Crystalline silicon
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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.[1][2] 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.[3]: 29
Overview
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Classification
[edit]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
[edit]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[3]: 17, 18
- 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
[edit]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.[5][6]
Comparison of technical specifications
[edit]Categories | Technology | η (%) | VOC (V) | ISC (A) | W/m2 | t (μm) |
---|---|---|---|---|---|---|
Thin-film solar cells |
a-Si | 11.1 | 6.3 | 0.0089 | 33 | 1 |
CdTe | 16.5 | 0.86 | 0.029 | – | 5 | |
CIGS | 20.5 | – | – | – |
Market share
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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%.[3]: 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.[3]: 41
Efficiency
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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.[7][8]
- 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.[3]: 6
Energy costs of manufacture
[edit]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 (kWh) per kilogram of silicon.[9]
The energy requirements of this process per unit of silicon metal produced may be relatively inelastic. But major energy cost reductions per (photovoltaic) product have been made as silicon cells have become more efficient at converting sunlight, larger silicon metal ingots are cut with less waste into thinner wafers, silicon waste from manufacture is recycled, and material costs have reduced.[3]: 29
Toxicity
[edit]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.[10]
Cell technologies
[edit]PERC solar cell
[edit]Passivated emitter rear contact (PERC) solar cells [11] 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.[12]
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.[13] 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.[14]
HIT solar cell
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A HIT solar cell is composed of a mono thin crystalline silicon wafer surrounded by ultra-thin amorphous silicon layers.[15] The acronym HIT stands for "heterojunction with intrinsic thin layer". HIT cells are produced by the Japanese multinational electronics corporation Panasonic (see also Sanyo § Solar cells and plants).[16] Panasonic and several other groups have reported several advantages of the HIT design over its traditional c-Si counterpart:
1. An intrinsic a-Si layer can act as an effective surface passivation layer for c-Si wafer.
2. The p+/n+ doped a-Si functions as an effective emitter/BSF for the cell.
3. The a-Si layers are deposited at much lower temperature, compared to the processing temperatures for traditional diffused c-Si technology.
4. The HIT cell has a lower temperature coefficient compared to c-Si cell technology.
Owing to all these advantages, this new hetero-junction solar cell is a considered to be a promising low cost alternative to traditional c-Si based solar cells.
Fabrication of HIT cells
[edit]The details of the fabrication sequence vary from group to group. Typically in good quality, CZ/FZ grown c-Si wafer (with ~1ms lifetimes) are used as the absorber layer of HIT cells. Using alkaline etchants, such as, NaOH or (CH3)4NOH the (100) surface of the wafer is textured to form the pyramids of 5-10μm height. Next, the wafer is cleaned using peroxide and HF solutions. This is followed by deposition of intrinsic a-Si passivation layer, typically through PECVD or Hot-wire CVD.[17][18] The silane (SiH4) gas diluted with H2 is used as a precursor. The deposition temperature and pressure is maintained at 200o C and 0.1-1 Torr. Precise control over this step is essential to avoid the formation of defective epitaxial Si.[19]
Cycles of deposition and annealing and H2 plasma treatment are shown to have provided excellent surface passivation.[20][21] Diborane or Trimethylboron gas mixed with SiH4 is used to deposit p-type a-Si layer, while, Phosphine gas mixed with SiH4 is used to deposit n-type a-Si layer. Direct deposition of doped a-Si layers on c-Si wafer is shown to have very poor passivation properties.[22] This is most likely due to dopant induced defect generation in a-Si layers.[23] Sputtered Indium Tin Oxide (ITO) is commonly used as a transparent conductive oxide (TCO) layer on top of the front and back a-Si layer in bi-facial design, as a-Si has high lateral resistance.
It is generally deposited on the back side as well fully metallized cell to avoid diffusion of back metal and also for impedance matching for the reflected light.[24] The silver/aluminum grid of 50-100μm thick is deposited through stencil printing for the front contact and back contact for bi-facial design. The detailed description of the fabrication process can be found in.[25]
Opto-electrical modeling and characterization of HIT cells
[edit]The literature discusses several studies to interpret carrier transport bottlenecks in these cells. Traditional light and dark I-V are extensively studied [26][27][28] and are observed to have several non-trivial features, which cannot be explained using the traditional solar cell diode theory.[29] This is because of the presence of hetero-junction between the intrinsic a-Si layer and c-Si wafer which introduces additional complexities to current flow.[26][30] In addition, there has been significant efforts to characterize this solar cell using C-V,[31][32] impedance spectroscopy,[31][33][34] surface photo-voltage,[35] suns-Voc[36][37] to produce complementary information.
Further, a number of design improvements, such as, the use of new emitters,[38] bifacial configuration, interdigitated back contact (IBC) configuration[39] bifacial-tandem configuration[40] are actively being pursued.
Mono-silicon
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