Missing Excitons: How Energy Transfer Competes with Free Charge Generation in Dilute-Donor/Acceptor Systems

Area of Science:

• Organic photovoltaics
• Photochemistry
• Energy transfer dynamics

Background:

• Energy transfer is crucial for charge generation in organic photovoltaics.
• Typically, donor-acceptor energy transfer enhances device performance.
• However, specific molecular arrangements can lead to detrimental energy transfer pathways.

Purpose of the Study:

• To investigate a scenario in organic photovoltaics where excitation energy transfer (EET) suppresses free charge generation.
• To elucidate the competition between charge generation and EET in dilute donor-acceptor systems.
• To determine the role of photochemical driving force in optimizing charge yield.

Main Methods:

• Time-resolved microwave conductivity (TRMC) to detect free charge generation.
• Transient absorption spectroscopy to study exciton dynamics.
• Analysis of energy transfer pathways and charge-transfer state formation.

Main Results:

• Dilute donor molecules in an acceptor host exhibit ultrafast excitation energy transfer (EET).
• This EET suppresses the yield of free charge carriers.
• An optimal photochemical driving force for free charge generation was observed, but with low yield.
• Transferred excitons efficiently form charge-transfer states, competing with free charge generation.

Conclusions:

• Excitation energy transfer to the host matrix can be detrimental to organic photovoltaic performance.
• A competition exists between long-range electron transfer (producing free charge) and EET (producing localized charge-transfer states).
• The findings challenge models where localized charge-transfer states are solely intermediates between excitons and free charge.