Influence of Electrodes Materials on External Quantum Efficiency of Bulk Photovoltaic Effects in Electrode/Mn-Doped BiFeO₃/SrRuO₃ Structures

Recently, the bulk photovoltaic effect (BPVE) of ferroelectrics has been rediscovered because of their potential for novel solar cell applications [1]. The BPVE has two mechanisms, 'shift current' and 'ballistic conduction' [2]. The 'shift current' is a recently discovered mechanism that can generate current when the charge carriers exhibits an asymmetric shift due to a phase charge during the inter-band transition. Shift current therefore causes photovoltages above the bandgap. Indeed, we have already reported 852 V generation in Pt/Mn-doped BiFeO₃(BFMO)/Pt coplanar capacitor [3]. In ballistic conduction, as the excited carriers descend to the conduction band minimum, anisotropic carrier motion is induced in a noncentrosymmetric crystal. It is known that the thermalization length in BaTiO₃ reaches 100 nm [4], which means that excited carriers can be extracted without recombination. Therefore, an improvement in external quantum efficiency is expected. In this study, we have investigated the external quantum efficiency of photovoltaic effects in ferroelectric BFMO epitaxial thin films with ITO, graphene and CNT electrodes.
To study the BPVE of BFMO thin films, single crystalline thin films are suitable. For this purpose, we used a vicinal SrTiO₃(001) (STO) substrate whose (00l) planes are tilted by 4^o along the [110] direction. 30 nm thick SrRuO₃ (SRO) bottom electrode and 20-300 nm thick BFMO thin films were grown on the STO substrate by rf planar magnetron sputtering, resulting in a BFMO/SRO/STO heteroepitaxial structure. On the BFMO/SRO/STO, ITO, graphene and CNT dots were fabricated as transparent electrodes, respectively, resulting in ITO, graphene or CNT/BFMO/STO capacitor structures. The bulk photovoltaic effects of the BFMO capacitors was evaluated by measuring the I-V characteristics under blue-violet laser illumination (λ = 405 nm), and the open circuit voltage (V_OC) and short circuit current (I_SC) were evaluated. In addition, the light polarization was changed by rotating the λ/2 plate.
The I_SC of the ITO/BFMO/SRO capacitor structure with a BFMO film thickness below 100 nm exhibited a markedly disparate tendency in comparison with capacitors with a BFMO film thickness exceeding 100 nm. This indicates that the ballistic conduction mechanism is the dominant process below 100 nm, and that the thermalization length of BFMO is approximately 100 nm, which is consistent with the value observed in BaTiO₃ [4]. Furthermore, with the deceasing thickness of the BFMO film, enhancement of the external quantum efficiency has been observed, reaching a maximum of 2.4% in a 20-nm-thick BFMO film capacitor. Conversely, the light polarization angle (φ) dependence of V_OC and I_SC in graphene/BFMO/SRO capacitors exhibits a double sinusoidal curve. However, the light polarization angle dependence of I_SC differs between 100- and 300-nm thick BFMO capacitors. It is hypothesized that ballistic conduction is the dominant mechanism in the 100-nm-thick BFMO capacitor, despite the ‘shift current’ being the dominant mechanism in the 300-nm-thick BFMO capacitor. Furthermore, the photovoltaic effect on metallic- and semiconducting-CNT/BFMO/SRO capacitors will be discussed at the conference.

References
[1] J. E. Spanier et al., Nat. Photonics 10, 611 (2016).
[2] S. Pal et al., J.Phys.: Condens. Matter., 32, 485701 (2020).
[3] S. Nakashima et al., Sci. Rep., 10, 15108 (2020).
[4] Z. Gu et al., PRL, 118, 096601 (2017).