Lead Replacement in CH₃NH₃PbI₃ Perovskites

DOI: 10.1002/aelm.201500089 The advantages of APbI 3 perovskites in the solar cells fi eld are prominent in their optical and electronic properties. The small direct band gap of 1.55–1.61eV leads to a very board absorption range almost over the entire visible light region. [ 8,9 ] The direct band gap p–p transition, which was reported from a series of ab initio calculations, [ 9–11 ] makes these perovskites with better light absorption abilities than that of common p–s transition type thinfi lm solar cells materials such as CdTe. Moreover, the excitons generated by light absorption in perovskite are easy to be dissociated into free carriers at room temperature (the binding energy of exciton is few meV). [ 12,13 ] In addition, small carrier effective mass estimated from density function theory (DFT) calculations [ 14,15 ] is competitive to monocrystal silicon, with experimentally obtained long carrier lifetime and long carrier diffusion lengths exceeding a micrometer [ 16,17 ] Such long carrier diffusion lengths enable the solar cell application of thin APbI 3 fi lm with thickness of ≈500 nm by further improving their optical absorption effi ciency. To further explore the potentials of such materials, work on the underlying mechanisms of those advantages are still ongoing. [ 18–21 ] These work mainly focused on the investigations of functionalities for each part consisting of AMX 3 , namely, A, M, X, and/or their interactive behaviors. For instance, DFT calculations were employed to investigate the role of the organic cations A + in APbI 3 perovskites. [ 9,22 ] It was found that by choosing molecular cations A + with different ionic radius, the lattice volume can be easily altered, thus making the band gap tuning of perovskites more fl exible. [ 22,23 ] Also, the random orientation of polar molecular cation A + in APbI 3 will bring out nanoscale charge localization in the two band extremes according to Ma and Wang. [ 24 ] The charge localization in valance band maximum (VBM) and conduction band minimum (CBM) have no overlapping, suggesting that two separated “high-ways” for electrons and holes are responsible for the long carrier lifetime observed in the experiments. [ 25 ] Rensmo and co-workers showed that a shift of the valence band edges will occur for the APbBr 3 perovskite due to an intrinsic binding energy difference of the halide ions. [ 26 ] Besides, the solar cells using mixed-halide perovskites exhibited long carrier diffusion distances and high effi ciencies. By different mixture ratio of halide, the band gap of perovskites can be dramatically changed. [ 27 ]