Preparation and characterization of narrow bandgap ferroelectric (K,Ba)(Ni,Nb)O3−δ films for mesoporous all-oxide solar cells

Ferroelectric (K,Ba)(Ni,Nb) O 3 − δ films have triggered intense studies for applications in photovoltaic device due to their efficient ferroelectric polarization-driven carrier separation and above-bandgap generated photovoltages. However, they are suffered from a challenge of preparation limiting novel device architectures. Meanwhile, the bandgap for most of ferroelectric materials reported so far is still too large to be considered for desirable spectral absorption. Here, we propose a unique strategy to successfully synthesize the (K,Ba)(Ni,Nb) O 3 − δ films with the lower bandgap of about 1.45 eV. A new cell structure of utilizing (K,Ba)(Ni,Nb) O 3 − δ as a active layer is explored to interface with electron-transporting TiO2. Such mesoporous-ferroelectric combination solar cell is beneficial for facilitating the extraction of photocarriers. Under standard AM 1.5G irradiation, the optimized (K,Ba)(Ni,Nb) O 3 − δ film solar cell exhibits a higher open-circuit voltage of 1.27 V than those of previous reports on ferroelectrics. Furthermore, a fill factor of 64% and a power conversion efficiency of 0.2% are achieved via the polarization switching modulation. The present results provide a novel synthetic approach toward developing high performance solar cells based on lead-free ferroelectric films.


Introduction
Since the discovery of the ferroelectric photovoltaic (FPV) effect for the (KNbO 3 ) -x 1 (BaNi 1/2 Nb 1/2 d -O 3 ) x (KBNNOx) solid solutions, these materials have been increasingly investigated for photovoltaic (PV) and photocatalytic devices [1][2][3]. Under illumination, ferroelectric (FE) materials exhibit a reversibly switchable photocurrent and the above bandgap open circuit voltages [4][5][6][7][8][9]. Compared to the PV effect of traditional p-n junction devices, [10,11] for the FPV effect, photon-generated electron-hole (e-h) pairs are separated without the presence of a gradient for the electrochemical potential [12,13]. While the FPV effect is derived from the polarization switching-modulated energy band of the heterostructure, which can be also summarized as the polarization-induced build-in field [14]. Recent studies have confirmed that PV parameters of FE-based solar cells, including open-circuit voltage (V OC ), short-circuit current (J SC ), and power conversion efficiency (PCE) [15] can be dramatically enhanced by tuning FE bandgap (E g ) and switching FE polarization [16][17][18][19]. However, the wide E g of reported FE absorbers exceeds the optimum value of solar spectrum center, which corresponds to the maximum theoretical PCE, [20] and leads to the poor sunlight absorption capabilities. The wide E g of typical ferroelectric perovskites is ascribed to the metal-oxygen bonds at B sites and the large difference in their electronegativities. Due to the merit of the narrow bandgap of 2.7eV, BiFeO 3 (BFO) has been widely studied for PV applications [9,21]. Nevertheless, the photovoltaic efficiency has been stillinhibited by the wide bandgap, which only allows the use of 8%-20% of the solar spectrum. Therefore, lowering the E g value of novel Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. materials without affecting the ferroelectric properties is a cost-effective way to obtain PV devices with higher PCE. Fortunately, followed above method, the lower bandgap of 1. 39 [1,22]. Therefore, chemical modified the perovskite B site by transition metal is a promising guide for engineering the value of E g [23][24][25][26]. Nevertheless, the potential of semiconducting FE films in actual solar energy conversion is remained highly expectant.
In this work, we demonstrate a unique route to synthesis light absorbers to develop FE-based solar cells by exploring the properties of KBNNOx films. The present method is based on forming nucleation sites (deionized water as the growth medium) and tailoring Ni 2+ -V O -Nb 5+ and Ni 2+ -V O -Ni 2+ configurations in the film to realize effectively tuning of bandgap and FE properties for KBNNOx. The film crystallizes in two different transition-metal cations of Nb 5+ and Ni 2+ on the perovskite B sites, as shown in figure 1(a). It is apparent that ferroelectricity is driven by the Nb 5+ ions and E g is controlled by the interaction between Ni 2+ and Nb 5+ via V O . The simultaneous occurrence of the two mechanisms represents the successfully synthesis of the semiconducting KBNNOx films accompanying optimum ferroelectricity-optical absorption combinations for solar cell applications. Furthermore, we used KBNNOx as a film sensitizer to interface with electron-transport material (TiO 2 ) and hole-transport material (HTM, p-type NiO) to achieve a kind of all-oxide heterojunction cell. Under illumination, the optimal KBNNOx solar cell exhibits a PCE of 0.2% and a fill factor (FF) up to 64% by applying FE polarization, which is firstly reported for KBNNO1 PV devices. The present results provide critical insights into both understanding and optimizing the FEPV properties in KBNNOx.  the KBNNOx precursors were hydrolyzed by the addition of deionized aqueous solution in a ratio of water/ alkoxide=0.3.

Ferroelectric perovskite solar cells fabrication
Fluorine-doped SnO 2 (FTO) glass were cleaned sequentially in deionized water, acetone, isopropanol, and ethanol. For the mesoporous-type device, 40 nm compact TiO 2 (c-TiO 2 ) blocking layer was spin-coated on the FTO substrate at 3000 rpm for 30 s and baked at 450°C for 1 h. The c-TiO 2 precursor was synthesized with the molar ratio of titanium isopropoxide/HCl=1.5/0.13. Next, the diluted TiO 2 paste (Dyesol 30NR-D) was deposited on the c-TiO 2 layer and sintered at 550°C for 1 h, resulting in the formation of mesoporous TiO 2 (m-TiO 2 ) layer. The KBNNOx precursor was grown on the m-TiO 2 layer for twelve times with 2500 rpm for 30 s. Each coating layer was sintered at 320°C for 5 min. Then, all the films were baked at 550°C for 30 min. Similarly, a 60 nm thick NiO layers were prepared and pre-annealing at 280°C for 5 min. The NiO precursors with 0.1 M concentration was prepared by dissolving nickel (II) acetate tetrahydrate [Ni(CH 3 COO) 2 ·4H 2 O, 98%] and monoethanolamine in ethanol. The samples were annealed at 500°C for 30 min. Then, 50 nm thick Au electrodes were sputtered on top of the NiO layer with fringe shape, resulting in a cell area of 0.01 cm 2 . Note that the planar configuration device without m-TiO 2 layer was deposited on c-TiO 2 layer. The procedures for fabricating the c-TiO 2 , KBNNOx, NiO, and Au layers are the same as mesoporous-type device.

Characterization
Crystallographic structure of the films was characterized by x-ray diffraction (XRD) with Cu Kα radiation (D/MAX-2550 V, Rigaku Co.). Temperature dependent Raman spectra were measured using a micro-Raman spectrometer with a spectral resolution of 0.65 cm −1 (Jobin-Yvon LabRAM HR Evolution). A Nd:YAG laser with a wavelength of 532 nm was taken as the exciting source. The absorption spectra were measured by the ultraviolet-visible-near infrared (UV-vis-NIR) spectrophotometer (cary500, USA Varian) equipped with integration sphere. Scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy of the devices were performed (FEI Nova Nano). Ultraviolet photoelectron spectroscopy characterizations were investigated by monochromatized HeI radiation at 21.22 eV. The photovoltaic outputs were excited by solarsimulated AM1.5 sunlight (100 mW cm −2 , Sol 3A solar simulator, Newport Orial). The current density-voltage characteristics were recorded using Keithley 2400. The present study has demonstrated that the co-presence of ethoxide and methoxide in precursor could effectively alleviate this problem. Moreover, few works to date have been reported on the influence of hydrolytic process on the development of microstructure in KBNNOx films. Figure 1(a) displays the atomic structures of the KNO and KBNNO3.3 with the aided of the first-principles density functional theory calculations. Note that the theoretical information about the detailed calculation and analysis of the above materials could be found in previous work [1][2][3]22]. Figure 1(b) provides the fabrication parameters, such as distillation, deionized water of hydrolysis, and hydrolysis temperature. Furthermore, as shown in figure 1(c), the OH − ions attached to the substrate in a highdensity monolayer might result in the formation of a small number of seeds, which acted as nucleating sites for KBNNOx. As an example, the seeds subsequently grown by progressive incorporation of precursors from the KBNNO1 solution could form three-dimensional nanometre-sized crystals ( figure 1(d)). The reasonable chemical states of the as-grown KBNNO1 films were confirmed by x-ray photoemission spectroscopy, which are shown in figure S1 available online at stacks.iop.org/NJP/21/013011/mmedia. Finally, the corresponding high resolution TEM (figure 1(e)) image shows that the d-spacing is about 0.284 nm, which is in accordance with the (111) plane of KBNNO1 films. Note that the perfect lattice fringe suggests the high quality of the present films.

Design and characterization of KBNNO1 solar cells
To investigate the absorption properties of KBNNOx films, as an example, KBNNO1 planar and mesostructured heterojunctions were constructed (figures 2(a) and (b)), which were abbreviated as NiO/KBNNO1/c-TiO 2 /FTO and NiO/KBNNO1/m-TiO 2 /c-TiO 2 /FTO, respectively. TiO 2 has been recognized as one of the material of possessing the stabilization, environment-friendly characteristics, and high electron mobility for applications in dye-sensitized and perovskite devices. Similar to the dye-sensitized solar energy conversion, the incorporation of mesoporous composites for KBNNO1/c-TiO 2 interface can increase the surface area of absorbing layer, which enables a more effective photon-generated carriers separation and collection [27,28]. The hole mobility of p-type NiO is 25 cm 2 V −1 s −1 . It is regarded as one of the excellent inorganic HTMs for dye-sensitized and FE-based PV devices [29][30][31][32].  To demonstrate the crystalline quality of the light-absorbing layers, the crystalline structures of the four kinds of as-fabricated films (KNO-KBNNO3) were characterized by XRD, as shown in figure 3(a). Typical single peaks are observed for the end member KNO and doped KBNNOx films. A closer inspection of the shoulders suggests the doped KBNNOx samples belong to an average orthorhombic crystal symmetry. It is clear that the XRD reflections shift toward lower 2θ angles with increasing x, suggesting an increase in the unit cell volume ( figure 3(b)). To further illustrate the crystallization, Raman spectra for temperature range of 77K<T<793K were measured for end number KNO and doped KBNNO1 films in the figures 3(c), (d) and S3. Additional stoichiometries could be available in figures S4 and S5. Figures 3(c) and (d) show that a red-shift trend of phonon center positions are obtained caused by lattice thermal expansion. Moreover, KNO and KBNNO1 films are polycrystalline, and the observed signal from Raman spectra is also an average of many oblique angles, which is not strictly associated with phonon wave vectors (neither parallel nor perpendicular to the specific crystallographic axes) [33][34][35]. Furthermore, according to the group theoretical analysis, the Raman spectrum is mainly identified as (i) a Fano-type interference dip at around ∼190 cm −1 due to E(TO 2 , LO 2 ) + A 1 (TO 2 ,LO 2 ); (ii) a broad E(TO 1 ) mode zone centered at 240 cm −1 ; (iii) a sharp A 1 (TO 1 ) mode centered at 279 cm −1 ; (iv) another sharp mode (E + B 1 )(TO 4 ,LO 4 ) at 293 cm −1 ; (v) a E(TO 3 ) at 530 cm −1 ; (vi) a A 1 (TO 3 ) at 600 cm −1 ; (vii) a mixed low intensity A 1 (LO 3 ) at 830 cm −1 . By contrast, the peaks in the KBNNO1 film are much broader, and a peak at ∼430 cm −1 becomes clearer. Moreover, an obvious characteristic for KBNNO1 sample is that the mode at 830 cm −1 was still clearly visible at high temperature, which indicates the presence of a new mode due to the breathing of the octahedra when the B sites were occupied by different cations of Ni 2+ and Nb 5+ . for KBNNO1 film, respectively. Note that all above Raman features except for the relative intensities of KNO and KBNNO1 thin films are fully consistent with the characteristics of KNO and KBNNO1 ceramics, [3,36] which can be confirmed on the successful preparation of KNO and KBNNO1 thin films.  Figure 4(a) shows the UV-vis-NIR absorption spectra of the KBNNO1, KBNNO2, and KBNNO3 films. It is found that all the samples exhibit double absorbance peaks, as remarked by I and II, indicating that these films have both UV-vis and NIR absorption, as compared with previous reported FE absorbers including the wideacceptably FE active layers BiFeO 3 , Bi 2 FeCrO 6 , and Pb(Zr,Ti)O 3 [17,18,[37][38][39]. The obvious NIR absorption is attributed to oxygen vacancies (V O ) derived from the Ni 2+ doping in perovskite B site, which is similar with Pb(Ti 1−x Ni x )O 3−x solid solutions [40,41]. The theoretical results show that there are two stable configurations for the V O in KBNNOx, Ni 2+ -V O -Nb 5+ and Ni 2+ -V O -Ni 2+ . Thus, it indicates that the combination of metaloxygen bond results in the lower bandgaps owing to the introduction of the Ni 3d. Whereas, an extra density of states peak, contributed by the d-orbitals of the six-fold-coordinated Ni, is observed in the valence band maximum from Ni 2+ -V O -Nb 5+ [2]. Therefore, the E g value of the Ni 2+ -V O -Nb 5+ configuration is lower than that of Ni 2+ -V O -Ni 2+ configuration, which can correspond to the new absorbance I and II, respectively. It is noteworthy that the above films have a high concentration of V O to form recombination centers due to K + loss during the sintering process, which hinder the motion of the electron/hole. Therefore, it is necessary to reduce the amount of V O while preserving the beneficial effects of V O in reducing the bandgap and enhancing the visible-light absorption [1]. Here, we demonstrate that the hydrolysis at   5(e)), respectively. It illustrates that the PV output increases with increasing the scan bias, which can be extracted from the data of figure 5(f) that J SC ≈0.23 mA cm −2 , V OC ≈1.27 V, PCE≈0.2%, and FF≈64%, respectively. The reason for such a increased PV output can be attributed to enhanced light absorption and a possible reduction of the photocharge recombination rate caused by the migration of V O and polarization flipping. As shown in figure S6(a), the polarization-electric field (P-E) loop of the KBNNO1 mesoporous-type solar cell exhibits a linear behavior over the voltage range of −1 to 1 V, which means that no polarization flipping occurs. Figure S6(a) also shows that the polarization flipping can be triggered in KBNNO1 films with increasing the voltage. These results are consistent with the above PV output. It demonstrates that the present P-E results are solid evidences to reflect the impaction of PV output induced by the polarization flipping. The detailed physical mechanisms will be discussed in the following.  It has been revealed that the photocurrent is consistent and repeatable. The stability of the photoswitching behavior is related to the ferroelectric polarization stability of KBNNO1 layers and the reliability of the e-h transmission channel. It is worth emphasizing that the V O plays a significant role in the obtained PV effect. Because of KBNNO1 ferroelectrics with low V O concentration of 2.5%, the unstable domain state can be suppressed by polarization. Thus, the magnitude of depolarization field shows little change and high stability of the photocurrent over multiple cycles can be observed [22,44]. On the contrary, for those ferroelectrics with a higher V O concentration, the domain state after removing the poling field is unstable, where the hopping electrons will recombine with the V O [44]. Thus, it leads to the monotonous decrease in the magnitude of the photocurrent under the illumination. Here, a decay of photocurrent was also observed. Therefore, it is necessary to control the amount of V O to ensure the stability and enhanced photocurrent output. Furthermore, the rise and decay times can be deduced to be about 0.12 and 0.12 s from figure 6(b), respectively, which exhibit little temporal change of photocurrent. In our case, the high-sensitivity photoresponse may result from the interface (e.g. TiO 2 /KBNNO1 and NiO/KBNNO1) of the mesostructured carrier transporters in PV device. It is clear that the e-h pairs could be much easier to transport and collect at the interface regions due to the ETM and HTM layers are closely coupled in the mesostructured devices [45]. Those studies on highsensitivity photoresponse and fast e-h separation for photovoltaic device with KBNNO1 absorber may find new materials for future applications in perovskite-based photodetector.

Operation mechanism and hysteretic behavior
For the mesostructured KBNNO1 heterojunctions, the intrinsic build-in fields (  E bi ) generated at the interfaces between FE and carrier transporting-layer, which is the primary driving force for separation of the e-h pairs. However, switchable polarization and redistribution of V O [21,37,46,47], could also play an important role in enhancement of the PV effect. To better understand the contributions of above factors, the different scanning voltage range and direction were applied on KBNNO1 solar cells during the photocurrent measurement under illumination. In the measurement process, all applied voltage was on the Au electrode and FTO electrode was always grounded. In order to prevent the breakdown and irreversible reversal of these devices, the maximum positive voltage (V m ) was carefully controlled below 8 V. Forward and reverse scans are recorded as the voltage scanned from −V m to V m and V m to −V m , respectively. The V m dependent J SC , V OC , and PCE were extracted from the J-V characteristics under reverse (forward) scan, which can also indicate the J-V hysteretic behavior, as presented in figures 7(a)-(c), S6(b) and (c). Apparently, the J-V curves of the reverse scan exhibit a enhance tendency with V m increasing, and finally reach the saturated state. However, the extracted J SC , V OC , and PCE from the forward-scanned J-V characteristics are lower than those of the reverse scanning, resulting in lower PV output. Moreover, because of the completely polarization of the KBNNO1 light absorber layer under reverse scan from +8 to −0.3 V, the maximum PCE is measured up to 1%.
The phenomena of the above reverse and forward rectifying signal of the KBNNO1 PV devices can be explained by modulated energy band due to FE polarization and V O , as reported by Maksymovych et al [48]. Electromigration of the V O and switchable FE polarization can change the width of the depletion regions inside the FE absorber, which could regulate the J-V curves at different scanning history, as previously discussed [49][50][51]. To clarify the different interface band and PV effects for KBNNO1 heterojunctions under varied scanning voltage range and direction, the detailed rectification process for photovoltaic properties and electronic band structure for devices at short-circuit condition are described sequentially in figures 7(e)-(g), respectively. As shown in figure 7(e), without applying the electric field, the V O can be distributed homogeneously inside the polycrystalline KBNNO1 grains. In this case, KBNNO1 films have no selfpolarization. The interface band structures of KBNNO1 are regulated only by the two transporting-layers at the interface regions. Grinberg et al have demonstrated that the KBNNO1 material is a n-type semiconductor because V O serves as electron donors [1,52,53]. Thus, the p-n and + n -n junctions are formed at the interfaces of NiO/KBNNO1 and TiO 2 /KBNNO1. The  E bi1 and  E bi2 have the same direction. The total build-in field can be written as , which is the primary force for separation of the photon-generated e-h pairs. By controlling the voltage pulses applied on the solar cell, KBNNO1 grains can be polarized. Meanwhile, the charged V O can move toward the other end of the ferroelectric layer to achieve a new equilibrium state. It has been reported that the high concentration V O layer in the interface can reduce the barrier height of the n-type films, which is similar to an extra build-in field (  E VO ) applied on the heterojunction. In the scanned process, although the applied voltage can be decreased to 0, the remained asymmetric distribution of V O and  P r could modify the interface band structures of KBNNO1, resulting in varied PV output. Under the reverse scanned from V m to 0, the positive bias leads to V O move toward TiO 2 /KBNNO1 interface and create  P r pointing toward TiO 2 side. The accumulation of V O and FE polarization charges induce a heavily doped n + layer, which leads to a downward-bending for KBNNO1 energy bands in the TiO 2 /KBNNO1 interface region. Meanwhile, the NiO/ KBNNO1 interface region is depleted, thus reducing the KBNNO1 energy band at this end [19,44,[54][55][56][57]. To quality the impact of V O and polarization for PV output, an additional build-field field derived from the combination of V O and polarization can be defined as  E VO +  E P , which has the same direction with  E bi and enhances the PV effect. Because large reserve scanning voltage will increase  P r and the density of V O in the TiO 2 /KBNNO1 interface region, a monotonic enhancement of PV parameters as V m increases is obtained.
In contrast, under a forward bias scanned from −V m to 0, the negative bias on the NiO electrode promotes the accumulating of V O at the NiO/KBNNO1 interface and generates P r pointing toward the NiO side. Thus, the it reduces the net build-in field. As a result, in the NiO/KBNNO1 interface region, a recombination between the photon-generated holes and the V O -trapped electrons will be generated before collecting by the NiO. Additionally, the recombination rate decreases with increasing scanning voltage, until the scanning voltage is close to the breakdown voltage. Furthermore, when V m is large enough to make |  E P +  E VO |>|  E bi |, the inverted net build-in field makes e toward the NiO side and h toward the TiO 2 side. That is to say, NiO and TiO 2 layers become the energy barriers, only the photocarriers with higher energy can overcome this barrier hight and be collected by electrodes ( figure 7(g)). In conclusion, even though the forward scanning with large V m can improve the PV output, the J SC and V OC are less than those of observed in the reverse scanning. The above mechanisms have provided deep insights into how V O and polarization affect the PV effect in KBNNO1 solar cells.

Conclusions
It is noteworthy that the strategy reported here is summarized from the novel single-phase (KNbO 3 ) -x 1 (BaNi 1/2 Nb 1/2 d - 0.1, 0.2, and 0.3) films as a technological breakthrough for the preparation of KNbO 3 -based films. Furthermore, the bandgap tunability has been firstly established by applying growth medium (deionized water) as the reaction nucleation sites in KBNNOx films. It is found that the bandgaps of the new films can be substantially reduced to 1.45 eV, matching well with the solar spectral center (∼1.5eV). Subsequently, we successfully combine the mesostructured electron-transporting TiO 2 and hole-transporting p-type NiO layers with perovskite KBNNO1 in dye-sensitized PV device. Such structural device takes advantage of the large area of light-absorption layer and good ferroelectric properties of KBNNO1 in photoelectric conversion devices, which can contribute to a high PV output for KNbO 3 -based solar cells. Moreover, the intrinsic FE polarization and oxygen vacancies electromigration in KBNNO1 can also be beneficial to the transformation from Schottky-like barrier to Ohmic contacts, thus tailoring the PV performance. Therefore, the present work opens a new avenue to discover and design optimal ferroelectric film based solar cells with improved efficiency. Finally, the high-sensitivity photoresponse and fast e-h separation for photovoltaic device with KBNNO1 absorber layers will open up new materials for future developments of novel generation perovskite-based photodetector.