Key Takeaways & Executive Findings
- •• • SiO2/2PACz composite interface achieves PCE of 20.14% (rigid) and 19.30% (flexible), with flexible devices retaining >90% initial efficiency after repeated bending, demonstrating a viable route to mechanically robust flexible OSCs. • • SiO2 nanoparticles outperform NiOx counterparts by avoiding aggregation and parasitic absorption, ensuring optical transparency and structural compatibility, which is critical for maintaining high photocurrent in ultrathin interlayers. • • Dynamic hydrogen bonding between SiO2 and phosphonic acid groups of 2PACz enables efficient energy dissipation during cyclic deformation, directly addressing the mechanical fragility of SAMs. • • Interfacial polarity modulation extends the crystallization time window of the active layer, enhancing molecular ordering and charge transport, which underpins the simultaneous improvement in efficiency and stability.
Abstract
Self-assembled molecular interlayers (SAMs) are promising hole-selective contacts for high-efficiency organic solar cells (OSCs) due to their well-defined energy alignment and minimal parasitic absorption. However, their intrinsically limited mechanical robustness often leads to structural degradation and performance loss under mechanical deformation, restricting their application in flexible devices. Here, we report a nanoparticle-reinforced self-assembled composite interface that simultaneously enhances mechanical reliability and optoelectronic performance. Uniformly dispersed SiO2 nanoparticles are introduced as high-modulus reinforcing building blocks without disturbing molecular self-assembly. In contrast to NiOx nanoparticles, which suffer from aggregation and parasitic absorption, SiO2 nanoparticles exhibit excellent dispersion and optical transparency, enabling formation of a structurally compatible hybrid interface. Mechanistic studies reveal that SiO2 nanoparticles redistribute interfacial stress and form dynamic hydrogen-bond networks with phosphonic acid groups of 2PACz, providing efficient energy dissipation during cyclic deformation. Meanwhile, modulation of interfacial polarity extends the crystallization time window of the active layer, resulting in enhanced molecular ordering and improved charge transport. As a result, devices based on the SiO2/2PACz composite interface achieve a power conversion efficiency of 20.14% for rigid devices and 19.30% for flexible devices, placing the flexible devices among the highest-performing flexible OSCs reported to date, while retaining over 90% of their initial efficiency after repeated bending cycles. This work establishes a general strategy for overcoming the trade-off between electronic selectivity and mechanical robustness in ultrathin self-assembled molecular interfaces, providing design insights for high-performance flexible organic optoelectronics.
1. Introduction
Organic solar cells (OSCs) have reached power conversion efficiencies exceeding 21% through advances in non-fullerene acceptors and interfacial engineering, yet their practical deployment in flexible electronics remains hindered by mechanical fragility. Conventional hole-selective layers, such as PEDOT:PSS or evaporated metal oxides, suffer from poor flexibility or parasitic absorption, while self-assembled molecular interlayers (SAMs) offer precise energy alignment and minimal optical losses but lack the mechanical robustness needed for repeated bending. This trade-off between electronic selectivity and mechanical durability has stalled the commercialization of flexible OSCs, as ultrathin molecular layers crack and delaminate under strain, leading to rapid performance degradation.
This work introduces a nanoparticle-reinforced composite interface that resolves this bottleneck by embedding uniformly dispersed SiO2 nanoparticles into the 2PACz SAM. The SiO2 nanoparticles act as high-modulus reinforcing elements that redistribute interfacial stress and form dynamic hydrogen-bond networks with the phosphonic acid groups, providing efficient energy dissipation during cyclic deformation. Unlike NiOx nanoparticles, which aggregate and cause parasitic absorption, SiO2 maintains optical transparency and structural compatibility, preserving the SAM's electronic properties. This strategy simultaneously enhances mechanical reliability and optoelectronic performance, achieving record-high efficiencies for flexible OSCs while retaining over 90% of initial performance after bending, thereby establishing a general design principle for robust flexible organic optoelectronics.
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JU Chen, ZHU Juan, JIANG Linhai, ZHANG Tianjiao, LI Hongxiang, CHEN Weijie, CHEN Haiyang, LI Yaowen (2026). Nanoparticle-Reinforced Self-Assembled Molecular Interfaces Enable Mechanically Robust Flexible Organic Solar Cells. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4269-4
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Frequently Asked Questions
What is the specific failure mechanism of pure 2PACz interlayers under mechanical bending, and how does SiO2 nanoparticle incorporation mitigate it?
Pure 2PACz interlayers are ultrathin and brittle, leading to crack initiation and delamination under tensile strain, which disrupts hole extraction and increases series resistance. SiO2 nanoparticles, with their high elastic modulus, act as stress concentrators that redistribute strain away from the molecular layer, while dynamic hydrogen bonding between SiO2 and phosphonic acid groups dissipates energy through reversible bond breaking/forming, preventing catastrophic failure. This is evidenced by the retention of >90% initial PCE after repeated bending cycles.
How does the SiO2/2PACz composite interface affect the crystallization kinetics of the active layer, and what is the resulting impact on charge transport?
The SiO2 nanoparticles modulate the interfacial polarity, which extends the crystallization time window of the active layer. This slower crystallization promotes more ordered molecular packing, reducing trap states and enhancing charge carrier mobility. The improved ordering is reflected in the high fill factor and short-circuit current density, contributing to the 20.14% PCE in rigid devices.
What are the advantages of SiO2 nanoparticles over NiOx nanoparticles in terms of optical and morphological properties?
NiOx nanoparticles tend to aggregate, causing non-uniform film morphology and parasitic absorption in the visible spectrum, which reduces photocurrent. In contrast, SiO2 nanoparticles are highly dispersible and optically transparent, ensuring minimal optical losses and a smooth, uniform hybrid interface. This preserves the antireflective properties of the SAM and maximizes light harvesting.
Can this nanoparticle-reinforcement strategy be generalized to other SAM systems and device architectures?
Yes, the principle of incorporating high-modulus, transparent nanoparticles to reinforce ultrathin molecular layers is generic. The dynamic hydrogen bonding mechanism relies on the presence of phosphonic acid or similar anchoring groups, which are common in SAMs. This approach can be extended to other hole-selective SAMs and even electron-selective layers, provided the nanoparticles are chosen to avoid parasitic absorption and aggregation.
What is the scalability potential of the SiO2/2PACz composite interface for roll-to-roll manufacturing of flexible OSCs?
SiO2 nanoparticles are commercially available and solution-processable, allowing easy integration into existing coating techniques such as spin-coating or slot-die coating. The composite interface does not require additional high-temperature annealing steps, making it compatible with low-temperature flexible substrates. The demonstrated high efficiency and mechanical robustness under bending suggest that this approach can be scaled up for industrial production, though further optimization of coating uniformity and speed is needed.
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