**5.1 Emitter diffusion**

Emitter diffusion is one of the crucial thermal steps in the industrial solar cell fabrication. The n-type emitter of the crystalline p-type silicon solar cells is formed by phosphorus (P) diffusion. In the diffusion process, the Si wafers are sent in a furnace and exposed at 800–900°C to phosphoryl chloride (POCl3) and O2 which results in PSG deposition on the Si wafer surfaces. This step is called as pre-deposition, where the PSG [28] acts as a source of phosphorus (P) dopants to diffuse into the Si wafer. The next step is drive-in, where the supply of dopant gases is disconnected and P from the PSG layer diffuses further into the Si wafer. Hannes et al. [29] illustrates for the optimum process feasibility for photovoltaic applications, three different effects have to be considered. Firstly, the in-diffusion of P from the PSG and its presence in electrically active and inactive states in the Si wafer, which increases Shockley-Read-Hall (SRH) recombination. Secondly, the gettering of impurities into the Si layer towards the PSG layer. Finally, the metal contact formation with the P-doped Si emitter draws out the generated power.

The diffusion process is quantified by sheet resistance which depends on the depth of p-n junction and P concentration profile. The sheet resistance has units of Ω/cm (commonly measured as Ω/□) and is measured using a four-point probe system. The definition of sheet resistance is illustrated in Eq. (1).

$$R = \frac{\rho l}{A} = \frac{\rho l}{W \ast D} = \frac{\rho}{D} = \rho\_{sheet} \tag{1}$$

where *R* = resistance of a rectangular section (Ω); *ρ* = resistivity (Ω cm); *l* = length of the rectangular section (cm); *A* = area of the rectangular section (cm2 ); *W* = width of the rectangular section (cm); *D* = depth of the rectangular section (cm) and *ρsheet* = resistance for given depth (*D*) when l = W (Ω/□).

The earlier values of emitter sheet resistance were 30–60 Ω/□ with p-n junction depths of >400 nm and high P surface concentration. With improvements in the front-side silver (Ag) contacting paste, the emitter sheet resistance is now in the range of 90–110 Ω/□ with junction depth of around 300 nm and lower P surface concentration. Shifting to larger sheet-resistance allows to capture more light in the UV and blue spectrum, while also reducing the surface recombination to improve the Voc. It should be noted that the diffusion process occurs on the FS (directly exposed to the gases) and also on the edges and RS. If the edge isolation process is not carried out (as discussed in Section 4.3), the emitter will be short-circuited with the substrate.

**Figure 12** shows the POCl3 diffusion process in a closed quartz-tube system. POCl3 is a liquid source supplied to the process tube by bubbling it with a carrier gas N2. By mixing O2 with the POCl3, there will be an epitaxial growth of PSG layer as indicated in Eq. (2) [30].

$$\text{4POCl}\_3 + \text{3O}\_2 \rightarrow 2\text{P}\_2\text{O}\_5\\(\text{PSG}) + 6\text{Cl}\_2\tag{2}$$

At the Si surface, 2P2O5 is reduced to elemental phosphorus during the drive-in step as shown in Eq. (3) [30].

$$2\text{P}\_2\text{O}\_5 + 5\text{Si} \rightarrow 4\text{P} + 5\text{SiO}\_2\tag{3}$$

Chlorine which is a by-product during the pre-deposition cleans the wafers and quartz-tube by forming complexes with metals. PSG is used as source for driving in the P atoms into Si surface. During the drive-in process, POCl3 is switched off and only O2 is added to build up a thin oxide layer beneath the PSG to enhance the diffusion of P atoms into Si surface.

Inside the diffusion tube there are five heating zones as illustrated in **Figure 13**. The zones are:


Typically the temperatures of each heating zone are adjusted to obtain equal emitter sheet resistance for all wafers across the boat.

Environment of diffusion process should be very clean and hence quartz material is used for the tubes. Cleanliness of the tubes and loading-area maintenance also affects the process results. Since in gas-phase diffusion there is no residue in the tube, it results in a cleaner process. By half pitch loading in the low pressure (LP) conditions [31], the throughput can be increased. Commonly 1,000 wafers are loaded in a single tube and with five diffusion tubes in a batch-type diffusion system, a throughput of up to 3,800 wafers/h can be achieved for solar cell manufacturing.

#### **Figure 12.**

*(a) Schematic representation of the batch-type diffusion process and (b) representative image of a batch-type diffusion equipment. Source: centrotherm GmbH.*

**Figure 13.** *Heating zones inside the diffusion tube.*

#### *Industrial Silicon Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.84817*

An inline diffusion system where the wafers are transported on a belt with phosphoric acid as the source of P dopants was also used in commercial production [32]. However, compared to the inline process, the batch process is more clean, effective and efficient. For n-type solar cells or advanced solar cells concepts like PERT, the p-type batch diffusion is based on boron (B) dopant sources like boron tribromide (BBr3) [33, 34].

#### **5.2 Anti-reflective coating (ARC) deposition**

A bare Si surface reflects >30% of the light incident. As discussed in Section 4, the texturing process improves the light-capturing. It is desirable to reduce the reflectance further which is obtained by depositing an ARC layer. TiOx was one of the earliest material to be used as an ARC layer for solar cells, however since it could not provide adequate surface passivation it was eventually replaces by SiNx:H [37]. Thermally grown silicon oxide (SiO2) was also employed as the passivating material in the record breaking passivated emitter rear locally diffused (PERL) cells [37]. High thermal budget and long process time made SiO2-based passivation unsuitable for mass-production of solar cells [37]. A comprehensive review of various ARC and passivating material for solar cell applications is discussed in [37].

The plasma enhanced chemical vapour deposition (PECVD) process is suitable for depositing an ARC layer of SiNx:H which not only reduces the reflection but also passivates the front-side n-type emitter and the bulk thus improving the solar cell efficiency [36, 37]. A schematic of a batch-type PECVD system is shown in **Figure 14**. The wafers are loaded in a graphite boat with the front-sides facing each other. An RF plasma based on process gases ammonia (NH3) and silane (SiH4) operating at a temperature of 400–450°C deposit the hydrogenated SiNx:H layer as per Eq. (4) [35]. The hydrogen incorporated in the SiNx:H film diffuses into the bulk during the firing step (discussed in next section) and passivates the dangling bonds to improve the solar cell performance [36, 37].

$$\text{3SiH}\_4 + \text{2NH}\_3 + \text{N}\_2 \rightarrow \text{Si}\_3\text{N}\_4 + \text{9H}\_2\tag{4}$$

The refractive index (RI) of the SiNx:H film is controlled by the ratio of SiH4/ NH3 gas, while the thickness depends on the deposition duration. The SiNx:H-based ARC can minimize the reflection for a single wavelength and the wavelengththickness is given by [38],

$$t = \frac{\lambda\_0}{4n\_1} \tag{5}$$

where *t* = thickness of the SiNx:H ARC layer, *λ*0 = wavelength of incoming light and *n*1 = refractive index of the SiNx:H layer.

Based on the relationship, the ARC is also called as a 'quarter wavelength ARC'. For solar cells, the RI and thickness are selected to minimize the reflection at a wavelength of 600 nm as it is the peak of the solar spectrum. The thickness and RI of the ARC is selected to be the geometric mean of materials on either side, i.e., glass/air and Si. The typical thickness of the SiNx:H ARC is 80–85 nm with RI of 2.0–2.1 giving the solar cell a color of blue to violet blue. A representative image of textured multi-crystalline solar cell deposited with SiNx:H is shown in **Figure 15(a)**, while the variation of SiNx:H color based on its thickness is shown in **Figure 15(b)**. It is important to note that there is a dependence on the surface texture and ARC color for given deposition parameters. There is a variety of solar modules where the color of the solar cells is darker unlike the typical blue color. A typical ARC

**Figure 14.**

*(a) Schematic diagram of batch-type PECVD process for SiNx:H deposition and (b) graphite boat for loading Si wafers in the PECVD furnace.*

**Figure 15.**

*(a) Representative image of SiNx:H coated multi-crystalline solar cell, (b) variation of SiNx:H layer based on its thickness.*

deposition stage in a solar cell manufacturing line consists of two PECVD systems, each with four tubes and a throughput of up to 3,500 wafers/h.

SiNx:H is not suitable for passivating p-type Si and hence dielectrics like Al2O3 are used for RS passivation for cell architecture like PERC cells [8] or for p-type emitters in n-type solar cells. For PERC solar cells, the Al2O3 passivating layer is capped by a SiNx:H to protect it from the Al-paste during the firing process and also serve as an internal reflector for the long wavelength light. Commercial PECVD and atomic layer deposition (ALD)-based systems are available for depositing Al2O3 with throughput of up to 4,800 wafers/h [39].
