Optically and electrically excited intermediate electronic states in donor:acceptor based OLEDs

Abstract

Thermally activated delayed fluorescence (TADF) emitters consisting of donor and acceptor molecules are potentially highly interesting for electroluminescence (EL) applications. Their strong fluorescence emission is considered to be due to reverse intersystem crossing (RISC), in which energetically close triplet and singlet charge transfer (CT) states, also called exciplex states, are involved. In order to distinguish between different mechanisms and excited states involved, temperature-dependent spin-sensitive measurements on organic light-emitting diodes (OLEDs) and thin films are essential. In our work we apply continuous wave (cw) and time-resolved (tr) photoluminescence (PL) spectroscopy as well as spin-sensitive electroluminescence and PL detected magnetic resonance to films and OLED devices made of three different donor:acceptor combinations. Our results clearly show that triplet exciplex states are formed and contribute to delayed fluorescence (DF) via RISC in both electrically driven OLEDs and optically excited films. In the same sample set we also found molecular triplet excitons, which occurred only in PL experiments under optical excitation and for some material systems only at low temperatures. We conclude that in all investigated molecular systems exciplex states formed at the donor:acceptor interface are responsible for thermally activated DF in OLEDs with distinct activation energies. Molecular (local) triplet exciton states are also detectable, but only under optical excitation, while they are not found in OLEDs when excited states are generated electrically. We believe that the weakly bound emissive exciplex states and the strongly bound non-emissive molecular triplet excited states coexist in the TADF emitters, and it is imperative to distinguish between optical and electrical generation paths as they may involve different intermediate excited states.

The role of spin in the degradation of organic photovoltaics

“…The situation is very different at 77 K, where the S 1 lifetime increases ( k HT decreases) for all blends but significantly more for α-6T (122 ps) and TAPC (171 ps) dilute blends than for m-MTDATA (61 ps). This means higher ISC yields are expected for α-6T and TAPC than m-MTDATA at 77 K. However, the difference in k HT is not sufficiently strong to explain the lack of ISC triplets in m-MTDATA TREPR experiments, from which we conclude that efficient HT from triplets to CTs also occurs when E CT < E T1 76 (illustrated in Fig. 3b ).…”

Spin‐ and Voltage‐Dependent Emission from Intra‐ and Intermolecular TADF OLEDs

“…These measurements elucidate properties of triplet states which are involved in the light generation mechanisms of the devices and the temperature dependence reveals the TADF activation energy. [ 19,22 ] The idea of ELDMR experiments is to probe EL, while applying a static magnetic field B that induces a Zeeman splitting of triplet states. By applying resonant microwaves, the following resonance condition is fulfilled.…”

“…Meanwhile, it is still an important point of discussion how the spin degree of freedom influences the first order forbidden RISC rate in operational TADF OLEDs. Since pCNBCzoCF 3 shows both intra‐ and intermolecular CT states, it is an interesting model system to employ inherently spin sensitive methods that just recently were applied for the first time to donor:acceptor based intermolecular TADF OLEDs, [ 19,20 ] but not yet to intra‐molecular emitters or their combination. In this study, we investigate the TADF characteristics of the building blocks of warm white OLEDs based on the intramolecular CT emission from pristine pCNBCzoCF 3 and devices based on emission from exciplex states formed between pCNBCzoCF 3 and m‐MTDATA.…”

Effect of Bulky Functional Groups on Donor and Acceptor Interactions and their Photoluminescence Properties

“…[13,14] Intrinsically π deficient conjugated systems along with electron rich molecules are often used in organic photovoltaics [15,16] and organic light emitting diodes that made use of thermally activated delayed fluorescence molecules (TADF). [17,18] Both are important classes of photoactive materials. However, whether such interactions can modify the optical properties of the supramolecules are not well studied.…”

“…Some heteroaromatic molecules are intrinsically π deficient such as 1,3,5‐triazine which is speculated to give rise to lone‐pair‐π interactions with water molecules and amine [12] and formation of new supramolecular networks and crystal structures [13,14] . Intrinsically π deficient conjugated systems along with electron rich molecules are often used in organic photovoltaics [15,16] and organic light emitting diodes that made use of thermally activated delayed fluorescence molecules (TADF) [17,18] . Both are important classes of photoactive materials.…”