Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
Metal-halide CH3NH3PbX3 perovskites recently opened a new route towards low-cost manufacture of photovoltaic cells [1]. Converting sunlight into electrical energy depends on several factors among which a broad absorption across the solar spectrum and attractive charge transport properties are of primary importance [2]. Our goal is to propose solid-state physics concepts [3]. Using modelling and extensive analysis of published experimental data,we show in this presentation (figure 1a), that the appropriate absorption and transport properties are afforded by the multi-bandgap and multi-valley nature of their band structure. The strength of the optical transitions calculated at various points of the Brillouin zone and the connectivity of the Fermi surface was used to analyse recent dynamical studies [3].
We also investigate the nature of the photoexcited species [2]. Inspection of the dielectric responses over a wide frequency range allows clarifying the nature of the photoexcited species created in these materials. In particular, the Wannier-like exciton evidenced at low temperature becomes almost entirely screened at room temperature, due to collective rotational motion of the organic cations. The approaches developed for the reorientation dynamics of CN- in alkali-cyanides is used to understand such a dynamics in hybrid perovskites [4]. In mixed-halides perovskites, the insertion of a halide of another size is expected to prevent such collective reorientations, thus reducing exciton screening and enhancing optical absorption. At ambient conditions, charge carriers are rotational polarons, i. e. quasiparticules dressed by the reorientation of neighbouring cations (figure 1b). This picture is also consistent with the counterintuitive increase of carrier mobility experimentally evidenced upon doping. Finally, we further suggest CH3NH3SnI3-xBrx as a good alternative to circumvent toxicity.
a) (left) Electronic band diagram of the high temperature Pm3m cubic phase of CH3NH3PbI3 taking spin-orbit coupling into account. Carrier localization and transport after optical excitation are sketched. (right) Fermi surface in the first Brillouin zone. R and M are connected along the edges, highlighting the saddle point nature of M secondary optical band gap. b) Artist view of a quasiparticle hole dressed by the interaction with neighbouring rotating cations and travelling across the crystal.
[1] Burschka, J. et al. Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature. 499, 316 (2013). Liu, M. et al. Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature. 501, 395 (2013). [2] Stranks, S. D. et al. Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber Science. 342, 341 (2013). Xing, G. et al. Long-range balanced electron- and hole-transport lengths in organic-inorganic CH3NH3PbI3. Science. 342, 344 (2013). Antonietta, M. et al Hybrid solar cells: Perovskites under the sun. Nature Mater. 12, 1087 (2013). [3] Even, J. et al, Electronic model for self-assembled hybrid organic/perovskite semiconductors: Reverse band edge electronic states ordering and spin-orbit coupling. Phys. Rev. B. 86, 205301 (2012). Even, J. et al,Importance of spin–orbit coupling in hybrid organic/inorganic perovskites for photovoltaic applications. J. Phys. Chem. Lett. 4, 2999 (2013). Even, J. et al,DFT and k • p modelling of the phase transitions of lead and tin halide perovskites for photovoltaic cells. Phys. Status Solidi RRL. 8, 31 (2014). [4] Ortiz-Lopez, J. Dipolar Reorientation and Order-Disorder Behavior of Pure and Mixed Alkali Cyanides. Phys. Rev. Lett. 50, 1289–1292 (1983). Lynden-Bell, R. M, Translation-rotation coupling, phase transitions, and elastic phenomena in orientationally disordered crystals. Rev. Mod. Phys., 66, 721-761 (1994).
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