Triplet-to-Singlet energy transfer as a novel triplet harvesting mechanism: Photophysics and prototype OLED devices

This work is focused on the investigation of a new mechanism for triplet harvesting in organic molecules, which is based on the energy transfer between triplet and singlet electronic states of different moieties. Excited triplets are electronic states of multiplicity 1, which are not so enabled to contribute to the radiative process in organic molecules since the transition between the excited state and the ground state (a singlet state, with spin multiplicity 0) is spin-forbidden. Therefore, the lifetime of the triplet state is usually very long (μs or even ms time range) and triplets tend to lose their energy by non-radiative processes due to collisions with other molecules and vibrations. This fact is paramount for the performance of organic light emitting diodes (OLEDs). In the way OLEDs work, upon charge recombination, 75 % of the created excited states are dark triplets and only 25 % are emissive singlet states, hence the internal efficiency of the device is limited to 25 %. Mechanisms to harvest triplet states in organic materials are thus of great importance to improve device efficiencies. Herein, a perylene bisimide unit of strong luminescence yield is used as the acceptor (A) for energy transfer from two thiophene derivatives used as energy donors (D) in a D-A-D molecular structure. The donor units form triplets in high yield once excited. However, the strong overlap between the phosphorescence of the donor and the S0-S1 absorption of the perylene promotes fast energy transfer from the triplet state of the donor to the singlet state of the perylene unit, from where emission occurs. In this way, triplets are harvested and become able to contribute to the emission. This novel, and often ignored mechanism is investigated in detail using steady-state and time-resolved luminescence spectroscopy techniques and applied in prototype OLEDs