Proceedings of Perovskite Thin Film Photovoltaics (ABXPV17)
Publication date: 18th December 2016
Hybrid metal halide perovskites (stoichiometry AMX3) have recently emerged as low-cost active materials in PV cells with power conversion efficiencies in excess of 20%. We discuss how parameters essential for photovoltaic operation, such as crystallinity, photostability, charge carrier mobility and diffusion lengths are altered with substitutions of the organic A cation (e.g. methylammonium versus formamidinium), the metal M cation (e.g. Pb2+ or Sn2+) and the halide X anion (I- versus Br-). We focus on two 3D perovskite systems that have attracted interest lately, lead-free ASnI3 (optical bandgap ~1.3 eV) and the mixed organic lead iodide/bromide system APb(BryI1-y)3 whose band gap can be tailored between ~1.5 eV (FAPbI3) and ~2.3 eV (FAPbBr3). We show that unintentional hole doping in tin iodide perovskites introduces fast recombination pathways that limit the charge-carrier diffusion length. However, changes in crystal structure appear to subtly influence the relative alignment of dopant levels with respect to the valence band, offering a route to reduced background hole densities [1]. In addition, we demonstrate that such hole doping introduces a radiative quasi-monomolecular charge recombination channel that supports efficient light emission even in the low charge-carrier density regime [2]. In addition, we demonstrate that charge-carrier diffusion and recombination in FAPb(BryI1-y)3 depends on a complex interplay between changes in morphology and electronic bandstructure with bromide fraction y [3]. In particular, a “stability gap” that leads to photo-induced halide segregation in the central region (y=0.3–0.5) is associated with low crystallinity and charge-carrier mobility [4,5]. We show that the replacement of a small fraction of FA with caesium (e.g. FA0.83Cs0.17Pb(I0.6Br0.4)3) lifts this instability allowing for high charge-carrier mobilities (21 cm2/(Vs)) and diffusion lengths [4]. We find that the substitution range of 10-30% caesium fraction is associated with higher crystallinity which correlates with improved optoelectronic properties and photo-stability [5].
References
[1] Parrott, Milot, Stergiopoulos, Snaith, Johnston, Herz, J. Phys. Chem. Lett. 7, 1321 (2016).
[2] Milot, Eperon, Green, Snaith, Johnston, Herz, J. Phys. Chem. Lett. 7, 4178 (2016).
[3] Rehman, Milot, Eperon, Wehrenfennig, Boland, Snaith, Johnston, Herz, Adv. Mater. 27, 7938 (2015).
[4] McMeekin, Sadoughi, Rehman, Eperon, Saliba, Hörantner, Haghighirad, Sakai, Korte, Rech, Johnston, Herz, Snaith, Science 351, 151 (2016).
[5] Rehman, McMeekin, Patel, Milot, Johnston, Snaith, Herz, Energy Environ. Sci. ASAP (2017).
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