Efficient charge generation by relaxed charge-transfer states at organic interfaces

Area of Science:

• Organic electronics
• Photovoltaics
• Materials science

Background:

• Organic solar cells rely on interfaces between electron-donating (D) and electron-accepting (A) materials to generate charge carriers upon illumination.
• Efficient organic solar cells necessitate a high yield of charge carrier generation and minimal energy loss.
• The interfacial charge-transfer state (CT1) plays a crucial role in the charge generation process.

Purpose of the Study:

• To investigate the role of the lowest energy emissive interfacial charge-transfer state (CT1) in the charge generation process.
• To determine the quantum yield and electric field dependence of charge generation upon excitation of the charge-transfer (CT) state manifold.
• To assess the impact of exciting higher energy D, A, or CT states on internal quantum efficiency (IQE).

Main Methods:

• Measuring the quantum yield of charge generation.
• Analyzing the electric field dependence of charge generation.
• Exciting the charge-transfer (CT) state manifold via weakly allowed, low-energy optical transitions.

Main Results:

• Internal quantum efficiency (IQE) was found to be largely independent of the excitation energy of D, A, or CT states above CT1.
• High IQE, exceeding 90%, was observed in the best performing material systems.
• Efficient charge generation was achieved without requiring excess electronic or vibrational energy.

Conclusions:

• The lowest energy emissive interfacial charge-transfer state (CT1) is a key factor in efficient charge generation in organic photovoltaics.
• Optimized organic photovoltaic devices can achieve high internal quantum efficiency irrespective of the initial excitation energy.
• The findings suggest that excess energy is not a prerequisite for high charge generation yields in state-of-the-art organic solar cells.