Conjugated polymers have emerged as a cornerstone in the development of organic photovoltaics (OPVs), particularly in the design of bulk heterojunction (BHJ) architectures. Their tunable electronic and optical properties make them ideal for optimizing light absorption, charge generation, and transport in OPV devices. The success of BHJ OPVs relies heavily on the careful selection and engineering of conjugated polymer donors paired with suitable acceptors, including non-fullerene acceptors (NFAs), to achieve high power conversion efficiencies (PCEs), fill factors (FFs), and open-circuit voltages (Voc).
Material design for conjugated polymers in BHJ OPVs begins with the strategic manipulation of their bandgap. Low-bandgap polymers, typically with optical bandgaps below 1.8 eV, are favored for their ability to harvest a broader range of solar photons, particularly in the near-infrared region. By tailoring the polymer backbone through donor-acceptor (D-A) copolymerization, researchers can fine-tune the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. For example, incorporating strong electron-withdrawing units like benzothiadiazole or diketopyrrolopyrrole into the polymer backbone can lower the bandgap while maintaining sufficient Voc by balancing energy level offsets with the acceptor.
Non-fullerene acceptors have revolutionized OPVs by overcoming the limitations of traditional fullerene derivatives, such as weak absorption and limited energy level tunability. NFAs, such as ITIC and Y6 derivatives, exhibit strong light absorption, adjustable energy levels, and enhanced morphological stability when blended with conjugated polymers. The compatibility between the polymer donor and NFA is critical for forming a nanoscale interpenetrating network in the BHJ, which facilitates efficient exciton dissociation and charge transport. Recent advances in NFA design have pushed PCEs beyond 18% in single-junction OPVs, with some laboratory-scale devices approaching 20%.
Key performance metrics in OPVs include PCE, FF, and Voc. PCE is determined by the product of Voc, short-circuit current density (Jsc), and FF, divided by the incident light power. Achieving high Voc requires minimizing the energy loss between the polymer donor’s HOMO and the acceptor’s LUMO while maintaining sufficient driving force for charge separation. Strategies such as reducing the energetic disorder and optimizing interfacial layers have been employed to enhance Voc. FF is influenced by charge carrier mobility, recombination losses, and the BHJ morphology. High FF values, often exceeding 75%, are attainable through balanced hole and electron mobility, achieved by selecting polymers with high crystallinity and optimizing blend ratios.
Despite these advancements, challenges remain in improving the stability and scalability of conjugated polymer-based OPVs. Stability issues arise from photo-oxidation, morphological degradation, and interfacial reactions under environmental stressors like moisture, oxygen, and thermal cycling. Encapsulation techniques and the development of more robust materials, such as cross-linkable polymers or inert side-chain engineering, have shown promise in extending device lifetimes. Scalability concerns involve the reproducibility of high-performance polymers and NFAs across large-area fabrication methods like roll-to-roll printing. Solution processability must be balanced with performance, requiring careful selection of solvents and additives to control film morphology without compromising efficiency.
Recent research has focused on ternary blends and multi-component systems to further enhance OPV performance. By incorporating a third component—either a second donor or acceptor—light absorption can be broadened, and energy loss minimized. For instance, ternary systems combining wide-bandgap and low-bandgap polymers with an NFA have demonstrated improved photon harvesting and reduced recombination. However, optimizing the composition and morphology of such systems remains complex, requiring precise control over phase separation and domain purity.
In conclusion, conjugated polymers play a pivotal role in advancing BHJ OPVs through tailored material design. The synergy between low-bandgap polymers and NFAs has led to remarkable improvements in PCE, FF, and Voc. However, addressing stability and scalability challenges is essential for transitioning laboratory-scale successes into commercially viable technologies. Future directions may involve exploring novel polymer architectures, advanced encapsulation methods, and scalable processing techniques to unlock the full potential of OPVs in renewable energy applications.