When talking about solar technology, most people think about one type of solar panel which is crystalline silicon (c-Si) technology. While this is the most popular technology, there is another great option with a promising outlook: thin-film solar technology.
Thin-film solar technology has been around for more than 4 decades and has proved itself by providing many versatile and unique applications that crystalline silicon solar cells cannot achieve. In this article, we provide you with a deep review of this technology, the types of solar panels, applications, and more.
Overview: What are thin-film solar panels?
Thin-film solar panels use a 2nd generation technology varying from the crystalline silicon (c-Si) modules, which is the most popular technology. Thin-film solar cells (TFSC) are manufactured using a single or multiple layers of PV elements over a surface comprised of a variety of glass, plastic, or metal.
The idea for thin-film solar panels came from Prof. Karl Böer in 1970, who recognized the potential of coupling thin-film photovoltaic cells with thermal collectors, but it was not until 1972 that research for this technology officially started. In 1980, researchers finally achieved a 10% efficiency, and by 1986 ARCO Solar released the G-4000, the first commercial thin-film solar panel.
Thin-film solar panels require less semiconductor material in the manufacturing process than regular crystalline silicon modules, however, they operate fairly similar under the photovoltaic effect. This effect causes the electrons in the semiconductor of the thin-film PV module to move from their position, creating an electric flow, that can be harnessed into electricity through an external circuit.
Thin-film solar panels are manufactured using materials that are strong light absorbers, suitable for solar power generation. The most commonly used ones for thin-film solar technology are cadmium telluride (CdTe), copper indium gallium selenide (CIGS), amorphous silicon (a-Si), and gallium arsenide (GaAs). The efficiency, weight, and other aspects may vary between materials, but the generation process is the same.
What are the different types of thin-film solar technology?
There are several types of materials used to manufacture thin-film solar cells. In this section, we explain the different types of thin-film solar panels regarding the materials used for the cells.
Cadmium Telluride (CdTe) Thin-Film Panels
Cadmium Telluride (CdTe) thin-film solar technology was introduced to the world in 1972 by Bonnet, D. and Rabenhorst, H. when they evaluated a Cadmium sulfide (CdS)/CdTe heterojunction which delivered a 6% efficiency. The technology has been improved to reduce manufacturing costs and increase efficiency.
CdTe solar cells are manufactured using absorber layers comprising a p–n heterojunction, which combines a p-doped Cadmium Telluride layer and an n-doped CdS layer that can also be made with magnesium zinc oxide (MZO). To depose materials on the substrate, manufacturers use the vapor-transport deposition or the close-spaced sublimation technique.
On top of the absorber layer, CdTe thin-film solar cells include a Transparent Conductive Oxide (TCO) layer usually made with fluorine-doped tin oxide (SnO2:F) or a similar material. The electrical contact for these cells is made with zinc telluride (ZnTe), and the materials are placed over a metal or carbon-paste substrate.
CdTe thin-film solar panels reached a 19% efficiency under Standard Testing Conditions (STC), but single solar cells have achieved efficiencies of 22.1%. This technology currently represents 5.1% of the market share worldwide, falling second only under crystalline silicon solar panels that hold 90.9% of the market. The cost for CdTe thin-film solar panels rounds the $0.40/W.
Copper Indium Gallium Selenide (CIGS) Thin-Film Panels
The first progress for Copper Indium Gallium Selenide (CIGS) thin-film solar cells was made in 1981 when the Boeing company created a Copper Indium Selenide (CuInSe2 or CIS) solar cell with a 9.4% efficiency, but the CIS thin-film solar cell was synthesized in 1953 by Hahn, H. In 1995, researchers at the National Renewable Energy Laboratory (NREL) embedded Gallium into the CIS matrix to create the first Copper Indium Gallium Selenide (CIGS) thin-film solar cell with a reported efficiency of 17.1%.
Manufacturing for Copper Indium Gallium Selenide (CIGS) thin-film solar panels has improved throughout history. Currently, CIGS thin-film solar cells are manufactured by placing a molybdenum (Mo) electrode layer over the substrate through a sputtering process. The substrate is usually manufactured with polyimide or a metal foil.
The absorbing layer is manufactured by combining a p-n heterojunction. The P-doped layer is made with copper indium gallium selenide (CIGS), placed above the electrode, and the CdS n-doped buffer is formed by chemical-bath deposition.
To protect the absorbing layer of the CIGS thin-film solar panel, a layer of Intrinsic Zinc Oxide (i-ZnO) is placed above the CdS buffer. The materials are finally covered with a thick AZO compound layer made with Aluminium doped Zinc Oxide (Al: ZnO), acting as the TCO layer to protect the cell.
The first CIGS thin-film solar panel manufactured by NREL reported a 17.1% efficiency, but the most efficient one ever created reported an efficiency of 23.4% and was made by Solar Frontier in 2019. The CIGS technology could be even more promising in the future since these materials can achieve a theoretical efficiency of 33%.
CIGS modules are not as popular for regular applications, being mostly used for space applications due to their resistance to low temperatures and great performance under low-intensity light conditions found in space. The cost is relatively more expensive than for other technologies, with a current price slightly above $0.60/W, but future manufacturing generations promise to reduce the cost for these panels.
While CIGS thin-film solar panels have not become as popular as CdTe panels in the market, CIGS technology still holds 2.0% of the PV market share. Considering that thin-film solar modules only hold around 10% of the market, This is still quite popular as a thin-film solar technology.
Amorphous Silicon (a-Si) Thin-Film Panels
The first observation of doping in Amorphous Silicon (a-Si) was achieved in 1975 by Spear and LeComber, a year later in 1976 it was demonstrated that Amorphous Silicon (a-Si) thin-film solar cells could be created. Great expectations have surrounded this technology, but the material represents several challenges like weak bonds, a relatively poor efficiency, and several others.
Unlike other thin-film solar panels, amorphous silicon (a-Si) modules do not include an n-p heterojunction, but a p-i-n or n-i-p configuration, which differs from the n-p heterojunction by adding an i-type or intrinsic semiconductor. There are two routes to manufacture amorphous silicon (a-Si) thin-film solar panels, by processing glass plates or flexible substrates. Efficiency for a-Si solar cells is currently set at 14.0%.
Disregarding the route taken to manufacture amorphous silicon (a-Si) thin-film solar panels, the following steps are part of the process:
First, the substrate is conditioned, the TCO and back reflector are placed under the deposition process, and then thin hydrogenated amorphous silicon (a-Si:H)-based layers are placed onto the electrodes, and the cells are connected in a monolithic series via laser scribing and silicon layers. The module is finally assembled and encapsulated, applying framing and electrical connections.
While manufacturing amorphous silicon (a-Si) requires an inexpensive material in low quantities, the price is relatively expensive, since the conductive glass for these panels is expensive and the process is slow, making the total cost of the panel to be set at $0.69/W. This technology currently holds 2.0% of the retail market for PV modules.
Gallium Arsenide (GaAs) Thin-Film Panels
The first Gallium Arsenide (GaAs) thin-film solar panel was made by Zhores Alferov and his students in 1970. The team persisted to create the gallium arsenide semiconductor, until they made a breakthrough in 1967, three years later they created the first gallium arsenide (GaAs) solar cell. Around 10 years later in 1980, the technology was being researched for specific applications like spaceships and satellites.
The manufacturing process for GaAs thin-film solar cells is more complex than for regular thin-film solar cells.
The first step is to grow the material. During this step, GaAs buffers are grown on Si substrates by being submitted to several temperature changes and different chemical processes, to finally create the layers for the cell.
After the GaAs buffer grows, the substrate is processed for the fabrication of the cell. The first step is to deposit a Platinum (Pt)/Gold (Au) layer (10/50 nm) which will serve as the bonding material and electrode for the GaAs solar cell, and then a bonding process is performed on the substrate.
After the bonding process is completed, the GaAs epitaxial layer that grew on the Si substrate is placed over the new substrate. To complete the assembly process a Pt/Titanium (Ti)/Pt/Au layer of 20/30/20/200 nm is deposited on the top contact layer through electron beam evaporation.
Since GaAs PV cells are multijunction III-V solar cells composed of graded buffers, they can achieve high efficiencies of up to 39.2%, but the manufacturing time, cost for the materials, and high growth materials, make it a less viable choice for terrestrial applications. The rated efficiency for GaAs thin-film solar cells is recorded at 29.1%.
The cost for these III-V thin-film solar cells rounds going from $70/W to $170/W, but NREL states that the price can be reduced to $0.50/W in the future. Since this is such an expensive and experimental technology, it is not mass-produced and is mainly destined for space applications, holding the lowest market share.
Thin-film vs. Crystalline silicon solar panels: What’s the difference?
Before comparing the different types of thin-film solar panels against crystalline silicon solar panels (c-Si), it is important to remark that there are two main types, monocrystalline silicon (mono c-Si) and polycrystalline silicon (poly c-Si) solar panels.
In this section, we compare several aspects of both types of crystalline silicon solar panels against the different types of thin-film solar panels.
|Solar Cell Technology||Crystalline Silicon (c-Si)||Thin-Film|
|Type of Technology||Monocrystalline Silicon (mono c-Si)||Polycrystalline Silicon (poly c-Si)||Cadmium Telluride (CdTe)||Copper Indium Gallium Selenide (CIGS)||Amorphous Silicon (a-Si)||Gallium Arsenide (GaAs)|
|Temperature Coefficient (average)||-0.446%/ºC||-0.387%/ºC||-0.172%/ºC||-0.36%/ºC||-0.234%/ºC||0.09%/°C|
|Highest Recorded Efficiency||26.7%||24.4%||22.1%||23.4%||14.0%||29.1%|
|Price Range||$0.16/W - $0.46/W||$0.24/W||$0.40/W||$0.60/W||$0.69/W||$50/W|
|Applications||Residential / Commercial / Industrial||Commercial / Industrial||Mostly building-integrated photovoltaics||Mostly space applications|
There are many differences regarding crystalline silicon and thin-film solar panel technology. One important difference is how the temperature affects the efficiency of each technology, c-Si solar cells are more affected by temperature than thin-film technologies. In other words, c-Si modules are likely to have higher thermal PV losses than their thin-film counterparts, making them ideal for locations with high or extreme temperatures.
Another difference is efficiency. GaAs PV modules have the highest efficiency, but the manufacturing cost is too expensive, which is why the technology is currently destined for space applications only. The efficiency for c-Si PV modules has stood as the best balance between efficiency and costs for commercial, industrial, utility-scale and especially residential applications. This is why they hold the highest market share.
Crystalline silicon technology is currently cheaper than thin-film solar technology, making it more viable regarding the cost. Considering the coefficient temperature and longer durability for thin-film solar panels, thin-film solar panels can be a better choice in the long run.
Thin-film solar panel applications: When to use them?
Thin-film solar panels have many interesting applications, and they have been growing in the last decade. Below you will find some of the most popular applications for thin-film.
One application starting to become widely popular worldwide is the Building-Integrated Photovoltaic (BIPV) highly dependent on thin-film solar technology. There are two main branches of this technology, solar shingles or solar roof tiles, and solar windows or solar glass.
The goal for both applications is to provide the means to keep aesthetics for homes and buildings while allowing the possibility of solar power generation. This technology integrates thin-film solar technology to provide a certain generation efficiency, which can be used just like with regular c-Si solar panels.
One of the most important applications for thin-film solar technology, specifically Copper Indium Gallium Selenide (CIGS) and Gallium Arsenide (GaAs) technology is the space applications. The technology provides many advantages like being extremely lightweight, highly efficient, having a wide temperature of operation range, and even the damage resistance against radiation, making it ideal for these applications.
Rooftop of vehicles and marine applications
One common application for thin-film solar panels is the installation of flexible PV modules on vehicle rooftops (commonly RVs or buses) and the decks of boats and other vessels. This application allows the installation of modules on curved surfaces, provides solar power generation while keeping practicality and aesthetics for the vehicles and vessels.
An advantage of thin-film solar technology is its portability and size. The technology has been installed for years in calculators, but with much improvement, now there is a possibility of having solar power in remote locations with foldable solar panels, solar power banks, solar-powered laptops, and more.
Due to its versatility, an important focus of thin-film solar technology is commercial applications. While c-Si solar modules hold the largest market share, efficiency for thin-film solar panels is growing and manufacturing processes are becoming cheaper, which could lead to thin-film solar panels becoming the norm for most installations.
Another important focus for thin-film solar panels is the industrial level applications, specifically at the utility scale. Since thin-film solar panels degrade at a much slower pace, they offer a potential alternative to the traditional c-Si solar panels, sometimes providing a better investment over time.
Rounding up: Pros and cons of thin-film solar panels
Thin-film solar panels have many pros, while only holding a few cons to them. These are the most important pros and cons of this technology.
- Higher resistance to degradation.
- Lower thermal losses at extreme temperatures due to the low-temperature coefficient.
- High efficiency for most technologies (CdTe, CIGS, and especially GaAs)
- Ideal for portable and BIPV applications.
- Promising research and development with much more ground to cover.
- Requires less material to create PV modules.
- Thin-film solar panels are lighter than c-Si PV modules.
- Higher retail cost.
- Less availability in the market.
- More installation space is required to achieve the same generation capacity as c-Si modules (Except for GaAs PV modules).
Thin-film solar technology might not be as popular as crystalline silicon, but it has an incredibly promising future. This technology opens possibilities that are not available for c-Si panels, like BIPV applications, portable modules, and even high-efficiency space applications with CIGS and GaAs PV modules.
While c-Si technology will probably keep having the largest market share due to its currently high rated efficiency, low manufacturing prices, and other pros attached to it, thin-film technology is still a valuable option to consider. As a matter of fact, the market share for thin-film solar has grown in the last decade, and it could keep it up in the following one.
With further research and breakthroughs for thin-film solar cells, this technology could be adapted to even more applications in the future and potentially increase its market value not only in large-scale applications but also in small commercial and residential sectors in the next 10 years.