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Chao Zheng1 Oleg Rubel1

1, McMaster University, Hamilton, Ontario, Canada

As the low-cost and high efficient solar cell absorbers, halide hybrid perovskites increasingly catch researchers attention. However, the instability of the materials is still not solved. Based on our latest research, the ionization energy of A site molecule can be a very important factor which determines the thermodynamical stability of hybrid halide perovskites [1]. Meanwhile, the size of the molecule determines the stable phase at room temperature and the bandgap. Here we present a new three-membered ring radical (CH2)2NH2 which demonstrates a low ionization energy, that implies a good stability, and owns a reasonable size that generates suitable bandgap for photovoltaics. We use density functional theory (DFT) method to research the three-membered cyclic cation based perovskite’s structural, stability, electronic properties.

The choice of the exchange–correlation functional is important for DFT to accurately predict the correct polymorphism transition order of halide hybrid perovskites. It can shed light on predicting the room temperature favorable phase. From the experimental point of view, CH3NH3PbI3 prefers the orthorhombic phase as the low-temperature structure. However, the hexagonal phase demonstrates the lowest total energy at 0 K when the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional is used, which contradicts to the experimental observation. We found that when the van der Waals correction vdW(D3) is added to the PBE, the order of polymorphism of CH3NH3PbI3 is consistent with the experimental data. We suggest the PBE+vdW(D3) method should be a prefered choice for structural and total energy calculations of halide hybrid perovskites.

It is found from the calculations that the ionization energy of the three-membered radical (CH2)2NH2 is lower than both CH3NH3 and HC(NH2)2. The lower ionization energy of the organic cation, the more stable the halide hybrid perovskites will be. Then, we use PBE+vdW(D3) method to estimate the stability of (CH2)2NH2PbI3, and we found that the reaction enthalpy of (CH2)2NH2PbI3 is lower than other mainstream perovskites (CH3NH3PbI3, HC(NH2)2PbI3). It is reported that compared with CH3NH3PbI3, CH3NH3PbCl3 is more stable at ambient condition. Our calculated result demonstrates that the stability of (CH2)2NH2PbI3 is even better than CH3NH3PbCl3.

We use G0W method to calculate the bandgap of cubic (CH2)2NH2PbI3 which is 1.49 eV that is 0.09 eV higher than the calculated value of HC(NH2)2PbI3, but 0.17 eV lower than CH3NH3PbI3. We also found that the energy difference between low-temperature and high-temperature phases correlates with the high-temperature transition point. Based on this finding, we conclude that (CH2)2NH2PbI3 will be stable at cubic phase above approximately 190 K.

[1] C Zheng, and O Rubel. "Ionization energy as a stability criterion for halide perovskites." J. Phys. Chem. C, 2017, 121 (22), pp 11977–11984

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