Donor-acceptor bulk heterojunction (BHJ) design is a cornerstone of organic photovoltaics (OPVs), enabling efficient light absorption, exciton dissociation, and charge transport. The BHJ structure consists of an interpenetrating network of electron-donating (donor) and electron-accepting (acceptor) materials, forming nanoscale phase-separated domains critical for device performance. The selection of donor and acceptor materials, control of morphology, and optimization of phase separation are key factors in achieving high power conversion efficiencies (PCEs).

The donor material in a BHJ must exhibit strong light absorption, high hole mobility, and appropriate energy levels to facilitate exciton dissociation. Conjugated polymers like P3HT, PTB7-Th, and PM6 are widely used due to their tunable bandgaps and favorable electronic properties. These materials absorb sunlight efficiently and generate excitons, which must diffuse to the donor-acceptor interface for charge separation. The highest occupied molecular orbital (HOMO) level of the donor should align closely with the acceptor’s HOMO to maximize open-circuit voltage (Voc), while the lowest unoccupied molecular orbital (LUMO) offset between donor and acceptor must be sufficient for exciton dissociation, typically around 0.3 eV or higher.

Acceptor materials in BHJs are categorized into fullerene derivatives and non-fullerene acceptors (NFAs). Fullerene acceptors like PCBM were historically dominant due to their high electron mobility and isotropic charge transport. However, NFAs such as ITIC, Y6, and their derivatives have surpassed fullerenes in performance, offering broader absorption, tunable energy levels, and reduced energy losses. NFAs often exhibit stronger crystallinity and better compatibility with donors, enabling finer control over morphology. The LUMO level of the acceptor must be sufficiently lower than the donor’s LUMO to ensure exciton dissociation, while the HOMO level should be deep enough to minimize recombination losses.

Morphology plays a pivotal role in BHJ performance. An ideal morphology features nanoscale phase separation with domain sizes comparable to the exciton diffusion length (typically 10–20 nm). Larger domains reduce the interfacial area for charge separation, while excessively small domains hinder charge transport. The crystallinity of both donor and acceptor materials influences charge mobility, with higher crystallinity generally improving conductivity but potentially leading to excessive aggregation. The miscibility of donor and acceptor materials determines the blend’s stability and phase separation behavior, with low miscibility often causing large-scale phase separation and high miscibility leading to poorly defined domains.

Optimizing phase separation involves several strategies. Solvent selection is critical, as different solvents affect the drying kinetics and molecular packing of the blend. High-boiling-point solvents like chlorobenzene or additive processing with compounds such as 1,8-diiodooctane (DIO) can slow drying, allowing for better self-assembly of donor and acceptor domains. Thermal annealing and solvent vapor annealing are post-deposition treatments that enhance molecular ordering and phase separation. Thermal annealing promotes crystallinity by providing energy for molecular rearrangement, while solvent vapor annealing can selectively swell one component, refining the morphology.

Advanced BHJ designs incorporate ternary blends and cascade structures to further improve performance. Ternary blends introduce a third component, either a second donor or acceptor, to broaden the absorption spectrum or improve charge transport. For example, adding a low-bandgap polymer or small-molecule acceptor can harvest near-infrared light, increasing short-circuit current density (Jsc). The third component can also act as a morphology modifier, optimizing phase separation and reducing recombination. Careful selection is necessary to prevent unfavorable energy level alignment or excessive disorder.

Cascade energy structures employ multiple donors or acceptors with graded energy levels to facilitate stepwise charge transfer. This design reduces energy losses by minimizing the driving force required for exciton dissociation while maintaining efficient charge transport. For instance, a high-bandgap donor can transfer excitons to a lower-bandgap donor, which then interfaces with the acceptor, ensuring minimal voltage loss. Cascade structures are particularly effective in tandem or multi-junction OPVs, where each subcell targets a specific portion of the solar spectrum.

Characterization techniques such as grazing-incidence X-ray diffraction (GI-XRD) and resonant soft X-ray scattering (R-SoXS) are indispensable for probing BHJ morphology. GI-XRD reveals the crystallinity and molecular packing of donor and acceptor phases, while R-SoXS provides insights into domain size and purity. Transient absorption spectroscopy (TAS) and photoluminescence quenching measurements help quantify exciton dissociation efficiency and charge recombination dynamics. These tools guide the rational design of BHJs by correlating morphological features with device performance.

The stability of BHJ OPVs remains a challenge, as morphological degradation over time can reduce PCE. Strategies to enhance stability include crosslinking the donor or acceptor materials to lock the morphology, using more robust NFAs with higher glass transition temperatures, and incorporating interfacial layers to protect the active layer from environmental factors like oxygen and moisture. Encapsulation techniques further extend device lifetimes by preventing extrinsic degradation.

Recent advancements in BHJ design focus on reducing non-radiative recombination and energy losses. Materials with high photoluminescence quantum yields and low energetic disorder minimize voltage losses, enabling PCEs exceeding 18% in single-junction OPVs. The development of new NFAs with tailored aggregation behavior and optimized energy levels continues to push the boundaries of efficiency. Additionally, machine learning approaches are being explored to predict optimal donor-acceptor pairs and processing conditions, accelerating material discovery.

In summary, donor-acceptor BHJ design in organic photovoltaics relies on careful selection of materials, precise control of morphology, and innovative strategies like ternary blends and cascade structures. The interplay between donor and acceptor properties, phase separation, and charge dynamics dictates device performance, with advanced characterization techniques providing critical insights. Continued progress in material design and morphological engineering holds promise for further improvements in efficiency and stability, bringing OPVs closer to commercial viability.