A New Framework for Understanding Recombination-Limited Charge Extraction in Disordered Semiconductors

Area of Science:

• Organic electronics
• Photovoltaics
• Charge transport phenomena

Background:

• Recombination of free charges is a primary performance limitation in organic semiconductor devices.
• Transient charge extraction (CE) experiments are commonly used to determine recombination kinetics.
• Existing CE methods often overlook recombination during the extraction process, particularly in low-mobility materials.

Purpose of the Study:

• To investigate the validity of transient charge extraction (CE) experiments in low-mobility organic semiconductor devices.
• To quantify the impact of recombination during extraction on measured recombination rate constants.
• To develop and validate a new analytical model for accurate determination of recombination kinetics.

Main Methods:

• Transient drift-diffusion simulations were employed to model charge carrier behavior.
• Numerical simulations were used to validate the proposed analytical model.
• The analytical model was applied to correct experimental data for bimolecular recombination rate constants.

Main Results:

• Recombination during transient extraction leads to incomplete charge extraction (CE).
• This incomplete CE results in apparent carrier density-dependent recombination rate constants and overestimated recombination orders.
• The observed effects are dependent on charge carrier mobilities and the device's resistance-capacitance time constant.

Conclusions:

• The standard transient charge extraction (CE) experiment is not fully valid for low-mobility organic devices due to recombination effects.
• An analytical model accounting for recombination during extraction provides accurate bimolecular recombination rate constants.
• This work offers a method to correct experimental data and improve the understanding of recombination in organic photovoltaics.