A new family of symmetrical fluorene derivatives with different types of substituents attached to the C-2 and C-7 positions of the fluorene core synthesized by the Sonogashira coupling reactions is reported. The electronic structures and the properties of the compounds investigated by means of photoelectron emission spectroscopy, UV–Vis absorption and photoluminescent spectroscopy as well as by DFT and TD-DFT theoretical calculations are discussed. It is shown that the nature of substituents influences the π-conjugation of the molecules. No intermolecular charge transfer within the investigated wavelength range is observed. The applicability of the synthesized compounds in organic light-emitting diodes (OLEDs) based on exciplex emission is demonstrated. The advanced co-deposition technique with the tuned OLED architecture was applied and resulted in improved OLED parameters.

Organic light-emitting diodes (OLEDs) have emerged as one of the dominant technologies in displays due to their high emission efficiency and low power consumption. However, the development of blue color emitters has fallen behind that of red and green emitters, posing challenges in achieving optimal efficiency, stability, and accessibility. In this context, thermally activated delayed fluorescence (TADF) emitters hold promise as a potential solution for cost-effective, exceptionally efficient, and stable blue OLEDs due to their potential high efficiency and stability. TADF is a principle where certain organic materials can efficiently convert both singlet and triplet excitons, theoretically achieving up to 100% internal quantum efficiency. This research focused on diphenyl sulfone derivatives with carbazole groups as TADF compounds. Quantum chemical calculations and photoluminescence properties show the potential TADF properties of the molecules. New materials exhibit glass transition temperatures that would classify them as molecular glasses. Depending on the structure of the molecule, the photoluminescence emission is in the blue or green spectral region. Organic light-emitting diodes were fabricated from neat thin films of emitters by the wet casting method. The best performance in the deep blue emission region was achieved by a device with a turn-on voltage of 4 V and a maximum brightness of 178 cd/m². In the blue-green emission region, the best performance was observed by an OLED with a turn-on voltage of 3.5 V, reaching a maximum brightness of 660 cd/m².

The thermal behavior of a three-layer structure—glass/ITO/TAPC/CBP/BPhen—in an OLED system was investigated using in situ spectroscopic ellipsometry during controlled heating from room temperature to 120 °C over 60 min, simulating the ageing process and analyzing degradation kinetics. Variations in Ψ and Δ spectra were observed across the entire 0.7–5.9 eV spectral range, with five distinct anomalies, particularly in the UV region. An anomaly at approximately 66 °C was attributed to the glass transition temperature T_g of BPhen, while another two at around 82 °C and at around 112 °C corresponded to the first-order phase transition of TAPC and T_g of CBP, respectively. The origins of the remaining anomalies at 91 °C and 112 °C were explored in this study, with a focus on interphase layer formation and morphological changes that emerge during heating. These findings provide insights into the stability of OLEDs under thermal stress.

The thermal behavior of a three-layer structure – glass/ITO/TAPC/CBP/BPhen – in an OLED system was investigated using in situ spectroscopic ellipsometry during controlled heating from room temperature to 120°C over 60 minutes, simulating the ageing process and analysing degradation kinetics. Variations in Ψ and Δ spectra were observed across the entire 0.7-5.9 eV spectral range, with five distinct anomalies, particularly in the UV region. An anomaly at approximately 66°C is attributed to the glass transition temperature Tg of BPhen, while another two at around 82°C and at around 112°C correspond to the first-order phase transition of TAPC, and Tg of CBP, respectively. The origins of the remaining anomalies at 91°C and 112°C are explored in this study, with a focus on interphase layer formation and morphological changes that emerges during heating. These findings provide insights into the stability of OLEDs under thermal stress.

Spectroscopic ellipsometry is inherently model dependent, and in buried organic multilayer stacks, reduced optical sensitivity and strong parameter correlations can lead to non-unique solutions. The thermal evolution of an OLED-relevant ITO/PEDOT:PSS/TAPC/CBP/Ag stack is investigated using in situ spectroscopic ellipsometry complemented by ex situ Mueller-matrix measurements. Measurements from the glass-substrate side, including backside reflection, yield small Ψ–Δ amplitudes due to interplay between limited refractive-index contrast and organic layer thicknesses, allowing multiple optical models to fit the data with comparable statistical quality. Model ambiguity is resolved by applying physically constrained modeling based on independently determined room-temperature optical constants, substrate characterization, and the temperature-dependent optical response of PEDOT:PSS and TAPC established in Part I. Within this framework, the time- and temperature-dependent optical properties of the CBP layer and the CBP/Ag interphase are extracted during prolonged annealing at 50 °C, revealing relaxation-driven refractive-index changes and interfacial reorganization.