Infrared reflection absorption spectroscopy diagnostic of the alkyl chain length-dependent molecular orientation of evaporated thin films

Abstract Molecular orientation control of organic semiconductor molecules may improve the performance of organic electronic devices. We investigated the effect of the alkyl-chain length of perylene derivatives (PTCDI-C n ), such as N , N ′-dipentyl-3,4,9,10-perylenedicarboximide (PTCDI-C5), N , N ′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8), and N , N ′-ditridecyl-3,4,9,10-perylenedicarboximide (PTCDI-C13), on the molecular orientation. The molecular orientation angles of PTCDI-C n , perylenetetracarboxylic dianhydride (PTCDA), and zinc phthalocyanine (ZnPc) films during evaporation were monitored by in situ infrared reflection absorption spectroscopy. The ZnPc film grown on the short alkyl chain PTCDI-C5/PTCDA-ITO substrate could be precisely tuned to attain the lying-down orientation owing to the strong π – π interaction between the PTCDI-C5 and ZnPc molecules, whereas ZnPc film grown on the longer alkyl chain PTCDI-C8/PTCDA-ITO or PTCDI-C13/PTCDA-ITO substrate dipslayed the standing-up orientation. For lying-down orientation, the light-absorbance was 1.17-fold larger than for the standing-up counterparts (1.00 and 1.04 for PTCDI-C8/PTCDA-ITO and PTCDI-C13/PTCDA-ITO, respectively). Precise control of lying down molecular orientation is of interest for organic electronics applications.