Content area
Full text
Keywords:
electroluminescence; light-emitting electrochemical cells; multiresonance; purely organic emitters; thermally activated delayed fluorescence
Abstract
We designed and synthesized two new ionic thermally activated delayed fluorescent (TADF) emitters that are charged analogues of a known multiresonant TADF (MR-TADF) compound. DiKTa. The emission of the charged derivatives is red-shifted compared to the parent compound. For instance. DiKTa-OBuIm emits in the green (λPL = 499 nm. 1 wt % in mCP) while DiKTa-DPA-OBuIm emits in the red (λPL = 577 nm. 1 wt % in mCP). In 1 wt % mCP films, both emitters showed good photoluminescence quantum yields of 71% and 61%. and delayed lifetimes of 316.6 ps and 241.7 ps. respectively, for DiKTa-OBuIm and OBuIm. leading to intersystem crossing rates of 2.85 × 103s-1 and 3.04 × 103s-1. Light-emitting electrochemical cells were prepared using both DiKTa-OBuIm and DiKTa-DPA-OBuIm as active emitters showing green (λmax = 534 nm) and red (λmax = 656 nm) emission, respectively.
Introduction
Light-emitting electrochemical cells (LEECs) are thin film light-emitting devices typically consisting of an emissive layer containing ionic species that facilitate charge transport and an emissive semiconductor material. The emissive layer is sandwiched between two air-stable electrodes [1]. Upon application of an external bias the ions in the active layer migrate to the corresponding electrodes, resulting in the formation of electrical double layers (EDLs) at the interface of the electrodes. The EDLs facilitate charge injection into the emissive layer regardless of the energy levels of the electroactive species and work function of the electrodes. Injection of electrons and holes creates oxidized and reduced species near the anode and cathode, respectively. These oxidized and reduced species are stabilized by the ions to form a p-i-n junction in the bulk of the emissive layer and emission takes place within the intrinsic region [2-6].
Two families of widely investigated emitters for LEECs are ionic transition metal complexes (iTMCs) [7-10] and conjugated polymers (CPs) [4]. From the early use of ruthenium(II) complexes, a significant amount of research has focussed on developing high-performance iTMC-based LEECs [11,12], with iridium(III) complexes typically showing the greatest potential. A detracting feature of many iTMC LEECs is the use of scarce noble metal complexes. Despite the enormous number of...