tailieunhanh - Organic Light Emitting Diode Part 2

Tham khảo tài liệu 'organic light emitting diode part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Organic light emitting diodes based on functionalized oligothiophenes for display and lighting applications 13 in which electronic de-excitation via intermolecular interactions and internal conversion processes - which are the most important non radiative relaxation channels in oligothiophene-S S-dioxides Lanzani et al. 2001 Della Sala et al. 2003 Anni et al. 2005 -are strongly reduced. Functionalization with the bulky cyclohexyl groups has several effects. First the large intermolecular distances due to the bulky substituents reduce the intermolecular interactions. Second as shown by DFT ground state and TD-DFT excited state molecular geometry optimizations Mazzeo et al. 2003 a the molecular distortion is increased both in the ground and in the excited state. In the first singlet excited state the thiophene branches lie in two different planes making the formation of non radiative aggregates unlikely. Third the flexibility of the branches is strongly reduced. The calculations show for example that while compounds 8 and 9 are very flexible and can exist in different conformations of similar energy for compound 11 only one ground-state energy minimum is found. Thus the cyclohexyl substituents stabilize the conformation and make the molecule more rigid. All factors lead to enhanced photoluminescence in the solid state. The luminance of 10500 cd m2 reached with the device based on compound 11 is one of the highest values reported so far in the literature for devices with spin coated active layers. Recently OLEDs using as emitting layers nicely engineered branched oligomers compounds 12-13 containing a dibenzothiophene-S S-dioxide core and triarylamine branches have been reported Huang et al. 2006 . The thiophene-S S-dioxide group was introduced for its beneficial effect on the electron affinity of the molecules while the triaryl amino groups were introduced because of their beneficial effect on charge holes transport and film forming properties. The molecular .