Time-Resolved EPR Study on Photoinduced Charge-Transfer Trap State in Thiophene-Thiazolothiazole Copolymer Film

Abstract

To shed a light on fundamental optoelectronic properties of conjugated polymer films applicable to the organic photovoltaics (OPV), field-effect transistors (FET) and light-emitting diodes (LED), we have characterized interspin separation and exchange coupling of interchain charge-transfer (CT) states in a pristine film of thiophene-thiazolothiazole copolymer by using the time-resolved EPR method. It has been indicated that the CT state is generated at the disordered regions of the polymer films as deep trap sites via the singlet−exciton diffusion in polymer amorphous phase. These characteristics of the trapped charges may limit the device performances in the OPV, FET and LED applications and thus are informative for the device developments.

Side-Chain Engineering of Conjugated Polymers: Distinguishing Its Impact on Film Morphology and Electronic Structure

“…Due to it appearing in emission, it is definitely spin-polarized, hence with the energy levels populated far from the Boltzmann equilibrium. It is very narrow, comparable to spin-correlated radical pairs observed in both biological systems , and materials used in organic electronics. − And last but not least, the time profiles have a similar behavior to those of the triplet states (for full two-dimensional (2D) datasets, see the SI), making a species with the same spin multiplicity highly likely. , A possible interpretation is, therefore, the presence of two separated but still Coulombically bound charges, resulting from short-lived charge separation within the pristine polymer.…”

“…A few apparent characteristics of the small emissive signal seen at the center of (some of) the triplet signals of PCDTBT shall be discussed, shaping the ground for further investigations. Generally, such features have been observed in other polymers as well. ,, Possible origins are radical–triplet pairs between a stable radical, e.g., from a defect and a light-induced triplet state, as well as paramagnetic states with higher spin states, such as quintets arising from interacting triplet states. , For a more detailed discussion of light-induced paramagnetic species with higher spin multiplicity see ref .…”

“…Generally, such features have been observed in other polymers as well. 53,63,64 Possible origins are radical− triplet pairs between a stable radical, e.g., from a defect and a light-induced triplet state, as well as paramagnetic states with higher spin states, such as quintets arising from interacting triplet states. 54,55 For a more detailed discussion of lightinduced paramagnetic species with higher spin multiplicity see ref 42.…”