In the quest for cleaner energy, a clever twist on solar cell design is breaking efficiency records. The secret? Adding a second polymer donor to create a ternary organic solar cell.
Imagine a solar panel that is not just efficient but also lightweight, flexible, and semi-transparent, capable of turning your window into a power generator. This is the promise of organic solar cells (OSCs). For years, scientists have been perfecting a binary recipe, blending one electron-donating polymer with one electron-accepting molecule. Now, a breakthrough strategy is pushing the boundaries: the ternary organic solar cell, which uses two polymer donors and one acceptor to unlock unprecedented performance levels.
Breaking the 20% power conversion efficiency barrier
Two polymer donors working in synergy with one acceptor
Flexible, semi-transparent solar cells for diverse surfaces
Most high-tech OSCs today are binary, consisting of a single donor and a single acceptor material mixed together in a bulk heterojunction. While this has led to impressive progress, the inherent narrow absorption windows of organic semiconductors means a single material can only capture a limited portion of the sun's broad spectrum 7 . This places a fundamental cap on the current, and thus the power, that the cell can generate.
The ternary approach is an elegant solution. By introducing a third component—often a second donor with complementary properties—scientists can create a cell that harvests light more effectively 7 . But the benefits go beyond just capturing more light.
The second donor can be chosen to absorb light in a wavelength range that the primary donor misses, leading to a broader absorption profile and a higher short-circuit current density (JSC) 8 .
The active layer of a solar cell isn't just a chemical soup; its nanoscale structure, or morphology, is critical. A well-chosen second donor can improve the blend's microstructure, leading to more efficient charge transport and a higher fill factor (FF) 1 .
Limited absorption spectrum and charge transfer pathways
Enhanced absorption and optimized charge transfer pathways
A landmark study published in Energy & Environmental Science in early 2025 perfectly illustrates the power of the two-donor strategy 1 . The research team introduced a novel wide-bandgap polymer donor, P(BTzE-BDT), into the high-performing binary system PM6:BTP-eC9.
The researchers first synthesized the new polymer donor P(BTzE-BDT), designed to have a wide bandgap and complementary absorption to the host donor PM6.
Ternary solar cells were fabricated with a standard structure. The key step was preparing the active layer solution, where a small, optimized amount (5%) of P(BTzE-BDT) was added to the PM6:BTP-eC9 blend.
The researchers used advanced techniques to analyze the ternary blend films, studying their light absorption, molecular packing, and nanoscale phase separation.
The final step was to measure the photovoltaic performance of the completed devices, comparing the ternary cells directly against the binary control under simulated sunlight.
The results were striking. The binary reference cell based on PM6:BTP-eC9 already showed a high efficiency of 18.8%. However, the ternary cell with just 5% P(BTzE-BDT) achieved a champion power conversion efficiency (PCE) of 20.0% 1 .
This improvement wasn't one-dimensional. The team observed a simultaneous increase in both JSC and FF. Their analysis revealed why: the guest donor P(BTzE-BDT) was highly compatible with PM6. It promoted more intensive molecular packing and reduced the domain size within the active layer, creating a more optimal morphology for charge generation and transport while suppressing energy-wasting recombination 1 .
| Device Type | PCE (%) | VOC (V) | JSC (mA cm⁻²) | FF (%) |
|---|---|---|---|---|
| Binary (PM6:BTP-eC9) | 18.8 | Data not specified | Data not specified | Data not specified |
| Ternary (with 5% P(BTzE-BDT)) | 20.0 | Data not specified | Increased | Increased |
Crucially, the team also tested "thick-film" devices with an active layer of 300 nm, a step toward the scalable production needed for commercialization. Here, the ternary device again dominated, achieving a PCE of 18.2% compared to 16.3% for the binary device, proving its superior performance potential in practical applications 1 .
| PM6 : PSEHTT Ratio | PCE (%) | VOC (V) | JSC (mA cm⁻²) | FF (%) |
|---|---|---|---|---|
| 100 : 0 | 15.83 | 0.870 | 24.39 | 74.6 |
| 95 : 5 | 16.28 | 0.872 | 25.09 | 74.4 |
| 90 : 10 | 16.66 | 0.875 | 25.66 | 74.2 |
| 85 : 15 | 14.37 | 0.870 | 23.10 | 71.5 |
This chart illustrates how the Power Conversion Efficiency (PCE) changes with different concentrations of the guest donor, showing a clear optimal range before performance declines.
Creating a champion ternary solar cell is like being a master chef; it requires the right ingredients and a precise recipe. Below are some of the key "research reagents" and their functions in the two-donor, one-acceptor system.
| Material Type | Example | Function in the Device |
|---|---|---|
| Host Polymer Donor | PM6 | Serves as the primary electron-donating matrix; typically has strong and efficient absorption in one part of the solar spectrum 1 8 . |
| Guest Polymer Donor | P(BTzE-BDT, PSEHTT | The second donor, added in a smaller amount, provides complementary light absorption and helps optimize the active layer morphology 1 8 . |
| Non-Fullerene Acceptor | BTP-eC9, L8-BO | The electron-accepting molecule; modern non-fullerene acceptors (NFAs) have strong near-infrared absorption and tunable energy levels 1 8 . |
| Solvent & Additive | Chloroform, 1,8-Diiodooctane (DIO) | The solvent dissolves the active materials, while a small amount of additive (like DIO) fine-tunes the molecular packing and phase separation during film formation 2 . |
Widely used polymer donor with strong absorption in the visible spectrum and excellent charge transport properties.
Novel wide-bandgap polymer with complementary absorption to PM6, improving light harvesting and morphology.
The journey of ternary organic solar cells is just beginning. The successful integration of two polymer donors, as demonstrated in the PM6:P(BTzE-BDT):BTP-eC9 system, provides a clear blueprint for surpassing the 20% efficiency milestone 1 . The future of this technology lies not only in chasing higher lab-scale records but also in addressing the challenges of scalability, stability, and cost.
Innovations are already underway, such as the development of easily-scalable polymer donors like PPT-3, which can be synthesized in 20-gram batches and used to create large-area, semi-transparent solar modules for power-generating windows 5 .
Furthermore, the unique properties of these ternary blends are finding applications beyond electricity generation, such as in the rapid photodegradation of water pollutants, showcasing their potential as multifunctional green materials .
As research continues to refine the selection of polymer pairs and unravel the complex physics within the ternary layer, we move closer to a future where solar energy is harvested from the surfaces all around us—our windows, our vehicles, and even our personal devices—powered by these efficient and versatile organic blends.
Semi-transparent solar windows
Lightweight solar panels for vehicles
Flexible solar chargers for devices