Multiple strategies to greatly enhance the photovoltaic characteristics of BiFeO₃-based films

More recently, multiferroic BiFeO₃ has attracted widespread interest due to its potential photovoltaic applications and features including an above-bandgap photovoltage and switchable photocurrent. However, there are still some problems with BiFeO₃-based photovoltaic devices. In particular, its poor photocurrent density (∼μA cm⁻²) leads to a low power conversion efficiency, which seriously hinders its practical application in photovoltaic devices. Herein, multiple strategies were introduced to enhance the photovoltaic characteristics of BiFeO₃-based films. An elaborate photovoltaic structure of Au/BPFCO-g/Au-NPs/FTO was constructed by (Pr, Co) gradient-doped BiFeO₃ (BPFCO-g) multilayer film coupling with the Au nanoparticle (Au-NP) layer. The results and analyses show that the photocurrent density was greatly enhanced in this photovoltaic device. For instance, its photocurrent density (J_sc) is approximately 727 times higher than that of the pure BiFeO₃ film (J_sc ∼0.011 mA cm⁻²), reaching a staggering value of 8 mA cm⁻². Moreover, its photocurrent intensity remained almost unchanged after a long-term durability measurement of 1800 s, exhibiting excellent stability and repeatability. A possible mechanism for the enhanced photocurrent density was proposed in the paper. Firstly, gradient doping in the BiFeO₃ film introduces a gradient distribution of oxygen vacancies, creating a built-in electric field E_bi-gv. Secondly, gradient doping simultaneously leads to a continuous contraction of the lattice, inducing another built-in electric field (the flexoelectric field E_flexo). Fortunately, these two built-in electric fields have the same orientation and jointly enhance the separation of the photogenerated carriers. Finally, the Local Surface Plasmon Resonance (LSPR) effect from the Au-NP layer not only enhances the light absorption of the BPFCO-g film, but also greatly enhances the energy of the photogenerated carriers, further improving their separation efficiency. In all, the photocurrent density of such a photovoltaic heterostructure is tremendously enhanced under the multifactorial coupling effect.