Thin-film Solar Overview | Cost, types, application, efficiency Comments Off on Thin-film Solar Overview | Cost, types, application, efficiency

Thin-film solar cells (TFSCs) are the second-generation solar cells that have multiple thin-film layers of photovoltaic or PV materials. This is the reason why thin-film solar cells are also known as “Thin-film Photovoltaic Cell.” These solar cells have a very thin layer of thickness (few nanometers) compared to conventional P-N junction solar cells. These layers are usually 300 – 350 times smaller than the layers of standard silicon panels.

Thin-film solar cells are developed by assembling thin-film solar cells. Typically, these solar cells are created by depositing several layers of photon-absorbing materials layers of photovoltaic or PV materials on a substrate, including plastic, glass, or metal. Thin-film solar cells have widespread commercial usage in several technologies such as copper indium gallium diselenide (CIGS), cadmium telluride (CdTe), and amorphous thin-film silicon (a-Si, TF-Si).

These solar cells are capable of converting solar energy to electrical energy by applying the principle of the photovoltaic effect. The thickness of the film thickness has variances from a few nanometers (nm) to tens of micrometers (µm). Due to this, thin-film solar cells are way thinner than the other contemporary technology, the conventional, first-generation crystalline silicon solar cell (c-Si). Crystalline silicon solar cells have wafers of up to 200 µm thick. 

Compared with the crystalline cells, thin-films are more flexible and lighter in weight. These are used in developing integrated photovoltaics and as semi-transparent, photovoltaic material that is possible to laminate and use in windows. Rigid thin-film cells have other commercial applications (inserting or interleaving between two panes of glass) in some of the largest photovoltaic power stations in the world.

The thin-film technology has been relatively economical, though have lesser efficiency compared to the traditional c-Si technology. However, due to constant research and development, this technology has significantly improved over the years. The efficiency for CdTe and CIGS cells is now over 21 percent, which has outperformed multi-crystalline silicon, the dominant material that is currently found in most solar PV systems. 

Under the laboratory condition, life-testing of thin-film modules shows that the degradation of these cells are faster compared to conventional PV, though the expected lifetime of these cells is 20 years or more. Despite all these testing and development, the market share of thin-film cells has not gone beyond 20 percent during the last two decades and has dipped to about 9 percent in recent years considering the worldwide installations of photovoltaic solar cells. 

There are various other thin-film technologies that have come up, and are still in their early stages of research or have limited commercial availability. These technologies are often terms as the “third-generation photovoltaic cells,” including dye-sensitized, organic, quantum dot, perovskite, micromorph, copper-zinc, tin sulfide, and nanocrystal solar cells.

One of the main obstacles that came in the way of large-scale production and expansion of photovoltaic (PV) systems has been the steep price of the solar cell modules. Later, researchers developed one of the solutions to reduce this cost is by creating thin-film solar cells. These solar cells save both materials and energy when producing cells and modules. 

The concept or the motivation to develop thin-film solar cells was coined dates back to the inception of photovoltaics. It was an idea based on achieving low-cost photovoltaics that is suitable for mass production and development and energy-significant markets. The key to the idea was using active material that was relatively cheaper. As the sunlight carries lesser energy compared with combustion-based energy sources, photovoltaic (PV) modules must be cheap enough to produce energy that can be competitive. It was assumed that thin-films was going to be the answer to that low-cost requirement. 

Thin-film solar cells were originally developed in the 1970s by researchers of the Institute of Energy Conversion at the University of Delaware in the USA. Subsequently, these solar cells became well-known during the late 1970s, when solar calculators got their power from a small strip of amorphous silicon that came on the market during that time. These days, the material exists in very large modules that are used in sophisticated building-integrated installations and in the systems for charging vehicles.

Until the 1980s, the applications of thin-film solar cells were still limited to using the small strips of silicon material in calculators and watches. Later, during the early 21st century, the potential for thin-film applications increased significantly due to their flexibility, which enabled their installations on curved surfaces and use in building-integrated photovoltaics.

The increased applications of thin-film cells encouraged to set a more ambitious long-term goal of nearly $1.5/W for systems, at operating conditions, or around $1.2/Wp for module efficiencies under standard measurement. Although the long-term goal of PV was also to be competitive in the markets such as for U.S. utilities, an increasing number of interim markets allowed the PV to evolve toward the goal when generating considerable revenue and profits. 

During that time, many consumers found PV useful, and valued its efficiency and paid for PV electricity at greater than 6 cents/kWh. In today’s PV market, the electricity of about 150 MW/year is sold at prices that are nearly 7 times the goal, which is $8/Wp. The annual sales of PV systems are nearly $1.2 billion. 

It is, however, still a fact that both standard and rigid photovoltaics (e.g., classic crystalline silicon panels), have higher efficiency compared to thin-films. Except for cadmium telluride thin-films, non-flexible photovoltaic cells have higher yields and faster payback times, and also they are more durable due to their sturdy construction. There are certain advantages of both types of solar cells, and it depends on what consumers prefer, and the highest efficiency of these solar cells for a particular application.

Thin-film solar cells have continuously improved and provided increased efficiency, and thus, it was predicted that these solar cells could take over the market of the classic inflexible photovoltaic technologies. Thin-film sheet can increasingly be used to generate electricity in places where it could be a challenge to use photovoltaic cells. It has been possible to use thin-film cells on curved surfaces of buildings or vehicles, or even on clothing to charge handheld devices. Such unique applications of thin-films could pave the way to achieve a sustainable energy future.

It was expected that the thin-film technology would take more leaps towards a significant advancement in the market and to surpass the predominant traditional crystalline silicon (c-Si) technology in the long-term. However, the market share of thin-film solar cells has been declining during the past several years now. In 2010, amid the shortage of traditional PV modules, thin-film had 15 percent share in the overall market, which dropped to 8 percent in 2014, and further came down to 7 percent from 2015. Though the good news is that thin-film cells have the potential to grow over 16% from 2016 to 2024. It is because the governments in the countries all over the world are now initiating grid integration of renewable technologies, that is likely to drive the thin-film solar cell market.   

Thin-film solar cells provide more efficient ways to generate electricity from sunlight than any other solar cells. 

It is comprise of amorphous solar cells and are characterized by its simple manufacturing process, lightweight, and flexibility in applications. 

Features

• The installation process of thin-film solar panels is way easier and takes less effort than the conventional silicon panels.
• Thin-film solar panels are lightweight and flexible, and thus can be applied in the areas where traditional solar panels cannot be installed.
• These solar cells contain a significantly lesser quantity of silicon, and therefore emissions during their production are also quite low as compared to the production of standard solar panels.

The structural features and functioning of thin-film solar cells are almost similar to standard silicon wafer cells. The only difference is in the usage of different thin, flexible layers and the application of the basic solar substance. The arrangement of the thin, flexible layers helps in producing a thin form of cells that is more efficient than the traditional silicon wafer cells.

In terms of efficiency, however, the thin-film modules have lesser efficiency compared to the solar modules available on the market. Particularly, they are lesser yields than silicon solar panels, and due to which thin-film cells are much less expensive. Though thin-film modules are suitable for use in large and flat areas.

In addition, thin-film cells can also be used in dim or weak lighting conditions, and they are less heat-sensitive. Moreover, the manufacturing process these solar cells is simple and requires low resource costs. Amorphous solar modules are much cheaper than the crystalline solar modules. 

The level of efficiency of thin-film modules is between 6 and 10%. It means for these solar cells to achieve the same performance as the crystalline modules, thin-film modules need to be installed in a comparatively larger area. 

The performance of thin-film solar modules is reduced due to degradation. There are basically three types of degradation: the initial degradation or light-induced degradation,  age-related degradation, and potential-induced degradation.

More details about these three types of degradations are below:

• In case of initial or light-induced degradation in thin-film solar panels, manufacturers of these modules calculate this initial degradation by measuring the nominal output of modules.
• Age-related degradation is often more evident in thin-film modules as these modules are susceptible to this type of degradation compared to crystalline. It means, these solar cells are less resistant to aging and have a shorter lifetime. This is the reason why manufacturers give shorter guarantees for these modules.
• Potential-induced degradation is about performance degradation in solar cell modules. Even though potential- and light-induced degradation happens only once, but they can equally affect all types of modules.

Applications

• Implementation of thin-film solar panels needs a larger area. These modules can be installed in commercial/institutional buildings, forest areas, streets, and in large rooftops/open spaces.
• Thin-film solar panels are suitable for use in solar farms.
• These solar panels can also be used in street lights and traffic.
• It is possible to install these solar panels on the rooftop of buses/RVs to power small appliances, fans, Wi-Fi modems, and others. The solar cells can also help in maintaining the temperature of a bus.
• Installation of thin-film solar panels can be carried out in large-sized steel water tanks to provide electricity for pumping water.

Thin-film modules react to changes in temperatures. However, the reaction is lesser than the crystalline modules.

These solar cells can generate constant solar output at comparatively high temperatures, as they are less sensitive to the change in lighting conditions. The cells can also maintain the same level of performance in weak or diffuse lighting conditions.

In addition, thin-film solar panels can be installed in areas that do not have a direct south-facing orientation. The modules are suitable for PV mounting on flat roofs as well as on large solar PV systems, where a large number of solar modules can be installed.

The structure and function of thin-film solar cells are closely linked with any standard solar cells. It means the basic science behind thin-film solar cells is the same as conventional silicon-wafer cells.

A typical thin-film solar cell does not have a metal grid for the electrical contract, unlike most single-crystal cells. Instead, the cells have a thin layer of a transparent conducting oxide, like tin oxide. These oxides are highly transparent and conduct electricity quite easily. An additional antireflection coating might be applied on the top of the device unless the transparent conducting oxide does a similar function.

Conventional solar cells use silicon in the p-type and n-type layers. The latest generation of thin-film solar cells has thin layers of either copper indium gallium diselenide (CIGS) or cadmium telluride (CdTe) instead. The Nanosolar company, based in San Jose, California, has has been able to develop the CIGS material as an ink comprising nanoparticles. A nanoparticle has at least one dimension less than 100 nanometers (one-billionth of a meter, or 1/1,000,000,000 m). The four elements, existing as nanoparticles self-assemble in a uniform distribution and make sure that the atomic ratio of the elements is correct.

Photovoltaic or PV cells depend on substances known as “semiconductors.” Semiconductors work as insulators in their pure form, and they can conduct electricity when heated or combined with other materials. Mixing a semiconductor with phosphorus can develop an excess of free electrons, which is known as an n-type semiconductor. A semiconductor mixed or doped with other materials, including boron, develops excessive “holes” or spaces that accept electrons. This is known as a p-type semiconductor.

There are basically two fundamental configurations of a CIGS solar cell. The CIGS-on-glass cell needs to have a layer of molybdenum to create an effective electrode. This additional layer is not essential in the CIGS-on-foil cell as the metal foil functions as the electrode. A layer of zinc oxide (ZnO) works as the other electrode in the CIGS cell. Existing in between are two more layers, which are cadmium sulfide (CdS) and the semiconductor material. These two layers work as the n-type and p-type materials, which are required to create a current of electrons.

The structure of CdTe solar cell is similar in structure as above. In this solar cell, one electrode is made from a layer of carbon paste infused with copper, and the other from tin oxide (SnO2) or cadmium stannate (Cd2SnO4). In this case, the semiconductor is cadmium telluride (CdTe), and along with cadmium sulfide (CdS), it creates the pt-type and n-type layers that are required for the PV cell to work. 

A PV cell connects p-type and n-type materials, with a layer in between, which is known as a “junction.” Even when the light is absent, a moderate number of electrons move across the junction from the n-type to the p-type semiconductor and produce a small voltage. When the light is present, photons dislodge a large number of electrons that flow across the junction to create a current. This current can be used to provide power for electrical devices, starting from light bulbs to smartphone chargers.

Now, about the efficiency of thin-film solar cells compared to conventional solar cells. Theoretically, the maximum efficiency for silicon-wafer cells is nearly 50 percent; which means half of the energy hitting the cell is converted into electricity. However, in reality, on average, silicon-wafer cells achieve 15 to 25 percent efficiency. In comparison, thin-film solar cells have become more competitive. The efficiency of CdTe solar cells has been a little more than 15 percent, and CIGS solar cells have 20 percent efficiency.

The structure of polycrystalline thin-film cells contains many tiny crystalline grains of semiconductor materials. The materials present in these polycrystalline thin-film cells have different properties than the ones in silicon. Polycrystalline seems to function better to create the electric field with an interface between two different semiconductor materials. This type of interface is termed as “heterojunction” or “hetero” as its formation is from two different materials, compared to the “homojunction,” which is formed by two doped layers of the same material, similar to the one in silicon solar cells.

A typical polycrystalline thin film contains a quite thin layer with less than 0.1 microns on its surface, and it is called the “window” layer. The role of a window layer is to absorb light energy from the high-energy end of the spectrum. So, it must be thin enough and have enough band gap or width (2.8 eV or more) to let all the light through the interface to the absorbing layer. The absorbing layer must have a high ability to absorb photons for high current. It should also have a suitable bandgap to provide a steady voltage. 

There have been health concerns regarding the use of cadmium in thin-film solar cells. It is because cadmium is a highly toxic substance, just like mercury, and can accumulate in food chains. This is a gray area for a technology that aims to stand out as part of the green revolution. Several organizations, like the National Renewable Energy Laboratory as well as other agencies, are currently looking to find ways to make cadmium-free thin-film solar cells. Several of these technologies are said to be as efficient as the ones that use Cadmium.

There are four types of thin-film solar cells: 

• Cadmium Telluride (CdTe) 
• Amorphous Silicon (a-Si)
• Copper Indium Diselenide (CIS) 
• Gallium Arsenide (GaAs)

Cadmium Telluride (CdTe)

Cadmium telluride (CdTe) thin-film solar cells are the most common type of thin-film solar cell. They are more economical compared to the standard silicon thin-film cells. The highest level of efficiency that Cadmium telluride thin-films have recorded is more than 18 percent. The efficiency is measured by the percentage of photons that hit the surface of the cell and transformed into an electric current. Until 2014 CdTe thin-film technologies had the lowest carbon footprints and the fastest payback time of any thin-film solar cell technology available on the market.

CdTe has a nearly ideal bandgap of 1.44 eV and also has a quite high level of absorbing ability.  Although CdTe is most commonly used in PV devices without being alloyed, it can easily be alloyed with mercury, zinc, and a few other elements to vary its properties. Similar to Copper indium diselenide (CIS), the films of CdTe can be produced using low-cost techniques.

Thin-film technology that has the closest to potential to commercialize is based on CdTe. Even though this type of solar cells have high efficiency, however, commercial module efficiency is likely to be in the range between 7% and 9% range. It is because there is a higher possibility of a lag in the first phase of production in incorporating leading-edge lab results in initial manufacturing. 

There is a common perception that that CdTe is the easiest among the thin films to fabricate. So far, over a dozen methods have been tried to make 10% cells. This experimentation allows the potential manufacturers to pick the method that is least expensive. Currently, the most popular methods are screen printing/sintering, high-rate sublimation, and electrodeposition. Their low capital costs provide a significant advantage for CdTe. 

An underlying problem with CdTe modules is that they have much lesser efficiency than CdTe cells. The main issue lies in the use of a thin n-CdS layer to form the junction with p-CdTe. The CdS layer must be thin enough to allow high energy light that is above the 2.45-eV CdS band gap, to reach the junction. However, the CdS must have adequate quality and coverage to make a steady and high-voltage junction. It has been a challenge to achieve such conflicting goals in cells. Though this is a method of making the best cells, this has not been possible to in case of modules so far.  

The main obstacle in the research in CdTe technology is the ability to achieve high current and high voltage with thin CdS. There are two other issues, which are stability and cadmium. When it comes to stability, many CdTe cells and modules have been developed with great stability. However, under extreme stress, the cells degrade. The mechanisms of degradation could be oxidation at the contact, copper diffusion to the junction, or humidity-driven corrosion at the contact. 

In the current scenario, it seems that these stress-driven mechanisms will not limit the reliability of CdTe. Also, issues related to cadmium are another area of concern. A very amount of  cadmium is used in CdTe modules, that is enough to create health hazards. It is still unknown if commercial CdTe modules will be considered as hazardous waste because some modules are said to have passed the test of the U.S. Environmental Protection Agency, whereas others have not. For example, fluorescent lights containing mercury and computer screens containing lead that do not pass this test.  

The strategies of In-plant waste-handling at PV plants have been excellent, where suppliers like ASARCO has agreed to take back (for re-smelting) any in-plant wastes. In addition, the strategies of engineering and management to minimize worker hazards have been great too. Ultra-modern engineering controls have been implemented, and biomonitoring has been in use for years with no significant increase in workers’ exposure to hazards. Tests have been conducted to test the level of toxicity of CdTe devices by ingestion by the National Institutes of Health. These tests have shown that the material is quite indigestible, and usually does not enter the body by this route. 

Also, there are ongoing examinations of cost-effective ways to assure future product recycling of these modules. Two companies that have so far achieved success in developing the technology for stripping and recycling CdTe films. However, the concern still prevails about cadmium in CdTe modules, and it is still viewed as manageable. PV offers certain advantages like offsetting other sources of energy. Considering that, the issue about cadmium should be seen from the same point of view as other similar issues, such as mercury in fluorescent lights. 

Because of the nature of thin-film CdTe modules, the application of cadmium in PV is always going to remain a small fraction (below 10%) of the existing use of cadmium in the world. It is likely to remain so even in the future, CdTe modules are able to provide a significant fraction (more than 10%) of the total electricity generated in the world. 

Overall, the CdTe technology has the following underlying issues:

(1) The technology has the possibility of commercialization in the future, and is likely to encounter the issues of typical start-ups;

(2) It does not remain stable under severe stress, although the stability is perfect outdoors; and 

(3) It is important to up the efficiency of modules, as it needs solution of processing challenges associated with using thin CdS in manufacturing. 

If these issues are resolved over the next 10 years, CdTe technology has a great potential to achieve the long-term goals related to cost, performance, and stability could be considered as the leading thin film.

Amorphous Silicon (a-Si)

Amorphous silicon (a-Si) thin-film cells are the earliest and most mature type of thin-film. These solar cells are produced by using noncrystalline silicon, unlike typical solar-cell wafers. Amorphous silicon is less expensive to manufacture compared to crystalline silicon as well as most other semiconducting materials. This type of silicon is also popular due to its abundance, nontoxicity, and cost-effectiveness. However, the average efficiency level of these cells is significantly low, which is 10 percent.

In the 1980s, amorphous silicon used to be considered as the “only” available thin film. However, by the end of that decade, and in the early 1990s, many had written off this solar cell. Despite that, amorphous silicon technology has made notable progress and developed as a sophisticated solution or multijunction modules or cells to most of its problems. Now, seems that that commercial, multijunction a-Si modules may have the range of efficiency between 7% and 9% in the near future. 

As stated earlier, amorphous silicon is a useful and attractive solar cell material because it is abundant and non-toxic material. This solar cell requires a low-processing temperature and makes scalable production a possibility upon a flexible, low-cost substrate with the requirement of little silicon material. The bandgap of amorphous silicon (1.7 eV), it is capable of absorbing a broad range of the light spectrum, which includes infrared and even some ultraviolet and can perform really well in dim light. It allows the cell to produce power in the early morning, or late afternoon and even on cloudy and rainy days, which is a contrast to crystalline silicon cells. In fact, crystalline silicon cells have a significantly lesser level of efficiency when exposed at low and indirect daylight.

The efficiency of a-Si cells usually drops significantly of nearly 10 to 30 percent during the initial phase of operation, which is called the Staebler-Wronski effect (SWE). Simply put, SWE is a typical loss in electrical output because of changes in photoconductivity and dark conductivity due to prolonged exposure to sunlight. This degradation is totally reversible upon annealing at or above 150 °C. In contrast, traditional c-Si solar cells hardly ever show such degradation effect.

It is a reality that all PV technologies go through tough phases, and some (with adequate investment) come with realistic chances to succeed. However, there are still a number of serious issues with amorphous silicon technology. Currently, the best cell efficiencies (with stability) are nearly 12%, which is almost 50% lower than copper indium-gallium selenide (CIGS). However, outdoors, CIGS loses as much as 20% of its output (because of operating temperature), whereas a-Si loses around 5%. 

Still, the issue of low efficiency (modules with below 10% efficiency) could prevent the cost of a-Si from dropping below $0.5/Wp manufacturing cost. In the range of $0.5-$1.5/Wp, a-Si may have a significant impact on the PV marketplace. However, the future of this solar cell will remain limited if it is not able to overcome the obstacle of 10% efficiency in power modules. These days, the multi-band gap and multijunction designs are driven by the demand to develop thin layers to minimize the Staebler-Wronski Effect. Due to this, major research efforts are focusing on the component cells and their optimization. 

In recent times, the efficiencies of solar cells have improved drastically, primarily due to the success of a handful of organizations like United Solar. The efficiency of their solar cell has improved from 10% to more than 12% in the last three years. The efficiencies of modules have also gone up, and a lot of effort has been put in to bring amorphous silicon into multi-MW production. Another issue with amorphous silicon that its manufacturing cost is associated with initial capital investment and with applicability in multi-junctions. The capital costs are in the range of $3/Wp of annual capacity amortized over 7 years cost are more than $0.4/Wp ($35/m2 ), which is considered quite high. 

There are two proposed solutions to this cost problem are higher rates and simultaneous batch processing of multiple modules. Notable progress has been made when it comes to rates that are 3-10 times higher than those used in production. This is a potential solution that has been proved by a small company, EPV. It has shown lower-performance same-band-gap multijunction modules using a batch process that can produce 48 modules in the same reactor. 

Using these lower-cost approaches, if it is possible to maintain or improve module performance, it will help amorphous silicon  to progress to low cost significantly. The second issue, germane use or application, which is also a serious concern because germane in gas form costs nearly $4000/kg. Poor usage of around 5%-25% for existing amorphous silicon processes, the cost can be more than $10/m². 

Copper Indium Diselenide (CIS) 

Copper Indium Selenide (CIS) is a p-type or absorbing layer material. Photovoltaic cells or (PV Cells) that are CIS-based and used for generating solar energy are fabricated from a p-type or positively charged CIS layer below an n-type or negatively charged layer. The p-type layer can be produced by the vapor deposition of thin-film physical/chemical of a CIS. 

CIS cells are developed using a thin layer of CuInSe² on plain glass or flexible metal backing. Another common variation of CIS cell is the copper indium gallium diselenide CIGS. The level of efficiency of CIS cells is usually around 14% similar durability as silicon solar cells. The efficiency can even reach more than 20% under standard test conditions. As of January 2019, the highest conversion efficiency of 23.35% was achieved by a company with New Energy and Industrial Technology Development Organization.

The improved level of efficiency proves that the best CIS cell is nearing the highest level of efficiency of a polycrystalline silicon cell. This is a strong case that shows thin films can perform well. However, CIS cells have major obstacles to overcome to be successful in the market. However, CIS cells have an advantage over conventional silicon solar as they are much less expensive to manufacture and are considerably more versatile and flexible.

The cost of CIS can be lesser than Si cells because the former is a thin-film technology. These cells have an open circuit voltage of 5 V DC and a short circuit current of 95mA. Maximum power of these cells is 3.9V and 64 mA (.25 watts).

Most PV solar cells that are CIS-based can be produced from single crystals of CIS, as the recent technology allows thin film deposition of CIS (using sputtering techniques). Companies like ‘American Elements’ produces many standard grades when applicable, such as ACS, Reagent and Technical Grade; Mil-Spec (military grade); Food, Agricultural and Pharmaceutical Grade; Optical Grade, and others by following the applicable American Society for Testing and Materials (ASTM) testing standards. 

The production effort of CIS always ran into difficulties due to reasons starting from poor adhesion between the CIS and the bottom contact (molybdenum) to irreproducible deposition of the CIS. These issues prevented the efforts from achieving high output at high efficiency. Though over the years, with continued research and development, critical CIS issues have been addressed, and most of the manufacturing problems are said to be resolved. 

Meanwhile, the federal laboratory the National Renewable Energy Laboratory (NREL) found a way to produce high-quality CIS cells and reached 17.7% efficiency in the mid-1990s. They did so by including gallium and graded layers in their cells, which made it possible to achieve both improved morphology (larger grains) and better electronic properties. The work of NREL was based around the previous efforts by Boeing (now discontinued) and by EuroCIS, the European consortium of universities. These days, CIS and its alloys with gallium and sulfur, have regained their significance, while some corporations like Siemens started commercial production of CIS modules. 

The long and challenging history of CIS is an example of unexpected obstacles that may happen and can be overcome. Outdoor reliability has never been a problem with CIS cells. Earlier, the tests carried out by NREL shows that the efficiency of CIS modules and a 1-kW system (SSI) have been excellent, and there has not been any degradation of any modules. Improved stability and proven efficiency over the years have made CIS a quality thin film. 

Copper Indium Gallium Selenide (CIGS) 

Copper Indium Gallium Selenide (CIGS) is a variant of CIS and comprises a thin layer of copper indium gallium diselenide Cu(In, Ga)Se2 (CIGS). The efficiency of CIGS solar cells is up to 10% with almost the same durability as silicon solar cells. Though CIGS cells are part of thin-film technology, the cost of these cells can be lesser than crystalline silicon. In addition, these cells are more versatile and less sensitive to temperature. 

CIGS cells have got this name because they use an absorber that is made with copper, indium, gallium, selenide (CIGS), whereas the gallium-free variants of the semiconductor material are known as CIS. CIGS is one of the three mainstream thin-film technologies besides cadmium telluride (CdTe) and amorphous silicon (a-Si), with a lab-efficiency of more than 22.4% percent. Though, as of April 2019, the current conversion efficiency for a laboratory CIGS cell went up to 22.9%. Also, the market share of CIGS is likely to reach $8.5 billion by 2024, which will be a compound annual growth rate (CAGR) of 17.3%.

CIGS cells have more efficiency than a-Si cells. However, these cells are less efficient and more costly compared with CdTe solar cells. Another issue with CIGS cells is that Indium is a metal that is not easily available and is supplied mostly from China. Despite this, CIGS cells have the edge over CdTe cells as they are relatively non-toxic, whereas the cadmium used in CdTe cells is highly toxic chemical.

The traditional methods of fabricating CIGS involves vacuum processes such as co-evaporation and sputtering. In 2008, Tokyo Ohka Kogyo Co., Ltd. (TOK) and IBM announced that they had developed a new, non-vacuum, solution-based manufacturing process for CIGS cells and their target was to achieve 15% or more efficiency level. 

The method of hyperspectral imaging is being used to characterize CIGS cells. Researchers from the Institute of Research and Development in Photovoltaic Energy (IRDEP), in collaboration with Photon were able to figure out the splitting of the quasi-Fermi level with photoluminescence mapping while using the electroluminescence data to derive the external quantum efficiency (EQE). Moreover, the EQE of a microcrystalline CIGS solar cell could be determined at any point in the field of view, through a light beam induced current (LBIC) cartography experiment.

Currently, CIGS-based thin-film solar cell modules have the highest-efficiency alternative for large-scale, commercial thin-film solar cells. During the early years, several companies had confirmed about module efficiencies of over 13%. Since then, the efficiency of CIGS has taken leaps to reach the current level of 22.9% efficiency. Back then, most of the companies were using ideas and intellectual properties that the NREL CIGS Group developed during the past two decades of research. The research NREL mainly revolved around the development of the “three-stage process.” 

The “three-stage process” enables the formation of a CIGS thin-film layer that has a proper composition and structure to allow the charges generated by sunlight (i.e., electrons and holes). The sunlight needs to exist long enough in the CIGS layer of the device before being separated and collected at the front and back contacts. This process of separation and collection is critical for exhibiting high conversion efficiency.

The high conversion efficiency both in laboratory settings and in the field have made CIGS a leader among alternative cell materials in thin-film technologies. Traditionally, CIGS cells have been costlier than other types of solar cells on the market, and for that reason, they were not widely used for long. 

Various deposition techniques for CIGS solar cells in the future 

In the future, CIGS solar cells may be produced through various techniques such as chemical vapor deposition, co-evaporation, electrospray deposition, and film production. The electrospray deposition technique involves the spraying of ink (with the assistance of electric field) containing CIS nano-particles directly onto the substrate and then sintering in an inert environment. 

The chemical vapor deposition (CVD) segment had the major market share in 2017 and is expected to grow at CAGR of 7.92% from 2019-2024. CVD processes include atmospheric pressure metal-organic CVD, plasma-enhanced CVD, low-pressure MOCVD, and aerosol assisted MOCVD.

Gallium Arsenide (GaAs)

The composition of Gallium Arsenide (GaAs) contains two base elements: gallium and arsenic. When these two separate elements bind together, they form the GaAs compound, which displays numerous interesting characteristics. Gallium arsenide is a semiconductor that has greater saturated electron velocity and electron mobility than silicon. 

A semiconductor is a material that has electrical conductivity between a conductor and an insulator, and its ability to conduct electricity may vary with the increase and decrease in temperature. This makes GaAs useful in many applications. Another major feature of gallium arsenide is that it has a direct band gap, which means it can efficiently emit light.

Also, because GaAs has higher electron mobility than silicon, it can be used in various ways that silicon cannot. Transistors made of this material can run at frequencies over 250 GHz. These transistors generate less noise when operating at the same high frequencies as their silicon counterparts. Gallium arsenide also has a higher breakdown voltage. Breakdown voltage is the minimum (reverse) voltage used that can partially make the component electrically conductive (or conduct in reverse).

Considering these factors, GaAs has been suitable for many electrical applications ranging from the common to the extraordinary. Some of these applications include satellites, cellular phones, satellites, and satellite communication, radar systems, micro, and nano-scale semiconductors, and even nano-based solar power.

GaAs is also used for single-crystalline thin-film solar cells. Even though GaAs cells relatively expensive, they hold the record for the highest-efficiency of 29.1% in 2018, produced by the company Alta Device. The most common use of GaAs is 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). These cells are also used in concentrator photovoltaics, which is an emerging technology and ideal for locations that receive more sunlight. It uses lenses to focus sunlight on a much smaller and less expensive GaAs concentrator solar cell.

It is possible to apply some unique methods on the nanoscale to fabricate gallium arsenide heterostructures. This does require another compound to be present. A few common methods of creating these structures are metalorganic vapor phase epitaxy and molecular beam epitaxy. These processes allow these compounds to grow in a crystalline form. Some of the common metals for combining heterostructures with GaAs are manganese and aluminum. 

Safety

GaAs contains both gallium and arsenic. Gallium is said to have been found as non-toxic. However, many sources find this information to be non-conclusive. Contact with Gallium may cause skin diseases such as skin irritations or even dermatitis. On the other hand, arsenic, which is both a toxic chemical and carcinogen, has been found to be stable in this compound. Due to this, arsenic does not put its users in any immediate danger. It can also pass through the digestive system with negligible arsenic absorption.

As stated earlier, GaAs thin-film solar cells have reached nearly 30 percent efficiency in laboratory environments. However, they are still relatively expensive to produce. The cost has been a major constraint in the way of expanding the market for GaAs solar cells. They are commonly used for spacecraft and satellites.

Technology

The earlier solar panel technology used silicon semiconductor for producing p-type and n-type layers and has several disadvantages. However, in the case of thin-film layer technology, the silicon semiconductor material is replaced by either cadmium telluride (CdTe) or copper indium gallium diselenide (CIGS).

The National Renewable Energy Laboratory (NREL) has classified several thin-film technologies as emerging photovoltaics, though most of those technologies are still in the research and development phase, and yet to be commercially applied. Many of these technologies use organic materials, such as organometallic compounds and also inorganic substances. 

These technologies have a lack of efficiency, and also the stability of the absorber material has often been too short for commercial applications. A lot of research is being carried out on these technologies as they have the potential to achieve the goal of producing low-cost and energy-efficient solar cells.

These emerging photovoltaic technologies are called third-generation photovoltaic cells, which include:

• Organic solar cell
• A dye-sensitized solar cell or “Grätzel cell”
• Copper zinc tin sulfide solar cell (CZTS)
• Quantum dot solar cell
• Perovskite solar cell

The achievements in the research and development of perovskite cells have received accolades as the research efficiency soared above 24 percent in 2019. They also offer a broad spectrum of low-cost applications. In addition, another new technology, concentrator photovoltaics (CPV) applies highly-efficient, multi-junction solar cells combining optical lenses and a tracking system.

Efficiency

Over the years, gradual improvement in the efficiency of thin-film solar cells started when the first modern silicon solar cell was invented in 1954. Subsequently, with constant trial and error, the level of efficiency increased to 12 to 18 percent by converting solar radiation into electricity by the early 2000s.

The thin-film materials have the potential to reach a high efficiency of over 20%. The highest efficiency of 22.4% was achieved in the case of Copper Indium Gallium Selenide (CIGS) by the company Solar Frontier. The company achieved this level of efficiency through joint research with the New Energy and Industrial Technology Development Organization (NEDO) of Japan. Solar Frontier achieved the conversion efficiency on a 0.5 cm2 cell using its CIS technology. This is an improvement of 0.6 percentage points more than the industry’s earlier thin-film record of 21.7%.

Also, for Gallium Arsenide (GaAs), which is an expensive technology, the highest efficiency has been 28.9% for all single-junction cells, and it is a world record. However, it is still not possible to completely rely on these performance metrics.

Earlier, the thin-film cell prototype with the best efficiency yielded 20.4% by the First Solar, compared to the best traditional solar cell prototype efficiency of 25.6% from Panasonic.

Solar cells made using newer materials tend to be less efficient than bulk silicon, though they are less expensive to manufacture. The quantum efficiency of those cells is also lower because of the reduced number of collected charge carriers per incident photon.

Thin-film solar cells have one major disadvantage, which is lesser efficiency in converting sunlight into electricity compared to silicon wafers. Researchers at Colorado State University Next Generation Photovoltaics Center, with a collaborated effort with scientists at Loughborough University in the United Kingdom, have found that adding selenium to the mix can improve the efficiency of thin-film solar cells to around 22%. Fortunately, laboratory efficiency of more than 22% has been achieved by some of the leading companies that had been experimenting on these solar cells for years.

Even the scientists were not able to answer why adding selenium to the mix increased the efficiency of thin-film cells. Their experiment showed that selenium has the ability to overcome the effects of atomic-scale defects in the cadmium telluride crystals. Electrons generated when sunlight enters the selenium-mixed solar panel does not get trapped and lost in the defects and increases the amount of power extracted from each solar cell. The results of the research were published in the journal Nature Energy.

Laboratory test results, however, often do not translate into commercially into viable products. Through research like this, funded partly by the National Science Foundation, is one of the reasons that keeps bringing down the price of solar energy and making it a challenge for fossil fuels to keep up. 

Advantages

• Among all other types of solar panels, thin-film panels have the maximum potential for mass production. It is because these solar cells rely on different photovoltaic substances such as amorphous silicon, copper indium gallium selenide, and cadmium telluride, and do not rely purely on molten silicon when it comes to production.
• Easy to handle
• More flexible than traditional solar cells
• Available as thin wafer sheets
• Thin-film solar panels many applications such as powering Wi-Fi, a portable heating device for shavers, hot water showers, and as a non-conventional power source. 
• Thin-film panels are not affected by the environment, such as by shade or high temperatures. 
• Cheaper than traditional solar panels.

Disadvantages

• Less efficient (only 20 to 30% of light is converted into electricity)
• Complex structure
• More space needed considering the current level of efficiency. Nearly 50% more space is required for installing thin-film solar cells to generate the same amount of electricity as traditional solar panels. 
• Heat retention is high. It is because thin-film solar cells are usually applied directly to a surface, and they retain more heat, which does not allow to cool panels easily. 
• The cost of fabricating thin-film cells makes it large-scale production a difficult proposition. 
• Need more care when handling.

Manufacturing

These days, it is possible to have large-scale production of thin-film solar panels. To produce these solar panels, manufacturers first spray the photovoltaic (PV) substances onto a solid surface similar to glass, and from which a solar panel is made.

The manufacturing process depends on various PV substances such as amorphous silicon (a-Si), copper indium gallium selenide (CIGS), and cadmium telluride (CdTe). Unlike the conventional solar panels, thin-film solar panels do rely on quality molten silicon ingots for production.

The following are the leading manufacturers of thin-film PV:

• First Solar – First Solar is a leading company in producing the CdTe thin-film solar cells. As of now, First Solar has only served the commercial market. The company holds the world record for CdTe cell efficiency at 18.7%. 
• Solar Frontier – Solar Frontier is one of the top manufacturers for CIGS thin-film. The company has achieved the world record for thin-film conversion efficiency at 22.4%.
• SoloPower – SoloPower is a company based in San Jose, California, and well-known for ultra-lightweight thin-film design, and applying for building-integrated photovoltaics. The company is one of the largest manufacturers of CIGS solar cells. 
• Sharp – Sharp Solar is a leading producer of thin-film cells in the world and has been in for more than 50 years. The is the top producer a-Si solar cells. 

Cost of thin-film solar

Thin-film solar cells are cheaper than traditional solar cells that are made from crystalline silicon. On the other hand, thin-film cells, for example, CdTe-based solar cells need far less raw material (up to 100 times less), and lesser manufacturing cost than silicon cells. Thin-film cells also absorb sunlight at nearly the ideal wavelength. Due to this, the power generated by thin-film solar cells is the least expensive available today.

When it comes to cost, most manufacturers are aware that the cost of most solar panels is a barrier for most solar panels to be more accessible for the general public. Keeping this issue in mind, manufacturers have been putting in efforts to reduce costs. The current cost of the thin-film solar cells ranges from $0.50 to $1.00/watt. Many manufacturers have set a target to bring down the cost under $0.70/watt of peak power. It will be cost-effective for residential users to have solar panels at their home, particularly compared with the traditional solar panel, where the average price per watt for solar panels is between $2.58 to $3.38 silicone cell (in the US).

Market Share

According to the latest research report on ‘Thin-film Solar Cell market’ by Market Study Report, LLC,  the Thin-film Solar Cell market will register a 9.8% CAGR in terms of revenue, the global market size will reach US$ 9950 million by 2024, from US$ 6230 million in 2019. 

Market Share

According to the latest research report on ‘Thin-film Solar Cell market’ by Market Study Report, LLC,  the Thin-film Solar Cell market will register a 9.8% CAGR in terms of revenue, the global market size will reach US$ 9950 million by 2024, from US$ 6230 million in 2019. 

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