Organic Photovoltaics and Charge Transport Mechanisms

Organic photovoltaics harness conjugated polymers and small molecules to convert sunlight into electrical energy through a sequence of light absorption, exciton generation, charge separation and charge transport. At the heart of this technology lies the bulk heterojunction architecture, in which electron-donating and electron-accepting materials are intermingled at the nanoscale to form domains that facilitate exciton dissociation into free charges. The morphology, crystallinity and phase purity of these domains govern the pathways for hole and electron transport towards their respective electrodes. Charge mobility in organic semiconductors is intrinsically linked to molecular packing, energetic disorder and interfacial energetics. Strategies to control the kinetics of phase separation, to finely tune interfacial molecular orientation and to stabilise morphology under operation have led to power conversion efficiencies now exceeding 18 per cent in laboratory devices. Recent advances also address sustainable processing, with eco-solvent systems and ambient fabrication techniques enabling scalable production. The interplay between thermodynamic driving forces and kinetic constraints in film formation remains a central theme, guiding design rules that seek an optimal balance between crystallisation, demixing and interfacial alignment for maximised device performance and operational stability.

Research from Nature Portfolio

In situ and ex situ scattering studies have elucidated the coupling and competition between crystallisation and phase separation in bulk heterojunction blends. By mapping kinetic routes under thermal annealing, a unified morphological model has been proposed, revealing how different phase diagrams and annealing protocols determine domain purity and donor–acceptor interfacial orientation. This framework provides concrete guidelines for processing devices with enhanced charge transport and long-term stability. Complementing this, a systematic examination of interfacial energetics has dissected the energetic driving force for charge generation. By varying donor–acceptor combinations in planar heterojunctions, researchers identified a threshold energy difference between the singlet excited state and the charge transfer state that optimises field-independent charge separation, thereby minimising voltage losses and enhancing photocurrent extraction.

Organic Photovoltaics and Charge Transport Mechanisms publication trend

The graph below shows the total number of articles in organic photovoltaics and charge transport mechanisms across all publications each year (not limited to Nature Index journals).

Technical terms

Bulk heterojunction: A nanoscale blend of donor and acceptor materials in organic solar cells that maximises the interface area for exciton dissociation.

Exciton diffusion length: The average distance an exciton travels before recombining, critical for designing domain sizes in active layers.

Charge transfer state: A bound electron–hole pair at the donor–acceptor interface that precedes full charge separation.

Phase separation: The process by which donor and acceptor materials demix to form distinct domains during film formation.

Crystallinity: The degree of molecular order within polymer or small-molecule domains, influencing charge mobility and recombination.

Power conversion efficiency (PCE): The ratio of electrical power output from a solar cell to the incident solar power on its surface.

Heterojunction: The interface between two semiconducting materials with differing energy levels, essential for charge separation.

References

1. The coupling and competition of crystallization and phase separation, correlating thermodynamics and kinetics in OPV morphology and performances. Nature Communications (2021).
2. Anatomy of the energetic driving force for charge generation in organic solar cells. Nature Communications (2019).
3. Suppressed Degradation Process of Green‐Solvent Based Organic Solar Cells Through ZnO Modification With Sulfhydryl Derivatives. Advanced Energy Materials (2024).
4. Phase separation and domain crystallinity control enable open‐air‐printable highly efficient and sustainable organic photovoltaics. InfoMat (2024).
5. Driving fiber diameters to the limit: nanoparticle-induced diameter reductions in electrospun photoactive composite nanofibers for organic photovoltaics. Advanced Composites and Hybrid Materials (2023).