Photoemission study of frontier orbital alignment at a metal-organic interface as a function of conjugation length of oligothiophene derivatives

We report an ultraviolet photoemission spectroscopy (UPS) study of solid fi lms of two molecules, 5,5'-bis(dimesitylboryl)-2,2'-bithiophene (BMB-2T) an d 5,5'-bis(dimesitylboryl)-2,2':5'2'terthiophene (BMB-3T) which have been c haracterized as potential electron transport materials in organic light-emi tting devices. Using the frontier orbital positions at a metal-organic inte rface, determined from the UPS measurements, together with the optical band gaps of the molecules, the electron injection barriers for the two oligoth iophene derivatives are found to be significantly different. This barrier i s estimated to be similar to0.2-0.3 eV lower for BMB-2T than for BMB-3T at a metal-organic interface. (C) 2001 American Institute of Physics.