Flexibility Enhancement of Flexible Solar Cell Devices by Nano-Mechanical Analysis of The

However, the results of the latter process, such as domed, pyramidal, or columnar structures, can play the role of nano- or microdefects on the surface of the silicon substrate, causing cracks to propagate and reduce the mechanical properties of the entire material. the product. Defect analysis was also performed from SEM images of fracture surfaces and FEA simulations. Encapsulation effect (a) Normalized PCE under ambient conditions (b) Normalized PCE of Al2O3 deposited devices immersed in water.

Introduction

In Chapter 3, the bending strength and critical bending radius of non-, dome-, and pyramid-textured silicon are measured by 4-point bending tests in the OM system. In Chapter 4, the mechanical properties and moisture permeability of thin Al2O3 layers were measured using a tensile test and an electrical calcium test. Based on the tensile results, the critical bend radius was calculated, and cyclic bending was performed at a radius greater than the critical bend radius.

Research background

Flexibility of ultra-thin silicon solar cell

• Flexible ultra-thin silicon
• Mechanical properties of surface textured silicon

38] reported flexural strength of nanotextured silicon samples is measured using three-point bending test and has higher value than planar samples even including micro V-notch. As the nanotexture depth increases, the stress distribution improves and the flexural strength from the three-point bending test improves. The bending tests are classified by three-point bending test and four-point bending test.

Figure 2-1. Best research-cell efficiency chart. Copy right 2020. NREL

Encapsulation for flexible solar cell

• Flexible encapsulation
• Water vapor transmission rate
• Griffith’s theory
• Atomic layer deposition
• Amorphous Al 2 O 3 by ALD

WVTR is the representative value of the performance of the barrier film and means the rate of diffusion of water vapor through the materials. Ca optical test and Ca electrical test are used Ca layers to measure water vapor permeability. The optical calcium test tracks the optical transmission changes of the calcium layer resulting from the reaction of calcium with water vapor.

Meanwhile, water vapor passing through the barrier film diffuses into the gas volume, resulting in homogeneous calcium degradation (Figure 2-9). In the initial region named after the lag region, the change in conductivity is negligible because water vapor does not pass through the barrier film. The transition period is the region where a small amount of water vapor diffuses and reacts with the calcium layer.

Finally, a certain amount of water vapor is continuously supplied to calcium so that the conductance decreases linearly. Where Rw and Aw are the transmission rate of water vapor through the barrier film and the window area, respectively. Water vapor passes rapidly through organic material due to internal free volume, and the WVTR value for organic film is ~100 g/m2day.

As mentioned previously, the water vapor size is approximately 3Å, so a nanosized defect can act as a diffusion path of water vapor. Before fracture, solid has elastically stored energy in (A) and crack propagates when the released elastically stored energy is at least equal to. the energy required to generate a new crack surface. Thermal ALD uses water vapor as oxidant and the high density film can be deposited at high growth temperature.

Figure 2-6. Encapsulation effect (a) Normalized PCE in an ambient condition (b) Normalized PCE of the Al 2 O 3 -deposited devices immersed in water

Flexibility of ultra-thin silicon of flexible silicon solar cell

• Introduction
• Preparation of surface textured silicon
• Flexural strength of surface textured silicon
• Weibull distribution
• Stress concentration analysis
• Theoretical analysis
• Stress distribution analysis by FEA simulation
• Critical bending radius of textured silicon
• Conclusion

The characteristic strength of the flexural strength was in the order of non-, dome- and pyramid-textured silicon samples. Surface texturing forms surface notches that act as stress concentrators; thus, the flexural strengths of the dome and pyramid textured samples are lower than those of the non-textured sample. The schematic of the rectangular parallelepiped with a single notch shown in Figure 3-5 (a) describes stress-concentration factor K as.

The measured stress concentration factors of the cube and pyramid textured samples, which were calculated using the ratio of the characteristic strength 𝜎0 to that of the untextured sample, were 1.34 and 1.54, respectively. 75° for the cube-textured sample and  = 70.5° for the pyramid-textured samples did not affect the stress concentration at the peaks of the surface textures. However, we did not present this quantitatively because the stress concentration factor equation is only valid up to h/r = 20 (a depth of 10 um and a tip radius of 28 nm corresponding to pyramid texturing) .

In the surface textured samples, domes and pyramids were also distributed along the thickness direction; thus, the stress concentration dispersion was also effective along the thickness direction. The critical bend radius for the dome-textured sample was 26.1% larger than that of the untextured sample and 26.7% smaller than that of the pyramid-textured sample. In the wavelength range between 700 nm and 900 nm, the reflectance of the dome-shaped surface was lower than that of the pyramid-shaped surface [86].

These results were influenced by the stress concentration of the geometries of the surface textures.

Figure 3-1. Preparation of surface textured samples (a) Pyramid- and (b) Dome-texturing

Flexible encapsulation

Introduction

• Optical properties of Al 2 O 3
• Mechanical properties of Al 2 O 3

Hole nanoindentation was performed to confirm changes in mechanical properties and optical properties of the Al2O3 thin film depending on the growth temperature. In the case of the conventional nanoindentation, it is limited to measuring the accurate properties due to the influence of the substrate in the case of thin film because the sample is limited to the substrate. However, hole nanoindentation measures the mechanical properties of free-standing films that are not bonded to the substrate.

The mechanical properties of the Al2O3 thin films are investigated by indenting the center of the free-standing film through hole nanoindentation. The gold layer is selectively etched with gold etchant, causing a thin Al2O3 film to float in water. After the floating Al2O3 thin film is transferred to the hole substrate, free-standing Al2O3 thin films are obtained.

Where F, 𝜎0, a, 𝛿, E and v are the applied force, film stress, membrane diameter, deflection at the center point, Young's modulus and Poisson's ratio. Where 𝜎𝑚 and R are the maximum stress at the center point of the film radius and tip, respectively [96]. The fracture strength can be calculated from equation 25, however, since it is an equation for elastic deformation, the fracture strength may be overestimated.

Based on these optical and mechanical results, experiments with Al2O3 thin films deposited at 80℃ were carried out in the following study.

Figure 4-1. Optical properties of PEALD-Al 2 O 3 thin films depending on temperature (a) Refractive index (b) Calculated density

• Sample preparation for push-to-pull tensile test
• Plastic deformation by e-beam irradiation
• Tensile properties of Al 2 O 3 thin films
• Critical bending radius

In both compression and tension, amorphous silicon had electron beam-assisted plastic deformation at room temperature. In previous research, brittle ceramic materials show plastic deformation in very thin nanowires by surface diffusion. According to this paper, the e-beam not only causes plastic deformation of the amorphous silica sphere, but also densification. a) Compression of silica particles (b).

Under beam off (c) Beam state (d) Beam on/off compression plot (e) Atomic bond switching mechanism [97]. Al2O3 thin films have plastic deformation and elastic modulus decreases in SEM due to beam penetration. Therefore, many dangling bonds which are applied by plastic deformation can be formed in thin films.

When the elastic modulus of film and substrate are similar, the neutral plane is in the center surface and stress and critical bending radius can be expressed by following the following equation. Where Ef, Es, tf and ts are the elastic modulus of film and substrate and the thickness of film and substrate, respectively. Because the thickness of PET substrate is very thick compared to the thin Al2O3 film, the difference in elastic modulus is negligible.

Tensile test results of 100 nm-thick Al2O3 thin films (a) Beam on (b) Beam off condition. a) Stress-strain curves of PEALD-Al2O3 thin films (b) SEM images before and after fracture.

Figure 4-5. Tensile test using push-to-pull device

• WVTR of single Al 2 O 3 layer
• WVTR of Al 2 O 3 film after cyclic bending

Mechanism of WVTR degradation

• Atomic bond-switching in amorphous oxide
• Optical calcium test

Because microcracks do not form after cyclic bending, water vapor diffuses through nanoscale defects that cannot be seen in the OM image. The oxidation is initiated at a specific time within 1 hour after 1,000 cycles of bending at 10 mm radius. These results may be evidence for the formation of pinholes, which are water vapor diffusion pathways in the material rather than microcracks.

The sample that bends at 20 mm radius after 100,000 cycles has smaller and more pinholes than the sample that bends at 10 mm radius after 1,000 cycles. Since applied load is so small at 20 mm bend radius, bond shift does not occur at initial low bending cycles. The reason why the oxidation aspects are different between 10mm and 20mm bend radius, applied load on the material is different as bend radius.

Since applied strain at 10 mm bend radius is greater than 20 mm bend radius, larger pinholes are generated. In previous research, defect density was analyzed depending on bend radius and the number of cycles. Since PC substrate is etched by acetone, the point etched by the defect can be observed after thin film deposition when exposed to acetone.

As a result of cyclic bending, the defect density increases as the bending radius decreases and the number of cycles increases.

Figure 4-13. Crack propagation at the notch tip [108]

Application

• Optical Mg test
• Perovskite solar cell encapsulation

Hybrid organic-inorganic halide perovskite solar cells (PSCs) are based on methylammonium lead iodide (CH3NH3PbI3). Perovskite solar cells have advantages of high power conversion efficiency (PCE), cost-effective materials and manufacturing process [11]. When CH3NH3PbI3 is exposed to water vapor, the color changes from dark brown to yellow due to the formation of PbI2.

To improve the stability of the perovskite solar cell, exposure of water vapor is prevented by encapsulation. In this study, perovskite solar cell was encapsulated by PEALD-Al2O3 and PCE was measured over time. Perovskite solar cells were heated to 30℃ (hereafter referred to as reference cell) and 60℃ (hereafter referred to as heated cell) under dark conditions, and the efficiency was measured in ambient condition.

In this research, PSC was encapsulated using PEALD-Al2O3 thin film and accelerated tests were performed at 30℃, 90% RH in dark conditions. As shown in Figure 4-18, the color of the encapsulated PSC changed from dark brown to light brown due to water vapor, while the PCE decreased rapidly.

Figure 4-17. Optical magnesium tests encapsulated by 25 nm thick-Al 2 O 3 at 85℃, 85% RH

Conclusion

Summary

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