the improvements in organic solar cells operation and efficiency are due to the introduction of the bulk heterojunction architecture (blend of donor and acceptor molecules). For this two organic materials with different properties are used in order to structure the device, and sandwiched between two electrodes with different work functions (high work function for the anode and low work function for the cathode). The key difference between the previous single layer devices is that here exists an additional site where the exciton can be separated, this is the interface formed between the two materials, Figure 14. Anode Acceptor Donor Cathode Glass Figure 14: the architecture of organic solar cell (own design) In this chapter, the materials used in the different solar cell layers is presented, followed by the presentation of the research that was done in order to optimize the active layer, the cathode (bottom electrode) and anode (top electrode) is shown. 3.2 Introduction to the materials For this project a solution blend of PCPDTBT (D) and PC 70 BM (A) which gives us an absorption peak in the red infrared part of the solar spectrum, and a solution blend of P3HT (D) and PC 60 BM (A) which gives us a absorption peak in the blue green part of the solar spectrum. 18

20 Table 1, shows the organic molecules and their HOMO and LUMO level, as well their mobility, which was used for this project. Table 1: Organic molecules Material Molecular structure HOMO (ev) LOMO (ev) Mobility (cm2/vs) P3HT 5 3 (P type) PCPDTBT (P type) PC 60 BM (N type) 0.21 PC 70 BM (N type) 0.1 The values are used from Ref. [9] The mobility or electron mobility characterizes how quickly an electron can move through a semiconductor when it is pulled by an electric field. There is also a so called hole mobility, which stands for that of semiconductors, where there is an equal amount of holes. The term carrier mobility stands for generally for both electron and hole mobility in semiconductors [10]. The work function is the work that needs to be applied in order to remove an electron from an uncharged solid. Table 2 shows the work function for the different materials there were used in this project. 19

21 Table 2: Work function for material Material Atomic/Molecular structure Work Function (ev) Reference ITO Ti PEDOT:PSS MoO x Cs 2 CO ZnO PH HTL Solar ,25 20

22 At present day, ITO is the standard inorganic anode material and its chemical composition is In 2 O 3 /SnO 2. The typical weight distribution of In 2 O 3 and SnO 2 is 90% to 10% [15]. Titanium is used in many areas and even though it is not rare, it is still with a content of 0.565% at 9 place the element abundance in the continental crust. Titanium is extracted from ilmenite or rutile. [12]. The titanium are deposits by e beam and then the titanium oxides on air so it get a thin layer of TiO X on top of the layer of titanium. Since the discovery of conducting polymers (PEDOT:PSS) by H. Shirakawa, A.G. MacDiarmid and A.J. Heeger in 1977, research activities in the field of polymers have significantly increased. Many applications today use PEDOT in its polymerized cationic form with polystyrene sulfonic acid (PSS), which is called as PEDOT:PSS or PEDT:PSS [15]. 3.3 Cathode fabrication (bottom electrode) Since the objective is to make semi transparent solar cells semi transparent bottom electrodes are required. Different materials were investigated in order to achieve the best result. Since there are used different materials, there are also different approaches to deposit them. To find out if a material is suitable for use as a bottom electrode, the sheet resistance of the material was extracted by using a Four Point Probe Resistivity Measurement System. Figure 15, shows the installation of the Four Point Probe Resistivity Measurement System from SDU. Figure 15: Four Point Probe Resistivity Measurement System form SDU 21

23 To find out the resistivity of the materials, it was deposited on a piece of glass. Applying a current through the outer probes, the voltage can be measured by the inner probes and from the datasheet [21] the sheet resistance (R S ) can be extracted. Where the sheet resistance R S unit is given in Ω per square; U (in mv) is the voltage drop across the inner two probes; I (in ma) is the current flow between the outer two probes; ρ(rho) is the geometric factor for thin film measured on four point probe, which equals to ; this give us R S = U/I [21]. Appendix I, shows the measurements with the Four Point Probe Resistivity Measurement System for the different materials. The resistivity of commercial glass coated ITO has been measured just after purchase (New) and also after two months (Old), in order to verify changes in the resistivity. The Cryofox ITO was deposited be RF sputtering using the Cryofox Explorer 600 machine from SDU [22]. Appendix II, describes the process recipe for fabrication of bottom electrodes on a BK 7 glass wafer. Step 1 to 9 shows how the bottom electrodes are fabricated on the BK 7 glass wafer by photolithography and step 10 to 12 describes how the photoresist lifts off and forms the bottom electrodes, e.g. ITO/Ti deposition. The different techniques for depositing the investigated electrodes are described in the next section Material deposition Different materials for bottom electrodes were investigated in order to determine the optimum solution. Appendix III is a summary of the materials that have been investigated. The electrodes have to be defined on the commercial ITO coated glasses, and the process is described in Appendix IV: The step 1 shows a figure of the commercial ITO coated glass. In step 2 normal tape are place on the area where the bottom electrodes has to be, this should protect the ITO to be removed and in step 3, the non covered ITO is removed from the glass, with the help of Zinc powder and Hydrochloric acid. In step 4 the sample is cleaned with acetone and distilled water before there in step 5 remove the tape. Clean the glass again and then the commercial ITO bottom electrodes are ready for used. Additionally, Appendix III describes the difference methods used for bottom electrodes fabrication. Under Description are descript which of the material deposition systems was used for the different materials. Three of these depositions methods (E beam, RF sputtering and thermal evaporated) can be classified as Physical vapor deposition (PVD) [23]. 22

24 PVD is a variety of vacuum deposition methods used to deposit thin films, done by means of condensation of a vaporized form of the desired material on the wafer [23]. Figure 16 shows how the process for the PVD takes place. Source (Solid) Gas phase Solid phase (Changes in physical morphology) Electron beam Figure 16: PVD process flow diagram (own design from [23]) The electron beam is one of the PVD deposition methods. From Figure 17 can be seen an ingot solid of the material are placed on a water cooled hearth. The electrons from the electron source emitted from a heated thermionic filament are guided as a beam with the help of magnetic fields onto the material surface. When the electron beam hits the surface the material is heated up (and molt) and evaporates. The evaporated material is now traveling to the wafer where it forms a thin film [23]. The whole assembly resides in high vacuum conditions as well as where the evaporation takes place. The thickness of the material that gets on the wafer is monitored by a quartz microbalance and the deposition rate is regulated by the beam intensity [23]. Wafer Evaporated Molten Material Solid Material Electron source Figure 17: Illustration of an Electron beam deposition system (own design from [23]) 23

25 RF sputtering RF sputtering is also a PVD deposition method (see Figure 18), in which atoms of a solid (Target) is dissolved out by bombardment with energetic ions and pass into the gas phase. By the alternating field of the ions, in this case argon ion and electrons are accelerated alternately in both directions [23]. In order to obtain a thin film, the wafer is brought into the vicinity of the target so that the ejected atoms condense on this and form the thin film. To get this to work, the gas pressure in the process chamber must be very low, which the target atoms reach the wafer without colliding with gas particles [23]. There are different kinds of sputtering. In this project, RF sputtering is used. The difference in RFsputtering relation to the other is instead of the DC electrical field, a radio frequency alternating field is applied. The required RF voltage source is connected in series with a capacitor and the plasma. The capacitor serves to isolate the DC component and to maintain the plasma electrically neutral [23]. Sputtering gas (Argon) RF Target Plasma Electrode Wafer Heater Vacuum pump Figure 18: Illustration of an RF sputtering deposition system (own design from [23]) Thermal evaporated Thermal evaporation, which is illustrated in Figure 19, is a PVD deposition system. The solid material are placed in dimple boat resistance heater which is connected to a high current. With the help of this high current, can the dimple boat resistance heater and the source material on it to be heated to 24

26 temperatures near the boiling point. Here, resolved individual atoms evaporate and travel through the vacuum chamber. Because of the arrangement between the evaporation source and the wafer, the evaporated material hits the opposing cool down wafer and is deposited there. Thereby it forms on the wafer a thin film of the evaporated material [23]. Wafer Evaporated Solid Material High current ~ Figure 19: Illustration of a Thermal evaporation deposition system (own design from [23]) For this deposition the Edwards R500 electron beam evaporator was used with thermal evaporation capabilities from SDU, see Figure 20. Figure 20: The Edwards R500 25

27 3.3.2 Spin coating Spin coating is a procedure to deposit thin films on a wafer or a substrate. The used materials have to be in liquid form [24]. The wafer is fixed by vacuum suction on a rotating plate. With a metering device over the center of the wafer the desired amount of solution is applied. The acceleration speed and the time are being set at the spin coater, and the solution is uniformly distributed across the wafer surface. Any excess material is thrown off from the wafer [24]. In order to obtain a solid thin film it is necessary to anneal the now finished deposition. This is done with the help of a hot plate. The hot plate temperature and annealing time are key parameters for this step. Liquid material Wafer Sample holder Figure 21: Illustration of a spin coating deposition system (own design) Appendix V shows the description how to make the solar cell bottom electrodes by spin coating. The step 1 is cleaning a normal piece of glass. In step 2 the covering of the part of the glass, with tape, so it forms electrodes when it in step 3 spin coated the material on it. Step 4 are the annealing of the material before it in step 5 remove the tape. All of these steps are performed in the glovebox, in an inert Nitrogen environment. Appendix III under Description shows for the material there are used for this project the acceleration speeds, the acceleration times and the annealing times Characterization Different materials have been used as bottom electrodes (Appendix III) and single solar cells (P3HT:PC 60 BM and PCPTDBT:PC 70 BM) are made on this bottom electrodes to see which of them gives the best performance. 26

28 The Aluminium/Titanium bottom electrodes for solar cells give the best result (Table 3), however, this electrode only function as a non semi transparent reference. The reason why was to use them as reference to our next best result, namely the Cryofox ITO with TiO X (Table 6). The fact that the Aluminium/Titanium bottom electrodes gives a better result are that it reflect more of the incoming light back into the solar cell, increasing absorption. The attempts to used commercial ITO (see Table 4 and 5 for their result) was not so successful because of the used tape for the protection of our ITO not to be removed (see Appendix IV for the steps), which leave residual glue from the tape and small particles of the zinc powder on the ITO. Therefore, we have decided to use our Cryofox ITO instead. For the bottom electrodes number 5 to 8 described in Appendix III, it did not obtained successful results, and this is the reason why there are no table with their performances. The idea of using the PH1000 as bottom electrodes was taken from the literature [27]. In this article, the PH1000 is successfully used instead of ITO. But in the article they have use ZnO X on top of the PH1000, as electron conductive layer, while here only Cs 2 CO 3 and TiO X were used. The conclusion is that for the bottom electrodes fabrication it will be the Cryofox ITO and TiO X, because this give us the best device performance and it is semi transparent (see Appendix III (4)) for the fabrication. Table 3: The Aluminium/Titanium bottom electrode for solar cell P3HT mean stdev PCPDTBT mean stdev

29 Table 4: Commercial ITO with TiOx P3HT mean stdev PCPDTBT mean stdev Table 5: Commercial ITO with Caesium Carbonide P3HT mean stdev PCPDTBT mean stdev Table 6: Cryofox ITO with TiOx P3HT mean stdev PCPDTBT mean stdev

30 Appendix II shows the photolithography steps to make bottom electrodes (ITO/TiO X ). This are making on a BK 7, 4 glass wafer with a thickness on 500μm. After the fabrication the wafer gets cut in smaller pieces so the solar cells are with 7 electrodes and a dimension of 2 cm 1.5 cm. Before the using of the small glass pieces for fabrication of solar cells, there have to be cleaned. The first step is to clean them in acetone and IPA and then use the plasma cleaner [26], Figure 22. Metal surface with contaminations before plasma cleaning Carry off the contamination After the plasma cleaning without any contamination Figure 22: The process of a plasma cleaner [26] 3.4 The blend of the organic material for active layer For this project a solution blend of PCPDTBT (D) and PC 70 BM (A) which gives the absorption peak in the red infrared part of the solar spectrum, and a solution blend of P3HT (D) and PC 60 BM (A) which gives the absorption peak in the blue green part of the solar spectrum are used. To optimize the active layers there performed a systematic investigation P3HT:PC 60 BM The mixture of P3HT:PC 60 BM was prepared, based on previous experiments realized at NanoSYD, SDU. Literature shows that by mixing PC 70 BM with P3HT or PCPDTBT (instead of PC 60 BM), better solar cells are achieved, one with a mixing ratio 1:0.8 [28] and another mixing ratio 1:0.7 [29]. Therefore, PC 70 BM was tested in the experiments presented in this thesis. The decision was to make two mixtures, one with mixing ratio 1:1 and one with mixing ratio 1:0.8. Table 7, shows the performance from these two mixtures. It also shows that the mixtures was being measures two times, the reason for this was that the performance of a blends sometimes improves over time and also this time it improves. The result from Table 7 shows that the mixture P3HT:PC 60 BM give the better performances, showing in Table 8. Therefore, the conclusion is that P3HT:PC 60 BM are used in this project. 29

31 Table 7: P3HT:PC 70 BM mixture P3HT:PC70BM 1:1 14/03/2013 mean stdev P3HT:PC70BM 1:0.8 14/03/2013 mean stdev P3HT:PC70BM 1:1 03/05/2013 mean stdev P3HT:PC70BM 1:0.8 03/05/2013 mean stdev The verified 1:1 ratio are used and with the help of a precise weighting of each of the two materials (P3HT and PC 60 BM), 200mg of each where mixed into 10mL of 1,2 dichlorobenzene in a glass vial. P3HT 1,2-dichlorobenzene PC 60 BM Figure 23: The blend of P3HT:PC 60 BM (Own design) 30

32 Afterwards, the glass vial was sealed airtight, so that the material does not react with the air and glass vial was wrapped with aluminium foil to protect the material from the Sun. Next, the glass vial is placed in a glovebox and steered in a hot plate at acceleration speed 600 RPM and a temperature of 65, and leaved overnight PCPDTBT:PC 70 BM The verified 1:3 ratio [18] are used and with the help of a precise weighting, 100mg of PCPDTB T and 300mg PC 70 BM were mixed into 20mL of chlorobenzene in a glass vial and doped with 3 vol% diiodooctane [18]. PCPDTBT Chlorobenzene 3 vol% diiodooctane PC 70 BM Figure 24: The blend of PCPDTBT:PC 70 BM (Own design) The next step is the same as like for the P3HT material, the glass vial was sealed airtight and wrapped the glass vial with aluminium foil. The glass is steered in the glovebox overnight (600 RPM, 65 C). Both molecules PC 70 BM and PC 60 BM were used in this optimization. In order to optimize the active layer, three parameters can be changed: the annealing times, annealing temperature and the active layer thickness. As a first step, the PCPDTBT:PC 60 BM layer was deposited with a acceleration speed of 1500 RPM and annealed for 5 minutes, varying the annealing temperature for the different samples. Appendix VIII shows the performance result and that the best result for the temperature is 130 C. The next step for the optimization was to keep the temperature on 130 C and change the annealing time. Appendix IX shows the performances of the annealing time and only 5 minutes is enough time for the annealing time on the hot plate. 31

33 Now only one parameter is missing to being optimized, the active layer thickness, through the acceleration speed variation. Appendix X shows the performance results for the acceleration speed and the best result is 2000RPM. In conclusion, the best acceleration speed was 2000RPM, the annealing time was 5 minutes and the temperature was 130 C. For the optimizations of the PCPDTBT:PC 70 BM blends see the section 4.1.2: PCPDTBT:PC 70 BM solar cell. 3.5 Anode fabrication (top electrode) In section 2.2: Why organic solar cells?, are descript that for a single organic solar cell is in this case an active layer of organic material sandwich between two metal electrodes with different work functions. In this work, the PEDOT:PSS is the anode (top electrode) of the solar cells devices. This section will give more details about it PEDOT:PSS The deposition parameter for this material has to be optimized [30]. For that, three parameters are investigated, as for the active layer, namely annealing time, annealing temperature and layer thickness. The annealing time of 2.5 minutes and the temperature of 140 C, were previously optimized by researchers at NanoSYD. In this work, only the acceleration speed (thickness change) has to be optimized and the performances from this optimization is shown in Appendix XI. The best result is to have the acceleration speed of 1000RPM. The Poly(3,4 ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) has several functions, not only due to its hole transporting and electron blocking ability, but also can be used to smooth the electrodes surface and induce light due to its transparency [19]. Due to the transparency and conductivity, we used PEDOT:PSS as the top electrode in this project. 32

34 Figure 25: Chemical structure of PEDOT:PSS [15] The standard electron blocking layer of PEDOT, which is called CPP PEDOT, has a relevantly high hydrophobicity, that hinders the formation of a uniform thin layer on top of the active layer. For this reason it mix CPP PEDOT with a high conductivity version (HTL Solar [19]), which besides increasing the PEDOT mixture conductivity, also enhances its adhesion to the active layer [20]. HTL Solar CPP PEDOT Figure 26: The blend of PEDOT:PSS (Own design) HTL Solar and CPP PEDOT are mixed with 1:1 ratio and finally the PEDOT:PSS material glass vial get sealed airtight and put in to a ultrasonic water bath for 10 minutes to blend the mixture properly. 33

35 Chapter 4: Results and discussion In this chapter, the results for the fabricated and investigated devices are described. The result are split in this chapter in sub sections to get a better overview. The first section are about the single solar cells that are optimized prior to tandem solar cell fabrication. The second section are about the tandem solar cells that are investigated. The third section are about the transparency of the optimized single and tandem solar cells and the last section are about the encapsulation of solar cells. 4.1 Single solar cells Before fabrication of tandem solar cells, the single solar cells with both active layers were optimized separately, in order to determine the ideal parameters for these materials. In this section, the results for the single solar cells are presented and compared P3HT:PC 60 BM solar cell In section 3.4.1: P3HT:PC 60 BM blends of the P3HT:PC 60 BM used for the fabrication for the P3HT:PC 60 BM solar cells are described, and the fabrications step are shown in Appendix VI A. The mixture of P3HT:PC 60 BM was prepared, based on previous experiments realized at NanoSYD, SDU. In order to optimize the performances of the P3HT:PC 60 BM solar cell, only the changes on the annealing time from 5 minutes to 10 minutes were done and the obtained performances are shown in Table 8. Table 8: The performances of P3HT:PC 60 BM P3HT:PC 60 BM 5 min mean stdev P3HT:PC 60 BM 10 min mean stdev

36 6 Current density [ma/cm^2] Voltage [V] Figure 27: I V curve of a P3HT:PC 60 BM cell (annealing times 5 minutes) 5 Current density [ma/cm^2] Voltage [V] Figure 28: I V curve of a P3HT:PC 60 BM cell (annealing times 10 minutes) The conclusion are that the two different annealing times it gives us the same result within the stdev (Table 8). Figure 27 shows a I V curve of one cell from a P3HT:PC 60 BM solar cell with the annealing time of 5 minutes and Figure 28 with the annealing time of 10 minutes. 35

37 4.1.2 PCPDTBT:PC 70 BM solar cell The thickness of the active layer is important for the performed of a solar cell and therefore the acceleration speed has to be optimized, since different speeds result in different thicknesses. Therefore the active layers thicknesses were measured for different acceleration speeds. For the measurement a surface profilometer Dektak 150 from Veeco was used. Table 9 shows the thicknesses of the PCPDTBT:PC 60 BM and the PEDOT:PSS layer, always with the acceleration speed 1000RPM. It shows that the PEDOT:PSS not always has the same thickness on the active layer as expected. Table 9: Thickness measurement for PCPDTBT:PC 60 BM PCPDTBT:PC 60 BM Acceleration speed 1500RPM 2000RPM 3000RPM 5000RPM Active layer ~220nm ~150nm ~100nm ~60nm Active + PEDOT layer ~300nm ~210nm ~190nm ~130nm PEDOT (calculated) 80nm 60nm 90nm 70nm Also for the PCPDTBT:PC 70 BM solar cell measurements was made to find out the thickness of the active layers (Table 10). The first attempts used blends of PCPDTBT and PC 70 BM with a verified 1:3 ratio mixed into 10mL chlorobenzene, but these blends were not very semi transparent and therefore the optimization of the active layer thicknesses, by varying the acceleration speeds was made. Table 10 shows the results and it can be seen that this blend results gives thicker solutions then for the PCPDTBT:PC 60 BM blend with same acceleration speed (Table 9). Therefore, the PCPDTBT:PC 70 BM blends were mixed with additional 10mL chlorobenzene and this has given us a more transparency result, see Table 10. Table 10: Thickness measurement for PCPDTBT:PC 70 BM PCPDTBT:PC70BM Acceleration speed 1500RPM 2000RPM 3000RPM 4000RPM Active layer ~250nm ~220nm ~200nm ~180nm PCPDTBT:PC70BM+10mL chlorobenzene Acceleration speed 1500RPM 2000RPM 3000RPM 5000RPM Active layer ~200nm ~150nm ~100nm ~50nm The performance of the solar cells with the blends of PCPDTBT:PC 70 BM and their different thicknesses are shows in Appendix XII and Appendix XIII shows the results for the solar cells made of 36

38 PCPDTBT:PC 70 BM with the additional 10mL chlorobenzene, which improves both the electrical performance of the solar cells and its semi transparency. The finally fabrication steps for the PCPDTBT:PC 70 BM solar cell are showing in Appendix VI B and the best result for the PCPDTBT:PC 70 BM are there with acceleration speed of 5000RPM and the performance are listed in Table 11. Figure 29 shows the I V curve for this a single cell. Table 11: The performance of PCPDTBT:PC 70 BM Voc (V): 0.48 ±0.04 Jsc (ma/cm2): 3.17 ±0.42 FF (%): ±3.19 PCE (%): 0.50 ± Current density [ma/cm^2] Voltage [V] Figure 29: I V curve of a PCPDTBT:PC 70 BM cell 37

39 4.2 Tandem solar cell A tandem solar cell is composed of two or more solar cells with different materials, which are stacked on one another monolithically. The purpose of this arrangement is to increase the efficiency of the entire assembly. Thereby a broader spectrum of sunlight can be absorbed compared to a single cell, which leads to a higher efficiency. A so called sandwich solar cell was fabricated to see if it was possible to mount this two single solar cells (P3HT:PC 60 BM and PCPDTBT:PC 70 BM) together and used for the cathode (bottom and top cell) ITO/TiOx, solving the issue of depositing ITO on the organic layers, since the sputtering process damages the PEDOT:PSS and active layer. The fabrication steps of the sandwich solar cell are shown in Appendix VI C. But this did not work since the two single solar cells were easily released from each other again and therefore, this approach was discarded. However, the main issue, which was to optimize the cathode fabrication for the top cell, therefore, different materials was tested as material candidates for the cathode of the top cell, which has to be deposited on top of the organic layers of the bottom cell. The following sections describe the materials and result from tandem solar cells using this material as cathode MoO x layer A transparent layer of MoO X was applied both as buffering to prevent sputtering damage to the organic active layer [31] and as a hole conducting transport interlayer. The MoO X is used as the middle part in the tandem devices [32]. The MoO X thin films were deposited by system thermal evaporation, descripted in section : Thermal evaporation. The first step was to optimize the thicknesses of the layer and its morphology, in order to evaluate the deposition conditions. The tooling factor for the thermal evaporation system was determined by deposition of different layer thicknesses. The uniformity of the layer is estimated by surface roughness values extracted from the AFM images, measured by use of the Dimension TM 3100 Atomic Force Microscopy (AFM), SDU. 38

40 a) b) c) Figure 30: 10µm 10µm AFM images of MoO X a) 14nm thickness of MoO X b) 22nm thickness of MoO X c) 44nm thickness of MoO X. Figure 30 shows AFM images of MoOx films with various thicknesses. The roughness of the surface of the AFM images was extracted by using a software called Scanning Probe Image Processor (SPIP) which calculates the root mean square (Sq) values of the numerical vertical height of the surface to present the roughness level. Figure 30: a, shows the AFM image of a surface with 14nm MoO X and the Sq is 1.63nm got from SPIP. b, shows the AFM image of a surface with 22nm MoO X and the Sq is 0.85nm got from SPIP and c, shows the AFM image of a surface with 44nm MoO X and the Sq is 0.61nm got from SPIP. If the thickness of MoO X is too low, clusters are formed on the surface, which makes it not suitable for the application required here. For thickness larger than 44nm, the thin film of MoO X starts to get a low roughness; therefore, the minimum thickness used in these experiments was 50nm. Figure 31, shows the MoO X layer (50nm or 100nm) as protection layer for the organic active layer and for the subsequent sputtering of 50nm ITO. 39

41 Figure 31: Illustration of the MoO X on PEDOT:PSS and with ITO Table 12, shows the performances of the MoO X and ITO deposited on the single PCPDTBT:PC 70 BM solar cell (Figure 31). The performances of the single solar cell with MoO X decrease a little, but after the deposition of ITO it decreases significantly. Table 12: Optimization of MoOx PCPDTBT:PC 70 BM with 50nmMoOx mean stdev PCPDTBT:PC 70 BM with 50nm MoOx and 50nm ITO mean stdev PCPDTBT:PC 70 BM with 100nm MoOx and 50nm ITO mean stdev The second approach was to use the MoO X layer instead of the PEDOT:PSS, as the MoO X can be used as a substitute for the PEDOT:PSS (see Figure 32). To find out if it is better to deposits the MoO X on the active layer without using of PEDOT:PSS, a layer of HTL Solar (Appendix XIV) or PH1000 (Appendix XV) was spin coated on the top of the MoO X since 40

42 they are not an electron blocking layer. The thicknesses of the MoO X layer are 50nm or 70nm. The result from these tests they are rather conductive electrodes and therefore, give the bad result. Figure 32: Illustration of the MoO X with HTL Solar or PH1000 The thickness of 50nm MoO X, 50nm ITO and 8nm TiOx was used in the tandem cells, as shown in Figure 33 and the performance are shown in Table 13. Figure 33: Illustration of the tandem with MoO X It looks like that the sputtering process slightly damages the active layer for the voltage give a low result (see Table 13), and also only few of the cells was working. Figure 34 shows the I V curve of one of the cells that works and it looks more like a straight line. 41

43 Table 13: Tandem solar cell with 50nm MoOx, 50nm ITO and 8nm TiOx Voc (V): 0.21 ±0.01 Jsc (ma/cm2): ±1.10 FF (%): ±0.90 PCE (%): 1.17 ± Current density [ma/cm^2] Voltage [V] Figure 34: I V curve of the tandem cell The first step was to deposit ITO on MoO X in two steps: the first one with a low deposition rate and the second one with a higher deposition rate, in order to avoid damaging of the organic layers. In the literature, the first layer (20 nm of ITO) was deposited at a sputtering power density of 0.05 W/cm2, followed by a further 40 nm layer of ITO prepared at a sputtering power density of 0.3 W/cm2 [31]. Since the sputtering power density cannot be changes in the Cryofox Explorer 600 the deposition rate was changed instead. The first layer of 20 nm ITO was deposited at a deposition rate of 0.2nm/s, followed by the deposition of 40 nm ITO with a deposition rate of 0.7nm/s. Table 14, shows the electrical performance of the tandem solar cell with a 50nm MoO X layer and the two different layer of ITO (as described above), but the results are not satisfactory. Also here only few of the cells were working. Figure 35 shows the I V curve of one of the cells that works, but this time it looks more like an I V curve that is expected of a solar cell. 42

44 Table 14: Tandem with 50nm MoO X 2 layers of ITO and 8nm TiO X Voc (V): 0.16 ±0.03 Jsc (ma/cm2): 0.55 ±0.14 FF (%): ±0.93 PCE (%): 0.03 ± Current density [ma/cm^2] Voltage [V] Figure 35: I V curve of a single cell In conclusion, the use of MoO X in the tandem solar cell does not result in satisfactory results. This can be attributed to the non uniformity of the MoOx layer which fails in protecting the organic layers from the bottom solar cell during the ITO sputtering ZnO as top cell cathode ZnO is a material widely used in the literature about organic solar cells, including tandem solar cells. This material has a low work function, and therefore acts as a hole blocking, electron transport layer and has been used as an interlayer for organic solar cells [33, 34]. Here, this material has been investigated as cathode for the top cell. The tandem solar cell structure is shown in Figure 36. A solution containing ZnO nanoparticles is spin coated with an acceleration speed on 1500RMP [34] and annealed for 10 minutes at 150 C [33]. 43

45 Figure 36: Illustration of the tandem with ZnO X The concentration of nanoparticles on the spin coated solution was the first parameter to be optimized. A pure solution of ZnO (NanoArc ZN 2225, 40% in 1,2 propanediol monomethyl ether acetate, colloidal dispersion with dispersant) or the solution dissolved in a solvent (1,2 Propanediol monomethyl ether acetate, 99%, stab. with 50ppm BHT) with different ratios was tested (1:1 and 1:9, own selected). The morphology of the ZnO layers spin coated on a glass substrate was investigated by AFM and are shown in Figure 37. Three different concentrations were measured: pure solution (40%), 1:1 (20%) and 1:9 (4%). a) b) c) Figure 37: 10µm 10µm AFM 3D images of samples with ZnO a) pure solution b) 1:1 c) 1:9 44

46 The roughness was extracted from the AFM images with SPIP software. The Sq for the the pure ZnO (Figure 37 a) is 108nm, for the mixture of the ZnO and the solving with a verified 1:1 ratio (Figure 37 b) 65.6nm and for the mixture of the ZnO and the solving with a verified 1:9 ratio (Figure 37 c) 61nm. The conclusion is that the roughness of the layer is too high for using as cathode on a solar cell. In order to optimize it, the annealing times after ZnO spin coated were increased from 10 minutes to 20 minutes and also a change on the temperature from 150 C to 200 C was performed, but only for the pure ZnO and the mixture with the verified 1:9 ratio, because only this two was given a performances for the tandem solar cells (annealed for 10 minutes for 150 C). AFM measurements were performed on the new set of samples, annealed for 20 minutes. The roughness for the sample spanned from the pure ZnO solution is 54.7nm and for the 1:9 mixture is 58nm. The measurements were repeated also for the layers annealed at 200 C for the pure ZnO, giving a roughness of 61.2nm and for the 1:9 mixture the found roughness was 87nm. The samples annealed for 20 minutes for 150 C have a lower roughness as the previous samples and have therefore been used in the tandem cells. Table 15 shows the electrical performance of the tandem solar cells with the pure ZnO annealed for 10 and 20 minutes and the mixture of ZnO and the solving mixture ratio 1:9, also with the annealing time of 10 and 20 minutes. The performances are still too low and therefore not usable; this also stems the roughness of ZnO. Table 15: Electrical performance of the ZnO tandem solar cells Tandem: ZnO 10min mean stdev Tandem: ZnO 20min mean stdev Tandem: ZnO/solving 1:9 10min mean stdev Tandem: ZnO/solving 1:9 20min mean stdev

47 4.2.3 TiO x as top cell cathode Since the experiments with the other material have failed, titanium dioxide was tested instead. In these experiments, the order of the top and bottom cells (P3HT:PC 60 BM and PCPDTBT:PC 70 BM) was investigated, by fabricating two different cells, with opposite bottom and top cells. The results from these experiments are presented in Appendix XVI and the best result are given by having the P3HT:PC 60 BM as the bottom solar cell and PCPDTBT:PC 70 BM as the top solar cell. The cross section of this device is shown in Figure 38, and the electrical performances are presented in Table 16. Figure 38: Illustration of the tandem solar cell with TiO X Table 16: The characterization of the tandem with TiO X Voc (V): 0.39 ±0.08 Jsc (ma/cm2): 1.62 ±1.04 FF (%): ±3.79 PCE (%): 0.13 ±

48 Current density [ma/cm^2] Voltage [V] Figure 39: I V curve of a single cell As can be seen from Table 16, the efficiencies are still very low, and this can be attributed to that the thermal evaporation of TiO X damaged the active layer, since the process for the thermal evaporation heats up the sample, damaging the active layer, too prove this a test setup was made (Figure 40). Table 17 shows the results from a single solar cell after thermal evaporation of TiO X, as illustrated in Figure 40. The result from this test setup should have giving the same as for the single solar cells, see section 4.1: Single solar cells, it is not the case and therefore, it proves that the thermal evaporation of TiO X damaged the active layer. Figure 40: The test setup of the thermal evaporation of TiO X 47

49 Table 17: Investigation of TiO X P3HT:PC 60 BM PEDOT:PSS TiOx mean stdev PCPDTBT:PC 70 BM PEDOT:PSS TiOx mean stdev Cs 2 CO 3 as top cell cathode Since the tandem solar cell with the TiO X as top cell cathode does not give good solar cell performances, the next material to be tried was caesium carbonate (Cs 2 CO 3 ). As described in the previous section, the order of the bottom and top solar cells were systematically changed in order to determine which cell should be fabricated on top (Appendix XVII), and the results show that the optimum case occurs when the P3HT:PC 60 BM is the bottom active layer and PCPDTBT:PC 70 BM is the top active layer (Figure 41). Figure 41: The finally design of the tandem solar cell with Cs 2 CO 3 48

50 Table 18 shows the electrical characterization for the tandem solar cell with the material Cs 2 CO 3, spin coated with an acceleration speed of 800RPM for 45secnds and annealed for 20 minutes at 130 C. Figure 42, shows the I V curve of one of the tandem cells. Table 18: The characterization of the tandem with Cs 2 CO 3 Voc (V): 0.56 ±0.03 Jsc (ma/cm2): 2.19 ±0.44 FF (%): ±2.49 PCE (%): 0.47 ± Current density [ma/cm^2] Voltage [V] Figure 42: I V curve of one single cell Since this tandem gives the best result is its fabrications step of these tandem solar cell devices shown in Appendix VI D. These are the optimum materials investigated in this research. 4.3 Transparency of solar cells The main part of this project are to made semi transparent solar cells. In this case a measurement of the transparency of the finally optimized single and tandem solar cell was made. To measure the transparency the TFProbe spectroscopic ellipsometer was used, it sent from a light source visible light through the solar cell and the result are recorded for the detector. 49

51 4.3.1 Single solar cells Section 4.1: Single solar cells descript the optimizations and fabrications of the finally two single solar cells there are used for this project. Figure 43 shows the picture of the P3HT:PC 60 BM solar cell and Figure 44 shows the picture of the PCPDTBT:PC 70 BM solar cell, in this two pictures can been see that there are semi transparent, since the text are readable through the solar cells. Figure 45 shows the transparency measurement of these two single solar cells, transmission in percent over the wavelength. 100% transmissions are the transparency of glass and 0% are no transparency. The result shows that the PCPDTBT:PC 70 BM solar cells are more transparent as the P3HT:PC 60 BM solar cells and this also depends on their active layer thickness. Figure 43: Picture of the P3HT:PC 60 BM solar cell Figure 44: Picture of the PCPDTBT:PC 70 BM solar cell 50

52 Figure 45: Transparency of the single solar cells Tandem solar cell Section 4.2.4: Cs 2 CO 3 as top cell cathode descript the optimizations and fabrications of the finally tandem solar cells there are used for this project. Figure 46 shows the picture of this tandem solar cell and it also shows that it is semi transparent, since the text is readable through the solar cells. Figure 47 shows the transparency measurement of the tandem solar cells and the result are that the tandem solar cell are not so transparency as the single solar cells, however this is understandable since the tandem solar cell are composed of these two single solar cells. Figure 46: Picture of the tandem solar cell with Cs2CO3 as top cell cathode 51

53 Figure 47: Transparency of the tandem solar cell 4.4 Encapsulation of solar cells One of the main problems of organic solar cells is the degradation due to oxygen and moisture, which significantly reduces the lifetime. The encapsulation of bulk heterojunction solar cells can prevent this. Therefore, the tandem solar cells fabricated in this project also had to be encapsulated. For the encapsulation with the glue DELO KATIOBOND LP VE , the material has been spin coated on the tandem solar cell with an acceleration speed of 4000RPM for 45 seconds. The sample was illuminated under 1 Sun with a standard solar generator setup for one hour. Another kind of encapsulation was tested, using PMMA and SiO X. The PMMA used for this encapsulation is called PMMA 950 A4 from NanoChem and was spin coated on the tandem solar cell with an acceleration speed of 2000RPM for 30 seconds and annealed for 4 minutes at 90 C. The SiO X (50nm) was deposited by RF sputtering by using the Cryofox Explorer 600. Appendix XVIII shows the electrical performances of the tandem solar cells with these two different encapsulation methods, measured before and after constant illumination. As can be seen in the results, the tandem solar cells are decreases due of the encapsulation processes, and therefore, alternative encapsulation methods should be investigated in future work, otherwise, these two encapsulation methods functioning which shows the increase of the performance from the tandem solar cells after the illumination. 52

54 Chapter 5: Conclusions and outlook This master thesis has focused on understanding of the architecture of the organic bulk heterojunction for single and tandem solar cells and increasing the devices efficiency through controlling the organic materials with the ultimate goal of achieving a improve the power conversion efficiency. 5.1 Conclusions Organic solar cells are promising due to their low material and processing costs, flexibility, low weight and mechanical durability. The control over the structuring of the active layer in the organic solar cells has been investigated over the years. For this project the organic material P3HT (D), PCPDTBT (D), PC 60 BM (A) and PC 70 BM (A) was used and investigated for single solar cells. Before fabrication of tandem solar cells, the single solar cells with both active layers were optimized separately, in order to determine the ideal parameters for these materials. The first step for this project was to find and optimized for the single cells they cathodes (bottom electrodes), therefore, the investigation of different semi transparent materials was made. The result was that the bottom electrodes materials for the single solar cell are by used of the Cryofox Explorer 600, by RF sputtering deposits 50nm of ITO and by e beam deposits 8nm of TiOx. In this project the anode (top electrode) of the solar cells devices are the PEDOT:PSS, a mixture out of HTL Solar and CPP PEDOT (1:1 ratio). After the conclusion of the cathode (bottom electrodes), the Cryofox ITO and TiOx, are then used for the single solar cells and the optimization of the P3HT:PCBM and PCPDTBT:PCBM was made with this cathode. For the P3HT:PCBM solar cell the best performance with the PC 60 BM prepared, based on previous experiments realized at NanoSYD, SDU. Literature shows that by mixing PC 70 BM with P3HT or PCPDTBT better solar cells are achieved, therefore, the investigation for the optimization of active layer by used of the PC 70 BM was made. The performance of the P3HT:PCBM solar cell with the PC 70 BM was not giving a better result as there with the PC 60 BM, therefore, the mixture P3HT and PC 60 BM (1:1 ratio) was used and the three parameters, namely layer thickness (get from the acceleration speed), annealing time and annealing temperature, gives the best performance if the acceleration speed was 1500RPM, the annealing time was 10 minutes and the temperature was 140 C. Also for the PCPDTBT:PCBM solar cell was mixture with the PC 60 BM or PC 70 BM, but for this active layer the PC 70 BM gives the best performance. The PCPDTBT and PC 70 BM was mixture with 1:3 ratio 53

55 and doped with 3 vol% diiodooctane. The best performance was if the acceleration speed was 5000RPM, the annealing time was 5 minutes and the temperature was 130 C. For the tandem solar cell the cathode of the top cell has to be investigated, since the ITO deposition by sputtering on top of the organic layers causes damage. Therefore, different materials were tested as material candidates for the cathode of the top cell. The material there give the best performance result as the cathode of the top cell was the caesium carbonate (Cs 2 CO 3 ). The order of the bottom and top solar cells were systematically changed in order to determine which cell should be fabricated on top and the results show that the optimum case occurs when the P3HT:PC 60 BM is the bottom active layer and PCPDTBT:PC 70 BM is the top active layer. The main part of this project was to made semi transparent solar cells and the transparency for the single solar cells and the tandem solar cell, there was made for this project, was successful. The fast degradation hinders the practical use of the cells and therefore an encapsulation with a high barrier effect has to be applied in order to increase their stability. It had been found that an encapsulation with PMMA/SiOx prologues the lifetime of the solar cells and a very simple method of encapsulating the solar cells has been found by using the commercial product from DELO adhesives (the glue), but this are only investigated and optimized for the single solar cells and therefore, it have to be optimized for the tandem solar cell. 5.2 Outlook The encapsulation for the tandem solar cell have to be optimized either with the encapsulation there already are been used and works for the single cells or investigate of a new encapsulation method. Finally a shelf lifetime research should be done with accelerated life time testing, to find out where the limits of the current encapsulations are. ZnO is a material widely used in the literature about organic solar cells, including tandem solar cells. This material has a low work function, and therefore acts as a hole blocking, electron transport layer and has been used as an interlayer for organic solar cells. Here, this material has been investigated as cathode for the top cell, but it needs to be more optimized and also more investigated. For the optimization investigate more in for the three parameters, namely annealing time, annealing temperature and layer thickness. Otherwise, use a different transparent material as anode, instead of PEDOT:PSS, for the bottom cell, with the same criteria as MoOx. Maybe also make for the MoOx material more optimization. Get a better control over the deposit thickness of the MoOx layer, hoping to get an increasing of the Opencircuit voltage (V OC ). 54

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59 List of figures Figure 1: Doped silicon crystal lattice e.g. phosphorus or boron [6]... 8 Figure 2: PN junction [36]... 8 Figure 3: Photons conversion process into electricity [6]... 9 Figure 4: New flexible solar modules are integrated into existing or new buildings [7] Figure 5: Solar cell irradiance above Earth s atmosphere and at the sea level [7] Figure 6: The Best Research Solar Cell Efficiencies from NREL [8] Figure 7: Light emitting diodes or Photovoltaic device [7] Figure 8: Relation between the energy and the spatial boundaries [7] Figure 9: Process of generation and recombination in BHJ solar cell [7] Figure 10: Loss processes in a standard solar cell [7] Figure 11: I V characteristic of a solar cell [7] Figure 12: The energy level in a donor and an acceptor interface (Own design) Figure 13: Zoom in on the emerging PV efficiencies from NREL [8] Figure 14: the architecture of organic solar cell (own design) Figure 15: Four Point Probe Resistivity Measurement System form SDU Figure 16: PVD process flow diagram (own design from [23]) Figure 17: Illustration of an Electron beam deposition system (own design from [23]) Figure 18: Illustration of an RF sputtering deposition system (own design from [23]) Figure 19: Illustration of a Thermal evaporation deposition system (own design from [23]) Figure 20: The Edwards R Figure 21: Illustration of a spin coating deposition system (own design) Figure 22: The process of a plasma cleaner [26] Figure 23: The blend of P3HT:PC 60 BM (Own design) Figure 24: The blend of PCPDTBT:PC 70 BM (Own design) Figure 25: Chemical structure of PEDOT:PSS [15] Figure 26: The blend of PEDOT:PSS (Own design) Figure 27: I V curve of a P3HT:PC 60 BM cell (annealing times 5 minutes) Figure 28: I V curve of a P3HT:PC 60 BM cell (annealing times 10 minutes) Figure 29: I V curve of a PCPDTBT:PC 70 BM cell Figure 30: 10µm 10µm AFM images of MoO X a) 14nm thickness of MoO X b) 22nm thickness of MoO X c) 44nm thickness of MoO X Figure 31: Illustration of the MoO X on PEDOT:PSS and with ITO Figure 32: Illustration of the MoO X with HTL Solar or PH Figure 33: Illustration of the tandem with MoO X Figure 34: I V curve of the tandem cell Figure 35: I V curve of a single cell Figure 36: Illustration of the tandem with ZnO X Figure 37: 10µm 10µm AFM 3D images of samples with ZnO a) pure solution b) 1:1 c) 1: Figure 38: Illustration of the tandem solar cell with TiO X Figure 39: I V curve of a single cell Figure 40: The test setup of the thermal evaporation of TiO X Figure 41: The finally design of the tandem solar cell with Cs 2 CO

60 Figure 42: I V curve of one single cell Figure 43: Picture of the P3HT:PC 60 BM solar cell Figure 44: Picture of the PCPDTBT:PC 70 BM solar cell Figure 45: Transparency of the single solar cells Figure 46: Picture of the tandem solar cell with Cs2CO3 as top cell cathode Figure 47: Transparency of the tandem solar cell List of Tables Table 1: Organic molecules Table 2: Work function for material Table 3: The Aluminium/Titanium bottom electrode for solar cell Table 4: Commercial ITO with TiOx Table 5: Commercial ITO with Caesium Carbonide Table 6: Cryofox ITO with TiOx Table 7: P3HT:PC 70 BM mixture Table 8: The performances of P3HT:PC 60 BM Table 9: Thickness measurement for PCPDTBT:PC 60 BM Table 10: Thickness measurement for PCPDTBT:PC 70 BM Table 11: The performance of PCPDTBT:PC 70 BM Table 12: Optimization of MoOx Table 13: Tandem solar cell with 50nm MoOx, 50nm ITO and 8nm TiOx Table 14: Tandem with 50nm MoO X 2 layers of ITO and 8nm TiO X Table 15: Electrical performance of the ZnO tandem solar cells Table 16: The characterization of the tandem with TiO X Table 17: Investigation of TiO X Table 18: The characterization of the tandem with Cs 2 CO

61 Appendix I: Resistivity measurements Spacing between probes: 0.04 inch (1.016mm) Rs=4.5324*U/I U (mv) I (ua) Rs (Ω 2 ) Mean Rs(Ω 2 ) STDEV New Commercial ITO E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E+01 Old Commercial ITO E E+01 Cryofox ITO E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E+02 Commercial ITO/TiOx E E E

62 Cryofox ITO/TiOx E E+02 Gold E E+03 PH E E+05 PEDOT:PSS E E+02 HTL solar E E E E E E E E E E E

63 Appendix II: The process recipe of photolithography STEP PROCESS DESCRIPTION CONFIGURATION 1 Wafer type Glass wafer, BK 7, 4, 500μm thick, double side polished 2 Deposit Adhesion Promoter HMDS processing, for getting good photoresist adhesion in the following step. 3 Spin coating Photoresist EBS11 spin coater, Photoresist: AZ 5214E, thickness = 1.5 μm; 1. Automatic resist dispense for about 4s 2. Spin at 500RPM for 5s 3. Spin at 4000RPM for 30s 4 Prebake Hot plate 90 C for 60 seconds Hotplate 5 UV Exposure KS mask aligner, exposure time for 20 seconds UV 6 Inversion Bake Hot plate 140 C for 120 seconds 7 Flood Exposure KS mask aligner, exposure time for 120 seconds Without mask Hotplate UV 8 Development AZ 351B mix with DI water, ratio 1:4 Agitation for 60 seconds 9 Wash and Airblow Drying Rinse in water (fine rinse bath) for 2 min Spin dry 62

64 The Metal deposition STEP PROCESS DESCRIPTION CONFIGURATION 10 Metal deposition The metal deposition depends on which of the bottom electrodes we would use 11 Lift off Put the sample into Acetone for a certain while to remove the photoresist until only the electrode are back 12 Wash and Airblow Drying Rinse in water (fine rinse bath) for 2 min Spin dry 63

65 Appendix III: The material for bottom electrodes Bottom electrode BK7 glass wafer Bottom electrode BK7 glass wafer 1. The Aluminium/Titanium bottom electrode for solar cell LAYER MATERIAL DESCRIPTION CONFIGURATION (from the bottom up) 1 Titanium Deposited 3nm with E beam 2 Aluminium Deposited 80nm with E beam 3 Titanium Deposited 20nm with E beam 2. Commercial ITO with TiOx LAYER MATERIAL DESCRIPTION CONFIGURATION (from the bottom up) 1 ITO Commercial ITO coated glass, to get to this step see Appendix IV 2 Titanium Deposited 8nm with E beam 64

66 3. Commercial ITO with Caesium Carbonide LAYER MATERIAL DESCRIPTION CONFIGURATION (from the bottom up) 1 ITO Commercial ITO coated glass, to get to this step see Appendix IV 2 Caesium Carbonide Glovebox 1. Spin coat with an acceleration speed 800RPM for 45 seconds 2. Hot plate 130 C for 20 min 4. Cryofox ITO with TiOx LAYER MATERIAL DESCRIPTION CONFIGURATION (from the bottom up) 1 ITO Deposited 50nm with RFsputtering 2 Titanium Deposited 8nm with E beam 5. HTL Solar LAYER MATERIAL DESCRIPTION CONFIGURATION (from the bottom up) 1 HTL Solar Glovebox 1. Spin coat with an acceleration speed 1000RPM for 45 seconds 2. Hot plate 140 C for 5 min 6. PH1000 LAYER MATERIAL DESCRIPTION CONFIGURATION (from the bottom up) 1 PH1000 Glovebox 1. Spin coat with an acceleration speed 1000RPM for 45 seconds 2. Hot plate 120 C for 30 min 65

67 7. PH1000 with Caesium Carbonide LAYER MATERIAL DESCRIPTION CONFIGURATION (from the bottom up) 1 PH1000 Glovebox 1. Spin coat with an acceleration speed 1000RPM for 45 seconds 2 Caesium Carbonide 2. Hot plate 120 C for 30 min Glovebox 1. Spin coat with an acceleration speed 800RPM for 45 seconds 2. Hot plate 130 C for 20 min 8. PH1000 with TiOx LAYER MATERIAL DESCRIPTION CONFIGURATION (from the bottom up) 1 PH1000 Glovebox 1. Spin coat with an acceleration speed 1000RPM for 45 seconds 2. Hot plate 120 C for 30 min 2 Titanium Deposited 8nm with Thermal evaporated 66

68 Appendix IV: The process recipe for commercial ITO coated glass STEP PROCESS DESCRIPTION CONFIGURATION 1 Sample ITO coated glass Thickness 25 mm 25 mm 1.1 mm 2 Tape Covering Place the tape in the area where we want our electrodes 3 ITO Removal Removed the ITO we not need with Zinc powder and Hydrochloric acid 4 Cleaning Cleaning with Acetone Rinse in water Blow drying with clean dry air 5 Removal Remove now the tape there was being place in step 2 PICTURE OF INDIUM TIN OXIDE COATED GLASS 67

69 Appendix V: The process recipe for spin coating in Glovebox STEP PROCESS DESCRIPTION CONFIGURATION 1 Sample Normal piece of glass Thickness 2mm 2mm 1mm 2 Tape Covering Place the tape in the area where we need it to be use In this configuration is the tape place so we get electrodes 3 Spin coating Spin coat the material which we would use 4 Annealing Hot plate Depend on which material there is being used Hotplate 5 Removal Remove now the tape there was being place in step 2 68