Chapter 4: Results and Discussion
4.1 The Silicon Thin Chip by Etching
After etching for 49 hours, the 675μm-thick silicon wafers has been leveled down to chips with the thickness of 30 μm. The 30μm-thick silicon chips are thin enough to be bendable, and the flexible electronics in this experiment are base on these silicon thin chips. The XRD data reveals the silicon thin chips remaining single-crystalline in microstructure. The mobility of the silicon thin chips is an important parameter that determines whether the samples can be excellent devices or not, thus we measure the Hall mobility in this experiment. The Hall mobility of the silicon thin chips is 773 cm2 /V-S, which is better than that of amorphous silicon and polycrystalline silicon.
(a) (b)
Figure 4-1 The ultrathin silicon chips after etching. (a) optical image of 30μ m-thick silicon chip (b) optical image of 30μm-thick silicon chip with electrode film.
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Figure 4-2 The croess-section SEM image of the 30μm-thick ultrathin silicon chip.
The 30μm-thick silicon thin chips have good flexibility and little brittle nature, so we choose the silicon chips with a thickness of 30μm to fabricate our electronic devices. The etching process levels down the silicon wafer thickness from 675μm to 30μm, and the thickness of the chips deviates within 5μm.
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(a)
(b)
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(c)
Figure 4-3 X-Ray diffraction measurement of ultrathin silicon chips. (a) The database of silicon. (b) X-Ray diffraction diagram of bulk 675μm-thick silicon wafer.
(c) X-Ray diffraction diagram of ultrathin 30μm-thick silicon chip.
Comparing (b) with (C) , it is obvious that the ultrathin 30μm silicon chips are still single crystalline.
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Figure 4-4 The Hall mobility of ultrathin silicon chip is 773 cm2/V-S.
The Hall mobility shown in Figure 4-4 is 773 cm2/V-S, which is close to
commercial single crystalline silicon wafer, and the XRD data shows that the structure of thin silicon chip are still single crystalline.
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4.2 Optimal Conditions of Anisotropic Texturization
4.2-1 Surface Morphology of the Pyramidal Structure and Its Reflectivity Analysis
As mentioned in chapter 3-2, we have determined the optimized etching solution of silicon wafer is 1.19 % of TMAH including 50 % of IPA under the temperature of 80 °C. The anisotropic texturization usually leads to pyramidal structures; moreover, the higher pyramids go along with more surface areas. Therefore, we discuss the optimal texturing time for growing the optical pyramidal structures, and analyze the reflectivity by N&K analyzer and the surface energy by contact angle. Figure 4-5 indicates the morphology of silicon wafers etched in 1.19 % of TMAH and 50 % of IPA at 80 °C with different texturing times, and the silicon wafers are all carried out in vertically direction.
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Figure 4-5 The AFM and SEM texturing morphology of silicon wafers etched in 1.19 % of TMAH and 50 % of IPA at 80 °C with varying time. (a) and (b) are the morphology of silicon wafers textured for 30 min; the texturing time is 60 min for (c) and (d); 90 min for (e) and (f); and 120 min for (g) and (h).
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According to figure 4-5, longer etching time gives rise to higher density of pyramidal structures on the silicon wafer and faster etching rate of the (100) and (110) crystallographic planes compared with the etching rate of the (111) crystallographic plane. The morphology of pyramidal structures can be observed from the SEM. The different etching time results in the different average height of the pyramidal structure shown in figure 4-6. With longer etching time, the process leads to a faster etching rate and gives rise to the growing of higher pyramidal structures.
Figure 4-6 Average height of pyramidal structures with different etching time.
The highest density and optimal pyramids result from the 120 min processing time, and the average height is 3.01 μm.
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The etching mechanism of TMAH follows alkaline silicon etching and can be classified into three separate steps [54-56]:
The mechanism reactions takes place on the silicon surface. In the first step, equation 2, TMAH molecular dissociate into hydroxyl ions. In the second step (Eq. 3), the silicon atoms at surface react with these hydroxyl ions and form oxidized silicates Si(OH)
2
2+ and four electrons are injected from each silicon atom into the conduction
band. Simultaneously, water is reduced to provide more hydroxyl ions which are bonded to the silicate formed in second step. The reaction of equation 4 and 5 produce soluble silicic acid, with hydrogen gas as a byproduct. As shown in previous equations, it means the water will present and the silicon will be etched. When the concentration of water increases, more hydrogen forms and therefore more bubbles present. However, the bubbles will stick on the silicon surface and affect the etching rate as discussed in chapter 3-2. The bubbles on the surface will block the reactants and products from diffusing thus a depletion region forms. The hydrogen generated on the silicon surface forms bubbles which can reduce the natural upward flow between the solution and silicon surface and causes some difficulties in etching rate.
We add the IPA as the surfactant to diminish the adherence of hydrogen bubbles on
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the etched surface. Therefore, roughness increases during etching time due to the progressive etching rate in (100) and (110) directions, and finally pyramids with surrounding (111) planes leave behind.
In addition, the texturization efficiency depends on the concentration of etching solution, temperature and etching time. It provides an alternative method to create uniform and reproducible pyramidal texture on silicon wafer with the help of isopropyl alcohol (IPA) surfactant and agitation. Since the amount of water and IPA present in the solution increases in same time and less hydrogen produced when TMAH concentration decreases, therefore the roughness increases. Initially, a small number of pyramids forms at the surface, as etching process continues, new pyramids form and begin to superimpose over previous hillocks. The number and the size of the pyramids are observed by SEM and the average height is observed by AFM.
With the growing of pyramidal structures, the difference of etching rate results in higher pyramids and leads to a lower reflectivity shown in figure 4-7. By textural effects, the reflectance of pyramidal structures is lowered within all wavelengths. The higher pyramids leads to a lower reflectivity hence the light absorption is increased, and this absorption can be further improved by an optimized texturization [51]. In this experiment, the best condition of anisotropic texturization is etching under the condition of 1.19 % TMAH and 50 % of IPA solution at 80 0C for 120 min. For the sake of electrode manufacturing, we coat 5 nm-thick Cr and 10 nm-thick Au on the optimal texturing silicon substrate. The resulting reflectivity is relatively lower than untreated sample shown in figure 4-8. The reflection of the front surface needs to be minimized and the incident light must shine into the valley of the pyramids to increase the ability of light collection and promote high efficiency of silicon solar cells. This improvement was recently achieved by an anisotropic textured surface covered by a gold film [57], and shows a large decrease of the reflectance over the entire spectrum.
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This difference is explained by light trapping [58-60] improvement for following detection.
Figure 4-7 Comparison of the reflectivity of silicon surface with pyramidal structures during with different texturing time.
Figure 4-8 The reflectivity of silicon substrate with and without texturization
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process by N&K analyzer. The reflectivity of TMAH-textured silicon substrate with a thin gold film is down to 2.00 % at 400 nm light wavelength.
4.2-2 Surface Energy of the Pyramidal Structure
The definitions of surface energy involve consideration of the behavior of liquids in contact with solids materials and the formation of droplets. One convenient way of quantifying this behavior is to measure the contact angle formed by the liquid-solid interfaces shown in figure 4-9. The total surface energy is based on the Young–Dupre equation, which is described by the interfacial tension between the liquid (L) and the polymer surface (S) [61]:
The equation 6 is together with the Lifshitz–van der Waals (LW) and Lewis acid–base (AB) theories. The equation 7 is the surface tension (γi) of a phase i, and γi+ and γi– are the electron acceptor and electron donor parameters, respectively. The contact angle θ is determined using three different liquids with water, ethylene glycol and diiodomethane. The surface properties of optimal textured process are studied by the drop contact-angle technique shown in table 4-1. The total surface energy can be calculated from the contact angles of water, ethylene glycol, and diiodomethane.
According to the Young–Dupre equation, the contribution of van der Waals (γSLW)
surface tension is broader than acid–base (γS+, γS– ), and therefore the textured surfaces provide the apolar interface in table 4-2. The longer etching time leads to higher surface energy and relatively unstable surface due to larger surface area and surface tension [62]. However, when coating with a gold thin film, the total surface energy
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becomes lower which represents more stable surface. When we drop the liquid, the water drop become spreading out over the surface and the contact angle tends to be 5.97 degree, and it means the textured surface is super-hydrophilic.
Figure 4-9 The drop of DI water on the gold thin film coating on the textured surface. The contact angle of water droplet and film is 30.2 degree which means the hydrophilic feature of optimal texturing substrate etched on the conditions of 1.19 % TMAH and 50 % of IPA solution at 80 0C for 120 min.
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Table 4-1 The measurement of contact angle with different etching time by 1.19 % TMAH and 50 % of IPA solution at 80 0C.
Table 4-2 The measurement of surface tension and total surface energy with different etching time.
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4.3 Electrical Properties of flexible devices
Because the devices are made from single crystal silicon, they provide great electrical properties. The electrical properties are shown in the figures below.
4.3-1 Electrical Properties of MSM Photodetector
I-V measurements of MSM photodetectors were carried out using a halogen lamp incident from the object lens of optical microscopy in a dark environvent, and the wavelength is ranged in visible light region. Since our goal is to detect the light, the ratio of the light current (photo current, IL) to dark current (ID) is the main characteristic we concern with. We focus on this issue in this section. The definitions of dark current and photo current are the current responses without and without light illuminating respectively.
We use a calibration photodiode to measure the power of the halogen lamp. The calibration photodiode has a filter that only wavelengths between 300nm~ 800nm can pass through the filter and absorbed by the photodiode. While the photo current generated by the photodiode equals 1 mA, the power of the incident light is equivalent to 100 mW/cm2. The photodiode operates at reverse-biased region.Figure 4-10 shows the halogen lamp power intensity. In the following experiment, the power of the lamp which can be controlled by a roll knob equals to 1.46 mW/cm2 if not mentioned.
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Figure 4-10 Power intensity of the halogen lamp measured by the calibration photodiode.
Figure 4-11 shows I-V characteristics of MSM photodetectors. From Figure 4-12, the ratio of photo current density to dark current density that operates at 5V is 562.
The photo current and the dark current at 5 V are 2.7397 μA and 0.00486 μA respectively. In the small voltage region (~0.1V), the electric field which results from applied voltage may not be strong enough to separate the photo-induced electrons. As the applied voltage gets larger and larger, the corresponding electric field gets stronger at the same time, hence more electrons can be collected and finally results in larger photo current.
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Figure 4-11 The I-V characteristic of the MSM photodetector with interdigitated lines.
Figure 4-12 The dark and light I-V characteristic of the MSM photodetector with interdigitated lines.
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Good mechanical flexibility is extremely important for flexible electronics applications. We evaluate flexibility by performing repeatedly frontward and backward bending tests. Afterward we measure the device bent for 1~20000 times to check the stability. As shown in Figure 4-13, the dark current density and photo current density at 5V are very stable with different bending times. We also measure the durability with different bending radius (0.5~10 cm) to test the critical deformation condition. As shown in Figure 4-14, the devices deformed from 10 cm to 0.5 cm in radius perform little change in dark current density and photo current density at 5V. In Figure 4-15 we can find out the device is considerably stable, and the difference of current density between the devices unbent and bent for 100 times is around 2.1%.
Figure 4-13 The durability of the MSM photodetector at 5V.
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Figure 4-14 The dark and light I-V characteristic of the MSM photodetectors bent with various curvature radius.
Figure 4-15 The difference of I-V characteristic between MSM photodetectors unbent and bent for 100 times.
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4.3-2 Electrical Properties of PN Diode
Figure 4-16 shows the I-V measurement of a PN diode, and the ratio of ON/OFF current is 25120. From Figure 4-17, the ratio of photo current density to dark current density operation at -1V is 6.31. The photo current and the dark current at -1V are 0.00126 μA and 0.0002 μA respectively. In the small voltage region (~0.001V), the electric field which results from applied voltage may not be strong enough to separate the photo-induced electrons. As the applied voltage gets larger and larger, the corresponding electric field gets stronger at the same time, hence more electrons can be collected and finally results in large photo current.
Figure 4-16 The I-V characteristic of the PN diode.
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Figure 4-17 The dark and light I-V characteristic of the PN diode.
Good mechanical flexibility is extremely important for flexible electronics applications. We evaluate flexibility by performing repeatedly frontward and backward bending tests. Afterward we measure the device bent for 1~20000 times to check the stability. As shown in Figure 4-18, the dark current density and photo current density at -1V are very stable with different bending times. We also measure the durability with different bending radius (0.5~10 cm) to test the critical deformation condition. As shown in Figure 4-19, the devices deformed from 10 cm to 0.5 cm in radius perform little change in dark current density and photo current density at -1V. In Figure 4-20, we can find out the device is considerably stable, and the difference of current density between the devices unbent and bent for 100 times is around 0.7%. Figure 4-21 shows the current-time responses of MSM photodetectors at 1Vand PN diodes at -5V. The current of MSM photodetector and PN diode are
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stable during scanning time, and the performance of MSM photodetector is relatively stable compared with the PN diode.
Figure 4-18 The durability of the PN diode.
Figure 4-19 The dark and light I-V characteristic of PN diodes bent with various curvature.
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Figure 4-20 The difference of I-V characteristic between PN diodes unbent and bent for 100 times.
Figure 4-21 The current-time responses of MSM photodetectors and PN diodes.
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4.3-3 Electrical Properties of NPN Transistor
Figure 4-22 and 4-23 show the I-V measurements of NPN transistors in log and linear scale. The ratio of photo current density to dark current density operation at 1V is 8.47. The photo current and the dark current at -1V are 4.858 μA and 0.5734 μA respectively. As the applied voltage gets larger and larger, the corresponding electric field gets stronger at the same time, hence more electrons can be collected and finally results in large photo current.
Figure 4-22 The I-V characteristic of the NPN transistor in log scale.
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Figure 4-23 The I-V characteristic of the NPN transistor in linear scale.
4.3-4 Electrical Properties of Solar Cell
Figure 4-24 shows the I-V measurement of a flexible solar cell carried out under the illumination condition of AM 1.5 (1000W/m2 full-spectrum solar simulator at room temperature). Figure 4-25 shows a short-circuit current, Isc, of -2.0182 mA, and open-circuit voltage, Voc, of 0.1088 V, a fill factor of 0.25, electrode area of 0.0363 cm2 and an overall solar-energy conversion efficiency (η) of 1.5%. The properties of the 30 μ m-thick solar cells are mainly ranged in 1~1.6% for solar-energy conversion efficiency (η) and 0.08~0.11 V for open-circuit voltage (Voc).
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Figure 4-24 The I-V characteristic of the solar cell in linear scale (-10 V~5V).
Figure 4-25 I-V characteristic of the solar cell in linear scale (0V~0.12V) .
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Good mechanical flexibility is extremely important for flexible electronics applications. We evaluate flexibility by performing repeatedly frontward and backward bending tests. Afterward we measure the device which is bent for 1~20000 times to check the stability. As shown in Figure 4-26, dark current density and photo current density at 0.1V are very stable with different bending times. We also measure the devices with different bending radius (0.5~10 cm) to test the critical deformation condition. As shown in Figure 4-27, the devices deformed from 10 cm to 0.5 cm in radius perform little change in dark current density and photo current density at 0.1V.
Figure 4-28 shows the current-time responses of solar cell and PN diode at 0.1V, and the current of solar cell is more stable compared with the PN diode.
Figure 4-26 The durability of the solar cell.
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Figure 4-27 The dark and light I-V characteristic of solar cell bent with various curvature.
Figure 4-28 The current-time responses of solar cells.
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Figure 4-29 Optical images of flexible solar cells with conductive copper tapes.
The types of modules reported in this study may create new possibilities for single-crystalline silicon photovoltanics, particularly in the applications that benefit from thin, lightweight construction, mechanical flexibility or the unusual optical properties of the flexible solar cells designs. In most cases, we chose materials that have potentials for long lifetime and high reliability. The procedures are compatible with substrates, encapsulation, adhesive and optical materials used in existing photovoltaic systems. Similarly, the designs of advanced single-crystalline silicon cell can also incorporate with enhancement techniques for improved performance.
Although the main focuses presented here are based on module capabilities and designs rather than the cost or performance, these approaches of ultrathin cell geometry designs provide notable features in the efficient silicon usage. The process suggested here reduces the requirements for purity of the silicon, thus lowers the cost of silicon source materials. Low-cost doping and etching techniques are suitable for high-performance solar cells and module fabrication, together with other means to reduce cost or increase performance, are therefore important areas for further work.
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Chapter 5 Conclusions
This study presented here demonstrate the degree to which extreme mechanical properties can be achieved in fully formed, high-performance integrated circuits by the use of optimized structural configurations and multilayer layouts, even with intrinsically brittle but high-performance inorganic electronic materials. Such designs offer the possibility of direct integration of electronics with biological systems, medical prosthetics and monitoring devices [63-67], complex machine parts, or with mechanically rugged, lightweight packages for other devices. Further development of the mechanical concepts provides, for example, expanded ranges of flexibility [68], extends such electronic systems to other material types, and exploit them in new classes of devices all appear to represent promising directions for future research.
The excellent reliability of silicon, together with its high natural abundance and good efficiency in solar cells, suggest its continued use in production of solar energy, on massive scales, for the foreseeable future. Although organics, nanocrystals, nanowires and other new materials hold significant promise [69-73], many opportunities continue to exist for research into unconventional means of exploiting silicon in advanced photovoltaic systems. Here, we describe modules that use large-scale of ultrathin silicon chip solar cells created from bulk wafers. The resulting devices can offer useful features, including high degrees of mechanical flexibility, high quality ultrathin designs.
The experimant presents materials and processing approaches to achieve high performance bendable single-crystalline silicon electronic devices, as a result, expand the range of application possibilities for flexible electronics.
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