We are pleased to inform that a credit scheme to promote rooftop solar power generation systems has been established with preferential terms through financial assistance from Asian Development Bank (ADB). The credit line will be managed by the Ministry of Finance and Mass Media (MOFMM) and funds will be channeled to the beneficiaries through selected Participating Financial Institutions (PFI).
The foremost challenge in the crystalline silicon solar PV industry is to cheapen solar electricity. Manufacturers try to tackle this challenge by improving the efficiency and cutting down the module price.
One obvious approach to slash the cost of solar PV electricity is minimizing the number of light photons that escape from the cell without participating in the generation of electricity. We have already discussed two such strategies. Back reflector is another approach used to improve the efficiency by enhancing the internal reflectivity and promoting light trapping inside the cells.
The idea of back reflector is not new at all…
Crystalline silicon absorbs light and generates electron-hole pairs when light hits and shines on it. Electron-hole pairs then get separated leading to a photocurrent in a case of a short circuit and open-circuit voltage in a case of an open circuit. However, photons with energy less than 1.12 eV (bandgap of silicon) are not absorbed by the valance electrons as the valance electrons of silicon atoms need at least 1.12 eV of energy to jump on to its conduction band. As such, low-energy photons cannot generate electron-hole pairs. They just go through the lattice and escape.
What does it theoretically mean?
1.12 eV is equivalent to the energy of light photons whose wavelength is approximately 1100 nm. This implies that the photons with wavelengths longer than 1100 nm do not generate electron-hole pairs or a photocurrent in a cell. Further, it is worth noting that silicon is, of course, an excellent material for mass production of solar panels but it is not good at absorbing light or promoting its valance electrons to its conduction band since silicon is an indirect bandgap semiconductor material. Photon absorption and subsequent electron transition to the conduction band must, therefore, be assisted by phonons (discrete unit of vibrational energy in the lattice). Owing to this discouraging requirement, silicon atoms in the lattice miss a significant number of near-infrared light photons. These low-energy photons go through the cell and leave without getting absorbed. In particular, it has been observed that the penetration depth of the photons whose wavelengths range between 900-1100 nm is up to 3 mm whereas the typical thickness of silicon solar cells is about 180 µm! [1]
See the gap!
What does it imply?
The thickness of silicon wafers needs to be at least 16 times that of the wafers used in conventional solar cells in order to optimize the absorption of near-infrared light.
Inefficient light absorption has been one of the major barriers that hinder the development of efficient, low-cost solar PVs. In order to enhance the light absorption and efficiency, we need to use thick silicon wafers. Technology can endure the use of thick wafers but not the economy!
As we discussed in the previous articles, silicon wafers still account for about one half (51%) of the total cell price [2]. The thicker the wafer the greater the fabrication cost!
Two opposite requirements!
How to succeed in?
Even though fabrication cost falls with the decreasing wafer thickness, light absorption steadily drops with the decreasing wafer thickness. A wise way to address this complicated issue is to promote the light absorption capacity while keeping the wafer thickness at an affordable level.
Light absorption can be improved by employing some light management mechanisms that improve the internal reflection, minimize the rear side’s absorption, and enhance the light trapping capacity inside the cell. That is the idea of a back reflector. Light that is not absorbed by the cell reaches the back reflector and get reflected back. This increases the chance of unabsorbed photons getting absorbed by valance electrons leading to an increase in the photocurrent. Especially, when back reflector joins with the textured front surface, it forms an excellent light trapping scheme in which previously unabsorbed photons bounce back and forth between the back reflector and textured front surface [3]. Most of the unabsorbed photons bounce back and forth several times until they get absorbed by an electron. This notably extends the optical path length of the cell while keeping the geometric length of the cell the same. In other words, the light trapping schemes virtually widen the wafer thickness which is the most expensive component of a typical solar cell. An excellent model to lift the conversion efficiency whilst cutting the amount of crystalline silicon and the fabrication cost!
Screen printed silver and aluminium paste is still being widely used as the rear side metallization technique to create back contact in the solar PV industry. The screen-printed paste is then fired to obtain the back surface field passivation (BSF). The back contact adds structural strength, seals off rear side of the cell and act as a conductor. Not only that but also it acts as a back reflector and closely work with the textured front surface to trap light inside. But any technology has its own limits and restrictions. It is a universal rule. Al-BSF technology has also a technical limit in improving efficiency. The tireless effort to find more-efficient alternatives to the Al-BSF technology has given rise a wide range of new solar PV species with higher efficiencies. PERC is one of the best alternatives to Al-BSF technology. It has already been able to commercialize with an efficiency enhancement of about 1% thanks to its superior performance.
Today, BSF technology leads the solar PV market with the highest market share. However, its share would probably drop to about 50% by 2020 and would be less than 10% by 2028 as per recent estimations. This would be largely due to the increasing penetration of more-efficient new PV technologies such as PERC, PERL, and PERT into the PV market. [4] In addition, HIT and back contact solar PV technologies seem to be another efficient PV technologies competing in the market. All these technologies would finally, offer increasingly improved efficiency while cutting down the wafer thickness and the module price making the solar PV the most affordable energy technology.
Reference
[1] Eisenlohr, J., Benick, J., Peters, M., Bläsi, B., Goldschmidt, J. C., & Hermle, M. (2014). Hexagonal sphere gratings for enhanced light trapping in crystalline silicon solar cells. Optics express, 22 (101), A111-A119.
[2] Solanki, C. S., & Singh, H. K. (2017). Anti-reflection and Light Trapping in c-Si Solar Cells. Springer.
[3] Blakers, A. W., Wang, A., Milne, A. M., Zhao, J., & Green, M. A. (1989). 22.8% efficient silicon solar cell. Applied Physics Letters, 55 (13), 1363-1365.
[4] Fischer, M., Cells, H, Q. (2018). Trends & challenges in c-Si PV-an update of the ITRPV 9th edition. World solar congress-Shanghai.
As we realized in the previous article, crystalline silicon is not good at absorbing light efficiently. It is highly reflective and thus it losses a considerable portion of incident sunlight. In order for us to minimize the reflective loss and thus to raise the light absorption and efficiency of the cell, we need to tone down the reflectivity as much as possible. Use of anti-reflective coatings is an effective strategy but is not sufficient enough to make silicon solar PVs more efficient. Surface texturing (Textured front-side) is another attractive approach used to reduce the reflective loss with or without an anti-reflective coating. It makes the surface rougher leaving anti-reflecting features on the surface. The shape of the features may be square pyramids, pillars, or cones [1].
It is observable that any rough surface reduces the reflective loss by redirecting a great deal of reflected light back towards the cell with the help of anti-reflecting features on the surface. Unlike in a smooth surface, a textured surface, therefore, exhibits excellent light trapping properties and low reflective loss. Obviously, low reflective loss and improved light trapping capacity means high penetration of sunlight on to the solar cell. Also, increase in the path length of light inside the cell boosts the probability that the free electrons in the lattice capture light photons to jump on to the conduction band. As a consequence, both of these phenomena enhance the short-circuit photocurrent, and open-circuit-voltage of the cell leading to improved overall efficiency.
Various techniques such as photolithography, reactive ion etching, and laser texturing have been developed for surface texturing [2]. Today, alkaline texturing is widely being used for monocrystalline silicon solar cells. This etching process leaves randomly sized, distributed, square pyramid-shaped features as it is an anisotropic process. Since alkali surface texturing is anisotropic, it is not suitable for the texturing of polycrystalline silicon solar cells that consist of many randomly oriented grains. Therefore, acid-based isotropic wet etching techniques are used for texturing of polycrystalline silicon solar cells [3].
Texturing techniques have also been evolving with solar photovoltaic technology. While 30% of incoming solar radiation is reflected into the surrounding air by the surface of a polished silicon wafer, it can be reduced to about 15% even without an anti-reflective coating [1]. Some texturing processes have been reported to bring down the reflectance to about 10% and even less than 5% for multicrystalline silicon solar PVs with the assistance of silver etchants.
Power of texturing is observable, proven and therefore, is unquestionable. While texturing itself is capable of reducing the reflectance significantly, it can be used along with anti-reflective coatings to further reduce the reflective loss leading to much efficient photovoltaics on your rooftop.
Reference
[1] Kelvii Wei, G. U. O. (2017). Surface texturing for silicon solar energy by wet acid. J Nanosci Adv Tech, 2(1), 24-29.
[2] Do, K. S., Kang, M. G., Park, J. J., Kang, G. H., Myoung, J. M., & Song, H. E. (2013). Surface Texturing of Crystalline Silicon Solar Cell Using Silicon Nanowires. Japanese Journal of Applied Physics, 52(9R), 092301.
[3] Battaglia, C., Cuevas, A., & De Wolf, S. (2016). High-efficiency crystalline silicon solar cells: status and perspectives. Energy & Environmental Science, 9(5), 1552-1576.
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A solar cell is a sustainable factory built to produce solar electricity. It relies on nothing but sunlight which is one and only raw material it needed to generate electricity!
Just like any other factory does, solar cells need more raw material in order to produce more. The greater the solar intensity received the greater the solar power they generate. Traditionally, three different strategies are employed to raise the number of light photons that participate in the generation of electricity: Anti-reflective coatings, textured front-sides, and metallic back reflectors. They are all straightforward concepts. But their individual contribution in the operation cannot be underestimated at all. Let us see: Why?
Bare crystalline silicon solar cells reflect a substantial portion of incoming sunlight since pure crystalline silicon is highly shimmering. This results in low photocurrent and low module efficiency. The degree of reflectance may vary with the wavelength. This reflective effect reduces the number of light photons going through the front side on to the cell. On average, as much as 30% of IR and 50% of the UV portion is lost due to the reflection [1].
Is it possible to reduce the reflective loss to improve the light absorption capacity and efficiency of photovoltaics?
Yes! This is how the idea of an anti-reflective coating was born.
Reducing the reflection means increasing the number of photons transmitting on to the cell and increasing the overall efficiency. This is done by applying an anti-reflective coating. While various systems such as SiO2, TiO2, Al2O3, ZnO, ZnS, and Si3N4 have been proven to be anti-reflective, SiO2, Si3N4, and TiO2 are the most common materials being used in the industry [1, 2, 3].
Without anti-reflective coatings, solar PVs would not be economically viable. Moreover, it has been demonstrated that the reflective loss can more efficiently be reduced by applying a double layer anti-reflective coating. An efficiency enhancement of 214% and 60% have been reported with a double layer anti-reflective coating (SiO2/ TiO2) and with a single layer SiO2 coating as compared to as-grown silicon solar cells [1]. As it can be seen, the role of an anti-reflective coating is crucial to manufacture crystalline silicon solar cells at a lower cost with higher efficiency.
In next articles, we will see the duty, and significance of a textured front-side, and metallic back reflector in improving the efficiency.
Reference
[1] Ali, K., Khan, S. A., & Jafri, M. M. (2014). Effect of double layer (SiO2/TiO2) anti-reflective coating on silicon solar cells. Int. J. Electrochem. Sci, 9, 7865-7874.
[2] Al-Turk, S. (2011). Analytic Optimization Modeling of Anti-Reflection Coatings for Solar Cells (Doctoral dissertation).
[3] Du, Q. G., Alagappan, G., Dai, H., Demir, H. V., Yu, H. Y., Sun, X. W., & Kam, C. H. (2012). UV-blocking ZnO nanostructure anti-reflective coatings. Optics Communications, 285(13-14), 3238-3241.
As its name implies, polycrystalline silicon (multicrystalline silicon/ polysilicon) is made up of many small single crystals and is a form of silicon with high purity. So, polycrystalline silicon contains so many grains and also grain boundaries.
Polycrystalline ingots are grown in quartz crucibles which is then cut into square-shaped polycrystalline silicon substrates. Note that the manufacturing cost of solar cell modules includes the cost of silicon substrate (50%), module processing (30%) and cell processing (20%) [1]. As we can see it, the latter costs the least whilst the former costs as much as one half of the total cost. Therefore, the market price is largely determined by the cost of polycrystalline feedstock. This is why reducing the cost of silicon substrate remains one of the biggest changes in the PV industry. Any attempt to reduce the cost of silicon substrates is, in turn, an attempt to reduce the overall cost of solar PV modules and is an effort of expediting the growing share of solar electricity in the energy sector.
The cost of crystalline solar cells is constantly falling.
Anyway…
It is worth knowing that both single crystalline and polycrystalline solar PV technologies are already proven, economically viable means of converting solar radiation into electricity.
High purity silicon is too bad at conducting electricity as it is not a conductor but a semiconductor while the material needed to fabricate solar PVs must show some crucial properties like electrical conductivity. Simply, high purity crystalline is not a good candidate to be used as a light-harvesting semiconductor material in the solar PV industry.
Purity is good. But not here!
But… we are still not unlucky!
Even though high purity crystalline silicon is not originally conducting, we can make it conducting by introducing an insignificant amount of impurities like boron and phosphorus to the high purity crystalline silicon framework. This process is called doping by which extrinsic or doped silicon is formed. Although the amount of impurity is extremely negligible, it impressively improves the electronic, and optical properties needed for the semiconductor industry and plays a vital role in crystalline silicon solar cells.
There are two types of doping: p-type doing & n-type doping. P-type doping is done by introducing dopants with 3 valance electrons such as boron, aluminium and indium. Once they have been integrated into the lattice structure, p-type dopants catch an additional electron creating a hole (lack of an electron) in the valance band of silicon atoms. This increases the hole-concentration and in turn, the conductivity as well since the creation of holes makes the electrons in the valance band mobile. n-type doping is exactly the opposite of p-type doping. It is done by introducing elements with five valance electrons (phosphorous, arsenic, antimony) into the lattice. In this case, each dopant atom contains one more valance electrons than the silicon atoms. Four valance electrons of the dopant atoms combine with four electrons of the outer shell of silicon atoms. The additional electron of the dopant is loosely bounded to the positively charged nuclei and thus free to move and acts as charge carriers. Especially, they require much less amount of photon energy in order to promote to the conduction band (compared to the energy required by intrinsic silicon). This greatly enhances the light-absorbing capacity of the material and thus the efficiency when doped semiconducting material is used to manufacture solar cells.
Reference
[1] Dalapati, G. K., Masudy-Panah, S., Kumar, A., Tan, C. C., Tan, H. R., & Chi, D. (2015). Aluminium alloyed iron-silicide/silicon solar cells: A simple approach for low cost environmental-friendly photovoltaic technology. Scientific reports, 5, 17810.
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As we discussed in the previous article, single crystalline & polycrystalline silicon solar cells are made of single crystalline and polycrystalline silicon, respectively. Let us briefly discuss now how they are produced.
The crystalline silicon production process is an intermediate state between a small bear (sand/ quartz) and solar electricity.
Silicon which is the key raw material in crystalline silicon solar PV industry is extensively available in the form of silica (silicon dioxide) on and in the earth’s crust. But both monocrystalline and polycrystalline silicon solar cells require ultra-pure silicon which is not available in nature. So, silicon must be extracted from silica and then needs to be purified up to solar grade silicon.
Single crystalline silicon wafer:
Silicon is extracted from sand or quartz by heating with a reducing agent which is usually high purity coke or coal. In this process, an excessive amount of silica is used to reduce the formation of silicon carbide. During the process, oxygen combines with carbon to produce carbon dioxide and escape from the furnace leaving molten silicon. Extracted silicon in this reaction is, about 99% pure which is, however, not pure enough for the fabrication of solar cells. It needs to be purified further to obtain solar grade silicon. Today, chemical purification techniques are widely used for the purification process.
The next step is to obtain cylindrical ingots of high purity silicon. While several methods are available to grow single crystalline silicon, Czochralski process is widely used due to its intrinsic advantages over other techniques. First, ingots are obtained through the Czochralski method and then are sliced into silicon wafers using a diamond saw. The good fact about this process is that it is more advantageous than other techniques. The bad fact is that this process leaves a great deal of waste silicon. The amount of waste silicon depends on the shape of the wafer being produced. Approximately one-half of silicon is wasted if the wafer is going to be circular-shaped. But it becomes notably high if the ingot is cut into the rectangular shape or hexagonal shape. This wastage is overwhelming since the process of making silicon ingots consumes a huge amount of energy thereby increasing the overall cost of the final product. This is one of the strongest stumbling blocks which hinders fabrication of low-cost s-Si solar cells.
Anyway, as its name implies, single crystalline silicon is made of a single crystal. The entire sample is made of an unbroken, high purity single crystal of silicon. Hence, single crystalline silicon contains no grains or grain boundaries.
s-Si solar cells are more efficient than pc-Si solar cells
Recombination mechanisms which reduce the photocurrent and thus the efficiency of a solar cell occur more frequently at grain boundaries. Also, electrons experience less freedom to move when there are many grains. Since s-Si contains neither grains nor grain boundaries s-Si solar cells are more efficient than their pc-Si counterparts.
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Nobody will have a comfortable life in an oven-like world.
Nobody is willing to live in a sandy desert.
Nobody would like to die in a heat wave.
Nobody would be healthy in a severely contaminated world.
As we discussed in our very first articles, anthropogenic climate change which fueled by the extensive use of fossil fuels is creating an environmental catastrophe which is now turning to be life-threatening to our mother Earth.
We are now in the vicinity of a horrendous tragedy.
We just started reaching it at a snail’s pace with the industrial revolution. Blindly, we are now making an all-out effort to reach it in the name of development. Intensified cyclones, sea level rise, desertification, frequent floods, droughts, and many other extreme weather conditions signpost a devastating disaster. All these terrible disaster-carriers are asking us to take appropriate actions.
If we really want to decelerate the deadly change in the climate system and alleviate its evil effects, we must shift to potential alternative energy technologies ASAP.
Solar energy is the best thirst-quencher to slake our excessive thirst for energy.
Naturally, no green technology would be attracted by the general public unless it is affordable, efficient, stable, durable, and environmentally friendly. Therefore, unattractive green technology would not have a far-reaching impact on the environment or economy.
Crystalline silicon solar PV is such amazing technology that carries all these salient characteristics and therefore, it has become the fastest-growing technology used to harvest energy from the tireless Sun’s radiation. They are increasingly affordable and soundly efficient at converting sunlight into electricity and, what’s more, they reach us with a limited performance warranty of about 25 years plus with a limited product warranty of about 10 years.
So,
Being a proven and potential technology, crystalline silicon solar PV is our plan A to replace fossil fuels and solve the maturing energy crisis/ climate change. There is currently no plan B…
And nobody knows if there will be…
So…
It is time to discuss the crystalline silicon PV technology in details. First, we will briefly discuss two most prominent crystalline silicon solar PV technologies. Then we will discuss the pros and cons of both technologies and finally, we will move on to discuss how to decide the best technology that fits one’s specific requirements.
Appearance:
The big brother in the solar PV industry, single crystalline silicon solar cells can easily be distinguished from polycrystalline silicon solar cells. They are moderately circular-shaped whereas polycrystalline silicon solar cells are rectangular-shaped. Also, they can easily be distinguished by the color. Single crystalline silicon solar cells appear black in color due to the way sunlight interacts with the single crystalline silicon layer. Polycrystalline silicon solar cells, on the other hand, are blue in color. This is largely due to the effect of anti-reflecting coating which used to improve the light absorption and light-to-electricity conversion efficiency.
Key raw material used:
Crystalline silicon is the key raw material in both technologies. However, as their names imply, single crystalline and polycrystalline silicon solar cells are made of single crystalline silicon (s-Si) and polycrystalline silicon (pc-Si), respectively.
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