Just outside the earth’s atmosphere, the sun shines with a power of 1367 watts on a square meter of surface facing the sun. This sunlight radiates continuously from the sun to the earth. On a cloudless day in summer, 1000 watts per square meter arrive at the earth’s surface in the Netherlands – the earth’s atmosphere has absorbed the rest. But power does not say anything about the amount of energy you can generate with it. After all, for this you need to know how many hours the sun is actually shining. If you take into account day and night, the seasons and the clouds, in the Netherlands we have an average of 1000 of these hours of full sunshine each year. In summer of course we have more sun than in winter, and the Dutch weather is also characterised by many cloudy days, but if we convert this all into hours of full sun, then the amount of sun in an entire year is the 1000 hours mentioned here. By way of comparison: in the Sahara, at the level of the equator, there are around 3000 effective hours of sunshine per year, which is (only) three times as much as in the Netherlands.
Measured over an entire year, approximately 1000 hours of 1000 watts = 1000 kWh ‘solar energy’ is irradiated on a square meter of surface in the Netherlands. Which part of your domestic energy consumption can you cover with this? An average Dutch household consumes around 3000 kWh of electricity per year. In addition, an average Dutch household consumes about 1300 m3 of natural gas per year, mainly for heating purposes. With an energy density of 31 MJ per m3 natural gas, the gas delivers a little bit more than 11,000 kWh of ‘cosy warmth’ in that year – so almost four times as much thermal energy as electrical energy.
Let’s dive somewhat deeper into the electrical part, and the role of solar cells. The currently popular solar cells made of polycrystalline silicon convert sunlight with an efficiency of 15% into useful electricity. 1000 kWh of solar energy therefore corresponds to 150 kWh of useful electricity. So if you have 20 m2 of surface area available on your roof – preferably facing south where the sun shines most powerfully – you can theoretically get all your family’s electrical energy from the sun by using solar cells. Apart from the fact that not every Dutch family has such a large roof, there are still a few hurdles to take in practice. The moment that the energy becomes available may not be at the same time as the moment that the energy is needed – which is an important hurdle. For example, solar cells do not work in the evening or at night when you want to watch TV or be able to read a book. And the sun shines more in summer than in winter, while in winter there is a greater need of electrical artificial lighting. To bridge this gap, energy storage is important, for example in the form of batteries. As an alternative, you can deliver any surplus of electricity to the power grid, in order to be used by other people.
A solar cell converts (sun)light directly into electricity via the photovoltaic effect – abbreviated to ‘PV’. Light particles or photons from sunlight fall on a solar panel, where semiconductor materials such as silicon absorb these light particles. Due to this absorption electrons are, so to speak, released from their (silicon) atoms so that they can move freely through the material. And moving charged particles form the definition of electrical current. In a sense, a solar cell and an LED (light-emitting diode) are each other’s opposite: with an LED lamp, electrical energy is converted into light, whereas a solar cell converts light into electrical energy.
Crystalline silicon solar cells
Silicon has always been the preferred material for solar cells. The basis for this is formed by slices of monocrystalline or polycrystalline silicon with a thickness of approximately 0,3 millimeter. For silicon, the minimal energy gap that has to be crossed in order to release an electron from a silicon atom (known as the band gap) is 1.1 electronvolt (eV), after which this electron can move freely through the material. Visible sunlight has a wavelength between 400 and 800 nm with corresponding energy levels of the light particles between 1.5 and 3.1 eV, so sunlight has sufficient energy for such as crossing. After this ‘liberation action’, a positively charged electron hole stays behind in the silicon atom. Such an electron hole can move through the material if that hole is filled by an electron nearby – and in turn that particular electron will leave a hole behind. In short, when a light particle is absorbed by the silicon, the energy of that light particle is transferred to the set of the mobile electron and the mobile electron hole. If this electron releases its energy outside the solar cell, then you can employ this energy in a useful way – the principle of solar energy.
The p-n junction
But there is one problem … The electrons and electron holes generated this way have the ability to immediately ‘recombine’ to light or energy, making the absorption action up to now meaningless. Therefore, it is necessary to separate the electrons from the electron holes, and this is the role of an essential part of the solar cell: the p-n junction. This is an interface between silicon with excess of free electrons (n-type silicon) on the one hand, and silicon with many electron holes (p-type silicon) on the other hand. In n-type silicon, a small part of the silicon atoms with four valence electrons has been replaced by phosphorus with five valence electrons. This additional electron does not participate in the bond between atoms, and can move as a free electron through the lattice. In p-type silicon, a small part of the silicon atoms has been replaced by boron with three valence electrons. This boron atom likes to have another electron attached to it, and withdraws this electron from a neighbouring silicon atom, leaving an electron hole behind. This electron hole also can be considered as a charged particle that can move freely through the lattice.
What happens if you bring such an n-type silicon in close contact with a p-type silicon? At the interface, the electrons and electron holes present combine to a neutral ‘something’ under the emission of energy. All mobile charge carriers disappear in this small area. The only that remains are immobile positive remainders at the n-side and immobile negative remainders at the p-side. This charge distribution leads to a built-in voltage on the p-n junction. The internal electrical field forms a driving force to direct the positively charged mobile particles into one direction, and the negatively charged mobile particles into the opposite direction. In brief, this interface is the ideal place to separate the electrons from the electron holes.
So the free electrons created by the light absorption move to the n-side, and the electron holes to the p-side. The flow of electrons is routed outside the cell from the ‘n-side’ to the ‘p-side’, where the electrons can provide useful electrical energy. At the moment that such an electron recombines with an electron hole at the ‘p-side’, the circle is complete.
The theoretically maximum efficiency for these crystalline silicon solar cells – the fraction of sunlight that can be converted to electrical energy – is about 33%. This is partly due to the band gap of 1.1 eV for silicon. In any case, photons with less energy than 1.1 eV – or a wavelength larger than 1127 nm, in the near infrared region – do not have sufficient energy to be absorbed by the silicon. Moreover, photons above 1.1 eV – for example in visible light – have more energy than is necessary to release one electron from silicon, with the remaining energy being converted to (useless) heat. In addition, energy can be lost in case of premature recombination of electron and electron hole within the silicon, so before these particles have the chance to leave the solar cell. In practice, these solar cells have an efficiency of 15-20%.
Besides silicon as a functional material, a solar panel consists mainly of glass and aluminium. A transparent layer of glass protects the underlying silicon from wind and weather, but allows sunlight to pass through. The supporting frame is made of aluminium. Thin lines of silver paste connect the solar cells to each other electrically, and the current is further dissipated by copper cables. Ethylene vinyl acetate films glue the various parts together, and a polyvinyl fluoride backsheet gives the panel a waterproof finish. The well-known (dark) blue colour of these solar cells is due to a 70 nanometer thin silicon nitride transparent anti-reflection coating, which is applied via CVD (chemical vapour deposition).
The first generation of crystalline silicon solar cells still dominates the commercial market today. But the second-generation solar cells, where the magic word is ‘thin film’ and that partially compensate for the disadvantages of their predecessors, are close on their heels. Manufacturing techniques such as CVD and PVD (physical vapour deposition) which are already known from microelectronics, coatings and displays, are used for these thin-film systems. Such a system is no longer self-supporting, so you have to apply the thin film to a substrate, such as well-chosen flexible plastic films – also large rolls – or glass or metal foils.
Traditional first-generation solar cells contain relatively large amounts of silicon. If you reduce the thickness of the silicon layer from 0.3 mm to 3 μm, then you can save on the amount of silicon. Since silicon is applied with (PE)CVD at a relatively low temperature of about 200 °C, the material does not crystallise but remains in the amorphous state. Because of such a low application temperature, the silicon can be applied to many different substrates. The efficiency of solar cells based on amorphous silicon is significantly lower than that of their crystalline counterparts, and has a value of about 10%.
Another type of second-generation solar cell contains the elements copper, indium, gallium and selenium, and is known as the CIGS solar cell, as the abbreviation for these four elements. The interface between a 50 nm thin CdS layer (n-type semiconductor) and a 2.5 μm thin CIGS layer (p-type semiconductor) forms the p-n junction here. The n-type semiconductor is further extended with a transparent (doped) conductive ZnO layer which also serves as an electrode to transport the electrons out of the cell. The incident sunlight enters the ‘n-side’ through this transparent conductor. At the other side, a 1 μm thin layer of molybdenum metal forms the electrode at the ‘p-side’ where the electrons (re)enter the solar cell. Just below this electrode is the substrate onto which this micron-thin solar cell is applied. The efficiency of the CIGS solar cell is close to that of the first generation of crystalline silicon solar cells.