# Wet-Deposited TADF-Based OLED Active Layers: New Approaches towards Further Optimization
## 1 Introduction  
Organic light-emitting diodes (OLEDs) play an important role in display technology nowadays and have started to become an important field of research for lighting. Since [PERSON]'s and [PERSON]'s pioneer work [1] in a thermal evaporated double-organic-layer device, composed of tris(8-hydroxyquinolate)aluminum(III) (electron transport and emissive layer--ETL and EML, respectively) and an aromatic diamine (hole transport layer--HTL), fast growth of the academical and industrial research has been noted due to the possibility of obtaining a practical and feasible device [2]. OLED research can be divided into five main technologies: first--fluorescent emitters [3]; second--phosphorescent emitters [4]; third--thermally activated delayed fluorescent (TADF) emitters [5; 6; 7]; fourth--hyperfluorescent emitters [8]; and, most recently, fifth--usage of nonresonant energy transfer process to pump up OLEDs efficiency [9; 10]. Also, solution-processable protocols (i.e., spin-coating [11], blade coating [12], inkjet [13], screen printing [12], and slot-die coating [14]) have been adopted to achieve easy up-scaling devices, coupled with urgent cost-, energy-, and material-saving demands [15]. In particular, TADF dyes have played an important role in highly efficient OLEDs' emissive layers (EMLs) due to their ability to ease optoelectronic feature tunability via simple chemical structure modifications [16; 17]. Such an advantage enables the achievement of external quantum efficiencies (EQEs) up to the theoretical limit for a simple architecture device, and in a wide gamma of emitting colors at the entire visible spectrum [18; 19; 20; 21; 22]. As usual, TADF molecules are composed of an electron donor (D) and acceptor (A) moietieschemically bonded, which is capable of creating a charge-transfer (CT) state, and, with an orthogonal orientation between D and A parts, it can create a small energy barrier between the first excited singlet and triplet states (\(\Delta\)E\({}_{\text{ST}}\approx 0\)) [23]. When these organic dyes have those characteristics, they can undergo a thermally activated reverse inter-system crossing (rISC), enabling TADF emitters to theoretically achieve internal quantum efficiencies (IQE) of 100%, according to the singlet/triplet spin statistics [23; 24]. However, TADF molecules present strong triplet-triplet annihilation (TTA) pathways and sensitivity to the local chemical environment (solvatochromism) due to their active triplet states involved in the delayed fluorescence electronic structure reorganization [25; 26; 27; 28; 29]. Those characteristics imply that the use of TADF molecules for OLED's EML can only be made in a host: guest system at small concentrations, diminishing emitting losses via a nonradiative recombination process. Globally, a compromise between an efficient TADF process and the final device figures of merit must be considered for their emitting diodes [30; 31]. The OLED fabrication techniques were almost based on thermal evaporation methods as they can, in a well-controlled process, give highly efficient and reliable devices (due to the best molecular conformation of the deposited layers, specifically those containing small molecules). The focus is, naturally, displays. To achieve an optimum electrical charge balance and confine the exciton recombination region in the emissive layer, a complex OLED structure needs to be made. Additionally, for efficient emitters (transition metals complexes and TADFs), the abovementioned emission quenching implies the use of a co-evaporation process to achieve the host: guest concept, avoiding quenching and promoting an efficient energy transfer from host to guest (emitter). In the end, efficient OLEDs made by thermal evaporation process are particularly expensive and the process is prone to faults as the masks can be minimally misaligned. Despite such issues, thermal evaporated OLEDs are still the industry standard for displays. A completely different framework has to be created towards a low-cost fabrication technique.