Publication date: 3rd July 2020
In this work we demonstrate how an PbS QDs can stabilize the formamidinium perovskite without affecting its original properties. The formamidinium has the lowest bandgap in the perovskite material, but Its black cubic phase, desired for the PV application is not stable at ambient temperature, and a yellow hexagonal one is stable in ambient conditions. [1] We embed the PbS QDs in the perovskite matrix, as they have the same lattice parameter of the perovskite, and upon a screening on the concentrations and size of the QDs, we found that band gap is preserved, the grains become bigger, the film more stable in air and the solar cell enhances the efficiency, the voltage and the stability. The different mechanism behind the stabilization are investigated by DFT calculations. The isolated effect of the strain on the bulk is evaluated by forcing that the bulk perovskite matches with the PbS lattice: black (cubic) and yellow (hexagonal) bulk phases are forced to match the lattice constant of PbS on (100) and (111) surfaces, respectively, which are the most compatible crystallographic planes in each case. Secondly, the introduction of the surface energy inverts the trend destabilizing the yellow phase. Finally the chemical bonds between the FAPI and the PbS stabilize the black phase. In conclusion the stabilization is due to two distinct, but both needed, mechanisms: first, the structure stabilization that destabilizes the yellow phase due to its large surface energy and, second, the crucial chemical stabilization by chemical bonds between the PbS and formamidinium perovskite that stabilizes the black phase. [2] So the QDs could be used for the bulk epitaxial growth of the perovskite or other materials with metastable phase, which can be stabilized by the chemical bonds creation between the two different materials.
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