Small molecules with donor-acceptor-donor (D-A-D) or acceptor-donor-acceptor (A-D-A) architectures have emerged as critical materials for organic photovoltaics (OPVs) due to their tunable electronic properties, well-defined molecular structures, and compatibility with solution processing. These materials offer advantages such as high purity, reproducibility, and the ability to fine-tune energy levels through molecular design. Unlike polymer-based systems, small molecules eliminate batch-to-batch variations and enable precise control over optoelectronic properties. This article discusses the design rules, energy level engineering, and bulk heterojunction (BHJ) compatibility of D-A-D and A-D-A small molecules for OPV applications, focusing on non-polymer blends.

The design of D-A-D and A-D-A small molecules follows fundamental principles to optimize light absorption, charge generation, and transport. The donor (D) and acceptor (A) units are selected based on their electronic properties, with donors typically being electron-rich (e.g., oligothiophenes, benzodithiophenes) and acceptors being electron-deficient (e.g., diketopyrrolopyrrole, benzothiadiazole). The D-A-D or A-D-A arrangement creates an intramolecular charge transfer (ICT) state, which enhances absorption in the visible to near-infrared range. The conjugation length and planarity of the molecule influence the extinction coefficient and charge carrier mobility. For example, increasing the conjugation length can redshift absorption but may also introduce steric hindrance, reducing crystallinity.

Energy level tuning is critical for maximizing open-circuit voltage (Voc) and charge separation efficiency. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels must align with those of the acceptor or donor in the BHJ to minimize energy losses. In D-A-D molecules, the HOMO is typically localized on the donor units, while the LUMO resides on the acceptor. By modifying the donor or acceptor strength, the HOMO-LUMO gap can be adjusted. For instance, incorporating stronger acceptors like isoindigo lowers the LUMO, while alkylated donors like triphenylamine raise the HOMO. The Voc in OPVs is proportional to the energy difference between the HOMO of the donor and the LUMO of the acceptor, typically requiring a minimum offset of 0.3 eV for efficient charge separation.

Bulk heterojunction compatibility is another key consideration. Small molecules must form a nanoscale phase-separated network with fullerene or non-fullerene acceptors to facilitate exciton dissociation and charge transport. The molecular packing and crystallinity of D-A-D or A-D-A small molecules influence the morphology of the active layer. Side-chain engineering is often employed to balance solubility and intermolecular interactions. Branched alkyl chains improve solubility but may reduce crystallinity, while linear chains enhance packing but can lead to excessive aggregation. Thermal annealing or solvent additives are commonly used to optimize the BHJ morphology.

The choice of acceptor in the BHJ is crucial. Fullerene derivatives like PCBM have been widely used due to their high electron mobility and isotropic charge transport. However, non-fullerene acceptors (NFAs) such as ITIC or Y6 are now preferred for their tunable energy levels and stronger absorption. When paired with D-A-D or A-D-A small molecules, NFAs enable higher Voc and better spectral coverage. The complementary absorption profiles of the small molecule donor and NFA can enhance the short-circuit current density (Jsc). For example, a D-A-D molecule with absorption peaking at 600 nm paired with an NFA absorbing at 800 nm can cover a broader solar spectrum.

Charge transport properties are influenced by the molecular orientation and interfacial interactions in the BHJ. High hole mobility is desirable for efficient charge extraction, and this can be achieved by promoting π-π stacking in the small molecule film. Crystallinity can be controlled by the choice of solvent and processing conditions. For instance, chlorobenzene or chloroform with high boiling points may facilitate slower drying, leading to larger crystalline domains. Additives like 1,8-diiodooctane (DIO) can further refine the morphology by selectively dissolving one component, promoting finer phase separation.

Stability is another critical factor for OPV applications. D-A-D and A-D-A small molecules must resist photo-oxidation and thermal degradation. Incorporating robust donor and acceptor units with high oxidation potentials can enhance stability. For example, fused-ring acceptors with rigid structures exhibit better resistance to degradation than flexible chains. Encapsulation techniques and interfacial layers are also employed to protect the active layer from environmental factors.

Recent advances in D-A-D and A-D-A small molecules have demonstrated power conversion efficiencies (PCEs) exceeding 12% in single-junction OPVs. Key breakthroughs include the development of low-bandgap materials with broad absorption and low energy losses. For instance, A-D-A small molecules with benzodithiophene cores and rhodanine end groups have achieved PCEs above 10% when paired with NFAs. The systematic optimization of molecular weight, side chains, and end-group functionalization has further improved performance.

In summary, D-A-D and A-D-A small molecules represent a promising class of materials for OPVs due to their tunable optoelectronic properties and compatibility with BHJ architectures. Design rules focus on optimizing donor-acceptor combinations, energy level alignment, and morphological control. By avoiding polymer blends, these small molecules offer reproducible and scalable solutions for next-generation organic solar cells. Future research will likely explore novel acceptor units, ternary blends, and advanced processing techniques to push PCEs closer to the 20% threshold.