BiFeO_(3)系固溶体钙钛矿窄禁带铁电半导体光伏材料

Narrow bandgap ferroelectric semiconductors within BiFeO_(3)-based solid solution perovskites

铁电半导体能够单片式集成半导体p-n结光伏与铁电体光伏,有望突破太阳能光伏电池能量转换效率的Shockley-Queisser理论极限.本研究采用Mn^(4+)、Co^(2+)等磁性离子替代技术在BiFeO_(3)系固溶体钙钛矿中成功创制出一系列窄禁带室温铁电半导体,应用传统固相反应法制备了0.50BiFeO_(3)-0.25A_(1)MnO_(3)-0.25A_(2)TiO_(3)(A_(1)=Ca、Sr、Ba,A_(2)=Sr、Ba、Pb)和0.49BiFeO_(3)-0.26BaTiO_(3)-0.25(Sr_(1–x)Ba_(x))(Co_(1/3)Nb_(2/3))O_(3)单相钙钛矿陶瓷.室温拉曼散射光谱测试表明这些固溶体钙钛矿空间中心反演对称破缺,与应用Vegard规则估算它们是室温铁电相的结果一致.紫外-可见-近红外波段吸光度测试表明它们是禁带宽度E_(g)在0.75~1.0 eV的直接带隙半导体.变温电学测试表明其电阻率为~10^(6)Ωcm(40°C)量级、缺陷激活能E_(a)~0.5 eV.与卤化物钙钛矿相比,E_(g)~0.9 eV的BiFeO_(3)系固溶体钙钛矿对太阳能光谱的吸收已深入近红外波段.通过分析铁电半导体禁带宽度与磁性离子d电子组态、μ×r_(A)/r_(B)系综描述符(μ、r_(A)和r_(B)分别为原胞的约合质量、A位和B位离子半径)之间的因果关系,为下一步优选集成半导体p-n结光伏和铁电体光伏太阳能电池的功能元,提供了材料预测设计的物理模型.

Solar energy is a clean and renewable source of energy.Various solar photovoltaic(PV)cell technologies have been developed,out of which perovskite PV cell technology is growing rapidly.Using organometal halide perovskite as an optical absorption component,the power conversion efficiency(PCE)of heterojunction solar PV cells increased from 3.8%in 2009 to 25.5%recently.Nonetheless,the bottleneck for CH_(3)NH_(3)PbI_(3)-based solar cells entering into commercial market is their poor stabilities relating to environmental,thermal,and ultraviolet light influence.In contrast,transition metal oxide perovskites are thermodynamically and chemically stable enough for solar cells’commercialization,with more advantages such as bandgap(E_(g))adjustability from 0 to 6 eV,processing being compatible with transparent conducting oxides,and ferroelectric bulk PV effect.Owing to a wide E_(g),traditional ferroelectric perovskite oxides exhibit poor visible optical absorption,low electrical conductivity,and thus extremely low PCE.As an alternative to p-n junction PV effect,ferroelectric bulk PV effect provides a new separation mechanism for photoexcited carriers,which is closely related to spatial inversion symmetry breaking and its resulting spontaneous electric polarization.Different from p-n junction,the active space for photo-excited carrier separation spans the whole ferroelectric body,and thus an above-E_(g) open-circuit voltage is produced.By narrowing E_(g) while maintaining ferroelectricity of oxide perovskites,ferroelectric semiconductors become available to monolithically integrate the p-n junction and ferroelectric bulk PV effects,which could,in principle,go beyond the Shockley-Queisser theoretical PCE limit of conventional p-n junction solar cells.Bismuth ferrite(BiFeO_(3))has been experimentally demonstrated to exhibit ferroelectric PV effect.Meanwhile,it has a high ferroelectric Curie temperature(T_(C))of 830°C and an intermediate E_(g) of~2.2 eV,providing a large chemical space for solid solution perovskite oxides to trade off both target properties of narrow E_(g) and T_(C)>400 K.In this report,0.50BiFeO_(3)-0.25A_(1)MnO_(3)-0.25A_(2)TiO_(3)(A_(1)=Ca,Sr,Ba,A_(2)=Sr,Ba,Pb)and 0.49BiFeO_(3)-0.26BaTiO_(3)-0.25(Sr_(1−x)Ba_(x))(Co_(1/3)Nb_(2/3))O_(3)(x=0 and 0.4)ferroelectric semiconductors with E_(g) of~0.9 eV were created by substituting Fe^(3+)with Mn^(4+)or Co^(2+)in BiFeO_(3)-based solid solution perovskites.Ceramic samples were prepared using a refined solid-state reaction electroceramic processing.X-ray diffraction measurements showed them crystallized in a single pseudo-cubic perovskite phase,while Raman scattering characterizations illustrated a breaking of spatial inversion symmetry at room temperature.Optical absorbance measurements found that these Mn^(4+)-or Co^(2+)-substituted BiFeO_(3)-based solid solution perovskites have a direct bandgap in a range of 0.75–1.0 eV.Compared with CH_(3)NH_(3)PbI_(3) perovskites,their optical absorption reaches near-infrared band of solar spectra.Temperature-dependent resistance measurements showed their resistivity on the magnitude of order of~10^(6)Ωcm with thermal excitation energy(E_(a))of~0.5 eV.Combined with observation of frequency-dependent dielectric properties,A-site vacancies were proposed responsible for electrical conduction and dielectric relaxation.Through data-mining causal relationships between E_(g) and the filling number of d electron of B-site cations,μ×r_(A)/r_(B) ensemble descriptor of oxide perovskites(μ,r_(A),and r_(B) are the reduced mass of primitive cell,A-site cation radius,and Bsite cation radius,respectively),a physical model was proposed to predictively design chemical compositions of ferroelectric semiconducting oxide perovskites with E_(g)~0.9 eV for applications of solar PV cells.This essay provides an opportunity for developing novel solar PV cells to integrate monolithically p-n junction and ferroelectric bulk PV effects,with PCE beyond the Shockley-Queisser theoretical limit.