RGB emission using a dimesitylboryl-bithiophene derivative as a universal host and pentacene derivatives as the red emitters

Phenyl, ethynyl-silyl and ethynyl-alkyl derivatives of pentacene have been optically characterized and their use as potential red emitters in organic light emitting devices is investigated. Tuning of the red emission wavelength and photoluminescence quantum efficiency (φ_pl) is achieved by modifying the substituent and its position on the pentacene backbone. A red shift in the emission maxima (λ_max) is observed upon addition of more phenyl groups or changing from a phenyl to an ethynyl-R due to an increase in π-conjugation. For example, the λ_max of 6,13-diphenylpentacene (DPP) is 617 nm compared to 630 nm for 5,7,12,14-tetraphenylpentacene (TPP). Similarly, the diethynyl pentacene derivatives have a red shifted λ_max (638 nm), relative to that of DPP, due to the greater conjugation associated with the triple bond of the ethynyl group. DPP is explored as a red emitter in a universal blue host due to its ideal red chromaticity and good φ_pl. Red and green emission is achieved in multi-layered devices through the incorporation of an emitting layer based on a blue-emitting/electron transporting universal host, 5,5′-his(dimesitylboryl)-2,2′-bithiophene (BMB-2T), doped with fluorescent red and green emitters, respectively. Blue emission can be obtained from the host BMB-2T, or from the adjacent hole transporter. A hole-blocking layer is used for the latter case to force electron and hole recombination in the hole transporting layer. The host and guest molecules are selected in order to take advantage of two electroluminescence mechanisms, energy transfer from host to guest and direct carrier recombination on the guest molecules. Hence, one can tune the emission color while maintaining high device efficiency. This approach is also technologically advantageous because it minimizes the number of materials used, reduces cross contamination and production costs.