**2. Fabrication of porous silicon nanowires (PSiNWs)**

Najar et al. detailed the fabrication of PSiNWs using Ag-assisted electroless etching method from an n-type Si wafer (100) with a resistivity of 0.01–0.02 ohm cm [33]. The Si wafers were cleaned using acetone followed by ethanol for 5 min in an ultrasonic bath. Next, the wafers were immersed in a piranha solution H<sup>2</sup> SO4 /H<sup>2</sup> O2 (3:1) for 20 min to remove the organic deposits from the surface. The cleaned wafers were transferred into HF/AgNO<sup>3</sup> solution with a concentration of 4.8/0.005 M for Ag-deposition, followed by rinsing with de-ionized water. Then, the treated Si samples were etched in the HF/H<sup>2</sup> O2 solution for 45 min. Finally, samples were rinsed again for 10 min with HNO<sup>3</sup> solution to dissolve the excessive Ag nanoparticles (Ag NPs), leaving behind traces of Ag for catalyzing the etching reaction. Four samples were etched using HF concentration of 1.8, 2.8, 4.8, and 5.8 M, respectively, with a fixed H<sup>2</sup> O2 concentration of 0.5 M.

#### **2.1. PSiNWs: Physical properties**

dimensional systems. It has potential impact in realizing nanoscale interconnects and functional device elements in future nanoscale electronic and optoelectronic devices [12, 13]. The field of SiNWs synthesis represents an exciting and rapidly expanding research area. Considerable efforts have been devoted to the development of versatile and controllable methods for the synthesis of SiNW. These methods can be broadly classified as: (i) bottom-up and (ii) top-down approaches. The bottom-up approach involves the construction of desirable nanostructures

This method is useful for the fabrication of low-dimensional heterostructure-based devices in large quantities [14–18]. Using bottom-up, SiNWs were first obtained by vapor-liquid-solid (VLS) method [19], followed by an etching step to create nanowires. The VLS method has been implemented in a variety of techniques, such as pulsed laser deposition (PLD) [20, 21], gas-phase molecular beam epitaxy (GS-MBE) [22], chemical vapor deposition (CVD) [23, 24],

Top-down approach seeks to fabricate SiNWs from high-quality single crystal silicon wafer or thin film. Silicon nanowires have also been realized using lithographically defined patterns, or spin-coating of nanospheres as etched mask [28] followed by etching of the nanowires using plasma processing technique. Metal-assisted chemical etching method (MACE) is a simple, cost-effective, and powerful semiconductor etching technique that can produce high aspect ratio semiconductor nanostructures. By combining with metal patterning lithography or nonlithographic patterning methods, accurate control of the nanowire orientation (vertical vs. slanted), size (nano vs. microscale), shape, architecture, density, length, and doping char-

The fabrication of silicon nanowires using the metal-assisted electroless etching method has also been adopted [29–33]. The silver (Ag) ions in an ionic solution of hydrofluoric acid (HF)

crystal wafers [34, 35] The effects of various process parameters such as the etchant concen-

ties of the SiNWs have also been investigated [36]. The fabrication of nanowires using this method does not require complex sample preparation steps. Furthermore, this technique is effective, having high throughput and low cost. In this chapter, we will present the synthesis

Najar et al. detailed the fabrication of PSiNWs using Ag-assisted electroless etching method from an n-type Si wafer (100) with a resistivity of 0.01–0.02 ohm cm [33]. The Si wafers were cleaned using acetone followed by ethanol for 5 min in an ultrasonic bath. Next, the wafers

concentration of 4.8/0.005 M for Ag-deposition, followed by rinsing with de-ionized water.

O2

of PSiNWs and the last progress of hybrid solar cells based on these nanostructures.

SO4 /H<sup>2</sup> O2

its from the surface. The cleaned wafers were transferred into HF/AgNO<sup>3</sup>

, etching time, and postetch treatment on the morphology, and optical proper-

) have been used to prepare the arrays of SiNWs from single

(3:1) for 20 min to remove the organic depos-

solution for 45 min. Finally, samples

solution with a

from the basic components, i.e., from the atomic level to the nano- or microscale wires.

laser ablation [25, 26], and oxide-assisted growth techniques [27].

acteristics can be achieved readily at wafer scale.

O2

**2. Fabrication of porous silicon nanowires (PSiNWs)**

and hydrogen peroxide (H<sup>2</sup>

164 Emerging Solar Energy Materials

O2

were immersed in a piranha solution H<sup>2</sup>

Then, the treated Si samples were etched in the HF/H<sup>2</sup>

tration of H<sup>2</sup>

Najar et al. studied the etching morphology of the synthesis of PSiNWs [32]. **Figure 1(a)** shows the scanning electron microcopy image of the vertically oriented PSiNWs having an average length of 13 μm after 75 min of wet etching. The nanowires distribute uniformly on the sample. **Figure 1(b)** shows the top view with tips of NWs congregates together, and the diameter of the congregated bundles is of several micrometers. These congregated bundles are uniformly distributed on the entire samples and confirmed from the cross-section images. The transmission electron microscopic (TEM) image of an individual nanowire is shown in **Figure 1(c)**, which shows that the nanowire has a diameter of ~150 nm.

A typical 2D bright-field TEM image used for volume reconstruction of the sample is presented in **Figure 2(a)**–**(c)**. The three successive longitudinal slices obtained from the reconstructed volume revealed a distribution of irregular size/shape vacuum spaces and silicon frame, constituting the PSiNWs.

A typical 3D-tomography observation was conducted using a TEM, and 3D reconstructions were achieved using a simultaneous iterative reconstruction algorithm of consecutive 2D slices in **Figure 2(d)** [32]. The pore sizes present a distribution from 10 to 50 nm, with an estimated measurement error of 10%, and these pores go inside the nanowire showing similar structure to porous silicon. An average distance between two neighboring pores of lower than 5 nm has been observed. These mesoporous structures are expected to show strong quantum confinement effects.

It is noted that mesopores are mostly found near the top of the nanowires with an average pore size between 10 and 40 nm. AgNPs were detected at the bottom section of the nanowires by

**Figure 1.** (a) Cross-sectional images of SEM micrograph of PSiNWs etched for 75 min, (b) top view micrograph of PSiNWs, (c) TEM image of individual nanowire, and inset of the FFT image [32].

**Figure 2.** Panels (a), (b), and (c) represent three longitudinal slices extracted from the volume reconstruction of the PSiNW nanostructures revealing an area of highly porous medium. The bare scale is 50 nm. (d) 3D tomography TEM image of nanowires [32].

In this way, the higher H<sup>2</sup>

using various HF concentrations [33].

using Ag-assisted HF/H<sup>2</sup>

H2 O2

[Ag<sup>+</sup>

O2

O2

In **Figure 4(a)**, the electron transports from Si to Ag<sup>+</sup>

and the band edge bending in space charge layer (SCL) (ECB,

ions) is significantly reduced in the electrolyte. Low [Ag<sup>+</sup>

I-IV at the Si/electrolyte interface. Electrons at EF

doping level not only up shifts EF

Hence, a large amount of Ag<sup>+</sup>

concentration during the etching of highly doped n-Si wafers.

coming a potential energy barrier ∆Φ, consisting of the excitation barrier (ECB-EF

Since the scratching of growing Ag NPs is limited by the diffusion of Ag<sup>+</sup>

concentration increases the amount of out-diffused Ag<sup>+</sup>

electroless etching technique. (a-ii)–(d-ii) The corresponding SEM micrographs, prepared

S

toward ECB to reduce ∆Φ, but also makes the SCL thinner

increase of doping concentration increases the amount of weak defective points in the silicon lattice; both are important factors for the initiation of additional etching pathways in addition to the vertical etch, which explains the observation of the increased porosity with increasing

**Figure 3.** (a-i)–(d-i) The schematic illustration of the proposed qualitative etching model for the formation of PSiNWs

The heavy doping silicon wafer-induced porosification can be understood by a thermodynamic model that describes charge transport across the silicon-electrolyte interface driven by potential energy (**Figure 4(a)**). To et al. present a model to explain the etching mechanism [39].

mismatch of ECB,S with EAg+, electrons are apt to relaxing to surface states (ESS) aligned with EAg+ (the process III) followed by the resonant migration to EAg+ (the process IV). The increase in the

to promote electron tunneling through the SCL (**Figure 4(b)**), to facilitate the process I and II.

nuclei to grow and scratch the bulk Si. The rest of the nongrown Ag nuclei will undergo a random scratching in a short period of time to porosify SiNWs and the bulky wafer (**Figure 4(c)**).

ions nucleate on the surface and [Ag<sup>+</sup>

], the random porosification tends to be faster than the NP scratching. Consequently, the

, and the

167

, the process I)

ions with low

ions, which is composed of the processes

Hybrid Silicon Nanowires for Solar Cell Applications http://dx.doi.org/10.5772/intechopen.74282


] can support only a portion of Ag

] (i.e., concentration of Ag<sup>+</sup>

of Si wafers migrate to the interface by over-

EDS analysis in STEM mode. Based on the SEM and TEM analyses, the following qualitative model for the formation of PSiNWs is proposed. In HF/H<sup>2</sup> O2 solution, part of the Ag is oxidized into Ag<sup>+</sup> by H<sup>2</sup> O2 [37]. The Ag<sup>+</sup> catalyzes the dissolution of Si through the formation of SiF<sup>6</sup> 2−.

It is apparent that the transport of Ag<sup>+</sup> is isotropic while the AgNPs merely drop downward. Hence, the AgNPs facilitated the nanowire formation through vertical chemical etching, while the isotropic diffusion of Ag<sup>+</sup> promoted lateral chemical etching and pore formation as illustrated in **Figure 3(a**-**i** and **a**-**ii)** [33].

The Ag<sup>+</sup> may be chemically reduced by the n-type silicon nanowires [38] forming Ag particle that redeposited onto the sidewalls forming new Ag nucleation sites, and therefore the new localized lateral etching pathway. Furthermore, the nucleation of the AgNPs on the side walls would also reduce the Ag<sup>+</sup> concentration locally and accelerates the Ag<sup>+</sup> diffusion in the lateral direction. The laterally diffused Ag<sup>+</sup> will continue to catalyze the chemical etching of the sidewalls of Si. Since the upper Si nanowires are exposed longer to the solutions, larger Si volume dissolves at the top (and creating more mesopores) than the bottom of the nanowires (**Figure 3(b**-**i** and **b**-**ii)** and **(c**-**i** and **c**-**ii)**), being consistent with the TEM image as shown in **Figure 2(a)**. Furthermore, increasing the HF concentration accelerates the process described in our qualitative model, with the Ag Ag<sup>+</sup> + e<sup>−</sup> reversible reaction that continues to penetrate the nanowires laterally to catalyze the HF/H<sup>2</sup> O2 chemical reaction forming mesopores. Eventually, the mesopores merged leading to a reduction in length and pore density, with the formation of conical PSiNWs as illustrated in **Figure 3(d**-**i** and **d**-**ii)**.

**Figure 3.** (a-i)–(d-i) The schematic illustration of the proposed qualitative etching model for the formation of PSiNWs using Ag-assisted HF/H<sup>2</sup> O2 electroless etching technique. (a-ii)–(d-ii) The corresponding SEM micrographs, prepared using various HF concentrations [33].

In this way, the higher H<sup>2</sup> O2 concentration increases the amount of out-diffused Ag<sup>+</sup> , and the increase of doping concentration increases the amount of weak defective points in the silicon lattice; both are important factors for the initiation of additional etching pathways in addition to the vertical etch, which explains the observation of the increased porosity with increasing H2 O2 concentration during the etching of highly doped n-Si wafers.

EDS analysis in STEM mode. Based on the SEM and TEM analyses, the following qualitative

**Figure 2.** Panels (a), (b), and (c) represent three longitudinal slices extracted from the volume reconstruction of the PSiNW nanostructures revealing an area of highly porous medium. The bare scale is 50 nm. (d) 3D tomography TEM

Hence, the AgNPs facilitated the nanowire formation through vertical chemical etching,

that redeposited onto the sidewalls forming new Ag nucleation sites, and therefore the new localized lateral etching pathway. Furthermore, the nucleation of the AgNPs on the side walls

sidewalls of Si. Since the upper Si nanowires are exposed longer to the solutions, larger Si volume dissolves at the top (and creating more mesopores) than the bottom of the nanowires (**Figure 3(b**-**i** and **b**-**ii)** and **(c**-**i** and **c**-**ii)**), being consistent with the TEM image as shown in **Figure 2(a)**. Furthermore, increasing the HF concentration accelerates the process described

+ e<sup>−</sup>

Eventually, the mesopores merged leading to a reduction in length and pore density, with the

O2

may be chemically reduced by the n-type silicon nanowires [38] forming Ag particle

concentration locally and accelerates the Ag<sup>+</sup>

O2

catalyzes the dissolution of Si through the formation of SiF<sup>6</sup>

is isotropic while the AgNPs merely drop downward.

will continue to catalyze the chemical etching of the

reversible reaction that continues to pen-

chemical reaction forming mesopores.

promoted lateral chemical etching and pore formation as

solution, part of the Ag is oxidized

2−.

diffusion in the lat-

model for the formation of PSiNWs is proposed. In HF/H<sup>2</sup>

[37]. The Ag<sup>+</sup>

It is apparent that the transport of Ag<sup>+</sup>

illustrated in **Figure 3(a**-**i** and **a**-**ii)** [33].

eral direction. The laterally diffused Ag<sup>+</sup>

in our qualitative model, with the Ag Ag<sup>+</sup>

etrate the nanowires laterally to catalyze the HF/H<sup>2</sup>

formation of conical PSiNWs as illustrated in **Figure 3(d**-**i** and **d**-**ii)**.

while the isotropic diffusion of Ag<sup>+</sup>

would also reduce the Ag<sup>+</sup>

into Ag<sup>+</sup>

The Ag<sup>+</sup>

 by H<sup>2</sup> O2

image of nanowires [32].

166 Emerging Solar Energy Materials

The heavy doping silicon wafer-induced porosification can be understood by a thermodynamic model that describes charge transport across the silicon-electrolyte interface driven by potential energy (**Figure 4(a)**). To et al. present a model to explain the etching mechanism [39]. In **Figure 4(a)**, the electron transports from Si to Ag<sup>+</sup> ions, which is composed of the processes I-IV at the Si/electrolyte interface. Electrons at EF of Si wafers migrate to the interface by overcoming a potential energy barrier ∆Φ, consisting of the excitation barrier (ECB-EF , the process I) and the band edge bending in space charge layer (SCL) (ECB, S -ECB, the process II). Owing to the mismatch of ECB,S with EAg+, electrons are apt to relaxing to surface states (ESS) aligned with EAg+ (the process III) followed by the resonant migration to EAg+ (the process IV). The increase in the doping level not only up shifts EF toward ECB to reduce ∆Φ, but also makes the SCL thinner to promote electron tunneling through the SCL (**Figure 4(b)**), to facilitate the process I and II. Hence, a large amount of Ag<sup>+</sup> ions nucleate on the surface and [Ag<sup>+</sup> ] (i.e., concentration of Ag<sup>+</sup> ions) is significantly reduced in the electrolyte. Low [Ag<sup>+</sup> ] can support only a portion of Ag nuclei to grow and scratch the bulk Si. The rest of the nongrown Ag nuclei will undergo a random scratching in a short period of time to porosify SiNWs and the bulky wafer (**Figure 4(c)**). Since the scratching of growing Ag NPs is limited by the diffusion of Ag<sup>+</sup> ions with low [Ag<sup>+</sup> ], the random porosification tends to be faster than the NP scratching. Consequently, the

indicating the dominant effect of vertical etching enhanced by Ag catalyst with preferential etching along the low atomic density plane in [001]. An increase in aspect ratio from about 9 to 12 and 27 has been calculated for NWs samples etched with 1.8, 2.8, and 4.8 M HF concentration

Hybrid Silicon Nanowires for Solar Cell Applications http://dx.doi.org/10.5772/intechopen.74282 169

**Figure 5.** (a)–(h) SEM and TEM micrographs of PSiNWs fabricated with increasing HF concentration from 1.8, 2.8, 4.8,

up to 5.8 M [33].

**Figure 4.** (a) Quasi-quantitative energy diagram at the silicon-electrolyte interface, at the beginning of one-MACE of n++ type Si(100). The energy is referred to standard hydrogen electrode (SHE) potential. Heavily doped n-Si has Fermi energy (EF ), valence, and conduction band in the bulk (EVB and ECB) and at the interface (EVB,S and ECB,S), and space charge layer (SCL) with a width WSCL. ΔΦ is an energy barrier between EF and ECB,S. EAg+/Ag, EAg+, and EAg represent the potential energy of the redox pair, Ag<sup>+</sup> and Ag in the electrolyte, respectively. λ is the reorientation energy. The light blue sphere represents an electron at EF , and the I-IV processes denote a subsequent series of electron migration from the bulk to interface. (b) Energy diagram with variation of doping levels in n-Si(100). (c) Schematic of the one-MACE evolution. For clarity, only one nongrown Ag nucleus scratching the wafer in the mpSi is shown, marked by a red dashed line. Within a period of time Δt, the evolution of the porosification and mesoporous SiNWs are denoted by ΔTP and ΔTNW, respectively [39].

electrochemical porosification tends to generate a hybrid structure composed of an array of mpSiNWson mpSi (**Figure 4 (a)**–**(d)**). On the contrary, the light and media doping increases ∆Φ to prevent Ag<sup>+</sup> ions from severely consuming. High [Ag<sup>+</sup> ] substantially supports most Ag NPs grow and scratch the wafer to create SiNWs, and effectively suppresses the porosification. This model can also account for the creation of mesoporous SiNWs via one-MACE of p++ type Si.

#### **2.2. HF concentrations vs. morphology of PSiNWs**

The modification of HF or H<sup>2</sup> O2 concentrations affects the morphology of PSiNWs. In the presence of Ag catalyst, an increase in HF or H<sup>2</sup> O2 concentration in electroless etching method is analogous to an increase in the current density in electrochemical-based methods [40, 41]. In both cases, increasing the H<sup>2</sup> O2 or HF concentration increases the oxidation rate and dissolution rate, respectively, resulting in nanostructures with varying optical properties. In **Figure 5(a)**–**(h)**, Najar et al. shows the cross-sectional SEM micrographs and TEM images acquired from the NW samples grown with HF concentrations of 1.8, 2.8, 4.8, and 5.8 M in a fixed H<sup>2</sup> O2 concentration of 0.5 M [33].

The variation of NWs length and NWs diameter with increasing HF concentrations was measured and analyzed using SEM and TEM, and the results are tabulated in **Table 1**. The NWs length shows significant changes, while the NWs diameter varies from 80 to 210 nm for samples indicating the dominant effect of vertical etching enhanced by Ag catalyst with preferential etching along the low atomic density plane in [001]. An increase in aspect ratio from about 9 to 12 and 27 has been calculated for NWs samples etched with 1.8, 2.8, and 4.8 M HF concentration

electrochemical porosification tends to generate a hybrid structure composed of an array of mpSiNWson mpSi (**Figure 4 (a)**–**(d)**). On the contrary, the light and media doping increases ∆Φ

**Figure 4.** (a) Quasi-quantitative energy diagram at the silicon-electrolyte interface, at the beginning of one-MACE of n++ type Si(100). The energy is referred to standard hydrogen electrode (SHE) potential. Heavily doped n-Si has Fermi

interface. (b) Energy diagram with variation of doping levels in n-Si(100). (c) Schematic of the one-MACE evolution. For clarity, only one nongrown Ag nucleus scratching the wafer in the mpSi is shown, marked by a red dashed line. Within a period of time Δt, the evolution of the porosification and mesoporous SiNWs are denoted by ΔTP and ΔTNW,

), valence, and conduction band in the bulk (EVB and ECB) and at the interface (EVB,S and ECB,S), and space charge

and Ag in the electrolyte, respectively. λ is the reorientation energy. The light blue sphere

, and the I-IV processes denote a subsequent series of electron migration from the bulk to

grow and scratch the wafer to create SiNWs, and effectively suppresses the porosification. This model can also account for the creation of mesoporous SiNWs via one-MACE of p++ type Si.

O2

is analogous to an increase in the current density in electrochemical-based methods [40, 41].

solution rate, respectively, resulting in nanostructures with varying optical properties. In **Figure 5(a)**–**(h)**, Najar et al. shows the cross-sectional SEM micrographs and TEM images acquired from the NW samples grown with HF concentrations of 1.8, 2.8, 4.8, and 5.8 M in a

The variation of NWs length and NWs diameter with increasing HF concentrations was measured and analyzed using SEM and TEM, and the results are tabulated in **Table 1**. The NWs length shows significant changes, while the NWs diameter varies from 80 to 210 nm for samples

] substantially supports most Ag NPs

and ECB,S. EAg+/Ag, EAg+, and EAg represent the potential

concentration in electroless etching method

concentrations affects the morphology of PSiNWs. In the

or HF concentration increases the oxidation rate and dis-

ions from severely consuming. High [Ag<sup>+</sup>

O2

O2

**2.2. HF concentrations vs. morphology of PSiNWs**

layer (SCL) with a width WSCL. ΔΦ is an energy barrier between EF

presence of Ag catalyst, an increase in HF or H<sup>2</sup>

concentration of 0.5 M [33].

The modification of HF or H<sup>2</sup>

In both cases, increasing the H<sup>2</sup>

to prevent Ag<sup>+</sup>

respectively [39].

energy (EF

energy of the redox pair, Ag<sup>+</sup>

168 Emerging Solar Energy Materials

represents an electron at EF

fixed H<sup>2</sup>

O2

**Figure 5.** (a)–(h) SEM and TEM micrographs of PSiNWs fabricated with increasing HF concentration from 1.8, 2.8, 4.8, up to 5.8 M [33].


**Table 1.** Effect of HF concentration on PSiNWs length, NWs diameter, and aspect ratio analyzed from SEM micrographs [33].

(**Figure 5(a)**, **(c)**, and **(e)**). However, the aspect ratio reduces to about 9 for NWs etched with (**Figure 5(g)**) signifying a change in etching mechanism with faster lateral chemical etch.

### **2.3. Effect of HF concentration on PL**

It is known that the mesoporous silicon nanostructure gives rise to strong visible emission [42]. Optical properties of the PSiNWs samples have been investigated using PL spectroscopy. **Figure 6(a)** shows the PL spectra of the samples etched with HF concentrations of 1.8, 2.8, 4.8, and 5.8 M with a constant H<sup>2</sup> O2 concentration of 0.5 M. It is observed that the asymmetric PL spectra have two dominant peaks around 700 and 760 nm. The sample etched with 4.8 M HF concentration has an additional hump at 670 nm. The highest PL peak at 760 nm is related to the emission from Si–O bond [42, 43].

In our case, we observe in **Figure 6(a)** that the increase in integrated intensities of the broadband PL emissions correlates well with the increase in pore densities for samples etched using 1.8–4.8 M HF concentrations. With an HF concentration of 5.8 M, a reduction in PL integrated intensity is again consistent with a reduction in the pore density of PSiNWs.

After HNO<sup>3</sup> and HF treatments for 5 min to remove the oxide, all samples show similar PL peak wavelength at 800 nm (**Figure 6(c)**). However, samples without HF treatment show a significant blueshift with PL wavelength peak at 700 nm (**Figure 6(a)**). This blueshift and increase of PL intensity have been attributed to the presence of defect states in Si-Ox.

The quantum confinement effect dictates that the characteristic size of the Si-nanocrystallites should be less than the Bohr radius of the free exciton of bulk Si, which is around 5 nm [46]. From the TEM analysis, Si-nanocrystallites of <5 nm has been observed (**Figure 7(a)**). The inset of **Figure 7(a)** shows the calculated fast-Fourier transform (FFT) of PSiNWs prepared using 4.8 M HF concentration. The FFT revealed the spatial frequencies corresponding to {111}, {220}, and {311} d-spacing of Si crystal structure. The presence of these d-spacing implies that the PSiNW was oriented along the <110> direction. This was expected as the (001) Si wafer was used to fabricate the NWs, and etching was vertical with respect to the substrate surface. Also, the energy-filtered TEM analysis of the top region of the PSiNW, as shown in **Figure 7(b)**, confirms the existence of Si nanocrystallites and silica amorphous structures, indicating the possible strong PL emission from the highly quantum-confined Si nanocrytallite structures. This prediction correlates positively with our previous discussion on the effect of pore density on PL enhancement. The above argument further confirms our observation that samples etched with HF concentration of 5.8 M (i.e., with low pore and Si-nanocrystallites densities) results in a decrease in PL intensity. The evolution of PL peak intensity with HF concentration is hence consistent with the physical changes in the PSiNWs developed in our etching model.

**Figure 6.** PL spectra of PSiNWs as a function of HF concentration etched for 45 min (a): 1.8, 2.8, 4.8, and 5.8 M, (b) after

Hybrid Silicon Nanowires for Solar Cell Applications http://dx.doi.org/10.5772/intechopen.74282 171

treatment followed by HF (5%), and (c) with and without HF treatment [33].

HNO<sup>3</sup>

This has been correlated with the redshift of PL spectrum that suggests the interfacial between Si and surface oxide, as well as the quantum confinement effect, and plays a critical role in the light emission process (i.e., the presence of nanocrystallites with size <5 nm that has been confirmed by HRTEM).

Several mechanisms have been proposed to explain the origin of strong PL emission for the indirect bandgap silicon material. These include (i) the quantum confinement effects of free excitons within the Si-nanocrystallites and (ii) SiOx/Si interface defects and/or defect states in the surface oxide, related to Si-O bond [42, 44, 45]. Comparing the PL spectra before and after the HF treatment, we believe that existence of oxide plays a significant role in changing the emission property of the nanowires.

(**Figure 5(a)**, **(c)**, and **(e)**). However, the aspect ratio reduces to about 9 for NWs etched with (**Figure 5(g)**) signifying a change in etching mechanism with faster lateral chemical etch.

**Table 1.** Effect of HF concentration on PSiNWs length, NWs diameter, and aspect ratio analyzed from SEM micrographs [33].

NWs length (μm) 1.3–1.8 2–2.2 4.8–5.3 1.6–1.8 NWs diameter (nm) 160–210 100–180 80–200 150–200

It is known that the mesoporous silicon nanostructure gives rise to strong visible emission [42]. Optical properties of the PSiNWs samples have been investigated using PL spectroscopy. **Figure 6(a)** shows the PL spectra of the samples etched with HF concentrations of 1.8, 2.8, 4.8,

spectra have two dominant peaks around 700 and 760 nm. The sample etched with 4.8 M HF concentration has an additional hump at 670 nm. The highest PL peak at 760 nm is related to

In our case, we observe in **Figure 6(a)** that the increase in integrated intensities of the broadband PL emissions correlates well with the increase in pore densities for samples etched using 1.8–4.8 M HF concentrations. With an HF concentration of 5.8 M, a reduction in PL integrated

peak wavelength at 800 nm (**Figure 6(c)**). However, samples without HF treatment show a significant blueshift with PL wavelength peak at 700 nm (**Figure 6(a)**). This blueshift and

This has been correlated with the redshift of PL spectrum that suggests the interfacial between Si and surface oxide, as well as the quantum confinement effect, and plays a critical role in the light emission process (i.e., the presence of nanocrystallites with size <5 nm that has been

Several mechanisms have been proposed to explain the origin of strong PL emission for the indirect bandgap silicon material. These include (i) the quantum confinement effects of free excitons within the Si-nanocrystallites and (ii) SiOx/Si interface defects and/or defect states in the surface oxide, related to Si-O bond [42, 44, 45]. Comparing the PL spectra before and after the HF treatment, we believe that existence of oxide plays a significant role in changing the

increase of PL intensity have been attributed to the presence of defect states in Si-Ox.

and HF treatments for 5 min to remove the oxide, all samples show similar PL

intensity is again consistent with a reduction in the pore density of PSiNWs.

concentration of 0.5 M. It is observed that the asymmetric PL

1.8 M 2.8 M 4.8 M 5.8 M

~9 ~12 ~27 ~9

**2.3. Effect of HF concentration on PL**

the emission from Si–O bond [42, 43].

O2

**PSiNWs physical parameters HF concentration**

and 5.8 M with a constant H<sup>2</sup>

After HNO<sup>3</sup>

Aspect ratio

(longest length/largest diameter)

170 Emerging Solar Energy Materials

confirmed by HRTEM).

emission property of the nanowires.

**Figure 6.** PL spectra of PSiNWs as a function of HF concentration etched for 45 min (a): 1.8, 2.8, 4.8, and 5.8 M, (b) after HNO<sup>3</sup> treatment followed by HF (5%), and (c) with and without HF treatment [33].

The quantum confinement effect dictates that the characteristic size of the Si-nanocrystallites should be less than the Bohr radius of the free exciton of bulk Si, which is around 5 nm [46]. From the TEM analysis, Si-nanocrystallites of <5 nm has been observed (**Figure 7(a)**). The inset of **Figure 7(a)** shows the calculated fast-Fourier transform (FFT) of PSiNWs prepared using 4.8 M HF concentration. The FFT revealed the spatial frequencies corresponding to {111}, {220}, and {311} d-spacing of Si crystal structure. The presence of these d-spacing implies that the PSiNW was oriented along the <110> direction. This was expected as the (001) Si wafer was used to fabricate the NWs, and etching was vertical with respect to the substrate surface. Also, the energy-filtered TEM analysis of the top region of the PSiNW, as shown in **Figure 7(b)**, confirms the existence of Si nanocrystallites and silica amorphous structures, indicating the possible strong PL emission from the highly quantum-confined Si nanocrytallite structures. This prediction correlates positively with our previous discussion on the effect of pore density on PL enhancement. The above argument further confirms our observation that samples etched with HF concentration of 5.8 M (i.e., with low pore and Si-nanocrystallites densities) results in a decrease in PL intensity. The evolution of PL peak intensity with HF concentration is hence consistent with the physical changes in the PSiNWs developed in our etching model.

**Figure 7.** (a) HRTEM and (b) EFTEM analyses of the top section of a PSiNW etched using 4.8 M concentration of HF [33].

the Si/Ag interface and notable improvements in the fabrication reproducibility. With an opti-

**Figure 8.** Embedded metal electrode for highly efficient organic-inorganic hybrid nanowire solar cells, and I-V charac-

of up to 16.1% with an open-circuit voltage of 607 mV and a short-circuit current density of

using a conventional electrode. The microgrid electrode significantly minimizes the optical and electrical losses. This reproducibly yields a superior quantum efficiency of 99% at the main solar spectrum wavelength of 600 nm. In particular, the solar cells exhibit a significant increase in the fill factor of 78.3% compared to that of a conventional electrode (61.4%), this is because of the drastic reduction in the metal/contact resistance of the 1 μm-thick Ag electrode. Hence, the use of this embedded microgrid electrode in the construction of an ideal carrier collection path presents an opportunity in the development of highly efficient organic-

Liu et al. fabricated a hybrid silicon nanowire/polymer heterojunction solar cell combined with a polypyrrole-based supercapacitor as shown in **Figure 9**, to harvest solar energy and store it [53]. By efficiency enhancement of the hybrid nanowire solar cells and a dual-functional titanium film serving as a conjunct electrode of the solar cell and supercapacitor, the integrated system is able to yield a total photoelectric conversion to storage efficiency of 10.5%, which is the record value in all the integrated solar energy conversion and storage system. This system may not only serve as a buffer that diminishes the solar power fluctuations from light intensity, but also pave its way toward cost-effective high-efficiency self-charging power unit. Finally, an integrated device based on ultrathin Si substrate is demonstrated to expand its

feasibility and potential application in flexible energy conversion and storage devices.

Organic-inorganic hybrid solar cells based on n-type crystalline silicon and poly(3,4 ethylenedioxythiophene)-poly(styrenesulfonate) exhibited promising efficiency along with a low-cost fabrication process (**Figure 10**). Zhang et al. fabricated ultrathin flexible silicon substrates, with a thickness as low as tens of micrometers, which were employed to fabricate hybrid solar cells to reduce the use of silicon material [54]. To improve the light-trapping ability,

. This power conversion efficiency is more than twice as high as that of solar cells

hybrid solar cells exhibit a power conversion efficiency

Hybrid Silicon Nanowires for Solar Cell Applications http://dx.doi.org/10.5772/intechopen.74282 173

mized microgrid electrode, the 1-cm<sup>2</sup>

inorganic hybrid solar cells.

34.0 mA/cm<sup>2</sup>

teristic of the solar cell [52].

However, SiO<sup>2</sup> alone should not give rise to the PL emission; rather, defect states would contribute to PL emission. Indeed, Najar et al. observed Si1+ (Si2 O) and Si3+ (Si2 O3 ) using X-ray hotoelectron spectroscopy (XPS) signatures, which may originate from dangling bonds and volumetric stress that distorted the PSiNWs forming localized defect states.

Therefore, the emissions from surface/defect states in oxide may contribute to the red PL emission, in agreement with the results obtained in [47], in addition to the contribution from quantum confinement effect.
