Perovskite solar cells (PSCs) are promising for next-generation photovoltaics due to their high power conversion efficiency (PCE), lightweight nature, flexibility, and low cost. However, their commercialization is hindered by issues with long-term operational stability and mechanical endurance, primarily stemming from defects accumulating at layer interfaces. The interface between the perovskite and the electron-transporting layer (ETL) is particularly critical, with high defect concentrations and film fracturing leading to efficiency and stability losses. While strategies like adding interfacial layers, creating interpenetrating interfaces, using scaffolding, and introducing additives have been explored to mitigate these issues, introducing self-assembled monolayers (SAMs) offers a simpler approach. However, defects in these SAMs due to substrate roughness and film deposition can negatively impact flexible PSCs. Silane coupling agents, forming cross-linked SAMs, have shown promise in improving stability and mechanical tolerance, but a trade-off between passivation quality and series resistance remains. This research focuses on addressing this trade-off by utilizing liquid crystal elastomers (LCEs) to create a toughened charge transfer channel at the interface, enhancing both performance and reliability of flexible PSCs.

The literature extensively discusses the challenges of interfacial defects in perovskite solar cells and strategies to address them. Studies have shown that interfacial defects, particularly deep-level defects, lead to hysteresis and degradation. Various methods, including the addition of interfacial layers, the creation of interpenetrating interfaces, the use of scaffolding techniques, and the introduction of additives, have been investigated to reduce these defects. Self-assembled monolayers (SAMs) have emerged as a simple and effective method for minimizing charge-transport losses and suppressing charge recombination. However, challenges remain, especially regarding defects stemming from substrate roughness and film deposition processes in flexible devices. The use of silane coupling agents to form cross-linked SAMs has shown promise in improving operational stability and maintaining mechanical tolerance, but trade-offs between passivation quality and series resistance persist. Liquid crystalline elastomers (LCEs) have demonstrated utility in various applications due to their unique combination of anisotropy and elasticity. This research leverages the properties of LCEs to create a novel interface within PSCs.

The researchers synthesized a thiol-terminated liquid crystal elastomer (LCE) interlayer via a photopolymerization process involving liquid crystalline diacrylate monomers and dithiol-terminated oligomers. The synthesis involved a pre-polymerization reaction between a diacrylate monomer (RM257) and a dithiol (1,3-propanedithiol), followed by an oxygen-mediated thiol-acrylate click reaction upon UV exposure. The resulting LCE interlayer was characterized using techniques such as Fourier-transform infrared spectroscopy (FTIR), polarized optical microscopy (POM), and grazing incidence wide-angle X-ray scattering (GIWAXS) to confirm its structure and alignment. The influence of the LCE interlayer on perovskite crystal growth was evaluated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Contact angle measurements were conducted to understand the effect of the LCE on the SnO2/perovskite interface. Photoluminescence (PL) and time-resolved PL spectroscopy were used to investigate charge transfer and recombination dynamics. Kelvin probe force microscopy (KPFM) was employed to analyze surface potentials and assess the uniformity of the charge transfer channels. The electrical conductivity was investigated through I-V measurements of devices with and without the LCE interlayer. Space charge limited current (SCLC) analysis and thermal admittance spectroscopy were used to determine trap density. X-ray photoelectron spectroscopy (XPS) was employed to examine the chemical state of the interface. Devices were fabricated using a typical planar n-i-p architecture, incorporating the LCE interlayer between the SnO2 electron transport layer and the perovskite absorber. The performance of both rigid and flexible devices was evaluated by measuring their current-voltage (J-V) characteristics, external quantum efficiency (EQE), and operational stability under different conditions (continuous illumination, bending cycles). The flexible devices were integrated into a wearable haptic device to demonstrate their practical application.

The aligned LCE interlayer significantly improved the performance and stability of both rigid and flexible perovskite solar cells. Rigid devices achieved a maximum power conversion efficiency (PCE) of 23.26%, with a Voc of 1.17 V and a fill factor (FF) of 0.803. Flexible devices reached a PCE of 22.10%, maintaining 86% of their initial efficiency after 5000 bending cycles. The LCE interlayer effectively suppressed non-radiative interfacial recombination, facilitated efficient charge carrier transfer, and reduced trap density. The unencapsulated LCE-based devices exhibited exceptional operational stability, maintaining >80% of their initial efficiency for over 1570 h under flowing N2. The LCE interlayer also enhanced the crystallinity and uniformity of the perovskite film, suppressing phase segregation and improving the long-term stability. The improved interfacial contact provided by the LCE led to a reduction in hysteresis and enhanced reproducibility. The devices showed a smaller slope of 1.50 kT/q (trap-assisted recombination suppressed) compared to the reference device (1.92 kT/q). The LCE-based flexible devices exhibited a minimal efficiency loss (5%) compared to their rigid counterparts. Finally, the integration of the LCE-based flexible PSCs into a wearable haptic device demonstrated their suitability for practical applications, showcasing reliable power supply for a pain sensation system in virtual reality, with stable photovoltage even under mechanical bending.

The results demonstrate the effectiveness of the aligned LCE interlayer in addressing the key challenges associated with the stability and performance of flexible perovskite solar cells. The enhanced charge transfer, reduced recombination, and improved interfacial contact significantly improved the PCE and long-term stability of the devices. The suppression of phase segregation, a major cause of degradation in perovskite solar cells, further highlights the benefits of the LCE interlayer. The superior mechanical flexibility and stability of the LCE-based devices make them ideal candidates for wearable applications. The integration of the solar cells into the wearable haptic device showcases the potential of this technology for powering flexible electronics and sensors. The findings contribute significantly to the advancement of flexible and reliable perovskite solar cell technology, paving the way for their wider adoption in various applications.

This research successfully demonstrated a novel approach to enhance the performance and reliability of flexible perovskite solar cells using an aligned liquid crystal elastomer (LCE) interlayer. The LCE interlayer effectively improved charge transfer, minimized charge recombination, and suppressed phase segregation, leading to high PCEs and exceptional stability in both rigid and flexible devices. The successful integration into a wearable haptic device highlights the practical implications of this work. Future research could explore different LCE materials and alignment techniques to further optimize device performance and investigate large-scale fabrication methods for commercial viability.

While this study demonstrates significant improvements in perovskite solar cell performance and stability, some limitations exist. The long-term stability tests were conducted under specific controlled conditions (flowing N2), and the performance under real-world conditions might differ. The scalability and cost-effectiveness of the LCE fabrication process should be further evaluated for mass production. Further research could explore the impact of different LCE compositions and processing parameters on the device performance and stability. The study focuses on a specific perovskite composition (CsFAMAPbI3), and investigations with other perovskite materials would be valuable to assess the general applicability of the LCE interlayer approach.