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Abstract Flexoelectricity refers to the mechanical-electro coupling between strain gradient and electric polarization, and conversely, the electro-mechanical coupling between electric field gradient and mechanical stress. This unique effect shows a promising size effect which is usually large as the material dimension is shrunk down. Moreover, it could break the limitation of centrosymmetry, and has been found in numerous kinds of materials which cover insulators, liquid crystals, biological materials, and semiconductors. In this review, we will give a brief report about the recent discoveries in flexoelectricity, focusing on the flexoelectric materials and their applications. The theoretical developments in this field are also addressed. In the end, the perspective of flexoelectricity and some open questions which still remain unsolved are commented upon.

Hybrid perovskite solar cells (PSCs) have been intensively studied in recent years because of their high efficiency and low costs. For PSCs, the electron transport layer (ETL) is a key for its photoelectric conversion efficiency. Here we demonstrate the application of amorphous InGaZnO4 thin films as ETL for efficient PSCs by pulsed laser deposition (PLD). The PSC device using such InGaZnO4 amorphous film as ETL has achieved an efficiency of 15.1%. The outstanding performance is attributed to the excellent properties of amorphous InGaZnO4 oxide thin films, including high electron mobility and high transparency, what is more, the electronic properties of the films can be controlled by changing the partial pressure of oxygen in the deposition chamber and post-deposition annealing process. Our result will be helpful for preparation of large area PSCs and other opto-electric devices at low temperature by physical vapor deposition method.

Flexoelectricity couples strain gradient to polarization and usually exhibits a large coefficient in the paraelectric phase of the ferroelectric perovskites. In this study, we employed the relaxor 0.3Pb(In1/2Nb1/2)O3-0.35Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 (PIN-PMN-PT) single crystals to study the relationship between flexoelectric coefficients and the crystal structure. The flexoelectric coefficients in PIN-PMN-PT single crystal are found to vary from 57 μC/m at orthorhombic/monoclinic phase to 135 μC/m at tetragonal phase, and decreases to less than 27 μC/m in the temperature above Tm. This result discloses that ferroelectricity can significantly enhance the flexoelectricity in this kind of perovskite. [ABSTRACT FROM AUTHOR]