Characterization and Photovoltaic Properties of BiFeO 3 Thin Films

Bismuth ferrite (BiFeO3) thin films were prepared by a spin-coating method. Crystal structure and optical properties of the BiFeO3 films were evaluated using X-ray diffraction. The lattice constants, crystallite size, and energy gap of BiFeO3 films depended on the concentration of the BiFeO3 precursor solution. BiFeO3/CH3NH3PbI3 photovoltaic devices were fabricated to investigate photovoltaic properties of BiFeO3. Current density–voltage characteristics of the photovoltaic devices showed rectifying behavior, indicating that BiFeO3 worked as an electron transport layer in CH3NH3PbI3-based photovoltaic devices.

The purpose of the present work is to characterize BiFeO 3 thin films prepared by a simple spin-coating method and investigate photovoltaic properties of BiFeO 3 /CH 3 NH 3 PbI 3 photovoltaic devices.The crystal structures of the BiFeO 3 thin films prepared from precursor solutions with different concentrations were characterized by X-ray diffraction (XRD).The optical properties of the BiFeO 3 films were investigated by collecting optical absorption spectra.The photovoltaic properties of the BiFeO 3 /CH 3 NH 3 PbI 3 devices were investigated by measuring current density-voltage (J-V) characteristics and incident photon to current conversion efficiency (IPCE).
The crystal structure of the BiFeO 3 thin films was characterized by an X-ray diffractometer (D2 PHASER, Bruker Corporation, Billerica, MA, USA) with CuKα radiation.The film thickness of the BiFeO 3 films was checked by atomic force microscopy (AFM) (SPI-3800N/SPA-400, SII Nanotechnology Inc., Chiba, Japan).The optical absorption spectra of the BiFeO 3 and CH 3 NH 3 PbI 3 films were collected using ultraviolet-visible-near-infrared spectrophotometer (V-770, Jasco Corporation, Tokyo, Japan).The J-V characteristics of the photovoltaic devices were recorded using a potentiostat (HSV-110, Hokuto Denko Corporation, Hyogo, Japan).The devices were irradiated under simulated AM 1.5 (100 mW•cm −2 ) conditions through the bottom of FTO-coated glass substrate by solar simulator (XES-301S, San-Ei Electric Co., Ltd., Osaka, Japan).The effective area of the devices was 0.090 cm 2 .IPCE spectra of the devices were collected using an IPCE measurement system (QE-R, Enli Technology Co., Ltd., Kaohsiung City, Taiwan).All measurements were performed at room temperature.

Results and Discussion
Figure 1a shows XRD patterns of the BiFeO 3 thin films.It was confirmed that the BiFeO 3 films were polycrystalline with a rhombohedral system.BiFeO 3 is usually classed as rhombohedral space group R3c structure with lattice parameter of a rh = 0.5634 nm and interaxial angle of α rh = 59.348 • [31].To later discuss lattice distortion in the present BiFeO 3 films, the XRD patterns of the BiFeO 3 films were indexed on the basis of hexagonal unit cell of BiFeO 3 , as shown in Figure 1b.Moreover, the film thicknesses of the 0.10 M, 0.20 M, and 0.30 M BiFeO 3 films were 224, 336, and 367 nm, respectively; i.e., the film thickness of BiFeO 3 films was proportional to the concentration of BiFeO 3 precursor solution.This behavior was almost the same as a previous report [32].The lattice constants of the BiFeO 3 were estimated from Equation (1): Here, a, c, d, and (hkl) are the lattice constants of aand c-axes, interplanar lattice spacing, and Miller indices, respectively.As shown in Figure 1c, the estimated lattice constants were compatible with those of BiFeO 3 bulk [31,33].On the other hand, the lattice constant of the a-axis slightly increased with increasing concentration of BiFeO 3 precursor solution, while that of the c-axis significantly increased at a concentration of 0.20 M. The results suggest that stress in the BiFeO 3 films relieved at concentration of 0.20 M. Crystallite sizes (D) of the BiFeO 3 were also estimated from the XRD patterns using Scherrer's equation: Here, λ, β, and θ are the X-ray wavelength of 0.154184 nm, full width at half maximum of the diffraction peak, and Bragg angle of the diffraction peak, respectively.In the present study, the averaged D values were estimated from D values of 012, 104, and 110 diffraction peaks of the BiFeO 3 .Figure 1d shows the averaged D of the BiFeO 3 .The D increased from 42 to 55 nm with increasing concentration of BiFeO 3 precursor solution.
Figure 2a shows optical absorption spectra of the BiFeO 3 thin films.The absorption cut-off wavelengths of the films were approximately 565 nm, in agreement with a previous report [34][35][36].The absorption spectrum of the 0.10 M BiFeO 3 film showed a large absorbance in the wavelength from 300 to 450 nm, which is likely associated with lattice distortion in the film.Energy gaps of the BiFeO 3 films were calculated by Tauc formula: Here, h, ν, α, A, E g , and n are the Plank constant, light frequency, absorption coefficient, proportional constant, energy gap, and power index which depends on the nature of the transition, respectively.In the present study n = 2 was used for the BiFeO 3 because BiFeO 3 is a direct transition semiconductor [36,37].The E g were estimated by extrapolating the linear part of the Tauc plots to meet (hνα) 2 = 0, as shown in shown in Figure 2c.The estimated E g values corresponded to previous reports [6,9,36,37].On the other hand, the estimated E g decreased with increasing concentration of BiFeO 3 precursor solution, indicating that E g depended on the concentration of the BiFeO 3 solution.Furthermore, the largest E g was obtained for the 0.10 M BiFeO 3 film, which would be a result of lattice distortion in the films.
Here, λ, β, and θ are the X-ray wavelength of 0.154184 nm, full width at half maximum of the diffraction peak, and Bragg angle of the diffraction peak, respectively.In the present study, the averaged D values were estimated from D values of 012, 104, and 110 diffraction peaks of the BiFeO3.
Figure 1d shows the averaged D of the BiFeO3.The D increased from 42 to 55 nm with increasing concentration of BiFeO3 precursor solution.Here, λ, β, and θ are the X-ray wavelength of 0.154184 nm, full width at half maximum of the diffraction peak, and Bragg angle of the diffraction peak, respectively.In the present study, the averaged D values were estimated from D values of 012, 104, and 110 diffraction peaks of the BiFeO3.Figure 1d shows the averaged D of the BiFeO3.The D increased from 42 to 55 nm with increasing concentration of BiFeO3 precursor solution.Figure 2a shows optical absorption spectra of the BiFeO3 thin films.The absorption cut-off wavelengths of the films were approximately 565 nm, in agreement with a previous report [35][36][37].The absorption spectrum of the 0.10 M BiFeO3 film showed a large absorbance in the wavelength from 300 to 450 nm, which is likely associated with lattice distortion in the film.Energy gaps of the BiFeO3 films were calculated by Tauc formula: Here, h, ν, α, A, Eg, and n are the Plank constant, light frequency, absorption coefficient, proportional constant, energy gap, and power index which depends on the nature of the transition, respectively.
In the present study n = 2 was used for the BiFeO3 because BiFeO3 is a direct transition semiconductor [37,38].The Eg were estimated by extrapolating the linear part of the Tauc plots to meet (hνα) 2 = 0, as shown in shown in Figure 2c.The estimated Eg values corresponded to previous reports [6,9,37,38].
On the other hand, the estimated Eg decreased with increasing concentration of BiFeO3 precursor solution, indicating that Eg depended on the concentration of the BiFeO3 solution.Furthermore, the largest Eg was obtained for the 0.10 M BiFeO3 film, which would be a result of lattice distortion in the films.To discuss the lattice distortion in the BiFeO3 films, a simple calculation was attempted using Hooke's law: To discuss the lattice distortion in the BiFeO 3 films, a simple calculation was attempted using Hooke's law: where ∆ε, σ, E are the strain, stress, and Young's modulus, respectively.In the present study, previously reported Young's modulus of BiFeO 3 was used [39].The lattice strains for aand c-axes (∆ε a and ∆ε c ) are given as follows: and Here, a' and c' are the lattice constants of BiFeO 3 bulk [33].The calculated σ values for aand c-axes of BiFeO 3 as a function of film thickness are shown in Figure 3.The σ for a-axis slightly increased with increasing film thickness, while that for c-axis significantly increased: the stress for c-axis strongly depended on film thickness.Furthermore, a sign reversal of the σ for c-axis occurred at a film thickness of 367 nm (0.30 M), considering that the 0.10 and 0.20 M BiFeO 3 films were wholly under compressive stress, while the 0.30 M BiFeO 3 was partially under tensile stress.Accordingly, a series of variations in crystal structure and optical properties were attributed to compressive stress relaxation in the BiFeO 3 films that depended on film thickness.
Coatings 2016, 6, 68 5 of 9 where Δɛ, σ, E are the strain, stress, and Young's modulus, respectively.In the present study, previously reported Young's modulus of BiFeO3 was used [39].The lattice strains for a-and c-axes (Δɛa and Δɛc) are given as follows: and Here, a' and c' are the lattice constants of BiFeO3 bulk [33].The calculated σ values for a-and c-axes of BiFeO3 as a function of film thickness are shown in Figure 3.The σ for a-axis slightly increased with increasing film thickness, while that for c-axis significantly increased: the stress for c-axis strongly depended on film thickness.Furthermore, a sign reversal of the σ for c-axis occurred at a film thickness of 367 nm (0.30 M), considering that the 0.10 and 0.20 M BiFeO3 films were wholly under compressive stress, while the 0.30 M BiFeO3 film was partially under tensile stress.Accordingly, a series of variations in crystal structure and optical properties were attributed to compressive stress relaxation in the BiFeO3 films that depended on film thickness.Figure 4a shows J-V curves of the FTO/BiFeO3/CH3NH3PbI3/spiro-OMeTAD/Au and FTO/CH3NH3PbI3/spiro-OMeTAD/Au device photovoltaic devices under light irradiation.A clear rectifying behavior was seen for the FTO/BiFeO3/CH3NH3PbI3/spiro-OMeTAD/Au device, while a linear J-V curve was seen for the FTO/CH3NH3PbI3/spiro-OMeTAD/Au one, ensuring that the BiFeO3 acted as an electron transport layer in the present device.From these J-V curves, a short-circuit Figure 4a shows J-V curves of the FTO/BiFeO 3 /CH 3 NH 3 PbI 3 /spiro-OMeTAD/Au and FTO/CH 3 NH 3 PbI 3 /spiro-OMeTAD/Au device photovoltaic devices under light irradiation.A clear rectifying behavior was seen for the FTO/BiFeO 3 /CH 3 NH 3 PbI 3 /spiro-OMeTAD/Au device, while a linear J-V curve was seen for the FTO/CH 3 NH 3 PbI 3 /spiro-OMeTAD/Au one, ensuring that the BiFeO 3 acted as an electron transport layer in the present device.From these J-V curves, a short-circuit current density (J sc ) of 0.290 mA cm −2 , open-circuit voltage (V oc ) of 0.531 V, fill factor (FF) of 0.414, and conversion efficiency (η) of 0.064% were obtained for the FTO/BiFeO 3 /CH 3 NH 3 PbI 3 /spiro-OMeTAD/Au device.In contrast, a J sc of 0.652 mA cm −2 , V oc of 0.014 V, FF of 0.227, and η of 0.002% were obtained for the FTO/CH 3 NH 3 PbI 3 /spiro-OMeTAD/Au one.However, the η was quite small compared to the previous report [9].As one of the possible reasons for the low conversion efficiency, external factors such as temperature, humidity, and fabrication process were considered [40].An IPCE spectrum of the FTO/BiFeO 3 /CH 3 NH 3 PbI 3 /spiro-OMeTAD/Au photovoltaic device is shown in Figure 4b.The device showed a broad IPCE spectrum in the wavelength range between 320 and 780 nm, indicating that the generation of free electrons and/or free holes occurred in the CH 3 NH 3 PbI 3 layer under visible light irradiation.An optical absorption spectrum of the same device is also shown in Figure 4b.From the optical absorption spectrum, E g of the BiFeO 3 and CH 3 NH 3 PbI 3 were estimated to be 2.78 and 1.57 eV, respectively.
Coatings 2016, 6, 68 The photovoltaic mechanism based on energy levels is discussed.Figure 5 shows an energy level diagram of the FTO/BiFeO3/CH3NH3PbI3/spiro-OMeTAD/Au photovoltaic device, where previously reported and estimated values were used for the energy levels [16,21,22].The electronic charge generation occurs in the CH3NH3PbI3 layer light irradiation from the FTO bottom side.Then, the electrons from the CH3NH3PbI3 are transferred to the FTO electrode through the BiFeO3 electron transport layer.Similarly, the holes are transferred to Au electrode through the spiro-OMeTAD hole transport layer.It has been known that Voc of inorganic/organic solar cells is related to the energy gaps between the conduction band of the inorganic semiconductor and the valence band of the organic semiconductor.According to the energy level diagram in Figure 5, the theoretical Voc of the BiFeO3/CH3NH3PbI3 photovoltaic device can be calculated by the following equation [41]: Here, e, Ec, EH, and Vloss are the elementary charge, conduction band of inorganic semiconductor, the highest occupied molecular orbital (HOMO) of the organic semiconductor, and the empirical voltage loss for exciton dislocation at a heterojunction, respectively.Usually, the Vloss is 0.3-0.5 V [8,41].In the present study, the theoretical Voc of the BiFeO3/CH3NH3PbI3 photovoltaic cell was 0.9-1.1 V.These values are greater than the present Voc, as shown in Figure 4a.The obtained low Voc was considered The photovoltaic mechanism based on energy levels is discussed.Figure 5 shows an energy level diagram of the FTO/BiFeO 3 /CH 3 NH 3 PbI 3 /spiro-OMeTAD/Au photovoltaic device, where previously reported and estimated values were used for the energy levels [16,21,22].The electronic charge generation occurs in the CH 3 NH 3 PbI 3 layer by light irradiation from the FTO bottom side.Then, the electrons from the CH 3 NH 3 PbI 3 are transferred to the FTO electrode through the BiFeO 3 electron transport layer.Similarly, the holes are transferred to Au electrode through the spiro-OMeTAD hole transport layer.It has been known that V oc of inorganic/organic solar cells is related to the energy gaps between the conduction band of the inorganic semiconductor and the valence band of the organic semiconductor.According to the energy level diagram in Figure 5, the theoretical V oc of the BiFeO 3 /CH 3 NH 3 PbI 3 photovoltaic device can be calculated by the following equation [41]: Coatings 2016, 6, 68 7 of 9 Here, e, E c , E H , and V loss are the elementary charge, conduction band of inorganic semiconductor, the highest occupied molecular orbital (HOMO) of the organic semiconductor, and the empirical voltage loss for exciton dislocation at a heterojunction, respectively.Usually, the V loss is 0.3-0.5 V [8,41].
In the present study, the theoretical V oc of the BiFeO 3 /CH 3 NH 3 PbI 3 photovoltaic cell was 0.9-1.1 V.These values are greater than the present V oc , as shown in Figure 4a.The obtained low V oc was considered to be due to large V loss associated with trap states in the CH 3 NH 3 PbI 3 layer, presumably generated due to partial separation of PbI 2 from the CH 3 NH 3 PbI 3 [23,24,[26][27][28].In addition, it is considered that grains of the present CH 3 NH 3 PbI 3 were small.In fact, a correlation between grain size and V oc of

Conclusions
Crystal structures and optical properties of BiFeO3 thin films prepared from precursor solutions with different concentrations were investigated.Polycrystalline BiFeO3 films with a hexagonal system were obtained.The lattice constants, crystallite size, and energy gap of the BiFeO3 films depended on concentration of BiFeO3 precursor solution.The variations in structural parameters and optical properties were attributed to compressive stress relaxation in the BiFeO3 films that depended on film thickness.BiFeO3/CH3NH3PbI3 photovoltaic devices were also fabricated to investigate their photovoltaic properties.The BiFeO3/CH3NH3PbI3 photovoltaic devices showed a rectifying behavior, and a small conversion efficiency was obtained for the present BiFeO3/CH3NH3PbI3 device.From these combined results, BiFeO3 is anticipated as an electron transport layer in solar cells.

Conclusions
Crystal structures and optical properties of BiFeO 3 thin films prepared from precursor solutions with different concentrations were investigated.Polycrystalline BiFeO 3 films with a hexagonal system were obtained.The lattice constants, crystallite size, and energy gap of the BiFeO 3 films depended on concentration of BiFeO 3 precursor solution.The variations in structural parameters and optical properties were attributed to compressive stress relaxation in the BiFeO 3 films that depended on film thickness.BiFeO 3 /CH 3 NH 3 PbI 3 photovoltaic devices were also fabricated to investigate their photovoltaic properties.The BiFeO 3 /CH 3 NH 3 PbI 3 photovoltaic devices showed a rectifying behavior, and a small conversion efficiency was obtained for the present BiFeO 3 /CH 3 NH 3 PbI 3 device.From these combined results, BiFeO 3 is anticipated as an electron transport layer in solar cells.

Figure 1 .
Figure 1.(a) X-ray diffraction (XRD) patterns of BiFeO3 thin films grown on glass substrates; (b) Schematic illustration of hexagonal unit cell of BiFeO3; (c) Lattice constants and (d) crystallite sizes of BiFeO3 as a function of concentration of BiFeO3 precursor solution.The crystallographic information data of (b) was downloaded from Ref. [34].

Figure 1 .Figure 1 .
Figure 1.(a) X-ray diffraction (XRD) patterns of BiFeO3 thin films grown on glass substrates; (b) Schematic illustration of hexagonal unit cell of BiFeO3; (c) Lattice constants and (d) crystallite sizes of BiFeO3 as a function of concentration of BiFeO3 precursor solution.The crystallographic information data of (b) was downloaded from Ref. [34].

Figure 3 .
Figure 3. Stresses along a-(filled circle) and c-axes (filled triangle) of BiFeO3 as a function of film thickness of BiFeO3.

Figure 3 .
Figure 3. Stresses along a-(filled circle) and c-axes (filled triangle) of BiFeO 3 as a function of film thickness of BiFeO 3 .