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Abstract
Since the domain wall photovoltaic effect (DW-PVE) is reported in BiFeO3 film, the investigations on photovoltaic properties in ferroelectrics have appealed more and more attention. In this work, we employed two Fe doped KTa1-xNbxO3 (Fe:KTN) single crystals in tetragonal phase and orthorhombic phase, respectively, possessing similar net polarization along [001]C direction, to quantize the contribution on photovoltaic properties from bulk photovoltaic effect (BPVE) and DW-PVE in Fe:KTN. The results show that there are significant enhancements of open-circuit voltages (VOC = –6.0 V, increases over 440%) and short-circuit current density (JSC = 18.5 nA cm–2, increases over 1580%) in orthorhombic Fe:KTN with engineer-domain structure after poled, corresponding to 14.2 mV and 2.2 mV for the single domain wall and bulk region under illumination of 405 nm light (100 mW). It reveals that DW-PVE plays a major role in KTN-based ferroelectrics, indicating an orthorhombic Fe:KTN single crystal is one of the potential photovoltaic materials.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, the management of energy shortage and environmental pollution is more and more urgent on the road of sustainable development. The photovoltaic (PV) effect, as a potential energy source to replace fossil fuels and reduce carbon emissions, has been widely studied to promote the development of advanced solar cells that generate a high voltage [1–5]. However, the below-bandgap open-circuit voltages (Voc) largely limit the further development of semiconductors in the field of PV effect [6]. Researchers have focused on exploring new systems of PV materials for a long time. As one potential material system, ferroelectric materials possess large PV responses and appeal to extensive studies. In particular, as representative material systems, the BiFeO3 and BaTiO3-based material systems have received more and more attention due to their remarkable PV properties [7–15]. Recently, researchers have combined the two systems in one and achieve a remarkable enhancement of PV property [16]. Compared with traditional semiconductor photovoltaic materials, ferroelectric photovoltaic materials have two obvious advantages: one is the high open-circuit voltage, the other is not limited by the inherent energy gap of the material [17]. Experiments have confirmed that there are two main contributions on photovoltaic effect in ferroelectric materials, bulk photovoltaic effect (BPVE) and domain wall photovoltaic effect (DW-PVE), respectively [18–20]. Since the DW-PVE is discovered from the multidomain BiFeO3 film, dawn is bright to the researched of photovoltaic effect. The researches demonstrate that domain walls acting as nanoscale photovoltage generators can significantly enhance the PV responses, resulting in a steady photocurrent as well as an above-bandgap photovoltage [17–19].
Recently, the PV response consisting of a potential step and a local PV component in the 90° DW region is obtained in BaTiO3 single crystal [20], which realizes a superimposition of BPVE and DW-PVE. Later off, the methodology to reveal the contributions of the BPV and the DW-PV effects quantitatively is also reported, which also indicates a design strategy to improve ferroelectric photovoltaics [19,20]. Since the potential steps crossing domain walls are cumulative, the construction of nanoscale and complex domain structures is one of the efficient ways to improve the PV response and has appealed extensive studies [5,16,18–20]. The investigations of PV are also focused on other ferroelectric material systems, such as K1−xNaxNbO3 (KNN) and Bi1−xNaxTiO3 (BNT) based ferroelectrics [21–24].
KTa1-xNbxO3 (KTN) single crystal has appealed widen studies for the excellent optical performance [25–27], such as giant broadband refraction [25], etc. Besides, KTN single crystal also possesses good ferroelectric [28], piezoelectric [29], and electrocaloric [30] properties and there are nanoscale domain structures in KTN single crystal. However, similar with LiNbO3, BaTiO3 et al ferroelectric materials, there is a large bandgap (over 3 eV) in pure KTN. Thus, there are few studies about the PV effects in KTN single crystals. Recent researches show that Fe doping can significantly decrease the bandgap of ferroelectric, [4,31] therefore, Fe doping may make KTN as one of potential PV materials.
In this paper, we design and grow two 0.5mol% Fe-doped KTN single crystals in the tetragonal phase (Fe:KTa0.47Nb0.53O3, Fe:KTN53) and orthogonal phase (Fe:KTa0.41Nb0.59O3, Fe:KTN59), respectively. A large Voc = –6.0 V is obtained in Fe:KTN59, which shows engineer-domain structure with dense domain walls after poling. By contrast, Fe:KTN53 possesses a single domain structure after poled, representing small Voc = 1.1 V. In addition, the contribution of PV from BPVE and DW-PVE are quantitatively analyzed, corresponding 14.2 mV and 2.2 mV for single domain wall and bulk region.
2. Experimental design
The KTN single crystals were grown via an improved top-seeded solid growth (TSSG) method, and 0.5mol% Fe is doped in the raw materials. The dimensions of the samples are 3.0[100]C(x)× 3.0[010]C(y) × 0.5[001]C(z) mm3. ITO electrode was sputtered on the upper surface of 3.0[100]C(x)× 3.0[010]C(y) mm2 by pulsed laser deposition (PLD) method and the gold electrode was sputtered on one surface. Temperature dependence of dielectric permittivity (ɛr) was measured by using an inductance, capacitance, resistance (LCR) meter (E4980A, Agilent Technologies) at 1 kHz, applying a probing voltage of 1 V to both sides of crystal facets. The heating rate is 1 K min–1. The polarization microscope images were observed by a polarizing microscope (Axioskop40, Zeiss). The vertical piezo-response force microscopy (V-PFM) images are obtained by using a commercial microscope (Cypher ES, Asylum Research) with conductive Pt/Ir-coating probes (EFM, Nanoworld). The applied voltage is 500 mV. Current-voltage curves were tested by the electrometer (6517B, Keithley) under 405 nm laser. Besides, the samples were poled at TC for 1 h with a voltage of 1000 V, and the voltage was removed after the samples cooled to room temperature.
3. Results and discussion
Since KTN growth is a cooling process and the Nb compositions (x) increases with the decease of growth temperature, there are different x for different positions in one as-grown crystal. Thus, two samples are selected from top (small x) and bottom (large x) position from one as-grown crystal. The Nb compositions (x) of samples in this work are defined by the linear relationship between TC and x in KTN [29–31]. It should be noted that KTN is one of the critical relax-like ferroelectric materials, wherein the Curie temperature (TC) is similar with the maximum relative dielectric permittivity temperature (Tm). As shown in Fig. 1(a), the dielectric permittivity versus temperature curves illustrate that the Tm for the two samples are 118° and 160°, corresponding to x = 0.53 and x = 0.59, respectively. The orthorhombic–tetragonal phase transition temperature of Fe:KTN53 and Fe:KTN59 are 17° and 50°, respectively. Thus, Fe:KTN53 and Fe:KTN59 are in the tetragonal phase and orthorhombic phase at room temperature (∼25°), respectively. The Fe:KTN53 and Fe:KTN59 possess normal P–E loops with similar shapes. The samples represent similar maximum polarization Pmax, as shown in Fig. 1(b). Based on Fig. 1(a), Fe:KTN53 and Fe:KTN59 is in the tetragonal phase and orthorhombic phase, respectively, corresponding to spontaneous polarization along [001] (six equivalate directions) and [011] (twelve equivalate directions). It indicates that spontaneous polarization PS of Fe:KTN59 is about $\sqrt 2$ times that of Fe:KTN53. Meanwhile, a similar Pmax reveals that the two samples possess nearly the same net polarization (Pnet) along the [001]C direction.
Fig. 1. (a)Temperature dependence of relative dielectric permittivity (ɛr) of the two Fe:KTN single crystal, measured at 1 kHz. (b) P–E loops of two Fe:KTN samples at room temperature.
Since the samples are poled along the [001]C direction, Fe:KTN53 is in a single domain state, while Fe:KTN59 displays an engineer-domain state. To confirm the above indication, Fig. 2 shows the image of the domain for the two samples under polarization microscopy. When the included angle between polarizer direction and poling direction is 45°, the horizon is bright for both samples. Wherein, there is only one domain wall in the test region for Fe:KTN53 [Fig. 2(a)], while there are a lot of domain walls in Fe:KTN59, as shown in the dashed frame in Fig. 2(c). When the poled direction of samples turns to polarizer or analyzer direction, Fe:KTN53 displays an extinction view as shown in Fig. 2(b), while Fe:KTN59 still represents a bright view with dense domain walls, indicating that Fe:KTN59 is in the engineer-domain state.
Based on the theory of DW-PVE [17–20], the domain walls would provide a substantial contribution to the PV response in Fe:KTN59. To reveal the influence of domain walls on the PV effects in KTN, J–V curves of the two samples are measured under 405 nm light (100 mW), as shown in Fig. 3. Fe:KTN53 shows opposite directions of VOC and JSC to the Fe:KTN59, attributed to the opposite contribution directions between BPVE and DW-PVE, which will be analyzed in the next section. Meanwhile, the open-circuit voltage (VOC) of the Fe:KTN53 and the Fe:KTN59 is 1.1 V and –6.0 V respectively. The short-circuit current density (JSC) values are –1.1 nA cm–2 and 18.5 nA cm–2, respectively. Distinctly, Fe doped KTN sample with domain walls along the poled direction possesses significantly large VOC and JSC than that without domain walls, corresponding to over 440% and 1580% enhancements of VOC and JSC, respectively. Therefore, the results reveal that the DW-PVE is effective in KTN based samples, significantly improving both VOC and JSC. In addition, Fe:KTN53 shows a nonlinear J–V curve, which is caused by the existence of leakage of current. Compared with other PV materials, [4,32,33] the photocurrent density is relatively small. Since 405 nm light is enough to overcome the bandgap, the low photocurrent density is attributed to the small absorption coefficient of Fe:KTN.
Fig. 2. The polarization microscopy images of domains under different test angles for poled Fe:KTN53 (a, b) and Fe:KTN59 (c, d), respectively. Wherein, P1 and P2 (blue) represent the polarization directions of polarizer and analyzer; the C (orange) represents the crystallography direction and the P (red) represents the poled direction.
Fig. 3. (a) The Current density (J)–voltage (V) characteristics curves of Fe:KTN53 (a) and Fe:KTN59 (b), measured under 405 nm light with 100 mW power.
The above phenomena reveal the influence of domain walls on the PV response. To investigate the effect of the domain walls, Fig. 4(a) shows the schematic of Fe:KTN59 photovoltaic device structure, displaying the orientations of P after poled based on Figs. 2(c) and 2(d). Since Fe:KTN59 is in the orthogonal phase at room temperature, the domain structures are in the engineer-domain state along the poled direction. To show the details of the microdomain structure of poled Fe:KTN59, Fig. 4(b) represents the amplified image of the red dashed frame region in Fig. 2(d). The average width of domains in Fe:KTN59 is about 1 µm, as shown in Fig. 4(b). In addition, on the basis of the principle of polarization microscopy, the bright view indicates that the P has included angle with both polarizer and analyzer. Thus, the polarization states of Fig. 4(b) are shown in Fig. 4(c), existing as 90° domain structure, and the grey regions represent domain walls. The directions of dipoles turn gradually in domain wall regions, as shown in Fig. 4(d). As reported in BaTiO3 single crystal and BiFeO3 films [18–20], there are opposite electric fields in the domain wall region (EDW) compared with that in the bulk region (EB), corresponding to the opposite voltages, as shown in Fig. 4(d). Combining with the results of VOC and JSC, it indicates a relation between voltages caused by BPVE (VB) and DW-PVE (VDW), $|{{V_{DW}}} |\gg |{{V_B}} |$. The VDW can be estimated by
where VOC(DW) corresponds to VOC of Fe:KTN59, VOC(B) corresponds to VOC of Fe:KTN53, ${n_{DW}}$ is the number of the domain walls along the poled direction. Since the average width of the domain is about 1 µm and the sample thickness is 0.5 mm, thus, the ${n_{DW}}$ of Fe:KTN59 is about 500 in this work. Based on the Eq. (1), the VDW is approximately 14.2 mV in Fe:KTN59. Likewise, the value of VB can be calculated by ${{|{{V_{OC}}(B)} |} / {{n_{DW}}}}$, about 2.2 mV (the thickness of domain walls is just several nanometers and can be ignored). As shown in Fig. 4(d), the energy of valence band maximum (VBM) and conduction band minimum (CBM) decrease slightly in the bulk regions, while remarkably enhance across the domain walls, corresponding to the total energy increase gradually with crossing more and more domain walls.Fig. 4. (a) Schematic diagram of Fe:KTN59 single crystal photovoltaic device structure in upper-poled condition; (b) The local micro-domain structure under polarization microscopy, cutting form the red dashed frame in Fig. 2(d); (c) Schematic of polarization state for Fig. 4(b); (d) Schematic of the valence band maximum (VBM) and conduction band minimum (CBM) containing both bulk PV and DW-PV effects.
To illustrate detailed characteristics of domains, Fig. 5 shows the V-PFM images of as-grown Fe:KTN53 and Fe:KTN59 with a 3×3 µm2 size, measured at room temperature. It is clear that Fe:KTN53 shows strip-type domain structures with about 1µm width. While Fe:KTN59 possesses more complex domain structures, representing as both strip-type and “sawtooth-like” structures, wherein the size of “sawtooth-like” domains is just about one hundred nanometers, even more small. The complex domain structures may be attributed to that the phase transition temperature (from orthorhombic to tetragonal phase) of Fe:KTN59 is near room temperature. As Figs. 5(a) and 5(e) shown, the height retraces (within ±4 nm) images reveal that the measured response of domain structures are not caused by the surface roughness. Combining both the amplitude and phase images, Fe:KTN53 displays 180°–state domains, corresponding to the similar amplitudes and opposite phases [180° difference, as shown in Fig. 5(d)]. Diversely, Fe:KTN59 shows different amplitudes for domains, revealing different piezoelectric responses, which is attributed to various orientations of spontaneous polarization in the orthorhombic phase of KTN. In addition, the differences of phase are still 180°as shown in Fig. 5(h). Therefore, Fe:KTN59 possesses a smaller size and more complex domains, which can turn into regular engineer-domain structure with small widths after poled, leading to high VOC and large JSC under illumination via DW-PVE.
Fig. 5. The V-PFM images of Fe:KTN53 and Fe:KTN59. (a, e) Height retrace; (b, f) Amplitude images; (c, g) Phase images; (d, h) The details of phase derived from red dotted lines in Fig. 5(c, g). The tests are measured at room temperature.
4. Conclusion
In summary, two Fe doped KTN single crystals, being in tetragonal and orthorhombic phases with similar Pnet along [001]C direction, are fabricated in this work in order to investigate the effects of domain walls on the photovoltaic effect. The results reveal that Fe:KTN59 (orthorhombic phase at room temperature) possesses about 1 µm width engineer-domain structure after poled, showing opposite direction of VOC and JSC compared with the results of Fe:KTN53 in a single domain state. Meanwhile, significant enhancements of PVE (over 440% and 1580% enhancements of VOC and JSC) are achieved in Fe:KTN59, due to the contribution from DW-PVE. The voltage caused by single domain walls is estimated at 14.2 mV under illumination of 405 nm light (100 mW), while 2.2 mV for the single bulk region. Therefore, the investigations of the photovoltaic effect in both tetragonal and orthorhombic Fe:KTN single crystal reveal one way to furtherly explore the DW-PVE and indicate that orthorhombic Fe:KTN is one potential photovoltaic materials. Besides, the remarkable enhancements provide a way to improve the photovoltaic properties by building in engineer-domain structure.
Funding
National Natural Science Foundation of China (11674079, 51802055); China Scholarship Council (201906120225).
Disclosures
The authors declare no conflicts of interest.
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