Performance Optimization Of Tin Halide Perovskite Solar Cells Via Numerical Simulation
Organic-inorganic hybrid perovskite solar cells have attracted great attention in the photovoltaic research community in recent years due to its ease of processing, low cost of production, superb light-harvesting characteristics, and relatively high efficiency which make it more preferable over other existing solar cell materials. Lead-based perovskites (CH3NH3PbX3, X= Cl, I, Br) solar cells have recently attained a high efficiency of ~19.3% which far surpasses the efficiencies of most thin film and organic solar cells. Therefore, the presence of lead, which is a toxic material in these solar cells poses serious challenge to our health and environment. ‘Tin’ is non-toxic and stands as a replacement to ‘lead’ for commercial purposes. Thus, there is a drive to use non-toxic materials such as tin-based perovskites. Unfortunately, the tin-based perovskite solar cells recently produced have low efficiencies (). In order to improve the performance of tin-based perovskite solar cells, a numerical simulation was done. First, known experimental results were reproduced. Based on the work reproduced we developed a new configuration with a reduced acceptor doping concentration of the absorber layer which showed an increase in efficiency > 18%. A device simulator, the Solar Cell Capacitance Simulator (SCAPS) was used to solve the poisson and hole and electron continuity equations in order to obtain information concerning the device properties of the tin-based perovskite (CH3NH3SnI3) solar cells.
TABLE OF CONTENTS
LIST OF FIGURES-ix
LIST OF TABLES-xi
1.1 General Background-1
1.2 The Sun and Solar Radiation-2
1.2.1 The Air Mass (AM)-3
1.3 The Solar cell-4
1.3.1 Evolution of Solar Cells-4
126.96.36.199 First generation-5
188.8.131.52 Second generation-5
184.108.40.206 Third generation-6
1.5 Objectives of the research-7
1.6 Research Methodology-8
1.7 Scope and Organization of the Study-8
2.1 Working principle of photovoltaic solar cells-9
2.2 Solar cell device characterization parameters-9
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2.2.1 Short-Circuit Current Density,
2.2.2 The open-circuit voltage,
2.2.3 Fill factor, FF-11
2.2.4 Power Conversion Efficiency,
2.3 Junctions in solar cell device-11
2.4 Perovskite solar cells-12
2.4.1 Properties of organic-inorganic hybrid perovskite materials-13
2.5 Prior work on the organic-inorganic hybrid perovskite solar cells-15
2.5.1 Perovskite solar cells device architecture-16
2.5.2 Charge carriers in perovskite material (CH3NH3PbX3)-17
2.5.3 Operational principle of perovskite solar cells-18
3.2 Description of working principles of SCAPS-21
3.3 Derivation of the governing equations in SCAPS-22
3.3.1 Poisson equation-22
3.3.2 Continuity equations-23
3.3.3 Carrier transport equations-24
3.4 Generation (Gn, Gp) and recombination (Rn, Rp)-30
3.5 Absorption Coefficient α-31
3.6 Real device analysis-31
3.5 Simulation Device Structure-32
The absorption profile below is as adopted from experiment.-35
1.1 General Background
The limited resources of conventional nonrenewable energy sources such as gas, coal and petroleum, along with the growing opposition to nuclear power generation, motivated by concerns about safety issues and radioactive waste disposal, have led to an increasing demand for safer, cleaner, and, most importantly, renewable sources of energy.
Photovoltaic (PV) technologies offer such a solution and have already been used for many years . Initially, the use of PVs was for power generation on satellites and space crafts  and later also for terrestrial applications .
The population growth, which is particularly large in off-grid rural areas not connected to the state electrical networks ( about 2.5 billion worldwide ), is another factor favoring the development of the PV industry. The interest in solar cells as an alternative energy source for terrestrial applications is further driven by social concerns about modern improved living standards as well as the high human desire to save money. There is also a need to protect our health and environment. These can be achieved through the use of energy that is produced from environmentally-benign PV technology, instead of the conventional fossil fuels, which produce environmentally-harmful greenhouse gases.
The promising PV potentials above have increased research interests and concerns on the sustainability and use of PV systems in solving global energy deficiencies . These have resulted not only in the further improvement of the efficiencies of silicon solar cells  along with considerable reduction of the solar energy cost, but also in the development of new PV materials and novel solar cell devices. The increase in production volume and new cost efficient solar cell technologies rendered this achievement possible. Currently a number of promising options for future developments of PV technology are available. The commercialization of these technologies has been hindered by the high cost per peak watt of solar cell modules. Even though researchers have made efforts over the years, improved performance, lower costs and reliability AMU, LORETA |Dept. of Theoretical Physics (AUST) 2 of PV systems remain major concerns . Hence, policy goals by state agents pertaining improved energy security and diversity, reduced emissions of greenhouse gases and increased levels of technology growth have spawned PV technologies in the past years.
1.2 The Sun and Solar Radiation
The sun, our closest star, is the origin of most of the energy maintaining life on earth and produces the necessary gravitational attraction to keep our planet in a nearly circular orbit. It has a mass of 1.99 x 1030 kg and a radius of 6.96 x 108 m . The earth-sun distance, R, is approximately 1.5 x 1011 m. A simple model assumes that the sun is spherical and the spectrum of the Sun’s solar radiation is close to that of a blackbody , whose surface temperature is at about T 6000 K. This surface temperature is maintained by energy generated through continuous nuclear fusion of hydrogen into helium in the interior. The interior temperature is, approximately 107K. As a result of interior temperature, the surface (photosphere) radiates electromagnetic waves in all directions. The spectral distribution is changed considerably when the sunlight penetrates through the earth’s atmosphere. Even for a clear sky, the light intensity is attenuated by at least 30% due to scattering by molecules, aerosols and dust particles and adsorption by its constituent gases like water vapor, ozone or carbon dioxide.
Figure 1.1: Solar irradiance spectrum
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The figure above shows the radiations outside the earth’s atmosphere (AM0) and at the surface (AM1.5). The dashed line in fig. 1.1 indicates the radiation distribution expected from the sun if it were a perfect blackbody at a temperature of 6000 K .
1.2.1 The Air Mass (AM)
The degree of attenuation is highly variable because of the constantly changing position of the sun and the corresponding change of the light path through the atmosphere. These effects are conveniently described by defining an air mass number (AMm). Air mass according to IEEE Standard Dictionary of Electrical and Electronics Terms  is the mass of air between a surface and the sun that affects the spectral distribution and intensity of sunlight. Air mass is the relative path length of light through the earth’s atmosphere in relation to the zenith point (Fig.1.2); the zenith point is the path length vertically upward at 900 and is defined as AM1. Hence, AM1 is the spectral distribution and intensity of sunlight on earth at sea level with the sun directly overhead and passing through a standard atmosphere. AM0 is the spectral distribution and intensity of sunlight outside the earth’s atmosphere. Air mass 0 is above the earth’s atmosphere along the zenith point. Solar cells are tested at AM1.5, which corresponds to the sun at a 48.20 from the zenith point, with a temperature of 25 0C. For any given angle θ, with respect to the overhead position, the air mass takes the value AMm, in which the air mass number, m, is represented as m = 1/cosθ and thus measures the atmospheric path length relative to path length when the sun is directly overhead. AM2 is the solar radiation at ground level when the sun is 60.1° above the horizon. The most widely used terrestrial standard is the AM1.5 spectral distribution (for θ = 48.2°), which is plotted as the terrestrial curve in Fig. 1.1. This terrestrial standard allows a meaningful comparison of different solar cells tested at different locations  .
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Figure 1.2: Illustration of various air mass (AM) positions and the zenith point. [Adapted from M.Pagliaro et al. (2008)].
1.3 The Solar cell
An example of a solar cell is a p-n junction semiconductor device which converts the solar energy (sunlight) directly into electricity by the photovoltaic effect. Solar cells are described as being photovoltaic irrespective of whether the source is sunlight or an artificial light. The operation of a PV cell is based generally on the following three steps:
1. The absorption of light, generating electron-hole (exciton) pairs.
2. The separation of excitons into free charge carriers (electrons and holes).
3. The transportation of separated charge carriers to their respective anode and cathode of the solar cell, and extraction of those carriers to an external circuit.
1.3.1 Evolution of Solar Cells
The advancement in solar cell technology is in order to produce a cheap (easy to produce), high efficiency and long lifetime (stable) solar cell which is a better replacement for energy generation from fossil fuel. Research has been going on, in order to meet these ultimate goals in photovoltaic technology which has led to the discoveries of new materials and new techniques in solar cells fabrication. Solar cells are categorized into three main groups known as generations based on their order of appearance in the market .
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Figure1.3: A schematic representation of the Evolutions of Solar cells Technology
220.127.116.11 First generation
The solar cells in this category are mainly made of silicon wafer. Bell Laboratories developed the first silicon solar cell in 1954 ,  with an efficiency of 6%. Since then, research on improving the efficiency and lowering the cost of these solar cells has been abundant. Silicon solar cells are the most widely used of all solar cells, and they are also the most efficient in terms of single crystalline cell photovoltaic devices. Also, silicon is the most abundant element on earth, only second to oxygen. This type of solar cell is the most widely used with the highest reported cell efficiency (single crystal cell) of ~ 28% . There are three types of silicon, used in this generation of solar cells: single crystalline silicon (c-Si), multi-crystalline silicon and amorphous silicon (a-Si). However, c-Si is expensive and involves high cost of fabrication. This has increased recent research interests into the next generation of thin film solar cells.
18.104.22.168 Second generation
This is also known as thin film solar cells. In an effort to reduce the fabrication costs of the present technology based on Si, and to increase material utilization, thin film materials have been the subject of intensive research. This second generation materials had been developed to reduce production costs of solar cells without jeopardizing their energy output . Three main types of materials have emerged as the most promising candidates for this generation of solar cells. These are hydrogenated amorphous silicon (a-Si:H), cadmium telluride (CdTe) as well as copper indium diselenide (CuInSe2) and its related alloys like Copper Indium Gallium diselenide AMU, LORETA |Dept. of Theoretical Physics (AUST) 6 CuInxGa1-xSe2 (CIGS). The highest recorded efficiencies of CIGS and CdTe thin film single cells are as high as 20% and 17%, respectively .
Although these thin film solar cells have a competitive edge on the first generation solar cells because of lower costs and good efficiencies, they have some drawbacks. Most of the material that these cells are made of are both becoming increasingly rare and more expensive (indium) or are highly toxic (cadmium). To mass produce these solar cells would also require new facilities, which would greatly increase the cost of production. Because of these drawbacks, a different generation of solar cells has been inspired.
22.214.171.124 Third generation
This generation of solar cells includes new concepts in their development. These new concepts are in order to solve the major challenges facing the first and second generation of solar cells which are the high costs of first generation solar cells and toxicity and limited availability of materials for second generation solar cells. This new generation includes Organic/polymer and dye-sensitized solar cells. Organic photovoltaic solar cell (OPV) technology employs semiconducting polymers as low cost materials alternatives to inorganic photoactive semiconductors (silicon, CdTe and CIGS). The third generation of solar cells is the cheapest of all the other solar cell generations. The efficiencies gotten so far for dye-sensitized and polymer single cells are ~ 11% and 8% respectively. This indicates that the efficiency of organic solar cell generation is generally very low. Furthermore, organic photovoltaic is technologically immature and its wide spread applications are limited by several instabilities issues that are associated to its solar cell degradation mechanisms in different environments. Hence, OPV and dye-sensitized technology are relatively low to make these cells competitive in a commercial market .
With the relentless effort of researchers in photovoltaics, a new type of solar cell which is based on organic-inorganic hybrid solar cell known as perovskite was discovered in 2006 by Miyasaka and his co-workers , . This new material has a highest reported efficiency of ~19.3% . These hybrid solar cell technologies such as perovskite based solar cells are classified as such because their photoactive layer is made of organometallic material. This work will focus purely on this organic-inorganic perovskite based solar cell which also shall be limited to a particular type known as methylammonium tin tri-iodide (CH3NH3SnI3).
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At present, the perovskite based solar cells are regarded as promising candidates of hybrid solar cells due to their ease of fabrication, strong solar absorption coefficient and low non-radiative carrier recombination rates for such simply prepared materials, plus its ability to capitalize on over 20 years of development of related dye-sensitized and organic photovoltaic cells . Recent results in literature suggest that a vapour-deposition  and a sequential solution deposition route  approaches are required to yield good quality films and high efficiency (η ≥ 15%) solar cell devices for lead perovskite. However, the presence of lead in the most efficient ones which contain CH3NH3PbI3 poses a serious challenge to the continued adoption of lead perovskite solar cells for commercial purposes due to the toxicity of lead which is a key component in this perovskite. This has led to a search for a non-toxic substitute for lead. Tin can be a viable substitute for lead since it is non-toxic and also in the same group as lead in the periodic table. Also, the relatively high absorbance of lead-free perovskite solar cells has demonstrated that its performance is not linked to the presence of lead and also beckons even higher efficiencies for non-toxic, abundant low cost solar cells . The highest efficiencies of tin-halide perovskite as reported by  is 6.4%.
1.5 Objectives of the research
Though Sn based perovskite solar cells are non-toxic compared to their lead counterparts, they cannot be directly used to replace lead perovskite solar cells. This is due mainly to their low efficiency, which makes them not as competitive as lead containing perovskite solar cell systems. Therefore, tin-based perovskite solar cells require efficiency improvements prior to their acceptability for applications in terrestrial energy production. Hence, the contribution of this study is to explore ways in which the efficiency of Sn-based perovskite can be improved. A numerical simulation approach will be used to study the characteristics of tin-based perovskite solar cells that are relevant for their performance improvement.
Our study is guided by the following specific objectives:
Develop a planar architecture of a tin-based perovskite solar
Reproduce the experimental result using our developed architecture
To investigate the influence of interlayer properties on the performance of the solar cells.
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Improve the efficiency of the obtained result
1.6 Research Methodology
The method adopted in this work is totally computational modeling. The results obtained will be compared with the experimental data that were reported earlier by Noel et al.. Attempts will be made to optimize the layer thicknesses in order to improve the performance of the solar cell. The modeling is carried out using a numerical software package called Solar Cell Capacitance Simulator (SCAPS). SCAPS  is a 1D numerical simulation program developed at University of Gent, Belgium. This is used to predict the changes in Sn-perovskite based solar cell performances associated to the incorporation of the absorber and the buffer (TiO2) layers. An optimum value of the absorber thickness will be determined.
Due to the short diffusion length (Ln) of ~30nm of the charge carriers of the absorber as reported by , there is bound to be recombination of charge carriers generated in the absorber, if the absorber thickness is larger than Ln. In that case, the charges recombine before they get extracted at the electrodes . This causes loss of charges and hence low solar cell efficiency. Reducing the thickness of the absorber can minimize this recombination effect. Furthermore, the different device parameters will be computed. These include the efficiency, fill factor FF, short circuit current Jsc and the open circuit voltage Voc. These will also be compared with the experimental results in the literature, 
1.7 Scope and Organization of the Study
This thesis is organized into five chapters. In Chapter 2, a literature survey is presented on the relevant theoretical aspects of present research. The typical solar cell designs are presented along with the advantages of perovskite solar cells. Attention is also focused on the critical relationship between the material properties and the fabrication conditions of these solar cells.
In Chapter 3, the numerical modelling of the device structure is performed based on a planar hetero-junction device architecture. The results obtained are compared with experiments from literature . In chapter 4, a detailed result analysis is carried out and presented. Finally, in Chapter 5, the most significant results are summarized. Concluding remarks and suggestions for future work are also presented in this chapter of the report.