Charge-Carrier Dynamics, Mobilities and Diffusion Lengths of 2D-3D Lead Halide Perovskites
Leonardo Buizza a, Zhiping Wang a, Timothy Crothers a, Rebecca Milot a, Henry Snaith a, Michael Johnston a, Laura Herz a
a Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU, United Kingdom
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, Leonardo Buizza, 056
Publication date: 11th February 2019

Perovskite solar cells have improved drastically over the past decade, overcoming hurdles of temperature- and water-induced instability to achieve efficient, stable devices. Three-dimensional (3D) perovskites have excellent properties including high charge-carrier lifetimes and mobilities, strong absorption and good crystallinity – ideal for photovoltaic devices. However, 3D perovskite materials struggle especially with moisture-induced degradation [1]. The addition of large, hydrophobic organic cations can lead to the formation of two-dimensional (2D) perovskite structures. Devices made with 2D perovskites show much greater stability, but with far lower power conversion efficiencies than their 3D cousins [2]. Materials combining 2D and 3D structures have thus recently become one the most promising candidates for use in solar cells [3, 4]. In order to fully understand the optoelectronic properties of these 2D-3D hybrid systems we look at BAx(FA0.83Cs0.17)1-xPb(I0.6Br0.4)3 across the composition range 0 ≤ x ≤ 80 %. We find that small amounts of butylammonium (BA) help to improve crystallinity and passivate grain boundaries, thus reducing monomolecular charge-carrier recombination, and boost charge-carrier mobilities. Excessive amounts of BA lead to poor crystallinity and inhomogeneous films forming, greatly reducing charge-carrier transport capabilities. For low amounts of BA the benevolent effects of reduced recombination and enhanced mobilities lead to outstanding diffusion lengths.

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