Charge Transport Mechanisms in Organic Semiconductor Devices

Charge transport in organic semiconductor devices is governed by a complex interplay of molecular structure, morphology, energetic landscapes and external perturbations. Unlike inorganic counterparts, organic materials rely on weak van der Waals interactions and π-conjugated systems, giving rise to mixed hopping and band-like transport regimes. Charge carriers often localise into polaronic states, whose mobility is influenced by dynamic disorder arising from thermal motions and static disorder due to impurities or structural defects. Trapping phenomena—where carriers are immobilised in deep energy states—further modulate conductivity and device stability. In thin-film transistors, light-emitting diodes and solar cells, optimising molecular packing, reducing energetic disorder and balancing hole and electron transport are essential for achieving high performance. Advances in de-doping techniques and molecular design have demonstrated control over trap densities, while refined deposition strategies and post-processing methods enable tuning of crystallinity and phase separation. Overall, a multiscale understanding—from quantum localisation of polarons to macroscopic drift-diffusion models—is critical for the rational design of next-generation organic electronic devices.

Research from Nature Portfolio

Recent studies have demonstrated novel molecular approaches to suppress charge trapping by spatially decoupling the highest occupied and lowest unoccupied molecular orbitals. By engineering stacking motifs and side-chain architectures, electron traps induced by impurities are minimised, resulting in orders-of-magnitude enhancement in balanced transport for wide-bandgap semiconductors. This strategy opens pathways to efficient blue-emitting diodes and ambipolar transistors with broadened trap-free energetic windows.

Other work has challenged the conventional view of trace oxygen as purely detrimental. A gentle plasma de-doping method reveals that residual oxygen can pre-emptively fill donor-like traps, explaining predominant p-type behaviour in many organic systems. Reversible re-doping under light and oxygen allows precise, non-destructive tuning of polarity, conductivity and threshold voltages, greatly expanding the accessible property space without chemical alteration of the host material.

Charge Transport Mechanisms in Organic Semiconductor Devices publication trend

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

Technical terms

Polaron: A charge carrier (electron or hole) locally coupled to lattice or molecular deformations, forming a composite quasiparticle that influences mobility and localisation.

Energetic disorder: Variation in site energies within the semiconductor due to structural, electronic or chemical inhomogeneities, broadening the density of states and affecting transport pathways.

Trap state: A deep energy level, often arising from impurities or defects, that captures and immobilises carriers, reducing conductivity and increasing recombination.

Exciton: A bound electron–hole pair generated upon photoexcitation; its dissociation into free charges is crucial for photovoltaic and photodetector applications.

Non-Langevin recombination: A recombination mechanism in which the rate of free-charge encounter is lower than predicted by classical Langevin theory, often due to spatial separation or energetic barriers between carriers.

Drift-diffusion simulation: A numerical method solving coupled transport and Poisson equations to model charge carrier drift under electric fields and diffusion due to concentration gradients, used to predict device performance.

References

1. From generation to collection – impact of deposition temperature on charge carrier dynamics of high-performance vacuum-processed organic solar cells. Energy & Environmental Science (2024).
2. Elimination of charge-carrier trapping by molecular design. Nature Materials (2023).
3. Efficient Nanoscale Exciton Transport in Non‐Fullerene Organic Solar Cells Enables Reduced Bimolecular Recombination of Free Charges. Advanced Materials (2023).
4. Origin, Nature, and Location of Defects in PM6:Y6 Organic Solar Cells. Advanced Energy Materials (2023).
5. Unraveling the crucial role of trace oxygen in organic semiconductors. Nature Communications (2024).
6. Quantum localization and delocalization of charge carriers in organic semiconducting crystals. Nature Communications (2019).