The Difference Between Thin Film and Crystalline Silicon Solar Panels

If the sources are to be believed, it was French physicist Alexandre-Edmond Becquerel who discovered the photovoltaic effect in 1839 by accidentally operating electrodes in a conductive liquid placed under light. American inventor Charles Fritts first prepared photovoltaic solar cells around 1883. His method was to coat the surface of selenium with a thin layer of gold, and the maximum efficiency of the cell was less than 1%. Of course, the high cost of selenium and gold discounted his achievement.

The story continues. In 1888, Russian physicist Aleksandr Stoletov assembled a photovoltaic cell based on the photoelectric effect discovered by Heinrich Hertz in 1887; in 1905, Albert Einstein explained the photoelectric effect; in 1946, American engineer Russell Shoemaker Ohl patented the junction solar cell; and research eventually led to the invention of the transistor.

Bell Labs is known for developing the first effective solar photovoltaic cell in 1954. Bell Labs' use of diffused semiconductor PN junctions brought efficiency improvements to solar cells, but not enough to be produced on a cost-effective scale. Four years later, the Vanguard I satellite was launched with solar cells installed on its hull to extend the satellite's mission time, which was usually determined by the life of the available batteries. The results proved that solar cells were effective and were therefore integrated into satellite designs of the time, such as Bell Labs' Telstar satellite.

Solar cells developed slowly over the next 20 years until Eliot Berman of Exxon Oil Corporation made a breakthrough in price and efficiency. Around 1969, Berman first noticed that solar cells could be made using semiconductor production processes. Instead of cutting and polishing semiconductors from scratch or applying anti-reflective coatings, the front side of waste semiconductor wafers was already anti-reflective. They just needed to be cut to the right size, printed with circuits, and used as anti-reflective surfaces.

Berman's method solved two costly production processes and made silicon wafer waste widely recyclable. He realized that perfect silicon wafers were not needed, and waste silicon wafers with a few defects had little effect on the performance of solar cells. To make a long story short, in 1973 Berman and his team made solar panels with a cost of $10/W and sold them for $20/W.

Crystalline silicon solar panels

Crystalline silicon (c-Si) solar cells are the most widely used solar cells, mainly because of the stability of crystalline silicon and the efficiency of 15%-25%. Crystalline silicon relies on mature process technology based on a large amount of data and has been proven to be reliable in general. However, crystalline silicon has poor light absorption ability, which may be an inherent defect of its ultra-small structure, so it must be quite thick and strong.

A basic crystalline silicon cell consists of 7 layers (Figure 1), with a transparent adhesive attached to a glass protective layer, and an anti-reflective coating underneath to ensure that all light passes through the silicon crystal layer. Similar to semiconductor technology, the N layer sandwiches the P layer, with two electrical contacts: the upper layer is positively charged and the lower layer is negatively charged.

There are two types of crystalline silicon: monocrystalline silicon and multicrystalline silicon. Monocrystalline silicon comes from a single crystal of high purity, cut from a wafer with a diameter of 150mm and a thickness of 200mm. Multicrystalline silicon is more popular and is manufactured in larger quantities, such as cutting silicon into strips and then into wafers. In either case, the amount of electricity generated by a silicon solar cell is about 0.5V, and multiple cells can be connected in series to increase the output voltage.

Thin-film solar panels

Even with discarded silicon wafers, silicon wafers are not necessarily cheap given their efficiency levels. Thin-film solar cells are cheaper than traditional solar panels, but they are also less efficient, with photovoltaic conversion rates between 20% and 30%.

Depending on the materials used, typical thin-film solar cells can be divided into the following four categories: amorphous silicon (a-Si) and thin-film silicon (TF-Si); cadmium telluride (CdTe); copper indium gallium selenide (CIS or CIGS) and dye-sensitized solar cells (DSC) plus other natural materials.

The structure of thin-film solar cells is not much different from that of silicon crystalline solar cells. It consists of a six-layer structure (Figure 2). In this structure, a transparent coating covers an anti-reflection layer, a PN junction below, and then a contact plate and a substrate. Obviously, the operating principle (photovoltaic) is the same as that of crystalline silicon cells.

Figure 2: The thin-film solar cell structure consists of six layers, which is not much different from the corresponding crystalline silicon structure, and the operating principle is also the photovoltaic principle.

Figure 2: The thin-film solar cell structure consists of six layers, which is not much different from the corresponding crystalline silicon structure, and the operating principle is also the photovoltaic principle.

Some people may think, and they may be right, that since the name is thin-film battery, the structure must be lighter and thinner than other battery technologies. Since the function and structure are the same, the only difference is the thickness and flexibility of each layer of thin-film and crystalline silicon solar cells and the photovoltaic material: if it is not silicon, it is either cadmium telluride (CdTe) or copper indium gallium selenide (CIGS).

Silicon vs. Thin Film

Crystalline silicon technology has been around for a while and has proven its worth, while thin-film technology is still in its infancy but has the potential to be lower cost with equivalent efficiency and reliability. So which one should we choose?

The advantages of crystalline silicon are high conversion efficiency, reaching 12%-24.2%, high stability, easy manufacturing, and high reliability. Time is another advantage: crystalline silicon modules have been produced since the 1970s, and monocrystalline silicon panels can withstand harsh environments and can be used in space flight.

Other advantages include heat resistance and low installation costs. Silicon is also more environmentally friendly when considering disposal/recycling times.

The downside is that crystalline silicon is the most expensive solar module in terms of initial cost. It also has a low solar absorption factor and the material is brittle and easily breaks.

Thin-film solar cells are cheaper than old-fashioned crystalline silicon solar cells, can be made on thin silicon wafers, are more flexible and easier to handle, and are less susceptible to damage from external impacts than crystalline silicon.

The main disadvantage of thin-film solar panels is their low efficiency, which can offset their price advantage in some applications. Its structure is also more complicated, and flexible thin-film cells require special installation skills, so at least for now, it cannot be used in aerospace.

application

Crystalline silicon and thin film solar panels can be used in many applications. According to their advantages and disadvantages, crystalline silicon cells are more used in situations requiring high efficiency, while thin film cells are often used in low-cost and more flexible situations.

Crystalline silicon solar panels are commonly used in energy harvesting systems and general designs (Figure 3), but are also used in specific applications such as the solar-powered Nuna 6 racing car made by the Dutch Nuon Solar Team (Figure 4).

Figure 4: The latest version of the Nuna series solar car, the Nuna 6, uses 1,690 monocrystalline silicon solar cells throughout the vehicle. The solar cells are used in conjunction with a 21kg lithium battery with an efficiency of 22%.

Launched in July 2011, Nuna 6 is the latest model in the Nuna series. Its 1690 monocrystalline silicon solar cells cover an area of ​​6m2. The cells are used together with a 21kg lithium battery. Nuna's solar cells have an efficiency of 22%. Nuna 6 weighs 145kg, which is much lighter than its predecessor.

Thin-film solar panels can also be used in outdoor energy harvesting systems.

SoloPower, based in San Jose, California, offers flexible thin-film solar panels that can be used on commercial rooftops. The panels are made of copper, indium, gallium, and selenium and integrated into a flexible solar cell foil (Figure 5). These panels are lighter and quicker to install than glass-wrapped crystalline silicon panels.

Figure 5: SoloPower’s flexible thin-film solar panels are lighter than crystalline silicon panels and can be easily installed on commercial building rooftops.

Summarize

It may feel that in the current market, thin-film cells are not only catching up with crystalline silicon cell components, but will surpass crystalline silicon in all aspects, including price and efficiency. One way to reduce the cost of thin-film solar cells is to use non-environmentally friendly materials, such as cadmium. Manufacturers claim that as long as they are well sealed and in use, they are safe. But for now, there is no recycling plan for such components.