A universal Urbach rule for disordered organic semiconductors.

Area of Science:

• Materials Science
• Solid-State Physics
• Organic Electronics

Background:

• Absorption onset sharpness in crystalline semiconductors is linked to temperature-dependent Urbach energies, quantifying disorder.
• The applicability of the exponential-tail model to disordered solids, particularly organic semiconductors, remains debated.
• Urbach energy analysis is routinely used for sub-gap absorption in organic semiconductors.

Purpose of the Study:

• To elucidate sub-gap spectral line-shapes in organic semiconductors and their blends.
• To investigate the role of static and dynamic disorder in absorption characteristics.
• To provide a model for absorption line-shapes in disordered systems and determine excitonic disorder energy.

Main Methods:

• Temperature-dependent quantum efficiency measurements were employed.
• Analysis focused on sub-gap absorption spectral line-shapes.
• A convolution model of Gaussian density of states with Boltzmann-like transitions was utilized.

Main Results:

• Sub-gap absorption in organic semiconductors is universally dominated by thermal broadening at low photon energies.
• The Urbach energy was found to equal thermal energy, irrespective of static disorder.
• A model explaining absorption line-shapes and a strategy for determining excitonic disorder energy were presented.

Conclusions:

• The Urbach energy in molecular solids is primarily governed by thermal broadening, not static disorder.
• Photo-physical properties are linked to static disorder, crucial for organic solar cell optimization.
• A revisited radiative open-circuit voltage limit for organic solar cells was proposed.