Solar PV Modules: Crystalline silicon (c-Si) vs Thin-Film
Choosing the right solar panels
The solar PV market is continually evolving, with new techologies bringing us solar panels with increased efficiency and higher wattage output – even while panels become smaller, thinner and lighter.
With so many variables, it can seem like a daunting and confusing prospect to choose the right panels. However, for practical reasons, what panel we choose tends to be based on the cost per watt, getting the best value for money that the current solar PV market can offer.
Solar panel colour
If the look and colour of your solar panels is important to you, you can choose all black panels, or the more familar blue tinted solar PV panels.
The guide below goes into more detail regarding the different solar panel technologies.
Crystalline silicon (c-Si) vs Thin Film solar panels
The majority of solar PV cells are made of silicon and classed as either monocrystalline or polycrystalline solar cell modules. There are also various thin film solar panel technologies.
Market prices update regularly, but as a comparison, a good quality multicrystalline solar panels has an average cost of £0.45 per watt, while the equivelant monocrystalline panels cost around £0.57 per watt *reflective of prices as of March 2017.
Solar panel market share: 63% Polycrystalline, 27% Monocrystalline, 10% Flat Film solar panels.
Monocrystalline Solar Panels
Monocrystalline solar cells have an efficiency of around 17-20%.
Monocrystalline are the most efficient type of solar panels because their cells are made of a single, cylindrical shaped crystalline silicon. This manufacturing process elimitates grain boundaries in the silicon crystal which would introduce discontinuities and detrimentally effect the electrical properties of the material.
Monocrystalline solar cells have an efficiency of around 17-20%. Monocrystalline cells are manufactured using the Czochralski process, which makes it more expensive to produce than polycrystalline or thin films solar panels.
The Czochralski Process
The process of making semiconductors and high-end monocrystalline solar panels makes high purity quartz such a necessity.
The Czochralski process is a way of growing a single crystal silicon ingot (the base for semiconductor chips and high efficiency solar panels) using molten polysilicon and a seed crystal to slowly grow a long ingot of single crystal silicon.
The Czochralski process requires the use of high purity quartz. Quartz has a high melting temperature (>2,000°C), and can withstand the temperatures needed to melt polysilicon (around 1,000°C). Quartz (SiO2) also shares the same Si element as polysilicon (Si), reducing the likelihood that the polysilicon becomes contaminated from a quartz crucible. Additionally, quartz is non-reactive so the crucible used in this process will not interfere with the chemical composition of the silicon ingot. This is crucial because even a tiny flaw in a wafer can interrupt the flow of electrons within the wafer.
Quartz in its raw form is non-reactive, however, mined quartz contains various metallic impurities, which when heated can travel from the quartz crucible to the silicon ingot, causing irregularities within the crystal. As such, very high purity quartz is required for the Czochralski process, especially within the semiconductor industry.
Monocrystalline solar cells have an even-colouring and uniform look, indicating high-purity silicon. They are made out of silicon ingots, which are cylindrical in shape. To optimize performance and lower costs, four sides are cut out of the cylindrical ingots to make silicon wafers.
A good way to differentiate between mono and polycrystalline solar panels is that polycrystalline solar cells look perfectly rectangular with no rounded edges.
Polycrystalline Solar Panels
Polycrystalline solar cells have an efficiency of around 15-17%.
Polycrystalline (aka Multicrystalline) solar cells are cast by melting polysilicon and pouring it directly into square low purity quartz crucibles. This is a much cheaper method of producing solar cells from polysilicon than using the Czochralski process (the method used to produce Monocrystalline panels), but creates many grain boundaries which causes the electrical properties of the material to be disrupted, lowering the average efficiency to around 15-17%.
Grain Boundaries in Multi-c-Si Cells
Polycrystalline solar cells are manufactured using a silicon crystal ‘seed’ placed in a vat of molten silicon. However, rather than carefully raising the silicon seed to form a large silicon crystal, the vat of silicon is allowed to cool. This process forms the visibile edges and grains in the Polycrystalline solar cell.
While Polycrystalline cells are less efficient than Monocrystalline solar cells, this difference has become less obvious, and the cheaper method of producing Polycrystalline cells has made them the preferred choice of solar panel for the majority of UK solar installations.
In November 2015 Trina Solar announced it had produced a Polycrystalline cell with efficiency of 21.25%, allowing them to produce polycrystalline modules with efficiencies between 18-20%.
The continual improvements in quality of multicrystalline wafers has helped pushed standard 60-cell polycrystalline panels from 240W to 260W in recent years.
Polycrystalline are now very close to Monocrystalline cells in terms of efficiency.
Thin Film Solar Panels
Depending on the technology, thin-film module prototypes have reached efficiencies between 7–13% and production modules operate at about 9%. Future thin film module efficiencies are expected rise to around 10–16%.
Thin film solar panels are the 2nd generation of solar cells, layering one or more thin films (TF) of photovoltaic material on a substrate such as glass, plastic or metal. They have a solid black appearance and may or may not have a frame. If the panel has no frame it is a thin film panel.
Thin film photovoltaic cells range in thickness from a few nanometers (nm) to tens of micrometers (µm) – much thinner than 1st generation crystalline silicon solar cells whose wafers can be up to 200 µm thick! Thin film solar cells are light and flexible, allowing them to be used to build semi-transparent, photovoltaic glazing material that can be laminated onto windows.
Until now, thin film solar panels have been less expensive than conventional crystalline silicon (c-Si) technology due to their lower efficiency. However, thin film performance has improved significantly, with efficiency for cadmium telluride (CdTe) and indium gallium diselenide (CIGS) solar cells now beyond 21% – outperforming the market dominant polycrystalline silicon cells currently used in the majority of solar PV systems.
Accelerated life tests conducted under laboratory conditions reveal a slightly faster degradation time for thin film modules compared to conventional polysilicon solar PV panels, but a lifetime of approximately 20 years should be expected.
Despite the progress of thin film panel technology, worldwid installed market share has never reached more than 20%, and by 2015 this had declined to around 7%. This trend is expected to change with the introduction of next generation thin film solar panels which will be incorporated into glass (bus stops, windows, tiles etc)
There are various photovoltaic materials which can be used to manufacture a thin film solar panel. The most common photovoltaic substances used are:
- Amorphous Silicon
Amorphous Silicon Solar Cells
Amorphous silicon (a-Si) is a non-crystalline allotropic form of silicon and is used as an alternative to conventional wafer crystalline silicon.
Amorphous thin film cells are usually fabricated using a technique called plasma-enhanced chemical vapor deposition which uses a gaseous mixture of silane (SiH4) and hydrogen to deposit at low temperatures, a very thin 1 micromettre (µm) layer of silicon on a variety of substrates, such as glass, plastic or metal, that has already been coated with a layer of transparent conducting oxide. Other methods used to deposit amorphous silicon on a substrate include sputtering and hot wire chemical vapor deposition techniques.
Amorphous silicon has a much lower electrical performance than crystalline silicon, but can be layered much thinner than crystalline silicon – requiring just 1% of the silicon needed for manufacturing a c-Si cell! This makes manufacturing much more cost effective. Amorphous silicon can also be deposited at a very low temperature (75°C), allowing it to be deposited onto a number of materials including glass, plastic and metal. Usually a-Si is used in devices that require very little power (pocket calculators etc) but recent improvements in technology have made them more attractive for large-area solar installation, mainly due to their thinness. The average efficiency of these cells is still ≤10%, however.Thin film amorphous silicon cells have been used since the late 1970s to power solar calculators.
While chalcogenide-based Cadmium Telluride (CdTe) and Copper Indium Selenide (CIS) thin films cells have been developed in the lab with great success, amorphous silicon cells are still an attractive option because they exhibit fewer problems than their more recent thin film counterparts, such as toxicity (because the source material is abundant and non-toxic) and humidity issues, and low manufacturing yields due to material complexity.
Excellent Low Light Performance
Amorphous silicon absorbs a very broad range of the light spectrum (due to its bandgap of 1.7 eV) including infrared and some ultraviolet light. They also perform very well during weak light, enabling amorphous silicon solar panels to generate power in the early morning, late afternoon, or on overcast days (contrary to crystalline silicon solar panels which are significantly less efficient in diffuse and indirect daylight).
However, the efficiency of an amorphous silicon cell suffer from the Staebler-Wronski effect (SWE) – which causes a significant drop of about 10 – 30% during the first six months of operation. This is because the defect density of hydrogenated amorphous silicon (a-Si:H) increases with light exposure, causing an increase in the recombination current, reducing the efficiency of the conversion of sunlight into electricity. Although this degradation is perfectly reversible upon annealing at or above 150 °C, conventional c-Si solar cells do not exhibit this effect in the first place.
- Cadmium Telluride (CdTe)
Cadmium Telluride Solar Cells
The efficiency of Cadmium Telluride cells is between 12-14%.
Cadmium telluride (CdTe) is the most popular thin film technology with over 50% of thin film solar PV panels using it today, and making up around 5% of the total solar panel market.
Cadmium telluride (CdTe) thin-film cells use cadmium telluride instead of silicon to absorb and convert sunlight into energy, and is the only thin film PV panel technology that currently competes with polysilicon panels.
The main problem with Cadmium telluride solar cells is that telluride is a very rare element, meaning that there is not enough supply to allow this technology to scale to an industrial output. In face, the rarity of tellurium (of which telluride is the anionic form) is comparable to that of platinum in the earth’s crust and contributes significantly to the cost of production. Another problem with the production of cadmium telluride cells is the toxicity of cadmium, which causes problems with waste disposal.
The efficiency of Cadmium telluride (CdTe) solar cell’s has increased significantly, making them on a par with CIGS thin film and multi crystalline silicon panels.
CdTe thin film solar panels also have the lowest energy payback time of all mass produced PV panels (time it takes for the panel to generate the same amount of energy as it took to manufacture the panel), and can be as short as eight months in favorable locations.
- Copper Indium Selenide (CIS) / Copper indium gallium selenide (CIGS)
Copper indium gallium selenide solar cells
Copper indium gallium selenide solar cells
A copper indium gallium selenide solar cell (CIGS) uses an absorber made of copper, indium, gallium, selenide to produce a semiconductor material. Gallium-free variants of the semiconductor material are abbreviated to CIS.
Copper indium gallium selenide solar cells have achieved efficiencies of above 20% under laboritory settings. CIGS solar PV panels had a 2% worldwide market share in 2013. Traditional methods of fabrication involve vacuum processes including co-evaporation and sputtering.
This material can be deposited on flexible substrate materials, producing highly flexible, lightweight solar panels. They are not very efficient, with an efficiency of <13%, but as of September 2014, current conversion efficiency record for a laboratory CIGS cell stands at 21.7%.
Its basic electronic structure is the p-i-n junction. The amorphous structure of a-Si implies high inherent disorder and dangling bonds, making it a bad conductor for charge carriers. These dangling bonds act as recombination centers that severely reduce carrier lifetime. A p-i-n structure is usually used, as opposed to an n-i-p structure. This is because the mobility of electrons in a-Si:H is roughly 1 or 2 orders of magnitude larger than that of holes, and thus the collection rate of electrons moving from the n- to p-type contact is better than holes moving from p- to n-type contact. Therefore, the p-type layer should be placed at the top where the light intensity is stronger, so that the majority of the charge carriers crossing the junction are electrons.
Tandem-cell using a-Si/μc-Si
A layer of amorphous silicon can be combined with layers of other allotropic forms of silicon to produce a multi-junction solar cell. When only two layers (two p-n junctions) are combined, it is called a tandem-cell. By stacking these layers on top of one other, a broader range of the light spectra is absorbed, improving the cell’s overall efficiency.
In micromorphous silicon, a layer of microcrystalline silicon (μc-Si) is combined with amorphous silicon, creating a tandem cell. The top a-Si layer absorbs the visible light, leaving the infrared part to the bottom μc-Si layer. The micromorph stacked-cell concept was pioneered and patented at the Institute of Microtechnology (IMT) of the Neuchâtel University in Switzerland, and is currently licensed to TEL Solar. A new world record PV module based on the micromorph concept with 12.24% module efficiency was independently certified in July 2014.*
Because all layers are made of silicon, they can be manufactured using PECVD. The band gap of a-Si is 1.7 eV and that of c-Si is 1.1 eV. The c-Si layer can absorb red and infrared light. The best efficiency can be achieved at transition between a-Si and c-Si. As nanocrystalline silicon (nc-Si) has about the same bandgap as c-Si, nc-Si can replace c-Si.
Tandem-cell using a-Si/pc-Si
Amorphous silicon can also be combined with protocrystalline silicon (pc-Si) into a tandem-cell. Protocrystalline silicon with a low volume fraction of nanocrystalline silicon is optimal for high open-circuit voltage. These types of silicon present dangling and twisted bonds, which results in deep defects (energy levels in the bandgap) as well as deformation of the valence and conduction bands (band tails).
Polycrystalline silicon on glass
A new attempt to fuse the advantages of bulk silicon with those of thin-film devices is thin film polycrystalline silicon on glass. These modules are produced by depositing an antireflection coating and doped silicon onto textured glass substrates using plasma-enhanced chemical vapor deposition (PECVD). The texture in the glass enhances the efficiency of the cell by approximately 3% by reducing the amount of incident light reflecting from the solar cell and trapping light inside the solar cell. The silicon film is crystallized by an annealing step, temperatures of 400-600 Celsius, resulting in polycrystalline silicon.
These new devices show energy conversion efficiencies of 8% and high manufacturing yields of >90%. Crystalline silicon on glass (CSG), where the polycrystalline silicon is 1-2 micrometres, is noted for its stability and durability; the use of thin film techniques also contributes to a cost savings over bulk photovoltaics. These modules do not require the presence of a transparent conducting oxide layer. This simplifies the production process twofold; not only can this step be skipped, but the absence of this layer makes the process of constructing a contact scheme much simpler. Both of these simplifications further reduce the cost of production. Despite the numerous advantages over alternative design, production cost estimations on a per unit area basis show that these devices are comparable in cost to single-junction amorphous thin film cells.
Gallium arsenide thin film cells
The semiconductor material gallium arsenide (GaAs) is also used for single-crystalline thin film solar cells. Although GaAs cells are very expensive, they hold the world record for the highest-efficiency, single-junction solar cell at 28.8%. GaAs is more commonly used in multi-junction solar cells for solar panels on spacecrafts, as the industry favours efficiency over cost for space-based solar power (InGaP/(In)GaAs/Ge cells). They are also used in concentrator photovoltaics, an emerging technology best suited for locations that receive much sunlight, using lenses to focus sunlight on a much smaller, thus less expensive GaAs concentrator solar cell.
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