**2. Crystalline silicon solar cells**

As mentioned above, c-Si is dominating the PV industry with a market share of 95%. In c-Si, thin film technologies are commonly applied to fabricate anti-reflection coatings (to reduce surface reflection loss) and passivation layers (to reduce carrier or surface recombination) during c-Si solar cell manufacturing. **Figure 1** shows schematics and fabrication flow steps for mainstream c-Si solar cells: a. Al back-surface field (Al-BSF) solar cell; b. passivated emitter and rear solar cell (PERC); c. n-type solar cell with a tunnel oxide passivating contact (TOPCon); d. silicon heterojunction solar cell (SHJ) contacted on both sides with intrinsic and doped bilayers at front and rear, respectively, and indium tin oxide (ITO). It can be seen that the deposition of ARC/passivation layers is a key step among all four crystalline silicon solar cell configurations.

The technique mainly used for these ARC/passivation layers is CVD, in particular, PECVD, LPCVD and ALD. PVD-sputtering is the main technique that is used for ITO layers in SHJ solar cells. The distinguishing feature between PVD and CVD is the states of vapor. In PVD, the vapor is made up of atoms and molecules that simply condense on the substrate; in CVD, the vapor undergoes a chemical reaction on the substrate which results in a thin film [3]. Films produced by CVD generally have better quality in terms of very high purity and density and better coverage on rough surfaces than those produced by PVD methods, although the process usually involves toxic and/or corrosive gases [7].

*Thin Film Deposition Technologies and Application in Photovoltaics DOI: http://dx.doi.org/10.5772/intechopen.108026*

#### **Figure 1.**

*Schematics and fabrication flow for a. Al back-surface field (Al-BSF) solar cell; b. localized rear contacts in the passivated emitter and rear cell (PERC); c. n-type solar cell with a tunnel oxide passivating contact (TOPCon); d. silicon heterojunction solar cell (SHJ) contacted on both sides with intrinsic and doped bilayers (i/n and i/p at front and rear, respectively) and indium tin oxide (ITO) [4–6].*

#### **2.1 PECVD**

Plasma-enhanced chemical vapor deposition (PECVD) is one of the most commonly used methods to deposit thin films in c-Si solar cell manufacturing. The current fabrication process involves PECVD deposited silicon nitride (SiNx) used as a front side anti-reflection coating being applied to Al-BSF, PERC and TOPCon solar cells. PECVD SiNx:H stacked with PECVD or atomic layer deposition (ALD) deposited AlOx provides excellent passivation and is involved as a standard step in the PERC and TOPCon fabrication processes [8, 9]. Amorphous silicon (a-Si) is another thin film material with excellent passivation property that is commonly deposited by PECVD [10]. **Figure 2** shows the schematics of a PECVD reactor. The PECVD deposition

**Figure 2.** *Schematic of a PECVD reactor.*

process normally takes place at a temperature range of 150–400°C and uses RF or DCgenerated plasma to initiate the reactant gases into reaction.

#### **2.2 LPCVD**

Compared with PECVD, low-pressure chemical vapor deposition (LPCVD) is another CVD thin film deposition technique but with a higher deposition temperature, lower deposition pressure and typically lower deposition rate. It uses heat to initiate a reaction of a precursor gas on the solid substrate. This reaction at the surface forms a solid phase material. The reactor is kept at low pressure to suppress any unwanted gas phase reactions, which also increases the uniformity. The temperatures can range from 400 to 900°C depending on the process and the reactive gases being used. **Figure 3** shows a schematic of an LPCVD reactor. LPCVD deposited films are typically more uniform, have fewer defects and exhibit better step coverage than films produced by PECVD.

LPCVD dominates in producing poly-Si for TOPcon solar cells in the PV industry. However, the technical difficulties of LPCVD are also required to be overcome to reduce the cost, in terms of higher deposition temperature, lower deposition rate and wrap-around deposition issues which increase the energy consumption for production [5, 11].

Basic CVD (PECVD and LPCVD) chemical reactions with silane gas (SiH4) are shown below:

Silicon nitride (SiNx):

$$\text{SiH}\_4 + \text{NH}\_x \rightarrow \text{SiN}\_x \ (+\text{H}\_2).$$

$$\text{Or } \text{SiH}\_4 + \text{N} \rightarrow \text{SiN}\_x \ (+\text{H}\_2).$$

Silicon oxide (SiOx):

$$\text{SiH}\_4 + \text{N}\_2\text{O} \rightarrow \text{SiO}\_x \,(+\text{H}\_2 + \text{N}\_2).$$

Silicon oxynitride (SiONx):

$$\text{SiH}\_4 + \text{N}\_2\text{O} + \text{NH}\_3 \rightarrow \text{SiON}\_x \,(+\text{H}\_2 + \text{N}\_2).$$

**Figure 3.** *Schematic of a PECVD reactor.*

*Thin Film Deposition Technologies and Application in Photovoltaics DOI: http://dx.doi.org/10.5772/intechopen.108026*

Hydrogenated amorphous silicon (a-Si:H):

$$\text{SiH}\_4 \rightarrow \text{Si} \ (+\text{H}\_2) .$$

Silicon carbide (SiCx):

$$\text{SiH}\_4 + \text{CH}\_x \rightarrow \text{SiC}\_x \ (+\text{H}\_2).$$

#### **2.3 ALD**

ALD is a widely used deposition technique in the field of solar cells, energy storage, catalysis and semiconductor technology. ALD deposition for AlOx has been successfully employed by the PV industry for its excellent film quality and process economics [12–14].

In ALD, thin films are built up in cycles, in which the surface is exposed to various vapor or gas-phase species in alternating, separated doses. One cycle typically involves using the precursor dose as the first half-cycle and the co-reactant as the second halfcycle. In each cycle, a sub-monolayer of material is deposited. The precursor molecules and co-reactants react neither with themselves nor with the surface groups created. The products generated during the gaseous reactions, as well as any unreacted precursor or co-reactant molecules, are removed from the ALD reactor in the purge and/or pump steps. This is necessary to avoid reactions between the precursor and coreactant molecules directly in the gas phase or on the surface, as this could lead to an undesired CVD component. The various steps in a typical ideal ALD cycle are shown in **Figure 4**. As shown, a typical cycle consists of four steps: (i) a precursor dosing step; (ii) a purge and/or pump step; (iii) a co-reactant step, where a small molecule is typically involved, such as water vapor; and (iv) a purge and/or pump step. This figure shows a schematic illustration of the self-limiting surface reactions during the two half-cycles, as well as the saturation of the surface coverage in every step of the cycle. The saturation of both half-cycles leads to a characteristic amount of growth per cycle (GRC) [15].

**Figure 4.** *Illustrations of the ALD cycle process.*
