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Graded structures with different architectures were obtained by spark plasma sintering from (Ba1-xSrx)TiO3 (BST, x = 0.10; 0.20; 0.30) powders. The presence of the composition gradient was confirmed by structural and compositional investigations using X-ray diffraction and electron microscopy combined with energy dispersive X-ray spectroscopy. The concentration gradient was either asymmetric (3 layers, starting with x = 0.10 and ending with x = 0.30) or symmetric (5 or 6 layers, starting and ending with x = 0.10, and with a single or double x = 0.30 layer in the middle, respectively). Electrical measurements reveal a decrease of the dielectric constant with increasing the number of the layers. It was found that the symmetric graded structure with 6 layers has the best thermal stability of both, the dielectric constant (variation of only 8% between zero and 100 °C) and the pyroelectric coefficient (6% variation between zero and 80 °C). In addition, an enhancement of the pyroelectric signal for frequencies above 100 Hz is obtained in symmetric structures, an effect that is attributed to the additive contributions of the signals originated from the layers with different Sr content.

The integration of ferroelectrics in perovskite solar cells is proposed as possible way to enhance charge collection efficiency. First results on solar cell manufactured with PbTiO3 (PTO) instead of TiO2 have shown negligible values for the power conversion efficiency (PCE). This is explained by the high serial resistance of sol-gel deposited PTO on F:SnO2 electrodes (FTO). Although PTO layer has remnant polarization of 22 μC/cm2, the high potential barrier (0.25 ± 0.05 eV) at the FTO/PTO interface and low carrier mobility (10−8 cm2 V−1 s−1) compared to TiO2 leads to high serial resistance. Better results were obtained with thinner PTO layers grown by pulsed laser deposition, with PCE values up to 0.6%. Further enhancement was obtained by replacing PTO with BaTiO3 (BTO), with PCE value reaching about 0.8% after poling the cell with +3 V. The most important finding was that the magnitude of the short circuit current increases with the amplitude of the poling voltage while the value of the open-circuit voltage remains about the same, around 0.9 V. This is explained through more efficient collection of the charges generated under illumination in the absorber layer due to the polarization that is present in the ferroelectric film. Keywords: Ferroelectric, Perovskite, Solar cell, Polarization

The concept of graded ferroelectrics became of significant interest for researchers working in the field of ferroelectric materials and related applications during the 1990s, when a number of published papers reported a series of intriguing phenomena observed in this type of structure. Among these are the shift of the hysteresis loop along the polarization axis, the presence of a giant effective pyroelectric coefficient, and the possibility of using the graded structures as ferroelectrics similar to semiconductor p–n junctions. Some pioneering experimental work was performed on simple structures composed of two layers with different values of polarization, called also ferroelectric bimorph. During the years, the interest for graded structures remained significant both from the point of view of the basic research as well as considering the potential applications in electronics, sensing, and energy. The concept of graded ferroelectric structures includes all structures in which a polarization gradient is present. This gradient can be obtained in several ways, such as (1) producing a concentration gradient along the direction normal to the electrodes; (2) producing a temperature gradient between the opposite electrodes of the capacitor; and (3) producing a strain gradient along the direction normal to the electrodes.