Bipolar polaron pair recombination in P3HT/PCBM solar cells

TL;DR

This study uses advanced magnetic resonance techniques to reveal that bipolar polaron pair recombination, rather than unipolar bipolaron formation, dominates spin-dependent charge transfer in P3HT/PCBM solar cells at low temperatures, providing new insights into organic semiconductor spin dynamics.

Contribution

The paper demonstrates that multi-frequency pEDMR can effectively analyze spin-dependent processes in organic semiconductors, specifically identifying bipolar polaron pair recombination as the key mechanism in P3HT/PCBM solar cells.

Findings

Bipolar polaron pair recombination governs spin-dependent response at low temperatures.

Recombination times differ for parallel and antiparallel spins.

Spectral distinction of polarons enhances analysis accuracy.

Abstract

The unique properties of organic semiconductors make them versatile base materials for many applications ranging from light emitting diodes to transistors. The low spin-orbit coupling typical for carbon-based materials and the resulting long spin lifetimes give rise to a large influence of the electron spin on charge transport which can be exploited in spintronic devices or to improve solar cell efficiencies. Magnetic resonance techniques are particularly helpful to elucidate the microscopic structure of paramagnetic states in semiconductors as well as the transport processes they are involved in. However, in organic devices the nature of the dominant spin-dependent processes is still subject to considerable debate. Using multi-frequency pulsed electrically detected magnetic resonance (pEDMR), we show that the spin-dependent response of P3HT/PCBM solar cells at low temperatures is governed by bipolar polaron pair recombination involving the positive and negative polarons in P3HT and PCBM, respectively, thus excluding a unipolar bipolaron formation as the main contribution to the spin-dependent charge transfer in this temperature regime. Moreover the polaron-polaron coupling strength and the recombination times of polaron pairs with parallel and antiparallel spins are determined. Our results demonstrate that the pEDMR pulse sequences recently developed for inorganic semiconductor devices can very successfully be transferred to the study of spin and charge transport in organic semiconductors, in particular when the different polarons can be distinguished spectrally.