Titanium Disulphide (TiS2) Dichalcogenide Thin Films as Inorganic Hole Transport Layer for Perovskite Solar Cells Synthesized from Ionic Liquid Electrodeposition

In high efficiency organic-inorganic perovskite solar cells formed as a multilayer structure, the hole transporting layer (HTL) at the perovskite absorber layer interface has a critical role. Organic HTLs based on Spiro-OMeTAD and PTAA have led to high efficiencies but displayed poor long-term stability and involves expensive purification processes that hinders universal low-cost commercialization goals for perovskite solar cells. Though as an inorganic alternative, transition metal chalcogenides have been investigated for HTL recently, the hot-injection method often used in synthesis has shown poor reproducibility and difficulty in scaling-up. In this work we demonstrate an ab initio facile inexpensive scalable synthesis of transition metal dichalcogenide (TiS2) by electrodeposition from ionic liquids as a low-cost inorganic HTL for perovskite solar cells. The TiS2 thin films were electrodeposited from choline chloride–urea eutectic based ionic liquid electrolytes at 80°C with Na2S2O3 as sulphur and TiCl4 as titanium source. From cyclic voltammetry studies the deposition potential of TiS2 was optimized at -0.8V vs Pt. The as-deposited TiS2 HTL exhibited polycrystalline structure with preferential growth along (001), (100), (002), (102), (110), (111) planes. The Raman spectroscopy of the films showed peaks around 225 cm−1 and 332 cm−1 attributed to the Eg and A1 g Raman modes respectively. The synthesized thin films demonstrated sharp optical bandgap edge along with bandgap tunability as the bandgap (direct) decreased from 1.53 eV to 1.49 eV, 1.40 eV, and 1.34 eV with gradual change in deposition potential from −0.8 V to −0.9 V, −1.0 V, and −1.1 V vs Pt, respectively. This aspect has potential for alignment of valance band edge in facilitating the hole transport at the perovskite-TiS2 interface. The absorption coefficient in visible-light range of the as-deposited TiS2 thin films likewise has shown a dependence on the synthesis potential which is highly conducive for application as an HTL in multilayer solar cell structure. The TiS2 thin films were observed to be p-type as shown from the Hall effect studies with a carrier mobility up to 14.4 cm2V−1s−1. A detailed study of the effect of the synthesis parameters on the structural, optical, band-edge, and electronic properties of TiS2 thin films suitable for application as HTL in perovskite solar cells is presented.