What are Perovskite Solar Cells?
Photovoltaic solar cells depends on semiconductors - materials in the middle ground between electrical insulators like glass and metallic conductors such as copper - to turn the energy from light into electricity. Light from the sun excites electrons in the semiconductor material, which flow into conducting electrodes and produce electric current. Primary semiconductor material silicon used in solar cells since the 1950s, because of its semiconducting properties align well with the spectrum of the sun’s rays and it is comparatively abundant and stable. Therefore, the large silicon crystals used in conventional solar panels need an expensive, multi-step manufacturing process which uses a lot of energy. Scientists had been searching for an alternative and harnessed the tun-ability of perovskites to create semiconductors with similar properties to silicon. Perovskite solar cells can be manufactured using simple, additive deposition techniques, like printing, for a fraction of the cost and energy. Because of the compositional flexibility of perovskites, they can also be tuned to ideally match the sun’s spectrum.
Unlike silicon solar cells, perovskites solar cells are less expensive and fabrication can be done by simple wet chemical process. Perovskite materials provide excellent light absorption, charge-carrier mobilities, and lifetimes, resulting in high device efficiencies with opportunities to realize a low-cost, industry-scalable technology. Actual mineral perovskite consists of calcium, titanium and oxygen in the form CaTiO3. Meanwhile, a perovskite structure is anything that has the generic form ABX3 and the same crystallographic structure as perovskite mineral.
The lattice arrangement of perovskite is demonstrated below. It can be demonstrated in various ways, because of many structures in crystallography. The easiest method to think about a perovskite is as a large atomic or molecular cation (positively-charged) of type A in the centre of a cube. The corners of the cube are then occupied by atoms B (also positively-charged cations) and the faces of the cube are occupied by a smaller atom X with negative charge (anion).
Based on the structure of atoms/molecules, perovskites can have an impressive array of interesting properties which includes superconductivity, giant magneto-resistance, spin-dependent transport and catalytic properties. Perovskites used in solid-state solar cells have following combination of materials in the usual perovskite form ABX3:
A = An organic cation - methyl-ammonium or formamidinium
B = A big inorganic cation - usually lead(II)
X3= A slightly smaller halogen anion – usually chloride or iodide
Perovskite solar cells have equivalent efficiencies of Cadmium Telluride (CdTe). Lead-based perovskite solar cells are comparatively better due to a range of factors such as strong absorption in the visible regime, long charge-carrier diffusion lengths and a tune-able band gap. Manufacturing process of these cells are comparability easy because of the ability to process at low temperatures and the high defect tolerance. The perovskite film itself is usually processed by either vacuum or solution methods. Film quality is very important. The dramatic rise in perovskite solar cell efficiency is still incredibly significant and impressive.
Basically, the active layer of a perovskite solar cell is deposited via either a one or two-step process. In the one-step process, a precursor solution like a mix of methyl-ammonium iodide and lead iodide is coated which converts to the perovskite film based on heating. A variation on this is the ‘anti-solvent’ method, in which the precursor solution is coated in a polar solvent, and then quenched during the spin coating process by a non-polar solvent. Exact timings of the quench and volumes of the quenching solvents is needed for the best performance.
In the two-step process, the metal lead iodide and organic components like methyl-ammonium iodide are spin-coated in separate, subsequent films. Alternatively, metal lead iodide films can be coated and galvanized in a chamber filled with the organic component vapor which is called "vacuum-assisted solution process".