Recently, simple perovskites have attracted great attention as the energy-absorbing material in solar cells. In computational and experimental studies they have shown several desirable properties but also challenges, such as instabilities and toxicity due to the presence of lead. As a solution to the problems connected to the simple perovskites, the double perovskites were suggested as a suitable material in the solar cells of tomorrow. In order to use double perovskites in an application it needs to be thoroughly understood. Within this thesis the octahedral tilting and the band gap of double perovskites have been studied, which is a step towards finding a material that is stable and exhibit the optimal band gap to absorb solar energy. It turns out that the octahedral tilting is not as prominent in all double perovskites as in the simple ones. The statement that the octahedral tilting is due to the presence of lone-pair electrons is considered and it agrees with observations - lone-pair electrons induce octahedral tilting. The effect of octahedral tilting on the band gap of the double perovskites is studied as well and it is concluded that the band gap increases with the tilting. As the global warming increases rapidly it is important to find green solutions and as solar cells are renewable and emission-free they are good candidates for further developement.
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