Solar panels fall into two main technological categories. The incubant, established tyoe are called crystalline silicon solar panels and the exciting but unproven type are known as ‘thin-film’ solar panels. To understand the advantages and disadvantage of each technology I’ll briefly explain how each type of solar panel is made. Crystalline silicon solar panels are made from 50 or so ‘solar cells’ connected together and encased in glass. Each solar cell is in fact a thin slice of large crystal of pure silicon (called an ingot). These large crystals are grown from a seed crystal surrounded by molten silicon at very high temperatures. The silicon used must first be extracted from silicon dioxide (also known as sand) and then purified to a very high level. Once the crystal is formed it can be sliced into wafers. The wafers are then specially treated to make a junction between a positive and negative type semiconductor, and then other layers such as the conductive contacts are added to make a working solar cell. This process has many steps and consumes a lot of energy. However, many companies have spent a lot of time refining the process to make it as efficient as possible so almost all parts of the process are now automated.
Thin film solar panels are made using a radically different process. The underlying physics is similar in that they still use a junction between a positive and negative doped semiconductor, however thin film solar panels have the potential to be made in much fewer steps than crystalline silicon. The idea is to take glass (or sometimes foil or plastic) and coat it directly with a series of layers, including the active semiconductor layers to produce a working solar cell. The glass is then encapsulated with a protective plastic and a second sheet of glass as protection. This process saves having to make lots of small cells and connect them together. The other advantage is that the layers are very thin, hence thin film solar cells. The active layers of the cell are only a few nanometers (billionths of a meter) compared to 0.2mm for each silicon wafer.
The important point of all this is that the manufacturing cost of thin film solar cells has the potential to be significantly lower than crystalline silicon. Unfortunately, there are some catches. Firstly, they are not as efficient as crystalline silicon. Crystalline silicon reaches 16 – 18% efficiency in modern solar panels, whereas the most efficient thin film solar panels on the market today are under 11%. The next drawback is reliability. Thin film solar panels have had less time to prove themselves and have been known to suffer from degradation meaning that their performance gets significantly worse over time.
Despite these drawbacks, several companies have managed to become very successful in manufacturing thin film solar cells. The most notable is called First Solar who are now one of the top two largest solar panels manufacturers in the world and have a significant advantage over rivals due to their low manufacturing costs. First solar make thin film solar cells made from cadmium telluride, one of a number of semiconductor materials that can be used for thin films. First Solar’s panels are less efficient but are very popular for large scale solar installations because of their low cost.
Before the financial crisis, when silicon was in short supply and very expensive, all thin film solar panels were a good idea. First Solar could not produce enough and billions were invested in a large number of thin film solar companies aiming to follow in their footsteps. Now that the silicon shortage is over and the price of crystalline silicon solar panels has fallen, the environment for thin film solar cells is more challenging. First Solar will remain a strong player as they have managed to get to high volume and have a reliable production process. Many of the 200+ start-ups hoping to replicate their success will struggle however. For thin film solar cells there are a wide range of different manufacturing processes and materials that can be used, and there is still a lot of research being done to improve our understanding of the underlying physics. This means that there is a lot of opportunity to invent a ‘unique’ technology and start a company but only the best thin film solar companies will make it however. They have to show not only that their technology is efficient and reliable, but also demonstrate that large scale production is feasible and low-cost. Many ideas that look good on paper or in the lab turn out to be impractical when it comes to volume manufacturing.
At present, it seems like crystalline silicon will retain a strong market share for the foreseeable future (it current represents 80-90% of the market) but I believe that eventually certain thin film technologies will begin to displace crystalline silicon. There is a lot of potential for efficiency improvement in thin film, as well as lower manufacturing cost. Some technologies, particularly that usce solution processing are really very exciting.
What does this mean for the UK solar industry? Very little actually. I would expect over 90% of the UK market will be crystalline silicon for a long time. The reason is that the UK market will be dominated by smaller rooftop applications (partly due to the structure of the feed.in tariff as discussed last week). In such space-constrained applications you want to use the most efficient technology to maximize the energy generated from the available area. For now, this means always choosing crystalline silicon as it’s efficiency is significantly above any thin film solar panel out there.
Keep an eye out for breakthroughs in solar technology as some are surely bound to occur, but beating high quality crystalline silicon solar panels made in China for cost, efficiency and reliability is not easy.