The ferroelectric properties of sample E were studied at different temperatures and frequencies. The P-E hysteresis loops were collected at 77 K and 300 K from a 145 µm thick sample with symmetric silver paste electrodes using four different frequencies (Figure 4.10a-b). At 300 K the sample shows very narrow loops with an almost linear response suggesting at best it is only weakly ferroelectric at room temperature. At 77 K the P-E loop was well defined with a remnant polarization Pr ≈ 5 C/cm2 and coercive field Ec ≈ 12 kV/cm. At both temperatures, the hysteresis loops were largely independent of the
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measuring frequency and essentially free of any leakage component associated with the lower bandgap.
Temperature dependent P-E loops were also collected in a probe station using an ITO(~100 nm)/FE-layer(107 µm)/Ag asymmetric electrode configuration designed for the photoresponse measurements (Figure 4.10c). The loops broaden with decreasing
temperature; at 77 K saturation started to appear at Pr ≈ 2 C/cm2 and Ec ≈ 5 kV/cm, value that were smaller than those collected from the regular testing fixture due to the restricted field strength that could be applied in the probe station.
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Figure 4.10 Ferroelectric P-E hysteresis loops of sample E. Frequency-dependent measurements of Ag/FE-layer (145 µm)/Ag geometry at (a) 300 K and (a) 77 K. (c) Temperature dependence of the ITO/FE-
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Figure 4.11 Photoresponses of positively poled (a) and negatively poled (b) sample E under AM1.5 at 77 K. Photoresponses of (c) positively poled and (d) negatively poled sample E under AM1.5 at 300 K.
Measurements of the photoresponse were made by poling sample E with a 268 V (25 kV/cm) pulse for 100 s at 77 K and exposure to AM1.5 illumination. Figure 4.11a-b shows the photocurrent versus time at zero bias under dark and illuminated conditions after positive and negative poling at 77 K. After poling no current was observed under dark conditions. For the positively poled sample the illumination induced a large negative current (−38 pA); however, this transient decayed and eventually changed sign to a small positive (0.5 pA) value. After the light was switched off, another transient current spike (10 pA) appeared and decayed to zero in the dark condition. The same photoresponse pattern was observed after re-illumination with smaller peak values and remained unchanged after 3 additional light on/off periods. After negative poling a positive current (38 pA) was observed after the initial illumination, the decay of the transient was faster than the positively poled sample but showed a similar reduction in magnitude after repeated
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modulation of the light. At 77 K, the direction of the poling only impacted the sign of the initial current spike after the first illumination, which flowed through the sample in a direction opposite to the ferroelectric polarization. Irrespective of the poling direction, the transient photoresponses of the second and third illumination periods were always negative when the light was switched on and positive when the light was turned off. In both cases no dark current was observed immediately after poling, implying negligible back switching or other relaxation processes.
The photoresponse measurements were repeated at 300 K, see Figure 4.11c-d.
Positive poling induced an initial dark current (−120 pA) that decayed to −10 pA after 5 minutes before the light was turned on. The transient dark current was only observed at 300 K and could arise from incomplete discharging after poling and/or back diffusion of mobile ions or vacancies; similar effects have been reported organic-inorganic hybrid perovskite polycrystalline solar cells.[101] The first application of light induced a transient current spike with a peak value of −110 pA; this transient was not present in subsequent illuminations where a slow decaying, quasi-steady-state photocurrent of −10pA (−8 nA/cm2) was observed. The value of this photocurrent is higher than that reported for a BaTiO3 + 5 wt% CaTiO3 ceramic (~ 2 nA/cm2) under 100 mW/cm2 of 403 nm illumination and 0.25%Mn-doped BaTiO3 single crystals (0.285 nA/cm2) under 14 mW/cm2 halogen lamp illumination.[102,103] The direction of the photocurrent was reversed when the poling direction was switched, consistent with a ferroelectric photovoltaic effect. In contrast to the 77 K measurements, where the current reversal only occurred during first illumination,
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the photoresponse at 300 K showed consistent switchability during subsequent exposures of light.
The reasons for the differences in the 77 K and 300 K photoresponses are unclear as there could be several contributing factors. The ferroelectric polarization should dictate the direction of the photocurrent and this was the case for the first illumination at 77 K and all the photo-excitations at 300 K. The change in the photoresponse of the subsequent 77 K measurements could arise from a Schottky barrier at the ITO - ferroelectric interface; it is also possible the formation of a space charge region during poling could mediate the currents at 300 K. For a polycrystalline ferroelectric with numerous grain and domain boundaries it is difficult to unambiguously relate the reversible photoresponse to the ferroelectric polarization or to a bulk photovoltaic effect. During poling the electromigration of VO•• could cause accumulation at the grain boundaries where the resultant space charge[97] could separate excited charge carriers throughout the sample in a forward or reverse direction determined by the poling voltages.
4.5 Conclusions
The phase stability, optical absorption, ferroelectric and photovoltaic responses of the Ba(Ti1−xNix)O3−x and (1 −x)BaTiO3-(x)[(1 −y)Ba(Ni1/3Nb2/3)O3-(y)“BaNiO2”] systems were investigated to elucidate their potential for applications in solar energy generating devices. The Ni2+-VO•• pair is identified as the main factor in determining the phase stability and absorption properties of both systems. Although Ni2+-VO••stabilizes a hexagonal 6H phase at high temperature, Ba(Ti0.99Ni0.01)O2.99 and (0.9)(BaTiO3)-(0.1)(BaNi0.5Nb0.5O2.75)
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can be transformed to a single-phase 3C structure by post-annealing. By controlling the Ni to Nb ratio and therefore the VO•• concentration, the optical absorption can be tuned to values between 1.3 eV to 3.2 eV suitable for potential ferroelectric photovoltaic applications. Ba(Ti0.99Ni0.01)O2.99 and (0.9)(BaTiO3)-(0.1)(BaNi0.5Nb0.5O2.75) show saturated hysteresis loops at room temperature and 77 K respectively. By alternating the direction of the poling voltage, a reversible transient photoresponse (38 pA) at 77 K was observed for (0.9)(BaTiO3)-(0.1)(BaNi0.5Nb0.5O2.75) with a reversible steady state photoresponses of 3 pA for Ba(Ti0.99Ni0.01)O2.99 and 10 pA for (0.9)(BaTiO3)- (0.1)(BaNi0.5Nb0.5O2.75) at 300 K under a AM1.5 light source. These results compare favorably to the photoresponses previously reported for BaTiO3 + 5 wt% CaTiO3 ceramics and Mn-doped BaTiO3 single crystals[102],[103]. The controllable absorption properties of Ni and Ni-Nb substituted BaTiO3 make these systems appropriate candidates for optimization of their ferroelectric and photovoltaic responses in thin film form.
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