Abstract
The ferroelectricity in the hybrid perovskite CH3NH3PbI3 is under debate because it results from the polar molecular cation CH3NH3+ while the molecular orientation was reported to be random. Here we predict that a Pb-free hybrid perovskite N(CH3)4SnI3 with non-polar molecular cation N(CH3)4+ has strong ferroelectricity with a spontaneous polarization of 16.13 μC cm−2. The large polarization results from the distortion of SnI6 octahedron induced by the large N(CH3)4+ and is independent of the molecular orientation, so the ferroelectricity is robust. The ferroelectric R3m perovskite structure of N(CH3)4SnI3 can be synthesized as the ground state under a hydrostatic pressure over 3 GPa and remains stable under ambient pressure. Given the strong ferroelectricity, good stability and high visible-light absorption, N(CH3)4SnI3 may be an ideal light-absorber semiconductor for high-efficiency solar cells because its ferroelectric polarization can facilitate electron-hole separation and produce large bulk photovoltaic effect, making the design of homogeneous bulk photovoltaic devices possible.
Highlights
The ferroelectricity in the hybrid perovskite CH3NH3PbI3 is under debate because it results from the polar molecular cation CH3NH3+ while the molecular orientation was reported to be random
The organic–inorganic hybrid perovskites (OIHPs) drew intensive attention in the past decade as the light-absorber material in the all-solid-state solar cells and the power conversion efficiency (PCE) of the solar cells based on the hybrid perovskites such as CH3NH3PbI3 or NH2CHNH2PbI3 is as high as 25.2%1–5, which is getting closer to that of the monocrystalline silicon[6], and higher than that of the commercialized thin-film solar cells based on CdTe (Eg = 1.5 eV) and CuInxGa1−xSe2 (Eg = 1.0–1.7 eV, PCE = 20%)[7,8]
The ferroelectricity of CH3NH3PbI3 was studied both experimentally and theoretically, the evidence of its ferroelectricity is still under debate and not robust[11,12]. For hybrid perovskites such as CH3NH3PbI3 and [(CH3)3NCH2I]PbI3, the A site is occupied by polar organic molecular cations, which can produce spontaneous polarization if the orientation of the polar molecular cations is ordered along a certain direction
Summary
The ferroelectricity in the hybrid perovskite CH3NH3PbI3 is under debate because it results from the polar molecular cation CH3NH3+ while the molecular orientation was reported to be random. Firstprinciples calculations showed that the polarization induced by the B-site ion displacement is very weak in CH3NH3PbI3, and OIHPs with non-polar molecular cation on A site have seldom been reported to have strong ferroelectricity so far[14]. We find unexpectedly that a Pb-free hybrid perovskite N(CH3)4SnI3 with the non-polar molecular cation N(CH3)4+ at A site and R3m structure can have robust and strong ferroelectricity with an intrinsic spontaneous polarization as large as 16.13 μC cm−2. As far as we know, N(CH3)4SnI3 is a hybrid perovskite with both strong ferroelectricity and suitable band gap for the visible light absorption, so it offers an ideal material candidate for fabricating stable and Pb-free hybrid perovskite solar cells with a high efficiency enhanced by ferroelectricity
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