**3. Commercial silicon solar cell technologies**

Si is the second most abundant material on earth after oxygen and has been widely used in the semiconductor industry. Metallurgical grade silicon (Mg-Si) of 98% purity is obtained by heating quartz (SiO2) with carbon at high temperatures of 1,500-2,000 [4]. Mg-Si is further purified to obtain solar grade silicon chunks of 99.99% purity. The refined solar grade Si chunks are then processed further to obtain mono-crystalline and multi-crystalline forms of Si ingots, which are a large mass of silicon. In mono-crystalline Si, the atoms are arranged in the same crystal orientation throughout the material. For solar cells, (100) orientation is preferred as it can be easily textured to reduce the surface reflection [5]. Multi-crystalline Si, as the name suggest has multiple grains of Si material with different orientations, unlike the mono-crystalline substrates. Mono-crystalline material have higher minority carrier lifetime compared to multi-crystalline Si and hence higher solar cell efficiencies for a given solar cell technology.

The Czochralski (Cz) method for making mono-crystalline Si ingots is illustrated in **Figure 2(a)**. High purity molten silicon with dopant is maintained above the melting point and then a seed crystal is pulled at a very slow rate to obtain an ingot of as large as 300 mm in diameter and 2 m in length [6]. The molten silicon can be doped with either p-type or n-type dopants to obtain the specific type of mono-crystalline Si ingot of up to 200 kg [2]. Wafers sawn from the ingots have circular edges and hence the shape is called a 'psuedo square'. Multi-crystalline silicon ingots are made by melting high purity Si and crystallizing them in a large crucible by directional solidification process [7] as demonstrated in **Figure 2(b)**. The process does not have a reference crystal orientation like the Cz process and hence forms silicon material of different orientations. Currently the multi-crystalline Si ingots weigh >800 kg [2] which are then cut into bricks and wafers are sawn further. Current size of mono-crystalline and multi-crystalline wafers for solar cell fabrication is 6 inch × 6 inch. The area of the mono-crystalline wafers will be little less due to the pseudo-square shape. The most widely used base material for making solar cells is boron doped p-type Si substrates. N-type Si substrates for also used

for making high efficiency solar cells, but have additional technical challenges like obtaining uniform doping along the ingot compared to p-type substrates.

A broad classification of different types of solar cells along with efficiency ranges is shown in **Figure 3**. The standard aluminum back-surface field (Al-BSF) technology is one of the most common solar cell technology given its relatively simple manufacturing process. It is based on full rear-side (RS) Al deposition by screen-printing process and formation of a p+ BSF which helps repel the electrons from the rear-side of p-type substrate and improve the cell performance. The manufacturing flow for Al-BSF solar cells is shown in **Figure 4**. The standard design of commercial solar cells is with grid-pattern FS and full area RS contacts.

The passivated emitter rear contact (PERC) solar cell improves on the Al-BSF architecture by addition of rear-side passivation layer to improve rear-side passivation and internal reflection. Aluminum-oxide is a suitable material for RS passivation with average solar cell efficiencies nearing 21% obtained in production [8]. An existing

**Figure 2.**

*Illustration of (a) Cz process for mono-crystalline ingots and (b) directional solidification process for multicrystalline ingots.*

**Figure 3.** *Broad classification of different types of solar cell.*

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

**Figure 4.** *Manufacturing flow of Al-BSF solar cells.*

Al-BSF solar cell line can be upgraded to PERC process by two additional tools (RS passivation layer deposition and laser for localized contact opening on the RS).

The remaining three cell architectures are mainly higher efficiency technologies based on n-type Si substrates. The a-Si heterojunction solar cell has a-Si layers on the FS and RS of n-type Si substrate to form 'heterojunctions' unlike the conventional high temperature diffusion-based p-n junction. Such technology allows processing at lower temperatures, but is very sensitive to the quality of the surface interfaces. a-Sibased heterojunction solar cell was commercially manufactured by Sanyo Electric, which is now taken over by Panasonic [9]. In the interdigitated back contact (IBC) solar cell design, both contacts are present on the rear-side eliminating the FS contact shading losses. Typically for IBC solar cells, the junction will also be located on the rear-side. One of the early manufacturers of the high efficiency n-type IBC solar cell is SunPower Corporation [10]. Bifacial cells, as the name suggests can capture light from both sides of the solar cells. This entails that the rear-side also has a grid-pattern contacts to enable light collection. An example of the bifacial technology is the BiSON solar cell developed and commercialized by ISC, Konstanz [11]. It should be noted that the indicated classification is not an exhaustive list of various other types of solar cell architectures which are in R&D phase, close to commercialization or already being manufactured. The subsequent sections will give an over-view of the process steps for manufacturing of Al-BSF solar cells.
