Excited state energy surfaces of flexible emitters for thermally activated delayed fluorescence

Organic Light-Emitting Diodes (OLEDs) technology has matured over recent years, reaching the commercialization level and being used in various applications. The required efficiency can be achieved by transforming triplet excitons into singlet states via Reverse InterSystem Crossing (RISC), which a general mechanism called thermally activated delayed fluorescence (TADF). Two prototypical molecules in the field, 2CzBN and 4CzBN, Carbazole Benzonitrile (donor-acceptor) derivatives, possess similar energy gap between singlet and triplet (∆EST, a key parameter in the RISC rate), but different TADF performance. In this sense, other parameter must be considered to explain these different behaviors. In this work, we theoretically investigate 2CzBN and 4CzBN and address the problem of how flexible donor-acceptor (D-A) or donor-acceptor-donor (D-A-D) molecular architectures affect the nature of excited state, and the oscillator strength. Furthermore, we analyze the RISC rates as a function of the conformation of the carbazole side groups, considering the S0, S1, T1 and T2 states. The oscillator strength of 4CzBN is higher than of 2CzBN, which, in turn, is almost vanishing, resulting in only 4CzBN being a TADF active molecule. We also note the presence of a second triplet state T2 lower in energy than S1, and that the reorganization energies, associated to the RISC processes involving T1 and T2, are both important factor in differentiating the rates in 2CzBN and 4CzBN. However, the 4CzBN RISC rate from T2 to S1 is surprisingly high with respect to the one from T1 to S1, although, according to EL-Sayed rules, since T2 (CT/LE) is more similar to S1 (CT) than in 2CzBN (LE, CT), this transition should be less favored. These insights are important to understand the photophysics of the TADF process and to design novel TADF emitters based on the benzo-carbazole architecture.

Computational characterization of carbazole-benzonitrile derivatives for applications in Organic Light Emitting Diodes

The technology of Organic Light-Emitting Diodes has reached such a high level of reliability that it can be used in various applications. The required light emission efficiency can be achieved by transforming the triplet excitons into singlet states through Reverse InterSystem Crossing (RISC), which is the main process of a general mechanism called thermally activated delayed fluorescence (TADF). In this thesis, we theoretically analyzed two carbazole-benzonitrile (donor-acceptor) derivatives, 2,5-di(9H-carbazol-9-yl)benzonitrile (p-2CzBN) and 2,3,4,5,6-penta(9H-carbazol-9-yl)benzonitrile (5CzBN), and addressed the problem of how donor-acceptor (D-A) or donor-acceptor-donor (D-A-D) flexible molecular architectures influence the nature of the excited states and the emission intensity. Furthermore, we analyzed the RISC rates as a function of the conformation of the carbazole lateral groups, considering the first electronic states, S0, S1, T1 and T2, involved in TADF process. The two prototype molecules, p-2CzBN and 5CzBN, have a similar energy gap between the first singlet and triplet states (∆EST, a key parameter in the RISC rate), but different TADF performances. Therefore, other parameters must be considered to explain their different behavior. The oscillator strength of p-2CzBN, never tested as emitter in OLEDs, is similar to that of 5CzBN, which is an active TADF molecule. We also note that the presence of a second T2 triplet state, lower in energy than S1 only in 5CzBN, and the reorganization energies, associated with RISC processes involving T1 and T2, are important factors in differentiating the rates in p-2CzBN and 5CzBN. For p-2CzBN, the RISC rate from T2 to S1 is surprisingly higher than that from T1 to S1, in disagreement with El-Sayed rules, due to a large reorganization energy associated to the T1 to S1, process; while the contrary occurs for 5CzBN. These insights are important for designing new TADF emitters based on the benzo-carbazole architecture.

Deposizione elettrochimica di film polimerici foto-attivi per la realizzazione di celle solari

Plastic solar cells bear the potential for large-scale power generation based on flexible, lightweight, inexpensive materials. Since the discovery of the photo-induced electron transfer from a conjugated polymer (electron-donor) to fullerene or its derivatives molecules (electron-acceptors), followed by the introduction of the bulk heterojunction concept which means donors and acceptors blended together to realize the fotoactive layer, materials and deposition techniques have been extensively studied. In this work, electrochemical-deposition methods of polymeric conductive films were studied in order to realize bulk heterojunction solar cells. Indium Tin Oxide (ITO) glass electrodes modified with a thin layer of poly(3,4-ethylenedioxythiophene) (PEDOT) were electrochemically prepared under potentiodynamic and potentiostatic conditions; then those techniques were applied for the electrochemical co-deposition of donor and acceptor on modified ITO electrode to produce the active layer (blend). For the deposition of the electron-donor polymer the electropolymerization of many functionalized thiophene monomers was investigated while, as regards acceptors, fullerene was used first, then the study was focused on its derivative PCBM ([6,6]-phenyl-C61-butyric acid methyl ester). The polymeric films obtained (PEDOT and blend) were electrochemically and spectrophotometrically characterized and the film thicknesses were evaluated by atomic force microscopy (AFM). Finally, to check the performances and the efficiency of the realized solar cells, tests were carried out under standard conditions. Nowadays bulk heterojunction solar cells are still poorly efficient to be competitively commercialized. A challenge will be to find new materials and better deposition techniques in order to obtain better performances. The research has led to several breakthroughs in efficiency, with a power conversion efficiency approaching 5 %. The efficiency of the solar cells produced in this work is even lower (lower than 1 %). Despite all, solar cells of this type are interesting and may represent a cheaper and easier alternative to traditional silicon-based solar panels.