Designing of Amorphous Silicon Solar Cells for Optimal Photovoltaic Performance

Transcription

1 Designing of Amorphous Silicon Solar Cells for Optimal Photovoltaic Performance Latchiraju Pericherla A Thesis submitted in part fulfilment of the requirements for the degree of Master of Engineering School of Engineering and IT Faculty of EHSE Charles Darwin University Darwin October 2013 Thesis Page 1

2 Acknowledgement I would like to thank my supervisor Professor Jai Singh for his guidance and help throughout my Thesis. I am grateful to my friends and family for their continuous support in this project. I would also like to thank my co-supervisor Monishka Narayan for her invaluable inputs in this Thesis. I would also like to thank David for his help, discussions and suggestions throughout this project. Thesis Page 2

3 Abstract Amorphous silicon solar cells are fabricated from thin layers of hydrogenated amorphous silicon (a-si:-h) deposited on a substrate. The process of fabrication is much more economical than that of crystalline silicon solar cells (C-Si). Recent studies indicate even further cost reduction potential compared to the crystalline silicon technology. First commercially available thin-film solar cells based on a-si:-h was produced for consumer applications such as pocket calculators and solar watches. In 1976, Carlson and Wronski developed the first a-si: H solar cell with efficiency 2-3%. Later on efficiency is increased to 13.1%. Now a-si:-h solar modules have entered to the market for power generation applications. Several manufacturing companies (Sanyo, BP Solar, Fuji Electric) with multimegawatt capacity have started producing a-si: H panels which are currently being installed. The performance of hydrogenated amorphous silicon solar cell (a-si: H) depends on its layer characteristics. It is important to understand the optimal structure, thicknesses of each layer and their function in the cell to design a cost effective, stable and efficient hydrogenated amorphous a-si: H silicon solar cell. In solar cells the photovoltaic performance can be enhanced by increasing the absorbance in some layer and reducing it in others. To achieve the same in a-si: H solar cell absorbance should be maximised in i-layer and minimised in other layers. Reflectance, transmittance and absorbance are calculated by using (SETFOS). In this project, semiconductor thin film optics simulation software (SETFOS) is used to design a single junction (p-i-n) hydrogenated amorphous silicon solar cell (a-si: H). Optimisation of absorbance of i-layer and optimal efficiency is achieved by using a software (SETFOS). The efforts on increasing the absorbance in i-layer can be made by optimising it with respect to the thickness of i-layer. Thesis Page 3

4 Contents Acknowledgement Abstract List of Figures Introduction Scope Historical developments Literature review Materials Conductors Insulators Semiconductors Doping of materials P-type semiconductor N-type semiconductor PN junction Depletion Region Types of solar cells Crystalline silicon solar cells Polycrystalline silicon solar cells GaAs Solar cell Thin film solar cells a Cadmium Telluride (CdTe) solar cell b Copper Indium Diselenide (CIS) Solar cells c Copper Indium Gallium diselenide d Graetzel solar cell e Nanocrystal photovoltaic cells f Organic photovoltaic cells g Amorphous Silicon solar cells Role of energy gap in solar cells Hydrogenated Amorphous Silicon solar cell Atomic structure Density of energy states Optical properties Thesis Page 4

5 2.6.4 Electrical properties Doping of a-si: H Deposition of thin-film silicon Plasma Enhanced Chemical Vapour Deposition (PECVD) Degradation of a-si: H solar cells Structure of a-si: H solar cells Structure of single junction a-si: H solar cell TCO layer p-layer i-layer n-layer Rear electrode Principle of operation of a-si: H solar cells Semiconducting thin film optics simulation software (SETFOS) Absorption module features Drift-diffusion module features Light absorption and charge-transport in thin films Solar cells performance evaluation Light scattering and rough interfaces Layer thickness optimization Current-matching by adjusting the active layer thicknesses I-V characteristics of organic solar cells Dark current Optical modelling of a-si: H solar cells Introduction Calculation of photovoltaic parameters Results Single junction p-i-n cells Discussion Single junction p-i-n cells Future Work Conclusion References Appendices Thesis Page 5

6 List of Abbreviations SETFOS a-si: H C-Si GaAs CdTe CIS (CIGS) STA (QDS) OLED OPV PV CVD PECVD RF PECVD GD I sc V oc FF V max I max TCO ITO Al Semiconductor Thin Film Optics Simulation Software Hydrogenated Amorphous Silicon Crystalline silicon Gallium Arsenide Cadmium Telluride Copper Indium Diselenide Copper Indium Gallium Diselenide Sustainable Technologies Australia Quantum dots Organic light-emitting diodes Organic solar cells Photovoltaic Chemical vapour deposition Plasma enhanced chemical vapour deposition Radio frequency plasma enhanced chemical vapour deposition Glow discharge Short circuit curret Open circuit voltage Fill factor Maximum voltage Maximum current Transparent Conducting Oxide Indium tin oxide Aluminium Thesis Page 6

7 Ag SiH 4 PH 3 B 2 H 6 Sn0 2 : F Ti0 2 : Al ZnO 2 : Al ZnO 2 : B σrt EGDM UNSW Silver Silane Phosphine Diborane Fluorine doped tin dioxide Aluminium doped titanium dioxide Aluminium doped zinc dioxide Boron doped zinc dioxide Room temperature conductivity Extended Gaussian disorder model University of New South Wales Thesis Page 7

8 List of Figures Figure 2.1 Illustration of energy bands (SZE LEE, 2012) Figure 2.2 Two-dimensional illustration of the crystal lattice of a p-type semiconductor (Jaeger, RC Blalock, 1997) Figure 2.3 Two-dimensional illustration of the crystal lattice of an n-type semiconductor (Jaeger, RC Blalock, 1997) Figure 2.4 N-type and P-type materials brought together (Wenham, 2003) Figure 2.5 Diffusion establishes built-in electric field (Wenham, 2003) Figure 2.6 Energy band gap of semiconductors and conversion efficiency (Dzhafarov 2013) Figure 2.7 Schematic representation of the atomic structure of (a) single crystal silicon (b) hydrogenated amorphous silicon (Zeman, 2013) Figure 2.8 The schematic representation of the distribution of density of allowed energy state of electron (a) crystalline silicon (b) a-si: H (Zeman, 2013) Figure 2.9 (a) Absorption coefficient as function of photon energy for a-si: H, p-type a-sic: H fabricated at Delft University the absorption coefficient of c-si is shown for reference. (b) Tauc plot with linear extrapolation to determine the Tauc optical band gap for a-si: H, p-type a-sic: H and a-sige: H Figure 2.11 Possible configurations of a phosphorous atom in a-si: H network Figure 2.12The schematic representation of RF PECVD deposition system (Zeman, 2013).. 32 Figure 2.13 The conversion efficiency in a-si: H-based solar cells (Zeman, 2013) Figure 3.1Single junction p-i-n solar cell Figure 3.2 I-V characteristics of a-si: H solar cell Figure 5.1 Absorbance of ITO, p-layer, i-layer, and layers as a function of the wavelength 45 Figure 5.2 Total absorbance in a cell Figure 5.3 Total reflectance in a cell Figure 5.4 Transmittance of whole solar cell Figure 5.5 Absorbance of a cell as a function thickness of i-layer Figure 5.6 Photocurrent as function with i-layer thickness Figure 5.7 I-V characteristics and efficiency obtained for a-si: H solar cell using Eq. (5.5).. 48 Thesis Page 8

9 1. Introduction Since 1976, the research interest in amorphous silicon solar cells has increased significantly. Even though the laboratory efficiency is more than 13% the commercially available modules efficiency is more than only 10%. 1.1 Scope The aim of this thesis is designing hydrogenated amorphous silicon solar cells for optimal photovoltaic performance by using SETFOS as a tool. The objectives of this project are: Learning the principle of operation of hydrogenated amorphous silicon solar cells. A single junction p-i-n type a-si: H solar cell absorbance, reflectance and transmittance are calculated by using SETFOS Optimisation of absorbance of i-layer, by using software (SETFOS) for optimal efficiency. The efforts on increasing the absorbance in i-layer can be made by optimising it with respect to the thickness of i-layer. Optical modelling of single junction p-i-n type a-si:-h solar cells I-V characteristics and conversion efficiency 1.2 Historical developments In 1839, the first photovoltaic effect was discovered by a French physicist Edmond Bequerel (Bube 1983). Fifty years later, an American inventor Charles Fritts, made first solar cells, which were made of thin wafers of selenium covered by a thin layer of gold. These cells efficiency is only 1% which was very low. In 1954, the crystalline silicon (c-si) was used to prepare solar cells. These cells efficiency is around 6%, made by Bell Laboratories in USA. The main significant use of solar cells happened in 1958 is to supply power to the satellite Vanguard I. Then, solar cells were majorly used for satellites and other space projects because of their high cost. In 1970, due to oil crisis in the Middle East, photovoltaic cells emerged as an alternative source of energy. So, researches took place to develop solar cell with two sides to improve their efficiencies and reduce manufacture costs. As a result, price droped from Thesis Page 9

10 A$200 per peak watt in 1960 to less than A$20 in the late Thus, solar cells were made economical for many more applications, particularly where electricity from mains power was unavailable or very expensive. However, the production cost of crystalline silicon solar cells is not possible to reach a reasonable price for domestic applications. The less expensive solar cells are made from hydrogenated amorphous silicon solar cells (a- Si: H) with layers deposited as thin films. However, these cells have an efficiency lower than crystalline silicon solar cells and thus relatively large number of solar panels is needed to supply the equal power compared crystalline silicon but the cost per device is less in amorphous silicon solar cells. In 1976, the first experimental a-si: H solar cell was announced by Carlson and Wronski which was made at RCA Laboratory (Carlson, Wronski 1976). Conversion efficiency of 2.4% was obtained when this single junction p-i-n a-si: H solar cell was deposited on a glass substrate coated with TCO and aluminium back contact. The concept of a stacked solar cell structure was introduced in order to increase the output voltage of a-si: H solar cells. Integrated type a-si: H solar cells were commercialised by Sanyo and Fuji Electric in 1976 and applied in consumer electronics such as watches, calculators, etc. The process of fabrication is much more economical that of crystalline silicon solar cells (C-Si). Recent studies indicate even further cost reduction potential compared to the conventional crystalline silicon technology. For developing and optimising a-si: H based alloys, a large research activity in the field of a-si: H solar cell was devoted. Low absorbing layer such as p-type hydrogenated amorphous silicon carbide was implemented which is usually denoted as hydrogenated amorphous silicon germanium (a-sige: H) and it eventually became an attractive low band gap material for stacked solar cells. Surface-textured substrates for optical absorption enhancement were introduced. Cells reached the initial efficiency in the range of 13% in the laboratory. Modules like a-si: H which is a commercial single junction was introduced to the market with efficiencies up to 5%. At the end of 1980, the annual production capacity of modules and a-si: H solar cells reached about 15 MW (Zeman, 2013). In order to achieve 10% stabilized module efficiency and high throughput process, the research and manufacturing effort was put in place. Alloy a-sige: H was optimised and implemented in tandem by several companies. At the end of 20 th century, the annual total production capacity for TF Silicon single and multi-junction modules reached around 30MW. Using the low temperature PECVD technique which emerged as a new candidate for the low band gap material in multijunction a-si: H based solar cells, Hydrogenated microcrystalline silicon were deposited. There Thesis Page 10

11 has been more focus on the increase of the deposition rate due to the introduction and implementation of μc-si: H in TF Si solar cells. For fabrication of solar cells, several new deposition methods such as RF PECVD, hot wire CVD, expanding thermal plasma CVD have been investigated at high deposition rates like 0.5 to 1.0 nm/s. Most improvements in stabilized solar cell efficiency are based on adapted cell designs and advanced light-trapping concepts. Many fundamental questions remain unanswered regarding the growth and the material properties of a-si: H and its alloys as well as the optical and electrical function of complete solar cell devices (Zeman, 2013). 2. Literature review 2.1 Materials Conductors Figure 2.1 Illustration of energy bands (SZE LEE, 2012). The metals have very low resistivity value because of its characteristics. The metals conduction band is partially filled or overlaps the valence band where the band gap is negligible as show in Fig. 2.1a. When kinetic energy is applied, the electrons at the top of the valence band or top most electrons in the partially filled band move to the next higher available energy level. Even small applied field in metal leads free electrons to move because there are many unoccupied states close to the occupied energy states (SZE LEE, 2012) Insulators As shown in Fig. 2.1c insulators conduction band is empty but valence band is occupied with electrons for all energy levels. The valence electrons in the valence band have strong bonds with neighbouring atoms. Insulators energy band gap is large which leads to have high Thesis Page 11

12 resistivity because of its characteristics. At room temperature these bonds are difficult to break so no current is in conduction as there are no free electrons. Even though small electric field or thermal energy is applied it is insufficient to raise the top most electrons in the valence band to the conduction band (SZE LEE, 2012) Semiconductors According to the characteristics of the semiconductors its energy gap is smaller on the order of 1eV. There are no electrons in the conduction band at T=0 K and all the electrons are in the valence band. As result, semiconductors are poor conductors if temperature is low. For instance, energy gap of Silicon is 1.1eV and 1.4eV for GaAs. At room temperature, thermal energy kt is good that energy excites valence electrons form the valence band to the conduction band. This potential energy can easily move electrons to many empty states which are in the conduction band and it results in moderate current (SZE LEE, 2012). 2.2 Doping of materials The intrinsic Silicon is a poor conductor at room temperature; its conduction can be increased by adding impurities to the semiconductor. This type of semiconductors called as doped semiconductors. Here, two types of impurities such as donor impurities and acceptor impurities can be used. A semiconductor, when it is doped with a donor impurity that material called as N-type semiconductor. When doped with acceptor impurity that is called as P-type semiconductor P-type semiconductor P-type semiconductors can be made by adding group three elements such as boron, gallium or indium to an intrinsic semiconductor. This group three elements are acceptor atoms, has three valence electrons. These three valence electrons are shared with the neighbouring atoms and it leaves hole as shown in Fig 2.2. In this manner a number of holes are created by adding impurity. Thus holes become greater than number of free electrons in the semiconductor. As a result, intrinsic semiconductor becomes p-type semiconductor because number of holes is far greater than the number of electrons. This type of semiconductor is called as p-type semiconductor because the positively charged carriers are in majority (Jaeger, RC Blalock, 1997). Thesis Page 12

13 In the p-type semiconductor electron-hole pairs are continually formed due to thermal excitement. Here, recombination process is done with free electrons because large number of holes is available. This process decreases the number of free electrons in the p-type semiconductor compared to the intrinsic semiconductor. For this reason, the current produced by the movement of free electrons in a p-type semiconductor is often neglected in the calculations. The acceptor atom is electrically neutral when the hole created due to the absence of its fourth valence electron is not filled by electron of an adjacent atom (Jaeger, RC Blalock, 1997). Figure 2.2 Two-dimensional illustration of the crystal lattice of a p-type semiconductor (Jaeger, RC Blalock, 1997) N-type semiconductor N-type semiconductors can be made by adding group five elements such as arsenic, phosphorus, antimony. This group five elements are donor atoms, has five valence electrons. These five valence electrons are shared by the neighbouring atoms and it leaves free electron as shown in Fig 2.3. After, number of holes created by adding impurity where free electrons are greater than number of holes in the semiconductor, the intrinsic semiconductor becomes N-type semiconductor because number of electrons is far greater than the number of holes. This type of semiconductor is called as N-type semiconductor because the negatively charged carriers are in majority (Jaeger, RC Blalock, 1997). Thesis Page 13

14 Figure 2.3 Two-dimensional illustration of the crystal lattice of an n-type semiconductor (Jaeger, RC Blalock, 1997). In the p-type semiconductor electron-hole pairs are continually formed due to thermal excitement. Here, recombination process is done with holes because large number of free electrons is available. This process decreases the number of holes in the N-type semiconductor compared to the intrinsic semiconductor. For this reason, the current produced by the movement of holes in a semiconductor of the N-type is often neglected in the calculations. The acceptor atom is electrically neutral when the fifth valence electron does not become free electron in the lattice (Jaeger, RC Blalock, 1997). Thesis Page 14

15 2.3 PN junction Figure 2.4 N-type and P-type materials brought together (Wenham, 2003). When silicon material doped with Boron that material becomes to P-type semiconductor and when silicon material doped with Phosphorus that material becomes to N-type semiconductor. The P-type material has large number of holes as majority carriers and the N-type material has large number of free electrons as majority carriers. PN-junction can be made when these materials are added together as shown in the figure 2.4. Here, if no electric field is applied externally, diffusion process distributes the electrons and holes uniformly near the junction. Thus, the free electrons in the N-type material diffuse across toward the P-type side and it leaves positively charged phosphorous ions near the junction in N-type material. Similarly, the holes in the P-type region diffuse towards the N- type region and it leaves behind negatively charged boron ions near the junction in P-type region as shown in figure 2.5. At the junction between N-type and P-type material these ions establishes electric field. This electric field occurs from the movement of positively charged ions of N-type material to the negatively charged ions of the P-type material. This build in electric field influences the free electrons and the holes near the junction. The free electrons move to P-type material near the junction and holes move to N-type material near junction. Hence, this built in electric fields causes some of the electrons and holes flow in opposite direction because of diffusion as shown in figure 2.5 (Wenham, 2003). Thesis Page 15

16 Figure 2.5 Diffusion establishes built-in electric field (Wenham, 2003) Depletion Region There are very few mobile electron and holes within the depletion region. After the diffusion, the mobile charges leaving their own charge and associated with the dopant atoms. As a result, the depletion region resistance is very high it can be modified by applying external electric field (Wenham, 2003). 2.4 Types of solar cells Since 1954, many solar cells have been developed by using variety of materials. A brief description of few of popular solar cells which were made of crystalline silicon (c-si) is given below Crystalline silicon solar cells From the beginning, crystalline silicon solar cells and modules have dominated photovoltaic (PV) technology. The reason behind crystalline silicon to be dominant in photovoltaic (PV) is the fact that microelectronics has developed silicon technology very impressively. P-type Czochralski silicon substrates have been dominated the global photovoltaic (PV) market for several decades. In most commercial arrays p-n junctions are made from crystalline silicon (c- Si). The efficiency of these cells is more than 20% and very stable. However the procedures Thesis Page 16

17 of growing and cutting crystalline silicon (c-si) are moderately expensive. Normally crystalline silicon (c-si) is processed by using Czochralski method, which consumes high electricity during the process. In this process high purity polycrystalline silicon is melted and a mono crystal is pulled out from the melt. The starting material is normally a p-type for fabricating a p-n junction. A small amount of boron (B) is incorporated into the crystal lattice during the crystal growth. This way one gets long cylinder shape ingots from which 300 µm thick-silicon wafers are cut and polished. More phosphorus (Ph) atoms are incorporated into the top layer of the wafer than boron atoms to make the top of such wafers as n-type. There are two processes normally used: a) diffusion method b) ion implantation During the diffusion process, wafers are placed in diffusion furnace and heated to a high temperature in the presence of phosphine gas. The phosphorus atoms from phosphine are incurred on the surface which diffuse slowly into the bulk of the material and form n-layer. In ion implantation, an ion implanter shoots individual ions at the surface of the wafer. The depth to which the ions penetrate can be controlled by changing the speed at which the ions hit the surface, so the thickness and doping characteristics of the top layer of the cell can be controlled. As in the ion implantation the energy of ions can be controlled accurately, this method can provide any desirable level of doping concentration and profile. Once p-n junction is formed, electrodes are then attached through thermal evaporation of a metal through a mask or photo-resist, and then a thicker metal layer is electroplated on top. Hence fabricated wafers are lastly coated with anti-reflection films by vacuum evaporation or sputtering (Pavel Stulik, 1998). The crystalline solar cells have been rapidly rising since The University of New South Wales (UNSW) in Sydney fabricated world recorded solar cells in 1984 that was 19% efficiency. Later, crystalline silicon solar cells efficiency (c-si) was increased to 25% which were also fabricated by UNSW research team (Solar progress, 2013) Polycrystalline silicon solar cells Polycrystalline silicon solar cells are one of the popular types of silicon solar cells. The Cz equipment consists of a vacuum chamber in which polycrystalline silicon pieces from single crystals are used as feedstock. Pieces are melted in a crucible and a seed crystal is first dipped Thesis Page 17

18 into the melt. Then the seed is slowly removed vertically to the melt surface whereby the liquid crystallises at the seed. Later the silicon is completely molten. The melt temperature is stabilised to accomplish the required temperature thereby to lower the seed into melt. This method is known as the Heat Exchange Method (HEM) and produces ingots 95% monocrystalline silicon (Pavel Stulik, 1998). The polycrystalline ingot is then cut. During the process of cutting silicon, silicon gets wasted. To avoid such wastage, there is different way of process. It is to pull out silicon ribbons directly from the molten state instead of first solidifying and then cut it. The production cost is cheaper but the efficiency is low (Pavel Stulik, 1998) GaAs Solar cell The conventional type GaAs solar cells efficiency is up to 25% and also the theoretical efficiency of GaAs is 26.8%, which are fabricated using hetero face windows. But the reported world record for single junction efficiency is 25.1%. These types of solar cells are very expensive so they are used mainly for space applications. a. Dual junction thin film photovoltaic cells The dual junction cell thickness (0.0055mm) can occupy an approximately equal to 4.2 square inches. These cells are capable of converting efficiency about 21%. These are efficient because having two solar cells one on top each other. The specific part of sun spectrum is converted by each solar cell. Gallium indium phosphide converts short wavelength into electricity form the top layer. Long wavelength passes through the indium phosphide layer and converted into electricity in gallium arsenide layer. Here, two cells are fabricated one on another, called as dual junction cells. These cells have higher efficiency than single junction cells (Ramninder, 2009). b. Triple-Junction cells The triple junction solar cells are very famous to use in space applications. The world recorded conversion efficiency 34% achieved in The U.S.A, Department of Energy (DOE) is first organisation to hold the world record. The different wavelength of light passes through triple junction to convert sunlight into electricity, it is designed to endure the energy of highly concentrated sunlight, and they require fewer cells to generate a given amount of power (Ramninder, 2009). Thesis Page 18

19 2.4.4 Thin film solar cells a Cadmium Telluride (CdTe) solar cell The thin film cadmium telluride (CdTe) solar cell technology made impact on commercialisation of solar energy production. Long term stability, ability to attract production scale capital investments and competitive performance demonstrates by large area monolithic thin film modules. The CdTe solar cells efficiency is comparatively high and has reached over 16% in the laboratory, but commercial modules have much lower efficiency, usually in the range of 10-11%. This thin film CdTe hetero junction solar cells have been fabricated in two different configurations called as substrate and superstate. Here, light enters through transparent conducting oxide (TCO) in both configurations. The efficiency of CdTe cells is much higher than the efficiency of the CdTe module which is a major problem. Only few groups are able to produce the efficiency of CdTe cells more than 15% (Pavel, 1998). Two major concerns about CdTe, one is stability and another one is cadmium. Many CdTe cells and modules have been made with good stability but a lot with poor stability. Also, cadmium is hazardous waste although a little amount is used to make CdTe cells b Copper Indium Diselenide (CIS) Solar cells Copper Indium Diselenide (CIDS) cells have efficiency close to 18%. These films are prepared by co-evaporation of the elements. These cells produce almost equal efficiency of polycrystalline silicon cells and also is less expensive compared to crystalline silicon solar cell (Pavel, 1998). However, there are yet to be resolved problems such as climate impact on the efficiency of Copper Indium Diselenide (CIS) solar cells. The impact of irradiance, cell temperature and relative air mass are parameters to be considered for producing low efficiency c Copper Indium Gallium diselenide Copper Indium Gallium Diselenide (CIGS) cells have efficiency close to 19%. These films are prepared by co-evaporation of the elements. National Renewable Energy Laboratory team has achieved a world record by holding an efficiency of 19.9%. The structure of CIGS cells Thesis Page 19

20 consists of four layers those are copper, indium, gallium and deselenide. The two layers in the middle are the responsible to produce electricity. Other two layers works as electrode which are one as cathode and another as anode. These are similar to the positive and negative terminal of a common battery. The manufacturing process of CIGS is less expensive and also very similar to that used to deposit reflective coatings on normal glass products like eye glasses. The CIGS cells have the highest efficiency and more stable, the materials used to make CIGS cells do not weaken the performance of cell compared with the other photovoltaic cells. CIGS are not expensive to manufacture as it takes less labour, less material, energy and capital costs d Graetzel solar cell Michael Graetzel is a Swiss scientist who discovered photo electrochemical device which produces electricity. The cell consists of three main parts, the top one is transparent and works as anode which is made of fluoride doped tin dioxide deposited on back of glass. Back of this conductive plate, titanium dioxide is deposited with an extremely high surface area. By using these materials, the reported efficiency is reasonably high, 11%. The major advantages of these cells are less expensive and are able to produce under non-ideal light conditions. Initially these cells are manufactures by Sustainable Technologies Australia (STA), Queanbeyan. STA are able to fabricate cells with efficiency 8-11% in small devices and 5-6% in 100cm 2 for modules (Pavel 1998) e Nanocrystal photovoltaic cells Nano crystal photovoltaic cells have Nano crystal coat on silicon substrate. This technology of using Nano crystal coating to form photovoltaic cells is called Nano crystal technology of photovoltaic cells. In the earlier methods the quantum dots (QDS) are prepared using molecular beam epistaxis and is an expensive method. Fabricating QDS by colloidal synthesis is more cost effective method. In Nano crystal technology a thin film of Nano crystals can be obtained from a process called spin coating. Here, required amount of QDS is placed on the flat substrate which is rotated at required speed so that the solution spreads uniformly. It is done so until the required thickness is gained. These cells are based on dye-sensitized colloidal TiO2. They are promisingly efficient in converting incident light energy to electrical energy. As these cells generate desired results not so expensively, they are invariantly economical (Ramninder 2009). Thesis Page 20

21 In Single-Nano crystal architecture, an array of single particles is placed in between the electrodes. For optimal performance it is proposed that these electrodes are placed at a distance of ~1 exciton diffusion length. There is an ongoing research done on this type of photovoltaic cells at many research institutes, including Stanford, Berkeley and the University of Tokyo. As these cells are still in research phase, in future cells based on this quantum dot technique may provide many good effects such as mechanical flexibility, low cost and clean power generation. The efficiency of these cells is expected go up to 65% f Organic photovoltaic cells The form of photovoltaic cells where organic materials are used to fabricate them is called Organic photovoltaic cells. The active part of these photovoltaic cells is fabricated from organic materials. These organic photovoltaic cells are comparatively inexpensive and the process of manufacture is easier than the inorganic photovoltaic cells. They can be extensively used in optoelectronic devices. Even though there is a steady improvement in power conversion efficiency of these cells, it remained less than 1% for many years. However, in last few years the performance of solid state organic photovoltaic cells have increased under white light illumination. With many economic and manufacture advantages, they still lay behind in terms of efficiency compared to photovoltaic cells based on inorganic materials (Wöhrle, 1991). The three forms of photovoltaic cells are: 1) Molecular photovoltaic cells 2) Polymer photovoltaic cells 3) Hybrid photovoltaic cells Any detailed description of these cells is out of scope of this project g Amorphous Silicon solar cells The following chapters are based on amorphous silicon which comes under thin film solar cells. Thesis Page 21

22 2.5 Role of energy gap in solar cells In general solar radiation has a wavelength which ranges from 200 nm to several µm. When such a solar photon of energy hυ greater than energy of energy gap E g is incident, the energy of photon is absorbed by electron and excited from valence band to the conduction band. This process leaves behind an excited hole in the valence band. At the edge of conduction band the excited electrons are relaxed immediately and the energy is dissipated in the a-si: H network in the form of heat. In order to reduce this loss a wide E g is needed. Because of wide E g the energy dissipated by higher energy photons can be utilized in a better photovoltaic effect. Also higher the energy gap higher the open circuit voltage, V oc. So the choosing higher energy gap also gives higher V oc. Therefore if there is a wider energy gap the photons of energy less than E g cannot get absorbed, resulting in a reduced photo generated current. On contrary, if the energy gap is narrow, the number of photons absorbed to give photo generated current is high, which results in low V oc. The performance of the cell is given by the product of V oc and J sc (Short circuit current), which depends on photo generated current and optimal value of E g, which in turn gives optimal performance of the cell. From Fig.2.6 the optimal energy gap of a single junction cell operating at AM1.0 is given as 1.5eV, which also depends on spectral profile of used radiation. Fig. 2.6 clearly shows that the usual energy gap of a-si:h is nearly 1.7eV. Hence a-si:h is reasonably suitable material for the manufacture of solar cells. Figure 2.6 Energy band gap of semiconductors and conversion efficiency (Dzhafarov 2013). Thesis Page 22

23 2.6 Hydrogenated Amorphous Silicon solar cell Atomic structure Figure 2.7 Schematic representation of the atomic structure of (a) single crystal silicon (b) hydrogenated amorphous silicon (Zeman, 2013). The above figure 2.7 shows the difference between single crystal silicon and a-si: H atomic structure. The figure 2.7.a shows the structure of single crystal silicon schematically. Here, each Silicon atom is covalently bonded with four neighbouring Silicon atoms and the lengths and the angles are equal between the bonds. In the atomic structure, coordination number is equal to the number of bonds that an atom has with its instant neighbours, so, it is called as coordination. Hence, the coordination number for all Silicon atoms is four in the single crystal Silicon. When the crystal lattice is reproduced by duplicating the unit cell and stacking the duplicates next to each other that atomic arrangement is defined as a structure with long range order. The real lattice structure of single crystal silicon is represented by diamond lattice unit cell. As figure 2.7.b shows that a-si: H does not reveal the structural order over a long range as is the event for single crystal silicon. The most Silicon atoms have covalent bonds with four neighbouring atoms when there is a similarity in atomic structure on a local atomic scale (Zeman, 2013). Thesis Page 23

24 The a-si: H is the same short range order as mono crystal silicon, but it lacks the long-range order. Small differences in bond angles and bond lengths between neighbouring atoms in a-si: H lead to a total loss of the ordered structure locally on a scale larger than a few atomic distances. As a result, the atomic structure of a-si: H is called as continuous random network. Because of the short range order in a continuous random network of a-si: H, the concept of semiconductor energy state bands, characterized by the conduction band and valence band, can still be used. The large deviations in bond angles and bond lengths between the neighbouring atoms in a-si: H results in strained or weak bonds. The energy of the weak bonds is higher than the energy of optimal silicon covalent bonds in ideal single crystal silicon. So, the weak or strained bonds can easily break and can form defects in the atomic network. In the continuous random network, the meaning of a defect is altered with respect to the crystal structure. In a crystal, a defect can form by the absence of any atom in lattice. In a-si: H, the silicon atoms are covalently bonded to only three silicon atoms and have one unpaired electron. So it is called as dangling bond. During the preparation of amorphous silicon with the mixture of silane, hydrogen can be incorporated in the network as hydrogen atoms bond with most of the dangling bonds. It is called as the dangling bonds are passivated by hydrogen, as shown in figure 2.7.b. Hydrogen passivation reduces the dangling bond density from about 1021 cm -3 in pure a-si (amorphous silicon that contains no hydrogen) to cm -3 in a-si: H, i.e. less than one dangling bond per million Silicon atoms (TU Delft, 2013).In the above figure 6.b shows the defects with unpassivated dangling bonds and with hydrogen passivated dangling bonds. As a result, the short range order in a-si: H network and the hydrogen passivation of the dangling bonds are accountable for semiconductor properties of amorphous silicon (Zeman, 2013) Density of energy states The below figure 2.8 shows the difference between single crystal silicon and a-si: H in terms of distribution of density of allowed energy states that are led by the atomic structure. The periodic atomic structure of single crystal silicon results in allowed energy states for electrons that are called energy bands and the excluded energy ranges that are called forbidden gaps or band gaps. In the single crystal silicon, the valence band and the conduction band are separated by a well-defined band gap as shown in figure 2.8.a. Here, at room temperature single crystal silicon band gap is 1.12eV. In the a-si: H, there is a continuous distribution of density of states and no well-defined band gap exists between the valence band and the Thesis Page 24

25 conduction band. Because the long range order disorder in the atomic structure of a-si: H, the energy states of the valence band and the conduction bands extended into the band gap and form states that are called band tail states as shown in the figure 2.8.b.The strained bonds in the a-si: H network formed by the energy state of electrons are represented by band tail states. The amount of disorder in a-si: H material is based on the width of the band tails measurement. If more disorder in a-si: H means that the band tails are broader. Additionally, the dangling bonds introduce allowed energy states that are located in the central region between the valence band and conduction band states. Here, the charge carriers are considered as free carriers. These states are non-localised which are called as extended states. So, the wave functions of the tail and defect states are localized within the structure hence these states are called localised states. Therefore, mobility that characterises transport of carriers through the localised states is strongly reduced (Zeman, 2013). Figure 2.8 The schematic representation of the distribution of density of allowed energy state of electron (a) crystalline silicon (b) a-si: H (Zeman, 2013). This feature of a severe drop of mobility of carriers in the localised states in a-si: H is used to describe its band gap. This band gap is designated by the term mobility gap (E m ) because the occurrence of a considerable density of states in this gap conflicts the classical concept of the band gap. These energy levels that separate the extended states from the localised states in a- Si: H is called the valence band and the conduction band mobility edges. This mobility gap of a-si: H is greater than the band gap of single crystal silicon and it has a typical value of 1.8 ev. As a result, this localised tail and dangling bond states have a large effect on the electronic properties of a-si: H (Zeman, 2013). Thesis Page 25

26 2.6.3 Optical properties The optical properties of a-si: H are generally characterised by its absorption coefficient and a value of the optical band gap. According to Delft University of Technology, the absorption coefficient of a-si: H as shown in figure 2.9.a, is fabricated at Delft which is a function of photon energy. Here, C-Si absorption coefficient is shown for reference. As per figure, it shows that a-si: H absorbs almost 100 times more than c-si in the visible part of the solar spectrum (Zeman, 2013). Because the disorder in the atomic structure of a-si: H, the higher absorption that performs like a direct gap semiconductor. As result, 1 μm thickness of a-si: H layer is sufficient to absorb 90% of usable solar light energy. In practice the thickness of a-si: H solar cells are less than 0.5 μm that is about 100 times less than the thickness of a typical single crystal silicon cell. Thus, it saves in both material and energy in fabrication of a-si: H solar cells. This a-si: H optical absorption can be somewhat be changed by varying its hydrogen content. It can be greatly changed by alloying with carbon or germanium. The absorption coefficient of hydrogenated amorphous silicon carbide (a-sic: H) and hydrogenated amorphous silicon germanium (a-sige: H) are shown in the figure 2.9 as a reference. The optical band gap is a valuable material parameter that allows comparison of a-si: H based materials regarding their light absorption properties. In general, a material with higher optical band gap absorbs less. The optical band gap (E opt ) is determined by extrapolating a linear part of the following function (Zeman, 2013). Versus the photon energy E to α (E) = 0, for α 10 3 cm -1. = B [E-E opt ] Here, α (E) is the absorption coefficient, n (E) is the refractive index, p and q are constants that define the shape of the density of extended states distribution for the conduction band and valence band, correspondingly, and B is a pre-factor. When (p=q=1/2) it is usually the case in crystalline semiconductors, Eq. 1 describes the corresponding optical band gap is called the Taucoptical gap. When (p=q=1) E opt is called the cubic optical gap. The Tauc gap of device quality of intrinsic a-si: H is in the range of 1.70 to 1.80 ev, the cubic gap of the same material is usually 0.1 to 0.2 ev smaller than the Tauc gap. The Tauc plot with linear extrapolation to determine the Thesis Page 26

27 Tauc optical gap for a-si: H (Eopt=1.77 ev), p-type a-sic: H (Eopt=1.95 ev), and a-sige: H (Eopt=1.52 ev) is shown in Figure 2.9.b (Zeman, 2013). Figure 2.9 (a) Absorption coefficient as function of photon energy for a-si: H, p-type a-sic: H fabricated at Delft University the absorption coefficient of c-si is shown for reference. (b) Tauc plot with linear extrapolation to determine the Tauc optical band gap for a-si: H, p-type a-sic: H and a-sige: H Electrical properties The a-si: H electrical properties are typically characterised in terms of dark conductivity and photoconductivity. To obtain information about the quality of a-si: H material for application in solar cells is based on the measurement of these two properties. Also it gives information about the mobility-lifetime product and the effect of impurities in a-si: H. The charge carrier s mobility in the extended states of a-si: H is about two orders of magnitude lower than in single crystal silicon. In an intrinsic a-si: H, the electron mobility is 10 cm 2 /V s and the hole mobility is 1 cm 2 /V s. The high mobility gap of a-si: H and the low values of electron and hole mobility result in a low dark conductivity, which in device quality intrinsic a-si: H is less than Ω -1 cm -1. Here, the material is more characterised by an excellent Thesis Page 27

28 photoconductivity that is higher than Ω -1 cm -1, while measured using the AM1.5 light spectrum and the incident power of 100mW/cm 2 (Zeman, 2013). 2.6 Doping of a-si: H The main purpose of doping or alloying is to change the type of electrical conductivity and its magnitude by mixing a controlled amount of special impurity atoms. The boron impurity and phosphorus impurity are used in a-si: H to make p-type and n-type materials same as in crystalline silicon. Here, the doped layers have two functions in a- Si: H. First, it creates an internal electric field across the intrinsic active participation a-si: H layer. The electric field should be high enough to ensure that the photo-generated carriers in intrinsic a-si: H layer are collected. The thickness of the intrinsic layer and the level of doping in the doped layer vary the strength of the electric field. Second, they establish low-loss ohmic electrical contacts between the a-si: H part of solar cell and the external electrodes. According to the Spear and LeComber from Dundee University reported in 1975 that amorphous silicon could be doped by adding of boron and phosphorus. It shows the difference conductivity of a-si: H by mixing the silicon source gas, silane (SiH 4 ), with phosphine (PH 3 ) or diborane (B 2 H 6 ) during the deposition using the glow discharge method. The a-si: H as a function of fraction of doping gases in a mixture with silane (SiH 4 ) room temperature conductivity (σrt) is shown in figure The conductivity of a-si: H could be varied by more than a factor 108. The activation energy decreases from ev in intrinsic material to about0.15 ev in phosphorus doped material and 0.3 ev in boron doped material. The continuous random network could easily incorporate impurity atoms, such as phosphorus and boron, with coordination that corresponded to the bonding configuration with the lowest energy. In the continuous random network the optimum number of covalent bonds (coordination), Z, for an atom with N valence electrons is, Z = 8 N for N >4 Z = N for N < 4. This prediction of the atom coordination in the continuous random network is known as the8- N rule and was introduced by (Mott, 1969). Thesis Page 28

29 Fig 2.10 Room-temperature conductivity σrt of n- and p-type amorphous silicon, plotted as a function of the ratio between the numbers of doping gases molecules and silane molecules in the gas mixture (Zeman, 2013). For instance, following the 8-N rule, a phosphorus atom with five valence electrons will incorporate itself in the continuous random network by forming three covalent bonds with neighbouring atoms as shown in the figure 2.11.a. The notation Tzq is used, in order to describe the configuration of the atoms in the structural network. Where T is the atom, z is the coordination number, and q is the charge state of the atom. Here, indicates a phosphorus atom that is covalently bonded to three neighbouring atoms and is neutral. According to 8-N rule, in a-si: H most of the phosphorus atoms are incorporated. They adopt the optimal threefold coordination that represents the non-doping state and thus is electrically inactive (Zeman, 2013). Thesis Page 29

30 The doping efficiency in a-si: H, which is distinct as the fraction of dopant atoms with fourfold coordination, is rather low. The doping efficiency at room temperature is almost unity in comparison to single crystal silicon, where, it is in the range of in a-si: H. It means that relatively high concentrations of phosphorous atoms need be introduced in order to obtain material with high conductivity. As shown in figure 2.11.c, in single crystal silicon, a phosphorous atom can also be incorporated as the neutral donor in the network but, in this configuration it is characterised by much higher energy than the optimal configuration and thus is unstable in the continuous random network. Most of the phosphorous atoms, which contribute to doping, are not neutral donors, but charged phosphorus atoms. The formation of the charged state is accompanied by formation of negatively charged dangling bond as shown in figure 2.11.b. If the and configuration is energetically more favourable than donor then it is called the defect-compensated donor (Zeman, 2013). Figure Possible configurations of a phosphorous atom in a-si: H network. (a) The non-doping state P30, (b) the defect-compensated donor state +, (c) the neutral donor (Zeman, 2013). The creation of the defect-compensated donors in the situation of phosphorus atoms and defect compensated acceptors in the situation of boron atoms is the major doping mechanism in a- Si: H. According to Street, it is known as the auto compensation model. The most important result of this model is that doping of a-si: H certainly leads to creation of dangling bonds. In comparison to intrinsic a-si: H, the doped a-si: H has two or three orders of magnitude larger defect density. In comparison of single crystal silicon, the diffusion length Thesis Page 30

31 of charge carriers in doped a- Si: H is very small. The a-si: H solar cells cannot function positively as a p-n junction but a comparatively defect free intrinsic layer has to be inserted between the p-type and n-type layers. Due to the underlying processes of the photovoltaic effect such as absorption of light and separation of photo-generated carriers take place in the intrinsic layer, it is called an active layer in a-si: H solar cell. Another important difference between single crystal silicon and a-si: H is that when increasing the concentration of boron and phosphorous doping atoms in a-si: H, the Fermi level does not move closer to the valence band mobility edge than 0.30 ev and 0.15 ev to the conduction band mobility edge, correspondingly. The purpose for this is that the shift of the Fermi level towards the band edges declines the possibility of forming the doping state formations and favours the nondoping configurations. Moreover, when the Fermi level is shifted near the band edges it is located in the band tail states, whose density rises exponentially towards the mobility band edges. Hence, any further shift near the band edges is attended by building up of a large space charge in the tail states that compensates the charge created by ionised doping atoms (Zeman, 2013). 2.8 Deposition of thin-film silicon The methods that are used for depositing thin films of a-si: H and μc-si: H can be divided in two groups. The first method is known as chemical vapour deposition (CVD) the second method is physical deposition method in which silicon atoms for a-si: H growth are obtained by sputtering a silicon target. 2.9 Plasma Enhanced Chemical Vapour Deposition (PECVD) The plasma enhanced chemical vapour deposition (PECVD) technique is most commonly used deposition method to produce device quality a-si: H for both laboratory and industrial scale. If the radio frequency is MHz then plasma decomposition of SiH 4 is called as RF PECVD or the glow discharge (GD) deposition. The figure 2.12 shows the schematic representation of RF PECVD deposition system. The role of the plasma is to provide a source of energy to dissociate the SiH4 molecules. This is done by collisions with electrons, which comes from secondary electrons in the plasma and build their energy by acceleration in an electric field. The a-si: H film growth is proficient by attachment of reactive particles of dissociated SiH4 molecules, called as radicals, to the surface of the growing film. For device Thesis Page 31

32 applications, the thickness of the a-si: H film is around half micro meter and it must be deposited on an appropriate substrate carrier. Here, the energy transferred to the SiH4 molecules in the collisions with electrons is radiated as visible light, thus the deposition method is called as the glow discharge. Figure 2.112The schematic representation of RF PECVD deposition system (Zeman, 2013) The RF PECVD deposition system is mainly contains of five parts: 1. The reaction chamber with the capacitatively coupled parallel electrodes. 2. DC or RF power source. 3. Gas handling system. 4. Vacuum pump system. 5. Substrate heating assembly. The device quality a-si: H normal deposition temperature is between 200 C and250 C. This temperature can be allowing wide range of low cost substrates such as glass, stainless steel and ceramic. But, scaling must be required for a careful design of reactor geometry in order to produce uniformly and homogeneous films. Doping and alloying of a-si: H can be done by adding appropriate gases to the source gas mixture. For example, adding B 2 H 6 to the source Thesis Page 32

33 mixture produces p-type a-si: H and adding PH 3 to the source mixture produces n-type a-si: H (Zeman, 2013). The major advantages of FR PECVD deposition technology are briefly: 1. The effective p- and n-type doping and alloying 2. large area deposition 3. Deposition of multi-layer structures by control of gas mixtures in a continuous process 4. low cost 5. The use of any cheap and arbitrarily shaped substrates 6. Easy patterning and integration technology 7. The low deposition temperature (100 C <Ts <400 C) 2.10 Degradation of a-si: H solar cells Staebler and Wronski reported in 1977 the first measurements in which they found a decrease in the photoconductivity during illumination and a decrease in the dark conductivity after illumination. This effect is called as the Staebler-Wronski effect (Staebler, Wronski, 1977). One of the most interesting and actively researched facets of amorphous silicon solar cells is the significant decline in their efficiency during their first few hundred hours of illumination. The below figure 2.13 shows the degradation effect of a single junction and a triple junction. In the single junction, loses is about 30% of its initial efficiency after about 1000h. In the triple junction module, loses is about 15% of its initial efficiency after about 1000h.. After the initial degradation the performance of the solar cells stabilises. The stabilised performance of high quality solar cells is 70-90% of their initial performance (Deng, Schiff, 2003).. Thesis Page 33

34 Figure The conversion efficiency in a-si: H-based solar cells (Zeman, 2013) 3. Structure of a-si: H solar cells 3.1 Structure of single junction a-si: H solar cell The a-si: H solar cell is signified by a single junction solar cell. The single junction solar cell can configure in two types such as p-i-n superstrate configuration and n-i-p substrate configuration. The deposition sequence of the a-si: H based layers reflected by configuration. When p-type layer is deposited first, then intrinsic layer and n-layer is deposited as the last one which is called as p-i-n configuration as shown in figure 4.1. When n-type layer is deposited first, then intrinsic layer and p-layer is deposited as the last one which is called as n- i-p configuration. The p-i-n configuration is used, if glass or another transparent material is used as a substrate. The n-i-p deposition sequence is used, when a stainless steel or another non-transparent material is used as a substrate. A single junction p-i-n a-si: H solar cell is deposited on glass substrate coated with a TCO. Here, TCO acts as anode which is top electrode and aluminium or silver is used as the bottom electrode which acts as cathode. Here, only the a-si: H intrinsic layer contributes to the current generation, the optimal optical design of the cell structure maximises absorption in the intrinsic layer and minimises absorption in all the other layers. Thesis Page 34

35 Figure 13.1Single junction p-i-n solar cell TCO layer Transparent Conducting Oxide (TCO) is the second layer of a-si: H solar cell. Basically it works as one of the electrode of the solar cell and it allows maximum transmission of the sun light into the p-layer. TCO should have very low series resistance with high conductivity and also any light absorbed in TCO does not contribute to electrical output so must be highly transparent. Thus, TCO should have both optimal electrical and optical properties. A TCO must be a suitable choice of the refractive index because it should allow lowest reflection of solar radiation from glass/tco and TCO/p-layer interfaces. There are many materials to use as TCO. But, Indium Tin Oxide (ITO) is best choice, so it is used as a top electrode. Also TCO works as an anode for solar cell. And there are several materials such as fluorine doped tin dioxide (Sn0 2 : F), aluminium doped titanium dioxide (Ti0 2 : Al) and aluminium or boron doped zinc dioxide (ZnO 2 : Al & ZnO 2 : B) p-layer This layer can be called as window layer because p-layer should be thin and non-absorbing and also it transmits maximum photos to i-layer. This layer can be made by mixing silane (SiH 4 ) gas with diborane (B 2 H 6 ) in the occurrence of a RF PECVD. For maximising optical energy band gap of p-layer, it can be done by mixing with carbon to form a-sic: H. This Thesis Page 35

36 increases layer to be more transparent. The materials used for this layer must have a wider energy gap and it allows the solar radiation pass through in to i-layer i-layer It is very important layer in a-si: H solar cell and it presents a main role. This layer is responsible for generating current in cell produced by charge carriers (electron and hole pair). For better performance of the photovoltaic solar cell, i-layer should have absorbed more photons. Normally this layer is thicker than other layers. This layer can deposit using silane and hydrogen gases in the occurrence of RF PECVD. Optimisation of i-layer absorbance is the main objective of this thesis. In later chapter more will be described on design of a-si: H n-layer This layer can be made by mixing silane (SiH 4 ) gas with phosphine (PH 3 ) in the occurrence of a RF PECVD. Also this is the last layer of a-si: H cell. The main purpose of n-layer is to create a built-in-field between p-layer and n-layer. This layer must be thin so that the unabsorbed photons in i-layer can be transmitted to the rear metal for being reflected back to i-layer. (Ramninder, 2009) Rear electrode This layer works as one of the electrode for a-si: H. After n-layer, this layer is deposited; usually aluminium (Al) and silver (Ag) are the popular material to use as back electrode. The purpose of this layer is act as reflecting mirror for unabsorbed photon in i-layer. Silver (Ag) is expensive material but it has higher reflectivity than aluminium (Al). However, silver is unstable because it can get oxidized easily. It has been established that this combination results in a highly reflective back contact that enhances the absorption of light in the long wavelength region (wavelengths above600 nm). 3.2 Principle of operation of a-si: H solar cells The light strikes on top of the solar cell. The incident photons get transmitted through TCO to p-layer, i-layer and n-layer where photons get absorbed. To excite an electron from valence band to conduction band, absorbed photon energy should be equal to optical band gap and hence generate free electron-hole pair. Most of the photons absorbed in i-layer and it is responsible to generate charge carriers, due to build in electric field that provided by p-layer and n-layer tends to move those electrons towards n-layer side electrode and holes move Thesis Page 36

37 towards p-layer side electrode. But, the carriers generated in the p-layer and n-layer does not contribute to the photovoltaic performance of solar cell. In order to get the Fermi energy level nearby the valence band in the p-layer and nearby the conduction band in the n-layer, p-layer and n-layer must be doped highly. It helps to make available a high concentration of charge carriers and ensures a high open circuit voltage (V oc ). Many recombination centres takes place for photogenerated charge carriers because the p- layer and n-layer are heavily doped. As it leads to recombination of photo generated carriers in the p-layer and n-layer, which makes these layers photovoltaic inactive layers. The conversion efficiency is very important parameter to describe the quality of the solar cell. This can be denoted by the ratio of electrical output power to optical input power. The spectrum of illuminate light is a factor of performance and efficiency of the cell. Usually, Air with mass one and one half (AM1.5) and intensity 100mW/cm 2 is standard laboratory measurement. Figure 14 I-V characteristics of a-si: H solar cell (Scientific, 2013) Figure 3.2 shows I-V characteristics of a-si: H solar cell under sun spectrum. Here, the short circuit current J sc or I sc, (J sc = J (v=0) ), open circuit voltage V oc, (V sc = V (j=0) ), P max =max(-vxj) where J=J max and V=V max, and fill factor FF=. The solar cell efficiency is defined as the ratio of the electrical maximum output power over the optical input power. Thesis Page 37

38 4 Semiconducting thin film optics simulation software (SETFOS) The SETFOS is very advanced software tool to design any kind of thin film solar cell. It is user friendly software. After installation of this software there are few materials available in the library list to design a solar cell. There is a possibility to add new material to design a cell by giving the input values like refractive index and extinction coefficient of that particular material. Once simulation is done it gives the results such as layer absorbance, reflectance transmittance of the whole solar cell and absorbance of individual layers. Also, It calculates current and voltage characteristics parameters such as short circuit current I sc, open circuit voltage V oc, and fill factor to obtain conversion efficiency of solar cell. SETFOS is a powerful scientific tool used to study and optimize organic light-emitting diodes (OLEDs) as well as organic solar cells (OPV) and other thin film devices. SETFOS provides insight into the solid state physics of the device. Due to its advanced analysis features, it is a highly efficient, predictive and descriptive. SETFOS is software tailored to develop novel optoelectronic thin film based technologies and enables you to inspect internal characteristics which are not possible by experiment, to assess device performance. SETFOS is CPU efficient software and is widely used in both industrial and academic research laboratories. There are three modules available in SETFOS. They are light absorption, driftdiffusion, and dipole emission simulations. They can be used individually or as a combination depending on the needs of researchers most significantly in the fields of optical multilayers, optoelectronics and renewable energies. The three versatile simulation tasks performed by SETFOS are namely multivariable sweep, optimization, and parameter extraction by least-square fitting. SETFOS provides us with three interactive views which are intuitive graphical input, optional script-based input, and customizable graphical representation of results (Fluxim 2013). Thesis Page 38

39 4.1 Absorption module features These features are available in SETFOS for simulation while absorption module is enabled. The main advantage of the software is giving many options under different conditions. Such as coherent thin film optics, polarization optics, rate profile of Photon absorption, maximum power conversion efficiency as key figure for fully coupled solar cell simulations, spectral penetration of external illumination, optics characteristics reflectance, transmittance, absorbance, incoherent substrate definition, arbitrary combination of coherent and incoherent thin film optics, arbitrary illumination spectrum and layer-specific absorbance s. The short circuit current of the solar cell depends on the layer specific absorbance calculation. To calculate the photon flux across the optical stack, allowed by transfer matrix formalism this is based on the optical model implementation of SETFOS absorption module (Fluxim 2013). 4.2 Drift-diffusion module features The features of drift diffusion module are generation diffusion, energy transfer of multiple excitons, charge drift-diffusion and trapping, temperature and density dependent mobility model, extended gaussian disorder model (EGDM), user-defined charge distribution initialization, customizable numerical methods, Tabulated electric field dependent mobility imported from external file, maximum power conversion efficiency as key figure for fully coupled solar cell simulations (Fluxim 2013). 4.3 Light absorption and charge-transport in thin films The features implemented in SETFOS to calculate light absorption and charge-transport in thin film solar cells are such as dark current curves, illuminated current curves, CELIV photocurrent, light-scattering and rough interfaces, and tandem solar cells, light absorption in any coherent and incoherent stack (Fluxim 2013). Thesis Page 39

40 4.3.1 Solar cells performance evaluation This software is a necessary research tool for both organic and inorganic solar cells and also coherent and incoherent layer structures treat by SETFOS extensively Light scattering and rough interfaces Scattering interfaces and rough interfaces can be enhanced light absorption. Maximum generated short circuit current is quantified by SETFOS. The scattering behaviour of an interface defined with use of analytical models Layer thickness optimization Calculate the layer thickness dependent absorption spectra that resolve interference effects Current-matching by adjusting the active layer thicknesses The optimization of tandem solar cells can be done in SETFOS I-V characteristics of organic solar cells Calculate recombination losses, peak-power characteristics, maximum short-circuit current by coupled optical-electrical calculations Dark current The dark current through a solar cell quantify by SETFOS. The calculation of I-V curves dominated by diffusion in no time due to a fast steady-state solver (Fluxim 2013). 5 Optical modelling of a-si: H solar cells Using semiconducting thin film optics simulation software (SETFOS), the optimal a-si: H solar cell is proposed and its photovoltaic parameters are calculated. The theoretical efficiency of the single junction (p-i-n) type a-si: H solar cell with the optimal design is projected to be 13.1%. 5.1 Introduction Thesis Page 40

41 Optimising the thickness of numerous absorbing and non-absorbing layers is one way of optimising the performance of a-si: H solar cell. This optimisation can be done with both the optical and electrical properties of material used. One of the software tool (SETFOS) is used to study, and maximise the integrated absorbance of the incident radiation in i-layer with respect to the thickness of various layers. By using this tool the optical design of a-si: H solar cells of structure TCO/p-i-n/rear metal contact have already been recommended. The thicknesses of other layers are optimised for a fixed thickness of 0.01 µm for p-layer and 0.03µm for n-layer. It has done because the contribution of photocurrent generation very low in a-si: H. The input parameters absorbance coefficient (n) and extinction coefficient (k) are obtained from the library of SETFOS. The values of optical constants of Aluminium (Al), ITO is chosen from library of SETFOS. Semiconducting thin film optics simulation software (SETFOS) is used to design a thin film a-si: H solar cell with structure of glass/tco/p-i-n/metal. By using this design, the thickness of i-layer is optimised. Since, it is introduced by Carlson and Wronski in The hydrogenated amorphous silicon (a-si: H) p-i-n solar cell has been comprehensively investigated. To reduce the cost of production and make it suitable for large scale terrestrial use, these researches are being continued. Computer simulation such as semiconducting thin film optics simulation software (SETFOS) can be used to optimise absorbance of layers. 5.2 Calculation of photovoltaic parameters Here, a single junction p-i-n type a-si: H solar cell structure is presented as TCO/p-i-n/rear metal contact. The active layer is i-layer and others considered as dead layers. Because the photons absorbed in i-layer contributes to the current generation. Thus, the maximum photocurrent density generated in the solar cell (J sc ) can be written as: J sc = qx dλ..5.1 Where P i-layer is the number of photons absorbed in i-layer, x=1. V oc = ln( +1) 5.2 Where T is temperature, k Boltzmann s constant, Q diode factor and J o reverse saturation current density measured in the dark. P m =V max J max.5.3 Thesis Page 41

42 J m is current corresponding to the voltage V m at maximum power point and V m is the voltage corresponding to the maximum power output. Fill factor of the solar cell is defined as: FF= 5.4 Conversion efficiency of the solar cell is: η = = Where P in is the total incident power from the sun on the earth surface and it can be calculated from the solar flux data as: P in =..5.6 Here, considered solar flux at Air mass one and one half (AM1.5) with intensity 100mW/cm 2 Table 5.1 Parameter Value Source [Ref] Thickness of the p-layer 10nm [20] Thickness of the n-layer Thickness of layer ITO layer Optical energy gap of a-si: H Refractive index of p-i-n layer Refractive index of the Al electrode Light spectrum used Dielectric Constant Donor doping level for p- layer Acceptor doping level for p- layer Donor doping level for p- layer 30nm 50nm 1.72eV n( ), k( ) n( ), k( ) A.M x10 22 m -3 1x10 23 m -3 2x10 22 m -3 [20] [4] [21] [22] [22] [3] [23] [24] [24] [24] Thesis Page 42

43 Acceptor doping level for n- layer Electron mobility Hole mobility 1x10 23 m cm^2/vs 0.5 cm^2/vs [24] [25] [25] 5.3 Results These are the results from SETFOS software which is used to design a-si: H solar cell for optimal photovoltaic performance Single junction p-i-n cells Using the semiconducting thin film optics simulation software (SETFOS), initially a single junction p-i-n type solar cell is designed with structure of TCO/p-i-n/rear metal contact. The refractive indexes n ( ), and extinction coefficients k( ) has been taken from the library of SETFOS to calculate absorbance, reflectance and transmittance of the solar cell. And the absorbance, reflectance and transmittance have been calculated for a-si: H solar cell as shown in figure The absorbance of individual layers of single junction cell with structure: glass/ito (0.05µm)/p (0.01µm)/I (0.5µm)/n (0.03µm)/Al, as a function of wavelength as shown in Fig 5.1. Thesis Page 43

44 Figure 5.1 Absorbance of ITO, p-layer, i-layer, and layers as a function of the wavelength Green colour represents as absorbance in the i-layer Yellow colour represents as absorbance in the p-layer Blue colour represents as absorbance in the n-layer Pink colour represents as absorbance in the ITO. Brown colour represents as absorbance in whole cell The i-layer thickness is 500nm Thesis Page 44

45 Figure 15.2 Total absorbance in a cell Figure 16 Total reflectance in a cell Thesis Page 45

46 Figure 17 Transmittance of whole solar cell In this design, thickness with 0.05 μm Indium tin oxide (ITO) is used as TCO, this is the optimal thickness of TCO for single junction p-i-n solar cell calculated previously (P.stulik, J.Singh 1996). And the thickness of p-layer, i-layer and n-layer are 0.01 μm, 0.05 μm and 0.03 μm. The absorbance in the i-layer and total absorbance in the whole single junction p-i-n solar cell are calculated as function of the wavelength of incident radiation by using the above thicknesses for ITO, p-layer, i-layer and n-layer, as shown in fig 5.2. Here, reflectance of the whole single junction p-i-n solar cell as function of wavelength is also plotted in Fig 5.3. Here, difference can be observed by looking at the fig 5.1, the i-layer absorbance is higher than other layers absorbance. When it is compared to the total cell absorbance, whole cell absorbance is only slightly higher than the absorbance of i-layer. The solar integrated absorbance is plotted by using SETFOS in the p-i-n single junction solar cell as a function of the thickness of its i-layer as shown in Fig 5.5 Thesis Page 46

47 Integrated absorbance in the i-layer Thickness of i-layer (nm) Figure 18 Absorbance of a cell as a function thickness of i-layer. Figure 19 Photocurrent as function with i-layer thickness Thesis Page 47

48 I-V characteristics of the single junction p-i-n solar cell are shown in Fig 5.7. By using the SETFOS, which gives current (I) and voltage (V) characteristic parameter values, and by using the equation 5.5 conversion efficiency is calculated. Here, integrated absorbance and conversion efficiency depend on the thickness of i-layer. The below plotted I-V curve with the thickness of 0.5 μm based on some experimental researchers (Iida, 1984). The conversion efficiency of below graph is 5.5%. This type of similar I-V graph can be obtained for any cell. Hence, if parameters like J sc, V oc, FF are obtained, efficiency can be calculated for any other cell easily. To minimise the effect of recombination and trapping of charge carriers, the width of i-layer shouldn t exceed the width of the depletion layer. Thickness of the a-si: H i-layer = 500nm Figure 207 I-V characteristics and efficiency obtained for a-si: H solar cell using Eq. (5.5) Open circuit voltage V oc = V Short circuit current J sc = ma Fill factor FF=0.69 Conversion efficiency η = 5.5 % Thesis Page 48

49 6 Discussion 6.1 Single junction p-i-n cells A single junction (p-i-n) type a-si: H solar cell is designed with SETFOS as a tool to achieve optimal photovoltaic performances. Based on few research papers, the solar cell layer structure has been designed. The material absorbance depends on the refractive index and extinction coefficient values. These values can be selected from the SETFOS library. The optical and electrical input values have given into the SETFOS as shown in table 5.1. There are three steps to follow in SETFOS for this type of design of solar cell. The first step can be achieved by selecting absorbance module. Once layer structure is done then start simulation, thus, it gives individual layer absorbance as function of wavelength. The second step is to give sweep variables as i-layer thickness (d) from (0-900) nm and select the optimisation module. After simulation is complete, the SETFOS gives optimised thickness of i-layer. Final step is to consider that optimised i-layer thickness (d) for the solar cell and enable driftdiffusion module before simulation. Once simulation has been processed, photovoltaic parameters like J sc, V oc, J m, V m, FF can be obtained from SETFOS. And the conversion efficiency is calculated based on these parameters. The absorbance of solar radiation in each layer is obtained from the SETFOS, particularly in the active i-layer of cell. The dependence of the integrated absorbance and conversion efficiency of a single junction p-i-n type solar cell with respect to the thickness of i-layer as shown in fig 5.5, as a result, absorbance and conversion efficiency increases with respect to the thickness of i-layer. But, the recombination and trapping processes involved in a-si: H is not considered into an account. However, it is reasonably well established that the recombination processes come to be important in i-layer of thicknesses greater than the depletion width of a cell. The both recombination and absorbance are enriched when the thickness of the i-layer increases beyond the depletion width, as result, the effect of increased recombination would be to decrease the conversion efficiency of a cell. In the fig. 5.7 the V-I characteristics for a single junction p-i-n solar cell of i-layer with thickness d=0.5 μm, gives the conversion efficiency of 5.5 %. This conversion efficiency is low compared to the recent highest efficiency 13.1% achieved experimentally. It may be considered that the quality of a thin film is determined majorly based on its refractive indexes and extinction coefficients and also other electrical parameters like acceptor and donor concentration of material, electron and hole mobility, recombination efficiency. There are Thesis Page 49

50 some important practical design approaches for making high efficient a-si: H solar cell. These methods lead to an enhancement of light absorption in the intrinsic layer of a-si: H and are commonly defined by the term light trapping. But, using SETFOS to design single junction p-i-n type solar cell, optical and electrical parameter should be accurate before simulation. Here, the simulation takes long time to give accurate results. To get a highest efficiency of p- i-n type a-si: H cell simulation has be done for several times. But, due to limited time frame for this thesis, the simulation hasn t been done for several times until to reach highest efficiency. Thesis Page 50

51 7 Future Work Design amorphous silicon solar cells for optimal photovoltaic performance by using SETFOS software. In this thesis part-a literature review and software training has been done. Later, in thesis part-b, the design of hydrogenated amorphous silicon solar cell has been done but the conversion efficiency is low compare to the present highest conversion efficiency. In further research, the conversion efficiency can be increased by taking all parts into consideration. This topic is very interesting and broad. Wide range of research is going on to increase the efficiency of solar cell. It is very challenging to reach present conversion efficiency and to improve that efficiency. Thesis Page 51

52 8 Conclusion In this Thesis, more focus has been built on the design and working principle of the single junction (p-i-n) type amorphous silicon solar cell and theory behind it. Here, Semiconducting thin film optics software (SETFOS) is used to design single junction p-i-n type a-si: H solar cell. By using SETFOS, absorbance, reflectance and transmittance of the solar cell has been calculated also absorbance of each layer has been calculated. For better performance of the solar cell absorbance should be high, reflectance and transmittance should be low. The efforts on increasing the absorbance in i-layer can be made by optimising it with respect to the thickness (d) of i-layer. The absorbance of i-layer is optimised for optimal photovoltaic performance. Here, the obtained conversion efficiency (5.5%) is low compared to the present highest conversion efficiency (13.1%). This is happened due to short time for doing this project. Also simulation always takes long time to run and give appropriate results. Moreover, the software which used in this project also expired before completion of this project. Finally, the conversion efficiency can be increased in further research. Thesis Page 52

53 9 References 1. Bube, R 1983, Fundamentals of solar cells: photovoltaic solar energy conversion, Access Online via Elsevier. 2. Carlson, D. E. and C. R. Wronski (1976). "Amorphous silicon solar cell." Applied Physics Letters 28(11): Chittick, R, Alexander, J & Sterling, H 1969, 'The preparation and properties of amorphous silicon', Journal of the Electrochemical Society, vol. 116, no. 1, pp Deng, X & Schiff, EA 2003, 'Amorphous silicon based solar cells'. 5. Dzhafarov, T 2013, 'Silicon Solar Cells with Nanoporous Silicon Layer'. 6. Fluxim, A 'Semiconducting thin film optics simulator SETFOS', Website: fluxim. com. 7. Gray, JL 1989, 'A computer model for the simulation of thin-film silicon-hydrogen alloy solar cells', Electron Devices, IEEE Transactions on, vol. 36, no. 5, pp Green, M. A., Emery, K., Hishikawa, Y., Warta, W. and Dunlop, E. D. (2013), Solar cell efficiency tables (version 42). Prog. Photovolt: Res. Appl., 21: doi: /pip Iida, H, Mishuku, T, Sakata, I & Hayashi, Y 1984, 'A milky tinoxide on glass (MTG) substrate thin undoped layer pin amorphous silicon solar cell with improved stability and relatively high efficiency', Electron Device Letters, IEEE, vol. 5, no. 3, pp Jaeger, RC & Blalock, TN 1997, Microelectronic circuit design, vol. 97, McGraw-Hill New York. 11. Largent R, WS 2003, Solar cells : resources for the secondary science teacher, Kensington, NSW : Key Centre for Photovoltaic Engineering, University of New South Wales, Faculty of Engineering, Kensington, NSW]. 12. McGuire, GE 1988, 'Semiconductor Materials and Process Technology Handbook for Very Large Scale Integration(VLSI) and Ultra Large Scale Integration(ULSI)', Noyes Data Corporation, Noyes Publications, Mill Rd. at Grand Ave, Park Ridge, New Jersey 07656, USA, Mott, N 1969, 'Conduction in non-crystalline materials: III. Localized states in a pseudogap and near extremities of conduction and valence bands', Philosophical Magazine, vol. 19, no. 160, pp Pavel Stulik, Designing Amorphous Silicon Solar cells for their optimal photovoltaic performance, Northern Territory University, PhD thesis, Faculty of Science, 1998 Thesis Page 53

54 15. Pawlikiewicz, AH & Guha, S 1990, 'Numerical modeling of an amorphous-siliconbased pin solar cell', Electron Devices, IEEE Transactions on, vol. 37, no. 2, pp Rech, B & Wagner, H 1999, 'Potential of amorphous silicon for solar cells', Applied physics A, vol. 69, no. 2, pp Scientific, C 2013, Solar Cell Voltage - Current Characterization, CSI, viewed 22 October 2013, < 18. Singh, R, Designing amorphous silicon solar cells for optimal photovoltaic performance 1st ed. Charles Darwin University: CDU Library. 19. sopra-sa Sopra. [ONLINE] Available at: [Accessed 01 July 13]. (SETFOS 3.3) 20. Sinha, S & Dubey, G 2000, 'Calculation of amorphous silicon solar cell parameters with different doping levels', in Energy Conversion Engineering Conference and Exhibit, 2000.(IECEC) 35th Intersociety, vol. 2, pp Staebler, D. and Wronski, C Reversible conductivity changes in discharge produced amorphous Si. Applied Physics Letters, 31, Stulik, Singh, P J, Optical modelling of a single-junction p-i-n type and tandem structure amorphous silicon solar cells with perfect current matching. Solar energy materials and solar cells, 46, Sze, SM & Lee, M-K 2012, Semiconductor Devices: Physics and Technology, Wiley Global Education. 24. Wöhrle, D & Meissner, D 1991, 'Organic solar cells', Advanced Materials, vol. 3, no. 3, pp Zeman, M 2013, Thin-film silicon solar cells, Delft University of technology, viewed 25th MAY 2013, ZHU, F, Studies of the optimal structure and photovoltaic of thin film hydrogenated amorphous silicon (a-si: H) solar cells. PhD thesis. Charles Darwin University: CDU Library. 27. Zhu, F & Singh, J 1993, 'An approach to study the effect of the band tail widths on the photovoltaic performance of pin a-si: H solar cells', Journal of non-crystalline solids, vol. 163, no. 1, pp Thesis Page 54

55 28. Zhu, F & Singh, J 1993, 'Study of the optical properties of amorphous silicon solar cells using admittance analysis', Journal of non-crystalline solids, vol. 152, no. 1, pp Thesis Page 55

56 10 Appendices Thesis Page 56

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