**2. General structure and properties of TiO2**

TiO2 belongs to the transition metal oxide family. There are four different polymorphs of TiO2 found in nature such as anatase (tetragonal), rutile (tetragonal), brookite (orthorhombic), and TiO2 (B) (monoclinic) [6], the most important of which are anatase and rutile. With calcination at high temperatures exceeding ~600°C, the brookite and anatase polymorphs will transform into the thermodynamically stable rutile polymorph [5].

The tetragonal anatase bulk unit cell has dimensions of a = b = 0.3733 nm and c = 0.9370 nm, and the rutile bulk unit cell has dimensions of a = b = 0.4584 nm, and c = 0.2953 nm (**Table 1**). In both structures, the octahedral distortions create the basic building units [7, 8]. The lengths and angles of octahedral coordinated Ti atoms, therefore, dictate stacking in both structures, as shown in **Figure 1**.

pressure at room temperature in solution and also the phase-selective reduction between ana-

phase changed into black color. In the case of P25 TiO2

. Copyright (2003), Elsevier [18].

in **Table 2**. These results show that a slight change occurred along the a and b directions, but there was significant expansion in the c direction, and as a result, the unit cell volume

phase was not changed,

Preparation of Blue TiO2 for Visible-Light-Driven Photocatalysis

http://dx.doi.org/10.5772/intechopen.73059

229

crystals (right). The black color corresponds to the visual

, the blue colored

are shown

phases. They showed that the white anatase TiO2

appeared as a result of the combination of white and black colors (**Figure 2**) [9].

The unit cell parameters and nanocrystalline size profiles of white P25 and blue TiO2

(white P25) (left) and blue TiO2

. Copyright (2016), Royal Society of Chemistry [9].

tase and rutile TiO2

TiO2

while the rutile TiO2

**Figure 2.** Schematics of TiO2

color of the reduced rutile TiO2

expanded significantly as well [10].

**Figure 1.** Bulk structures of anatase and rutile TiO2


**4.3 High-precision calibration technology**

Thrust or impulse measurement is to use the vibration differential equation of the measurement system to calculate thrust or impulse inversely. The calibration accuracy of system parameters will directly affect the accuracy of the thrust or impulse measurement. Therefore, high-precision system parameter calibration technology is one of the key technologies of thrust or impulse measurement. The difficulty of highprecision system parameter calibration is the suppression of environmental noise interference and the generation of high-precision constant force. In addition to the influence of environmental noise on the calibration accuracy of system parameters, the generation and acquisition method of high-precision constant force is an effective technical way to improve the calibration accuracy of system parameters.


*4.10. Immunohistochemistry (IHC)*

IHC staining was performed to detect protein expression of KLF5 (1:400, Proteintech), AR (1:800, 06-680-AF488, Millipore), Ki67 (1:2000, ab15580, Abcam), cyclin D1 (1:2000, ab134175, Abcam) and MYC (1:1000, 9106, Abcam) in tumor xenografts. Formalin-fixed paraffin embedded tissues were sectioned at 4 μm, deparaffinized in xylene, rehydrated in graded ethanol (100–75%) and repaired antigen by boiling the slides in a citrate buffer (10 mM trisodium citrate, pH 6.0) for 3 min using a pressure cooker. After treatment with 3% H2O2 for 10 min, tissue sections were blocked with 10% normal goat serum, incubated with primary antibodies at 4 ◦C overnight, and then with EnVision PolymerHRP secondary antibodies (MXB Biotechnologies, Fuzhou, China) at room temperature for 30 min. After chromogenic reaction using DAB (MXB Biotechnologies), nuclei were stained with hematoxylin (MXB Biotechnologies). Finally, tissue sections were dehydrated and mounted.


*3.3. Simulation and Analysis of the Gripper*

ADMAS 2018 software was used to carry out an analysis of motion simulation, exploring the movement characteristics of the gripper. It would provide the theoretical basis for realizing fast and undamaged control algorithms. The end-effector clamping principle is that constrain part controls the opening and closing of three fingers. In the simulation, only the relevant parts of the gripper were studied. Cherry tomatoes are non-standard spheres. So, the ball was used instead of actual fruit in the simulation model. The physical properties of cherry tomatoes were taken as the parameters of the ball. The fruit-holding process was idealized in simulation (Figure 8).

**Figure 8.** The Adams simulation model of gripper.

First, the ball was deactivated to verify the rationality of the gripper design. The constraint part moved from the chassis to the end of the mechanical finger. Figure 9 shows how the open range of the finger ends varies with the distance of the constrain part. The open range of the finger ends were from 72.8 mm to 9.8 mm, respectively, which met the actual grasping demand of cherry tomato sizes.

**Figure 9.** The simulation result of distance of finger end. Note: C is where the constraint part touches the finger. D is the position at which the finger ends converge to the minimum.

The ball diameters were set to three sizes, 20 mm, 30 mm, and 40 mm, respectively, to explore the motion parameters of the constraint part. The balls were activated in the simulation. The distance was 24 mm from the constraint part to the chassis, and it was set to the beginning position. The constraint part and fingers did not touch each other under this parameter. The simulation results are shown in Table 2. When the finger ends connected the ball, the distances between the constraint part and the chassis were 66.9 mm, 54.9 mm, and 46.2 mm, respectively. The moving distances of constraint part were 22.2 mm, 30.9 mm, and 42.9 mm, respectively. It is shown that the movement distance of the constraint part was related to the diameter of the ball when gripping. The larger the fruit sizes, the smaller the movement distance. There was a negative correlation between fruit sizes and the movement distances of the constraint part.

**Table 2.** Parameters of constraint part movement on holding cherry tomatoes.

Note: *r* is the fruit size; *L*1 is the beginning distance of the constraint part from the chassis; *L*2 is the distance from the constraint part to the chassis when the finger ends touch the fruit; Δ*L* is the moving distance.

The target required a holding time of 1s. In this paper, the maximum moving distance Δ*L* was set to 50 mm, and 50 mm/s was the minimum velocity of the constraint part. Based on this, for the convenience of calculation and electrical control, this section discussed the holding force of the finger ends on the three fruit sizes at six speeds. The speeds were 60 mm/s, 70 mm/s, 80 mm/s, 90 mm/s, and 100 mm/s, respectively.

From the mechanical structure of the gripper, it is easy to know that the gripping force of the finger ends on the fruit increases continuously from the touching to the gradual clamping process. Therefore, this work only needs to explore the maximum gripping force amplitude of contact moment between the fingers and the fruit. The simulation step of contact moment was adjusted to 10,000. The definition of the maximum gripping force amplitude of contact moment is:

$$F\_{\overline{s}} = \max[F\_{\Delta t}] \tag{13}$$

where Δ*t* is the time of contact moment, Δ*t* = 0.001 s; *F*Δ*t* is the gripping force of contact moment, N; [*F*Δ*t*] is the gripping force set of contact moment; *max*[*F*Δ*t*] is the maximum gripping force amplitude of contact moment. The simulation results are shown in Figure 10 and Table 3.

**Figure 10.** The simulation results: (**a**) the relationship between speed and time; (**b**) the relationship between speed and *Fs*

**Table 3.** Results of motivation simulation.

At the same speed, the holding time increased with the decrease in fruit size. Due to gripping small-size fruit, the moving distance of constraint part was long. Under the same fruit size, the larger the speed of constraint part was, the shorter the holding time was. The holding time of three kinds of cherry tomatoes was less than 1 s, which reached the expected target. At the same speed, *Fs* increased with fruit sizes. The minimum values of *Fs* were 0.2074 N, 0.9792 N, and 1.4509 N at the speed of 50 mm/s, respectively. At the same fruit size, *Fs* increased with the speed of the constraint part. When the constraint part speed was 0.1 m/s, *Fs* reached 10.1627 N [22,29] with 40 mm, which produced a risk of damaging the cherry tomatoes.

The indicators of time and gripping force should be considered to ensure a fast and undamaged grab. When the speed of the restrained part is 0.08 m/s, the comprehensive simulation results show that the clamping time were 0.5381 s, 0.3872 s, and 0.2761 s, respectively, and the maximum gripping force were 0.9717 N, 3.5077 N, and 4.0003 N, respectively, for three fruit sizes. This speed meets the requirement of fast and undamaged when picking. Therefore, 0.08 m/s can be used to the actual speed of the constrain part, then the program and algorithm can be designed.


**5. Outlook**

as identical concentrations of the divalent ions Mg2+ and Ca2+ lead to different experimental outcome [65]. This ion-specific effect suggests that, in addition to electrostatic forces, also hydrophobic interactions are likely to contribute to the selective filtering properties of the basal

214 Composition and Function of the Extracellular Matrix in the Human Body

Systematic permeability studies with artificial particles were very helpful to unravel the physical mechanisms which are responsible for the trapping of solutes in the basal lamina. However, most compounds which encounter the basal lamina layer under physiological conditions are small molecules rather than microparticles. To investigate the selective prop‐ erties of the basal lamina toward small molecules, a microfluidic setup (**Figure 6**) was recently introduced [66]. Here, customized peptides with tailored amino acid sequences and thus different net charges were used as diffusion probes. To ensure optimal comparability, the molecular weight of those oligopeptides was kept constant. The penetration behavior of those peptides into an ECM gel was visualized by fluorescent microscopy, and similar to the SPT experiments discussed above also the behavior of those molecules critically depended on their charge. Positively charged peptides accumulated at the gel/buffer interface, whereas nega‐

**Figure 5.** Single-particle tracking experiments can locally map the permeability of ECM gels.  For freely diffusing particles, the dependence of the MSD on time is linear and a diffusion coefficient *D* can be calculated according to the formula shown in the graph. For the calculation of *D,* only the first 10% of the MSD data is used to avoid errors arising from statistical uncertainties. *Lower panel*: Trajectories of a particle which transiently switches between a diffusing and a bound state and of a particle which alternates between a strongly and a weakly bound configuration. The states of motion can be distinguished based on the fluctuation amplitude of the particle. Both trajectories were obtained at a salt concentration of 1 M KCl. The histogram shows that in ECM gels, the fraction of mobile particles depends on the concentration, valency, and detailed species of the ions used. Adapted with permis‐

sion from Arends et al. [65]. Copyright ©2013 American Chemical Society.

lamina.

Here, we have summarized selected aspects of our current understanding how the biochemical composition of the basal lamina is mirrored in the complex microarchitecture as well as the multi-facetted material properties of the biopolymer network. Deciphering the physicochem‐ ical principles which dictate the microstructure, viscoelastic properties, and selective perme‐ ability of the basal lamina layer are not only interesting for cell biology studies [67, 68], drug

**Figure 6.** Illustration of a microfluidic setup used to probe the diffusive penetration of peptides from a buffer compart‐ ment into a basal lamina gel. Typical intensity profile images for positively and negatively charged peptides are shown next to the microfluidic channel. For positively charged peptides, an intensity peak is observed at the buffer/gel inter‐ face, whereas such an accumulation does not occur for negatively charged peptides. Similar results are obtained when those peptides are injected into the connective tissue of mice. Tissue immunostaining confirmed that the positively charged peptides colocalize with collagen IV, a main component of the basal lamina. Adapted with permission from Arends et al. [66]. © 2015 Arends et al. Published under CC BY license.

delivery questions and tumor treatment [69–72], but might also have strong implications for tattoo removal applications: Here, ink nanoparticles trapped in the skin tissue have to be mobilized, for example, by soaking the tissue in salt solutions, so that they can be washed out from the skin rather than removed by painful and scar-inducing laser treatment. The lessons learned from systematically unraveling the physical and chemical mechanisms, which give rise to the complex properties of the basal lamina, may also help in the rational design of artificial hydrogel systems for tissue engineering approaches: The synthesis of complex macromolecules with well-defined chemical properties may allow for constructing hydrogels with both tailored mechanical properties and selective permeability behavior.


Non-Woven Fabrics Technology

**85**

**Chapter 5**

**Abstract**

reduced pore size.

porosity, fogging

**1. Introduction**

Characteristics of Eco-friendly

for Automotive Application

*Seung Jin Kim and Hyun Ah Kim*

Kenaf Fiber-Imbedded Nonwoven

This study examined the physical properties of kenaf fiber-imbedded nonwoven for automotive pillar trim according to the blend ratio of the fibers and needle-punching process conditions. Kenaf-imbedded nonwoven specimens mixed with polypropylene (PP) and low-melt PET (LM PET) fibers were prepared via needle-punching, and their physical properties such as air permeability, water absorption, sound absorption coefficient, and porosity were investigated according to the various processing conditions. The kenaf-imbedded nonwoven treated with high needle depth in the needle-punching process and/or mixed with a large amount of LM PET exhibited the highest breaking and tearing strengths, due to the high weight of the nonwoven specimens. A high blend percentage of LM PET fibers reduced the pore size, which resulted in low air permeability and water absorption. The sound absorption coefficient of the kenaf-imbedded nonwoven specimens was highly dependent on its weight and thickness. Regarding the lamination treatment, the laminated nonwoven exhibited higher breaking and tearing strengths, thermal conductivity, and sound absorption coefficient than the non-treated one. In addition, the HDPE powder-treated nonwoven exhibited lower breaking and tearing strengths, air permeability, water absorption, and sound absorption, due to the

**Keywords:** kenaf, needle-punching, low-melt PET, sound absorption coefficient

The scientific name of kenaf is *Hibiscus cannabinus* L. The principal ingredients of kenaf fiber are cellulose, lignin, and pectin, and the hemicellulose distribution ranges between 10 and 22% according to the type of kenaf fiber. Research on the application to fashion textile materials with soft tactile hand by retting treatment of kenaf stem has been conducted by Ramaswamy et al. [1], Tao et al. [2], and Lee et al. [3, 4]. In particular, many studies have examined the spinning and fabric manufacturing technology using mixed fibers with cotton and kenaf, including Bel-Berger et al. [5], Weiying et al. [6], and Zhang [7]. Advanced composite materials mixed with kenaf and natural fibers with light weight, VOC-free, and good abrasion resistance are needed nowadays and have been studied for eco-friendly automotive materials [8–10]. In addition, the use of kenaf fiber in nonwoven

#### **Chapter 5**

## Characteristics of Eco-friendly Kenaf Fiber-Imbedded Nonwoven for Automotive Application

*Seung Jin Kim and Hyun Ah Kim*

#### **Abstract**

This study examined the physical properties of kenaf fiber-imbedded nonwoven for automotive pillar trim according to the blend ratio of the fibers and needle-punching process conditions. Kenaf-imbedded nonwoven specimens mixed with polypropylene (PP) and low-melt PET (LM PET) fibers were prepared via needle-punching, and their physical properties such as air permeability, water absorption, sound absorption coefficient, and porosity were investigated according to the various processing conditions. The kenaf-imbedded nonwoven treated with high needle depth in the needle-punching process and/or mixed with a large amount of LM PET exhibited the highest breaking and tearing strengths, due to the high weight of the nonwoven specimens. A high blend percentage of LM PET fibers reduced the pore size, which resulted in low air permeability and water absorption. The sound absorption coefficient of the kenaf-imbedded nonwoven specimens was highly dependent on its weight and thickness. Regarding the lamination treatment, the laminated nonwoven exhibited higher breaking and tearing strengths, thermal conductivity, and sound absorption coefficient than the non-treated one. In addition, the HDPE powder-treated nonwoven exhibited lower breaking and tearing strengths, air permeability, water absorption, and sound absorption, due to the reduced pore size.

**Keywords:** kenaf, needle-punching, low-melt PET, sound absorption coefficient porosity, fogging

#### **1. Introduction**

The scientific name of kenaf is *Hibiscus cannabinus* L. The principal ingredients of kenaf fiber are cellulose, lignin, and pectin, and the hemicellulose distribution ranges between 10 and 22% according to the type of kenaf fiber. Research on the application to fashion textile materials with soft tactile hand by retting treatment of kenaf stem has been conducted by Ramaswamy et al. [1], Tao et al. [2], and Lee et al. [3, 4]. In particular, many studies have examined the spinning and fabric manufacturing technology using mixed fibers with cotton and kenaf, including Bel-Berger et al. [5], Weiying et al. [6], and Zhang [7]. Advanced composite materials mixed with kenaf and natural fibers with light weight, VOC-free, and good abrasion resistance are needed nowadays and have been studied for eco-friendly automotive materials [8–10]. In addition, the use of kenaf fiber in nonwoven

was investigated by Moreau et al. [11], Yang et al. [12], and Tao et al. [13, 14]. Nonwoven has been used in the various industries because of its advantages of fast processing and competitive price. Recently, nonwoven has become one of the most common textile products in the automotive industry with sound absorption properties. The nonwoven fabrics used in the automotive industry require high functional quality and reliability. Many studies have examined the sound absorption property, including physical properties such as air permeability and wicking, of nonwovens. The studies carried out using natural jute [15] and coconut coir [16] fibers yielded good sound absorption properties. Kenaf, jute, and cotton fiber-imbedded nonwovens with PET and polypropylene (PP) fibers were used as industrial automotive padding materials and have significantly improved the sound absorption properties [17]. Nick et al. [18] investigated the acoustic behavior using three different composite materials: (1) cotton, bicomponent PET, and PP fibers; (2) flax, hemp, and PP fibers; and (3) lyocell, bicomponent PET, and PP fibers. The third composite material with lyocell fibers of 0.9 dtex exhibited the best sound absorption property. Lou et al. [19] studied the sound absorption property of nonwoven composed of low-melt PET (LM PET) and recycled PET particles mixed with PP fibers. The thick and low-density nonwoven specimens exhibited high sound absorption coefficients at low- and mid-frequency sound ranges. Lee et al. [20] examined the relationship between the acoustic absorption values of the recycled polyester nonwovens and the nonwoven processing conditions, including fiber and web properties. Byun et al. [21] investigated the sound absorption property of the PET nonwoven for automotive application according to the variation of the fiber fineness, density, and thickness of the three-layer nonwoven by substituting glass wool in order to improve the environmental and recycled capability. Kücük and Korkmaz [22] examined the effects of the physical parameters on the sound absorption properties of natural fiber-mixed nonwoven fabrics. They concluded that an increased thickness and decreased air permeability resulted in an increase of sound absorption properties. In addition, an increased amount of fiber per unit area resulted in an increase in sound absorption. On the other hand, Dubrovski and Brezocnik [23] studied the effects of the content of viscose and PET fibers and the porosity of the nonwoven structure on the vertical wicking rate of nonwovens. The results showed that higher-volume porosity gives higher vertical wicking rate. Soukupova et al. [24] studied the effect of the blend ratio of viscose and PET fibers on the wicking of the nonwoven and found that the capillary rise was higher for nonwoven fabrics containing more viscose fibers. Dubrovski and Brezocnik [25] predicted the model for the vertical wicking rate using the fiber density, fiber fineness, and nonwoven fabric density. Das et al. [26] and Tascan and Vaughn [27] examined the influence of fiber cross-sectional shape on the air permeability of nonwoven. Das et al. [26] found that the air permeability decreased with a higher proportion of noncircular fibers in the nonwoven fabrics, which was similar to Tascan and Vaughn's results [27].

In previous studies, LM PET and PP fibers as nonwoven materials were mixed with natural fibers such as cotton, lyocell, flax, jute, and coconut coir to enhance their physical properties such as the wicking rate, air permeability, and sound absorption property for automotive-acoustic materials. However, no detailed study has yet examined the physical properties of the kenaf fiber-imbedded nonwoven. Therefore, in this study, kenaf fiber-imbedded nonwoven specimens were produced with different processing conditions such as number of carding treatments, web layers, needle depth, and content ratio of LM PET, and their physical properties such as air permeability, water absorption, and sound absorption coefficient were measured in order to optimize the processing conditions for automotive pillar trim. Furthermore, the correlation between the breaking and tearing strengths and the

**87**

**Table 1.**

*Physical properties of staple fibers used.*

*Characteristics of Eco-friendly Kenaf Fiber-Imbedded Nonwoven for Automotive Application*

structural factors of the nonwoven were investigated according to the processing conditions. In addition, the effect of different processing conditions on the fogging

Kenaf, PP, and LM PET were used as raw materials of the nonwoven. **Table 1** presents the physical properties of the kenaf, PP, and LM PET staple fibers used. Ten kinds of kenaf-imbedded nonwoven specimen as a first batch of specimen were

**Figure 2** shows an image of the nonwoven machinery used. Three types of staple fiber supplied from material supply equipment were mixed and blended in the mixing tank shown in **Figure 1**. Specimen 1 in **Table 1** was mixed and blended by single opener in the mixing tank, and specimens 2–10 were prepared by multi-opener.

**Figure 1** shows the needle-punching nonwoven process to prepare the specimens.

As shown in **Table 2**, the basic blend ratio of kenaf, PP, and LM PET staple fibers was 40, 40, and 20%, respectively (specimens 1–9). The blend ratio of specimen 10 was changed to 30, 30, and 40%. The mixed and blended fibers were delivered to the first and second carding processes. The basic carding treatment was conducted twice, but specimen 4 underwent only one treatment. Lap forming was performed after the carding process. The layering of the carding lap was changed from two to four layers, as shown in **Table 2**. The needle-punching process was followed by the second web-forming process, as shown in **Figure 1**. The basic needle depth was 16 mm, but specimens 7 and 8 had a needle depth of 18.6 and 14.4 mm, respectively. The specimens underwent thermo-compression bonding after the needlepunching process, as shown in **Figure 1**. Polyethylene (PE) powder was added after the thermo-bonding process to enhance coherence between the fabric and PP foam when automotive pillar trim was fabricated, as shown in **Figure 3**. **Figure 3** shows the pillar trim in automotive interior. Kenaf-imbedded nonwoven is located between PP foam and fabric, which is placed on the inside of automotive pillar trim.

made with different processing conditions, as shown in **Table 2**.

Fabric is usually made by polyester (cation dyeable polyester).

among hot melt film, fabric, and kenaf-imbedded nonwoven.

**Physical properties Kenaf Low-melting PET**

Hot melt film was inserted between the fabric and the kenaf-imbedded nonwoven to enhance the adhesive force between them. In addition, the reason why low-melting (LM) PET is mixed to make nonwoven is to enhance the adhesive force

Fiber length (mm) 64.8 51.8 ± 5.0 64 Linear density (d) 8 4.53 ± 0.41 8 ± 0.5 Maker/origin Bangladesh Toray Chemical Han Kook Fiber

Breaking strength (gf/d) 4 3.52 ± 0.42 4 ± 0.5 Breaking strain (%) 200 44.0 ± 8.7 200 ± 20 Moisture regain (%) 11.8 — 0.1

**(LM PET)**

**Polypropylene (PP)**

Co. Ltd

*DOI: http://dx.doi.org/10.5772/intechopen.85367*

value of the nonwoven was investigated.

**2. Experimental**

**2.1 Specimen preparation**

structural factors of the nonwoven were investigated according to the processing conditions. In addition, the effect of different processing conditions on the fogging value of the nonwoven was investigated.

#### **2. Experimental**

*Generation, Development and Modifications of Natural Fibers*

which was similar to Tascan and Vaughn's results [27].

In previous studies, LM PET and PP fibers as nonwoven materials were mixed with natural fibers such as cotton, lyocell, flax, jute, and coconut coir to enhance their physical properties such as the wicking rate, air permeability, and sound absorption property for automotive-acoustic materials. However, no detailed study has yet examined the physical properties of the kenaf fiber-imbedded nonwoven. Therefore, in this study, kenaf fiber-imbedded nonwoven specimens were produced with different processing conditions such as number of carding treatments, web layers, needle depth, and content ratio of LM PET, and their physical properties such as air permeability, water absorption, and sound absorption coefficient were measured in order to optimize the processing conditions for automotive pillar trim. Furthermore, the correlation between the breaking and tearing strengths and the

was investigated by Moreau et al. [11], Yang et al. [12], and Tao et al. [13, 14]. Nonwoven has been used in the various industries because of its advantages of fast processing and competitive price. Recently, nonwoven has become one of the most common textile products in the automotive industry with sound absorption properties. The nonwoven fabrics used in the automotive industry require high functional quality and reliability. Many studies have examined the sound absorption property, including physical properties such as air permeability and wicking, of nonwovens. The studies carried out using natural jute [15] and coconut coir [16] fibers yielded good sound absorption properties. Kenaf, jute, and cotton fiber-imbedded nonwovens with PET and polypropylene (PP) fibers were used as industrial automotive padding materials and have significantly improved the sound absorption properties [17]. Nick et al. [18] investigated the acoustic behavior using three different composite materials: (1) cotton, bicomponent PET, and PP fibers; (2) flax, hemp, and PP fibers; and (3) lyocell, bicomponent PET, and PP fibers. The third composite material with lyocell fibers of 0.9 dtex exhibited the best sound absorption property. Lou et al. [19] studied the sound absorption property of nonwoven composed of low-melt PET (LM PET) and recycled PET particles mixed with PP fibers. The thick and low-density nonwoven specimens exhibited high sound absorption coefficients at low- and mid-frequency sound ranges. Lee et al. [20] examined the relationship between the acoustic absorption values of the recycled polyester nonwovens and the nonwoven processing conditions, including fiber and web properties. Byun et al. [21] investigated the sound absorption property of the PET nonwoven for automotive application according to the variation of the fiber fineness, density, and thickness of the three-layer nonwoven by substituting glass wool in order to improve the environmental and recycled capability. Kücük and Korkmaz [22] examined the effects of the physical parameters on the sound absorption properties of natural fiber-mixed nonwoven fabrics. They concluded that an increased thickness and decreased air permeability resulted in an increase of sound absorption properties. In addition, an increased amount of fiber per unit area resulted in an increase in sound absorption. On the other hand, Dubrovski and Brezocnik [23] studied the effects of the content of viscose and PET fibers and the porosity of the nonwoven structure on the vertical wicking rate of nonwovens. The results showed that higher-volume porosity gives higher vertical wicking rate. Soukupova et al. [24] studied the effect of the blend ratio of viscose and PET fibers on the wicking of the nonwoven and found that the capillary rise was higher for nonwoven fabrics containing more viscose fibers. Dubrovski and Brezocnik [25] predicted the model for the vertical wicking rate using the fiber density, fiber fineness, and nonwoven fabric density. Das et al. [26] and Tascan and Vaughn [27] examined the influence of fiber cross-sectional shape on the air permeability of nonwoven. Das et al. [26] found that the air permeability decreased with a higher proportion of noncircular fibers in the nonwoven fabrics,

**86**

#### **2.1 Specimen preparation**

Kenaf, PP, and LM PET were used as raw materials of the nonwoven. **Table 1** presents the physical properties of the kenaf, PP, and LM PET staple fibers used. Ten kinds of kenaf-imbedded nonwoven specimen as a first batch of specimen were made with different processing conditions, as shown in **Table 2**.

**Figure 1** shows the needle-punching nonwoven process to prepare the specimens. **Figure 2** shows an image of the nonwoven machinery used. Three types of staple fiber supplied from material supply equipment were mixed and blended in the mixing tank shown in **Figure 1**. Specimen 1 in **Table 1** was mixed and blended by single opener in the mixing tank, and specimens 2–10 were prepared by multi-opener.

As shown in **Table 2**, the basic blend ratio of kenaf, PP, and LM PET staple fibers was 40, 40, and 20%, respectively (specimens 1–9). The blend ratio of specimen 10 was changed to 30, 30, and 40%. The mixed and blended fibers were delivered to the first and second carding processes. The basic carding treatment was conducted twice, but specimen 4 underwent only one treatment. Lap forming was performed after the carding process. The layering of the carding lap was changed from two to four layers, as shown in **Table 2**. The needle-punching process was followed by the second web-forming process, as shown in **Figure 1**. The basic needle depth was 16 mm, but specimens 7 and 8 had a needle depth of 18.6 and 14.4 mm, respectively. The specimens underwent thermo-compression bonding after the needlepunching process, as shown in **Figure 1**. Polyethylene (PE) powder was added after the thermo-bonding process to enhance coherence between the fabric and PP foam when automotive pillar trim was fabricated, as shown in **Figure 3**. **Figure 3** shows the pillar trim in automotive interior. Kenaf-imbedded nonwoven is located between PP foam and fabric, which is placed on the inside of automotive pillar trim. Fabric is usually made by polyester (cation dyeable polyester).

Hot melt film was inserted between the fabric and the kenaf-imbedded nonwoven to enhance the adhesive force between them. In addition, the reason why low-melting (LM) PET is mixed to make nonwoven is to enhance the adhesive force among hot melt film, fabric, and kenaf-imbedded nonwoven.

#### **Table 1.**

*Physical properties of staple fibers used.*

#### **Table 2.**

*Processing conditions for ten kinds of kenaf-imbedded specimen.*

Therefore, to examine the effect of the polyurethane (PU)-laminating film treatment on the thermal conductivity, water absorption, and sound absorption properties of the kenaf-imbedded nonwoven, a second batch of specimens was prepared by the same procedure as that for the first batch. **Table 3** presents the eight types of nonwoven specimen as a second batch of specimens. Two types of specimen were prepared as nonlaminated (1–4) and laminated (5–8) by PU film. The blend ratio of kenaf, PP, and LM PET staple fibers was 40, 40, and 20% as a fixed blend ratio. The carding treatment was conducted twice, and the number of layers of the carding lap was fixed at three. The needle depth was fixed at 16 mm. In the thermo-compression bonding process, the surface temperature of the bonding roller was set at 170°C, and its velocity was fixed at 7.2 m/min. Specimens 1 and 3 were treated with powder after the thermo-compression bonding process and have different weight. Specimens 2 and 4 were non-treated and also have different weight. Specimens 5–8 were laminated by PU film after the needle-punching nonwoven process. The temperature of the laminating roller was 129°C, and the

**89**

**Figure 3.**

*Schematic diagram of pillar trim.*

*Characteristics of Eco-friendly Kenaf Fiber-Imbedded Nonwoven for Automotive Application*

doting temperature on the melting apparatus was set at 126°C and the feed speed of the laminating roller at 3.7 m/min. After laminating by PU film, aging was carried

The breaking strength and strain of the nonwoven specimens were measured using Testometric apparatus (Model Micro 350, England) according to KSK ISO 9073- 3: 2009. A specimen of width 50 mm and length 200 mm was prepared and elongated at a speed of 100 mm/min. The tearing strength of the nonwoven specimens was measured using Testometric apparatus (Model Micro 350, England) according to KSK ISO 9073-4: 2010. A specimen of width 75 and length 150 mm was prepared. In addition, the breaking strength, strain, and initial modulus of the nonwoven specimens prepared at machine direction (MD) intervals of 30 degrees were measured. The preparation of the specimens is shown in **Figure 4**. Furthermore, the orientation factor of fibers in the nonwoven specimens was calculated as the measured inclined angle (θ) of the 500 fibers in the nonwoven fabric as shown in **Figure 5**, and the distribution of the measured angles was analyzed in relation to the measured tensile property.

The thickness of the ten (first batch specimen) and eight (second batch specimen)

was measured, and 30

different nonwoven specimens was measured using the FAST-1 system. **Figure 6** shows an image and schematic diagram of the compression meter by FAST-1 system

assessments of each specimen were carried out for calculating the mean thickness.

[28]. The thickness (mm) at a compression force of 2 gf/cm2

*DOI: http://dx.doi.org/10.5772/intechopen.85367*

out at 60% of RH in the aging room for 24 hours.

**2.2 Measurement of physical properties**

*2.2.1 Tensile and tearing strengths*

*Image of the nonwoven machinery.*

*2.2.2 Thickness*

**Figure 2.**

**Figure 1.** *Needle-punching nonwoven process.*

*Characteristics of Eco-friendly Kenaf Fiber-Imbedded Nonwoven for Automotive Application DOI: http://dx.doi.org/10.5772/intechopen.85367*

*Generation, Development and Modifications of Natural Fibers*

*Processing conditions for ten kinds of kenaf-imbedded specimen.*

**No. of carding treatment** **Layer of web**

 40:40:20 3 Layers 3 Layers 16 mm O X 40:40:20 3 Layers 3 Layers 16 mm O O 40:40:20 4 Layers 4 Layers 16 mm O O 40:40:20 2 Layers 2 Layers 16 mm O O 40:40:20 3 Layers 3 Layers 18.6 mm O O 40:40:20 3 Layers 3 Layers 14.4 mm O O 40:40:20 3 Layers 3 Layers 16 mm X O 30:30:40 3 Layers 3 Layers 16 mm O O

**Needle depth**

40:40:20 3 Layers 3 Layers 16 mm O O

40:40:20 3 Layers 3 Layers 16 mm O O

**Thermal compression bonding**

**Powder treatment**

**No. Blend ratio**

1 Single opener

2 Multiopener

**Table 2.**

**(kenaf:PP:LM PET)**

Therefore, to examine the effect of the polyurethane (PU)-laminating film treatment on the thermal conductivity, water absorption, and sound absorption properties of the kenaf-imbedded nonwoven, a second batch of specimens was prepared by the same procedure as that for the first batch. **Table 3** presents the eight types of nonwoven specimen as a second batch of specimens. Two types of specimen were prepared as nonlaminated (1–4) and laminated (5–8) by PU film. The blend ratio of kenaf, PP, and LM PET staple fibers was 40, 40, and 20% as a fixed blend ratio. The carding treatment was conducted twice, and the number of layers of the carding lap was fixed at three. The needle depth was fixed at 16 mm. In the thermo-compression bonding process, the surface temperature of the bonding roller was set at 170°C, and its velocity was fixed at 7.2 m/min. Specimens 1 and 3 were treated with powder after the thermo-compression bonding process and have different weight. Specimens 2 and 4 were non-treated and also have different weight. Specimens 5–8 were laminated by PU film after the needle-punching nonwoven process. The temperature of the laminating roller was 129°C, and the

**88**

**Figure 1.**

*Needle-punching nonwoven process.*

**Figure 2.** *Image of the nonwoven machinery.*

doting temperature on the melting apparatus was set at 126°C and the feed speed of the laminating roller at 3.7 m/min. After laminating by PU film, aging was carried out at 60% of RH in the aging room for 24 hours.

#### **2.2 Measurement of physical properties**

#### *2.2.1 Tensile and tearing strengths*

The breaking strength and strain of the nonwoven specimens were measured using Testometric apparatus (Model Micro 350, England) according to KSK ISO 9073- 3: 2009. A specimen of width 50 mm and length 200 mm was prepared and elongated at a speed of 100 mm/min. The tearing strength of the nonwoven specimens was measured using Testometric apparatus (Model Micro 350, England) according to KSK ISO 9073-4: 2010. A specimen of width 75 and length 150 mm was prepared. In addition, the breaking strength, strain, and initial modulus of the nonwoven specimens prepared at machine direction (MD) intervals of 30 degrees were measured. The preparation of the specimens is shown in **Figure 4**. Furthermore, the orientation factor of fibers in the nonwoven specimens was calculated as the measured inclined angle (θ) of the 500 fibers in the nonwoven fabric as shown in **Figure 5**, and the distribution of the measured angles was analyzed in relation to the measured tensile property.

#### *2.2.2 Thickness*

The thickness of the ten (first batch specimen) and eight (second batch specimen) different nonwoven specimens was measured using the FAST-1 system. **Figure 6** shows an image and schematic diagram of the compression meter by FAST-1 system [28]. The thickness (mm) at a compression force of 2 gf/cm2 was measured, and 30 assessments of each specimen were carried out for calculating the mean thickness.

**Figure 3.** *Schematic diagram of pillar trim.*


*1.5.2 Desalting processes at a refinery*

They can be one and two-stage (respectively, with one or two electrostatics desalters) [20]. To increase the efficiency of the operation, the demulsifiers are added to the crude oil. Two-stage installations are used to reduce the flow of freshwater when washing crude oil [13]. The desalter of this design achieves 90% salt removal. However, 99% salt removal is possible with two-stage desalters [7, 20] as shown in **Figure 7**. A second stage is also essential since desalter maintenance requires a lengthy amount of time to remove the dirt and sediment which settle at the bottom. Therefore, the crude unit can be operated with a one stage desalter while the other is cleaned.


**7. Heart Aging**

Aging of the heart includes a number of physiological changes that increase the risk of developing diseases and conditions that are hazardous to health. Cardiac diseases are associated with age and have a detrimental impact on the whole organism. However, it remains unclear how cellular aging of heart tissue a ffects the appearance of heart diseases.

It is known that telomere length decreases with age. For human heart tissue, telomere loss is approximately 20 base pairs per year [39]. In addition to this, it was found that in old rats there was a tendency to reduce the length of terminal restriction fragments (TRF) related to the telomeric and subtelomeric region. Interestingly, only the heart showed a significant decrease in the average length of telomeres compared to the brain, kidney, lung and liver. [40]. A similar state of telomeres was found in coronary artery endothelial cells, where the T/C ratio (ratio of telomere length to centromeres) was reduced depending on the age of the donor [41]. At the same time, the reduction of the end sections of chromosomes is unlikely to be associated with cell division, as described above.

As for the age-dependent diseases, there is a correlation between the length of telomeres and the presence of one or another heart disease. For example, the risks of coronary heart disease are associated with telomeres shorter than 200 base pairs [42]. The mechanism of a possible causal link between short telomeres and ischemic disease has not been fully elucidated, but shorter telomeres are positively associated with the rapid formation of plaques observed in atherosclerosis and marked atherogenesis [42]. Indeed, direct measurement of telomere length in coronary endothelial cells supports the concept that telomere shortening in coronary endothelial cells with aging can contribute to the development of coronary endothelial dysfunction and the development of coronary heart disease in humans [41].

Dilated cardiomyopathy (DC) is characterized by an increase in cardiac ventricular volumes, thinning ventricular wall thickness, hypertrophy and impairment of cardiac function [43,44]. In an aging heart with DC, the forced entry of primitive cells, which express stem cell surface antigen c-kit, into irreversible quiescent state was identified by the expression of cell cycle inhibitor p16INK4a and by very short telomeres [45].

Telomeric shortening in CMs with age can be explained by the launch of miRNA-34a synthesis, the target of which is phosphatase 1 of the nuclear targeting subunit (PNUTS), which is involved in the maintenance of telomeres by interacting with TRF2. TRF2, together with PNUTS, is also involved in regulating the response to DNA damage and inhibiting the phosphorylation of Chk2 (checkpoint kinase), leading to apoptosis. Increased expression of PNUTS inhibits telomere depletion without telomerase involvement [46]. However, there are probably other molecular pathways that result in a reduction in telomere length. They can be associated, for example, with oxidative stress, leading to the accumulation of DNA damage in telomeric regions.

To determine the possible role of telomeres and telomerase in cardiac aging, mice with telomereinduced dysfunction were examined by knockout on telomerase subunit genes. Mice of the fifth generation (G5) with a *Terc* gene knockout (*Terc*-/-) suffered from severe left ventricular insufficiency and DC. Compared to WT mice, the masses of the heart and left ventricle were significantly reduced in G5 mice. Despite the decrease in heart weight in G5, hypertrophy was demonstrated, which was accompanied by a decrease in the number of CMs [47]. The phenotype was also characteristic for G4 *Tert*-/- mice: a decrease in body weight and endurance and an increase in free fatty acids and mitochondrial dysfunction. Mitochondrial dysfunction manifested as inhibition of PGC-1α and PGC-1β, key metabolic regulators. This led to a decrease of gluconeogenesis, a reduction in ATP synthesis, cardiomyopathy, and increased oxidative stress [48], which is a sign of tissue aging [49].

The tumor suppressor protein p53 is an important mediator of telomere dysfunction. An increase in this protein is observed in *Terc*-/- mice when telomeres reach a critically short length. These results are consistent with the notion that telomere loss in mice activates p53, which modulates both apoptosis and growth arrest [47]. In addition to these functions, the p53 protein is a link between telomere length and mitochondrial function: an increase in its synthesis leads to inhibition of the promoters *PGC-1*α and *PGC-1*β [48]. Thus, dysfunction of telomeres leads to premature aging of the heart, which manifests itself in the form of diseases dependent on age.

As described previously [50], premature aging of the cardiovascular system is induced by metabolic stress, obesity, hypertension, insulin resistance and type 2 diabetes [51–54]. Additionally, there is evidence that autophagy is important for longevity and health [55], and a change in autophagy contributes to heart aging [56–58]. Although it has been shown that autophagy inducers have a beneficial effect on life expectancy and slow down the aging of the cardiovascular system [56], there is still a contradiction between the protective and harmful effects of autophagy induction on aging [59]. There are many aging treatment options based on telomerase activation, NO modulation, antioxidants, PARP inhibition, senolytic therapy, plasma membrane redox system (PMRS) activators and stem cell therapy [55,60].

There are reports about the use of telomerase as a therapeutic tissue-specific target for diseases of the cardiovascular system [61,62]. Telomerase can be used to treat coronary heart disease due to protection against ROS [63,64]. With ischemia reperfusion injury, telomerase deficiency leads to heart failure [65]. Telomere depletion is a characteristic sign of cardiac hypertrophy. Shortening of telomeres in CMs is a marker of heart failure in humans, and shorter telomere length in CMs usually correlate with a reduced ejection fraction [66].

## **8. Heart Regeneration**

As we stated above, the regenerative potential of CMs, as well as telomerase activity, decreases in mammalian hearts shortly after birth [13].

In newborn mice (P1), in response to injury, accelerated differentiation of CM occurs [67,68]. Following the P1 period, the regenerative potential is quickly lost, and a similar injury on P7 leads to fibrosis instead of regeneration [67]. Tetraploidy, binucleation, diminished telomerase activity and telomere shortening during this period can be causative of a loss of proliferative potential of CMs in response to injury [13]. A recent study showed that co-cultivation of mononucleated and bi/multinucleated CMs from adult and newborn animals, respectively, led to the de-differentiation and proliferation of not only mononucleated, but also bi/multinucleated CMs, although to a lesser extent [69].

Telomerase expression in the mammalian heart was investigated using transgenic mice expressing green fluorescence protein (GFP) driven by the promoter for murine telomerase reverse transcriptase (mTert), which is a necessary and rate-limiting component of telomerase [19]. Local proliferation of *mTert-GFP*-expressing cells in the adult heart suggests the existence of a subpopulation of mTERT-positive cells that display a phenotype similar to stem cells. This observation is supported by the expression of the heart-specific transcription factors NKX2.5 and GATA4 in these cells, which are necessary for the differentiation into CM lineage. These factors are described as distinctive features of the native stem cells of the heart. A marked local increase in their number in response to trauma in the adult heart indicates their role in regeneration [19]. A similar increase in the number of stem-like cells with the surface antigen Sca-1 and c- kit and their proliferation was observed during a heart attack, which was accompanied by an increase in telomerase activity [70]. Telomerase delays growth arrest, aging and prevents cell death. It may also be involved in the fight against mechanical and oxidative stress [71], which increases with a concomitant increase in ROS during necrosis and inflammation [72].

Conditions of hypoxia can reduce the oxidative stress after induced myocardial infarction. This leads to an increase in proliferation, which further helps to reduce the area of fibrosis and improves systolic function [73]. In addition to these data, the possibility was found of increasing the regenerative potential of hearts at one week of postnatal development of mice after administration of the antioxidant *N*-acetyl-*L*-cysteine [21].

Cardiac muscle regeneration after an injury is complicated by an "irreversible" exit of CMs from the cell cycle. A forced expression of *Tert* in the cardiac muscle in mice is sufficient to restore telomerase activity and telomere length. This, in turn, can delay the exit from the cell cycle in the cardiac muscle, cause hypertrophy in postmitotic cells and contribute to CM survival [14].

To elucidate a role of telomerase in cardiac regeneration, *Tert* was overexpressed in mouse hearts by adeno-associated viral delivery [14,20]. Mice were subjected to experimental myocardial infarction (MI). Upon MI, *Tert*-expressing hearts showed attenuated cardiac dilation, improved ventricular function and smaller infarct scars concomitant with increased survival by 17% compared with controls. Cardiac transcriptome analysis revealed an increase of epidermal growth factor receptor (EGF) in *Tert*-expressing hearts. Signaling through EGF is cardio-protective, emphasizing defensive function of Tert. Tert therapy also leads to activation of pathways associated with extracellular matrix remodeling (an increase in serum MMP-9 and TGF-b). TGF-b has a pleiotropic effect on almost all cell types involved in the repair and heart remodeling after injury. Long-term activation of these genes may also be a consequence of enhanced heart regeneration, which requires matrix remodeling to integrate new CMs. Indeed, the CM can re-enter the cell cycle after injury. Expression of Ki-67 and the presence of phosphorylated forms of histone H3 (proteins, the maximum expression of which coincides with mitosis) was found to increase the number of proliferating CMs near the infarct zone in the Tert-treated group. According to these results, it is plausible that activation of Tert may assist cardiac regeneration [13].

After injury, sufficient telomere length is required for proliferation. In P1 G3 *Terc*-null newborn mice, the proliferative capacity is lower than that of WT controls. Instead of increasing proliferation, CMs of G3 *Terc*-/- mice grow hypertrophic. The aggravation of telomere shortening after cryogenic damage at P1 in the heart of G3 mice causes an increase of p21 levels compared to WTs, which indicates the activation of the DNA damage response. This leads to the cessation of the cell cycle. A reliable proliferative response was observed in CMs of p21-/- mice that were seven days old. This is the age at which the CMs of WT mice lose the ability to divide after injury. This observation further emphasizes the participation of telomerase inhibition through the expression of p21 in stopping cell cycle and inhibition of the regeneration reaction after injury in the early postnatal period [13].

The proliferative ability is obviously associated with increased telomerase activity relative to mammals; therefore, the *D. rerio* model is interesting from the point of view of the possibility of regeneration of CMs.

In a recent study, it was shown that cryogenic damage to the heart leads to an increase in telomerase activity in WT zebrafish, which was associated with an increase in *tert* gene expression. To determine the role of telomeres in regeneration, damage to the heart was performed with *tert*-/- and WT *D. rerio*. It was shown that the cardiac output was not restored, and the area of damage did not decrease in *tert*-/- fish, which indicates the need for telomerase during regeneration. However, the level of the inflammatory response, an important process for regeneration, was the same for WT and *tert*-/- fish. Similarly, dedi fferentiation of CMs in response to an injury occurs normally in the absence of telomerase. The length of telomeres increases only in WT CMs during regeneration, and this is characteristic of both actively proliferating cells and nondividing cells. Therefore, we can conclude that telomere elongation is important for CM regeneration [27].

Induction of polyploidy in the heart of zebrafish leads to a loss of regenerative potential after injury [29]. Consequently, for recovery of the myocardium, diploid CMs are required, the number of which decreases in the mammalian heart with age as a result of telomere depletion [13].


*Review* **A Brief Review of the Role of 2D Mxene Nanosheets toward Solar Cells Efficiency Improvement**

**T. F. Alhamada 1,2, M. A. Azmah Hanim 2,3,\*, D. W. Jung 4,\*, A. A. Nuraini 2 and W. Z. Wan Hasan 5**

	- Universiti Putra Malaysia, Serdang 43400, Selangor, Malaysia

**Abstract:** This article discusses the application of two-dimensional metal MXenes in solar cells (SCs), which has attracted a lot of interest due to their outstanding transparency, metallic electrical conductivity, and mechanical characteristics. In addition, some application examples of MXenes as an electrode, additive, and electron/hole transport layer in perovskite solar cells are described individually, with essential research issues highlighted. Firstly, it is imperative to comprehend the conversion efficiency of solar cells and the difficulties of effectively incorporating metal MXenes into the building blocks of solar cells to improve stability and operational performance. Based on the analysis of new articles, several ideas have been generated to advance the exploration of the potential of MXene in SCs. In addition, research into other relevant MXene suitable in perovskite solar cells (PSCs) is required to enhance the relevant work. Therefore, we identify new perspectives to achieve solar cell power conversion efficiency with an excellent quality–cost ratio.

**Keywords:** power conversion efficiency; solar cells; MXenes; electrodes; additives; HTL/ETL

#### **1. Introduction**

The development of innovative materials for efficient solar cells has garnered a lot of attention [1–10] because of the ever-increasing need for renewable and clean energy supplies [11–15]. Sunlight has been identified as the most prevalent, cheapest, and cleanest source of energy for meeting society's long-term energy requirements. Solar cells convert sunlight directly into electricity—the most efficient and practical method to utilise solar energy. Earth-rich silicon (Si)-based solar cells dominate the industry, with power conversion efficiencies (PCEs) of over 26 percent and a 25-year average module living standard [16–18]. However, since Si solar cells have high initial production costs, researchers are turning their attention to less expensive alternatives, such as perovskite solar cells (PSCs), organic solar cells (OSCs), quantum dot solar cells (QDSCs), and dye-sensitised solar cells (DSSCs) [19].

PSCs are the most feasible option among these new PV technologies for providing a PCE equivalent to maturing silicon solar cells. Furthermore, compared to traditional Si-based technologies, their lower costs, adjustable band gap, processability at low temperatures, long charge carrier diffusion lengths, high light absorption coefficients, lower exciton binding energy, numerous options for much simpler mass production processes lacking additional advantages, and increasing performance make it a more lucrative option [20–26]. Additionally, in contrast to traditional Si solar cells, PSCs operate well even in diffuse or weak light, making them suitable for specialised purposes [27]. Due to the development of various architectures, chemical compositions, manufacturing protocols, advances in

**Citation:** Alhamada, T.F.; Azmah Hanim, M.A.; Jung, D.W.; Nuraini, A.A.; Hasan, W.Z.W. A Brief Review of the Role of 2D Mxene Nanosheets toward Solar Cells Efficiency Improvement. *Nanomaterials* **2021**, *11*, 2732. https:// doi.org/10.3390/nano11102732

Academic Editor: Byungwoo Park

Received: 29 September 2021 Accepted: 14 October 2021 Published: 15 October 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

materials, and phase stabilisation techniques, efficiencies have increased dramatically since the first report on all-solid-state PSCs in 2012, from 9.7% in 2012 to 25.5% percent in 2021 [28–31]. Between the highest observed efficiency and its theoretical maximum, PSCs may be split into two categories: the normal (n-i-p) structure and the inverted (p-i-n) structure [32,33].

Furthermore, concerns regarding PSC stability have been highlighted because a thinfilm solar device must pass the IEC 61,646 environment stability test before it can be sold commercially [34]. A lot of research is now underway to improve the stability and performance of PSCs [19]. Scientists have been attempting to integrate perovskite into solar cells since the material's initial breakthrough in 2009. The solar cells in this material are more efficient than those in current solar modules [35]. On average, existing solar modules capture 15 to 18 percent of the sun's energy, while perovskite solar cells have an efficiency of up to 28 percent [36]. Dou's research team developed a sandwich-like material that mixes organic and inorganic components to form a composite structure that does not need lead and improves stability considerably. According to Yao Gao, the new organic–inorganic hybrid perovskite materials are cheaper and perform better than traditional inorganic semiconductors. Solar cells can be highly efficient using this new method; the authors made hybrid perovskite materials that are intrinsically more stable. These novel materials are better for the environment and safer for bioelectronic sensors on humans because the researchers removed hazardous lead [37].

Transition-metal nitrides, or carbides (MXenes), were first found in 2011 by Gogotsi and his coworkers as star materials from MAX phases, which are layered compounds resembling graphite with monoatomic A element layers sandwiched between electrically conductive and stiff MX-blocks [38–40]. It was proposed that the generated material be labelled MXenes to highlight the removal of the A element from the MAX phase and its twodimensional (2D) shape, related to graphene. The material has recently shown promising applications in solar cells [41–44], biomedical fields [45–47], light-emitting diodes [48–50], sensors [51–55], energy storage [56–62], catalysis [63–66], water purification [67–72], and electromagnetic applications [41–44,73]. The nanoengineering of these 2D materials is a hot topic right now. Due to its adjustable work function, high electrical conductivity, good transparency, and charge-carrier mobility, Ti3C2Tx (T stands for certain surface-terminating functional groups such as O, OH, and F) leads the current research on MXene in solar cells [74–76]. MXenes are currently divided into transition metals in either an out-plane or in-plane ordered form. Furthermore, most 2D transition-metal MXenes exist in the form of random solid solutions, which are characterised by two randomly distributed transition metals across the 2D structure. This review paper detailed the basic principles for the creation of each 2D transition-metal MXene structure, as well as their tunable characteristics depending on the transition-metal composition. 2D transition-metal MXenes vary from their counterparts mono-transition-metal MXenes, where two transition metals can occupy the metal sites.

Guo and his group included Ti3C2Tx as an additive in the photoactive layer of methylammonium lead iodide (MAPbI3) in the first research on MXene materials in perovskite solar cells, which was published in 2018 [77]. Since then, its application has been extended to the electrode, electron transport layer (ETL)/hole transport layer (HTL). The Ti3C2Tx functions on MXenes in solar cell applications may be classified into three categories: electrode [78], additive [77], and ETL/HTL [79,80]. Figure 1 below summarises the synthesis, properties, and application of MXene. The solar cells (SCs) in Figure 1 have been widely investigated [73].

**Figure 1.** MXene synthesis, properties, and their applications. Reprinted with permission from ref. [73]. Copyright 2021 John Wiley & Sons, Inc.

This article summarises all previously reported work on incorporating MXene into solar cells to improve solar power generation and operational stability. The next section defines the efficiency improvement of SC and how it is classified. Section 3 lists the types of roles that MXene mainly plays in solar cells. A conclusion and prospect are given in Section 4.

#### **2. The Efficiency Improvement of Solar Cells**

The conversion efficiency of a solar cell is a measurement of incident light that can be converted to electrical energy. The incident light's power is the denominator, while the solar cell's electrical power is the numerator; thus, this conversion efficiency may be expressed as a fraction [77].

The power conversion efficiency (PCE) of solar cells is one of the most significant parameters [81]. The PCE has improved rapidly since the PSC's introduction in 2009 [82]. The PCE of Kojima et al. initial's PSC was just 3.8 percent [83]. Im et al. [84] claimed a PCE of 6.5 percent in 2011, while Kim et al. [28] recorded a PCE of nearly 9 percent in 2012. In 2016, approximately 22% of PCEs were verified, to the best of our knowledge [85]. All of these remarkable outcomes and conclusions in such a short period of time demonstrated PSC's tremendous potential [86]. Below is the equation used to calculate the conversion efficiency:

$$\begin{array}{l}\text{Conversion efficiency (\%)} = \text{Generated electrical power (W)} / \text{Incident light} \\ \text{power (W)} \times 100 \end{array} \tag{1}$$

Fu et al. published a paper in 2019 that can be applied to various MXene compositions as possible electrodes for the creation of high-performance solar cells. Solar cells with a maximum power conversion efficiency (PCE) of 11.5 percent were delaminated from a few stacked Ti3C2Tx MXene-contacted Si layers [87]. The authors recently suggested integrating an inorganic 2D Cl-terminated Ti3C2 (Ti3C2Clx) MXene into the volume and surface area of CsPbBr3 lm to substantially decrease the superficial lattice tension. The faulty surface is healed, and a champion efficiency of 11.08 percent is obtained with an ultra-high open-circuit voltage of up to 1.702 V on the fully inorganic CsPbBr3-PSC, which is the greatest efficiency record for this kind of PSC to date. In addition, at 80 percent relative humidity for 100 days and 85 degrees Celsius for 1 month, the unencapsulated device performs nearly as well as the enclosed device [86]. Y. Zhang et al. used density functional theory calculations to thoroughly assess 64 2D transition-metal carbide (MXene) to determine that they were acceptable semiconductors for solar cells via material screening. Ti2CO2/Zr2CO2 and Ti2CO2/Hf2CO2 heterostructure solar cells, in particular, have really high power conversion efficiency of 22.74 and 19.56 percent, respectively (Most PCEs inside this paper were evaluated at AM-1.5G-illumination). This research opens the path for MXenes to be used as solar materials in the future [88]. According to Saeed et al., many new opportunities for creating effective indoor organic photovoltaics (OPVs) for practical applications can be explored. With the introduction of different optoelectronic methods to improve device performance under low indoor lighting with varied spectra, the indoor efficiency of OPVs (for PCE > 30%) has taken a quantum leap [89]. Saeed et al. demonstrated additional enhancements to dye-sensitised photovoltaic cells (DSPVs) in indoor solar applications for light energy recycling due to its outstanding light-harvesting performance under ambient lighting conditions. DSPVs' suitability for ambient energy harvesting is proven by their record high power conversion efficiency (PCE) of over 30% under indoor lighting circumstances, consistent device operation, cost-effectiveness, colorful aesthetics, and PCE retention of up to 99% [90].

#### **3. Applications of MXene in Solar Cells**

#### *3.1. MXene as Conducting Additives in Solar Cells' Photoactive Active Layer*

MXene as a photoactive layer additive in SCs is discussed in this section. Despite significant advances in PCE, charge-carrier recombination inside of the photoactive layer and at perovskite/ETL and perovskite/HTL interfaces still limits PSC performance. Improvements in charge-carrier management are essential to closing the gap between the existing PCEs and the theoretic efficiency frontier of CSs. Prior to mass manufacturing, the intrinsic instability of perovskite in humidity and at high temperatures, as well as the device's limited scalability, must be addressed. Two-dimensional nanomaterials with distinct characteristics have been investigated as additions in photoactive perovskite layers of the HTL/ETL of PSC in recent years. The use of additive engineering to enhance the surface coverage and crystallisation of perovskite films has proven to be successful.

Guo et al. investigated the inclusion of Ti3C2Tx in the MAPbI3-based perovskite absorber for the first time in 2018 [77], kicking off research on MXenes in solar cells. Their findings indicate that adding Ti3C2Tx to MAPbI3 may prolong the nucleation process, resulting in larger crystals. Furthermore, the Ti3C2Tx additive is extremely helpful in speeding electron transport across the grain boundary, similar to a carrier bridge [91–94]. This is measured by the reduced charge-transfer resistance for the Ti3C2Tx additive, as revealed by the electrochemical impedance spectra. The median power conversion efficiency (PCE) rises from 15.2 percent to 16.8 percent because of these factors. In addition to adding Ti3C2Tx to the photoactive MAPbI3 layer, similarly, Agresti et al. added Ti3C2Tx to the TiO2/ETL to fine-tune its work function (WF). This lowered it from 3.91 to 3.85 eV, which is beneficial for tuning the interfacial energy levels between the perovskite absorber and the TiO2/ETL, improving charge transfer and lowering the barrier height. The device achieves a PCE of 20.14 percent, which is 26.5 percent greater than the control device without the Ti3C2Tx addition, thanks to the double addition and optimisation of both the photoactive MAPbI3 and the TiO2 electron transport layer. Furthermore, the inclusion of Ti3C2Tx to the current density-voltage (JV) curves was shown to decrease hysteresis while enhancing the PSCs' long-term exposure stability. Recently, this group used density functional calculations to further investigate the MAPbI3 perovskite/Ti3C2Tx-based MXene interface. When the relative concentrations of the OH, O, and F termination groups were changed, the findings indicate that the work function interface displays highly nonlinear behaviour, and they offer a profound insight into the alignment of the energy level for the manufacture of high-performance materials [15].

Agresti et al. used Ti3C2Tx MXene in MAPbI3 PSCs to modify the work function of perovskite films and ETLs, resulting in a power conversion efficiency improvement of 26%, as compared to Ti3C2Tx-free control devices [95]. Di Vito and his colleagues used DFT to conduct first-principles calculations on a Ti3C2/MAPbI3 perovskite-coupled system, linking WF tuning to changes in the various concentrations of OH-, O-, and F-MXene-Terminations, and found that OH collections had the greatest impact in reducing work function [94].

Zhang and his colleagues used an in situ solution growth technique to synthesise MAPbBr3 nanocrystals (NCs) on the surface of multilayer MXene (Ti3C2Tx) nanosheets that form heterostructures in 2020 [96]. PSCs were manufactured utilising the C-TiO2/m-TiO2-TQD/TQD-Perovskite/Spiro-OMeTAD-Cu1.8S design to enhance PCE and device stability while retaining a champion hysteresis-free power conversion efficiency of 21.64% compared to 18.31% for control devices, with substantially better long-term air and light stability. The entire potential of MXene materials in SCs must be explored as a new area. Various groups, on the other hand, revealed different methods for making use of 2D MXene materials' higher electrical conductivity. 2D Ti3C2Tx MXene nanosheets were used as nanoscale additives in 2D Ruddles-den-Popper PSCs by Jin et al. The PCE of 2D PSCs rose from 13.69 percent (control device without MXene additive) to 15.71 percent [97] due to passivated trap states, optimal orientation, reduced charge transfer resistance, and enhanced crystallinity. Yang et al. utilised SnO2-Ti3C2 MXene nanocomposites as electron transport layers (ETLs) in planar PSCs [98].

Zhao et al. utilised Ti3C2Tx MXene nanosheets as a multifunctional additive in a two-step method to create extremely efficient planar PSCs in 2021. The findings indicate that single-layer Ti3C2Tx nanosheets improve the reactivity of the PbI2-layer by inducing the formation of a porous PbI2-layer, which increases the perovskite grain size and lowers the amount of residual PbI2 in the perovskite film. Random stacking of large PbI2 grains readily leads to the formation of pores, according to previous research [99]. The mechanism diagram to produce high-quality perovskite films is shown in Figure 2. Ti3C2Tx can also improve the WF of MAPbI3, allowing for better energy-level alignment between the perovskite layer and the ETL. Finally, by interacting with the under-coordinated Pb2+, the terminal collections on the surface of Ti3C2Tx play a critical role in the passivation of perovskite films. The maximum PCE of 16.45 percent and a PCE rate of 15.94 percent were obtained at the optimum Ti3C2Tx dose of 0.03 percent by weight. These values are about 18 percent better than those of pure PSCs, which had the greatest power conversion efficiency of 16.45 percent and a PCE rate of 15.94 percent. As a result, this research established Ti3C2Tx as an effective and feasible addition for the manufacture of greatly efficient two-stage produced PSCs, paving the path for their application to other 2D materials [100].

**Figure 2.** Mechanism diagram for the production of high-quality perovskite films processed in two steps, supported by the additive Ti3C2Tx. Reprinted with permission from ref. [100]. Copyright 2020 Elsevier B.V.

Larciprete et al. investigated pure environmental aging and the thermally driven breakdown of the mixed halide perovskite Csx (FA0.83MA0.17) (1x) Pb3 using X-ray photoelectron spectroscopy (I0.83Br0.17) and high-resolution ultraviolet. The scientists also looked at the impacts of the Ti3C2Tx MXene additive on photovoltaic stability as part of their research. Furthermore, the absence of any negative impact on PV stability, as well as a significant stabilising effect of the additional MXene, contribute to long-term aging. In the fresh samples, we observed a modest decrease in the initial halide migration rate, but this needs more investigation. In conclusion, we believe that our findings on Csx (FA0.83MA0.17) (1x) Pb3 (I0.83Br0.17) show severe criticality in the stability of certain mixed perovskites that are comparable to single-halide materials. As a result, it appears that the effectiveness of agents based on electronic and chemical stabilisation of their functional properties, as well as the creative development of device architectures capable of interacting with disruptive agents, are critical for the long-term use of mixed perovskite [101].

For the first time, Hou & Yu showed further improved IPSCs using Ti3C2Tx nanosheets as an additive in ZnO. The creation of the Zn–O–Ti bond enhances the PCE when ZnO is modified with Ti3C2Tx, because of the recently created charge transfer routes between both the passivated surface of ZnO films and the ZnO nanocrystals. Figures 3 and 4 illustrate energy level diagrams of the materials utilised in IPSCs. When compared to the control device that utilises pure ZnO as ETL, ITIC-based IPSCs with ZnO/Ti3C2Tx/ETL achieve an average power conversion efficiency of 12.20 percent, which is a 15.53 percent improvement (10.56 percent). PM6: Y6 IPSCs reach a champion power conversion efficiency of 16.51 percent based on the ZnO/Ti3C2Tx interface layer, compared to 14.99 percent for the reference device [102].

**Figure 3.** Schematic representation of the IPSCs configuration. Reprinted with permission from ref. [102]. Copyright 2020 Elsevier B.V.

**Figure 4.** (**a**) Chemical structures of PBDB-T and ITIC. (**b**) Band diagram of the materials used in IPSCs. Reprinted with permission from ref. [102]. Copyright 2020 Elsevier B.V.

According to Jin et al., a modest doping level of Ti3C2Tx nanosheets significantly enhanced the quality of 2D perovskite (BA) 2 (MA) 4Pb5I16 films and the photovoltaic performance of the associated device, with a PCE increase from 13.7 to 15.7 percent due to the increase in current. Figure 5a depicts the architecture of the current PSCs, as well as an example of Ti3C2Tx incorporation into a 2D perovskite film. Figure 5b shows the

JV curves of the devices constructed using the control, Ti3C2Tx0.1 mM, Ti3C2Tx0.3 mM, Ti3C2Tx0.5 mM, and Ti3C2Tx0.7 mM samples. The external quantum efficiency (EQE) spectrum displayed in Figure 5c supports this growth in short-circuit current density (Jsc). Furthermore, a steady power output compatible with the JV curves is shown by the photocurrent evaluated for much more than 5 min at a point of maximum power (0.80 V) (Figure 5d). The enhanced vertically directed growth, uniform phase distribution in the thin film, and the crystallinity, which eventually improves charge transfer, are primarily responsible for the Ti3C2Tx-doped components' superiority. Furthermore, owing to the superior crystallinity and passivation effect of the perovskite film, the components doped with Ti3C2Tx nanosheets had a greater moisture stability than the shell components [99]. We can conclude that MXene has many functions in solar cells. As an additive, it accelerates electron transport by acting as an "electron" bridge. Hence, by its addition, it influences the carrier transport materials' work function and other characteristics like conductivity. This research offers a viable approach for enhancing the efficiency of 2D perovskite film and expands the scope of Ti3C2Tx's photovoltaic applications [99].

**Figure 5.** (**a**) Schematic representation of devices with the structure glass/ITO/SnO2/2D perovskite/SpiroOMeTAD/Ag. (**b**) JV curves from devices with different amounts of Ti3C2Tx doping. (**c**) EQE spectra and integrated Jsc of the control and optimised Ti3C2Tx doping devices. (**d**) Stabilised power output and current density at a constant bias 0.80 V for the Ti3C2Tx dopant devices. Reprinted with permission from ref. [99]. Copyright 2021 Springer Nature Switzerland AG. Part of Springer Nature.

#### *3.2. Novel Metal Transparent Conductive Electrode*

In PSCs, MXene is used as an electrode. An electrode is one of the most essential components of a PSC for controlling the charge collecting process; it is important for long-term stability and affects the device's overall cost. Metal thin-film electrodes, nanostructured metal electrodes [103], carbon electrodes [104], and graphene electrodes [105], Ref. [106] are some of the newly described electrode materials for PSC.

The Ti3C2Tx MXene recently reported an electrical conductivity of up to 15,100 S cm−1 [107], as well as great transparency, good flexibility, and tunable WF [108–110]. Because of these characteristics, Ti3C2Tx may be used as an electrode in optoelectronic devices such as solar cells. The next sections elaborate on Si-wafer-based, organic, perovskite-based, and dye-sensitised solar cells, in that sequence. In quantum-dot-sensitised solar cells, the Ti3C2Tx MXene was also utilised to make the counter electrode (CE) (QDSCs). Chen et al. described a hybrid CE made up of hydrothermally produced CuSe nanoparticles on Ti3C2Tx-MXen nanosheets screen printed on graphite foil [111]. This composite CE offers higher electrical conductivity for electron transport and a greater specific surface area than CuSe and Ti3C2Tx-based CEs, allowing for more active centers for polysulfide electrolyte reduction. The device can obtain a PCE of 5.12 percent by employing a CuSe- Ti3C2Tx hybrid CE with an optimum mass ratio. Devices that utilise CuSe and Ti3C2Tx-based CEs, on the other hand, have a PCE of 3.47 percent and 2.04 percent, respectively. Similarly, Tian et al. used a simple ion-exchange technique at ambient temperature to produce CuS/Ti3C2 composite CEs, which exhibited a substantially higher electrocatalytic rate for polysulfide reduction than pure CuS [112]. The overall PCE of the QDSC based on this composite CE is 5.11 percent, which is 1.5 times higher than that of a device with pure CuS CE. The combined benefits of the Ti3C2 framework's high conductivity and the numerous catalytically active centers of the CuS nanoparticles are mostly responsible for the improved performance [15].

Cao et al. utilised 2D MXene material (Ti3C2) as a back electrode in non-precious metal PSCs and hole-transport materials in 2019 [78]. This increase in PCE was ascribed to the Ti3C2 electrode's superior charge extraction capacity and reduced square resistance when compared to carbon electrodes. Jiang and his colleagues recently reported that, by using a combination of one-dimensional carbon nanotubes (CNTs), two-dimensional Ti3C2-MXene nanosheets, and commercial carbon paste as the electrode material in CsP-bBr3-PSC, they were able to obtain a power conversion efficiency of 7.1% [19,113].

In dye-sensitised solar cells, the 2D-layered Ti3C2 counter electrode substantially surpassed V2C in 2021 when compared to the iodide redox couple. According to Xu et al., the catalytic activity of Ti3C2 may be enhanced by increasing the etching time suitably. A PCE of 6.2 percent was found in DSCs with a Ti3C2 counter electrode etched for 24 h. Furthermore, K + intercalation has the potential to substantially boost Ti3C2's catalytic activity, which is affected by the increased number of catalytic activity centers and the increased interlayer spacing for smooth iodide electrolyte transport. The PCE of the DSCs with the K + -Ti3C2 counter electrode was 7.11 percent, which was notably similar to the PCE of the conventional DSCs using Pt counter electrodes (7.2%) [114]. Chen et al. made the first effort to utilise MXene/CoS as an electrocatalytic CE for QDSSCs in their research. When compared to QDSSCs with bare MXene (4.25%) and bare CoS (5.77%) CEs, the QDSSCs with an Mxene/CoS/CE exhibit a substantial improvement in cell performance and provide a promising PCE of 8.1% [115].

Additionally, a fan was installed to aid in the construction of flexible OSCs. This study emphasises the significance of developing FTEs and demonstrates their essential importance in flexible OSCs. With a sheet resistance of 110 sq−1, the transparent Ti3C2Tx Mxene electrodes have the lowest sheet resistance to date. As a result, scientists and engineers should collaborate to develop FTEs with the high electrical and optical compromise needed for highly efficient flexible OSCs. Tang et al. [116] demonstrated a flexible non-fullerene OSC with Ag NW/Mxene component electrodes and PBDB-T: ITIC: PC71BM active layers utilising the Ag NW/Mxene component electrodes (Figure 6) [117].

**Figure 6.** (**a**) Schematic representation of the MXene/AgNW hybrid electrodes on PUA substrates. (**b**) AFM images of the MXene/AgNW PUA films. (**c**) Transmission spectra of pure PUA, MXene-PUA, Ag NW-PUA, optimised MXene/Ag NW-PUA, and ITO glass. (**d**) Energy level diagrams of the flexible OSCs. (**e**) JV curves of the flexible OSCs with PBDB-T: ITIC: PC71BM active layers. (**f**) Normalised PCE of the flexible OSCs with MXene/Ag NW electrodes as a function of the number of bending cycles. Reproduced with permission. Reprinted with permission from ref. [116,117]. Copyright 2019 American Chemical Society.

Ahmed et al. studied the application of single-layer delaminated 2-D-MXene (Ti3C2) created by the leaching method to replace both TCO and Pt as a conductive layer and a catalyst. Each test required at least five samples. To prevent human error and obtain the greatest possible conversion efficiency for reliable comparisons, a pre-built TCO Pt meter was utilised as the reference counter electrode (CE). Figure 7 depicts the whole procedure. Furthermore, Ti3C2 was adjusted in thickness for optimum conversion efficiency. At optimum thickness, the TCO/Pt/free MXen-based CE had a PCE of 8.68%, which was 4.03% higher than the conventional TCO/Pt-based counter electrode. The high efficiency is attributable to the high conductivity, the large number of accessible catalytic centers owing to the delaminated structure, and Ti3C2's excellent catalytic activity towards iodide and triiodide electrolytes [118].

Hence, we can conclude that MXene serves a variety of roles in solar cells. As an electrode, it improves the form of hybrid electrodes with other conducting nanomaterials, such as metallic nanowires or carbon nanotubes. In addition, it enhances transparency, increases flexibility, metallic conductivity, and influences the work functions.

**Figure 7.** Schematic representation of an experimental procedure. Reprinted with permission from ref. [118]. Copyright 2021 Elsevier Ltd. and Techna Group S.r.l.

#### *3.3. Mxene as Transfer Layer HTL/ETL in Solar Cells*

The Electron Transport Layer (ETL) and Hole Transport Layer (HTL) in perovskite solar cells play an essential role in increasing stability (PSCs) and photovoltaic performance. The ETL's primary function is to collect and transmit electrons from the perovskite layer while also preventing hole backflow, efficiently segregating charges, and reducing charge recombination [119]. The HTL's primary function is to collect and transport holes from the photoactive perovskite layer to the electrode while also acting as an energy barrier to inhibit electron transmission to the anode. Furthermore, the HTL efficiently divides the photoactive perovskite layer from the anode and isolates air moisture, which enhances the stability of PSCs by reducing deterioration and corrosion [120]. The HTL PSC performance of component prototypes with various Mo2C @ CNT nanocomposite loading (1, 1.5, and 2 wt.-percent) was also investigated. Then, the Mo2C-CNT @ PEDOT: PSS HTL-based device was utilised as an X-ray photodetector, with a maximum sensitivity of 3.56 mA/Gycm2. Figure 8a depicts the schematic structure of the ITO/HTL/CH3NH3PbI3/ETL/LiF/Al-PSC using Mo2C-CNT @ PEDOT: PSS as HTL in the ITO/HTL/CH3NH3PbI3/ETL/LiF/Al-PSC using Mo2C-CNT @ PEDOT: PSS as HTL. The architecture of this composite perovskite solar cell was studied using cross-sectional FESEM (Figure 8b), and the associated energy level diagram is presented in Figure 8c. The findings show that Mxene/CNT nanocomposites with a perovskite layer have the potential to improve the efficiency of SCs and photodetectors. A high PCE of 11.98 percent was obtained for the HTL containing 1.5 percent by weight Mo2C-CNTs mixed with PEDOT: PSS in a component architecture of ITO/HTL/CH3NH3PbI3/PCBM/LiF/Al, which is greater than the HTLs with Mo2C (9.82%) and CNT (10.61%) mix [121].

**Figure 8.** (**a**) Device architecture of the prepared ITO/HTL/CH3NH3PbI3/PCBM/LiF/Al prototype using Mo2C-CNTs @ PEDOT: PSS HTL and (**b**) FESEM cross-sectional image; (**c**) Energy level diagram for ITO/Mo2C-CNTs @ PEDOT: PSS/CH3NH3PbI3/PCBM/LiF/Al structure. Reprinted with permission from ref. [121]. Copyright 2021 Elsevier B.V.

According to Bati et al., the incorporation of 2D MXenes into the ETL of PSCs produces extremely effective photovoltaic (PV) components. A power conversion efficiency of over 21% is obtained with the optimum composition [122]. In a planar PSC with a regular structure, Zheng et al. examined a hybrid film of SnO2 nanoparticles and Ti3C2Tx MXene nanoflakes as an electron transport layer (ETL). The ETL and perovskite layer production procedures are shown in Figure 9. The results show that the film qualities of the upper perovskite layers can be controlled by changing the Ti3C2Tx/SnO2 ratios (2.02 wt percent in ETLs), such as crystallinity, crystal size, compactness, defect density, optical absorption, surface roughness, and so on, by changing the Ti3C2Tx/SnO2 ratios (2.02 wt percent in ETLs) [123].

**Figure 9.** Schematic representation of the manufacturing processes of the perovskite film and the ETL. Reprinted with permission from ref. [123]. Copyright 2021American Chemical Society.

J. Zhang et al. developed the Nb2CTx-MXene, which has outstanding photoelectric characteristics and can be utilised as the HTL in fabricating the inverted PVSCs. Enhancing the O-terminated functional groups on the Nb2CTx surface, oxygen plasma treatment altered the work function (WF) of Nb2CTx HTL. PVSCs with oxygen-plasma-treated Nb2CTx HTL have the greatest PCE of 20.74 percent and excellent stability. Figure 10 shows a schematic representation of the device construction as well as the structure of Nb2CTx MXene, as seen in Figure 10a. The PVSCs' current density–voltage curves (JV) are presented in Figure 10b for various scan directions. As demonstrated in Figure 10, the enhanced Jsc is attributed to the greater external quantum efficiency values (EQE) owing to more effective charge separation and collecting efficiency (Figure 10c). The Nb2CTx-HTL treated with oxygen plasma similarly produces flexible and large-area (0.99 cm2) PVSCs with PCE of 17.26 percent and 17.94 percent (Figure 10d,e). Furthermore, employing Nb2CTx treated with oxygen plasma as HTL, the flexible and large-area (0.99 cm2) PVSCs obtain the greatest PCE of 17.26 percent and 17.94 percent, respectively [124].

**Figure 10.** (**a**) The schematic diagram of the device structure and the structure of Nb2CTx MXene. (**b**) JV curves of PVSCs measured under different scan directions. (**c**) External quantum efficiency (EQE) and integrated Jsc curves of various PVSCs. JV curves of the flexible (**d**) and large-area (**e**) PVSCs using Nb2CTx-HTL treated with oxygen plasma. Reprinted with permission from ref. [124]. Copyright 2021 AIP Publishing LLC.

Wang et al. used a solution procedure at room temperature to show the potential of Ti3C2Tx Mxene as an ETL for efficient PSCs with traditional design. The authors modified the MXene surface using an oxygen plasma treatment and attempted to establish a link between the surface characteristics and MXene termination groups. The contact angle and topography measurements were used to study the surface tension of MXene and the morphology of the associated perovskite. The PbO interactions between perovskite and MXene were shown by high-resolution XPS spectra, which improved device stability [125].

Yang et al. found a superior match in energy levels between the ETL layer and the perovskite in the case of a hybrid of oxidised and pure Ti3C2Tx, with a champion PCE of 18.29 percent, compared to PSCs with pure Ti3C2Tx as ETL, with a PCE of 16.50 percent. The intersection of the baseline with the tangent line of the spectra determined the highest occupied molecular orbital (HOMO) and the highest energy levels, while the results of the UV-Vis absorption spectra calculated the lowest unoccupied molecular orbital (LUMO). The enhanced electron mobility in the ETL, which increases electron transport and decreases hole–electron recombination, is responsible for the improvement in PCE. This research shows that these materials have a lot of promise for use in low-temperature-produced PSC and other solar technologies [126].

To develop a new ZnO/Ti3C2Tx nanohybrid composite film, Hou & Yu utilised Ti3C2Tx, a representative of MXene, as an additive in zinc oxide (ZnO). By establishing the Zn–O–Ti bond on the ZnO surface, Ti3C2Tx nanosheets generate new electron transport routes between ZnO nanocrystals and passivates the ZnO surface. As a consequence, the PBDB-T: ITIC based photovoltaic devices with ZnO/Ti3C2Tx ETLs have a power conversion efficiency of 12.20 percent, compared to 10.6 percent for the comparable device utilising pure ZnO as the ETL, which is a 15.53 percent improvement. Furthermore, PM6: Y6-based IPSCs obtain a champion power conversion efficiency of 16.5 percent, compared to 15 percent for the reference device, demonstrating the ZnO/Ti3C2Tx—ETL's applicability [102]. Saranin et al. showed that by utilising MXenes as doping for the forming layers, it is possible to adjust the optoelectronic characteristics of inverted p-i-nperovskite components. When compared to reference cells, the MXene-based devices had a maximum PCE of over 19% and an average growth of +8%, which is a surprising result, given that the MAPbI3-based p-i-n cell used spin-coated NiO [127].

#### **4. Conclusions and Prospect**

From the discovery of MXene in 2011 up to now, MXene has achieved tremendous technological developments. In 2018, MXene entered into the development of solar cell production by enhancing the effectiveness of energy produced and the stability of solar cells. This review attempts to compile all previously published research on adding MXene into PSCs to enhance operational stability and solar energy collection. According to MXene's function, the most essential device parameters are given in Tables S1–S3 (Supplementary Materials).

The main conclusions of this work are:

semiconductors, supercapacitors, and structural materials, by controlling the composition of the 2D transition-metal MXenes phase. This level of control over their composition and structure is unique in the area of 2D materials, and it opens up new avenues for nanomaterial design. The addition of 2D transition-metal MXenes to the category of 2D materials has increased the design options for nanomaterials to satisfy the needs of growing technology.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/nano11102732/s1, Tables S1–S3: Summary of the key parameters for the solar cells employing MXenes.

**Funding:** This research was fully funded by the Universiti Putra Malaysia (UPM) Research Grant (UPM-GRANT Putra, UPM/GP-IPB/2020/9688700), the Ministry of Higher Education Malaysia (KPT) Research Grant (FRGS/2012/5524194), and Department of Mechanical Engineering, Jeju National University, Korea.

**Acknowledgments:** The authors would like to acknowledge the Ministry of Higher Education Malaysia (KPT), Universiti Putra Malaysia (UPM), and Jeju National University for providing necessary resources in completing this study.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**