Results and Discussion

4. Results and Discussion

4.1. Flexibility of Ultra-flexible Perovskite Solar Cells

4.2.3. Results and Discussion

The measurement of mechanical properties of functional constituent materials in flexible PSCs are too challenging due to thickness of each materials are below 100 nm, except perovskite material.

Accordingly, many studies carried out nanoindentation on precipitation and thick films. To investigate whether flexibility of flexible PSCs is dominated by perovskite materials, we carried out hole- nanoindentation tests for measurement of mechanical properties in constituent materials of flexible PSCs. Unlike nanoindentation on bulk materials which is restricted substrate effect, called 1/10 rule, it is performing nanoindentation on ultra-thin films which is suspended on hole-patterned Si substrate, as shown figure 4.14 [10]. The hole-pattern was fabricated by photo-resist (PR) process and Deep reactive-ion etching (Deep RIE) process, as shown in figure 4.15. The pattered hole sizes were different, which dependent on different sample thickness. The hole diameters were set to be below 1%

due to prevention of stress concentration on hole edge. Because of this, AI 4083, PC61BM, MAPbI3

thin films were transferred on to 4, 8, 50 μm-diameter hole patterned substrate as shown figure 4.16.

Figure 4.15. (a) the Blueprints of hole patterned mask, (b) Scanning electron microscopy image of hole-patterned substrate, and (c) cross-section SEM image of hole-patterned substrate.

Figure 4.14. (a) Schematics of experimental method of hole-nanoindentation, (b) Typical scanning electron microscopy image of before testing.

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Figure 4.16. The configuration of samples and transferred on hole-patterned substrates, (a) and (d) are MAPbI3, (b) and (e) are AI 4083, (c) and (f) are PC61BM.

Figure 4.17 shows typical force-indentation depth curves for AI 4083, MAPbI3, and PC61BM.

Unlike previous studies of nanoindentation, these were prepared by same conditions and procedures for as the fabrication process of flexible PSCs, and then transferred onto hole-patterned substrate which is assisted by suitable sacrificial layers. It was expected to have identical microstructure and thickness with flexible PSCs. From figure 4.17, the fracture strength and elastic modulus of each layers are measure using:

𝐹 = 𝜎₀(𝜋𝑎) (𝛿

𝑎) + 𝐸(𝑞³𝑎) (𝛿 𝑎)

3

𝜎_𝑓 = (𝐹𝐸 4𝜋𝑅)

1 2

Where F and δ are indentation force and displacement at center point of hole, respectively, σ0 is the pretension in the film, a is the patterned hole radius, q = 1/(1.05-0.15v-0.16v²) = 1.02 with v = 0.33 [6]

is a dimensionless constant based on Poisson’ ratio, v, R is the indenter tip ratius, E is the elastic modulus, and σf is the maximum stress at the center point [10-12]. The elasic modulus and fracture stress were divided by thin film thickness with 40 nm for AI 4083, 300 nm for MAPbI3, 60 nm for PC61BM. The fracture strength of each layers was estimated to be 0.87 (±0.09) GPa for AI 4083, 0.49 (±0.01) GPa for MAPbI3, and 0.92 (±0.09) GPa for PC61BM, respectively. The elastic modulus was measured as 5.06 (±0.61) GPa for AI 4083, 13.67 (±1.85) GPa, and 11.90 (1.63) GPa for PC61BM, respectively.

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0 100 200 300 400 500

0 2 4 6 8

Force, μN

Indentation Depth, nm

PC₆₁BM

Perovskite: MAPbI₃ PEDOT:PSS: AI4083

Figure 4.17. Typical force-indentation depth curves of MAPbI3, AI 4083, and PC61BM.measured by hole-nanoindentation.

Figure 4.18. Schematics of separation method about elastic and plastic deformation in hole-nanoindentation methods, (a) using fitting curves, and (b) using linear elasticity.

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Figure 4.19. Typical hole-nanoindentation curve and linear elasticity of AI 4083.

Figure 4.20. Typical hole-nanoindentation curve and linear elasticity of MAPbI3.

Figure 4.21. Typical hole-nanoindentation curve and linear elasticity of PC61BM.

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The mechanical flexibility has represented to elastic limit which is separation point between elastic and plastic deformation and revealed recoverable properties after deformation. It was described to figure 4.18. In the hole-nanoindentation tests, however, it is difficult to distinguish the brittle/ductile behavior of the materials. Therefore, I can be established that the change of linearity of elastic modulus at a certain proportional of indentation depth as a start of plastic deformation, as shown figure 4.19 ~ 21. Coincidentally, it was also confirmed that the fitting of above equation begins to shift from that point. By simple assumption for linear elasticity and fracture, fracture strain is 17.19% for AI 4083, 3.58% for MAPbI3, and 7.73% for PC61BM. As previously mentioned, perovskite material is the weakest of all the constituent materials of flexible PSCs. Figure 4.22 shows calculated critical bending radius based on elastic limit of constituent materials using following equation

𝑅𝑐 = 𝑡

2 × 𝜀× 100(%)

where t is the total thickness of devices and ε is elastic limit (i.e. yield/fracture strain). Based on the elastic limit of constituent materials by hole-nanoindentation tests, the calculated critical bending radius is 0.05 mm for AI 4083, 0.10 mm for PC61BM, and 0.22 mm for weakest MAPbI3 with 15 μm-thick

Figure 4.22. Calculation of critical bending radius based on elastic limit measured by hole- nanoindentation of constituent materials in flexible PSCs.

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NOA 88 substrate. It has a difference with repeatable bending tests with critical bending radius of above 0.5 mm. It is means that hole nanoindentation tests were overestimated mechanical properties rather than actual mechanical properties. Figure 4.23 show fracture strength of the perovskite materials studied so far [4, 5, 7-9]. Previous results by nanoindentations and computational simulations are similar to hole-nanoindentation results approaches 426 MPa for theoretical strength defined by E/30.

The possibly reasons of overestimation of hole-nanoindentation are (1) nanoindentation tests are measured only local volume, rarely contained various defects in underneath indenter tip, and the constituent materials in PSCs are brittle manner, so its stress depends on the probability that it contains crack with high stress concentration. (2) in uniaxial tensile state, it is that occur catastrophic failure after crack nucleation near multiple defects due to uniform stress field, but hole-nanoindentation is insufficient driving force to occur catastrophic failure after crack nucleation due to less existence of defects [12]. For these reasons, it is too hard to represent flexibility of constituent materials measured by hole-nanoindentation due to its overestimation. However, it is distinctly indicated that perovskite material is weakest layer in flexible PSCs since it was performed same testing method. This indicate that perovskite materials have dominate mechanical flexibility of flexible PSCs.

Figure 4.23. Distribution of fracture strength of perovskite materials with previous studies and hole- nanoindentation results.

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