First Author: Yang Yang, Chen Ruihao
Corresponding Author: Wang Hongqiang, Chen Ruihao
Affiliation: Northwestern Polytechnical University
Research Highlights:
1. In situ generated fluoride 1D “patch layer” at grain boundaries to enhance the moisture resistance of polycrystalline films and passivate defects;
2. F···H hydrogen bonds accelerate charge extraction and transfer, and stabilize ions to improve the durability of polycrystalline films;
3. Through the blade coating method, perovskite modules achieved an efficiency of 20.56% over a total area of 36 cm2, with excellent photostability.
1. Key to Perovskite Module Stability
Perovskite solar cells (PSCs) have achieved high power conversion efficiencies (PCE) comparable to commercially mature monocrystalline silicon solar cells due to their excellent optoelectronic properties and scalable solution processing. However, PSCs and modules currently face thermal decomposition issues caused by FA+ or MA+ ion salts in the perovskite film. Therefore, how to enhance long-term stability and control interfacial/surface charge recombination to enable PSCs to possess both high efficiency and high stability is an urgent issue to be addressed.
As the industrialization of organic-inorganic hybrid perovskite photovoltaic devices needs to be further advanced, considerations of efficiency and stability in module fabrication must be addressed. Professor Wang Hongqiang and Professor Chen Ruihao’s team at Northwestern Polytechnical University developed a one-dimensional lead iodide structure for polycrystalline perovskite films through a surface conversion repair strategy based on 1,3-diisopropyl-4,5-difluorobenzimidazolium iodide ions. Fluorine atoms can interact with another benzimidazoline to generate F···H hydrogen bonds that accelerate charge extraction and transfer in the intermolecular region. Moreover, fluorine combines with FA+ and stabilizes FA+ to improve the durability of polycrystalline films. Due to the stable chemical properties and specific electronic structure of the 1D structure, laboratory-scale devices achieved a high efficiency of 24.05% in the FA-based perovskite system. Unencapsulated cells under N2 atmosphere maintained 94.6% of the initial efficiency under AM 1.5G illumination after 500 hours.
3. Results and Discussion
Key Point 1: Introduction of Multifunctional F-ipr-I Salt
In this study, the multifunctional salt 1,3-diisopropyl-4,5-difluorobenzimidazolium iodide (abbreviated as F-ipr-I) was designed and introduced for passivating FA0.97Cs0.03PbI3 perovskite, generating a new type of 1D structure patch to repair grain boundary defects and prevent moisture/oxygen and ion migration (Figure 1a). The optimized F-ipr-I treated samples were referred to as “Target” in subsequent characterizations. After treatment with F-ipr-I, the morphology of the polycrystalline film changed significantly, especially enriched at grain boundaries, suggesting the formation of a new structure on the perovskite surface (Figures 1b, c). Meanwhile, a new low-angle diffraction peak appeared in the XRD pattern of the target film, indicating the generation of a new structure. By analyzing the single crystal of the new structure, it was found to have a one-dimensional (1D) structure, and the characteristic peaks in the simulated 1D structure XRD pattern matched those in the experimental XRD pattern (Figure 1d). As shown in Figure 1e, the average electrostatic surface potential (ESP) value of F-ipr+ is about 0.25 eV, exhibiting a more uniform surface potential distribution conducive to charge transfer. The F-iprPbI3 structure consists of 1D chains [PbI6]4- and surrounding F-ipr+ cations, which are arranged parallel along the 1D chains (Figures 1f, g). The intermolecular π-π close packing aids effective hole transport at the interface. Based on the 1D structure, a hydrogen bond network between the F of F-ipr+ and the H of adjacent F-ipr+ ions was discovered, which also facilitates charge transport within the grain boundaries of the perovskite film. The experimental results above demonstrate that the F-iprPbI3 patch will enhance device performance.
Figure 1 Study of the 1D structure patch on the perovskite surface
X-ray photoelectron spectroscopy (XPS) measurements were used to detect the chemical states in the target film. As shown in Figure 2a, in the target film, the N 1s peak shifts positively, consistent with the N-C-N structure of F-ipr+ ions. Notably, no F 1s signal was detected in the control film, while the F 1s peak appeared in the target film, confirming the presence of F-ipr+ ions in the target film (Figure 2b). Compared to the control film, the binding energies of I 3d and Pb 4f peaks in the target film were reduced, which was due to the longer Pb-I bond in the 1D structure relative to the original perovskite structure (Figures 2c, d). Interestingly, the intensity of the Cs 3d peak in the target film was significantly reduced compared to the control film, confirming that the 1D structure patch is located at the surface of the perovskite film (Figure S4). The binding energy changes in the C 1s spectrum further confirmed the formation of the 1D structure patch on the perovskite surface, which was evidenced by the elemental mapping from EDS analysis.
The in situ XPS spectroscopy was used to simulate and study how the 1D patch was formed on the surface of the perovskite film: In a vacuum chamber, a solution of F-ipr-I salt (1.0 mg mL-1) was pre-deposited on the perovskite film, followed by the annealing process at 80 oC. As shown in Figures 2e, f, the signals of N 1s and Pb 4f peaks in the perovskite film treated with F-ipr-I were collected at different annealing times (80 oC). At 0 min, N 1s represented the original F-ipr-I salt, while the Pb 4f peak signal almost disappeared due to being covered by the F-ipr-I salt. As the annealing time increased, N 1s began to shift towards lower binding energy until 8 min, indicating the emergence of the 1D patch structure. After 16 min, the N 1s peak showed little movement, consistent with (Figure 2c). Until 20 minutes, the characteristic peak intensity of the 1D structure remained unchanged, consistent with the Pb 4f signal (Figure 2f). The results above clearly indicate that the constructed 1D patch structure possesses good stability and is beneficial for the durability of PSCs.
Figure 2 Characterization of the 1D structure on the perovskite surface
Key Point 2: Performance of Small AreaCells and Modules
As shown in Figure 3a, in reverse scan (RS) mode, the target device achieved a high PCE of up to 24.05%, with a fill factor (FF) of up to 81.58%, a Jsc of 25.41 mA cm−2, and a Voc of 1.16 V, significantly surpassing the overall PCE of the champion control PSC (RS 20.70%), with a Jsc of 25.21 mA cm−2, a Voc of 1.08 V, and an FF of 76.03% under AM 1.5 G illumination. For forward scan (FS) mode, the PCE of the target device was 23.58%, higher than that of the control device (20.21%). Compared to the control module (RS 19.59% and FS 19.16%), the target module exhibited RS 22.82% and FS 22.75% with less hysteresis.
Figure 3 Performance Graph of Devices
Due to the advantages of the 1D patch structure, the operational stability of PSCs with and without F-ipr-I was tested under N2 conditions. The target device showed significant improvement, maintaining 94.2% of the original efficiency after 500 hours of aging under AM 1.5G illumination, while the initial efficiency of the control PSC rapidly dropped to 68.3% (Figure 3b). Furthermore, the presence of F-iprPbI3 enhanced the moisture resistance of the perovskite film and PSC, as verified by the increased water contact angle. Meanwhile, the target device exhibited long-term storage stability, maintaining 93.3% of the initial PCE over 3000 hours at 30% relative humidity and room temperature, while the control PSC rapidly declined to 67.5% of its initial efficiency within only 300 hours.
Key Point 3: Improvement of Perovskite Films Based on 1D Structure
To further understand the improvement mechanism of the 1D patch, scanning Kelvin probe microscopy (SKPM) was used to investigate the changes in surface contact potential (SP) of perovskite films without and after treatment with different concentrations of F-ipr-I (denoted as F-ipr-X; X represents different concentrations (0.5, 1.0, and 1.5 mg mL-1)). The average SP value of the original film was 719 mV, while after increasing the concentration of F-ipr-I treatment, the SP value increased from 740 mV (F-ipr-0.5) to 780 mV (F-ipr-1.0), then dropped significantly to 631 mV, demonstrating that the Fermi level of the perovskite film increased within the 1.0 mg range, but excessive F-ipr-I due to its poor conductivity lowered the Fermi level (Figures 4a, b). The increase in SP values near the perovskite surface is attributed to the passivation of electronic traps at the grain boundary (GB), further promoting effective hole transport in PSCs. After the repair with the 1D patch, this local difference at the GB was reduced, and it was nearly flat across the entire GB, indicating that the band bending at the GB was reduced due to the passivation of defects at the GB. Additionally, ultraviolet photoelectron spectroscopy measurements performed on the target and control films (Figure 4c) confirmed the repair of the 1D patch, with the work function changing from 4.72 eV to 4.39 eV, approaching the conduction band to expand the built-in electric field and generate high Voc.
Figure 4 Defect Healing Mechanism of 1D Structure
The valence band maximum (VBM) energy levels (EVBM) of the control and target films were -5.58 and -5.36 eV, respectively. The difference in EVBM (0.22 eV) values is consistent with the results of the XPS valence band spectra, contributing to the hole transport of 1D/3D devices that match the energy levels. The results of Jph–Veff tests and significantly reduced leakage current in intact devices indicate that the 1D patch will significantly suppress recombination in the perovskite film, aiding effective charge extraction (Figure 4d). Infrared spectroscopy (IR) was used to investigate the interactions between the 1D patch and FA+ ions (Figure 4e). To simplify this complex system, the authors directly mixed FAI and F-ipr-I salts to detect the interaction between the F atoms of F-ipr-I and the H atoms of FAI. Compared to pure FAI powder, the spectrum of the mixture of F-ipr-I and FAI powder showed a significant shift of the N–H vibration mode (3260 cm-1) to a lower wavenumber (3116 cm-1), forming hydrogen bonds N–H···F.
The strong hydrogen bond network effectively solidifies the grain boundaries and suppresses the decomposition of FA+ ions, enhancing overall stability. Furthermore, as characterized by TRPL (Figure 4f), the average decay time of 572 ns in the target film is longer than 320 ns in the control film, further proving that the 1D patch effectively reduces recombination losses. Mott-Schottky measurements were also used to evaluate the built-in potential (Vbi) in both devices, extracting 0.74 V from the control device to 0.87 V in the target device, which helps increase Voc and PCE (Figure 4g). The higher slope of the target device indicates that rapid charge transfer occurs at the perovskite/spiro-OMeTAD interface. These results demonstrate that the 1D patch effectively regulates energy levels and modulates charge extraction and transfer at the top interface of PSCs.
By using surface-converted 1D lead halide structure patches to passivate defects, the grain boundaries of perovskite films are repaired. The hydrogen bonds of fluorine in multifunctional designed molecules accelerate charge extraction and transfer, and stabilize FA ions to inhibit FA decomposition and migration. In small areas, laboratory-scale PSCs achieved an efficiency of 24.02% with high photostability. Additionally, large-area 1D patch repaired perovskite modules were fabricated through the blade coating process, achieving efficiencies of 20.56% (36 cm2) and 17.71% (100 cm2), with long-term operational stability. This novel surface repair strategy for scalable perovskite films and modules is expected to promote the industrialization of ultra-stable perovskite solar modules.
Ruihao Chen, et al. Patch-healed grain boundary strategy to stabilize perovskite films for high-performance solar modules, Nano Energy, 2023
https://doi.org/10.1016/j.nanoen.2023.108759
https://www.sciencedirect.com/science/article/abs/pii/S2211285523005967
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