Electronic properties and morphologies of thin films of organic molecules obtained by combinatorial deposition from gaseous phase

Postoji veliki interes za organske materijale zbog njihove primene u različitim organskim elektronskim uređajima. Međutim stabilnost oragnskih materijala ograničava njihovu primenu. Oni su podložni degradaciji ne samo za vreme operacije uređaja već i za vreme skladištenja. Jedan od glavnih uzroka je degradacija usled spoljnih uticaja: svetlosti, vlage i kiseonika. Ovaj problem može da se prevaziđe putem enkapsulacije uređaja, ali nijedan vid enkapsulacije nije savršen. U prvom delu rada biće izložena studija degradacije tankih filmova N,N′- bis(3-methylphenyl)-N,N′-bis(phenyl)benzidine (TPD) i 4,4′-bis(2,2-diphenylvinyl)- 1,1′-biphenyl (DPVBi) pod uticajem UV zračenja u vazduhu. Filmovi oba materijala su stabilni u vakuumu, ali degradiraju u prisustvu kiseonika. Dakle, nepohodan uslov za degradaciju, potrebno je istovremeno prisustvo UV svetlosti i kiseonika. Hemijska analiza osvetljenih filmova izvršena uz pomoć NMR, masene i infracrvene spektroskopije pokazala je prisustvo oksidovanih vrsta (nečistoća). Nečistoće su odgovorne za povećanu morfološku stabilnost osvetljenih filmova i gašenje fotoluminescencije. Mala količina nečistoća, 0.4 % (0.2 %) u slučaju TPD (DPVBi), izaziva pad intenziteta fotoluminescencije od 50 %. Ovo implicira netrivijalni mehanizam gašenja fotoluminescencije. Za oba molekula je nađeno da je rastojanja između nečistoća manje ili približno jednako dužini difuzije ekscitona što je neophodan uslov za gašenje fotoluminescencije. Predložen je mehanizam gašenja: ekscitoni difunduju u skokovima od jednog do drugog molekula DPVBi (TPD) slučajnim hodom putem Forsterovog rezonatnog transfera energije. Ako u toku svog vremena života eksciton dođe u blizinu nečistoće dolazi do Deksterovog transfera energije i gašenja fotoluminescencije. Rezultati studije za DPVBi molekul su važni zato što pokazuju da i mala količina kiseonika u sloju DPVBi može ozbiljno da utiče na fotoluminescentnu efikasnost uređaja. Štaviše, apsorpcija sopstvenog zračenja (kod oba molekula, DPVBi i TPD) će dodatno da doprinese brzini degradacije uređaja. Razumno je pretpostaviti da će i transportne osobine biti narušene u slučaju kada se ovi materijali koriste kao sloj za transport šupljina u organskim svetlećim diodama.There is an ongoing interest in organic materials due to their application in various organic electronic devices. However stability of organic materials limits their potential use. They are prone to degradation both during the working life and storage. One of the main causes is extrinsic degradation, under the influence of oxygen and moisture. This problem can be solved by encapsulation of devices. However no encapsulation is perfect. In the first part of this work a study of degradation of thin films of N,N′-bis(3- methylphenyl)-N,N′-bis(phenyl)benzidine (TPD) and 4,4′-bis(2,2-diphenylvinyl)-1,1′-biphenyl (DPVBi) under UV irradiation in air is given. Films of both materials are stable in vacuum, but readily degrade in the presence of oxygen. Thus, the necessary condition for degradation is the simultaneous presence of UV light and oxygen. Chemical analysis of irradiated films by NMR, mass and infrared spectroscopy revealed presence of oxidized species (impurities). These impurities are responsible for increased morphological stability of irradiated films and quenching of photoluminescence. Only small amount of impurities, 0.4 % (0.2 %) for TPD (DPVBi), causes 50 % decrease of photoluminescence. This implies a non-trivial mechanism of quenching. For both molecules it was found that distance between impurities is smaller or equal to exciton diffusion length, which is the necessary condition for quenching. Following mechanism of quenching is proposed: exciton diffuses by hopping form one DPVBi (TPD) to another through FRET in a random walk manner. If, during its lifetime, it comes to proximity of an impurity, a Dexter-type energy transfer occurs and PL is quenched. Findings of DPVBi study are important because they show that even a small amount of oxygen that penetrates a DPVBi layer would impair luminescence efficiency of a device. Moreover, the absorption of own radiation (for DPVBi and TPD both) would additionally contribute to the rate of degradation of a device. It is reasonable to expect that transport properties would also be affected when materials are used as a holetransporting layer in OLEDs