The Journey of an Element that converts Solar energy into Electricity

Introduction

Solar panel technology is advancing rapidly with greater efficiency and lower prices resulting in a huge increase in demand. However, despite the massive advancements in technology, basic solar panel construction hasn’t changed much over the years. Most solar panels are still composed of a series of silicon crystalline cells sandwiched between a front glass plate and an aluminium frame supported by rear polymer plastic back-sheet. When installed, the solar panels experience extreme conditions during their 25+ year life. Extreme variations in temperature, humidity, wind and UV radiation will place a solar panel under tremendous stress. Fortunately, most panels are well built to withstand the harsh weather. The focus of this article will be on the procedure of manufacturing the solar cell.

Raw material source

1] Sand: First, silicon gets extracted from the sand, but not just any sand. Typically, the sand used, called silica sand or silicon dioxide, is made from crushed quartz rock. Then the silica sand must be extracted using a method called Carbon Arc Welding (CAW), which removes the unnecessary oxygen and results in pure silica of 99%. Then the silica is further refined to become as pure silicon as near to 100%. The end result is very pure polycrystalline silicon that can be doped into either P-type or N-type silicon with small amounts of either boron or phosphorous.

2] Glass: The front glass sheet protects the PV cells against weather and from hail or airborne debris impacts. The glass is typically high strength tempered glass that is 3.0 to 4.0 mm thick and is designed to withstand mechanical loads and extreme changes in temperature. The IEC minimum standard impact test requires solar panels to withstand an impact of 1-inch (25 mm) diameter hail stones that travel up to 60 mph (27m/s). Tempered glass is also much safer than standard glass in the event of an accident or severe impact, as it breaks into tiny fragments rather than sharp pointy pieces. Most manufacturers use high transmissive glass that has a very low iron content and an anti-reflective coating on the rear side to reduce losses and improve light transmission to improve efficiency and performance.

3] Aluminium frame: The aluminium frame plays a critical role in both protecting the edge of the laminating section that houses the cells and providing a solid structure for the positioning of the solar panel. The aluminium moulded sections are designed to be extremely lightweight, stiff and able to withstand extreme stress and high wind and external forces loading. The aluminium frame can be silver or anodised black and the corner sections can be either screwed, pressed or clamped together depending on the panel manufacturer, providing different levels of strength and rigidity.

4] EVA film: EVA stands for ‘Ethylene Vinyl Acetate’ which is a highly transparent (plastic) layer of specially designed polymer used to encapsulate the cells and hold them in position during manufacture. The EVA material must be extremely durable and extreme temperature and humidity tolerant, it plays an important role in long-term performance by preventing moisture and dirt from entering. The lamination of either side of the PV cells provides some shock absorption and helps to protect the cells from vibrations and sudden impact from hail stones and other objects by connecting wires. A high-quality EVA film with a high degree of what is known as ‘cross-linking’ can be the difference between a long life or a failure of the panel due to water ingress. The cells are first encapsulated with the EVA during manufacture, before being assembled inside the glass and back sheet.

5] Back sheet: The back sheet is the backmost layer of common solar cell that act as a moisture barrier and ultimate outer skin to provide both mechanical and electrical insulation protection. The back-sheet material consists of various polymers or plastics including PP, PET and PVF offering different levels of protection, thermal stability, and long-term UV resistance. Depending on the manufacturer and module, the back-sheet layer is typically white in colour but is also available as clear or black. Instead of a polymer back sheet, some panels such as bifacial and frameless panels use a rear glass panel. The rear side glass is more durable and longer lasting than most back sheet materials. Thus, some manufacturers on dual glass panels offer a 30-year performance warranty

Mending silicon into ingots

Ingot: An ingot is a material cast into a shape for ease of transport and processing. Typically, an ingot is rectangular, which makes it stackable. Most commonly ingots are associated with metals.

Growing an ingot is the first step towards the production of silicon wafers. Once the ingot is fully grown, it will then be sliced to its specification, and a series of additional steps will still be performed before the final product comes up. Each step must be done perfectly so that the quality of the ingot is not affected.

Silicon wafers from ingots

Growing an ingot is the first step towards the production of silicon wafers. Once the ingot is fully grown, it will then be sliced to its specification, and a series of additional steps will still be performed before the final product comes up. Each step must be done perfectly, so that the quality of the wafers is not affected. Here’s a process of growing silicon ingots.

1] Growing a silicon ingot: Depending on various factors, including specification, size and quality, growing a silicon ingot can take about a week to a month. Most single crystal silicon wafers are grown using the CZ (Czochralski) method, while the remainder are grown using the FZ (Float-Zone) method. The first procedure to grow a silicon ingot is to heat the silicon to 1420 ° C, which is above the silicon melting point. Once the mixture of crystals and doping has been dissolved, the single silicon crystal seed is placed on top of the melt and hardly touches the surface. Keep in mind that the seed in the accomplished ingot must be of the same crystal orientation. Also, the doping must be uniform. To achieve this, molten silicon seed crystal and container must rotate in opposite directions. Once it reaches the requisite crystal growth conditions, the seed crystal can be removed from the melt. The growth will then begin with a quick pulling of the seed crystal. This will reduce the number of crystal defects inside the seed while it is still at the start of silicon wafer production.

2] Slicing: Once the ingot is fully grown it is ground to a diameter of rough size that is slightly larger than the final silicon wafer’s target diameter. The ingot is cut into a notch or flat, to indicate its orientation. The ingot proceeds to slicing after passing several inspections. Because of the silicon’s hardness, a diamond edge saw carefully slices the silicon wafers, so they are slightly thicker than the target specification. The diamond edge saw also helps minimize damage to the wafers, variation in thickness and defects in the bow and warp.

3] Polishing: Most prime grade silicon wafers go through 2-3 polishing stages, using gradually finer polishing compounds. Most of the time, only front side wafers are polished, excluding 300 mm wafers that are double side polished. Mirror finish is produced by polishing. Also, the polish distinguishes which side to use in the fabrication of devices. This surface must be free from damage to topography, micro-cracks, scratches and residual work.

Texturing

There are mainly 4 types of texturing:

1. Alkaline texturing
2. Laser texturing
3. Plasma texturing
4. Metal-assisted texturing

1] Alkaline texturing:

To manufacture solar cells, the PV industry relies on polycrystalline and monocrystalline silicon wafers. Together they account for nearly 90% of all wafer substrates used in the industry. Alkaline etching cannot be used to texture polycrystalline silicon because of different grain orientations within the same wafer, as this would result in non-uniform texturing on the surface as different grain etching at different rates. Monocrystalline silicon wafers with orientation are the most common type of monocrystalline wafer in the industry since it can be easily textured using an alkaline etchant.

2] Laser texturing:

Laser texturing is a two-step process that facilitates isotropic texturing of a silicon surface irrespective of the orientation of the crystallography. Firstly, the ablate silicon from the bulk regions using a pulsed laser. In order to allow direct control of the space between pits, laser pulses can be modulated with the scan speed or stage motion. Secondly, the silicon is then chemically etched using a solution of sodium hydroxide (NaOH) to smooth the ablated pits and remove any slag that may have been deposited during laser processing. Although laser textured silicon can achieve lower reflectivity than alkaline textured silicon wafers, this process is oppressed by the adverse effect of lifetime defects formed from the laser process.

3] Plasma texturing:

Plasma texturing, sometimes called reactive ion etching, is a technique that uses plasma ion bombardment to etch the surface of the silicon. Plasma precursors commonly used include gas mixtures of sulphur hexafluoride, nitrous oxide, and chlorine. Other mixtures such as gas mixtures nitrous trifluoride and oxygen were also noted. Although the resulting reflectance on mc-Si can be significantly lower, the process is costly and lower in utilisation when compared to acidic texturing. In addition, reactive ion bombardment of the surface can cause damage to the structure of the underlying silicon bulk.

4] Metal-assisted texturing:

Metal-assisted etching was used to etch surfaces in silicon to form a series of small and deep pores. Typically, the resulting low reflectance of raw silicon makes the surface look black and therefore the textured silicon through this process is sometimes referred to as ‘black silicon.’ The low reflectance also means that optical purposes do not require an anti-reflection coating.

Emitter diffusion

During the POCI3 diffusion process, the density of inactive P in the emitter caused by precipitation from P is strongly influenced by process parameters. Process gases such as POCI3-N2 and O2 thus play an important role in the formation of the Phospho-Silicate Glass (PSG) layer, which itself controls the formation of emitters, especially during the pre-deposition phase. The correlations between process parameters and phosphorus precipitation on the (PSG/Si) interface were found through a systematic pre-deposition phase investigation. The correlations between the pre-deposition process parameters and the PSG characteristics also lead to more realistic boundary conditions for P diffusion used in process simulations. In this work a systematic investigation was carried out to adapt and optimize the emitter profile for a screen printing process of an industrial type.

Edge isolation

The process of edge isolation removes the phosphorous diffusion around the cell ‘s edge, so that the front emitter is electrically isolated from the rear cell. A common way of doing this is to stack the wafers on top of each other and then use CF4 and O2 to etch plasma. During diffusion the entire surface of the wafer, including the rear of the solar cell and edges, is exposed to the dopant source. This creates a current path from the front junction to the rear of the device in the case of a phosphorous diffusion, effectively shunting the solar cell since these recombining carriers do not contribute to the power output. Therefore, after diffusion, an edge insulation process is required to remove the unwanted diffusion around the edges of the solar cell and to isolate the front and rear surfaces electrically.

Anti-reflective coating (To avoid optical losses)

The Anti-Reflective Coating on a solar cell increases the amount of light that is absorbed into the cell. This anti-reflective coating is much needed since the reflection of a bare silicon solar cell exceeds 30%. Silicon nitrite or titanium oxide is used for the thin AR Coating. The colour of the solar cell can be changed by varying the thickness of the anti-reflection coating. Anti-reflective coatings on solar cells are quite similar to those used on other devices, such as camera lenses.

The reflection coefficient from bare silicon for light incident from air is given by,

Where,
n = Refractive index of the semiconductor
k = Extinction index of the semiconductor

Now, consider
λ = Wavelength of light (in vacuum)
α = Absorption coefficient

Hence, the equation becomes,

Without AR coating, the solar panels get affected by few optical losses as stated below:

1. Irradiance loss
2. Efficiency loss
3. Heat loss

Screen print

Screen printed solar cells are the finest established, most mature solar cell manufacturing technology which was first developed in the 1970s. Currently screen printed solar cells dominate the market for terrestrial photovoltaic modules. Relative simplicity is the main advantage of screen printing process. Screen printing simply reproduces the same print over and over again using stencil. Solar PV cells are typically metallized by screen printing. This involves applying three different types of metallization paste onto the c-Si cell. The first paste is the front side silver used on the side facing the sun; it creates the collector gridlines and silver bus bars, and the second is silver or silver-aluminium rear side tabbing, and the third is aluminium rear side paste that reacts with silicon to create the back-surface area.

Firing

The silver front contact pattern is printed directly over the antireflective silicon nitride (ARC) coating. Therefore, it is necessary to penetrate the silver pattern through the ARC coating to make an electrical contact with the silicon. Once the cell is fired in an inline firing furnace, the electrical contact is made. During the co-firing process also, the rear contact is made. The firing process involves a peak firing temperature for 5 seconds or less over a range of 750 to 870°C. During the process, the paste etches and penetrates the ARC coating through the layer, forming an ohmic contact with the underlying silicon. However, optimisation of the firing temperature and time is important.

Testing

Testing is essential for companies that want to remain players in this fiercely competitive market while offering the lowest prices, accomplishing the promised performance data and life expectancy. Solar cells can only survive the tremendous price drop in recent years by improving quality, cost-effective production and improved safety. Solar cells are daily exposed to ecological extremes. Rain, hail, storms and huge temperature fluctuations should not impair the product’s functionality. To achieve a desired life expectancy without any damage they must be able to withstand the poundage of snow, ice, dust and rain. These testing expectations are high for both electrical and mechanical characteristics.

Module encapsulation

Compact photovoltaic module is a layered structure consisting of interconnected and encapsulated solar cells embedded between substrate surface and superstrate (back and front sheets), glass or plastic sheets, to create compact and sealed units that are protected from environmental influences. Strings of solar cells are encapsulated between two sheets of encapsulant foil. Encapsulant is a polymeric material that also ensures adhesion between substratum sheet and superstrate glass and solar cell strings among them. Photovoltaic module is source of electricity and from the electrical point of view encapsulant insulates electrically active solar cells and interconnecting wires on one side and also protects against direct electric contact in order to avoid electric hazard.

Conclusion

In conclusion, Solar cell needs go from several steps and procedures, namely:
Steps:

1. From silica sand to silicon ingots
2. From silicon ingots to wafers
3. From silicon wafers to cells
4. Finally, connection (in series or parallel) of cells to become a solar panel

Procedures (to improve its efficiency):

1. Texturing
2. Emitter diffusion
3. Anti-reflection coating

Hence, it has go through all these mentioned above, before it becomes the component that converts solar energy into electricity.