Fabrication of UV-Stable Perovskite Solar Cells with Compact Fe₂O₃ Electron Transport Layer by FeCl₃ Solution and Fe₃O₄ Nanoparticles

Abstract

Even though Fe₂O₃ is reported as the electron-transporting layer (ETL) in perovskite solar cells (PSCs), its fabrication and defects limit its performance. Herein, we report a Fe₂O₃ ETL prepared from FeCl₃ solution with a dopant Fe₃O₄ nanoparticle modification. It is found that the mixed solution can reduce the defects and enhance the performance of Fe₂O₃ ETL, contributing to improved electron transfer and suppressed charge recombination. Consequently, the best efficiency is improved by more than 118% for the optimized device. The stability efficiency of the Fe₂O₃-ETL-based device is nearly 200% higher than that of the TiO₂-ETL-based device after 7 days measurement under a 300 W Xe lamp. This work provides a facile method to fabricate environmentally friendly, high-quality Fe₂O₃ ETL for perovskite photovoltaic devices and provides a guide for defect passivation research.

1. Introduction

Organic–inorganic hybrid lead halide perovskites have attracted extensive attention [1,2]. Since the first reports in 2009, the power conversion efficiency (PCE) of PSCs has been improved to 25.7% within about one decade [3,4,5]. A typical planar PSC is composed of the structure of a cathode layer [6]. The ETL plays a significant role in electron extraction and transport from the perovskite absorber to the FTO [7]. To obtain highly efficient perovskite solar cells, a thin, transparent, and electrically conductive ETL without pinholes is crucial.

Currently, the most commonly used ETL material in PSCs is TiO₂, owing to its high chemical stability, innate transparency, inexpensiveness, and appropriate conduction band (CB) level aligning with the perovskite layer [8]. However, TiO₂-based devices are reported to suffer from hysteresis and high charge recombination, which severely restricts the wide use of the TiO₂ ETL and hinders the development of PSCs [9]. Moreover, the photocatalytic properties of TiO₂ could reduce the illumination stability of PSCs, resulting in poor UV light stability in PSCs [10]. Thus, a great deal of effort has been made to alleviate this problem. Meanwhile, many endeavors have been directed at searching for alternative semiconductor materials for ETLs, such as SnO₂, ZnO, and Nb₂O₅ [11].

As an n-type semiconductor, iron oxide (Fe₂O₃) has attracted increased attention in photovoltaic applications, due to its high chemical stability, low cost, and suitable energy band position [12]. Considering its ultraviolet stability and visible light absorption, Fe₂O₃ is one of the most promising candidates for the ETL in PSCs. However, only several studies have been reported on the application of Fe₂O₃ in PSCs [13,14,15,16]. Wang et al. applied spin-coated Fe₂O₃ as the ETL in PSCs, attaining a PCE of 10.7%, with stability over 30 days upon exposure to ambient air, indicating high stability but a poor efficiency [14]. Guo et al. reported the application of Ni-doped Fe₂O₃ ETL, achieving an efficiency of 14.2%. They also reported the application of γ-Fe₂O₃ ETL fabricated at room temperature. However, it is difficult to fabricate Fe₂O₃ films with good conductivity and crystallinity [15,16].

Herein, we report an Fe₂O₃ ETL fabricated with the water-dispersed Fe₃O₄ nanoparticles and FeCl₃ solution. It is found that the addition of FeCl₃ in Fe₃O₄ nanoparticles precursor reduces the defects and enhances the passivation ability. As a result, the improved electron transfer and suppressed charge recombination contribute to an improvement in the short circuit current density (J_sc) and open-circuit voltages (V_oc), eventually yielding a champion PCE of 12.61%.

2. Experimental Section

2.1. Preparation of Fe₂O₃ ETLs

The ITO substrates were rinsed by ultrasonic vibration with acetone, ethanol, and deionized water for 30 min, and then treated with UV–ozone irradiation for 15 min.

A total of 600 mg of 2.2 mM FeCl₃·6H₂O (Alfa Aesar, 97%) and 300 mg of 1.5 mM FeCl₂·4H₂O (Alfa Aesar, 99%) was dissolved in 5 mL deionized water. Next, 800 mg of polyglucose sorbitol carboxymethylether was dissolved in 10 mL deionized water. Then, both of the solutions were mixed in a three-neck bottle, and stirred vigorously (300 rpm) with nitrogen gas bubbling. Then, the bottle was immediately transferred to a water bath at 60 °C, and 900 μL of 28% ammonium aqueous solution was added (stirring at 800 rpm). The bottle was transferred to a cryogenic bath (containing cold water, ice water, and ethanol). After cooling to −5 °C (decline rate 0.28 °C min⁻¹), Fe₃O₄ nanoparticles solution was eventually obtained after workup by dialysis and filtration.

The Fe₂O₃ films fabricated with Fe₃O₄ nanoparticles were deposited on the substrates by spin-coating water-dispersed ten-nm-sized Fe₃O₄ nanoparticles with a concentration of 0.075 M at 5000 rpm for 30 s. The as-prepared layers were then annealed at 550 °C for 120 min in air. For the Fe₂O₃ films fabricated with FeCl₃ solution, the precursor solution was prepared by dissolving FeCl₃·6H₂O (Alfa Aesar, 97%) in deionized water with a concentration of 0.075 M. The Fe₂O₃ films were deposited by spin-coating the prepared precursor solution at 4000 rpm for 30 s and sintered at 550 °C for 120 min in air. For the Fe₂O₃ films fabricated with FeCl₃/Fe₃O₄ mixed solution, the mixed solution was prepared by dissolving FeCl₃·6H₂O in the as-prepared Fe₃O₄ solution with a concentration of 0.075 M. The Fe₂O₃ films were fabricated by spin-coating the mixed solution at 5000 rpm for 30 s, and then annealed at 550 °C for 120 min in air.

2.2. Fabrication of Perovskite Solar Cells

Perovskite solar cells were fabricated by a modified two-step method. Firstly, a PbI₂ solution with 600 mg mL⁻¹ in DMF was dropped on the ETL substrate with 3000 rpm for 30 s. A total of 50 µL of mixed solution (60 mg mL⁻¹ FAI, 6 mg mL⁻¹, MABr, and 6 mg mL⁻¹ MACl in isopropanol) was then rapidly dripped on the rotating substrate 10 s after the spin procedure started. The as-prepared film was heated at 150 °C for 10 min in air in order to obtain a dense perovskite film. After cooling to room temperature, the HTL solution (spiro-OMeTAD, 25 µL) was deposited by spin-coating at 2000 rpm for 30 s. The HTL solution consisted of 72.3 mg spiro-OMeTAD, 28.8 µL 4-tert-butylpyridine (TBP), and 17.5 µL of 520 mg mL⁻¹ lithium bis(trifluoromethylsulphonyl)imide (LiTFSI) in acetonitrile dissolved in 1 mL of chlorobenzene. Then, devices were oxidized in air for 36 h.

2.3. Characterization and Measurement

The surface morphology and cross-section of the samples were observed by a field-emission scanning electron microscope (FE-SEM, Hitachi, SU8010, Japan). The XRD results were measured with an X-ray diffractometer (XRD, Bruker, D8 Advance, Germany). The samples were also investigated by X-ray photoelectron spectroscopy (Thermo, Escalab 250Xi, USA). The photoluminescence (PL) and time-resolved photoluminescence (TRPL) were detected with a 530 nm laser (Edinburgh Instruments, LP320, UK). The absorption spectra were recorded on a UV–vis spectrophotometer (Shimadzu, UV-2600, Japan). The contact angle measurement was measured by DSA25E (KRÜSS, Germany). The current–voltage characteristics of the solar cells were tested with a Newport solar simulator and a Keithley 2400 Source Meter under AM 1.5G irradiation (100 mW cm⁻²). The electrochemical impedance spectroscopy (EIS) was measured with an electrochemical workstation (Autolab, PGSTAT 302 N, Switzerland) under AM 1.5G light condition with an alternative signal amplitude of 10 mV and in the frequency range of 0.1 Hz-40 kHz in glove box.

3. Results and Discussion

Figure 1a exhibits the schematic of different Fe₂O₃ films prepared by FeCl₃ solution, Fe₃O₄ nanoparticles, and FeCl₃/Fe₃O₄ mixed solution. Fe₂O₃ films prepared by FeCl₃ solution exhibit good compactness, but a large number of cracks and pin-holes after the annealing process. Fe₂O₃ films fabricated by water-dispersed Fe₃O₄ nanoparticles show better morphology. However, some aggregation is still found, owing to the gathered nanoparticles in the crystallization process, which could influence the nucleation process of perovskite film and suppress the charge transport at the Fe₂O₃/perovskite interface.

In order to further improve the planarity and compactness of Fe₂O₃ films, FeCl₃ solution was incorporated into the Fe₃O₄ nanoparticle precursor solution, which could simultaneously retain the advantages of the two methods and reduce the defects, thereby facilitating an efficient ETL. Figure S1 shows the top-view scanning electron microscopy (SEM) image of blank and clean ITO substrate, as previously reported. As shown in Figure 1b, the Fe₂O₃ film prepared by 0.075 M FeCl₃ solution shows a morphology with cracks and pin-holes, which could lead to direct contact between the perovskite absorber and ITO, resulting in aggravated charge recombination. Figure 1c shows the morphology of the Fe₂O₃ film fabricated by spin-coating water-dispersed ten-nm-sized Fe₃O₄ nanoparticles with a concentration of 6 mg mL⁻¹ (measured by Fe), which demonstrates a flat and compact surface except for a few gathered spots. Figure 1d depicts the morphology of the Fe₂O₃ film prepared by FeCl₃/Fe₃O₄ mixed solution. It can be observed that the as-prepared Fe₂O₃ film exhibits a pin-hole-free coverage, as a result of the cooperation between the nanoparticles and FeCl₃ solution in the annealing process.

Figure 2a shows the X-ray diffraction (XRD) pattern of the Fe₂O₃ films prepared by different methods. XRD analysis confirms that both the samples prepared by Fe₃O₄ nanoparticles and FeCl₃/Fe₃O₄ mixed solution display the same diffraction peaks, which match the standard α-Fe₂O₃ perfectly (JCPDS, No. 80-2377) [17]. XRD peaks at 22.5 and 24 degree may be the peaks of iron chlorate formed by the incompletely volatilized Cl in the crystallization process and the reduced iron. While the sample prepared by FeCl₃ solution displays an extra peak at low angle. The XRD results indicate that Fe₃O₄ is converted into Fe₂O₃ and that the FeCl₃/Fe₃O₄ mixed sample has better purity. X-ray photoelectron spectroscopy (XPS) measurements were carried out to elucidate the chemical composition of Fe₂O₃ films prepared by different methods.

Figure 2b illustrates the Fe 2p_3/2 peak of the as-prepared Fe₂O₃ films. The fitted curves are shown in Figure S2. The peaks center around 716 eV and 719 eV, corresponding to the binding energy of Fe²⁺ ions and Fe³⁺ ions, respectively [18]. The curves of the samples prepared by Fe₃O₄ nanoparticles and FeCl₃/Fe₃O₄ mixed solution show no obvious peak of Fe²⁺ ions, indicating that the Fe elements are converted into Fe₂O₃.

As shown in Figure 2c, a little peak of O-H can be observed in samples prepared by mixed solution, proving that there are intermediate products during the annealing process. To ascertain the influence of the different preparation methods on the surface energy, a contact angle test was carried out on the as-prepared Fe₂O₃ substrates. The contact angles are 13°, 17.4°, and 16° for Fe₂O₃ films prepared by FeCl₃ solution, Fe₃O₄ nanoparticles, and FeCl₃/Fe₃O₄ mixed solution, respectively. For the FeCl₃ prepared sample, the smallest contact angle could arise from its terrible morphology with large-area cracks and pin-holes, which could trap the perovskite precursor solution. It should be noted that the FeCl₃/Fe₃O₄ mixed sample has a smaller contact angle than that of Fe₃O₄ nanoparticles. Attributing this to the addition of FeCl₃ solution, the defects and aggregation of the Fe₃O₄ prepared films are passivated, leading to a compact and flat coverage of the Fe₂O₃ film prepared by FeCl₃/Fe₃O₄ mixed solution. The reduced defects and passivated surface of the Fe₂O₃ films make a great contribution to a smaller contact angle, which is conducive to the diffusion of perovskite precursor solution on the surface, thus, accelerating the nucleation process of perovskite films [19]. Figure S3 illustrates the UV–vis absorption spectra of Fe₂O₃ films prepared by different methods. The Fe₂O₃ film prepared by FeCl₃/Fe₃O₄ mixed solution shows a slightly higher absorption in almost the whole wavelength region, which could prevent the perovskite from degrading under UV irradiation and enhance the UV-stable ability.

Figure 3a shows the top-view SEM image of the perovskite layer deposited on the FeCl₃/Fe₃O₄ mixed sample, which exhibits compact surface and large grain size. Figure 3b shows the cross-sectional SEM image of the entire structure, from which we can see the perovskite layer is also compact and the thickness is about 500 nm. Figure S4 shows the XRD patterns of perovskite coated on as-prepared substrates, and all the peaks of the perovskite are presented with an asterisk. All of them display the same characteristic peaks of perovskite materials, which indicates excellent perovskite crystallinity [20]. Figure 3c presents the best current density–voltage (J-V) curves of the devices based on Fe₂O₃ films prepared by different methods. All samples were measured under AM 1.5G (from 1.2 V to 0 V, scan step of 0.04 V, and scan rate of 100 mV s⁻¹). The devices based on Fe₂O₃ films are also compared with the TiO₂-based device, as shown in Figure S5. The detailed photovoltaic parameters of the PSCs with the best PCE values including open-circuit voltage (Voc), short-circuit current density (J_SC), filling factor (FF), and PCE are summarized in Table S1. The device prepared with FeCl₃ displays the lowest PCE of 7.72% and the device based on Fe₃O₄ nanoparticles provides a PCE of 10.64%. Expectedly, the optimal device prepared by mixed solution exhibits overall superior performance, including a Voc of 0.98 V, Jsc of 23.45 mA cm⁻², and FF of 54.74%, resulting in a PCE of 12.61%. Compared with the device based on single Fe₃O₄ nanoparticles, Voc and Jsc are improved, which may be due to the reduced defects and passivated recombination with the addition of FeCl₃. The forward and reverse scanning tests were also carried out to investigate the hysteresis effect by (PCE_reverse − PCE_forward)/PCE_reverse. As shown in Figure S6, the mixed sample shows a minimum hysteresis of 0.09. As a contrast, the FeCl₃ prepared sample shows a hysteresis index of 0.15, and that of the Fe₃O₄ prepared sample is 0.10. It is indicated that PSC based on the mixed sample shows a better charge-transfer ability. Further characterizations were performed to evaluate the trap state density of the devices. We prepared electron-only devices with structures of ITO/ETL/perovskite/PCBM/Ag to quantitatively assess the trap state density in ETL, as shown in Figure S7. Compared with the Fe₃O₄-based device, the V_TFL of the mixed sample is reduced to 0.12 V. It is indicated that that addition of FeCl₃ can obtain high-quality Fe₂O₃ film with compact and flat coverage, contributing to passivating the surface defect and effectively filling the electron trap density, which can greatly improve the electrical properties and accelerate electron extraction and injection at the ETL/perovskite interface.

To investigate charge transport and recombination in perovskite solar cells, electrochemical impedance spectroscopy (EIS) was conducted. Figure 3d shows the Nyquist plots of the devices based on Fe₃O₃ ETLs prepared by different methods under AM 1.5 G illumination, and the fitted parameters are summarized in Table S2. The semicircle at high frequency is related to the transfer resistance (R_ct) at the interface and the semicircle at low frequency corresponds to recombination impedance (R_rec) of the device [21]. The device based on FeCl₃/Fe₃O₄ film exhibits a R_ct of 178 Ω and R_rec of 1023 Ω. The reduced R_ct is conducive to the enhanced the carriers transfer at the interface, and the increased R_rec is beneficial to the suppressed charge recombination. To further investigate the leakage capacity of Fe₃O₃ ETLs prepared by different methods, a leakage current test is carried out, as shown in Figure S8. The FeCl₃/Fe₃O₄-based sample shows the lowest leakage value, indicating a better leakage performance.

Photoluminescence (PL) was carried out to explore the carrier transport dynamics at the Fe₂O₃/perovskite interface, as shown in Figure 3e. All the samples display a typical emission peak at 788 nm, in agreement with the absorbance edge of the perovskite. The FeCl₃ prepared sample presents the lowest PL intensity. This could mainly be correlated to poor coverage of the prepared film, which could cause the direct contact between perovskite and ITO, resulting in an illusion of great electron transfer and extraction. A higher PL intensity is presented in the sample with Fe₃O₄-prepared films. It should be ascribed to the imperfect surface and interface. The FeCl₃/Fe₃O_4-prepared film demonstrates a PL quenching, indicating that the addition of FeCl₃ can passivate the surface defect and accelerate electron extraction and injection at the ETL/perovskite interface. To further demonstrate the charge transfer and extraction, the time-resolved photoluminescence (TRPL) was performed. Figure 3f shows the TRPL spectra and the fitting curves with a bi-exponential decay function [22]. It is clear that the average recombination lifetime (τ_ave) is prolonged from 7.76 ns to 20.28, and 12.29 ns for samples with FeCl₃, Fe₃O_4, and FeCl₃/Fe₃O_4-prepared films, respectively. Compared to the Fe₃O_4- prepared sample, the decreased carrier lifetime of the FeCl₃/Fe₃O_4-prepared sample indicates that the addition of FeCl₃ passivates defects of the Fe₃O₄ prepared films and greatly accelerates the charge separation and transport, leading to suppressed charge recombination.

The transmittance spectra of Fe₂O₃ films prepared by different methods are shown in Figure S9. The FeCl₃/Fe₃O₄-prepared film shows a high transmission, but it is still slightly lower than that of the TiO₂ film. Figure 4a shows the long-time stability test of controlled TiO₂ and mixed Fe₂O_3-ETL-based perovskite solar cell, which were tested under a 300 W Xe lamp with the condition of humidity of less than 20% and temperature of 25 ℃. In order to obtain more accurate stability test results, we used Au as the top electrode instead of the original Ag. The efficiency of the device prepared by controlled TiO₂ ETL decreases more than 70% after 7 days of continuous irradiation. As the most commonly used ETL material in PSCs, TiO₂ is reported as a serious issue that affects the stability of the PSCs. As a product of the TiO₂ photocatalytic effect, UV illumination can excite TiO₂ to generate strong oxidizing holes, which could cause the decomposition of perovskite into CH₃NH₂, HI, and PbI₂, and eventually result in the degradation of the stability [23,24,25]. The device prepared with mixed Fe₂O₃ ETL still has 70% efficiency, indicating a better stability performance. We speculate that it is due to the UV stability and lesser photocatalytic ability of Fe₂O₃, which slows the perovskite from degradation and, thus, enhances the UV-stable ability of PSCs. We also tested the TiO₂ and mixed-Fe₂O₃-based devices at the maximum power point (MPP) to investigate the stability under UV illumination (composed of 313 nm, 340 nm, and 351 nm) without encapsulation, as shown in Figure S10. Under the same conditions for 300 min, the mixed-Fe₂O₃-based device retains 86% of its initial current density, while the current density of the TiO₂-based device only retains 52%, indicating no UV reaction of Fe₂O₃ and perovskite, which makes a great contribution to the UV-stable devices. To further confirm our point of view, the XPS measurements were carried out to elucidate the valance change of Pb in the perovskite of controlled TiO₂ and mixed-Fe₂O_3-ETL-based perovskite solar cells, which were tested for long-time stability for 7 days. Figure 4b shows the peak of the XPS spectra centered at 141.4 eV, corresponding to Pb⁰ 4f_5/2 of controlled TiO_2-based sample, which is in good agreement with the literature values of 141.7 eV [26]. The peak is higher than that of the Fe₂O₃-based sample, confirming the presence of unsaturated Pb, which results from the degradation of perovskite and could be detrimental to the instability of the sample. For the Fe₂O₃-based sample, the peak of Pb⁰ 4f_5/2 is successfully suppressed, indicating improved stability of the perovskite. We think that the improvement of the stability should be ascribed to no UV reaction of Fe₂O₃, which protects perovskite from degradation under continuous irradiation.

4. Conclusions

In summary, we present a facile modification with FeCl₃ solution to optimize the Fe₂O₃ ETL prepared by water-dispersed Fe₃O₄ nanoparticles. The device efficiency is improved by more than 118% for the optimized device. The stability efficiency of the Fe₂O₃-ETL-based device is nearly 200% higher than that of the TiO₂-ETL-based device after 7 days measurement. The improved performance of the as-prepared solar cells is attributed to the reduced defects at the interface, enhanced passivation ability, excellent perovskite crystallization originating from the addition of the FeCl_3, and the UV-stable ability of the Fe₂O₃-based devices. This work is dedicated to broadening the scope of perovskite photovoltaic devices and provides a way for defect passivation in commercial applications.