Abstract
Mixed organic semiconducting thin films were prepared by vacuum-depositing copper phthalocyanine (CuPc) and rubrene of different mixing ratio onto oxide-terminated Si(100) wafers that were pre-modified with self-assembled monolayer (SAM) of either octadecyltrichlorosilane (OTS) or 4-phenylbutyltrichlorosilane (4-PBTS). The thin films were characterized by atomic force microscopy (AFM) and various synchrotron-based techniques such as x-ray photoemission spectroscopy (XPS), x-ray diffraction (XRD), and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy to obtain the information about the thin films in various aspects such as electronic structure, morphology, crystallinity, averaged molecular orientation, etc. The out-of-plane XRD data show that all the mixed films are composed of CuPc in predominately crystalline from and rubrene in exclusively amorphous form. More informative structural change can be obtained by examining grazing incidence x-ray diffraction (GIXD) images. For pure CuPc film, the a-b plane of CuPc crystallite is found to be parallel to the substrate surface. For the films of increasing rubrene percentage, the CuPC crystallite becomes tilted away from the substrate surface, forming elongated diffraction spots. For the mixed film of the highest rubrene percentage (90%), CuPc crystallites decrease in size, lose surface registry, and become powder-like samples, producing characteristic powder-diffraction rings. In consistence with the structural change documented by x-ray diffraction data, the average tilt angle of CuPc molecules, measured from the surface, also decreases dramatically by up to 10°, as recorded by NEXAFS data. Moreover, there seems to be very little chemical interaction between CuPc and rubrene molecules so as to effect observable changes in XPS N 1s spectra and N K-edge NEXAFS spectra (incidence angle of 55°) for CuPc of different mixed films. Organic field effect transistors based on the aforementioned mixed semiconducting films were fabricated. For mixed-film OFET, a modest mobility of at least 2 × 10–2 cm2/Vs can all be obtained, with the best value of 5.9×10–2 cm2/Vs obtained for the mixed film of 75% CuPc+25 % rubrene grown on 4-PBTS substrate. Comparing the AFM images with mobility data, we found that a larger width in the characteristic CuPc features in AFM image seems to give better electrical performance, suggesting a beneficial effect exerted by the larger CuPC grains.