Organic solar cells (OSCs) are a promising renewable energy technology, and material innovation is crucial for their advancement. Small molecules (SMs) offer defined structures and good assembly, but strong intermolecular interactions can limit performance and stability. Polymers, on the other hand, possess good thermal stability due to their high molecular weight (MW), but suffer from batch-to-batch variability and chain entanglement. To overcome these limitations, researchers have explored giant oligomeric acceptors, combining the advantages of SMs and polymers. These giant oligomers exhibit defined structures, high power conversion efficiencies (PCEs), and excellent stability. However, existing giant oligomer-based devices still utilize batch-varied polymers as donors, hindering reproducibility and commercial viability. This research aims to address this limitation by developing efficient giant oligomeric donors to create all-giant-oligomer OSCs (AGO-OSCs), leading to devices with completely defined structures and potentially superior stability. The key challenge lies in designing efficient giant dimeric donors (G-Dimer-Ds) and understanding the design principles governing their performance.

Previous studies have demonstrated the success of giant oligomeric acceptors, such as the “N-π-N” type giant dimeric acceptor (G-Dimer-A) and the “star-shaped” giant trimeric acceptor (G-Trimer-A), in achieving high-performance OSCs. These giant oligomers have shown excellent PCEs and stabilities, validating the design approach. However, a significant gap remains in the development of efficient giant oligomeric donors. While some dimerized donors, like dimeric porphyrins and BODIPY, have been reported, their performance in binary devices has been limited. This study leverages the success of G-Dimer-A design principles and focuses on combining highly efficient small molecule donors to synthesize G-Dimer-Ds. Critical parameters include the choice of linking groups, their positions, the balance between rigid aromatic groups, and flexible alkyl chains.

The researchers synthesized two isomeric dimeric rhodanine-based linkers with para- and meta-positions, creating G-Dimer-D1 and G-Dimer-D2. These linkers were designed to tune the dihedral angle between the two monomers, impacting their molecular interaction and thermal-driven assembly. The synthesis employed a modular design using the efficient SM donor MPhS-C6 as a monomer. The Knoevenagel reaction and subsequent condensation reactions yielded high yields (50-70%). The materials were characterized using various techniques including 1H NMR, MALDI-TOF-MS, DFT calculations, cyclic voltammetry (CV), UV-vis absorption spectroscopy, differential scanning calorimetry (DSC), grazing incidence wide-angle X-ray scattering (GIWAXS), transmission electron microscopy (TEM), and atomic force microscopy (AFM). In-situ UV-Vis-NIR spectroscopy during spin-coating and thermal annealing (TA) was used to study film formation and morphology. Organic solar cells were fabricated with the structure ITO/PEDOT:PSS/active layer/PFN-Br/Ag. Device performance was evaluated under AM 1.5 G (100 mW cm²) using a solar simulator, and J-V characteristics were recorded using a source-measure unit. External quantum efficiency (EQE), transient photocurrent (TPC) measurements, and electroluminescence quantum efficiency (EQEEL) were also performed. Charge carrier mobility was determined using a space charge limited current (SCLC) model. Contact angle measurements and calculations were used to determine surface energy, solubility parameters, and Flory-Huggins interaction parameters. Urbach energy (Eu) was determined through exponential fitting to FTPS-EQE spectra. Device stability was tested under simulated one-sun radiation in N2 atmosphere.

G-Dimer-D2 showed lower homo-molecular interaction and stronger thermal-driven assembly compared to G-Dimer-D1. DSC analysis revealed a lower melting point and enthalpy for G-Dimer-D2. GIWAXS measurements confirmed a dominant face-on packing mode for G-Dimer-D2 after TA, with improved crystallinity. In-situ UV-vis-NIR absorption studies showed simultaneous aggregation of donor and acceptor in G-Dimer-D2/DY blends, unlike the sequential aggregation in G-Dimer-D1/DY. G-Dimer-D2/DY exhibited the smallest energetic disorder (Ea = 21.8 meV) and slowest assembly, leading to an optimized morphology. AGO-OSCs based on G-Dimer-D2/DY achieved a PCE of 15.70% (V_oc = 0.858 V, J_sc = 24.40 mA cm⁻², FF = 75.00%), further improved to 16.05% (FF = 76.64%) by shortening alkyl chains. The hole and electron mobility were significantly higher in G-Dimer-D2/DY compared to G-Dimer-D1/DY. G-Dimer-D2/DY showed the lowest non-radiative voltage loss (ΔVnr). Long-term photostability tests indicated superior stability for AGO-OSCs compared to ASM-OSCs, with an extrapolated T80 of ~10,000 hours for G-Dimer-D2/DY. Light soaking analysis suggested that the burn-in is primarily due to changes in the disordered region of the blend, rather than the ordered crystalline regions.

The results demonstrate the successful design and synthesis of giant dimeric donors with tunable molecular assembly properties. The superior performance of G-Dimer-D2 in AGO-OSCs highlights the importance of balancing homo-molecular interaction and thermal-driven assembly capability for optimized morphology and charge transport. The reduced energetic disorder and non-radiative voltage losses contribute to the high efficiency. The improved photostability arises from the limited molecular migration associated with the high molecular weight of the giant oligomers. This study offers valuable insights into the design principles for high-performance giant donors and provides a new pathway for developing stable and efficient all-giant-oligomer OSCs.

This research successfully synthesized and characterized giant dimeric donors, demonstrating the importance of linker design in controlling molecular assembly. The resulting all-giant-oligomer organic solar cells exhibit PCEs exceeding 16% and exceptional long-term photostability. The findings provide a new strategy for designing high-performance and stable organic solar cells. Future research could explore other linker structures and monomer combinations to further improve efficiency and stability, as well as investigate the scalability of this approach for commercial applications.

While this study demonstrates significant advancements, some limitations exist. The current study focuses on a specific type of giant dimeric acceptor and donor, and the results might not be universally applicable to other systems. Further investigation into the long-term stability under various environmental conditions is necessary to fully assess the commercial viability of AGO-OSCs. The relatively high cost of synthesizing giant oligomers could also limit their widespread adoption.