**2. Processing and passivation of Si wafers used for solar cells**

It is known that crystalline silicon (c-Si) based solar cells dominate the solar energy industry. The modern silicon wafer production technology and processing sequence is the most mature and including hundreds of flawless process steps [15]. With increasing the electronic quality of c-Si, the efficiency of c-Si solar cells improved considerably [16]. Most generally, free carriers generated by the incident sunlight would be efficiently collected far away from the fast recombination centers while they move towards the device terminals. In particular, carrier recombination at the cell surfaces should be avoided, especially in thin wafers.

It is convenient to define an effective surface recombination velocity (*Seff* ), which for a symmetrically passivated sample at a low injection level takes the form [17]:

$$S\_{\rm eff} = \sqrt{D \left(\frac{1}{\tau\_{\rm eff}} - \frac{1}{\tau\_b}\right)} \tan\left(\frac{w}{2} \sqrt{\frac{1}{D} \left(\frac{1}{\tau\_{\rm eff}} - \frac{1}{\tau\_b}\right)}\right),\tag{1}$$

where *D* is the minority carrier diffusivity, *τeff* is the effective lifetime, *τ<sup>b</sup>* is the bulk lifetime of the wafer and *w* is the wafer thickness. If *Seff* is rather large, one equates [17]:

$$\frac{1}{\tau\_{\rm eff}} = \frac{1}{\tau\_b} + D\left(\frac{\pi}{w}\right)^2. \tag{2}$$

If the surface passivation is good and hence the surface recombination velocity is sufficiently small, the tangent term in Eq. (1) becomes linear and [17]:

$$\frac{1}{\tau\_{\rm eff}} = \frac{1}{\tau\_b} + \frac{2S\_{\rm eff}}{w} \,. \tag{3}$$

Therefore, carrier recombination at the wafer surfaces restricts its effective lifetime thus posing inherent limitation of using c-Si in solar cells.

The surface itself terminates an atomic order in c-Si such that Si atoms that reside on the surface are not fully bonded to four Si neighbors. This yields dangling bonds, which form surface defects and thus reduce the efficiency of solar cells. Therefore, reducing the number of these defects is clearly a necessary prerequisite for manufacturing higher efficiency silicon solar cells.

### *Ultrasonic Processing of Si and SiGe for Photovoltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.96939*

Over the last years different industries have therefore become increasingly drawn to sonochemistry because it provides a green and clean alternative to conventional technologies, particular in the areas of processing of silicon-based materials. The aim of this work is to provide a cohesive presentation of the related efforts. Two techniques related to ultrasonic cleaning of Si wafers and sonochemical modification of Si and SiGe surfaces in hydrocarbon solutions will be discussed. In both cases, the occurrence of cavitation and bubble implosion is necessary for ultrasonic cleaning and surface processing as it is known today. The use of higher ultrasonic frequencies to expand the range of ultrasonic cleaning and processing capabilities will be emphasized. Although exact mechanisms of an improved photoelectric behavior of Si-based micro- and nanostructures subjected to power ultrasound are not yet clarified in many cases, the likely scenarios behind the observed photovoltaic performances will be proposed to involve the surface chemistry of

oxygen and hydrogen molecules as well hydrocarbon chains.

*Solar Cells - Theory, Materials and Recent Advances*

at the cell surfaces should be avoided, especially in thin wafers.

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi *<sup>D</sup>* <sup>1</sup> *τeff* � 1 *τb* vu ! ut tan

> 1 *τeff*

sufficiently small, the tangent term in Eq. (1) becomes linear and [17]:

1 *τeff*

lifetime thus posing inherent limitation of using c-Si in solar cells.

for manufacturing higher efficiency silicon solar cells.

**138**

¼ 1 *τb* þ 2*Seff*

Therefore, carrier recombination at the wafer surfaces restricts its effective

The surface itself terminates an atomic order in c-Si such that Si atoms that reside on the surface are not fully bonded to four Si neighbors. This yields dangling bonds, which form surface defects and thus reduce the efficiency of solar cells. Therefore, reducing the number of these defects is clearly a necessary prerequisite

¼ 1 *τb*

*Seff* ¼

**2. Processing and passivation of Si wafers used for solar cells**

It is known that crystalline silicon (c-Si) based solar cells dominate the solar energy industry. The modern silicon wafer production technology and processing sequence is the most mature and including hundreds of flawless process steps [15]. With increasing the electronic quality of c-Si, the efficiency of c-Si solar cells improved considerably [16]. Most generally, free carriers generated by the incident sunlight would be efficiently collected far away from the fast recombination centers while they move towards the device terminals. In particular, carrier recombination

It is convenient to define an effective surface recombination velocity (*Seff* ), which for a symmetrically passivated sample at a low injection level takes the form [17]:

> *w* 2

0 @

where *D* is the minority carrier diffusivity, *τeff* is the effective lifetime, *τ<sup>b</sup>* is the bulk lifetime of the wafer and *w* is the wafer thickness. If *Seff* is rather large, one equates [17]:

> <sup>þ</sup> *<sup>D</sup> <sup>π</sup> w* � �<sup>2</sup>

If the surface passivation is good and hence the surface recombination velocity is

1 *D*

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1

*:* (2)

*<sup>w</sup> :* (3)

A, (1)

1 *τeff* � 1 *τb*

vu ! ut

In this respect, various surface-passivation layers have been employed. In particular, SiO2 surface passivation coating layer has been proven to offer outstanding passivation [18–20]. In general, two oxide growth methods are employed termed as the dry and wet oxidation. Dry oxygen and water vapor are used in the former and latter cases, respectively. Dry oxidation typically forms thin oxide layers in practical structures due to perfect Si–SiO2 interface developed in this case. In turn, wet oxidation yields greater growth rates, which is necessary for depositing thicker SiO2 layers.

Furthermore, a hydrogenated amorphous silicon a-Si:H layer can saturate the dangling bonds by hydrogen termination [21–23]. The atomic hydrogen is also able to effectively passivate dangling bonds at the c-Si/SiO2 interface thus drastically suppressing the interface state density and surface recombination velocities [24]. An important aspect is that the treatment in HF can produce an inversion layer on p-type Si surfaces while an accumulation of majority carriers is observed on n-type Si after treatment in either NH4F or HF [25]. These both are due to positive charges induced by electronegative surface groups such as –H, –O–H and –F bonded to the Si surface atoms �Sis– [26].

Most of the oxidizing solutions, e.g. H2SO4/H2O2, HCl, HNO3, RCA-2 (the mixture of HCl, H2O2 and H2O), lead to surface depletion of holes in p-Si and a weakly depleted majority carriers in n-Si that appear due to the positive fixed oxide charge. Surface processing in the RCA-1 solution, which contains the mixture of NH4OH, H2O2 and H2O, can be considered, in turn, noting a strong depletion in n-Si and a weak one on p-type Si surface thus implying the negative surface charge arisen from the dissociation of �Si–OH groups in the SiO2 film during the oxidation in the solution (�Si–OH ⇆ �Si–O� + H+ ) [25, 26].

Using an amorphous silicon layer should also prove useful in GexSi1–x/Si structures [27]. It is seen in **Figure 1** that the surface photovoltage (SPV) is enhanced as the structure is covered with a-Si (curves 3 and 4 compared with curves 1 and 2 at time *t* ¼ 0). Roughly a 10 times larger value of the SPV magnitude is observed in the

#### **Figure 1.**

*Time-dependent SPV of Si wafer (1), structures of GexSi1–<sup>x</sup> islands on Si (2), 10 nm a-Si/GexSi1–x/Si (3), and 10 nm a-Si/GexSi1–x/Si annealed for 5 minutes at 400 °C in an O2 ambient atmosphere (4). The concentration of Ge atoms in the islands is about 80%. Reprinted with permission from Podolian A, Nadtochiy A, Korotchenkov O, Romanyuk B, Melnik V, Popov V. Journal of Applied Physics. 2018;124:095703. Copyright 2018, AIP publishing.*

capped GexSi1–x/Si structure (curve 3 at time *t* ¼ 0) compared to that of bare GexSi1–x-on-Si islands (curve 2). This enlargement is even 5 to 10 times greater after subsequent sample annealing in O2 (curve 4 at time *t* ¼ 0).

bubbles during cavitation produces local transient temperatures of about 5000 K and pressures of about 500 atm, with heating and cooling rates exceeding 10<sup>10</sup> K/s [1, 38]. These conditions create high-velocity collisions between suspended particles and the estimated speed of colliding particles approaches almost the speed of sound in the liquid. Interestingly, sonochemical reactions in cavitational fields occur more slowly at elevated than at lower temperatures. Even so it is surprising, this counterintuitive property makes sense, because at higher temperatures more solvents

In cold liquids, ultrasound is able to drive reactions that normally occur only under extreme conditions. Examples [45] include intercalation, activation of liquid– solid reactions, and the synthesis of amorphous and nanophase materials. The sonochemical syntheses of nanophase metals, alloys, metal carbides, supported heterogeneous catalysts, and nano-colloids derives from the sonochemical decomposition of volatile organo-metallic precursors during cavitation, which produces

Various types of sonochemical cells using ultrasound baths and ultrasound horn systems have been reported [48–50]. Most frequently, an ultrasonic transducer and the ultrasound horn are placed directly in the solution. One example is shown in **Figure 2**. An oscillating rf voltage from an oscillator (1 in **Figure 2**) with the amplitude of *U*<sup>0</sup> is amplified by an amplifier (2) and applied to a Langevin transducer, which consists of a piezoelectric transducer (3), back (4) and front (5) masses. The vibrating transducer is loaded to a glass flask (6) filled with a reactant solution (7) thus delivering an acoustic power at a resonance frequency of the transducer-solution system. The lowest-mode resonance frequency is defined by the solution thickness *h* such that *h* ¼ *λ=*2 with *λ* the sound wavelength in the

The samples can freely levitate near the pressure antinode zone at operating frequencies in the dozens kHz range. They tend to reside on the flask bottom when

*Schematic of the sonochemical cell: 1 – Oscillator, 2 – Amplifier, 3 – Piezoelectric transducer, 4 – Back mass,*

vaporize into the bubble and, hence, cushion the collapse.

*Ultrasonic Processing of Si and SiGe for Photovoltaic Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.96939*

clusters of a few nm in diameter.

solution at the resonant frequency.

**Figure 2.**

**141**

increasing the frequency up to several hundreds of kHz.

*5 – Front mass, 6 – Boiling flask, 7 – Reactant solution, 8 – Sample.*

Presuming the use of effective hydrogenation, a-Si:H/c-Si heterojunction solar cells (HET) represent one of the most promising solar cell structures that enable high efficiencies due to high open-circuit values coming from the excellent passivation properties of a-Si:H combined with the beneficial effect of the *a*-Si:H/c-Si heterojunction on the built-in voltage and reduced charge carrier loss at the interface [28]. HET cells also have reduced costs compared with systems installed today based on conventional silicon technologies.

Moreover, in contrast to dielectric passivation materials such as SiO2 and amorphous Si nitride (a-SiNx:H) [29], a-Si:H is simultaneously suitable for good passivation and electrical conduction. However, the surface passivation quality worsens both at low and high processing temperatures because of the porous medium grown in the amorphous silicon phase at excess amounts of hydrogen [30] and growing crystalline Si film instead of forming a-Si:H [31, 32], respectively. As a consequence, discrepancy exists in the literature as to the passivation ability of a-Si:H [33–37].
