Pulsed excitation method provides path forward for tracing triplet excitons in OLEDs

Due to their potential for high-efficiency optoelectronic devices, the behavior of organic semiconductors has garnered increasing interest. Understanding the transport of excitons by diffusion of singlets and triplets is essential for developing efficient optoelectronic devices. Measuring the characteristically nonemissive triplets, however, continues to challenge the field.

Banappanavar et al. introduced a technique to measure exciton transport in the transverse direction of polymeric semiconductor thin films. By using pulsed excitation to measure angle-resolved delayed electroluminescence, the group’s method traces the position of the triplet exciton in the emissive layer of OLEDs.

“The triplet excitons are known to be very localized species in the organic polymers due to their parallel spin configuration and the mutual charge exchange between the molecules,” said co-author Dinesh Kabra. “The outcome of the work suggests that, in the case of well polymer chain packing, the triplet transport can be as good as the crystalline organic materials.”

Organic semiconductors exhibit singlet and triplet kinetics that share an important role in determining the performance of various optoelectronic devices.

The polyfluorene system exhibits an efficient triplet-triplet fusion process, which provides singlet excitons as delayed fluorescence for studying triplet kinetics.

Results suggest the diffusion is significantly anisotropic in thinner films. As the thickness of the film increases, anisotropy reduces in triplet transport.

The group found in thicker films, diffusivity approaches close to ultrahigh 0.001 square centimeters per second, nearing that of similarly based crystalline organic thin films. These findings resolve the decade old mystery of unusually thick emissive layer based efficient OLEDs.

The group hopes their findings stoke further interest in triplet behavior and looks to use its technique for singlet excitons and other class of semiconductors.

Source: “Novel optoelectronic technique for direct tracking of ultrafast triplet excitons in polymeric semiconductor,” by Gangadhar Banappanavar, Sumukh Vaidya, Urvashi Bothra, Lohitha R. Hegde, Kamendra Prakash Sharma, Richard H. Friend, and Dinesh Kabra, Applied Physics Reviews (2021). The article can be accessed at https://doi.org/10.1063/5.0054583 .