First-principles Study Of Novel Mixed-valence Double Perovskite Photovoltaic Materials

The progess of science and technology is always accompanied by the full use of energy.Whereas the over-exploitation of traditional energy has seriously damaged the ecological balance of the earth,green and pollution-free renewable energy are the mainstream nowadays.Among renewable energy sources,solar energy has attracted much attention due to its abundant reserves and readily to obtain.From the first generation of monocrystalline silicon solar cells to the second generation of thin film solar cells,then go further to the novel solar cells,the perovskite solar cells have emerged in recent years.According to the US national renewable energy laboratory（NREL）,its latest efficiency reached 25.7%,which is expected to become the main force of the third-generation solar cells.However,two key problems of perovskite materials greatly limit their commercialization:they tend to breakdown in moisture which makes them instable;lead toxicity of lead-based perovskite in high-efficiency perovskite solar cells.The current solutions are mainly to improve the instability of perovskite materials and to find alternative materials for perovskite solar cells.The mixed-valence double perovskite A₂B^IB^IIIX₆ has enormous explore space and multiple applications due to their diversity in selecting elements at different site.Thus,we considered from the perspectives of stability and photoelectric properties and carried out first-principles calculations of mixed-valence double perovskite materials,aiming to screen stable and lead-free perovskite photovoltaic materials with high efficiency.We mianly focused on the Au-based double perovskite Cs₂Au₂X₆（X=Cl、Br、I）which have been synthesized in experiment.The possibility to use them as the absorber in photovoltaic devices is proved by theoretical calculation.The structural stability,thermodynamic stability,electronic and optical properties were further studied.The results show that both of them are stable compounds and are indirect band gap semiconductors.These materials have long carrier diffusion length,high light absorption coefficient.Besides,they have strong optical anisotropy under different optical polarization directions which indicate us to optimize light absorption capacity artificially by adjusting crystal orientation.All these characteristics indicate that they have excellent photophysical properties.Finally,we predicted their photoelectric conversion efficiency（PCE）by the SLME model.The ideal efficiency of Cs₂Au₂Br₆ and Cs₂Au₂I₆are 30.2%and 32.6%respectively,which both exceed30%,thus they are suitable for solar cells as the absorbers.The golden triangle of photovoltaics indicates that the cost should be taken into account while ensuring efficiency and stability in solar cells.Therefore,we considered to replace the B^III site of mixed-valence double perovskite A₂Au B^IIIX₆which we studied above with 6 different elements.We totally studied 18 types of Cs₂Au B^IIIX₆ materials aiming to screen the potential photovoltaic materials as absorbers in solar cells.We consider from the perspectives of stability and photoelectric properties,Cs₂Au Sb Cl₆,Cs₂Au In Cl₆,Cs₂Au Bi Br₆ and Cs₂Au Bi Cl₆which satisfied the criterion we set were selected finally.There are all indirect band gap semiconductors with the values of 1.032 e V、1.65 e V、0.81 e V and1.52 e V respectively.These materials have long carrier diffusion length and their electrons-holes are not easy to recombine.The high light absorption coefficient with strong optical anisotropy indicates that they have excellent photoelectrical properties.Finally,we predicted their photoelectric conversion efficiency（PCE）by the SLME model.The SLME of Cs₂Au Sb Cl₆、Cs₂Au In Cl₆、Cs₂Au Bi Cl₆ are 31%,29.8%and 32.3%respectively,indicating they are potential absorbers in lead-free perovsike solar cells with high efficiency.