How a Toothpaste Ingredient Supercharged Solar Cells
10.4% Efficiency BreakthroughIn the quest for cheaper, more versatile solar energy, non-fullerene polymer solar cells (PSCs) have emerged as game-changers. Unlike traditional silicon panels, these lightweight, flexible devices can be printed like newspapers—but for years, their efficiency lagged behind.
Enter trifluoromethyl (–CF₃), a molecular fragment borrowed from pharmaceutical and toothpaste chemistry. In 2018, researchers achieved a stunning 10.4% power conversion efficiency (PCE) by strategically grafting this group onto a polymer backbone 1 2 . This leap wasn't accidental; it exploited fluorine's unique ability to rewire a material's electronic personality. Let's unravel how a tiny atomic tweak unlocked big energy gains.
The –CF₃ group's strong electron-withdrawing capability makes it ideal for modifying polymer electronic properties.
Lightweight, flexible, and printable - polymer solar cells offer unique benefits over traditional silicon.
Fluorine is the most electronegative element, making –CF₃ a powerful electron-withdrawing group. When attached to a polymer's side chains, it:
Lowers the polymer's energy state, boosting the open-circuit voltage (Voc)—a critical factor for efficiency 5 .
Increases the extinction coefficient, allowing thinner active layers to capture more photons 1 .
Promotes favorable molecular packing for efficient charge transport 3 .
The polymer in this breakthrough, PBZ-m-CF₃, is a wide-bandgap material (~1.99 eV). This contrasts with low-bandgap polymers that absorb infrared light but suffer from energy losses. Wide-bandgap polymers capture high-energy visible light and pair perfectly with non-fullerene acceptors (NFAs) like ITIC, which handle near-infrared photons 6 7 . This complementary absorption is key to covering more solar spectrum.
Created BDTP-m-CF₃, a benzodithiophene (BDT) unit with meta-trifluoromethyl and para-alkoxyphenyl side chains.
For comparison, synthesized PBZ1 (same backbone without –CF₃) 1 .
Copolymerized BDTP-m-CF₃ with difluorobenzotriazole (FBTZ) via Stille coupling to form PBZ-m-CF₃.
Purified the polymer to remove catalytic residues.
| Polymer | HOMO (eV) | Bandgap (eV) | Extinction Coefficient (×10â´ cmâ»Â¹) |
|---|---|---|---|
| PBZ-m-CF₃ | -5.49 | 1.99 | 6.51 (at 533 nm) |
| PBZ1 | -5.27 | 1.96 | 5.23 (at 539 nm) |
| Device | PCE (%) | Voc (V) | Jsc (mA/cm²) |
|---|---|---|---|
| PBZ-m-CF₃:ITIC | 10.4 | 0.94 | 18.4 |
| PBZ1:ITIC | 5.8 | 0.74 | 15.7 |
| Reagent/Material | Function | Role in Efficiency |
|---|---|---|
| BDTP-m-CF₃ monomer | Electron-rich unit with –CF₃ side chains | Deepens HOMO, enhances crystallinity |
| FBTZ (Difluorobenzotriazole) | Electron-deficient copolymer unit | Lowers LUMO, widens bandgap for complementary absorption |
| ITIC acceptor | Non-fullerene small molecule (narrow bandgap ~1.6 eV) | Absorbs near-infrared light; pairs with wide-bandgap donors |
| Toluene solvent | Processing solvent for active layer | Optimizes film morphology, prevents excessive aggregation |
The PBZ-m-CF₃ story is just one chapter. Recent advances show trifluoromethylation's broader potential:
(e.g., BTIC-CF₃-γ) achieve 15.59% PCE by extending absorption into infrared 3 .
Combine fluorinated wide-bandgap polymers with low-bandgap materials, exceeding 16.5% efficiency 3 .
Fluorinated polymers resist oxidation, extending device lifespan 5 .
While toluene processing is industry-friendly, replacing halogenated solvents remains a hurdle. Researchers are now designing –CF₃ polymers processable in non-toxic solvents without sacrificing morphology 6 .
Trifluoromethyl isn't just a chemical ornament—it's a precision tool for reengineering solar materials.
By lowering energy losses, guiding molecular assembly, and expanding light harvesting, this tiny fluorine bundle has pushed PSCs into double-digit efficiency territory. As research expands to 3D interpenetrated networks and multi-junction cells, the marriage of fluorine and photovoltaics promises to make solar energy thinner, cheaper, and ubiquitous. Who knew an atom from toothpaste could help power our future?