| Solar energy has been exploited for hundreds of years and it is everywhere in life,making Photovoltaic(Photovoltaic,PV)materials increasingly important.There are two biggest problems in making PV materials,that is the cost and efficiency.However,influenced by the material cost and production process of monocrystalline silicon solar cells,the price is still high.Therefore,it has become an urgent research content to find new PV materials,improving the conversion efficiency and reducing the cost.Due to its abundance and non-toxicity,iron pyrite FeS2 is readily available in nature and has good light absorption characteristics,which has attracted wide attention in recent years.However,although pyrite FeS2 has these good characteristics,its photoelectric conversion efficiency is very low.The main reason that restricts the photoelectric conversion efficiency of pyrite FeS2 is that its open circuit voltage(Voc)is too low.Therefore,it is necessary to find an effective method to improve the band gap value of pyrite FeS2.Doping method plays a crucial role in semiconductor band gap engineering,and it is also a very common method.First,we studied the case of doping O atoms into iron pyrite FeS2.Doping a certain concentration of O atoms into iron pyrite FeS2 can indeed significantly increase the band gap value.Comparing with pure FeS2,the conduction band minimum(Conduction Band Minimum,CBM)of the doped system FeO0.25S1.75 from ?-point shifts to near R-point,the valence band maximum(Valence Band Maximum,VBM)from near X-point shifts to M-point.The spin-up and spin-down channels are still perfectly symmetric,and FeO0.25S1.75 is still a nonmagnetic semiconductor.Subsequently,we studied the case of doping on both sides of the cation and anion of FeS2.The cations we selected were transition metal Zn,Ru and Os,and the anion was O.Our results show that Ru0.25Fe0.75O0.25S1.75 and Os0.25Fe0.75O0.25S1.75 systems indeed have larger band gap values.Ru0.25Fe0.75O0.25S1.75 has a indirect band gap,comparing with pure FeS2,the position of CBM changes from ?-point to R-point,and the position of VBM changes from near X-point to X-point precisely.The band structure of Os0.25Fe0.75O0.25S1.75 is very similar to that of Ruo.25Fe0.75O0.25S1.75,and the change of CBM and VBM is basically as same as that of Ru0.25Fe0.75O0.25S1.75.On the contrary,doping Zn and O atoms on both sides into pyrite FeS2 reduces the band gap of pyrite,making Zn0.25Fe0.75O0.25S 1.75 unsuitable for PV application.The CBM of FeS2 is contributed by the low intensity sulfur ppV*state,substituting O for S(at 12.5%),Ru or Os for Fe(at 25%)reduces the width of pp?*band,without producing gap states.In addition,doping Ru(Os)and O atoms also improved the optical properties of pyrite and increased the maximum absorption peak.Meanwhile,in the visible light region,the doping system Ru0.25Feo.75O0.25S1.75,Os0.25Fe0.75O0.25S1.75 and FeO0.25S1.75 not only have high light absorption,but also observed the red-shift phenomenon of the doping systems.Our results show that doping with Ru(or Os)and O atoms into pyrite is an effective way to enhance the band gap,making Ruo.25Fe0.75O0.25S 1.75 and Os0.25Fe0.75O0.25S 1.75 systems promising for PV applications.Finally,based on the study on FeO0.25S1.75 above,we further explored the band gaps of FeO0.25S1.75 and pure FeS2 under the pressure of 0 to 25GPa,and mainly discussed the band gap of FeO0.25S1.75 under the pressure of 13GPa.Our results show that the band gap of FeS2 is not very stable under the pressure range of 0 to 25GPa,but the overall trend appears to be downward.The maximum value of the band gap appears when it is not pressurized,and the minimum value appears under the pressure of 25GPa.The band gap of FeO0.25S1.75 under the pressure range of 0 to 25GPa increases first and then decreases,and reaches the maximum band gap value when the pressure is 13GPa,at this point,the band gap value is very close to the value of S-Q limit for semiconductor PV applications.The orbital hybridization of Fe-3d and S-3p orbitals is relatively strong,interaction between Fe and S atoms enhanced under the pressure of 13GPa.In our study on the mechanical stability of FeO0.25S1.75 under the pressure range of 10 to 16GPa,we find that systems of FeO0.25S1.75 is very stable since the Born stability conditions are all satisfied.Our results indicate that the band gap of FeO0.25S1.75 will significantly increase under a certain pressure(about 13GPa),and the maximum band gap value is very close to the value of S-Q limit for semiconductor photovoltaics applications,and it provides pyrite more possibilities for applying in the photovoltaic industry. |
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