Crystalline silicon or (c-Si) is the crystalline forms of silicon, either polycrystalline silicon (poly-Si, consisting of small crystals), or monocrystalline silicon (mono-Si, a continuous crystal). Crystalline silicon is the dominant semiconducting material used in photovoltaic technology for the production of solar cells. These cells are assembled into solar panels as part of a photovoltaic system to generate solar power from sunlight.
In electronics, crystalline silicon is typically the monocrystalline form of silicon, and is used for producing microchips. This silicon contains much lower impurity levels than those required for solar cells. Production of semiconductor grade silicon involves a chemical purification to produce hyper-pure 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 to 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.
Overview
Classification The allotropic forms of silicon range from a single crystalline structure to a completely unordered amorphous structure with several intermediate varieties. 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.
PV industry In photovoltaic industry, materials are commonly grouped into the following two categories: Crystalline silicon (c-Si), used in conventional wafer-based solar cells. Monocrystalline silicon (mono-Si) Polycrystalline silicon (multi-Si) Ribbon silicon (ribbon-Si), has currently no market Other materials, 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 established non-silicon materials, such as CdTe, CIGS Emerging photovoltaics Multi-junction solar cells (MJ) commonly used for solar panels on spacecraft 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.
Generations 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.
Arguably, multi-junction photovoltaic cells can be classified to neither of these generations. A typical triple junction semiconductor is made of InGaP/(In)GaAs/Ge.High-efficiency multi-junction solar cells
Comparison of technical specifications Categories Technology η (%) VOC (V) ISC (A) W/m2 t (μm) Thin-film solar cells a-Si 13.1 6.3 0.0089 33 1 CdTe 16.5 0.86 0.029 – 5 CIGS 20.5 – – –
Market share
In 2013, conventional crystalline silicon technology dominated worldwide PV production, with multi-Si leading the market ahead of mono-Si, accounting for 54% and 36%, respectively. For the last ten years, worldwide market-share of thin-film technologies stagnated below 18% and currently stand at 9%. In the thin-film market, CdTe leads with an annual production of 2 GWp or 5%, followed by a-Si and CIGS, both around 2%. 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.
Efficiency
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. Additionally, lab efficiency is always far superior to that of goods that are sold commercially.
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. 25.6% — mono-Si cell 20.4% — multi-Si cell 21.7% — CIGS cell 21.5% — CdTe cell
Both-sides-contacted silicon solar cells as of 2021: 26% and possibly above.
Modules
The average commercial crystalline silicon module increased its efficiency from about 12% to 16% over the last ten years. In the same period CdTe-modules improved their efficiency from 9 to 16%. The modules performing best under lab conditions in 2014 were made of monocrystalline silicon. They were 7% above the efficiency of commercially produced modules (23% over 16%) which indicated that the conventional silicon technology still had potential to improve and therefore maintain its leading position.
Energy costs of manufacture Crystalline silicon has a high cost in energy because silicon is produced by the reduction of high-grade quartz sand in an electric furnace. The electricity generated for this process may produce greenhouse gas emissions. This coke-fired smelting process occurs at high temperatures of more than 1,000 °C and is very energy intensive, using about 11 kilowatt-hours (kW⋅h) per kilogram of silicon.
The energy requirements of this process per unit of silicon metal produced may be relatively inelastic. However, significant reductions in energy cost per photovoltaic product have been achieved as silicon cells have become more efficient at converting sunlight, larger silicon ingots are cut with less waste into thinner wafers, silicon waste from manufacturing is recycled, and material costs have declined.
Toxicity
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 the copper strings of the cells, it contains about 36% 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.
Properties Crystalline silicon has become large in the semiconductor and solar spaces, primarily for its electronic properties, specifically its moderate valence band and high charge carrier mobility. But it is the mechanical properties of crystalline silicon wafers that have allowed this material to become scaled and commercialized, and mechanical and electronic properties go hand in hand. The ease of manufacturing crystalline silicon wafers are also largely owed to their mechanical properties.
The mechanical properties of pure, undoped monocrystalline silicon are included below. Due to its diamond cubic crystal structure, some of these properties are anisotropic depending on the crystallographic orientation along which they are measured, or depending upon the crystallographic plane along which the wafers are cut. +Mechanical Properties of Crystalline SiliconPropertyValueUnitMethodShear modulus64GPaVoigt-Reuss-Hill (VRH)Bulk modulus89GPaVRHYoung's modulus130GPaVRH along 168GPaVRH along 187GPaVRH along Poisson's ratio0.21noneUniversal anisotropy index0.23noneHardness13GPaBerkovich nanoindentation These mechanical properties are relevant both for the semiconductor industry, where silicon substrates are used as circuit supports, and for the PV industry, where silicon solar panels are often exposed to extreme weather conditions. In both cases, however, crystalline silicon is typically doped with certain other elements (Group 13 elements for more positive charge carriers, Group 15 elements for more negative charge carriers) to increase conductivity and shift the bandgap, which simultaneously modifies the mechanical properties. It has been shown that increasing dopant concentration can decrease the shear modulus, increase the bulk modulus, decrease the stiffness (elastic modulus), and shift the material from the brittle to the ductile regime. This results in overall increased toughness, which is useful for manufacturing, and increased elasticity and ductility.
In the PV industry, recent demand has risen for flexible solar cells that can be mounted on a variety of uneven or mobile surfaces and integrated into existing structures. While it is a relatively mature technology, crystalline silicon solar has maintained its lead as the most widespread and most efficient solar technology on the market, so researchers have long sought to adapt the technology to make it flexible and lightweight. This goal has remained elusive because it is a major challenge due to the inherent brittleness of crystalline silicon. In a new landmark study, researchers from China developed a method to create flexible crystalline silicon wafers. When crystalline silicon wafers are cut, the surface of these wafers are typically textured/micropatterned to increase the surface area and allow for better light absorption. This technique creates lots of sharp valleys at which cracks easily form when the wafer is bent, ultimately leading to brittle fractures. As a solution to this problem, the researchers blunted the tips at the edges of the wafer, eliminating the source of fractures and ultimately creating crystalline silicon solar cells that maintain their power conversion efficiency throughout bending cycles.
Cell technologies
PERC solar cell Passivated emitter rear contact (PERC) solar cells consist of the addition of an extra layer to the rear-side of a solar cell. This dielectric passive layer acts to reflect unabsorbed light back to the solar cell for a second absorption attempt increasing the solar cell efficiency.
A PERC is created through an additional film deposition and etching process. Etching can be done either by chemical or laser processing. About 80% of solar panels worldwide use the PERC design. Martin Green, Andrew Blakers, Jianhua Zhao and Aihua Wang won the Queen Elizabeth Prize for Engineering in 2023 for development of the PERC solar cell.
HIT solar cell
A HIT solar cell is composed of a mono thin crystalline silicon wafer surrounded by ultra-thin amorphous silicon layers. The acronym HIT stands for "heterojunction with intrinsic thin layer". HIT cells are produced by the Japanese multinational electronics corporation Panasonic (see also ). Panasonic and several other groups have reported several advantages of the HIT design over its traditional c-Si counterpart: An intrinsic a-Si layer can act as an effective surface passivation layer for c-Si wafer. The p+/n+ doped a-Si functions as an effective emitter/BSF for the cell.