Case study on OLED modeling with ADF

Mr. Kento Mori of Ryoka (now MOLSIS), reselling ADF in Japan, have conducted a case study on fac-tris(2-phenylpyridine)iridium (Ir(ppy)3), bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium (FIrpic) and bis(2-benzo[b]thiophen-2-yl-pyridine)(acetylacetonate)iridium(III) (Ir(btp)2(acac)), molecules of great interest in the development of Organic Light Emitting Diodes (OLEDs). The results were presented at the 2009 autumn meeting of the Society of Computer Chemistry, Japan (SCCJ). Molecules that are useful for OLEDs are known for a high quantum yield of triplet to singlet phosphorescence in the visible region. These formally spin-forbidden transitions are facilitated through spin-orbit coupling.

table

Left figure: typical green, blue, and red emitting phosphorescent materials used in OLEDs. Right table: radiative rate constants kr by ZORA-TDDFT calculations with the B3LYP hybrid functional. The experimentally observed trend is correctly reproduced.

The electronic excitations modeled in the table above are a singlet – triplet transitions important to the production of the three colors necessary for the composition of flat screen displays. Two major relativistic effects present in these complexes can be modeled with ADF’s ZORA method. The first is the scalar relativistic effect tied to the fast moving electrons in the core region of heavy atoms. The second relativistic effect is spin-orbit (SO) coupling, a phenomenon without which formally spin forbidden singlet – triplet transitions can not occur. The experimental trends for both zero field splitting (ZFS) and the radiative rate constants (i.e. radiative phosphorescence lifetimes) of a whole range of red, green, blue emitters are well reproduced by spin-orbit coupled ZORA time-dependent density functional theory (ZORA-TDDFT) in ADF. ADF is the only program that uses SO self-consistently (i.e. nonperturbatively) during both the SCF and TDDFT parts of the computation. Dr. Erik van Lenthe of SCM, involved in implementing these methods in ADF, provided useful feedback to the Ryoka group during their study.

For those who would like to see the original 17-slide presentation file of the study in Japanese, contact support@moslis.co.jp. A more recent powerpoint study on modeling organic electronics with ADF, in particular OLEDs, but also OFETs and OPVs is available.

spin-orbit coupling, relativity (with ZORA), fluorescence phosphorescence, excited states (with TDDFT)

Key concepts