Electronic and optical properties of lead-free hybrid double perovskites for photovoltaic and optoelectronic applications

Abstract

Developing of lead-free double perovskites have drawn significant interest for photovoltaics and optoelectronics as the materials have the potential to avoid toxicity and instability issues associated with lead-based organometallic perovskites. In this study, we report the optoelectronic properties of a new group of non-toxic lead-free organic-inorganic halide double perovskites composed of caesium (Cs), methylammonium (MA) or formamidinium (FA) with bismuth (Bi) and metal copper (Cu). We perform density functional theory investigations to calculate the structural, electronic and optical properties of 18 Pb-free compounds, ABiCuX₆ [A = Cs₂, (MA)₂, (FA)₂, CsMA, CsFA, MAFA; X = I, Br, Cl] to predict their suitability in photovoltaic and optoelectronic applications. We found that the considered compounds are semiconductors with a tunable band gap characteristics that are suitable for some devices like light emitting diodes. In addition to this, the high dielectric constant, high absorption, high optical conductivity and low reflectivity suggest that the materials have the potential in a wide range of optoelectronic applications including solar cells. Furthermore, we predict that the organic-inorganic hybrid double perovskite (FA)₂BiCuI₆ is the best candidate in photovoltaic and optoelectronic applications as the material has superior optical and electronic properties.

Figure 1

Unit cell of double perovskites Cs2BiCuI6 as an example of the crystal structure of the considered double perovskites ABiCuX6 [A = Cs2, (MA)2, (FA)2, CsMA, CsFA, MAFA; X = I, Br, Cl].

Figure 2

Comparison of the electronic band gap and dielectric constant of the selected group of double perovskites ABiCuX₆ [A = Cs₂, (MA)₂, (FA)₂, CsMA, CsFA, MAFA; X = I, Br, Cl]. (a) Electronic band gap of the materials determined by using GGA-PBE approach. (b) Calculated dielectric constant or static dielectric function for the considered double perovskites.

Figure 3

Calculated electronic properties of the considered double perovskite (FA)₂BiCuI₆. (a) Electronic band structure along the high symmetry direction of the Brillouin zone having path Γ(0,0,0)−F(0,0.5,0)−Q(0,0.5,0.5)−Z(0,0,0.5)− Γ(0,0,0). The bands calculated by GGA-PBE are indicated by blue color whereas the bands calculated by HSE06 are indicated by pink color. The valence band maximum (VBM) is seen at F point whereas the conduction band minimum (CBM) is observed at Z point of the Brillouin zone indicating that it is an indirect band gap semiconductor. (b) Calculated total and partial densities of states. The Cu-3d states (green color curve) and the I-5p (blue color curve) states are seen as the main contributors towards VBM whereas the Bi-6p (pink color curve) and I-5p (blue color curve) states are mostly contributed towards CBM.

Figure 4

Comparison of the optical properties of double perovskites ABiCuX₆ [A = Cs₂, (MA)₂, (FA)₂, CsMA, CsFA, MAFA; X = I, Br, Cl] along the incident electromagnetic radiation of energy from 0 to 5 eV. (a) Calculated dielectric function (real part). (b) Calculated absorption coefficient. (c) Calculated optical conductivity.