Influência da inserção dos íons Bi3+, La3+, Fe3+ e Mn3+ na distorção da rede cristalina, redução do bandgap e efeito fotovoltaico do KNbO3

Abstract

Sunlight is a renewable source of energy, virtually ilimitated and with potential to replace fossil fuels. The conversion of sunlight in electricity is possible due to photovoltaic effect present in materials used in solar cells, also known as photovoltaic cells. The asymmetry due to spontaneous electrical polarization promotes photovoltaic effect in ferroelectric materials, making possible the creation of photoferroelectric devices for energy harvesting. Although ferroelectric materials such as PZT and BiFeO3 are being extensively investigated, only little research is being done about the potential use of KNbO3 in such usage. This work investigates in an unprecedented way the synthesis and characterization of the compounds (1-x)KNbO3–xABO3, where A = Bi or La, B = Fe or Mn and x = 0; 0.01; 0.02; 0.04; 0.08; 0.16; 0.32; the distortion caused by insertion of the ions Bi3+ or La3+ and Fe3+ or Mn3+ into KNbO3 crystal lattice, its polarization and bandgap reduction, as well as its application in ferroelectric photovoltaic devices. X-rays diffraction and Raman spectroscopy revealed alterations in crystal lattice of KNbO3 when increasing x, as well as the maintenance of long range polar order (i.e. ferroelectricity) in all synthesized compositions. The bandgap of the compositions showed to be continuously narrowed when increasing x for all compositions, reaching maximum reduction of 0.9 eV for 0.68KNbO3–0.32BiFeO3. There was a clear alteration in the behavior of the I–V curve of KNbO3 compared to its solid solutions. Particularly, samples 0.68KNbO3–0.32BiFeO3, 0.68KNbO3–0.32LaFeO3, 0.92KNbO3–0.08BiMnO3 and 0.84KNbO3–0.16LaMnO3 showed diode behavior expected for solar cells. Sample 0.92KNbO3–0.08BiMnO3 resulted in the best combination of current and voltage results, 0.6 μA and 0.16 V, respectively. This result is the best reported among similar compounds already studied.