Second-generation thin-film materials

Cadmium telluride

Main article: Cadmium telluride photovoltaics

Cadmium telluride (CdTe) is a chalcogenide material that is the predominant thin film technology. With about 5 percent of worldwide PV production, it accounts for more than half of the thin film market. The cell's lab efficiency has also increased significantly in recent years and is on a par with CIGS thin film and close to the efficiency of multi-crystalline silicon as of 2013.^{[33]: 24–25} Also, CdTe has the lowest energy payback time of all mass-produced PV technologies, and can be as short as eight months in favorable locations.^[33]: 31 CdTe also performs better than most other thin-film PV materials across many important environmental impact factors like global warming potential and heavy metal emissions.^[34] A prominent manufacturer is the US-company First Solar based in Tempe, Arizona, that produces CdTe-panels with an efficiency of about 18 percent.^[35]

Although the toxicity of cadmium may not be that much of an issue and environmental concerns completely resolved with the recycling of CdTe modules at the end of their life time,^[36] there are still uncertainties^[37] and the public opinion is skeptical towards this technology.^[38][39] The usage of rare materials may also become a limiting factor to the industrial scalability of CdTe thin film technology. 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 module's cost.^[40]

${\displaystyle {\begin{pmatrix}\mathrm {Cu} \\\mathrm {Ag} \\\mathrm {Au} \end{pmatrix}}{\begin{pmatrix}\mathrm {Al} \\\mathrm {Ga} \\\mathrm {In} \end{pmatrix}}{\begin{pmatrix}\mathrm {S} \\\mathrm {Se} \\\mathrm {Te} \end{pmatrix}}_{2}}$

Possible combinations of Group-(XI, XIII, XVI) elements in the periodic table that yield a compound showing the photovoltaic effect: Cu, Ag, Au – Al, Ga, In – S, Se, Te.

Silicon

Possible crystal structures of silicon.

There are three prominent silicon thin-film architectures:

• Amorphous silicon cells
• Amorphous / microcrystalline tandem cells (micromorph)
• Thin-film polycrystalline silicon on glass.^[47]

Amorphous silicon

Main article: Amorphous silicon

Amorphous silicon (a-Si) is a non-crystalline, allotropic form of silicon and the most well-developed thin film technology to-date. Thin-film silicon is an alternative to conventional wafer (or bulk) crystalline silicon. While chalcogenide-based CdTe and CIS thin films cells have been developed in the lab with great success, there is still industry interest in silicon-based thin film cells. Silicon-based devices exhibit fewer problems than their CdTe and CIS counterparts such as toxicity and humidity issues with CdTe cells and low manufacturing yields of CIS due to material complexity. Additionally, due to political resistance to the use non-"green" materials in solar energy production, there is no stigma in the use of standard silicon.

This type of thin-film cell is mostly fabricated by a technique called plasma-enhanced chemical vapor deposition. It uses a gaseous mixture of silane (SiH₄) and hydrogen to deposit a very thin layer of only 1 micrometre (µm) of silicon on a substrate, 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.^[48]

a-Si is attractive as a solar cell material because it's an abundant, non-toxic material. It requires a low processing temperature and enables a scalable production upon a flexible, low-cost substrate with little silicon material required. Due to its bandgap of 1.7 eV, amorphous silicon also absorbs a very broad range of the light spectrum, that includes infrared and even some ultraviolet and performs very well at weak light. This allows the cell to generate power in the early morning, or late afternoon and on cloudy and rainy days, contrary to crystalline silicon cells, that are significantly less efficient when exposed at diffuse and indirect daylight.

However, the efficiency of an a-Si cell suffers a significant drop of about 10 to 30 percent during the first six months of operation. This is called the Staebler-Wronski effect (SWE) – a typical loss in electrical output due to changes in photoconductivity and dark conductivity caused by prolonged exposure to sunlight. 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.

Aerospace product with flexible thin-film solar PV from United Solar Ovonic

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.^[49]

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,^[50] and was 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.^[51]

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.^[52]

United Solar Ovonic roll-to-roll solar photovoltaic production line with 30 MW annual capacity

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.^[53] 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.^[47]

Gallium arsenide

Main article: Gallium arsenide

Gallium arsenide (GaAs) is a III-V direct bandgap semiconductor and is a very common material used for single-crystalline thin-film solar cells. GaAs solar cells have continued to be one of the highest performing thin-film solar cells due to their exceptional heat resistant properties and high efficiencies.^[54] As of 2019, single-crystalline GaAs cells have shown the highest solar cell efficiency of any single-junction solar cell with an efficiency of 29.1%.^[55] This record-holding cell achieved this high efficiency by implementing a back mirror on the rear surface to increase photon absorption which allowed the cell to attain an impressive short-circuit current density and an open-circuit voltage value near the Shockley–Queisser limit.^[56] As a result, GaAs solar cells have nearly reached their maximum efficiency although improvements can still be made by employing light trapping strategies.^[57]

GaAs thin-films are most commonly fabricated using epitaxial growth of the semiconductor on a substrate material. The epitaxial lift-off (ELO) technique, first demonstrated in 1978, has proven to be the most promising and effective. In this method, the thin film layer is peeled off of the substrate by selectively etching a sacrificial layer that was placed between the epitaxial film and substrate.^[58] The GaAs film and the substrate remain minimally damaged through the separation process, allowing for the reuse of the host substrate.^[59] With reuse of the substrate the fabrication costs can be reduced, but not completely forgone, since the substrate can only be reused a limited number of times.^[57] This process is still relatively costly and research is still being done to find more cost-effective ways of growing the epitaxial film layer onto a substrate.

Despite the high performance of GaAs thin-film cells, the expensive material costs hinder their ability for wide-scale adoption in the solar cell industry. GaAs is more commonly used in multi-junction solar cells for solar panels on spacecraft, as the larger power to weight ratio lowers the launch costs in 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.

An experimental silicon based solar cell developed at the Sandia National Laboratories

Third-generation (emerging) thin-film materials

The National Renewable Energy Laboratory classifies a number of thin-film technologies as emerging photovoltaics—most of them have not yet been commercially applied and are still in the research or development phase. Many use organic materials, often organometallic compounds as well as inorganic substances. Though many of these technologies have struggled with instability and low efficiencies in their early stages, some emerging materials like perovskites have been able to attain efficiencies comparable to mono crystalline silicon cells. Many of these technologies have the potential to beat the Shockley–Queisser limit for efficiency of a single-junction solid-state cell. Significant research has been invested into these technologies as they promise to achieve the goal of producing low-cost, high-efficiency solar cells with smaller environmental impacts.

Applications

Commercial module efficiences

It's important to note that the maximum efficiencies achieved in a laboratory setting are generally higher than the efficiencies of manufactured cells, which often have efficiencies 20-50% lower.^[66] As of 2021, the maximum efficiency of manufactured solar cells was 24.4% for mono crystalline silicon, 20.4% for poly crystalline silicon, 12.3% for amorphous silicon, 19.2% for CIGS, and 19% for CdTe modules.^[78] The thin film cell prototype with the best efficiency yields 20.4% (First Solar), comparable to the best conventional solar cell prototype efficiency of 25.6% from Panasonic.^[79][80]

A previous record for thin film solar cell efficiency of 22.3% was achieved by Solar Frontier, the world's largest CIS (copper indium selenium) solar energy provider. In joint research with the New Energy and Industrial Technology Development Organization (NEDO) of Japan, Solar Frontier achieved 22.3% conversion efficiency on a 0.5 cm² cell using its CIS technology. This was an increase of 0.6 percentage points over the industry's previous thin-film record of 21.7%.^[81]

Calculation of efficiency

Solar cell efficiencies of various cell technologies (including both single-crystal and thin film technologies) as tracked by NREL

The efficiency of a solar cell quantifies the percentage of incident light on the solar cell that is converted into usable electricity. There are many factors that affect the efficiency of a solar cell, so the efficiency may be further parametrized by additional numerical quantities including the short-circuit current, open-circuit voltage, maximum power point, fill factor, and quantum efficiency. The short-circuit current is the maximum current the cell can flow with no voltage load. Similarly, the open-circuit voltage is the voltage across the device with no current or, alternatively, the voltage required for no current to flow. On a current vs. voltage (IV) curve, the open-circuit voltage is the horizontal intercept of the curve with the voltage axis and the short-circuit current is the vertical intercept of the curve with the current axis. The maximum power point is the point along the curve where the maximum power output of the solar cell is achieved and the area of the rectangle with side lengths equal to the current and voltage coordinates of the maximum power point is called the fill factor. The fill factor is a measure of how much power the solar cell achieves at this maximum power point. Intuitively, IV curves with a more square shape and a flatter top and side will have a larger fill factor and therefore a higher efficiency.^[82] Whereas these parameters characterize the efficiency of the solar cell based mostly on its macroscopic electrical properties, the quantum efficiency measures either the ratio of the number of photons incident on the cell to the number of charge carriers extracted (external quantum efficiency) or the ratio of the number of photons absorbed by the cell to the number of charge carriers extracted (internal quantum efficiency). Either way, the quantum efficiency is a more direct probe of the microscopic structure of the solar cell.^[83]

Schematic of a solar cell I-V curve.

Increasing efficiency

Some third-generation solar cells boost efficiency through the integration of concentrator and/or multi-junction device geometry.^[61] This can lead to efficiencies larger than the Shockley–Queisser limit of approximately 42% efficiency for a single-junction semiconductor solar cell under one-sun illumination.^[84]

A multi-junction cell is one that incorporates multiple semiconducting active layers with different bandgaps. In a typical solar cell, a single absorber with a bandgap near the peak of the solar spectrum is used, and any photons with energy greater than or equal to the bandgap can excite valence-band electrons into the conduction band to create electron-hole pairs. However, any excess energy above the Fermi energy will be quickly dissipated due to thermalization, leading to voltage losses from the inability to efficiently extract the energy of high-energy photons. Mutli-junction cells are able to recoup some of this energy lost to thermalization by stacking multiple absorber layers on top of each other with the top layer absorbing the highest-energy photons and letting the lower energy photons pass through to the lower layers with smaller bandgaps, and so on. This not only allows the cells to capture energy from photons in a larger range of energies, but also extracts more energy per photon from the higher-energy photons.^[85]

Concentrator photovoltaics use an optical system of lenses that sit on top of the cell to focus light from a larger area onto the device, similar to a funnel for sunlight. In addition to creating more electron-hole pairs simply by increasing the number of photons available for absorption, having a higher concentration of charge carriers can increase the efficiency of the solar cell by increasing the conductivity. The addition of a concentrator to a solar cell can not only increase efficiency, but can also reduce the space, materials, and cost needed to produce the cell.^[86]

Both of these techniques are employed in the highest-efficiency solar cell as of 2023, which is a four-junction concentrator cell with 47.6% efficiency.^[1]

Increasing absorption

Multiple techniques have been employed to increase the amount of light that enters the cell and reduce the amount that escapes without absorption. The most obvious technique is to minimize the top contact coverage of the cell surface, reducing the area that blocks light from reaching the cell.

The weakly absorbed long wavelength light can be obliquely coupled into silicon and traverses the film several times to enhance absorption.^[87][88]

Multiple methods have been developed to increase absorption by reducing the number of incident photons being reflected away from the cell surface. An additional anti-reflective coating can cause destructive interference within the cell by modulating the refractive index of the surface coating. Destructive interference eliminates the reflective wave, causing all incident light to enter the cell.

Surface texturing is another option for increasing absorption, but increases costs. By applying a texture to the active material's surface, the reflected light can be refracted into striking the surface again, thus reducing reflectance. For example, black silicon texturing by reactive ion etching (RIE) is an effective and economic approach to increase the absorption of thin-film silicon solar cells.^[89] A textured back reflector can prevent light from escaping through the rear of the cell. Instead of applying the texturing on the active materials, photonic micro-structured coatings applied on the cells' front contact can be an interesting alternative for light-trapping, as they allow both geometric anti-reflection and light scattering while avoiding the roughening of the photovoltaic layers (thereby preventing increase of recombination).^[90][91]

Besides surface texturing, the plasmonic light-trapping scheme attracted a lot of attention to aid photocurrent enhancement in thin film solar cells.^[92][93] This method makes use of collective oscillation of excited free electrons in noble metal nanoparticles, which are influenced by particle shape, size and dielectric properties of the surrounding medium. Applying noble-metal nanoparticles at the back of thin-film solar cells leads to the formation of plasmonic back reflectors, which allow broadband photocurrent enhancement.^[94] This is a result of both light scattering of the weakly-absorbed photons from the rear-located nanoparticles, plus improved light incoupling (geometric anti-reflection) caused by the hemispherical corrugations at the cells’ front surface formed from the conformal deposition of the cell materials over the particles.^[95]

In addition to minimizing reflective loss, the solar cell material itself can be optimized to have higher chance of absorbing a photon that reaches it. Thermal processing techniques can significantly enhance the crystal quality of silicon cells and thereby increase efficiency.^[96] Layering thin-film cells to create a multi-junction solar cell can also be done. Each layer's band gap can be designed to best absorb a different range of wavelengths, such that together they can absorb a greater spectrum of light.^[97]

Further advancement into geometric considerations can exploit nanomaterial dimensionality. Large, parallel nanowire arrays enable long absorption lengths along the length of the wire while maintaining short minority carrier diffusion lengths along the radial direction.^[98] Adding nanoparticles between the nanowires allows conduction. The natural geometry of these arrays forms a textured surface that traps more light.

Market-share

In 2013 thin-film technologies accounted for about 9 percent of worldwide deployment, while 91 percent was held by crystalline silicon (mono-Si and multi-Si). With 5 percent of the overall market, CdTe holds more than half of the thin-film market, leaving 2 percent to each CIGS and amorphous silicon.^{[33]: 18–19}

Thin film solar on metal roofs

Thin film solar running down standing seam metal roof

With the increasing efficiencies of thin film solar, installing them on standing seam metal roofs has become cost competitive with traditional Monocrystalline and Polycrystalline solar cells. The thin film panels are flexible and run down the standing seam metal roofs and stick to the metal roof with Adhesive, so no holes are needed to install. The connection wires run under the ridge cap at the top of the roof. Efficiency ranges from 10-18% but only costs about $2.00-$3.00 per watt of installed capacity, compared to Monocrystalline which is 17-22% efficient and costs $3.00-$3.50 per watt of installed capacity. Thin film solar is light weight at 7-10 ounces per square foot. Thin film solar panels last 10–20 years^[117] but have a quicker ROI than traditional solar panels, the metal roofs last 40–70 years before replacement compared to 12–20 years for an asphalt shingle roof.^[118][119]

Cost

In 1998, scientists at the National Renewable Energy Laboratory (NREL) predicted that production of thin-film PV systems at a cost of $50 per m² could someday be possible, which would make them extremely economically viable. At this price, thin-film PV systems would yield return on investment of 30% or greater.^[120]

To help achieve this goal, in 2022 NREL began administering the Cadmium Telluride Accelerator Consortium (CTAC) with the objective of enabling thin-film efficiencies above 24% with a cost below 20 cents per Watt by 2025, followed by efficiencies above 26% and cost below 15 cents per Watt by 2030.^[121]

Life cycle analysis

Life cycle analysis (LCA) is a family of approaches that attempt to assess the total environmental impact of a product or process in an objective way, from the gathering of raw materials and the manufacturing process all the way to the disposal of the outcome and any waste products. Though LCA approaches aim to be unbiased, the outcome of LCA studies can be sensitive to the particular approach and data used. It is therefore important that LCA findings clearly state the assumptions made any which processes are included and excluded. This will often be specified using the system boundary and life cycle inventory framework. Due to the emerging nature of new photovoltaic technologies, the disposal process may sometimes be left out of a life cycle analysis due to the high uncertainty. In this case, the assessment is referred to as "cradle-to-gate" rather than "cradle-to-grave," because the calculated impact does not cover the full life cycle of the product. However, such studies may miss important environmental impacts from the disposal process, both negative (as in the case of incineration of end-of-life cells and waste products) and positive (as in the case of recycling). It's also important to include the effect of balance of service (BOS) steps, which include transportation, installation, and maintenance as they may also be costly in terms of materials and electricity.^[75]

LCA studies can be used to quantify many potential environmental impacts, from land use to transportation-related emissions. Categories of environmental impacts may be grouped into so-called impact factors for standardized quantitative comparison. For solar cells, perhaps the most important impact factor is the total lifetime greenhouse gas (GHG) emission. This is often reported in terms of the global warming potential (GWP), which gives a more direct indication of the environmental impact.^[61]

Another important measure of environmental impact is the primary energy demand (PED) which measures the energy (usually electricity) required to produce a particular solar cell. A more useful measure may be the cumulative energy demand (CED), which quantifies the total amount of energy required to produce, use, and dispose of a particular product over its entire lifetime. Relatedly, the energy payback time (EPBT) measures the operational time needed for a solar cell to produce enough energy to account for its cumulative energy demand. Similarly, the carbon payback time (CPBT) measures the operational time needed for a solar cell to produce enough electricity that the avoided carbon emissions from the same amount of electricity generated with the usual energy mix is equal to the amount of carbon emissions the cell will generate over its lifetime. In other words, CPBT measures the time a solar cell needs to run in order to mitigate its own carbon emissions.^[65]

These quantities depends on many factors, including where the solar cell is manufactured and deployed, as the typical energy mix varies from place to place. Therefore, the electricity-related emissions from the production process as well as the avoided electricity-related emissions from the solar-generated electricity during operation of the cell can vary depending on the particular module and application. The emissions from a cell may also depend on how the module is deployed, not just because of the raw materials and energy costs associated with the production of mounting hardware, but also from any avoided emissions from replaced building materials, as in the case of building-integrated photovoltaics where solar panels may replace building materials like roof tiles.^[77]

Though energy-usage and emissions-related impacts are vital for evaluation of and comparison between technologies, they are not the only important quantities for evaluating the environmental impact of solar cells. Other important impact factors include toxic heavy metal emissions, metal depletion, human toxicity, various eco-toxicities (marine, freshwater, terrestrial), and acidification potential which measures the emission of sulfur and nitrogen oxides.^[65][34] Including a wide range of environmental impacts in a life cycle analysis is necessarily to minimize the chance of passing environmental impact from a prominent impact factor like greenhouse gas emission to a less prominent but still relevant impact factor like human toxicity.^[75]

Greenhouse gas emissions

Using established first-generation mono crystalline silicon solar cells as a benchmark, some thin-film solar cells tend to have lower environmental impacts across most impact factors, however low efficiencies and short lifetimes can increase the environmental impacts of emerging technologies above those of first-generation cells. A standardized measure of greenhouse gas emissions, is displayed in the chart in units of grams of CO₂ equivalent emissions per kiloWatt-hour of electricity production for a variety of thin-film materials.^{[34][77][123]} Crystalline silicon is also included for comparison.

In terms of greenhouse gas emissions only, the two most ubiquitous thin-film technologies, amorphous silicon and CdTe, both have significantly lower global warming potential (GWP) than mono crystalline silicon solar cells, with amorphous silicon panels having GWP around 1/3 lower and CdTe nearly 1/2 lower.^[66][77] Organic photovoltaics have the smallest GWP of all thin-film PV technologies, with over 60% lower GWP than mono crystalline silicon.^[65]

However, this is not the case for all thin-film materials. For many emerging technologies, low efficiencies and short device lifetimes may cause significant increases in environmental impact. Both emerging chalcogenide technologies and established chalcogenide technologies like CIS and CIGS have higher Global warming potential than mono crystalline silicon, as do dye-sensitized and quantum dot solar cells. For antimony-based chalcogenide cells, favorable for their use of less-toxic materials in the manufacturing process, low efficiencies and therefore larger area requirements for solar cells are the driving factor in the increased environmental impact, and cells with modestly improved efficiencies have the potential to outperform mono crystalline silicon in all relevant environmental impact factors. Improving efficiencies for these and other emerging chalcogenide cells is therefore a priority.^[61] Low realized efficiencies are also the driving factor behind the relatively large GWP of quantum dot solar cells, despite the potential for these materials to exhibit multiple exciton generation (MEG) from a single photon. Higher efficiencies would also allow for the use of a thinner active layer, reducing both materials costs for the quantum dots themselves and saving on materials and emissions related to encapsulation material. Realizing this potential and thereby increasing efficiency is also a priority for reducing the environmental impact of these cells.^[34]

For organic photovoltaics, short lifetimes are instead the driving factor behind GWP. Despite overall impressive performance of OPV relative to other solar technologies, when considering cradle-to-gate rather than cradle-to-grave (i.e. looking only at the material extraction and production processes, discounting the useful lifetime of the solar cells) GWP, OPV constitute a 97% reduction in GHG emissions compared to mono crystalline silicon and 92% reduction relative to amorphous silicon thin-films. This is significantly better than the 60% reduction compared to mono crystalline silicon currently realized, and therefore improving OPV cell lifetimes is a priority for decreasing overall environmental impact.^[65] For Perovskite solar cells, with short lifetimes of only around 5 years, this effect may be even more significant. Perovskite solar cells (not included in the chart) typically have significantly larger global warming potential than other thin-film materials in cradle-to-grave LCA, around 5-8x worse than mono crystalline silicon at 150g CO_2-eq /kWh. However, in grade-to-gate LCA, Perovskite cells perform 10-30% lower than mono-crystalline silicon, highlighting the importance of the increased environmental impact associated with the need to produce and dispose of multiple Perovskite panels to generate the same amount of electricity as a single mono crystalline silicon panel due to this short lifetime. Increasing the lifetime of Perovskite solar modules is therefore a top priority for decreasing their environmental impact.^[66] Other renewable energy sources like wind, nuclear, and hydropower may achieve smaller GWP than some PV technologies.^[34]

It's important to note that although emerging thin-film materials don't outperform mono crystalline silicon cells in terms of global warming potential, they still constitute far lower carbon emissions than non-renewable energy sources which have global warming potentials ranging from comparatively clean natural gas with 517g CO_2-eq /kWh to the worst polluter lignite with over 1100g CO_2-eq /kWh. Thin-film cells also significantly outperform the typical energy mix, which is often in the range of 400-800g CO_2-eq /kWh.^[77]

The largest contributor to most impact factors, including the global warming potential, is nearly always energy use during the manufacturing process, greatly outweighing other potential sources of environmental impact such as transportation cost and material sourcing.^[77] For CIGS cells, for example, this accounts for 98% of the global warming potential, most of which is due to the manufacturing of the absorber layer specifically.^[61] In general, for processes that include metal deposition, this is often a particularly significant environmental impact hotspot.^[75] For quantum dot photovoltaics, hazardous waste disposal for the solvents used during the manufacturing process also contributes significantly.^[34] The level of global warming potential associated with electricity use can vary significantly depending on the location manufacturing takes place, in particular the proportion of renewable to non-renewable energy sources used in the local energy mix.^[77]

Energy payback time

In general, thin-film panels take less energy to produce than mono crystalline silicon panels,^[77] especially as some emerging thin-film technologies have the potential for efficient and cheap roll-to-roll processing.^[34] As a result, thin-film technologies tend to fare better than mono crystalline silicon in terms of energy payback time, though amorphous silicon panels are an exception. Thin-film cells typically have lower efficiencies than mono crystalline solar cells, so this effect is largely due to the comparatively lower primary energy demand (PED) associated with producing the cells.

The application in which the modules are used and the recycling process (if any) for the materials can also play a large role in the overall energy efficiency and greenhouse gas emissions over the lifetime of the cell. Integrating the modules into building design may lead to a large reduction in the environmental impact of the cells due to the avoided emissions related to producing the usual building materials, for example the avoided emissions from roof tile production for a building-integrated solar roof.^[77] This effect is especially important for thin-film solar cells, whose lightweight and flexible nature lends itself naturally to building-integrated photovoltaics.^[75] 70-90% lower emissions in portable charging applications.^[65] This effect holds for some other applications as well, for example organic photovoltaics have 55% lower emissions than crystalline silicon in solar panel applications but up to nearly Similarly, avoided emissions from recycling solar cell components rather than gathering and processing new materials can lead to significantly lower cumulative energy consumption and greenhouse gas emissions. Recycling processes are available for several components of mono crystalline solar cells as well as the glass substrate, CdTe, and CdS in CdTe solar cells.^[123] For panels without recycling processes, and particularly for panels with short lifetimes like organic photovoltaics, the disposal of panels may contribute significantly to the environmental impact, and there may be little difference in environmental impact factors if the panel is incinerated or sent to landfill.^[65]

Heavy-metal emission and human toxicity

Though material selection and extraction does not play a large role in global warming potential, where electricity usage in the manufacturing process is near universally the largest contributor, it often has a significant impact on other important environmental impact factors, including human toxicity, heavy-metal emissions, acidification potential, and metal and ozone depletion.

Human toxicity and heavy-metal emissions are particularly important impact factors for thin-film solar cell production, as the potential environmental and health effect of cadmium use has been a particular concern since the introduction of CdTe cells to the commercial market in the 1990s, when the hazards of cadmium-containing compounds were not well-understood.^[126] Public concern over CdTe solar cells has continues as they have become more common.^[127] Cadmium is a highly hazardous material^[128] that causes kidney, bone, and lung damage and is thought to increase the risk of developing cancer.^[129] Initially, all cadmium-containing compounds were classified as hazardous, although we now know that despite both Cd and Te being hazardous separately, the combination CdTe is very chemically stable^[25] with a low solubility and presents minimal risk to human health.^[126]

Feedstock Cd presents a larger risk, as do pre-cursor materials like CdS, and cadmium acetate, which are frequently used in other photovoltaic cells as well, and often contribute significantly to environmental impact factors such as human toxicity and heavy metal emission.^[61] These effects may be more pronounced for nanofabrication processes that produce Cd ions in solution, like the manufacture of quantum dots for QDPV.^[34] Due to these effects, CdTe solar cell production is actually seen to have lower heavy-metal emissions than other thin-film solar manufacturing. In fact, CdTe production has lower cadmium emission than ribbon silicon, multi-crystalline silicon, mono-crystalline silicon, or quantum dot PV manufacturing, as well as lower emission of nickel, mercury, arsenic, chromium, and lead.^[34] In terms of total heavy metal emissions, quantum dot PV has the highest emissions of PV materials with approximately 0.01 mg/kWh, but still has lower total heavy metal emission than any other renewable or non-renewable electricity source, as shown in the chart.

The desire to alleviate safety concerns around cadmium and CdTe solar cells specifically has sparked the development of other chalcogenide PV materials that are non-toxic or less toxic, particularly antimony-based chalcogenides. In these emerging chalcogenide cells, the use of CdS is the largest contribution to impact factors like human toxicity and metal depletion, though stainless steel also contributes significantly to the impact of these and other PV materials. In CIGS cells, for example, stainless steel accounts for 80% of the total toxicity associated with cell production and also contributes significantly to ozone depletion.^[61]

Another potential impact factor of interest for PV manufacturing is the acidification potential, which quantifies the emission of sulfur and nitrogen oxides which contribute to the acidification of soil, freshwater, and the ocean and their negative environmental effects. In this respect, QDPV has the lowest emissions, with CdTe being a close second.^[34]