Imagine a future where our phone screens, building facades, and even tents can easily generate electricity-a feat made possible by the immense potential of polymer solar cells (PSCs). Compared to traditional silicon-based solar panels, PSCs, with their unique advantages of being lightweight, flexible, and solution-printable for large-area device fabrication, have become a rising star in the new energy field. However, the core bottleneck to achieving commercial application lies in improving photoelectric conversion efficiency (PCE). Over the past decade, PCE has surged from around 1% to over 11%, and one of the key driving forces behind this is the design and optimization of high-performance polymer photovoltaic materials.
1. From Polythiophene to D-A Copolymers
Early research focused on polythiophene homopolymers such as P3HT, but their narrow absorption spectrum and high HOMO level limited efficiency. Researchers have broken through this limitation through molecular design: for example, introducing two-dimensional conjugated branches such as bisthiophene ethylene onto polythiophene not only broadened the absorption spectrum but also lowered the HOMO level by approximately 0.2 eV, significantly improving the open-circuit voltage and short-circuit current of the device, increasing the efficiency from 2.41% to 3.18%. Another strategy is to reduce the number of alkyl chains and introduce electron-withdrawing groups such as ester groups, which can also effectively lower the HOMO energy level and significantly improve Voc (e.g., PDGBT reaches 0.91 V) and efficiency (7.2%).
2. Benzodithiophene (BDT)
The truly revolutionary breakthrough came from the donor-acceptor (D-A) alternating copolymer structure. Among them, the benzodithiophene (BDT) unit stood out due to its large conjugated plane, high mobility, and easy structural modification. In 2008, Researcher Hou Jianhui pioneered the use of BDT in D-A polymer design in Yang's research group. Subsequently, the combination of BDT and thiophene[3,4-b]thiophene (TT) became a golden pairing for high-performance materials.
To further explore the potential of BDT-like polymers, two-dimensional conjugated branches and fluorination strategies can be adopted:
Introducing two-dimensional conjugated branches into the BDT unit greatly expands the π-electron conjugated area of the molecule. This not only enhances intermolecular interactions and charge transport capabilities but also effectively modulates the absorption spectrum and molecular energy level. For example, PBDTTT-C-T, PTB7-Th, and later PBDT-TS1, which achieved efficiency breakthroughs of over 10%, all benefited from this design.
Selectively introducing strongly electron-withdrawing fluorine atoms into the side chains or TT acceptor units of BDT can synergistically and significantly reduce the HOMO energy level of the polymer, thereby greatly improving the open-circuit voltage of the device. From PBT-OF to PBT-3F, as the number of fluorine atoms increases, Voc increases from 0.56 V to 0.78 V, and efficiency jumps from 4.5% to 8.6%.
3. Morphology Control
High performance depends not only on the material itself, but also on the microstructure of the bulk heterojunction formed by the donor/acceptor blend in the active layer. The morphology needs to be just right: if the phase region is too large, excitons will recombine before they can separate; if the phase region is too small, free charges will also easily recombine. Researchers have explored two approaches to control polymer blends:
Green solvent processing: To avoid toxic halogenated solvents, researchers explored the use of green solvents such as o-xylene and o-methyl anisole (MA), combined with specific additives (such as NMP), successfully replicating the excellent morphology similar to that of halogenated solvent systems and achieving a high efficiency of nearly 10%.
Molecular structure optimization: By designing to make the polymer backbone more linear, increasing the conjugated area, or finely adjusting the alkyl side chains, the crystallinity and molecular packing of the polymer can be actively controlled, thereby obtaining ideal blend morphologies.
As an important component of green energy, polymer photovoltaic materials are leading the trend of energy transformation with their unique properties and advantages. With continuous technological advancements and market expansion, polymer photovoltaic materials will demonstrate even broader application prospects and enormous market potential in the future. Let us look forward to polymer photovoltaic materials bringing cleaner, more efficient, and sustainable energy solutions to human society!
