Technology

Technology

The sun is the world’s most abundant renewable energy resource. Every hour, it supplies enough energy to power the world for an entire year. Photovoltaics (PV) use semiconductor materials to absorb solar radiation and convert it into electricity without fuel, water, emissions or moving parts. According to the International Energy Agency (IEA), solar PV was the second largest source of new power capacity (after onshore wind) and accounted for one third of new renewable additions in 2015.  By 2040, PV is expected to account for one fourth of global energy capacity additions. The transition to a low carbon economy is crucial to ensuring energy security, addressing water scarcity, and mitigating climate change.

 

Gross Capacity Additions by Technology (GW/Year)

Global Energy Portfolio

Source: Bloomberg New Energy Finance, New Energy Outlook 2015.

What is Thin Film PV?

Thin film PV was born out of the energy crisis of the 1970s. Determined to reduce the world’s reliance on fossil fuels, glass pioneers Harold McMaster and Norman Nitschke began exploring ways to commercialize and scale solar energy while also finding a new outlet for the glass industry. McMaster understood that scaling PV production was imperative to driving down the cost of solar electricity and meeting growing energy demands.

“If you can’t get your cost of production down below $1, it’s not worth it.” McMaster used to say, “You have to ask yourself, will this technology allow you to process 500 tons of glass a day? If it can’t do that, it’s a waste of time to do it.” –Harold McMaster, pioneer of commercial-scale PV

McMaster was convinced that PV cells could be created by coating large glass substrates with a layer of conductive metal, similar to the way tinted glass is manufactured. He envisioned integrating the panel-coating process into “float lines” in the glass industry so that every glass plant could turn out windows one day and produce solar panels the next. The successful development of the Vapor Transport Deposition process led to the large-scale production of thin film PV modules which could be manufactured nearly 600 times faster than conventional crystalline silicon modules.

Unlike the crystalline silicon PV batch process, thin film PV modules are manufactured in a single continuous process by depositing semiconductor material on inexpensive substrates such as glass or plastic. A sheet of glass can be transformed into a finished PV module in less than 3.5 hours compared to crystalline silicon wafers which can take up to three days.  By using compound semiconductors with a direct bandgap, such as gallium arsenide (GaAs), cadmium telluride (CdTe), copper indium disulphide (CIS) and lead sulphide (PbS), thin film modules absorb light up to 100 times more effectively than indirect bandgap materials such as silicon.

 

Shockley-Queisser Limit of Solar Cells

Shockley Queisser

Source: W. Shockley and H. J. Queisser, 1961, Detailed balance limit of efficiency of p‐n junction solar cells. Journal of applied physics, 32(3), 510-519.

Superior Performance

In addition to having the highest theoretical efficiencies among single-junction solar cells, thin film PV technologies have a proven real-world performance advantage over crystalline silicon modules. Beyond standard testing conditions of 25°C, all PV semiconductor technologies begin to incur increasing performance losses as temperatures rise. However, thin film technologies have less temperature-related losses due to their low temperature coefficient. As a result, thin film PV modules generate more energy in hotter field environments.

Thin film PV modules are also less sensitive to shading due to their monolithic design. When shading occurs, typical crystalline silicon technologies turn off disproportionately large portions of the module to protect it from damage. In a thin film module, only the shaded portion is impacted while the rest of the module will continue to produce power. By producing more usable energy per nameplate watt, thin film PV modules deliver a lower levelized costs of electricity in most climatic conditions.

CdTe – Cadmium Telluride

CdTe PV combines the performance advantages of thin film technologies with affordable high-volume manufacturing, making it ideal for commercial rooftop and utility-scale ground-mount applications. In addition to their performance advantage in hotter climates, CdTe PV modules are less sensitive to reductions in wavelengths caused by humidity which results in a global energy density advantage over crystalline silicon PV. Utility-scale CdTe PV applications have played a pivotal role in driving down the cost of solar electricity and transforming PV into mainstream energy source. According to Lazard’s latest unsubsidized levelized cost of energy comparison, utility-scale thin film PV electricity is less expensive than new conventional generation technologies in the U.S. Southwest. Utility-Scale thin film PV applications enable solar to be truly affordable and accessible to all.

First Solar CdTe Utility scale example

© First Solar: Utility-scale PV application

Calyxo CdTe Rooftop example

© Calyxo: Rooftop Application

CIGS – Copper Indium Gallium Selenide

CIGS PV modules are well-suited for vertical installation in building-integrated PV (BIPV) applications due to their high efficiency, physical flexibility, aesthetics, light weight and superior performance in diffuse light conditions. BIPV solutions help protect building facades from weathering while generating clean electricity. Like CdTe, CIGS modules have a low temperature coefficient which results in a superior energy yield at higher module temperatures. With buildings being one of the biggest consumers of energy worldwide, CIGS modules provide an innovative solution for net-zero buildings by replacing conventional building materials.

Manz AG Gebäudeintegrierte Photovoltaik3

©Manz: Building-Integrated CIGS PV Application

Manz AG Building Integrated PV CIGS Technology example

©Manz: Building-Integrated CIGS PV Application

PV technology tree

 

  • Crystalline Silicon: First generation PV technologies use wafer-based crystalline silicon as the active material. Crystalline silicon (c-Si) solar cells are divided into two categories: mono-crystalline and multi-crystalline silicon PV. These single junction silicon cells are limited by a theoretical efficiency of 27% and a practical efficiency of ~25%.

    • Mono-Crystalline Silicon: Mono c-Si cells use a higher crystal quality which improves the power conversion efficiencies, but requires more expensive wafers. Mono c-Si modules are more energy-intensive to produce than multi c-Si as they require additional recrystallization. Mono c-Si PV modules are ideally suited to space-constrained rooftop applications.
    • Multi-Crystalline Silicon: Multi c-Si cells are made of lower grade silicon ingots which results in a lower cost and efficiency than mono c-Si modules.
  • Hybrid Silicon Thin Film: Hybrid silicon thin film solar cells combine c-Si technology with a layer of wide-bandgap thin-film material such as III-V compounds (from Al, Ga, In, N, P, As, Sb elements), chalcogenides, metal oxides, or perovskites. Hybrid PV devices have a potential for low-cost manufacturing and high conversion efficiencies.

    • Hetero- and Multi- Junction: Multi-junction (MJ) solar cells use a stack of two or more single-junction cells with different bandgaps to capture a broader portion of the light spectrum, yielding higher cell and module efficiencies. Multi-junction solar cells have the potential to break the 50% conversion efficiency barrier, however, complex manufacturing processes and high material costs make multi-junction cells prohibitively expensive for large-area applications

      • Micromorph Si: Micromorph solar cells combines both hydrogenated amorphous and micro-crystalline silicon to absorb a broader portion of the solar spectrum. This multi-junction technology combines poly-crystalline silicon solar cells with the cost and absorption advantages of thin film amorphous silicon (a-Si).
      • Gallium Arsenide: High-efficiency multi-junction solar cells based on gallium arsenide (GaAs) use thin absorbing films and require wafers as templates for crystal growth. GaAs is a compound semiconductor that is near ideal for solar energy conversion, with strong absorption properties and a direct bandgap that closely matches the solar spectrum. Gallium arsenide solar cells have been used in space applications but its wider deployment was limited by its high cost which can be over 100 times more expensive than conventional crystalline silicon and thin-film technologies.
      • Perovskite/Silicon Tandem: As conventional crystalline silicon PV technologies are nearing their practical efficiency limit, perovskite/silicon tandem cells could deliver the potential for ultra-high efficiencies at affordable costs. Printing a perovskite stack on top of silicon cells will potentially enable higher efficiencies as the different layers absorb different portions of the sunlight. Researchers predict this technology could ultimately achieve a power efficiency of more than 35%.
      • Concentrated PV: CPV uses lenses and curved mirrors to concentrate sunlight onto small and highly efficient multi-junction solar cells. CPV systems often use solar trackers to follow the sun and sometimes use a cooling system to further increase their efficiency.
  • Conventional Thin Film: Commercial thin-film PV technologies include cadmium telluride (CdTe), copper indium gallium diselenide (CIGS), and amorphous silicon (a-Si). These semiconductor materials absorb light up to 100 times more efficiently than silicon, allowing the use of very thin films that are just a few microns thick. Their low use of raw materials provides a key cost and environmental advantage.

    • CdTe: Cadmium Telluride (CdTe) has amongst the highest theoretical efficiencies of any PV semiconductor material in a single junction configuration. Today, CdTe PV is recognized as a mature technology with over 15GW installed worldwide, a direct result of its ability to successfully translate record-breaking cell efficiencies over to the manufacturing line. CdTe’s unique chemical and thermal stability enables it to be evenly deposited on a sheet of glass in seconds, resulting in affordable, high-volume and high-quality manufacturing. CdTe PV is ideally suited to utility-scale applications which has played a pivotal role in driving down the cost of solar electricity and transforming PV into mainstream energy source.
    • CIS and CIGS: Similar to CdTe, CIS and CIGS semiconductors have direct energy bandgaps and high optical absorption coefficients which absorb a wide spectrum of light, making it possible to use just a few microns of semiconductor. Scientists say CIGS has great potential to become one of the most efficient, lowest cost PV technologies. CIGS have already recorded conversion efficiencies of over 20%. Certain CIGS modules are particularly well suited for flexible, portable and building-integrated PV (BIPV) applications because of their light weight and flexibility.
  • Emerging Thin Film: Unlike conventional thin film technologies which deposit the absorber material on a substrate in a single step, emerging thin film technologies often employ separate active material synthesis and deposition steps. This enables the use of flexible and lightweight plastic substrates.

    • Perovskites: Perovskite-based PV is an emerging technology that evolved from solid-state dye-sensitized cells. “Perovskite” refers to the crystal structure of the light-absorbing film, and the most widely investigated perovskite material is the hybrid organic-inorganic lead halide polycrystalline films. Perovskites have become one of the most promising emerging PV technologies, showing remarkable promise in terms of low cost and high efficiency. The efficiency of perovskite solar cells has improved dramatically in just five years, from 3.8% in 2009 to 21.1% in 2015.
    • Organic PV: OPV or “plastic solar cells” consist of several thin layers of carbon-based molecules or polymers to absorb light. These molecules can be deposited into thin films using low-cost deposition methods such as inkjet printing and thermal evaporation. However, OPV remains constrained by low efficiencies and poor long-term stability when exposed to excessive light, water and oxygen. Current OPV research is focused on increasing device efficiency and lifetime by optimizing the absorbers and improving the encapsulation. Substantial efficiency gains (around 11%) have been achieved in the laboratory. As various absorbers can be used to create colored or transparent devices, OPV is particularly well suited to the flexible architectural design and building-integrated PV (BIPV) market.
  • Dye-sensitized(DSSC): Dye-sensitized solar cells (also known as Grätzel cells) use dye as the photoactive material to produce electricity once it is sensitized by light. DSSCs have achieved certified efficiencies of 11.9% and may benefit from low-cost materials, simple assembly, and the possibility of flexible modules. Key challenges include limited long-term stability under illumination and high temperatures, and low absorption in the near-infrared.
  • Quantum Dot PV: QDPV use solution-processed nanocrystals to absorb light and hold the promise of significantly reducing material requirement due to their high absorption qualities. Although cell efficiencies remain relatively low at around 9%, quantum dot cells have attracted research interest due to their inherently low cost, versatility, and light weight.

 

Click on the chart below to learn more about the different PV technologies and their range of applications:

First generation PV technologies use wafer-based crystalline silicon as the active material. Crystalline silicon (c-Si) solar cells are divided into two categories: mono-crystalline and multi-crystalline silicon PV. These single junction silicon cells are limited by a theoretical efficiency of 27% and a practical efficiency of ~25%.

Mono c-Si cells use a higher crystal quality which improves the power conversion efficiencies, but requires more expensive wafers. Mono c-Si modules are more energy-intensive to produce than multi c-Si as they require additional recrystallization. Mono c-Si PV modules are ideally suited to space-constrained rooftop applications.

Multi c-Si cells are made of lower grade silicon ingots which results in a lower cost and efficiency than mono c-Si modules.

Hybrid silicon thin film solar cells combine c-Si technology with a layer of wide-bandgap thin-film material such as III-V compounds (from Al, Ga, In, N, P, As, Sb elements), chalcogenides, metal oxides, or perovskites. Hybrid PV devices have a potential for low-cost manufacturing and high conversion efficiencies.

Multi-junction (MJ) solar cells use a stack of two or more single-junction cells with different bandgaps to capture a broader portion of the light spectrum, yielding higher cell and module efficiencies. Multi-junction solar cells have the potential to break the 50% conversion efficiency barrier, however, complex manufacturing processes and high material costs make multi-junction cells prohibitively expensive for large-area applications

Micromorph solar cells combines both hydrogenated amorphous and micro-crystalline silicon to absorb a broader portion of the solar spectrum. This multi-junction technology combines poly-crystalline silicon solar cells with the cost and absorption advantages of thin film amorphous silicon (a-Si).

High-efficiency multi-junction solar cells based on gallium arsenide (GaAs) use thin absorbing films and require wafers as templates for crystal growth. GaAs is a compound semiconductor that is near ideal for solar energy conversion, with strong absorption properties and a direct bandgap that closely matches the solar spectrum. Gallium arsenide solar cells have been used in space applications but its wider deployment was limited by its high cost which can be over 100 times more expensive than conventional crystalline silicon and thin-film technologies.

As conventional crystalline silicon PV technologies are nearing their practical efficiency limit, perovskite/silicon tandem cells could deliver the potential for ultra-high efficiencies at affordable costs. Printing a perovskite stack on top of silicon cells will potentially enable higher efficiencies as the different layers absorb different portions of the sunlight. Researchers predict this technology could ultimately achieve a power efficiency of more than 35%.

CPV uses lenses and curved mirrors to concentrate sunlight onto small and highly efficient multi-junction solar cells. CPV systems often use solar trackers to follow the sun and sometimes use a cooling system to further increase their efficiency.

Commercial thin-film PV technologies include cadmium telluride (CdTe), copper indium gallium diselenide (CIGS), and amorphous silicon (a-Si). These semiconductor materials absorb light up to 100 times more efficiently than silicon, allowing the use of very thin films that are just a few microns thick. Their low use of raw materials provides a key cost and environmental advantage.

Unlike conventional thin film technologies which deposit the absorber material on a substrate in a single step, emerging thin film technologies often employ separate active material synthesis and deposition steps. This enables the use of flexible and lightweight plastic substrates.

Cadmium Telluride (CdTe) has amongst the highest theoretical efficiencies of any PV semiconductor material in a single junction configuration. Today, CdTe PV is recognized as a mature technology with over 15GW installed worldwide, a direct result of its ability to successfully translate record-breaking cell efficiencies over to the manufacturing line. CdTe’s unique chemical and thermal stability enables it to be evenly deposited on a sheet of glass in seconds, resulting in affordable, high-volume and high-quality manufacturing. CdTe PV is ideally suited to utility-scale applications which has played a pivotal role in driving down the cost of solar electricity and transforming PV into mainstream energy source.

Similar to CdTe, CIS and CIGS semiconductors have direct energy bandgaps and high optical absorption coefficients which absorb a wide spectrum of light, making it possible to use just a few microns of semiconductor. Scientists say CIGS has great potential to become one of the most efficient, lowest cost PV technologies. CIGS have already recorded conversion efficiencies of over 20%. Certain CIGS modules are particularly well suited for flexible, portable and building-integrated PV (BIPV) applications because of their light weight and flexibility.

Perovskite-based PV is an emerging technology that evolved from solid-state dye-sensitized cells. “Perovskite” refers to the crystal structure of the light-absorbing film, and the most widely investigated perovskite material is the hybrid organic-inorganic lead halide polycrystalline films. Perovskites have become one of the most promising emerging PV technologies, showing remarkable promise in terms of low cost and high efficiency. The efficiency of perovskite solar cells has improved dramatically in just five years, from 3.8% in 2009 to 21.1% in 2015.

OPV or “plastic solar cells” consist of several thin layers of carbon-based molecules or polymers to absorb light. These molecules can be deposited into thin films using low-cost deposition methods such as inkjet printing and thermal evaporation. However, OPV remains constrained by low efficiencies and poor long-term stability when exposed to excessive light, water and oxygen. Current OPV research is focused on increasing device efficiency and lifetime by optimizing the absorbers and improving the encapsulation. Substantial efficiency gains (around 11%) have been achieved in the laboratory. As various absorbers can be used to create colored or transparent devices, OPV is particularly well suited to the flexible architectural design and building-integrated PV (BIPV) market.

Dye-sensitized solar cells (also known as Grätzel cells) use dye as the photoactive material to produce electricity once it is sensitized by light. DSSCs have achieved certified efficiencies of 11.9% and may benefit from low-cost materials, simple assembly, and the possibility of flexible modules. Key challenges include limited long-term stability under illumination and high temperatures, and low absorption in the near-infrared.

QDPV use solution-processed nanocrystals to absorb light and hold the promise of significantly reducing material requirement due to their high absorption qualities. Although cell efficiencies remain relatively low at around 9%, quantum dot cells have attracted research interest due to their inherently low cost, versatility, and light weight.