Optimized Morphologies in Small-Molecule Organic Solar Cells

Small-molecule organic solar cells (SM-OSCs) have emerged as a promising alternative to polymer-based devices owing to their well-defined structures, facile purification and reproducible performance. The central challenge lies in engineering the active-layer morphology to balance light absorption, exciton dissociation and charge transport. Optimal morphologies feature finely tuned phase separation between donor and acceptor components, ordered π–π stacking for high charge mobility and hierarchical organisation across nanometre to micrometre scales. Strategies such as molecular fluorination, side-chain engineering and solvent selection have been instrumental in modulating crystallinity, domain size and vertical phase gradation. These advances have propelled power conversion efficiencies beyond 15 %, while reducing energy losses and enabling non-halogen solvent processing. The global significance of SM-OSCs extends to lightweight, semi-transparent and potentially low-cost photovoltaic modules suitable for building-integrated applications and portable electronics.

Research from Nature Portfolio

Recent studies have demonstrated that non-halogen solvent processing can yield reproducible and efficient SM-OSCs. A novel donor molecule featuring intramolecular chlorine–sulfur interactions promotes strong pre-aggregation in tetrahydrofuran, leading to well-defined nanoscale phase separation and improved π–π stacking. Devices processed from tetrahydrofuran achieved power conversion efficiencies of around 16 % under thermal annealing. Earlier work revealed that selective fluorination of end-groups produces a vertical phase gradient and hierarchical domain purity, achieving inverted device efficiencies above 11 %. The fluorinated molecules form highly ordered domains with enhanced vertical charge extraction. Foundational research on hierarchical morphology engineering combined narrow-bandgap acceptors with planar donors to create domains of tens of nanometres alongside larger donor- or acceptor-rich regions. This multiscale organisation facilitated balanced exciton splitting and charge collection, yielding efficiencies exceeding 14 % in fully solution-processed devices.

Optimized Morphologies in Small-Molecule Organic Solar Cells publication trend

The graph below shows the total number of articles in optimized morphologies in small-molecule organic solar cells across all publications each year (not limited to Nature Index journals).

Technical terms

Bulk heterojunction: interpenetrating network of donor and acceptor materials forming the photoactive layer.

Phase separation: spatial distribution of donor-rich and acceptor-rich domains at the nanoscale.

π–π stacking: face-to-face alignment of conjugated backbones facilitating charge transport.

Hierarchical morphology: organisation of domains across multiple length scales, combining nanometre and micrometre structures.

Non-halogen solvent processing: fabrication of active layers using environmentally benign solvents instead of chlorinated or aromatic solvents.

Crystallinity: degree of molecular ordering within donor or acceptor domains influencing charge mobility.

References

1. Side‐chain symmetry‐breaking strategy on porphyrin donors enables high‐efficiency binary all‐small‐molecule organic solar cells. SusMat (2024).
2. Efficient all-small-molecule organic solar cells processed with non-halogen solvent. Nature Communications (2024).
3. Fluorination-enabled optimal morphology leads to over 11% efficiency for inverted small-molecule organic solar cells. Nature Communications (2016).
4. All-small-molecule organic solar cells with over 14% efficiency by optimizing hierarchical morphologies. Nature Communications (2019).
5. 15.3% Efficiency All‐Small‐Molecule Organic Solar Cells Achieved by a Locally Asymmetric F, Cl Disubstitution Strategy. Advanced Science (2021).
6. Design of All-Small-Molecule Organic Solar Cells Approaching 14% Efficiency via Isometric Terminal Alkyl Chain Engineering. Energies (2021).