FAQ

Content:

General questions

Solar cells in society

Economical questions

Thin films and CIGS

Other types of solar cells


General questions

  • What does a solar cell do?

Solar cells transform sunlight directly into electricity – without consuming any material!

  • How large an area is needed to generate 1 kW of electricity from a solar cell system?

The area needed depends on how the modules are placed, their efficiency and the irradiation from the sun. For a rough estimate with an efficiency of 15 %, the surface needed is 1000 W/(15 % * 1000 W/m2)= 6.7 m2 with an irradiation of 1000 W/m2. In Sweden, we can get that irradiation in the middle of the day.

  • How do you think the efficiency of solar cells is going to develop?

The efficiency for the record solar cells might not increase that much. There are theoretical limits to the efficiency that cannot be passed without a lot of tricks. For a simple solar cell the theoretical limit is at 33 %. For the commercial solar cells, especially those based on thin films, the market is maturing and the large scale technologies are getting better and better. Therefore, the difference between the products (solar panels, or solar modules) and the record cells is steadily decreasing.

For CIGS, the record efficiency today (2016) is 22.3 %, achieved by the company Solar Frontier in Japan. The very best products at the market have an efficiency of 14 %, although this number is rapidly increasing (2016).

For silicon solar cells, which are dominating the market, the typical solar module efficiency is about 15 %, but for the more advanced modules it is possible to find products with more than 20 % efficiency (2016).

  • Are solar cells stable over time?                                             

Unprotected solar cells are affected by the environment which may decrease their efficiency; therefore the solar cells in all products are protected by glass or advanced plastic films. The solar cell modules are tested according to a special protocol (for example IEC61646) and will be certified if they pass the test. The test is made to correspond to about 25 years in operation. Products on the market normally have a warranty of 20 years. Some reasons for the solar modules to break could be that water is leaking in from the edges or that the protective glass, or a substrate glass cracks.

  • How do you protect the environment from dangerous compounds in solar cells?

While the glass and plastics are protecting the solar cells from the environment, they also provide a protection for possible leaks to the environment. The risk for pollution is very little with a glass encapsulation. It is rather during the extraction of the raw materials for the solar cells that environmental precautions must be taken. Since the modules contain glass and valuable elements, it will be profitable to recycle them when they have stopped working after 20-40 years.

  • What needs to be developed: electricity transmission, solar cells, or electrical storage?

The electrical grid in Sweden is very well developed, so electricity transmission is not a problem. Since 2015 it is possible to get a tax reduction for electricity that has been fed to the grid. It is possible to connect your solar cell system by making an agreement with your electricity supplier and also to get paid for the electricity through them. In general you get the spot price from the Nordpol market. The solar cells themselves are working very well and through market competition the price is soon under 1 kr/kWh in Sweden – which is what is needed to make solar cells installations economically viable for individuals within a 10 year period. Up to a certain level of installed power, storage is not needed for solar electricity on the grid, since hydro power can compensate for the intermittency of solar power. For larger shares of solar electricity (more than 10 TWh per year), other forms of storage are needed. This level depends also on the implementation of other energy sources, such as wind power.

  • Are there any solar cells that are flexible (for example on a roll)?

There are already flexible modules on the market that can be manufactured with conventional technology (silicon). These are mostly used on boats and you could basically walk on them. These can be bought in stores for boat equipment. There is also a production of flexible solar modules from thin film technology, for example by the companies Global Solar and Miasolé, but this is still in a small scale.

  • Is it difficult to use solar cells?

No. Even if manufacturing is fairly complicated, using them is very easy. The electricity output from a solar cell system is regulated automatically and the need of maintenance is very little.

  • What happens when it is dark or cloudy?

The electrical output delivered from the solar cell decrease with decreasing light intensity. When it is cloudy the power is therefore lower, about 20-50 % of full power with thin clouds and 5-10 % of full power when the clouds are really dark. During the night, the solar cell does not produce electricity at all. In a system not connected to the grid, these variations can be evened-out using a battery that stores the electricity.

  • Is there any way to store the energy from the solar power plants?

There are two main types of solar power plants that produce electricity; photovoltaic, i.e. solar cells, and thermal power plants. For the thermal ones, which work by heating up a medium with sunlight that is used in the power plants, the energy can be stored by storing the hot medium, for example from the day to the evening/night. For solar cells, the energy can be stored in the same way as other electricity, for example in batteries. There is also a lot of research on storing the solar energy in energy-rich compounds such as hydrogen gas, which can be used directly as a fuel.

  • Do solar cells work when it is cold?

Yes, solar cells actually work better at low temperatures. However, it is usually cold in the winter when the sun is low and, since it has to go through more of the atmosphere, the sunlight is weaker. Therefore, solar cells will give less power on a sunny day in January compared to a sunny day in July, but the difference is not as big as you might think. March is a great month for solar electricity!

  • What is the difference between solar cells and modules? How are they constructed in silicon solar cells and thin film solar cells respectively?

A solar cell is a single plate or coherent film of a material that is used to transform sunlight to electricity. The voltage from the solar cell depends on the material and is fairly low, typically 0.5-0.7 V, whereas the current depends on the cell’s area. By connecting cells in series the surface is divided into smaller segments, where every segment is a solar cell. Instead of one solar cell you may have 10 solar cells on the same surface, which gives 10 times higher voltage and 10 times smaller current compared to only one, big cell. This is advantageous since a low voltage and high current leads to more losses than a high voltage and low current with the same electrical power. These series connected cells are encapsulated for protection and are then called modules. For silicon solar cells, each cell is square and connected to the other cells by soldering. For thin film solar cells on the other hand, the cells are divided along the length of the module. This is because the film is applied one layer at a time and the cells are created by scribing thorugh the films in a series of steps.

  • What is the energy payback time for solar cells?

The energy payback time is the time that it takes for solar cells to produce as much energy as it took to produce them. This depends for example on the kind of solar cell and where they are installed. In sunny parts of Spain the energy payback time could be only around a year, but in Sweden it is probably about 2-3 years. Since there are no theoretical limit for how long the solar cells last they will be able to produce a net energy surplus for many years after this!

  • What material limits the power from a solar cell?

The power is affected by the current and voltage that the solar cell can produce, and for each there are different parts of the solar cell that are important. Both current and voltage are largely affected by recombination, which is the deexcitation of an electron where it returns to its ground level. This often occurs at surface boundaries or due to defects in the material. To decrease the recombination at surfaces one can passivate the material in different ways.

  • What do you think about when choosing materials for the solar cells, which properties do you want?

To make a solar cell you need materials that absorb sunlight and use the absorbed energy to excite electrons, as well as materials that separate the electrons and conduct them in a circuit. For the material that absorbs the sunlight you want the energy needed to excite an electron, which is material-dependent, to correspond to the energy of the sunlight, to make the power from the solar cell as high as possible. Then you want to match this material with materials that conduct the electrons with as small losses as possible, therefore you try to minimize recombination (see above). For the conductors you want as low resistance as possible, and for the window layer or the front contacts you want high transparency so as much as possible of the sunlight will reach the absorber layer. Furthermore, the materials should be chosen so they do not react with each other when the solar cell is used.

Solar cells in society

  • When will solar cells be used “seriously” in the Swedish electrical grid?

This is really difficult to answer and it mostly depends on if we and the politicians we choose make it easier to adopt solar cells. In 2009, the European Union put up a goal that 12 % of electricity production should come from solar cells in Europe in the year 2020. In 2013 only 0.06 % of Sweden’s electricity consumption was produced by solar cells, so the way it looks now Sweden will not be leading in solar energy and instead other countries such as Germany, Japan and China are much more progressive. In 2015, around 1.3 % of the total electricity usage in the world was produced by solar cells.

  • How many solar modules would be needed to provide electricity to the whole world?

The electricity production in the world is about 1013 kWh per year. If we assume an efficiency of 10 % for the solar cell system and that the solar cells are placed so the irradiation is 1500 kWh/m2 in average, about 70 000 km2 of solar cells would be needed to supply the whole world with electricity. This corresponds to less than 5 % of the surface of the Sahara desert or only 12 m2 per person. In a global perspective, the variations in the solar irradiation are also coinciding with the variations in electricity consumption. For example, in warm climates a lot of electricity is used to supply air conditioning systems.

  • Would all resources on the Earth be sufficient for all solar cells needed to cover our consumption of electrical energy?

For silicon solar cells there is probably no fundamental limit since silicon is one of the most common elements on the Earth. For CIGS, however, it is more difficult to say since the amount of indium available today would not be enough for so many solar cells, but with an increasing demand the extraction of indium could also increase.

  • What are the disadvantages for solar cells in Sweden compared to other renewable energy sources?

Solar electricity is only generated during the day, and mostly during the summer when our need for electricity is at its lowest. This is applicable to Sweden and other countries which like us have a need for heating a big part of the year.

  • When will the use of solar cells break through?

The expression “grid parity” works as a measurement of when the market for solar cells runs itself and there is no longer a need for different support systems. Grid parity means that a consumer installing solar cells and using that electricity will have the same electricity cost as someone who bought the electricity on the grid. In the first step, this presumes that the surplus can be sold during the day and bought back over the night. In many countries there are systems for longer time periods, for example selling the electricity surplus in the summer and buying electricity in the winter. Grid parity has already happened in many countries, for example in the south of Europe.

  • How do solar cells affect the environment?

Solar cells are one of the most environmentally friendly ways of producing electricity that we know today. They are silent, they do not lead to any emissions and they use sunlight as a fuel. The largest environmental impact occurs at the manufacturing. To make conventional silicon solar cells a lot of energy is needed. With thin film technology less energy and materials are used, but they are limited by the use of rare elements. When the solar cells stop working, a lot can be recycled.

Economical questions

  • What does electricity from solar cells cost?

The price for solar cells has been decreasing dramatically the past few years. The up-to-date prices could be found for example on Solarbuzz. The price depends on if there are any support systems, such as subsidies, if you can buy and sell electricity to the grid, how big your solar cell system is and also how complicated the installation is. For many systems the appearance is important and then the price per kWh can be high because of an expensive mounting structure. The price can therefore be somewhere around 0.90-5 kr/kWh, where the lowest price is for systems that got supported and all electricity is used within the building so it does not need to be sold to the grid.

  • Does it pay off to use solar cells to produce electricity?

Yes, a solar cell installation could pay off already within 10 years. The development of the profitability depends on the development of the electricity price, how well planned the installation is (shadowing, direction, angle towards the sun etc.), as well as the price for installation and modules of course. For a remote cabin where the option for electricity supply includes an expensive connection to the grid, solar cells have paid off for a long time.

  • What is the biggest cost for solar produced electricity?

Today, the cost for the solar modules themselves correspond to about a third of the price, the cost for cables, inverter and mounting construction another third and the labor for the installation about the last third.

  • Are there any subsidies for solar cells?

From January 1st 2015 there is an investment support for solar cells covering up to 30 % of the installation costs for companies and up to 20 % for others (2016). The support is limited and therefore not everyone that applies for it can get it, but the government has recently decided to increase their budget for solar cell investments. It is also possible to apply for “ROT” for the installation costs, although not together with the investment support, and it is also possible to get tax reductions of 0.60 kr per kWh for the surplus electricity fed to the grid.

Thin films and CIGS

  • Why do we use thin films to make solar cells?

1. Low material use.

2. Possibility to build monolithic modules and thereby connect the cells in series in the modules in a more convenient way for the manufacturing process (compared to the conventional silicon modules).

  • Are the CIGS solar cells transparent, i.e. can I see through them if they are applied on a window?

Generally they are not transparent, but it is not impossible to develop CIGS solar cells that are partly transparent. However, this would lead to a decreased efficiency. These cells are made by making holes in the active films down to the glass with a laser to make a series of transparent dots. The efficiency decreases depending on the total area of the dots.

  • What applications do you see for industrially made thin film solar cells? Will they be laminated between two glasses?

The commercial CIGS modules are laminated between two glasses, but there are researchers depositing CIGS cells on flexible substrates such as metal and plastic film to be able to manufacture products that can be bent.

  • When will you be able to buy Swedish CIGS solar cells?

Today there are no CIGS modules that are made in Sweden, but there are modules from other manufacturers in the world. The research at Ångström has resulted in a company that develops products and processes in Uppsala but has its large-scale manufacturing in Thalheim in Germany. This company is called Solibro GmbH. The largest CIGS manufacturer on the market is Solar Frontier (Japan).

  • Some sources claim that thin film solar cells are better at absorbing the sunlight; does that mean that these cells are better?

By this they mean the absorption per film thickness, which can be much higher in thin film materials. It is this property that allows the solar cells to be much thinner. However, the efficiency is not necessarily higher for the finished solar cell.

  • What method/methods are used to deposit the films in a thin film solar cell?

There are some different methods to deposit the layers. Sputtering is one where the substrate for the film is brought into a vacuum chamber with a low pressure of inert gas with very high energy. In the chamber there is also a target with the material for the film. When the gas particles hit the target the atoms will eject from the surface and attach to the surface, and thereby a film will start building up. In a coevaporation process the elements are instead evaporated in vacuum and hit the substrate where they react with each other.

  • What do you do at Ångström laboratory to inform the public and companies about your work?

The two most important ways to inform about our research is through this website and through different forms of direct contact. We are receive invitations to inform about our research in different contexts.

Other types of solar cells

  • What are nanocrystalline solar cells?

There are photo-electrochemical solar cells in which nanocrystalline materials are involved. These are also called Grätzel cells or dye-sensitized solar cells. These are not part of the research at the thin film department, but there are other research groups at KTH and Uppsala who look at this kind of solar cell.

  • How do the materials differ in light absorption between a conventional solar cell and a Grätzel cell?

A conventional solar cell has a p-doped and an n-doped side which meet in a pn-junction where the electrons are excited by the sunlight, separated due to the electrical field and thereby building an electrical potential that can generate electricity. In Grätzel cells, the light absorption occurs in a dye molecule where the electrons are excited to an orbital with higher energy and then separated by transferring to a semiconductor, for example titanium oxide, which the dye has been deposited on.

  • What are perovskite solar cells?

Perovskite solar cells are built from a material with the perovskite structure. They are a new type of solar cell which have been improved significantly and reached efficiencies of up to 20 % in only a few years. Advantages with these are that they have potential to become cheap to produce and could become easy to manufacture, while challenges are in the long-term stability and that lead is often used in the cells. Read more about this research at Uppsala University here.

  • What are organic solar cells, and what are their prospects for the future?

Organic solar cells are solar cells made from organic materials to absorb the sunlight and transform it to electricity. The biggest challenges for the organic solar cells today are to increase their efficiency and especially to extend their lifetime, since the organic solar cells today are very sensitive to moisture, oxygen and UV-light and are then easily degraded. If these problems could be overcome, there are good prospects for the organic solar cells to be a cheap and simple option.

  • Can you make a solar cell at home?

This depends on what kind of solar cell it is. To make silicon solar cells (the most common type) or CIGS solar cells (which we do) you need a lot of expensive equipment. On the other hand, a dye-sensitized solar cell could be made from a simple experimental kit. You can also buy silicon solar cells and solder them to a module.

Last modified: 2023-06-26