(The) thermal behavior analysis and evaluation of flexible CIGS thin film solar cell on Fe-Ni alloy substrates

다국어 초록 (Multilingual Abstract)

It necessitates the application of a bending flexible substrate to produce a flexible CIGS solar cell. To determine the materials of the bending flexible substrate, the characteristics such as price, thermostability, anti-corrosion, conformance to vacuum, and thermal expansion should be taken into account carefully. There are some commercial flexible substrates such as plastic materials, ultra-thin glass, and metal foil. For instance, a double-layer plastic, as it is vulnerable to high temperature at 550℃, is not appropriate for producing high-efficiency CIGS absorption layers, and thin glass has a disadvantage in durability. Metal foil has rough surfaces and requires an additional coating process to secure electric insulation. Nevertheless, metal foil is utilized and produced widely as it fits to roll-to-roll processing. Metal foil has several types; molybdenum, aluminum, titanium, chrome steel, stainless steel, Fe-Ni alloy material, and more.
What causes the transformation of a solar cell is the behavior difference of thermal expansion occurred between the substrate and the layer of semiconductor used in the solar cell. The thermal expansion influences on the efficiency of solar cells. Therefore, the substrate has to possess a behavior of thermal expansion that is similar with that of semiconductor layer. As regards Fe-Ni alloy materials, the thermal expansion properties can be adjusted through the change of compositions.
This study employed electroforming to manufacture Fe-Ni alloy materials of different compositions. A Thermo Mechanical Analyzer (TMA) helped figure out the thermal expansion coefficients of Fe-Ni alloy materials in each temperature range. To add to the calculation, the physical properties such as Poisson's ratio, yield stress, tensile stress, modulus of elasticity, and strain hardening were obtained through a tensile test. Meanwhile, the physical properties of dielectric layer, back contact and light absorbing layer were collected from the bibliographies. Algor was chosen for the finite element analysis program in this study.
To verify the result from a finite element analysis, a two-dimensional Mo substrate was calculated and its verification experiment was conducted. Furthermore, a residual stress tester checked on the residual stress of specimens. The absolute values from the finite element analysis of Mo/substrate structure and its verification experiment showed a difference. However, the size of residual stress of individual substrate compositions had a similar tendency. Two-dimensional CIGS/Mo/SiO2/substrate was modeled.
Looking into the residual stress of CIGS layer occurred while the temperature declined from 550℃ to room temperature, the smallest residual stress was found with the use of Fe-52wt%Ni substrate material. Consequently, it is considered the Fe-52wt%Ni is the most appropriate substrate material in terms of the residual stress.

목차 (Table of Contents)

• Chapter 1. Introduction 1
• Chapter 2. Theoretical background 8
• 2. 1. The composition of CIGS thin film solar cell 8
• 2. 1. 1. Substrate 9
• 2. 1. 2. Barrier Layer 11
• 2. 1. 3. Back Contact 12
• 2. 1. 4. Buffer Layer 13
• 2. 1. 5. Window Layer 15
• 2. 1. 6. Anti-reflection film and grid electrode 16
• 2. 2. Electroforming 20
• 2. 3. Electroformed Fe-Ni alloy 22
• 2. 4. Finite Element Analysis 26
• 2. 5. Residual stress of film 30
• 2. 5. 1. Measurements on the residual stress of film 32
• Chapter 3. Experiment and FEA modeling 35
• 3. 1. Analysis of physical properties and Fe-Ni substrate materials 35
• 3. 1. 1. Equipment for electroforming experiment 35
• 3. 1. 2. Composition of electroforming solution 39
• 3. 1. 3. Conditions of electroforming experiment 41
• 3. 1. 4. Compositional analysis of Fe-Ni substrate 42
• 3. 1. 5. Analysis of thermal expansion characteristics of Fe-Ni
• substrate materials 45
• 3. 1. 6. Tensile stress test on Fe-Ni substrate materials 51
• 3. 2. Modeling and result of finite element analysis 54
• 3. 2. 1. 2D modeling of Mo/Substrate structure 56
• 3. 2. 2. Production of Mo/Substrate specimen and measurement on
• residual stress 59
• 3. 2. 3. Comparison of result values between finite element
• analysis and verification experiment 65
• 3. 2. 4. 2D modeling of CIGS/Mo/SiO2/Substrate structure and
• result 67
• Chapter 4. Conclusion 72
• Reference 74