Crystalline silicon (c-Si) is an umbrella term for the crystalline forms of silicon encompassing multicrystalline silicon (multi-Si) and monocrystalline silicon (mono-Si), the two dominant semiconducting materials used in photovoltaic technology for the production of solar cells, that are assembled into a solar panel and part of a photovoltaic system to generate solar power from sunlight.
In electronics, the term crystalline silicon typically refers to monocrystalline form silicon, as the sole material used for producing microchips, containing much lower impurity levels than those required for solar cells. Production of semiconductor grade silicon involves a chemical purification to produce hyperpure polysilicon followed by a recrystallization process to grow monocrystalline silicon. The cylindrical boules are then cut into wafers for further processing.
Solar cells made of crystalline silicon are often called conventional, traditional, or first generation solar cells, as they were developed in the 1950s and remained the most common type up to the present time. Because they are produced from 160–190 µm thick solar wafers—slices from bulks of solar grade silicon—they are sometimes called wafer-based solar cells.
Solar cells made from c-Si are single-junction cells and are generally more efficient than their rival technologies, which are the second-generation thin film solar cells, the most important being CdTe, CIGS, and amorphous silicon (a-Si). Amorphous silicon is an allotropic variant of silicon, and amorphous means "without shape" to describe its non-crystalline form.:29
- 1 Overview
- 2 Mono-silicon
- 3 Multi-silicon
- 4 Not classified as crystalline silicon
- 5 Transformation of amorphous into crystalline silicon
- 6 See also
- 7 References
The allotropic forms of silicon range from a single crystalline structure to a completely unordered amorphous structure with several intermediate steps in between. In addition, each of these different forms can possess several names and even more abbreviations, and often cause confusion to non-experts, especially as some materials and their application as a PV technology are of minor significance, while other materials are of outstanding importance.
The photovoltaic industry, however, groups them into two distinct categories:
- Crystalline silicon (c-Si), used in traditional, conventional, wafer-based solar cells:
- Not classified as crystalline silicon, used in thin-film and other solar cell technologies:
- Amorphous silicon (a-Si)
- Nanocrystalline silicon (nc-Si)
- Protocrystalline silicon (pc-Si)
- Other non-silicon materials, such as CdTe, CIGS
- Emerging photovoltaics
- Multi-junction solar cells (MJ) commonly used for solar panels on spacecrafts for space-based solar power. They are also used in concentrator photovoltaics (CPV, HCPV), an emerging technology best suited for locations that receive much sunlight.
Alternatively, different types of solar cells and/or their semiconducting materials can be classified by generations:
- First generation solar cells are made of crystalline silicon, also called, conventional, traditional, wafer-based solar cells and include monocrystalline (mono-Si) and polycrystalline (multi-Si) semiconducting materials.
- Second generation solar cells or panels are based on thin-film technology and are of commercially significant importance. These include CdTe, CIGS and amorphous silicon.
- Third generation solar cells are often labeled as emerging technologies with little or no market significance and include a large range of substances, mostly organic, often using organometallic compounds.
Comparison of technical specifications
|Categories||Technology||η (%)||VOC (V)||ISC (A)||W/m²||t (µm)|
|Crystalline silicon cells||mono-Si||24.7||0.5||0.8||63||100|
|Thin film solar cells
In 2013, conventional crystalline silicon technology dominated worldwide PV production, with multi-Si leading the market ahead of mono-Si, accounting for 54 percent and 36 percent, respectively. For the last ten years, worldwide market-share of thin-film technologies stagnated below 18 percent and currently stand at 9 percent. In the thin-film market, CdTe leads with an annual production of 2 GWp or 5 percent, followed by a-Si and CIGS, both around 2 percent.:4,18 Alltime deployed PV capacity of 139 gigawatts (cumulative as of 2013) splits up into 121 GW crystalline silicon (87%) and 18 GW thin-film (13%) technology.:41
The conversion efficiency of PV devices describes the energy-ratio of the outgoing electrical power compared to the incoming radiated light. A single solar cells has generally a better, or higher efficiency than an entire solar module. Also lab efficiency is always significantly ahead of commercially available products in the market.
- Lab cells
In 2013, record lab cell efficiency was highest for crystalline silicon. However, multi-silicon is followed closely by Cadmium Telluride and Copper indium gallium selenide solar cells
- 25.0% – mono-Si cell
- 20.4% – multi-Si cell
- 19.8% – CIGS cell
- 19.6% – CdTe cell
These are all single-junction solar cells. For high concentration, multi-junction cells the record currently stands at 44.7 percent.:6
The average commercial crystalline silicon module increased its efficiency from about 12 to 16 percent over the last ten years. In the same period CdTe-modules improved their efficiency from 9 to 13 percent. Currently the best-performing modules under lab conditions are made of monocrystalline silicon. They are 7 percentage points above the efficeincy of currently produced modules (23% vs 16%) and indicate that the conventional silicon technology still has potential to improve and therefor maintain its leading position.:6
Best lab module efficiencies for multi-junction modules with concentrator technology reached 36.7 percent.:6
Energy payback time
The energy payback time (EPBT) describes the time span a PV system needs to operate in order to generate the same amount of energy that was used for its manufacture and installation. This energy amortization, given in years, is also referred to as break-even energy payback time. The EPBT depends vastly on the location where the PV system is installed (e.g. the amount of sunlight available) and on the efficiency of the system, namely the type of PV technology and the system's components.
In life-cycle analysis (LCA) from the 1990s, the energy payback time had often been cited to be as high as 10 years. Although the time span already decreased to less than 3 years in the early 2000s, the myth that "solar PV does not pay back the energy used to create it" seems to persists up to the present day.
The EPBT relates closely to the concepts of net energy gain (NEG) and energy returned on energy invested (EROI). They are both used in energy economics and refer to the difference between the energy expended to harvest an energy source and the amount of energy gained from that harvest. The NEG and EROI also take the operating lifetime of a PV system into account and an effective production life of 25 to 30 years is typically assumed, as many manufacturers now provide a 25-year warranty on their products. From these metrics, the Energy payback time can be derived by calculation.
|Global Solar Potential in kWh/m²/a|
|North-and Central Europe, Canada||3.3||2.1||2.4||1.7||1.1||–||1200 kWh|
|Southern Europe, USA, South America, India||1.8||1.2||1.3||0.9||0.7||0.8||1700 kWh|
|American Southwest, Australia, Africa, Middle East||1.5||<1.2||0.9||<0.9||<0.7||<0.8||1900 kWh|
|Source: Fraunhofer FHI, Energy Payback Time, presentation slides, and photovolatic report, p.30–32
Table: kWh/m²/a - kilowatt-hours per square metre per year, as Global Horizontal Irradiation
- EPBT improvements
The EPBT has always been longer for PV systems using crystalline silicon than thin-film technology. This is due to the fact, that silicon is produced by the reduction of high-grade quartz sand in electric furnaces. This carbo-thermic smelting process occurs at high temperatures of more than 1000 °C and is very energy intensive, using about 11 kilowatt-hours (kWh) per produced kilogram of silicon. However, the energy payback time has shortened significantly over the last years, as crystalline silicon cells became ever more efficient in converting sunlight, while the thickness of the wafer material was constantly reduced and therefore required less silicon for its manufacture. Within the last ten years, the amount of silicon used for solar cells declined from 16 to 6 grams per watt-peak. In the same period, the thickness of a c-Si wafer was reduced from 300 µm, or microns, to about 160–190 µm. Crystalline silicon wafers are nowadays only 40 percent as thick as they used to be in 1990, when they were around 400 µm.:29 The sawing techniques that slice crystalline silicon ingots into wafers have also improved by reducing the kerf loss and making it easier to recycle the silicon sawdust.
|Derate cell to module efficiency||8.5%||13.9%|
|Wafer thickness / layer thickness||190 µm||4.0 µm|
|Kerf loss||190 µm||–|
|Silver per cell||9.6 g/m2||–|
|Glass thickness||4.0 mm||3.5 mm|
|Operational lifetime||30 years||30 years|
|Source: IEA-PVPS, Life Cycle Assessment, March 2015 |
With the exception of amorphous silicon, most commercially established PV technologies use toxic heavy metals. CIGS often uses a CdS buffer layer, and the semiconductor material of CdTe-technology itself contains the toxic cadmium (Cd). In the case of crystalline silicon modules, the solder material, that joins together the copper strings of the cells, contains about 36 percent of lead (Pb). Moreover, the paste used for screen printing front and back contacts contains traces of Pb and sometimes Cd as well. It is estimated, that about 1,000 metric tonnes of Pb have been used for 100 gigawatts of c-Si solar modules. However, there is no fundamental need for lead in the solder alloy.
Monocrystalline silicon (mono c-Si) is a form in which the crystal structure is homogenous throughout the material; the orientation, lattice parameter, and electronic properties are constant throughout the material. Dopant atoms such as phosphorus and boron are often incorporated into the film to make the silicon n-type or p-type respectively. Monocrystalline silicon is fabricated in the form of silicon wafers, usually by the Czochralski Growth method, and can be quite expensive depending on the radial size of the desired single crystal wafer (around $200 for a 300 mm Si wafer). This monocrystalline material, while useful, is one of the chief expenses associated with producing photovoltaics where approximately 40% of the final price of the product is attributable to the cost of the starting silicon wafer used in cell fabrication.
Multicrystalline silicon (multi c-Si) is composed of many smaller silicon grains of varied crystallographic orientation, typically >1 mm in size. This material can be synthesized easily by allowing liquid silicon to cool using a seed crystal of the desired crystal structure. Additionally, other methods for forming smaller-grained polycrystalline silicon (poly-Si) exist such as high temperature chemical vapor deposition (CVD).
Not classified as crystalline silicon
These allotropic forms of silicon are not classified as crystalline silicon. They belong to the group of thin film solar cells.
Amorphous silicon (a-Si) has no long-range periodic order. The application of amorphous silicon to photovoltaics as a standalone material is somewhat limited by its inferior electronic properties. When paired with microcrystalline silicon in tandem and triple-junction solar cells, however, higher efficiency can be attained than with single-junction solar cells. This tandem assembly of solar cells allows one to obtain a thin-film material with a bandgap of around 1.12 eV (the same as single-crystal silicon) compared to the bandgap of amorphous silicon of 1.7-1.8 eV bandgap. Tandem solar cells are then attractive since they can be fabricated with a bandgap similar to single-crystal silicon but with the ease of amorphous silicon.
Nanocrystalline silicon (nc-Si), sometimes also known as microcrystalline silicon (μc-Si), is a form of porous silicon. It is an allotropic form of silicon with paracrystalline structure—is similar to amorphous silicon (a-Si), in that it has an amorphous phase. Where they differ, however, is that nc-Si has small grains of crystalline silicon within the amorphous phase. This is in contrast to polycrystalline silicon (poly-Si) which consists solely of crystalline silicon grains, separated by grain boundaries. The difference comes solely from the grain size of the crystalline grains. Most materials with grains in the micrometre range are actually fine-grained polysilicon, so nanocrystalline silicon is a better term. The term Nanocrystalline silicon refers to a range of materials around the transition region from amorphous to microcrystalline phase in the silicon thin film.
Protocrystalline silicon has a higher efficiency than amorphous silicon (a-Si) and it has also been shown to improve stability, but not eliminate it. A protocrystalline phase is a distinct phase occurring during crystal growth which evolves into a microcrystalline form.
Protocrystalline Si also has a relatively low absorption near the band gap owing to its more ordered crystalline structure. Thus, protocrystalline and amorphous silicon can be combined in a tandem solar cell where the top layer of thin protocrystalline silicon absorbs short-wavelength light whereas the longer wavelengths are absorbed by the underlying a-Si substrate.
Transformation of amorphous into crystalline silicon
Amorphous silicon can be transformed to crystalline silicon using well-understood and widely implemented high-temperature annealing processes. The typical method used in industry requires high-temperature compatible materials, such as special high temperature glass that is expensive to produce. However, there are many applications for which this is an inherently unattractive production method.
Low temperature induced crystallization
Flexible solar cells have been a topic of interest for less conspicuous-integrated power generation than solar power farms. These modules may be placed in areas where traditional cells would not be feasible, such as wrapped around a telephone pole or cell phone tower. In this application a photovoltaic material may be applied to a flexible substrate, often a polymer. Such substrates cannot survive the high temperatures experienced during traditional annealing. Instead, novel methods of crystallizing the silicon without disturbing the underlying substrate have been studied extensively. Aluminum-induced crystallization (AIC) and local laser crystallization are common in the literature, however not extensively used in industry.
In both of these methods, amorphous silicon is grown using traditional techniques such as plasma-enhanced chemical vapor deposition (PECVD). The crystallization methods diverge during post-deposition processing.
In aluminum-induced crystallization, a thin layer of aluminum (50 nm or less) is deposited by physical vapor deposition onto the surface of the amorphous silicon. This stack of material is then annealed at a relatively low temperature between 140 °C and 200 °C in a vacuum. The aluminum that diffuses into the amorphous silicon is believed to weaken the hydrogen bonds present, allowing crystal nucleation and growth. Experiments have shown that polycrystalline silicon with grains on the order of 0.2 – 0.3 μm can be produced at temperatures as low as 150 °C. The volume fraction of the film that is crystallized is dependent on the length of the annealing process.
Aluminum-induced crystallization produces polycrystalline silicon with suitable crystallographic and electronic properties that make it a candidate for producing polycrystalline thin films for photovoltaics. AIC can be used to generate crystalline silicon nanowires and other nano-scale structures.
Another method of achieving the same result is the use of a laser to heat the silicon locally without heating the underlying substrate beyond some upper temperature limit. An excimer laser or, alternatively, green lasers such as a frequency-doubled Nd:YAG laser is used to heat the amorphous silicon, supplying energy necessary to nucleate grain growth. The laser fluence must be carefully controlled in order to induce crystallization without causing widespread melting. Crystallization of the film occurs as a very small portion of the silicon film is melted and allowed to cool. Ideally, the laser should melt the silicon film through its entire thickness, but not damage the substrate. Toward this end, a layer of silicon dioxide is sometimes added to act as a thermal barrier. This allows the use of substrates that cannot be exposed to the high temperatures of standard annealing, polymers for instance. Polymer-backed solar cells are of interest for seamlessly integrated power production schemes that involve placing photovoltaics on everyday surfaces.
A third method for crystallizing amorphous silicon is the use of thermal plasma jet. This strategy is an attempt to alleviate some of the problems associated with laser processing – namely the small region of crystallization and the high cost of the process on a production scale. The plasma torch is a simple piece of equipment that is used to thermally anneal the amorphous silicon. Compared to the laser method, this technique is simpler and more cost effective.
Plasma torch annealing is attractive because the process parameters and equipment dimension can be changed easily to yield varying levels of performance. A high level of crystallization (~90%) can be obtained with this method. Disadvantages include difficulty achieving uniformity in the crystallization of the film. While this method is applied frequently to silicon on a glass substrate, processing temperatures may be too high for polymers.
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