Small Molecule Applications in Organic Photovoltaics

Organic photovoltaics based on small molecules have emerged as a versatile platform for sustainable solar energy conversion. These discrete molecular semiconductors offer precise structural control through tailored donor–acceptor motifs and π-conjugated backbones, enabling fine-tuning of optical absorption, energy levels and film morphology. Small molecules facilitate solution processing and reproducible batch synthesis, making them attractive for roll-to-roll fabrication of flexible, lightweight devices. Central to performance is the formation of an optimised bulk heterojunction, wherein a finely mixed donor–acceptor network ensures efficient exciton dissociation and charge transport. Recent advances in molecular design, including liquid crystalline donors for improved film uniformity, side-chain engineering for enhanced charge mobility and non-fullerene acceptors for broadened absorption, have driven power conversion efficiencies above 10 per cent. Alongside efficiency, control of crystallinity, phase morphology and photostability has been critical in translating promising small molecule systems towards real-world application.

Research from Nature Portfolio

Recent studies have demonstrated the impact of molecular ordering and morphology on device performance. One foundational report introduced a benzodithiophene–terthiophene–rhodanine donor exhibiting nematic liquid crystalline behaviour, achieving power conversion efficiencies approaching 9.3 per cent in thick films and demonstrating excellent processability for scalable fabrication. Investigations into benzo[1,2-b:4,5-b′]dithiophene-based acceptor–donor–acceptor small molecules revealed that subtle side-chain modifications on the central core significantly affect interchain spacing and charge transport, leading to bulk heterojunction cells with efficiencies near 7 per cent after solvent vapour annealing. Complementary work on p-DTS(FBTTh2)2:PC71BM blends elucidated two thermodynamically stable vertical phase morphologies, showing that solvent additives can promote a finely mixed donor–acceptor network with small nanocrystals, thereby enhancing fill factor and overall device efficiency.

Small Molecule Applications in Organic Photovoltaics publication trend

The graph below shows the total number of articles in small molecule applications in organic photovoltaics across all publications each year (not limited to Nature Index journals).

Technical terms

Bulk heterojunction (BHJ): An interpenetrating network of donor and acceptor materials in the active layer that facilitates exciton dissociation and charge transport.

Power conversion efficiency (PCE): The ratio of electrical power output from a photovoltaic device to the incident solar power input, expressed as a percentage.

Donor–acceptor (D–A) small molecule: A molecular design motif featuring electron-rich (donor) and electron-deficient (acceptor) segments to promote intramolecular charge transfer and broad absorption.

π-Conjugation: A system of alternating single and double bonds that allows delocalisation of electrons, enhancing optical absorption and charge mobility.

Crystallinity: The degree of structural order in a solid material, influencing the morphology, charge transport and stability of the active layer.

References

1. A molecular nematic liquid crystalline material for high-performance organic photovoltaics. Nature Communications (2015).
2. Side-chain Engineering of Benzo[1,2-b:4,5-b’]dithiophene Core-structured Small Molecules for High-Performance Organic Solar Cells. Scientific Reports (2016).
3. Peculiarity of Two Thermodynamically-Stable Morphologies and Their Impact on the Efficiency of Small Molecule Bulk Heterojunction Solar Cells. Scientific Reports (2015).
4. Crystallinity and Molecular Packing of Small Molecules in Bulk-Heterojunction Organic Solar Cells. Applied Sciences (2022).
5. Effects of Side-Chain Engineering with the S Atom in Thieno[3,2-b]thiophene-porphyrin to Obtain Small-Molecule Donor Materials for Organic Solar Cells. Molecules (2021).
6. Non‐fullerene all small molecule OBHJSCs with profound device characteristics. Nano Select (2020).