The ferroelectric-photovoltaic (FE-PV) device, in which a homogeneous ferroelectric material is used as a light absorbing layer, has been investigated during the past several decades with numerous ferroelectric oxides. The FE-PV effect is distinctly different from the typical photovoltaic (PV) effect in semiconductor p–n junctions in that the polarization electric field is the driving force for the photocurrent in FE-PV devices. In addition, the anomalous photovoltaic effect, in which the voltage output along the polarization direction can be significantly larger than the bandgap of the ferroelectric materials, has been frequently observed in FE-PV devices. However, a big challenge faced by the FE-PV devices is the very low photocurrent output. The research interest in FE-PV devices has been re-spurred by the recent discovery of above-bandgap photovoltage in materials with ferroelectric domain walls, electric switchable diodes and photovoltaic effects, tip-enhanced photovoltaic effects at the nanoscale, and new low-bandgap ferroelectric materials and device design. In this feature article, we reviewed the advance in understanding the mechanisms of the ferroelectric photovoltaic effects and recent progress in improving the photovoltaic device performance, including the emerging approaches of integrating the ferroelectric materials into organic heterojunction photovoltaic devices for very high efficiency PV devices.

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Arising applications of ferroelectric materials in photovoltaic devices

A general approach for considering primary ferroic (ferroelectric, ferromagnetic, ferroelastic) nanoparticle phase transitions was proposed in phenomenological theory framework. The surface stress, order parameter gradient, and striction, as well as depolarization, demagnetization, and de-elastification effects, were included into the free energy. The strong intrinsic surface stress under the curved nanoparticle surface was shown to play the important role in the shift of transition temperature (if any) up to the appearance of a new ordered phase absent in the bulk ferroic. Euler-Lagrange equations obtained after the Landau-Ginzburg-Devonshire free energy minimization were solved by direct variational method. This leads to the conventional form of the free energy with renormalized coefficients depending on nanoparticle sizes, surface stress, and electrostriction tensor values, and so opens the way for polar property calculations by algebraic transformations. Surface piezoeffect causes built-in electric field that induces an electretlike polar state and smears the phase transition point. The approximate analytical expression for the size-induced ferroelectric transition temperature dependence on cylindrical or spherical nanoparticle sizes, polarization gradient coefficient, correlation radius, intrinsic surface stress, and electrostriction coefficient was derived. Under the favorable conditions (radius of $5--50\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and compressive surface stress), spatial confinement induces a ferroelectric phase in incipient ferroelectric nanowires and nanospheres. The prediction of size-induced ferroelectricity in $\mathrm{K}\mathrm{Ta}{\mathrm{O}}_{3}$ nanorods with radius less than $5--20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ at room temperature could be useful for the next generation of devices based on three-dimensional nanostructures.

Phase transitions induced by confinement of ferroic nanoparticles

Grain size effects on the dielectric properties of BaTiO3 nanoceramics have been studied by using the modified Ginsburg-Landau-Devonshire (GLD) thermodynamic theory. Considering the existence of internal stresses, it is found that with decreasing grain size the transition temperature of cubic-tetragonal phase decreases, while those of tetragonal-orthorhombic and orthorhombic-rhombohedral phases increase. With further reducing grain size, our model predicts that the two ferroelectric structures of orthorhombic and tetragonal phases will become unstable and disappear at a critical size, leaving only one stable ferroelectric phase of rhombohedral structure. Consequently, a theoretical phase diagram of the transition temperature versus grain size is established wherein two triple points and a reentrance behavior are indicated. The results are compared with experimental data.

Size effect on the dielectric properties of BaTiO 3 nanoceramics in a modified Ginsburg-Landau-Devonshire thermodynamic theory

The influence of the mismatch effect on thin ferroelectric film properties has been studied in the phenomenological theory framework. The polarization dependent part of the surface energy that defined the boundary conditions for the Euler–Lagrange differential equation was written as a surface tension energy. The latter was expressed via the surface polarization and the tension tensor related to the mismatch of the substrate and film lattice constants and thermal expansion coefficients. The interfacial strain caused by the mismatch effect induces the additional surface polarization Pm via a piezoelectric effect that arises near the surface in any film. The new parameter Pm/PS (PS is the known value of the spontaneous polarization in the bulk ferroelectric material at T = 0 K) appeared in the derived phenomenological equations. So we calculate the influence of the parameter Pm/PS on the depth distribution of the dimensionless film polarization P/PS, its dependence on temperature, film thickness and applied electric field, as well as that on the hysteresis loop shape, coercive field values, phase diagram and average dielectric susceptibility temperature dependence. Non-zero Pm/PS values cause a mismatch induced thickness dependent internal electric field Em. We have shown that this field drastically influences all the properties. In particular, the polarization profile becomes asymmetrical, the polarization temperature dependence resembles that in the external electric field and there is a possibility of external field screening by the internal field Em. The asymmetry obtained for the hysteresis loop suggests that it is possible that the self-polarization phenomenon recently observed in some films is related to the mismatch effect. The thickness induced ferroelectric–paraelectric phase transition has been shown to exist when |Pm|/PS 1 could be the physical reason for the ferroelectric phase conservation in ultrathin film. The possibility of observing the peculiarities of the film property temperature and thickness dependences related to the mismatch effect is discussed.

https://iopscience.iop.org/article/10.1088/0953-8984/16/21/002/pdf

The internal electric field originating from the mismatch effect and its influence on ferroelectric thin film properties

One-dimensional (1D) ferroelectric nanostructures, such as nanowires, nanorods, nanotubes, nanobelts, and nanofibers, have been studied with increasing intensity in recent years. Because of their excellent ferroelectric, ferroelastic, pyroelectric, piezoelectric, inverse piezoelectric, ferroelectric-photovoltaic (FE-PV), and other unique physical properties, 1D ferroelectric nanostructures have been widely used in energy-harvesting devices, nonvolatile random access memory applications, nanoelectromechanical systems, advanced sensors, FE-PV devices, and photocatalysis mechanisms. This review summarizes the current state of 1D ferroelectric nanostructures and provides an overview of the synthesis methods, properties, and practical applications of 1D nanostructures. Finally, the prospects for future investigations are outlined.

One-Dimensional Ferroelectric Nanostructures: Synthesis, Properties, and Applications

The theory of size effects in nanocrystalline ferroelectric ceramics or nanoparticle powders allowing for surface tension and depolarization field is proposed. Surface tension was included in the free energy functional and extrapolation length was expressed via surface tension coefficient. The latter was shown to be dependent on temperature due to its relation to the dielectric permittivity of the nanoparticles. The depolarization field effect was calculated in the model taking into account the space-charge layer on the surface.

The Euler–Lagrange equation for inhomogeneous polarization was solved analytically in both paraelectric and ferroelectric phases. The temperature and size dependence of the spontaneous polarization and dielectric susceptibility of nanomaterials was obtained. The possibility to calculate these and other properties by minimization of the conventional free energy in the form of different power polarization series, but with coefficients that depend on particle size, temperature, contribution of depolarization field and surface tension coefficient, was demonstrated. A comparison with available experimental data is performed.

Effect of surface tension and depolarization field on ferroelectric nanomaterial properties

The rapid rise in energy prices in the world makes the problem of energy saving relevant for today. Drying of materials is one of the most energy-intensive heat-technological processes. Despite the high heat consumption for drying processes, the quality of the resulting material may not meet modern requirements, and the specific energy consumption will be too high. Therefore, consideration of new means and devices for drying materials using heat pumps is promising. 
The efficiency of the dryer depends on the degree of air recirculation. In dryers, complete recirculation of exhaust gases is impossible, since an increase in the moisture content of the air significantly reduces its drying capacity. But the heat pump can be effectively used as a means of removing moisture from the exhaust gases of the dryers, while it is possible to increase the proportion of their recirculation. 
The aim of the study is to determine the optimal value of the percentage of recirculation of the treated exhaust gases from the dryer, at which the greatest energy savings are achieved. 
The installation of a heat pump downstream of the dryer on the flue gas side effects both the humidity and temperature conditions by recirculating some of the flue gases treated in the heat pump to the dryer inlet. As the results of the study showed, the use of a heat pump to dry the exhaust wet gas and return it to the cycle makes it possible to increase the efficiency of the dryer, but energy is wasted to drive the heat pump compressor. In accordance with this, the consumption of primary fuel is also reduced to obtain the required amount of thermal energy for heating. 
The developed method makes it possible to determine the optimal proportion of exhaust air recirculation in dryers in order to achieve the greatest savings in primary fuel compared to conventional recirculation.

A. N. Morozovskaya

Papers

Effect of surface tension and depolarization field on ferroelectric nanomaterial properties

Optimization of heat pumps in dryers