Organic solar cells (OSCs) hold immense potential due to their flexibility, lightweight nature, and translucent properties, making them ideal for applications like portable energy sources, building-integrated photovoltaics, energy-efficient glass, and advanced agricultural systems. Unlike inorganic counterparts such as silicon-based cells, the challenge of managing the heterojunction interface in OSCs is far more complex. Ensuring proper microscopic morphology at these interfaces is vital for optimizing exciton/charge behavior and overall photovoltaic efficiency. However, during the fabrication process, metastable states can emerge within the active layer, leading to spontaneous diffusion and reaggregation of molecules at the interface. Over time, this disrupts phase separation, impairs charge transport, and reduces the lifespan of OSCs. Thus, strategies to suppress metastable formations are critical for enhancing the stability and longevity of these cells.
Under the leadership of Dr. Bao Xichang from the Qingdao Institute of Bioenergy and Process Engineering at the Chinese Academy of Sciences, the Advanced Organic Functional Materials and Devices Group has made significant strides in refining OSC technology through innovative approaches involving molecular peripheral functionalization. Their work has been documented in *Advanced Materials*.
This study focuses on fine-tuning the side-chain functional groups of the acceptor molecules (as depicted in Figure 1). By doing so, the orientation of guest molecules transitions from a mixed face-to-face/edge-to-face configuration to a predominantly face-to-face alignment, significantly boosting vertical electron transport capabilities. Single-crystal analyses and theoretical evaluations reveal that peripheral functionalization enables tighter π-π interactions between adjacent molecules via a conjugated platform, guiding molecular alignment. Additionally, an alloy-like aggregate state forms between the peripheral-functionalized guests and the host, fostering a more ordered molecular arrangement that minimizes defects and material losses in the acceptor phase. Consequently, the power conversion efficiency of doped OSCs reaches 19.12%, while the thermodynamic stability of the system improves markedly. This underscores the importance of controlling guest stacking and host-guest interactions to achieve optimal phase separation, inhibit metastable formations, and enhance cell stability.
Further exploration into reconfiguring the core skeletons of peripheral-functionalized guest molecules yielded high-quality alloy-like aggregate phases. Moreover, the size of these alloy-like crystalline domains can be systematically adjusted by varying the doping ratio of the guest molecules (as illustrated in Figure 2). In this study, guest molecules exhibit exceptional compatibility with the host receptor, ensuring nearly all guest molecules contribute to forming alloy-like phases. This alignment not only allows linear regulation of domain sizes but also enhances the crystallinity of both donor and acceptor polymers, facilitating hole and electron transport in the vertical direction. Furthermore, the alloy-like aggregates enveloped by the host receptor create a core-shell structure, enhancing energy transfer between components. For this distinctive aggregated state, the team proposes a novel driving mechanism based on the interplay of crystallinity, surface energy, and compatibility among receptor components. This unique structure significantly boosts charge transport and curbs charge recombination.
The study leverages this understanding to achieve a fill factor exceeding 80% (up to 81.1%) and an impressive energy conversion efficiency of 19.2%. Beyond efficiency gains, the formation of alloy-like phases is shown to drastically improve OSC stability, validating their role in suppressing metastable formations and prolonging device life.
Based on these findings, the increased regularity of molecular accumulations in alloy-like crystal domains contributes to a higher glass transition temperature (Tg), which in turn reduces the mobility of small molecules at the interface, maintaining active layer phase separation. Studies indicate that constructing oligomeric receptors—like dimers or trimers—from monomeric units can elevate the Tg of the receptor phase and enhance the thermal stability of OSCs. For instance, OSCs utilizing oligomeric receptors have achieved efficiencies up to 18%, with T80 values reaching 35,000 hours (equivalent to over 12 years of stable operation). Despite these advantages, the synthesis and purification of oligomeric receptors remain costly. The research team is currently exploring more economical methods to produce these materials while enhancing the intrinsic stability of anchored OSCs. They are also investigating in situ techniques and solutions tailored for large-area printing processes, aiming for cost-effective and user-friendly approaches to boost OSC stability, including thermal and mechanical resilience in flexible batteries.
This research has received support from the National Natural Science Foundation of China, the Shandong Energy Research Institute, and the Youth Innovation Promotion Association of the Chinese Academy of Sciences.
: Space orientation regulation of molecular orientation and host-guest aggregation states of functionalized molecules.
: Multi-factor driven guest distribution effects on OSC photovoltaic performance and stability.
: Mechanism analysis and future prospects of OSC aging.
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