Scanning Probe Microscopy of Intrinsic and Extrinsic Disorder in Organic Semiconductors

This dissertation is devoted to develop an understanding of intrinsic and extrinsic disorder, both structural and electronic, in organic semiconductors. Atomic force microscopy (AFM) and conducting atomic force microscopy (CAFM) were used to study the percolation characteristics of charge transport in disordered poly-3-hexylythiophene (P3HT): fullerene-C60 heterojunction thin films. Organic semiconductors N,N'-Di-[(1-naphthyl)-N,N '- diphenyl]-1,1'-biphenyl)-4,4 '-diamine (alpha-NPD) deposited on Au(111) and alpha-sexithiophene (alpha-6T) on C60/Au(111) substrates were studied by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) to investigate the intrinsic and extrinsic disorder, respectively. The observations can provide insights for the working principle of organic devices and help to improve the performance of future organic electronic applications.;Mixed films of P3HT and C60 were grown by vacuum codeposition. Films were characterized by ultraviolet-visible absorption spectroscopy, AFM, and CAFM. Absorption spectra show evidence for diminished crystallinity in mixed films. CAFM measurements show small local currents and only a limited number of charge percolation pathways through the film, especially when compared with related solution-processed bulk heterojunctions.;alpha-NPD thin films with different conformations grown on Au(111) substrate were studied by STM and STS. Three-lobe, windmill, ordered narrow-single-row and broad-single- row, and disordered patterns have been explored. Models of molecule conformation for these pattern have also been given. By utilizing STS, highest occupied molecular orbital (HOMO) level of disordered structures shifts to higher binding energy than the HOMO level of ordered broad-single-row structures has been observed, which can be explained by the change of the overlap of pi states of the backbone and phenyl/naphthyl groups.;Lastly, STM and STS measurements were carried out on alpha-6T films grown on C60/Au(111) substrates. The sexithiophene molecules adopt an upright orientation on this substrate with a unit cell that reflects both bulk-like alpha-6T packing and C60 substrate templating effects. Random apparent height variations in these films are assigned as molecular sliding defects but are observed to have negligible electronic impact on the energy of HOMO. However, the HOMO is observed to vary significantly in response to random changes in probe tip, implying that the origin of electronic disorder in this system is extrinsic to the molecular film.