# Wet-Deposited TADF-Based OLED Active Layers: New Approaches towards Further Optimization
## 1 Introduction
As the demand for OLEDs increases, the industry needs to move towards a new deposition process, as the complex and expensive thermal evaporation does not appear to be a solution; moreover, it is not compatible with the everyday more important concept of printed electronics, where the fabrication process is simple, low-cost, and with high-output production [32]. However, despite the first results, which indicate the easy possibility of producing reliable OLEDs by solution depositions, the overall figures of merit are poor. Those wet deposition methods are prone to increasing structural defects in the organic layers, some of them electrically active with a strong influence on trapping/detrapping of the electrical carriers, with an impact on the electrical balance efficiency [33; 34]. On other hand, the simplification of the device structure was achieved. The current main efforts are focused on the deposition parameters/procedures to obtain more \"trap-free\" organic layers (best molecular conformation) and in the host: guest optimal composition to promote the best energy transfer process and exciton recombination. The final goal is to achieve the best trade-off between efficiency and a low-cost simple OLED structure. Among the several OLED solution-deposition processes, spin-coating is one of the most interesting for research development. The choice of spin-coating for the deposition relies on the technique's simplicity and well-known good film formation in small-area emitters [35]. Several works in recent years show that this objective is possible, although the best results depend on a more complex structure [36; 37; 38; 39; 40; 41; 42]. As reported in several of those works, we need to play (besides the organic materials) with the solvents, concentrations, and deposition parameters as the fundamental keys [43]. In recent years, [PERSON] and his co-workers [44; 45; 46; 47; 48] have carried out comprehensive studies about how to optimize electrical and morphological properties of solution-processable mixed-host layers for efficient TADF OLEDs based on simple architectures. These mixed-host layers are composed of poly(9-vinylcabazole) (PVK) (a well-known transparent p-type polymer) and an ambipolar, p-type or n-type semiconductive small molecule for obtaining a balance of charge-carrier and density of injected electrons and holes into the EML. The electrical and morphological optimization of such kinds of layers enablesthe achievement of highly efficient solution-processable TADF OLEDs (EQEs beyond 15%) [45; 46]. In particular, a PVK:OXD-7 (1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl]benzene) mixed host with 2 PXZ-OXD (2,5-bis(4-(10H-phenoxazin-10-yl)phenyl)-1,3,4-oxadiazol) green-emitter TADF guest enabled [PERSON]'s group to achieve EQE up to 10% in a unique organic layer OLED, the best result for this material in such a type of simple architecture [44]. Moreover, this TADF emitter, as the previous works have carried out (and shown in the related references cited here) is particularly complicated to be used as an emitter, as the interactions with the organic environment induce a complex energy transfer process that results in an emission loss. Therefore, it is a challenge to develop and optimize a solution-deposited OLED, in a simple structure, that uses this TADF as an emitter. Nonetheless, a comprehensive study about how different solvents with specific physical-chemical properties and interactions must be performed to better understand the local chemical environment's influence on the OLED's morphological, electrical, and optical features. In this sense, herein, we exploit a detailed study about how four different solvents for processing OLED's EML, composed of PVK, 1,3-Bis(N-carbazolyl)benzene (mCP) (ambipolar semiconducting small molecule) and 2 PXZ-OXD, change their optoelectronic properties in solution-processed OLEDs by spin-coating technique.