The structural and electronic properties of interfaces composed of donor and acceptor molecules play important roles in the development of organic opto-electronic devices. Epitaxial growth of organic semiconductor molecules offers a possibility to control the interfacial structures and to explore precise properties at the intermolecular contacts. 5,6,11,12-tetraazanaphthacene (TANC) is an acceptor molecule with a molecular structure similar to that of pentacene, a representative donor material, and thus, good compatibility with pentacene is expected. In this study, the physicochemical properties of the molecular interface between TANC and pentacene single crystal (PnSC) substrates were analyzed by atomic force microscopy, grazing-incidence X-ray diffraction (GIXD), and photoelectron spectroscopy. GIXD revealed that TANC molecules assemble into epitaxial overlayers of the (010) oriented crystallites by aligning an axis where the side edges of the molecules face each other along the [1¯10] direction of the PnSC. No apparent interface dipole was found, and the energy level offset between the highest occupied molecular orbitals of TANC and the PnSC was determined to be 1.75 eV, which led to a charge transfer gap width of 0.7 eV at the interface.
Homoepitaxial growth of organic semiconductor single crystals is a promising methodology toward the establishment of doping technology for organic opto-electronic applications. In this study, both electronic and crystallographic properties of homoepitaxially grown single crystals of rubrene were accurately examined. Undistorted lattice structures of homoepitaxial rubrene were confirmed by high-resolution analyses of grazing-incidence X-ray diffraction (GIXD) using synchrotron radiation. Upon bulk doping of acceptor molecules into the homoepitaxial single crystals of rubrene, highly sensitive photoelectron yield spectroscopy (PYS) measurements unveiled a transition of the electronic states, from induction of hole states at the valence band maximum at an adequate doping ratio (10 ppm), to disturbance of the valence band itself for excessive ratios (≥ 1000 ppm), probably due to the lattice distortion.
Methylammonium lead triiodide (CH3NH3PbI3) is an essential material in prototype perovskite solar cells, and its intrinsic electronic structures have been of great interest. In this study, the clean surface of single crystal samples of CH3NH3PbI3 was elucidated by X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy (UPS), and photoelectron yield spectroscopy (PYS), and was closely compared with as-prepared single crystal surfaces with considerable native impurities. Low-energy UPS and PYS successfully revealed a presence of electronic states exceeding the Fermi level, which are presumably attributed to electrons transferring from the surface impurities into the conduction band of CH3NH3PbI3.
Methylammonium lead triiodide (CH3NH3PbI3) is a fundamental material used for prototypical perovskite solar cells. The electronic properties of the interface between CH3NH3PbI3 and hole transporting materials play a crucial role in the efficient performance of these solar cells. However, the intrinsic characteristics of the interfaces where these materials directly come into contact with each other have not yet been defined since previous studies were performed using polycrystalline thin films of CH3NH3PbI3, which were confirmed to contain a considerable amount of impurities. In this study, x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy were conducted to determine the interfacial electronic structure between CH3NH3PbI3 and 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD) on the clean interface formed on the impurity-free single crystal surface of CH3NH3PbI3. Spontaneous hole injection from CH3NH3PbI3 to spiro-OMeTAD occurred at the direct contact sites between these materials, a phenomenon that was confirmed to be hindered by the presence of impurities at the interface.
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