Electrodeposition in Ionic Liquids for Controlled Synthesis of New Class of Ionic Semiconductors Based on Binary and Ternary Chalcogenide Perovskites for Thin Film Photovoltaic Solar Cells
Authors: Asif Omar , Rastogi Alok C 
Published: December 2020

Thin film photovoltaic (PV) solar cells based on Cu(In,Ga)(S,Se)2 (CIGS), CdTe and Cu2ZnSnS4 (CZTS) have been in the forefront of the pursuit for sustainable and pervasive solar electricity utilization. Recently, researchers have focused on alternative semiconductor photo-absorbers that can be synthesized as thin films by simple low cost solution based methods. Efforts in this direction led to solar cells based on the hybrid organic-inorganic perovskite photo-absorbers having the general formula RNH3MX3 where R= CnH2n+1, M= Pb, Sn and X= Cl, I or Br showing significantly high (24.2%) conversion efficiency. However, the light induced instability, mechanical durability and thermal degradation problems though largely elucidated, still require intensive efforts towards alleviation. The key factors for high efficiency of hybrid perovskite cell are high absorption coefficient, defect-tolerance and large carrier diffusion lengths. This is attributed to their ionic character derived from higher cation-anion coordination resulting in enhanced Coulomb interaction. The entirely inorganic chalcogenide perovskites are the ionic semiconductor counterparts which are purported to have similar optoelectronic properties sans the stability issues but have not been investigated in detail. The perovskite ionic semiconductors with formula unit ABX3 (A=Ca, Ba or Sr ; B=Ti or Zr; X= S, Se) have direct 0.8-2.4 eV bandgap, high photo-absorption and large carrier mobility. These appear attractive for photovoltaic solar cell application but are needed to be investigated as thin films by emphasizing the synthesis-property correlations. In this work we have investigated electrodeposition synthesis in ionic liquid medium of the transition metal layered chalcogenide TiS2 and p-type CaTiS3 perovskite chalcogenide semiconductor thin films which have application in solar cells, the former as a hole conductor and the later as an efficient photo-absorber.
In this work we describe an ab initio study of scalable electrodeposition of TiS2 and CaTiS3 polycrystalline semiconductor thin films from choline chloride–urea eutectic based ionic liquid electrolytes at 80°C and establish their structure and optoelectronic properties dependent upon the electrodeposition variables. The ionic liquid electrodeposition, not studied in detailed for semiconductor layers, is unique due to large stable potential window, high solubility for metal salts and devoid of hydrogen interference usual in aqueous medium. Further, it enables electrodeposition of semiconductor with elements having high reduction potential like Ca and Ti. We synthesized these films by electrodeposition in a 3-cell assembly with Pt reference and counter electrode and transparent conducting glass (FTO) as substrate. Precursors TiCl4, CaCl2 and Na2S2O3 served as source of Ti, Ca and S and initial cyclic voltammetry (CV) studies with variable precursor composition have established deposition potentials of Ca and Ti as -0.68V and -0.79V vs Pt, respectively.
Detailed CV study with variable Ca:Ti precursor ratio coupled with film composition analysis establishes deposition potential and precursor composition for stoichiometric CaTiS3 film synthesis. X-ray diffraction showed as-electrodeposited films are polycrystalline. The Raman spectra includes numerous peaks in the 50-1200 cm-1 range which are interpreted as characteristic vibration modes of CaS at 170 and 220 cm-1. The variation in the intensity of the A1g mode at 359 cm-1 originating from TiS2 was used to calibrate the deposition potential relative to film composition. Tauc analysis of the uv-vis transmission spectra showed direct bandgap of 1.5 eV which is ideal for terrestrial solar radiation absorption. A correlation of the electrochemical deposition potential with the inclusion of secondary phases was noticed. This also resulted in the increase in the bandgap energy indicating possibility for adjusting bandgap for specific solar cell structures. The TiS2 layered chalcogenide with potential use as hole transport layer in chalcogenide perovskites was likewise electrodeposited. Detailed analysis of the reduction peaks in CV plots shows peak at -0.76V vs Pt corresponding to TiS2 formation and a rapidly rising current wave corresponds to under potential deposition. The direct electrodeposition of TiS2 is by reduction of Ti at substrate and instant sulfurization in presence of S2- ions. The granular growth is directed by FTO which provides electrochemical reduction and nucleation sites for TiS2 polycrystalline film growth as established by X-ray diffraction. All Raman peaks are interpreted belonging to TiS2 and partial phase mixing with the TiS3 depending on the electrodeposition potential was seen. Thus, the observed decrease in the bandgap with increase in potential can be used for integrating TiS2 in solar cells as hole conductor as well as an absorber depending on the configuration, This paper will report on the detailed effect of the variables used in ionic liquid electrodeposition and consequent structural and optoelectronic properties of the two important ionic semiconductors for assessment of their use in solar cells.