Perovskite solar cells
Today I will talk a little about a very hype topic in global materials science in the last few years – perovskite solar cells. They are being widely researched as a source of clean electricity.
The operating principle of solar panels is as follows: semiconductor material is used. Such materials in their electronic structure have a so-called band gap – an energy region that an electron in it cannot have. Lower in energy is the valence band – in it, under normal conditions, electrons are located. Above lies the unoccupied conduction band. If a material absorbs a photon with an energy equal to or greater than the bandgap, one of the electrons will be thrown up into the conduction band, leaving a hole (that’s what it’s called) in the valence band. If there are materials in the product that conduct electrons separately and holes separately, the electrons can go into an external circuit, powering some kind of load. Thus, light energy is directly converted into electrical energy.
One type of solar cell is called perovskite because its working material is metal halides with a perovskite-type structure. There are several reasons for the hype around them. These are already achieved high efficiency factors – efficiency (up to 25% of light energy is converted directly into electrical energy). The band gap, which corresponds to the energy of absorbed light particles, can be finely tuned to specific conditions. Such materials also have high light absorption coefficients (many photons are absorbed when illuminated) and low resistance. Additionally, it is cheap to create the batteries themselves using solution chemistry. Typically, dimethylformamide is used as a solvent. Unfortunately, all this happiness depends on the scaling of production processes for industrial conditions, as well as the low stability of materials.
The production of large solar cells requires the production of high-quality thin films, but the processes of nucleation and growth of perovskite crystals from solution are very difficult to control. Many approaches are being explored to obtain high-quality films – the addition of surfactants, vacuum treatment, elevated temperature, gas flows, and so on and so forth… Another method for growing high-quality films, which is very effective, is to obtain easily growing thin films from lead iodide ( Maybe someone saw an experiment with its deposition – beautiful yellow crystals) capturing reagents from the solution for the perovskite formation reaction that occurs when the film is heated.
The problem with stability is the following: the compound MAPbI3, usually studied as a working material for such batteries – methylammonium-lead triiodide – is unstable when heated and when irradiated with intense sunlight due to a phase transition caused by temperature – when heated, the crystal lattice of the substance is rearranged, and its properties are ruined. An alternative is systems based on the formamidinium cation: FAPbI3 or (FACs)PbI3 with greater thermal stability and potentially greater efficiency. The illustration shows the formulas of methylammonium and formamidinium, and the description gives the chemical formulas of the working substances. The addition of cesium further stabilizes the system. Unfortunately, the growth of films from such compounds turned out to be even more difficult to control than for MAPbI3.
Possible structure of the finished product – a perovskite solar cell – layer by layer: fluorine doped tin dioxide (transparent conductor), tin dioxide, perovskite, spiro-OMeTAD (conductive layer for hole transport), gold.
The main question that personally interests me is where is ecology in this whole scheme, taking into account the lead in the working material…
Author: Ivan Prikhno
