Bandgap tuning of bismuth ferrite by site engineering using samarium and cobalt for photovoltaic application

Abstract

Bismuth ferrite (BiFeO3) is one of the most promising candidates for photovoltaic application, due to its favorable bandgap energy (Eg) in the range of 2.0-2.7 eV which is smaller than many other ferroelectric perovskites (ABO3). Basically, BiFeO3 or BFO is a multiferroic material which simultaneously shows the ferroelectric and ferromagnetic properties in a single phase at room temperature. An enormous interest is grown to the scientific community to use BFO as functional materials for PV cells beside its wide spintronics applications. Thus, lowering the Eg of BFO to the optimum level is a promising way to obtain higher power conversion efficiency (PCE) of PV cells for maximum exploitation of solar spectrum. In this research, tune the Eg with rare earth metal Samarium (Sm) and transition metal Cobalt (Co) were doped in individually and simultaneously at A and B-site in pure BFO nanoparticles. The compositions chosen for this research include pure BiFeO3 (BFO), Sm-doped Bi(1-x)SmxFeO3 (x=0.05, 0.1 and 0.15) (BSFO), Co-doped BiFe(1-y)CoyO3 ( y=0.05, 0.1 and 0.15) (BFCO) and co-doped Bi(1-x)SmxFe(1-y)CoyO3 ( x=0.1 and y= 0.05, 0.1 and 0.15) (BSFCO) nanoparticles. The compositions were synthesized by sol-gel method. The thermal decomposition behavior of pure, doped and co-doped BFO xerogel powders was first investigated. The synthesized nanoparticles were then annealed at 400 to 800 °C to obtain proper crystallinity, also proved that 600 °C is the optimum annealing temperature. X-ray diffraction (XRD) shows the well-arranged crystalline with rhombohedral structure and appropriate peaks of pure, doped and co-doped BFO nanoparticles. XRD also revealed structural transformation from rhombohedral to orthorhombic symmetry in the synthesized nanoparticles. Crystallite size was decreased and lattice strain was increased with doping concentration. The crystal structure of BFO was constructed from the obtained crystallographic information file (CIF) of XRD results. It has shown to decrease in bond length (Fe–O) and increase in bond angle (Fe–O–Fe) with increasing doping concentration. Field emission scanning electron microscopy was conducted to characterize microstructural features. A significant reduction in particle size was found with the variation of doping concentration. The presence of Sm and Co dopants in BFO was semi-quantitatively identified by energy dispersive X-ray spectroscopy. The dielectric properties such as dielectric constant, dielectric loss, resistance, reactance, impedance, AC conductivity, AC resistivity, real and imaginary part of electric modulus and Cole-Cole plot were investigated for pure, doped and co-doped BFO nanoparticles as a function of frequency. The electric conductivity was found to increase with increasing dopants concentration. Similarly, electric resistivity was found to decrease with increasing dopants concentration. In addition to this, prospect of spintronic applications with respect to magnetic memory devices will also be studied to explore its multiple functionalities. The electric field dependent ferroelectric properties (P-E hysteresis loop) were investigated of the nanoparticles by using ferroelectric loop tracer. The values of maximum polarization, remnant polarization and coercive field were measured from the loops and found to enhance with increasing dopants concentration. The magnetic properties (M-H hysteresis loop) were measured of the samples by using vibrating sample magnetometer. The magnetic parameters like saturation magnetization, remnant magnetization and coercive field were calculated from the loops and found to increase significantly with increasing dopants concentration. The bandgap energy was calculated for pure, single-doped and co-doped BFO nanoparticles by employing UV-Vis-NIR spectrophotometry, also proven by photoluminescence method. In this pioneering work, the bandgap energy has shown to reduce to an optimum level of 1.40 eV required for the maximum efficiency of photovoltaic solar cells.