5. CHAPTER 4: MAPBI 3 BASED MEMRISTOR
4.7 Flexible HOIP Memristors
To expand the applicability of the HOIP memristors into flexible electronics, we fabricated them on flexible polyethylene terephthalate (PET) substrates. The device structure and the fabrication procedure adopted were the same as those for glass substrate.
For the convenience of spin coating, the PET substrate was stuck on a glass slide using scotch tape. The device schematic is shown in figure 4.11(a). The MAPbI3 layer had a thickness a 342 nm, which is the median value of 9 readings taken at different positions on the substrate using a profilometer. The XRD plot of MAPbI3 film on PET shows that the film is highly crystalline with a three-dimensional phase conformation at the peak of 14o, corresponding to the (110) plane (see figure 4.11(b)). The broad peak at ~26o corresponds to PET [22].
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Figure 4.11: (a) Schematic of the flexible memristor device. (b) XRD spectrum of the MAPbI3
film coated on the PET substrate, showing formation of desired (110) perovskite phase at 14°.
The FESEM and AFM images of the MAPbI3 films coated on glass and PET substrate are shown in figure 4.12 and 4.13 respectively. The FESEM image of the film coated on rigid glass substrate clearly shows polycrystalline grains (see figure 4.12(a)).
The average grain size was found to be 155 nm measured from 100 grains in an area of 1.8x1.3 µm2 using ImageJ software, where the grain size ranges between 51.5 nm, and 338.2 nm (see the grain size distribution in figure 4.12(b)). The film has an average RMS roughness of 1.46 nm, as obtained from the AFM image of 1x1 µm2 scan area (see figure 4.12(c)).
Figure 4.12: Morphological characterization of the fabricated MAPbI3 films on ITO coated glass substrates. (a) FESEM image taken over an area of 1.8x1.3 µm2 with a scale of 200 nm. (b) The
grain size distribution for 100 grains. The average grain size is 155 nm. (c) AFM image taken over an area of 1x1 µm2. The average RMS roughness is 1.46 nm.
From the FESEM image of the MAPbI3 film coated on flexible PET substrate (see figure 4.13(a)), the average grain size has been found to be 125 nm (measured from 100 grains in an area of 1.8x1.3 µm2), which is slightly higher than that for glass substrate.
The distribution in the figure 4.13(b) shows the grain sizes ranging between 38.9 nm, and TH-3069_166102004
CHAPTER 4: METHYL AMMONIUM LEAD IODIDE MEMRISTORS
348.6 nm. From the AFM image, an RMS surface roughness of 1.51 nm was obtained over an area of 1x1 μm2.
Figure 4.13: Morphological characterization of the fabricated MAPbI3 films on ITO coated PET substrates. (a) FESEM image taken over an area of 1.8x1.3 µm2 with a scale of 200 nm. (b) The
grain size distribution for 100 grains. The average grain size is 125 nm. (c) AFM images taken over an area of 1x1 µm2. The average RMS roughness is 1.9 nm.
4.7.1 I-V Characteristics
The I-V characteristics were taken by applying a linear voltage sweep, with a compliance limit of 1 mA. The representative device characteristics, given in figure 4.14(a), show a 𝑉𝐹𝑂𝑅𝑀𝐼𝑁𝐺 of 0.6 V, a 𝑉𝑆𝐸𝑇 of 0.45 V, a 𝑉𝑅𝐸𝑆𝐸𝑇 of -0.28 V, and an ON/OFF ratio of 610. The RS behaviour was repeatable, as shown in the endurance plot in figure 4.14(c). The inset of this figure shows a single pulse train applied to realize the endurance characteristics. The device maintained an ON/OFF ratio of 98 for 1000 such cycles. The HRS and LRS could be retained with a consistent ON/OFF ratio of 103 for 1350 seconds, measured at 100 mV (see figure 4.14(d)). The ON/OFF ratio obtained in case of pulse characterization is comparatively lesser (~6 times) that that in case of characterization using linear voltage sweep. It can be improved by using a higher pulse width or a higher pulse amplitude as discussed in section 4.5. However, this improvement in the ON/OFF ratio comes at the cost of device speed and/or power consumption.
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Figure 4.14: (a) I-V plots of flexible HOIP memristors. (b) Variation of 𝑉𝑆𝐸𝑇, 𝑉𝑅𝐸𝑆𝐸𝑇, and ON/OFF ratio from 10 different devices. (c) Endurance plot showing a consistent ON/OFF ratio
of 98 for 1000 cycles. (d) Retention plot showing ON/OFF ratio of 103 for 1350 seconds, measured at 100 mV.
4.7.2 Effect of Mechanical Stress
I-V characterization was carried out under applied tensile stress to understand the bending effect on the switching performance. To apply stress to the device during measurement, a simple setup has been made as shown in figure 4.15(a). A geometrical schematic used for bending radius calculation is shown in this figure 4.15(b). The degree of bending is defined in terms of bending radius (𝑅𝑏) and was calculated by the equation (4.2) below, where, a and b are the height and the distance between two edges of the substrate during bending.
𝑅𝑏 = 𝑏2
8𝑎+𝑎
2 (4.2)
The device performance has been compared for two different bending radii of 1.3 cm and 0.9 cm for the forming and switching steps as shown in figure 4.15(c) and (d) respectively.
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Figure 4.15: (a) A photograph of the measurement setup to characterize the devices under stress.
(b) A geometrical schematic of the device in the bending state. (c) Variation of forming step with different applied stress; 𝑉𝐹𝑂𝑅𝑀𝐼𝑁𝐺 tends to increase with bending. (d) Variation of switching
cycles with applied stress. While the ON/OFF ratio tends to reduce with increasing bending radius, only a small change is observed in the values of 𝑉𝑆𝐸𝑇 and 𝑉𝑅𝐸𝑆𝐸𝑇.
The 𝑉𝐹𝑂𝑅𝑀𝐼𝑁𝐺 was observed to increase with an increase in bending, being equal to 1.15 V for 𝑅𝑏=1.3 cm and 1.55 V for 𝑅𝑏=0.9 cm. On the other hand, 𝑉𝑆𝐸𝑇 and 𝑉𝑅𝐸𝑆𝐸𝑇 did not change significantly in the range of stress applied, as seen from the I-V plots in figure 4.15(d). However, the ON/OFF ratio tends to reduce with increasing bending radius, which may be due to the degradation of the HOIP layer and the subsequent decrease in 𝑅𝐻𝑅𝑆. Table 4.3 compares the performance of our device with similar flexible HOIP devices found in literature. From the table, we can see that our flexible memristors show promising results, which can be improved with further film optimization and device engineering.
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Table 4. 3: Bending radii and ON/OFF ratio of various flexible HOIP memristors
Device Structure Rb ON/OFF
ratio Ref.
PET|ITO|PEDOT:PSS|CH3NH3PbI3| PCBM|Ag
13 mm 112 Our
devices
9 mm 96
Au|MAPbI3|ITO|PET 15 mm 9 [10]
PET|ITO|PMMA|PeQDs:PMMA|
PMMA|Ag 7 mm 100 [23]
PET|ITO|CH3NH3PbI3|MoO3|Ag 9.2 mm 15 [24]
Ag|CH3NH3PbI3|Pt|COP 5 mm 105 [25]
PET|ITO|(CH3NH3)2PbI2(SCN)2|Al 8.8 mm 103 [26]
The next chapter presents two different types of RS obtained from the same 3D HOIP memristor under different operating conditions and a novel approach to simulate these HOIP memristor characteristics.