The discovery of a substantial short circuit photocurrent density (Jsc) in ferroelectric BiFeO3 (BFO)1,2 was a hallmark that stimulated revisiting the photoresponse of narrow gap polar materials. In conventional semiconductor photovoltaic devices, electron–hole pairs created by photon absorption are extracted by an internal electric field (Ein) present at device interfaces and/or by diffusion, and the resulting open circuit photovoltage (Voc) is limited by the energy gap.3 In recent years, attention is driven toward the so-called bulk photovoltaic effect (BPE), occurring in non-centrosymmetric materials, whose distinctive features are: the Jsc flows in the absence of any Ein, the Voc is not bounded by Eg, and the Jsc is sensitive to the polarization direction of the incoming light.4 Indeed, although in BFO, Voc was observed to be larger than the bandgap and first attributed to domain walls,5 subsequent experiments and the angular dependence of Jsc allowed to assess its BPE origin6 with amplitudes (Glass coefficients) that turned out to be in reasonable agreement with first principles calculations,7 although the detailed microscopic mechanisms of the observed photocurrent remain unsolved.8,9
Hexagonal manganites, such as h-LuMnO3, are ferroelectric with its polar axis along the hexagonal axis and have a narrow electronic bandgap (≈1.5 eV). Using Pt electrodes, h-LuMnO3 single crystals display a strong rectification, characteristic of a Schottky diode, and a large photoresponse. It is found that the short circuit photocurrent density Jsc along the polar axis is modulated (up to 25%) by the direction of the ferroelectric polarization P, leading to a short circuit photocurrent loop that mimics the ferroelectric polarization. However, a non-switchable Jsc persists. Diffusion photocurrent is shown to dominate current-in-plane measurements and contributes to the non-switchable Jsc.
This observation illustrates the dramatic role of the large optical absorption in hexagonal manganites. The accompanying optical dichroism might challenge disentangling a genuine bulk photovoltaic response in h-LuMnO3 single crystals, contributing to the non-switchable Jsc. Epitaxial thin films may offer a suitable alternative.
Oxides for new-generation electronics
Switchable photovoltaic response in hexagonal LuMnO3 single crystals
Yunwei Sheng, Ignasi Fina, Marin Gospodinov, and Josep Fontcuberta
Appl. Phys. Lett. 118, 232902 (2021)