p-Type Doping of Cs₂SnI₆ Halide Perovskites Using Charge Transfer Dopants

Perovskite solar cells (PSCs) have shown remarkable progress in recent years, surpassing the efficiency of conventional thin-film photovoltaic (PV) technologies such as CdTe and CIGS. The recent discoveries in organic-inorganic hybrid PSCs resulting from synergistic optimization of the perovskite absorber layer have generated remarkable development in device efficiency from 3.8% in 2009 to the present state-of-art 25.7%. Organic-inorganic halide perovskites have also shown 31.3% efficient tandem devices with Si bottom cells. Mixed-halide and mixed-cation systems have demonstrated high efficiency, improvement in stability and reduced hysteresis. The emergence of 2D perovskite passivation and 2D/3D heterojunctions, have shown promise by enhancing the long-term stability of Pb-based halide PSCs, however, addressing toxicity challenge is crucial for large scale adoption.<br/>A2BX6 is a defect variant of the general perovskite structure ABX3 where half of the vacant B-cations results in corner-sharing octahedron of anions [BX6]2- and A-cation occupying the sites between the octahedron in 12-fold coordination. Alteration and substitution of B-cations and/or halogen anions (Cs2TeI6, Cs2PtI6, Cs2PbI6, Cs2TiBr6, Cs2AgBiBr6, Cs2AgBiCl6, etc.) offer a multitude of features in these double perovskite structures with enhanced thermal and moisture stability. Cs2SnI6 double perovskite has emerged as a promising light absorber, with a 4+ oxidation state of tin and shorter Sn-I bond length (2.85 Å vs. 3.11 Å in CsSnI3), making it oxidation resistant and air-stable. Cs2SnI6 has a direct bandgap of 1.3-1.6 eV, and the closed-packed halide framework results in dispersed conduction and valence bands leading to low electron effective masses of 0.48m0 for electrons and 1.32m0 for holes. Cs2SnI6 exhibits electron and hole mobilities of 310 cm2V-1s-1 and 42 cm2V-1s-1, respectively, and is believed to be an ambipolar material due to formation of n-type iodide vacancies/tin interstitials or p-type cesium vacancies. Devices up to 5% PCE have been demonstrated with enhancements such as Cl, F, Rb, Ag, In doping; ZnO nanorod electron transport layer (ETL); ethylene diamine post-deposition treatments. However, state-of-art Cs2SnI6 devices show high bulk and interface recombination as indicated by low carrier lifetime and high dark saturation current. Power conversion efficiency of &gt; 5% has been demonstrated for planar p-i-n Cs2SnI6 devices; however, to increase the device efficiency further robust methods of defect passivation and improved doping density are needed. Here we will present the results of a controlled study on charge transfer doping using molecules such as F4TCNQ for Cs2SnI6. When applied to MAPb_0.5Sn_0.5I₃, the electrical conductivity increased by 5 orders of magnitude, the hole density increased by 1 order of magnitude, and the passivation of charge carrier traps increased the carrier recombination lifetime. The molecular dopant F4TCNQ also fits the criteria to be an effective p-type dopant for Cs₂SnI₆, due to its lowest unoccupied molecular orbital (LUMO) value being less than the valence band maximum (VBM) value, so as to facilitate effective charge transfer for holes. Data on film processing, doping density and carrier lifetime will be presented for Cs2SnI6 processed by doctor blade method with and without molecular dopants. The effect of the dopants on phase stability, absorption bandgap will also be studied.