Tin-based perovskite solar cell

A tin-based perovskite solar cell is a special type of perovskite solar cell, based on a tin perovskite structure (ASnX3, where 'A' is a 1+ cation and 'X' is a monovalent halogen anion). As a technology, tin-based perovskite solar cells are still in the research phase, and are even less-studied than their counterpart, lead-based perovskite solar cells. The main advantages of tin-based perovskite solar cells are that they are lead-free. There are environmental concerns with using lead-based perovskite solar cells in large-scale applications; one such concern is that since the material is soluble in water, and lead is highly toxic, any contamination from damaged solar cells could cause major health and environmental problems.

The maximum solar cell efficiency reported is 14.6% for a modified formamidinium tin triiodide-based (CH(NH2)2SnI3) composition with additional NH4SCN and PEABr, 5.73% for CH3NH3SnIBr2, 3% for CsSnI3 (5.03% in quantum dots), and above 10% for various compositions based on formamidinium tin triiodide (CH(NH2)2SnI3). Formamidinium tin triiodide in particular may hold promise because, applied as a thin film, it appears to have the potential to exceed the Shockley–Queisser limit by allowing hot-electron capture, which could considerably raise the efficiency.

Methylammonium tin triiodide (CH3NH3SnI3) has a band gap of 1.2–1.3 eV, while formamidinium tin triiodide has a band gap of 1.4 eV.

Self-doping
The main obstacle to viable tin perovskite solar cells is the instability of tin's oxidation state Sn2+, which is easily oxidized to the stabler Sn4+. In solar cell research, this process is called self-doping, because the Sn4+ acts as a p-dopant and reduces solar cell efficiency. The vacancy defects that promote this process are the subject of active research; folk wisdom holds that the process requires tin vacancies, but in CsSnI3, the primary hole contributors are instead Cs vacancies. In general, reducing tin vacancies is still ideal, because they impede charge carrier motion and lower efficiency.

Several techniques have been explored as a means of counteracting the self-doping of Sn-based perovskites. One method is the sealing of cells with polymers such as poly(methyl methacrylate) so that they are not exposed to oxygen. Alternatively, increasing the size of the organic component is believed to geometrically bar diffusion of oxygen. However, these techniques do not counteract Sn4+ ions formed during cell synthesis. Such ions can be with a chelating ligand, e.g. formamidinium chloride; the tin coordination complex can then be removed with gentle (<60 °C) heat. As long as the temperature to vaporize the complex is below that at which the perovskite loses mass, the perovskite film will remain intact after this processing step, save for the removed Sn(IV) ions.

Another option is adding reducing agents as sacrificial anodes: these may be as varied as maltol, gallic acid, or hydrazine. Tin-based reductants, such as the pure element or stannous halides, also act as a tin source, filling in Sn vacancies.

Annealing perovskite films during deposition also reduces self-doping.

Morphology of thin films
Another challenge of tin perovskite solar cells is to be found in the rapid crystallization of tin perovskite, often leading to poor morphology, high pinhole density and incomplete substrate coverage. The morphology of the tin perovskite thin films has been improved via vapor-assisted processing and hot antisolvent methods. Other studies suggest that the addition of methylammonium chloride into the precursor solution improves the morphology of the tin perovskite thin films.