Photoconductivity and Photovoltaic Studies on Polycrystalline Bi1−xM′xFe1−yM″yO3−δ (M′ = K+ and Cs+; M″ = Ti4+; x = y = 0 and 0.1) Thin Films

Abstract

The recent surge of interest in using all-inorganic materials like CsPbI3 for photovoltaic devices has brought significant interest in exploring the properties of BiFeO3 (BFO) by substituting Cs+ and K+ at the Bi3+ site. The aim of the present work is to study the parameters like controlling the lattice and surface oxygen vacancy (VO) defects which can significantly improve the photovoltaic device performances. Monovalent substituted and aliovalent co-substituted Bi1−xM'xFe1−yM"yO3−δ (M′ = K+ and Cs+; M" = Ti4+; x = y = 0 and 0.1) are grown on fluorine-doped tin oxide (FTO)/glass substrates by metal-organic chemical solution deposition method. Photoconductivity and photovoltaic properties of these films are studied in FTO/Bi1−xM'xFe1−yM"yO3−δ/Au capacitor configuration. Aliovalent substitution (for M' = K+ and Cs+, BK10FO and BCs10FO, respectively) at Bi3+ sites create VO in BFO thin films. Further, grain size reduction from 100 nm to 30 nm also results in the creation of surface VO defects. Tauc plot shows a reduction in the bandgap (Eg) values from 2.32 eV to 2.26 eV due to the formation of mid-bandgap VO defect states. Consequently, ferroelectric properties also get diminished. Aliovalent K+-Ti4+ co-substitution in BFO thin films brings back the Eg values to 2.3 eV, i.e., similar to pristine BFO thin films suggesting suppression of VO. This results in the revival of ferroelectric nature in aliovalent co-substituted thin films. Photoconductivity studies show two order enhancement in conductivity for BK10FO thin film devices compared to pure BFO. This enhancement is attributed to the mid-bandgap VO defect states due to aliovalent substitution. On the other hand, photoconductivity observed in BCs10FO devices is comparable with pristine BFO devices. Contrary to this, aliovalent co-substituted thin films show a decreasing trend of photoconductivity compared to M'+ substituted BFO suggesting that VO defects get suppressed due to charge compensation. Photoconductivity and photovoltaic device studies show similar behavior. BK10FO devices exhibit short circuit current density (JSC) ~0.45 µA/cm2, which is larger than that observed in BFO devices (JSC ~0.3 µA/cm2). But, BCs10FO devices show lesser JSC 0.07 µA/cm2. Open circuit voltage (VOC) observed in BFO devices is around 3 V which decreases to 0.1 V and 0.06 V for BK10FO and BCs10FO devices, respectively. This decrease in VOC is attributed to the reduction of the ferroelectric nature of the films. BFO, BK10FO, and BCs10FO devices show photovoltaic switching upon poling the films. Co-substituted thin films show a reduction in the JSC and betterment in the VOC compared to the monovalent substituted BFO thin films. This happens due to the suppression of VO defects by partial charge compensation in aliovalent co-substituted BFO films. This study explicitly brings out the role of VO on the photovoltaic and photoconductivity properties of BFO thin films.