光能轉換電能

光能轉換電能(photovoltaic)相關應用是一日益重要能源課題。近年染料敏化太陽能電池(dye-sensitized solar cell; DSSC)由於低成本及光伏轉換效率可達到20%受到許多重視，不過DSSC最大問題是轉換效率隨時間快速衰減及封裝不易。另一個近年備受關注光伏應用材料是有機-無機鈣鈦礦太陽能電池(organic-inorganic perovskite based solar cell)，例如CH₃NH₃PbX₃ (X=Cl, Br, I)，這些含鉛鈣鈦礦鹵化物的光伏轉換效率雖已達到21%，不過有機-無機鈣鈦礦太陽能電池最大問題是光伏轉換效率在大氣環境快速衰減及含鉛元素。另一方面，許多無中心對稱(non-centrosymmetric)無機鉛鈣鈦礦結構鐵電材料也具有光伏效應(photovoltaic effect)，例如 BaTiO₃ (BTO)及Pb(Zr,Ti)O₃ (PZT)，因為能隙(bandgap; E_g)過寬(≧3.5 eV)導致光伏電流太小(≦10⁹ A/cm²)及轉換效率太小(< 0.01%)等問題，使此類材料在光伏效應的應用發展受到限制。近年因為多鐵性(multiferroic) BiFeO₃ (BFO)材料能隙(E_g≦2.8 eV)明顯小於傳統鉛鈣鈦礦結構鐵電材料(E_g > 3.58 eV)，BFO比傳統鈣鈦礦氧化物具有更好光伏轉換效應的性質及具有光觸媒(photocatalytic)應用潛力，在光感測元件(optical sensor)、光觸媒元件(photocatalytic device)及光導元件具有相當的應用潛力。

Above-bandgap photovoltages in lead-free perovskite BiFeO₃ have been substantially studied for energy harvesting and photocatalytic applications.  However, most reported photocurrents and conversion efficiencies are still too small due to wide bandgaps.  Remarkably large and durable field-modulated external quantum efficiency (EQE) ~11% and power conversion efficiency (PCE) ~1% have been achieved under 405 nm irradiation in polycrystalline Nd-doped BiFeO₃ (BNFO) ceramic with a bandgap E_g~2.12-2.24 eV.  The photovoltaic voltage and current density can reach ~0.9 V and ~8.0 A/m² after poling (E=1 kV/cm) at irradiation intensity 10² mW/cm².  The p-n-junction layer between indium tin oxide (ITO) thin film and BNFO ceramic is the primary mechanism for the field-modulated photovoltaic responses in conjunction with the polarization-induced Schottky-barrier modulation.  The field-induced domain nucleation within the grains and nanodomains play important roles for the enhanced photocurrent and conversion efficiencies.