Molecular origin of differences in hole and electron mobility in amorphous Alq3--a multiscale simulation study

Area of Science:

• Materials Science
• Condensed Matter Physics
• Computational Chemistry

Background:

• Amorphous organic semiconductors like tris(8-hydroxyquinoline) aluminium (Alq(3)) are crucial in organic electronics.
• Understanding charge transport mechanisms, particularly mobility differences between electrons and holes, is vital for device optimization.
• Existing models often lack the multiscale detail to fully explain observed mobility disparities.

Purpose of the Study:

• To elucidate the molecular origins of the differing electron and hole mobilities in amorphous Alq(3) thin films.
• To correlate charge density localization with transport properties at a molecular level.
• To provide a comprehensive simulation-based explanation for mobility anisotropy.

Main Methods:

• Multiscale simulations integrating quantum mechanics, molecular mechanics, and lattice models.
• Inclusion of realistic disordered morphologies and polarized site energies for diagonal disorder.
• Quantum chemically calculated transfer integrals for off-diagonal disorder and Marcus theory for intermolecular transfer rates.
• Kinetic Monte Carlo simulations and Master Equation approach for mobility calculations.

Main Results:

• Electron mobility is higher than hole mobility in amorphous Alq(3) films.
• The difference arises from distinct charge density localization: radical anions are more delocalized than radical cations.
• Holes experience greater diagonal disorder and less favorable overlap for transfer integrals compared to electrons.

Conclusions:

• The molecular structure and charge localization significantly impact charge carrier mobility in amorphous Alq(3).
• Enhanced electron mobility is attributed to greater charge delocalization of the radical anion.
• These findings offer fundamental insights for designing organic electronic materials with tailored charge transport properties.