Open-Shell Guest Acceptors Manipulate Charge-Transfer Spin Dynamics in Organic Solar Cells

Area of Science:

• Organic electronics
• Photovoltaics
• Materials science

Background:

• Non-radiative energy loss in organic solar cells (OSCs) is linked to triplet-mediated recombination involving triplet charge-transfer (³CT) states.
• Traditional methods focus on energetic engineering to suppress this pathway.

Purpose of the Study:

• To explore a spin-manipulation strategy using intrinsically open-shell non-fullerene acceptors (NFAs) as spin-active guest acceptors.
• To reduce non-radiative recombination losses by manipulating charge-transfer (CT) state spin dynamics.

Main Methods:

• Incorporation of two (thio)barbituric acid-terminated open-shell NFAs (MAZ-1 and MAZ-2) into D18:N3 solar cells.
• Characterization using electron spin resonance (ESR) and variable-temperature ¹H NMR to confirm open-shell character and triplet states.
• Analysis of magneto-photocurrent responses and charge-transfer state decay dynamics.

Main Results:

• MAZ incorporation modulated CT-state spin evolution and suppressed triplet-related recombination.
• Weakened high-field magneto-photocurrent responses and accelerated decay of long-lived CT states were observed.
• Non-radiative energy loss was reduced by up to ~20 meV, leading to improved charge transport and morphology.

Conclusions:

• Open-shell guest acceptors provide an intrinsic spin-active platform for manipulating CT-state spin dynamics.
• This strategy effectively reduces non-radiative recombination losses in OSCs.
• Achieved a power conversion efficiency increase from 18.52% to 20.23%.