Tin-based perovskite solar cell

The main obstacle to viable tin perovskite solar cells is the instability of tin's 2+ oxidation state (Sn²⁺), which is easily oxidized to the stabler Sn⁴⁺.[10] In solar cell research, this process is called self doping,[11] 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.[12] In general, reducing tin vacancies is still ideal, because they impede charge carrier motion and lower efficiency.[13]

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.[14] Alternatively, increasing the size of the organic component is believed to geometrically bar diffusion of oxygen.[15] 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.[16]

Another option is adding reducing agents as sacrificial anodes: these may be as varied as maltol, gallic acid, or hydrazine.[17][18] Tin-based reductants, such as the pure element or stannous halides, also act as a tin source, filling in Sn vacancies.[18]

Annealing perovskite films during deposition also reduces self-doping.[19]