All-Conjugated diblock copolymer approach to improve single layer green electroluminescent devices

To improve both the stability and the efficiency of single spin-coatable active-layer polymer light emitting devices based on fluorene (F) and benzothidiazole (BT), that is, two of the monomers employed as building units in the most efficient electroluminescent polymers commonly used in organic electronics, we carry out a dual strategy. We prepare a triphenylamino (TPA) disubstituted F in order to improve hole injection, prevent oxidation and aggregation and we exploit the mechanism of resonant energy transfer through the macromolecular approach of a rod-rod-type all conjugated diblock copolymer formed by an alternated copolymer of TPA disubstituted F with dialkylsubstituted F (poly[(9,9-di-n-hexylfluorene)-alt-(9,9-bis(4-diphenylaminophenyl)-fluorene]) and an alternated copolymer between BT and F (poly(9,90-dioctylfluorene-alt-benzothiadiazole). We show that, by changing the relative length of the two blocks, we are able to address the electrooptical properties thanks to a different supramolacular organization, which can be governed by a proper chemical design, and the energy transfer mechanism tunability. The device performance and color coordinates are controlled by both the suppression of oxidation processes and the high thermal stability of the copolymers which allow stable green HDTV standard-matching electroluminescence to be obtained at ambient conditions for several hours. Two different copolymers are studied: a diblock amorphous copolymer and a more crystalline one.Afurther increase of the external quantum efficiency up to 5.5%, a luminous efficiency of 22.5 cd/A with a luminance of above 50 000 cd/m2, is obtained by the improvement of the cathode/polymer interfacial adhesion in the former copolymer upon thermal treatment above the glass transition temperature of PF8BT. On the contrary, the same treatment on the latter compound is detrimental (charge carrier unbalancing) due to a further increase of the crystallinity. The insertion of a poly(N-vinylcarbazole) interlayer prevents exciton quenching at the PEDOT:PSS interface.