Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV16)
Publication date: 28th March 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 the relevance of distinct charge-carrier recombination mechanisms, such as trap-mediated, bi-molecular (electron-hole) and Auger recombination, which show different dependences on composition and temperature [1,2]. We use these insights to predict charge-carrier diffusion lengths and radiative efficiencies in the limit of ultra-low trap-related recombination, which could potentially be reached through further advances in material processing. We find that for hybrid lead iodide perovskites with typical charge-carrier mobilities of ~30cm2/(Vs), charge-carrier diffusion lengths under solar irradiation are unlikely to exceed ~10μm even if all trap-related recombination is eliminated [3]. In addition, we show how parameters essential for photovoltaic operation, such as charge carrier mobilities 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 material systems that have attracted interest lately, lead-free CH3NH3SnI3 (optical bandgap ~1.3 eV) and the mixed organic lead iodide/bromide system FAPb(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 introduces fast recombination pathways in CH3NH3SnI3 that limit the charge-carrier diffusion length[4]. 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. Charge-carrier diffusion and recombination in FAPb(BryI1-y)3 exhibits a complex interplay between changes in morphology and electronic bandstructure with bromide fraction y[2]. In particular, a “stability gap” in the central region (y=0.3–0.5) is associated with low crystallinity and charge-carrier mobility[2]. 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 (21cm2/(Vs)) and diffusion lengths[5].
References:
[1] Milot, Eperon, Snaith, Johnston, Herz, Adv. Func. Mater. 25, 6218 (2015).
[2] Rehman, Milot, Eperon, Wehrenfennig, Boland, Snaith, Johnston, Herz, Adv. Mater. 27, 7938 (2015).
[3] Johnston and Herz, Acc. Chem. Res. 49,146 (2016).
[4] Noel, Stranks, Abate, Wehrenfennig, Guarnera, Haghighirad, Sadhanal, Eperon, Johnston, Petrozza, Herz, Snaith, Energy Environ. Sci. 7, 3061 (2014).
[5] McMeekin, Sadoughi, Rehman, Eperon, Saliba, Hörantner, Haghighirad, Sakai, Korte, Rech, Johnston, Herz, Snaith, Science 351, 151 (2016).
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