Polaron-mediated exciton dynamics of P(NDI2OD-T2) unveiled by transient absorption spectroscopy under electrochemical conditions

Abstract

Granted by the suitable characteristics of its electronic band structure, high electron mobility, and remarkable durability, P(NDI2OD-T2) stands out among conjugated polymers and is regarded as a prominent candidate for electron accepting materials in the third generation of photoelectrochemical and photovoltaic devices. However, a comprehensive understanding of exciton–polaron interactions, which provides the key to optimizing the efficiency of these devices, is lacking. In this study, we fabricated P(NDI2OD-T2) into a film working electrode of an electrochemical cell and changed its surrounding electrochemical environments by exerting different electric biases. Transient absorption (TA) spectroscopy was employed synchronously to interrogate the corresponding exciton dynamics. Careful inspection of the TA spectra of P(NDI2OD-T2) under different electrochemical conditions gives detailed insights into the interactions between excitons and polarons. Our results highlight that polarons have a significant influence on exciton behavior. Merely applying cathodic biases without efficiently doping the polymer yields excited-state dynamics similar to that in the neutral state. However, in the presence of polarons, the lifetime of excitons is significantly reduced by ∼5 times. Through analyzing the TA spectra and kinetics of P(NDI2OD-T2) at −1.0 and −1.2 V (vs Ag/Ag+ reference electrode), an efficient exciton–polaron quenching effect was identified. Furthermore, we report the absorption features of the excited states of polaron, i.e., charged excitons (trions) in P(NDI2OD-T2). We propose that bipolarons and such charged excitons may have similar electronic structures as they manifest resemblance in their absorption features. Our strategy of integrating ultrafast spectroscopy and electrochemistry to investigate the exciton–polaron interactions in polymers provides an effective methodology in research fields of photoelectrochemistry and photovoltaics, more broadly applicable beyond soft materials.