2D and Quasi-2D Lead Organohalide Perovskites with Tunable Band Gap and Enhanced Conductivity
Alberto Fraccarollo a, Leaonardo Marchese a, Maurizio Cossi a
a DISIT, Università del Piemonte Orientale
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Poster, Maurizio Cossi, 268
Publication date: 11th February 2019

The best PV performances obtained by hybrid organohalide perovskites come from systems with 3D structure, necessarily intercalated by small size organic cations: nonetheless, a large interest has grown recently for layered perovskites, with either pure 2D or mixed 2D/3D structures, which allow a much wider choice for the interlayer cations. [1]

Layered hybrid perovskites exhibit improved filmability and -more important- a much better stability to moisture, thanks to the hydrophobic barrier provided by the large organic cations. Presently, obtaining layered perovskites with efficiencies comparable to that of common 3D systems while keeping their improved stability is on the most interesting challenges in the field of perovskites PV. [2,3]

We present a DFT/GW theoretical study of the structure and electronic properties of layered hybrid perovskites, based either on PbI4 sheets (2D) or on (Methylammonium)n-1PbnI3n+1 slabs (quasi-2D, n=2, 3) and intercalated by divalent organic cations. [4,5]

We found that with a suitable choice of the intercalated cations, it is possible to modify the material band gap (what is not feasible in 3D systems) so that some organic-based electronic levels enter the inorganic band gap, allowing electronic transitions with energy in the optical/NIR region.

In addition, the presence of such intergap states strongly enhances the electrical conductivity along the direction perpendicular to the lead halide sheets, especially in the quasi-2D systems (n=2, 3). The low and anisotropic conductivity is one of the most serious limitations to the PV efficiency for the quasi-2D perovskites synthesized so far: we predict that using the proposed dications, possibly mixed with other mono- or di-valent organic cations, in the inter-slab region could actually improve the performance of such systems substantially.

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