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 films of two molecules, 5,5^′-bis(dimesitylboryl)-2,2^′-bithiophene (BMB-2T) and 5,5^′-bis(dimesitylboryl)-2,2^′:5^′2^′terthiophene (BMB-3T) which have been characterized as potential electron transport materials in organic light-emitting devices. Using the frontier orbital positions at a metal-organic interface, determined from the UPS measurements, together with the optical band gaps of the molecules, the electron injection barriers for the two oligothiophene derivatives are found to be significantly different. This barrier is estimated to be $\text{∼0.2-0.3\hspace{0.05em}}\mathrm{eV}$ lower for BMB-2T than for BMB-3T at a metal–organic interface.