– What are thin-film solar panels?

What are thin-film solar panels?

Thin-film solar panels are made with solar cells that have light-absorbing layers about 350 times smaller than that of a standard silicon panel. Because of their narrow design and the efficient semi-conductor built into their cells, thin-film solar cells are the lightest PV cell you can find while still maintaining strong durability.

Thin-film solar panels are typically made with one of the following technologies:

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

Cadmium Telluride (CdTe) solar panels

Cadmium Telluride (CdTe) is the most widely used thin-film technology. CdTe holds roughly 50% of the market share for thin-film solar panels. CdTe thin-film panels are made from several thin layers: one main energy-producing layer made from the compound cadmium telluride, and surrounding layers for electricity conduction and collection. CdTe contains significant amounts of Cadmium – an element with relative toxicity. First Solar is the top innovator and seller in this space.

CdTe is the most common thin-film solar technology, mainly because of First Solar’s utility-scale dominance. In 2016, First Solar hit a CdTe world-record cell efficiency of 22.1%, although its modules average 17%. The Series 6 module should produce 420 W, and its smaller Series 4 modules peaked at about 100 W

Advantages and disadvantages of cadmium telluride solar panels

One of the most exciting benefits of CdTe panels is their ability to absorb sunlight close to an ideal wavelength. Functionally, this means that CdTe solar panels can capture energy at shorter wavelengths than traditional silicon panels can, which matches the natural wavelengths of sunlight closely for optimal sunlight to electricity conversion. Additionally, cadmium telluride panels can be manufactured at low costs, as cadmium is abundant and generated as a by-product of key industrial materials like zinc.

The main concern with CdTe panels is pollution. Cadmium by itself is one of the most toxic materials known, and cadmium telluride also has some toxic properties. Currently, the general opinion on using cadmium telluride is that it is not harmful to humans or the environment in residential or industrial rooftop applications, but disposal of old CdTe panels continues to be a concern.

Amorphous Silicon (a-Si) solar panels

Amorphous Silicon (a-Si) is the second most popular thin-film option after CdTe. Amorphous Silicon is the most similar technology to that of a standard silicon wafer panel. Amorphous Silicon is a much better option than its counterparts (CdTe, CIGS) in terms of toxicity and durability, but it is less efficient and is typically used for small load requirements like consumer electronics. The quest for scale is always a hindrance for a-Si.

Amorphous Silicon (a-Si) is the oldest thin-film technology. It uses chemical vapour deposition to place a thin layer of silicon onto the glass, plastic or metal base. It is nontoxic, absorbs a wide range of the light spectrum and performs well in low light but loses efficiency quickly. One layer of silicon on an amorphous solar panel can be as thin as 1 micrometer, which is much thinner than a human hair.

Advantages and disadvantages of amorphous solar panels

Unlike many other thin-film panel options, amorphous silicon panels use minimal toxic materials. When compared mono- or poly-crystalline solar panels, amorphous panels use much less silicon. Amorphous silicon solar panels are also bendable and less subject to cracks than traditional panels constructed from solid wafers of silicon.

The ongoing challenge with amorphous solar panels is their low efficiency. Due to complicated thermodynamics and the degradation of amorphous silicon, among other factors, amorphous solar cells are less than half as efficient as mono- or poly-crystalline solar panels. The highest efficiency on record for a-Si is 13.6%. Attempts to raise the efficiency of amorphous panels by stacking several layers, each in tune to different wavelengths of light, has proven somewhat effective, but the overall efficiency of these types of thin-film panels is low compared to other options.

Copper Gallium Indium Diselenide (CIGS) solar panels

Laboratory CIGS cells have reached efficiency highs of 22.4%. However, these performance metrics are not yet possible at scale. The primary manufacturer of CIGS cells was Solyndra (which went bankrupt in 2011). Today, the leader is Solar Frontier. MiaSolé also manufactures CIGS panels in the U.S. and China.

CIGS solar cells are made from a compound called copper gallium indium diselenide sandwiched between conductive layers. This material can be deposited on substrates such as glass, plastic, steel, and aluminum, and when deposited on a flexible backing, the layers are thin enough to allow full-panel flexibility.

Advantages and disadvantages of CIGS solar panels

Unlike most thin-film solar technologies, CIGS solar panels offer a potentially competitive efficiency to traditional silicon panels. Solar Frontier has a 22.9% CIS cell efficiency record, while its full modules average lower and peak at 180 W. MiaSolé’s flexible CIGS thin-film modules average 16.5% efficiency and may peak at 250 W.

CIGS cells also use the toxic chemical cadmium. However, CdTe panels have a higher percentage of cadmium, and CIGS cells are a relatively responsible thin-film option for the environment. Even better, in some models, the cadmium is completely removed in favour of zinc.

The primary disadvantage of CIGS panels are their price. While CIGS solar panels are an exciting option, they are currently very expensive to produce, to the point where they can’t compete with traditional silicon or cadmium telluride panels. Production costs continue to be an issue for the CIGS solar panel market.

Gallium Arsenide (GaAs) solar panels

Gallium Arsenide (GaAs) is a costly technology. GaAs holds a world record of 29.1% efficiency for all single-junction solar cells and 31.6% for dual junction solar cells. GaAs is primarily used on spacecrafts and is meant for versatile, mass-scale installments of PV energy in unusual environments.

One of the leading companies in GaAs cells is Alta Devices. Alta Devices solar cells offer an exceptional combination of high efficiency, flexibility, thinness, and low weight. In addition, the product is highly configurable to meet your physical, mechanical and electrical requirements. These attributes make Alta Devices solar ideal for HALE aircraft, allowing them to fly longer, higher, and at more latitudes than competing solar technologies.

Advantages and disadvantages of GaAs cells

The backbone of our entire technology is gallium arsenide (GaAs), which is an III-V semiconductor with a Zinc-Blende crystal structure. GaAs solar cells were first developed in the early 1970s and have several unique advantages. GaAs is naturally robust to moisture and UV radiation, making it very durable. It has a wide and direct bandgap which allows for more efficient photon absorption and high output power density. Finally, it has a low-temperature coefficient and strong low light performance.

Like CIGS solar panels, the most significant disadvantage of GaAs cells is its price. GaAs solutions are costly, which will make them suitable for niche markets like space, HALE and small unmanned planes/drones. Based on its cost, GaAs cells can’t compete with traditional silicon or cadmium telluride panels.

Organic photovoltaic solar panels

Organic photovoltaic (OPV) cells use conductive organic polymers or small organic molecules to produce electricity. In an organic photovoltaic cell, several layers of thin organic vapour or solution are deposited and held between two electrodes to carry an electrical current.

Advantages and disadvantages of organic PV cells

The building-integrated photovoltaic (BIPV) market has the most to benefit from OPV cells. Due to the ability to use various absorbers in an organic cell, OPV devices can be coloured in several ways, or even made transparent, which has many applications in unique BIPV solar solutions. The materials needed to build organic solar cells are also abundant, leading to low manufacturing costs and subsequently, low market prices.

Like other thin-film options, organic photovoltaic cells currently operate at relatively low efficiencies. OPVs have been constructed with about 11% efficiency ratings, but scaling module production up while keeping efficiencies high is a problem for the technology. Much of the research currently surrounding OPVs is on how to boost their efficiency.

An additional issue with OPV technology is a shorter lifespan than both other thin-film options and traditional mono-or poly-crystalline panels. Cell degradation that doesn’t occur in inorganic modules is an ongoing struggle for organically-based photovoltaic products.

Efficiency Comparison Thin-film and crystalline silicon modules

Comparatively, a typical 60-cell crystalline silicon (c-Si) module averages a power output between 250 and 350 W with an efficiency between 17 and 18 %, with high-efficiency brands performing even better. One would need more thin-film modules and more area to produce the same power as a smaller group of c-Si. Crystalline silicon modules are just more consistently dependable for the majority of solar markets, and that’s why they are the dominant panel choice.

The future of the thin-film market

No single thin-film brand appears to be making a grab for c-Si’s market share in the United States. First Solar is expanding its CdTe manufacturing, but that’s because it has found that utility-scale sweet spot and is dominating globally. Most CIGS and CIS manufacturers market themselves as niche products.

MiaSolé semi-rigid CIGS modules were initially designed for commercial rooftops, but the company has since branched out into emerging markets like transportation and commercial trucks. When it comes to traditional solar applications, MiaSolé’s best play is its lightweight.

CIGS manufacturer Sunflare also works with nontraditional solar markets like transportation, marine and modular/tiny home applications. The company has been working to improve the manufacturing process at its plants in Sweden and China to increase thin-film adoption.

Source: A⁺ Solar Solutions / EnergySage / Solar Power World