Simpler Molecules for Cheaper Solar Power
In the relentless pursuit of cleaner energy sources, organic solar cells (OSCs) have emerged as a promising path forward. Imagine lightweight, flexible, and even semi-transparent solar panels that could be printed like newspapers onto various surfaces.
For decades, the efficiency of OSCs was stuck at low levels, but a scientific breakthrough in 2019, with a molecule named Y6, finally pushed efficiencies to a competitive 15% and beyond1 .
However, a hidden problem remained: the complex, expensive, and inefficient synthesis of the molecules at the heart of these cells. This is where the story of unfused synthesis begins—a clever chemical strategy that is simplifying molecules to slash costs and pave the way for a solar-powered future.
To appreciate the unfused revolution, one must first understand the "fused" design it seeks to replace. High-performance organic solar cells often use molecules with an A-D-A (Acceptor-Donor-Acceptor) or A₁-DA₂D-A₁ architecture1 .
The "D" or "DA₂D" core is typically a large, fused structure—a rigid, flat backbone where multiple rings share sides, like a panel in a intricately carved fence.
Fused Design
Unfused Design
Fused structures have rigid, chemically bonded rings while unfused designs use simpler blocks connected by non-covalent interactions.
This fused core is great for performance; its rigidity helps conduct electrical charges. However, from a manufacturing standpoint, it's a nightmare. Constructing these complex fused systems requires multiple synthesis steps and difficult cyclization reactions, which significantly lower the overall yield and drive up the cost1 .
Unfused-ring small molecule acceptors are the elegant solution. Instead of building a single, complex fused core, chemists connect smaller, simpler building blocks. The genius lies in using non-covalent intramolecular interactions, often called "conformation locks"1 .
Imagine the molecular blocks are like two pieces of wood that don't fit together perfectly. You can use a weak magnet (a non-covalent interaction) to hold them flush. Similarly, atoms like oxygen and sulfur in these molecules form weak attractive forces that lock the structure into a planar and rigid shape without needing a fused chemical bond1 .
This provides the best of both worlds: the simplicity and low cost of small parts, and the high performance of a rigid structure.
The theoretical advantages of unfused design are compelling, but it was concrete experiments that proved its viability. A landmark study, published in Materials Today Energy in 2021, designed and tested two novel unfused-ring acceptors named BDTC-F and BDTC-Cl1 .
The team used Cyclopentadithiophene (CPDT) as the donor. This unit has a sp3 hybridized carbon that acts like a "spacer," enhancing the molecule's solubility1 .
They selected a unit known as BDD. This component has a large planar structure and strong electron-withdrawing ability1 .
To complete the structure, they attached well-known end groups: IC-2F for BDTC-F and IC-2Cl for BDTC-Cl1 .
The key to the molecule's stability was the use of O⋯S non-covalent interactions. These weak forces between the oxygen atoms in the BDD and IC units and the sulfur atoms in the CPDT unit held the entire structure in a near-planar conformation, mimicking the rigidity of a fused system1 .
When blended with a common polymer donor called PM6, the new unfused molecules delivered impressive results1 :
Power Conversion Efficiency
Power Conversion Efficiency
This was a landmark achievement, demonstrating that unfused acceptors could indeed compete with their more complex cousins.
| Metric | BDTC-F | BDTC-Cl |
|---|---|---|
| Power Conversion Efficiency (PCE) | 10.05% | 12.0% |
| Open-Circuit Voltage (Vᵒc) | 0.89 V | 0.85 V |
| Short-Circuit Current Density (Jₛc) | 19.01 mA cm⁻² | 20.92 mA cm⁻² |
| Fill Factor (FF) | 59.5% | 67.4% |
Data sourced from 1
Beyond efficiency, the unfused molecules exhibited excellent thermal stability, with decomposition temperatures above 315°C, ensuring the solar cells could withstand operational stress1 .
Furthermore, devices based on BDTC-Cl showed remarkable air stability, retaining 89% of their initial efficiency after 120 hours, addressing a critical challenge for organic electronics1 .
The true success of this experiment was proving that high performance could be decoupled from synthetic complexity. By forgoing the fused core, the synthesis of these molecules was drastically simplified, which directly translates to lower production costs and a clearer path to mass production.
Creating these advanced materials requires a precise set of chemical tools. The following table details some of the essential reagents and their functions in the synthesis of organic electronic materials like unfused acceptors.
| Reagent / Tool | Function in Synthesis |
|---|---|
| Copper Bromide (CuBr₂) | A halogenation agent used to introduce bromine atoms onto molecular frameworks, a common step for further functionalization. |
| Potassium Fluoride (KF) | A reagent for halogen exchange, used to replace other halogens (like bromine) with fluorine atoms to fine-tune molecular properties. |
| 18-Crown-6 Ether | A "phase-transfer catalyst" that helps dissolve inorganic reagents (like KF) in organic solvents, facilitating the reaction. |
| Palladium Catalysts | Essential for cross-coupling reactions (e.g., Stille, Suzuki), which are key for connecting different aromatic rings to build the molecular backbone1 . |
| Chalcone Derivatives | A versatile building block in medicinal and materials chemistry; its flexible structure allows for the creation of various heterocycles and functional molecules. |
The global market for such synthesis reagents is growing, driven by the expansion of the pharmaceutical and biotechnology sectors, underscoring their fundamental role in innovation4 .
The shift toward unfused-ring acceptors represents a profound maturation in the field of organic photovoltaics. For years, the quest was dominated by a single metric: efficiency at any cost. The unfused synthesis paradigm introduces a more holistic and commercially aware principle: efficiency with practicality.
Low efficiency (<5%), limited applications
Breakthrough with Y6 molecule (15%+ efficiency) but complex synthesis
Simplified synthesis with competitive performance (12% efficiency)
Mass production, building integration, wearable electronics
Windows and facades that generate power
Flexible solar patches for portable devices
Lower costs through simplified manufacturing
Lower energy payback times and greener production
As research continues, the performance gap between unfused and fused acceptors is rapidly closing. With scientists now armed with a deeper understanding of non-covalent interactions and molecular design, the future of organic solar cells looks not only brighter but also simpler and more accessible, truly harnessing the power of sunlight for all.