In a drive to drop the prices of photovoltaic (PV) panels, crystalline silicon technology may have reached its limit. Thin film PV, however, still offers the opportunity to reduce manufacturing costs and hence unit prices. The capability to fabricate PV solar cells on a large scale and at a competitive price is a milestone still waiting to be achieved.
Silicon PV manufacture, although largely automated, still involves a large number of steps that need to be performed, including wafer manufacture, module assembly, conductor attachment, encapsulation, panel assembly and so on. Thin film bypasses many of these stages, as well as allowing the final configuration to be performed on the same basic substrate.
The idea of a spray-on thin film PV, which would allow manufacture without the use of special apparatus, has been around for many years but has not been seen as promising until recently. A further goal of “reel-to-reel” continuous production is being ardently pursued.
The main problem with thin film is the deposition of the film onto the substrate, and there have been various ways of doing this with materials such as amorphous silicon and cadmium telluride. The more promising materials, known as chalcopyrites, have achieved successful manufacture using vacuum deposition or “sputtering” as it is known, a process which requires vacuum machinery and a high energy usage.
Spray-on technologies typically exhibit lower efficiency than conventionally manufactured solar cells, but could provide a lower cost-per-Watt than conventional PV. On systems where size is not a limiting factor, this may prove to be a significant advantage.
Current thin film techniques
The two most prevalent thin film technologies with marketable products are amorphous silicon and cadmium telluride. CIGS or chalcopyrites are a new entry in the thin film field, with several companies now manufacturing commercially available product.
Amorphous silicon (a-Si) panels have been in production and available commercially for many years. The drawbacks of low efficiency and rapid initial degradation have largely been overcome, but usage remains limited. The biggest advantage of this technology is the ability to manufacture on flexible substrate. This type of thin-film cell is mostly fabricated by a technique called plasma-enhanced chemical vapor deposition. It uses a gaseous mixture of silane and hydrogen to deposit a very thin layer of only 1 µm of silicon on a substrate, such as glass, plastic or metal, that has already been coated with a layer of transparent conducting oxide. Other methods used to deposit amorphous silicon on a substrate include sputtering and hot-wire techniques.
Cadmium telluride (CdT )
Solar PV cells based on cadmium telluride (CdTe) represent the largest segment of commercial thin-film module production worldwide. Recent improvements have matched the efficiency of multicrystalline silicon while maintaining cost leadership . There are various methods to deposit CdTe on substrate, which includes close-spaced sublimation (CSS), vapor transport deposition (VTD), electrodeposition (ED), physical vapor deposition PVD, and sputtering. CSS appears to give the best results. close space sublimation appears to be the most promising technique.
Copper indium gallium selenide (CIGS)
Chalcopyrite (IGS) based panels have the highest efficiency of all thin film panels but there is a limited amount of commercially available product on the market as yet. CIGS based solar panels have been under development for many years, and a pilot production plant has been established in Stellenbosch for the production of a South African design. To date there is no record of commercial production or sale of any product.
CIGS panels are produced using vapour phase deposition or RF sputtering, a slow and expensive batch process that requires vacuum containers and high energy usage. Various other processes have been developed, such as the two-stage reactive transfer printing method, making use of thin film printing and other processes .
The spray-on option
The spray on option is often presented as a means of coating any surface under any conditions with a layer of photovoltaic material. The reality is that the process is mainly aimed at reducing costs of manufacture of solar cells, and requires controlled conditions for its implementation.
Two major challenges and concerns in thin film solar cell research are as follows:
The first is to produce suitable stable, environmentally-friendly and durable solar cell materials that can effectively convert a large portion of the incident solar radiation into electricity, and the second challenge is to devise a practical manufacturing technique to convert the solar cell materials into solar panels.
In this context, all or some layers of polymer, dye-sensitized, quantum dot, and thin film solar cells are examples of materials that may be processed in solution. The solution-processed materials may be transferred to the substrate by atomizing the solution and carrying the spray droplets to the substrate, a process that will form a thin film after evaporation of the solvent.
Spray coating is performed at low temperatures and ambient pressure using low cost equipment with a roll-to-roll process capability, making it an attractive fabrication technique, provided that fairly uniform layers with high charge carrier separation and transport capability can be made.
Thin-film deposition, using the spray pyrolysis technique, involves spraying a metal salt solution onto a heated substrate (Fig. 1). Droplets impact on the substrate surface, spread into a disk shaped structure, and undergo thermal decomposition. The shape and size of the disk depends on the momentum and volume of the droplet, as well as the substrate temperature. The film is usually composed of overlapping disks of metal salt being converted into oxides on the heated substrate.
Fig. 1 shows the process of spray coating of a solution-processed solar cell by ultrasonic atomisation, which is a method to convert the solar cell material solution into tiny droplet by high frequency vibration of a nozzle: (1) Ultrasonic nozzle tip, (2) 2D traveling arm, (3) solution inlet, (4) holder plate with controlled temperature, (5) substrate, (6) ultrasonic vibration transducer box, (7) function generator for ultrasonic piezoelectric transducer. The double arrows beside the plate show the direction of imposed vibration, which is lateral vibration.
During spray coating, these tiny droplets are carried to the substrate by a gas, such as air or nitrogen (to avoid degradation of the solar cell materials). The droplets create a thin liquid film on the substrate; as a result of the high temperature of the substrate, the solvent evaporates and a thin solid film of solar cell material forms. In Fig. 1, the substrate is also ultrasonically vibrated, to improve the uniformity of the film.
Spray coating is a multi-step method, comprising several steps such as atomising a liquid solution or mixture, droplet flight and evaporation, droplet impact on the substrate, droplet spreading, receding, recoiling, drying, solute adhesion and bonding to itself and to the substrate. To obtain a high quality and acceptable spray-coated layer, all of these processes have to be well understood and controlled. Parallel or post processing may be also helpful .
Early work on spray on solar was focused on polymer, quantum dot and dye sensitised solar cells as these were considered the most suitable for the process. More recent work has been done on nanoparticle materials and most of the conventional PV materials including CdTe and CIGS. There are many companies developing spray-on versions of their existing products. Spray on versions have been developed for flexible substrates and building integrated PV, as well as for conventional panels. Spray on deposition is also being used as an alternative to some the stages in conventional manufacture.
One of the problems facing the spray on technique is the need for a material in solution form. The most promising of the new thin film technologies is perovskite cells, named after the 19th century Russian mineralogist Lev Perovski. Unlike silicon-based PV cells, perovskite cells are soluble in a variety of solvents, which makes them easy to spray onto a surface link inks or paints. That potentially makes the cells much cheaper to manufacture and means that the light-gathering film can be attached to flexible materials, opening up a range of new applications.
Solar cells based on perovskites (a family of crystals with a common, distinctive structure) look very promising in this regard, for two main reasons. Firstly, they can be manufactured more cheaply, using widely available materials prepared at low temperatures. And secondly, the technology around them is advancing at a rapid pace. In a few short years, they have already reached efficiencies in excess of 19%, which is competitive with traditional cells based on crystalline silicon.
 M Eslamien: “Spray-on Thin Film PV Solar Cells: Advances, Potentials and Challenges”, Coatings, 2014.
 NREL: “Cadmium Telluride Solar Cells”, www.nrel.gov/pv/cadmium-telluride-solar-cells.html
 B Sang, et al: “Low cost copper indium gallium selenide by the FASST process”, 2008 33rd IEEE Photovoltaic Specialists Conference
 F Zabihi and M Eslamian: “Substrate vibration-assisted spray coating (SVASC): significant improvement in nano-structure, uniformity, and conductivity of PEDOT:PSS thin films for organic solar cells”, 2015, https://bit.ly/2u6EBdI
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