Characterization and Photovoltaic Properties of BiFeO₃ Thin Films

Abstract

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.

1. Introduction

Bismuth ferrite (BiFeO₃) has attracted much attention among researchers, because BiFeO₃ is used for potential applications, such as information storage, spintronics, and ferroelectric devices [1,2,3,4,5]. In addition, photovoltaic properties of BiFeO₃ bulks, BiFeO₃/semiconductor, and BiFeO₃/ferroelectric heterostructures have been extensively investigated [6,7,8,9,10,11]. However, photovoltaic properties are sensitive to fabrication process, electronic structure, and interfacial condition. Therefore, precise characterization of BiFeO₃ and investigation of BiFeO₃-based solar cells are of importance.

Very recently, BiFeO₃ films prepared by spin-coating method were used for CH₃NH₃PbI₃-based photovoltaic devices [9]. CH₃NH₃PbI₃ with perovskite structure is known as a fascinating light absorber, and the CH₃NH₃PbI₃-based solar cell is anticipated as a next-generation solar cell [12,13,14,15,16,17,18,19]. BiFeO₃ films act as electron transport layers because BiFeO₃ is an n-type semiconductor. However, optical and photovoltaic properties of the BiFeO₃/CH₃NH₃PbI₃ photovoltaic devices are still unknown.

The purpose of the present work is to characterize BiFeO₃ thin films prepared by a simple spin-coating method and investigate photovoltaic properties of BiFeO₃/CH₃NH₃PbI₃ photovoltaic devices. The crystal structures of the BiFeO₃ thin films prepared from precursor solutions with different concentrations were characterized by X-ray diffraction (XRD). The optical properties of the BiFeO₃ films were investigated by collecting optical absorption spectra. The photovoltaic properties of the BiFeO₃/CH₃NH₃PbI₃ devices were investigated by measuring current density–voltage (J–V) characteristics and incident photon to current conversion efficiency (IPCE).

2. Materials and Methods

BiFeO₃ thin films were prepared on glass substrates by spin-coating method in air atmosphere [20]. Solutions of BiFeO₃ with different concentrations were prepared from bismuth nitrate penehydrate (Bi(NO₃)₃·5H₂O) (99.5%, Wako Pure Chemical Industries, Ltd., Osaka, Japan), iron(III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O) (99.0%, Wako Pure Chemical Industries, Ltd.), citric acid (99.5%, Yoneyama Yakuhin Kogyo Co., Ltd., Osaka, Japan), and 2-methoxyethanol (99.0%, 2 mL, Wako Pure Chemical Industries, Ltd.). Citric acid was used as a chelating agent. The concentration of the precursor solution was varied from 0.05 to 0.30 M. Excess 10% Bi(NO₃)₃·5H₂O was added to compensate for Bi loss during annealing. The BiFeO₃ solution was dropped on the glass substrate, and was spun at 3000 rpm for 30 s by a spin coater (MS-A100, Mikasa Co., Ltd., Tokyo, Japan). Then, the BiFeO₃ precursor film was dried on a hotplate (ND-1, As One Corporation, Osaka, Japan) at 150 °C for 5 min. This process was repeated five times. Finally, the precursor film was annealed in an electric furnace (SMF-1, As One Corporation) at 500 °C for 1 h. Photovoltaic devices with BiFeO₃/CH₃NH₃PbI₃ structure were also fabricated by the spin-coating method. Concerning energy level of BiFeO₃/CH₃NH₃PbI₃ structure, BiFeO₃ and CH₃NH₃PbI₃ are a good combination, because BiFeO₃ is an n-type semiconductor [16,21,22]. The fabrication process of the BiFeO₃/CH₃NH₃PbI₃ photovoltaic devices is as follows [23]. First, 0.05 M BiFeO₃ precursor layer was spin-coated onto a fluorine-doped tin oxide (FTO)-coated glass substrate by the spin-coater at 3000 rpm for 30 s, and was dried onto the hot plate at 150 °C for 5 min. This process was repeated three times. Subsequently, the BiFeO₃ layer was annealed in an electric furnace at 500 °C for 30 min. After cooling to room temperature, a 0.10 M BiFeO₃ layer was spin-coated onto the 0.05 M BiFeO₃ layer at 3000 rpm for 30 s, and was dried onto the hot plate at 150 °C for 5 min. This process was repeated three times. The 0.10 M BiFeO₃ layer was annealed in the electric furnace at 500 °C for 2 h. In the present work, a one-step solution deposition method was employed to prepare CH₃NH₃PbI₃ layers [23,24,25,26,27,28,29]. The preparation process was as follows: a mixed solution consisting of methylammonium iodine (CH₃NH₃I, 98.8 mg, Showa Chemical Co., Ltd., Tokyo, Japan), PbI₂ (289.3 mg, Sigma-Aldrich, St. Louis, MO, USA), and γ-butyrolactone (500 μL, Wako Pure Chemical Industries, Ltd.) was prepared by stirring at 60 °C for 2 h in a water bath (BH-22, Masuda Corporation, Osaka, Japan,) attached with a magnetic stirrer (Heracles-20, Koike Precision Instruments, Hyogo, Japan). The CH₃NH₃PbI₃ layer was spin-coated on the BiFeO₃ layer at 2000 rpm for 60 s. This process was repeated four times. The spin-coated CH₃NH₃PbI₃ layer was annealed at 100 °C for 15 min. After cooling to room temperature, a hole transport layer was spin-coated on the CH₃NH₃PbI₃ layer at 4000 rpm for 20 s. As the hole transport material, 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene (spiro-OMeTAD) was used because spiro-OMeTAD can yield higher conversion efficiencies due to its amorphous structure and reasonable hole mobility [30]. The spiro-OMeTAD powder (36.1 mg, Luminescence Technology, Hsinchu City, Taiwan) was dissolved in chlorobenzene (Wako Pure Chemical Industries, Ltd., 500 μL). Solutions of lithium bis(trifluomethylsulfonyl) imide (Li-TFSI, 8.8 μL)and 4-tert-butylpyridine (14.4 μL, Sigma-Aldrich) were added into the spiro-OMeTAD solution. The Li-TFSI solution was prepared by dissolving Li-TFSI powder (260 mg, Tokyo Chemical Industry Co., Ltd., Tokyo, Japan) in acetonitrile (0.5 mL, Wako Pure Chemical Industries, Ltd.). Finally, gold (Au) counter electrodes were thermally deposited on the hole transport layer. For comparison, a photovoltaic device without BiFeO₃ layer was fabricated.

The crystal structure of the BiFeO₃ thin films was characterized by an X-ray diffractometer (D2 PHASER, Bruker Corporation, Billerica, MA, USA) with CuKα radiation. The film thickness of the BiFeO₃ films was checked by atomic force microscopy (AFM) (SPI-3800N/SPA-400, SII Nanotechnology Inc., Chiba, Japan). The optical absorption spectra of the BiFeO₃ and CH₃NH₃PbI₃ 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 cm². 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.

3. Results and Discussion

Figure 1a shows XRD patterns of the BiFeO₃ thin films. It was confirmed that the BiFeO₃ films were polycrystalline with a rhombohedral system. BiFeO₃ 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 BiFeO₃ films, the XRD patterns of the BiFeO₃ films were indexed on the basis of hexagonal unit cell of BiFeO₃, as shown in Figure 1b. Moreover, the film thicknesses of the 0.10 M, 0.20 M, and 0.30 M BiFeO₃ films were 224, 336, and 367 nm, respectively; i.e., the film thickness of BiFeO₃ films was proportional to the concentration of BiFeO₃ precursor solution. This behavior was almost the same as a previous report [32]. The lattice constants of the BiFeO₃ were estimated from Equation (1):

$$\frac{1}{d^2}=\frac{4}{3}\frac{h^2+hk+k^2}{a^2}+\frac{l^2}{c^2}$$

(1)

Here, a, c, d, and (hkl) are the lattice constants of a- and c-axes, interplanar lattice spacing, and Miller indices, respectively. As shown in Figure 1c, the estimated lattice constants were compatible with those of BiFeO₃ bulk [31,33]. On the other hand, the lattice constant of the a-axis slightly increased with increasing concentration of BiFeO₃ precursor solution, while that of the c-axis significantly increased at a concentration of 0.20 M. The results suggest that stress in the BiFeO₃ films relieved at concentration of 0.20 M. Crystallite sizes (D) of the BiFeO₃ were also estimated from the XRD patterns using Scherrer’s equation:

D = 0.9λ/βcosθ

(2)

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 [34,35,36]. 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:

(hνα)ⁿ = A(hν − E_g)

(3)

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 BiFeO₃ because BiFeO₃ 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 BiFeO₃ precursor solution, indicating that E_g depended on the concentration of the BiFeO₃ solution. Furthermore, the largest E_g 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:

Δɛ = σ/E

(4)

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 (Δɛ_a and Δɛ_c) are given as follows:

Δɛ_a = (a’ – a)/a

(5)

and

Δɛ_c = (c’ – c)/c

(6)

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 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/BiFeO₃/CH₃NH₃PbI₃/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/CH₃NH₃PbI₃/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/BiFeO₃/CH₃NH₃PbI₃/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 CH₃NH₃PbI₃ 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 BiFeO₃ and CH₃NH₃PbI₃ were estimated to be 2.78 and 1.57 eV, respectively.

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 by 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 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 BiFeO₃/CH₃NH₃PbI₃ photovoltaic device can be calculated by the following equation [41]:

$$V_{oc}=\frac{1}{e}(E_c-E_H)-V_{loss}$$

(7)

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 BiFeO₃/CH₃NH₃PbI₃ 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 CH₃NH₃PbI₃ layer, presumably generated due to partial separation of PbI₂ from the CH₃NH₃PbI₃ [23,24,26,27,28]. In addition, it is considered that grains of the present CH₃NH₃PbI₃ were small. In fact, a correlation between grain size and V_oc of CH₃NH₃PbI₃-based solar cells was recently reported by Kim et al., and they argued that trap states in CH₃NH₃PbI₃ layer decreased accompanied by enlargement of grains [42]. In our opinion, fabricating a completely pinhole-free CH₃NH₃PbI₃ film is significantly important to achieve high conversion efficiencies. In addition, to avoid deterioration of photovoltaic devices due to many external factors, further development of solar cells that are durable for external factors is required for the improvement of photovoltaic performance.

4. 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.