Fine-Tuned Molecular Design Pushes Organic Solar Cells Toward 19.67% Efficiency and Lasting Stability

Organic photovoltaic cells have long promised a future of lightweight, flexible, and low-cost solar energy, yet one stubborn obstacle has kept them from widespread commercialization: the trade-off between power conversion efficiency and long-term operational stability. A research team based at the Department of Chemistry of Southern University of Science and Technology (SUSTech) in Shenzhen now reports a molecular design strategy that appears to dissolve much of that trade-off, delivering devices that combine a power conversion efficiency of 19.67 percent with remarkably low energy loss and outstanding durability under both continuous illumination and elevated temperature. The study, published as an open access Research Article in CCS Chemistry, the flagship journal of the Chinese Chemical Society, demonstrates that subtle, deliberate adjustments to molecular fine structure can simultaneously optimize the crystallinity and miscibility of the active materials that determine how well an organic solar cell converts and retains energy.

The central challenge addressed by the team concerns non-fullerene acceptors, or NFAs, the electron-accepting molecules that have driven the recent surge in organic photovoltaic performance. In a bulk heterojunction solar cell, a donor polymer and an acceptor molecule are blended together to form an interpenetrating network through which photogenerated charges must travel. For efficient charge generation and transport, the acceptor molecules need to pack into ordered, crystalline domains with close π-π stacking, the parallel overlap of their conjugated π-electron systems that facilitates electron delocalization. Yet excessive crystallinity can cause the acceptor to separate from the donor polymer, degrading the miscibility of the blend and producing a coarse, unstable morphology. Historically, molecular designs that pushed crystallinity upward tended to sacrifice blend compatibility, and designs that preserved compatibility tended to leave behind disordered packing and large nonradiative energy losses, the fraction of absorbed photon energy dissipated as heat rather than converted into charge.

The SUSTech group, led by corresponding author Professor Feng He, approached this dilemma through what the researchers describe as fine molecular engineering. Rather than redesigning the acceptor scaffold wholesale, the team made selective, targeted modifications to a parent molecule designated 7C-2F, which is built around a 1,4-diazahexane core. From this starting point, three new non-fullerene acceptors were synthesized by independently controlling two structural variables: the length of the side chains attached to the molecular framework and the degree of π-conjugation in the terminal groups, the electron-deficient end units that largely govern the optical and electronic properties of the acceptor. This modular approach allowed the researchers to isolate the effect of each structural change on molecular packing, blend morphology, and device behavior.

The first structural variable, side chain shortening, produced two acceptors named 7C-BO-2F and 7C-eC9-2F. The second variable, extension of the π-conjugation of the terminal groups, produced a third acceptor named 7C-N2F. Although these two strategies act on different parts of the molecule, the study found that both converge on the same set of physical outcomes. Each modification improved molecular crystallinity, meaning the acceptor molecules adopted more ordered solid-state packing, and each shortened the π-π stacking distance, the spacing between adjacent conjugated planes in the crystalline domains. Closer stacking strengthens electronic coupling between molecules, which promotes efficient charge transport. Critically, the enhanced order also suppressed nonradiative energy loss, the dominant remaining channel through which modern organic solar cells squander potential voltage.

The photovoltaic results underscore how consequential these fine structural adjustments proved to be. When the acceptor 7C-eC9-2F was paired with the donor polymer PM6 in a conventional bulk heterojunction device, the system achieved a power conversion efficiency of 19.67 percent, a figure that places it among the highest-performing organic photovoltaic systems reported. Equally significant is the energy loss of only 0.539 electron volts, a remarkably small value for an organic solar cell. Nonradiative losses arise when excited molecular states decay without emitting photons or generating charges, and suppressing them is widely regarded as the principal route to raising the efficiency ceiling of organic photovoltaics. The fact that the same molecular features that tighten crystalline packing also curtail these losses illustrates the internal coherence of the design strategy: efficiency gains and morphological improvements flow from a single, unified structural logic rather than from competing optimizations.

Stability, the perennial weak point of organic solar cells, is where the new acceptors distinguish themselves most decisively. Devices based on 7C-eC9-2F were subjected to two standard accelerated aging tests. Under continuous illumination, the time required for the device efficiency to fall to 80 percent of its initial value, a benchmark denoted T80, reached 2728 hours. When the devices were held at 65 degrees Celsius, a temperature representative of real-world rooftop operating conditions, the T80 value reached 1242 hours. In both tests, the 7C-eC9-2F devices significantly outperformed the control systems included in the study. The researchers attribute this durability to the optimized molecular aggregation states and the clear, well-defined microstructure of the active layer, which maintains its charge-generating and charge-transporting architecture even under sustained thermal and optical stress. In effect, the same crystallinity that boosts efficiency also anchors the morphology against the gradual coarsening that typically degrades organic photovoltaic devices over time.

The improved performance stems from a convergence of factors that the authors identify as the mechanistic foundation of the work. Sophisticated molecular engineering of the acceptors produced optimized aggregation states, improved crystallinity, suppressed nonradiative recombination, and a well-defined microstructure. These properties collectively promote efficient charge generation and transport while ensuring morphological durability. The balance between crystallinity and miscibility, often treated as an unavoidable compromise in acceptor design, is here resolved by structural tailoring so precise that both properties improve together. The side chain and terminal group modifications act on the molecular fine structure in ways that tighten packing within acceptor domains without provoking the macroscopic phase separation that would destroy blend compatibility, a result the researchers link directly to the simultaneous gains in efficiency and stability.

Beyond the bulk heterojunction architecture, the team also evaluated the new acceptors in quasi-planar heterojunction, or Q-PHJ, devices, an alternative fabrication geometry in which the donor and acceptor layers are deposited in a more layered arrangement rather than as a fully intermixed blend. The acceptors delivered excellent photovoltaic performance in this structure as well, demonstrating good fabrication adaptability. This cross-process applicability matters for practical manufacturing, because different deposition and annealing protocols impose different demands on material systems. An acceptor that performs well across multiple device architectures offers device engineers substantially more flexibility than one optimized for a single fabrication route, and the Q-PHJ results suggest that the benefits of the fine structural tailoring are intrinsic to the molecules themselves rather than dependent on any particular processing window.

The research was carried out by a team whose co-first authors are Dr. Mingpeng Li, Dr. Waqar Ali Memon, and postdoctoral fellow Xue Lai, all senior visiting scholars in the Department of Chemistry at SUSTech, with Professor Feng He serving as corresponding author. The work was supported by the National Natural Science Foundation of China and the Shenzhen Natural Science Foundation. The study was published on 11 September 2026 under the title Subtle Structural Tailoring Enables Improved Crystallinity and Miscibility for High-Efficiency, Stable Organic Photovoltaics, and the authors report no conflict of interest. The article is available as diamond open access, with no fees for authors or readers, consistent with the publishing model of CCS Chemistry.

The broader significance of the study lies in the design guidance it offers for the next generation of organic photovoltaic materials. By demonstrating that selective control of side chain length and terminal group conjugation can raise crystallinity, shorten π-π stacking distances, suppress nonradiative energy loss, and preserve blend miscibility all at once, the work provides a practical molecular blueprint for overcoming the efficiency-stability trade-off that has constrained the field. The performance of the PM6:7C-eC9-2F system, with its 19.67 percent efficiency, 0.539 electron volt energy loss, 2728-hour illumination T80, and 1242-hour thermal T80 at 65 degrees Celsius, constitutes a concrete benchmark for what fine molecular engineering can achieve. As organic photovoltaics move from laboratory curiosities toward commercial devices, strategies of this kind, which treat molecular fine structure as the primary lever for both efficiency and intrinsic stability, are likely to shape the material systems that carry the technology into practical deployment.

Subject of Research: Organic photovoltaic materials and molecular engineering

Article Title: Fine-structure regulation improves crystallinity and compatibility to achieve efficient and stable organic photovoltaics

Article References: Fine-structure regulation improves crystallinity and compatibility to achieve efficient and stable organic photovoltaics. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: organic photovoltaics, non-fullerene acceptors, molecular engineering, solar energy, CCS Chemistry, energy loss, device stability, bulk heterojunction, Fine-structure, regulation, improves, crystallinity