Electrical and optical studies of gap states in self-assembled molecular aggregates

Abstract

Articles you may be interested inMolecular self ordering and charge transport in layer by layer deposited poly (3,3-dialkylquarterthiophene) films formed by Langmuir-Schaefer technique J. Appl. Phys. 116, 094311 (2014); 10.1063/1.4894515 Molecular and electronic structure of electroactive self-assembled monolayers Structure and optical properties of self-assembled multicomponent plasmonic nanogels Appl. Phys. Lett. 99, 043112 (2011); 10.1063/1.3615785 Energy level alignment at organic semiconductor/metal interfaces: Effect of polar self-assembled monolayers at the interface J. Chem. Phys. 128, 074705 (2008); 10.1063/1.2832306Conformations and charge transport characteristics of biphenyldithiol self-assembled-monolayer molecular electronic devices: A multiscale computational studyWe fabricated a variety of two-terminal devices using self-assembled monolayers ͑SAM͒ of solid-state mixtures comprised of molecular "wires" ͓1,4-methane-benzenedithiol ͑Me-BDT͔͒ and molecular insulator "spacers" ͓1-pentanethiol͔, which were prepared at various molar concentrations ratio, r of wires/spacers, and sandwiched between two gold electrodes. The devices' electrical transport was investigated at several r values using the bias voltage ͑V͒ dependencies of the conductance and differential conductance at various temperatures. In parallel, we also studied the UV-visible absorption and photoluminescence ͑PL͒ emission spectra of the SAM mixtures grown on silica transparent substrates. For r Ͼ 10 −3 we found that two-dimensional ͑2D͒ Me-BDT aggregates are formed in the SAM films leading to novel properties compared to SAM films of isolated Me-BDT molecules at concentrations 10 −8 Ͻr Ͻ 10 −4 , which we studied before ͓V. Burtman, A. S. Ndobe, and Z. V. Vardeny, J. Appl. Phys. 98, 034314 ͑2005͔͒. First, an Ohmic response in the current-voltage ͑I-V͒ characteristics is obtained up to V ϳ 0.5 V, which results in a new band in the differential conductance spectrum around V = 0. Second, a new subgap absorption band is formed at ϳ2.4 eV, which is related to a new yellow/red PL emission band. The novel optical and electrical properties of the 2D Me-BDT aggregates are explained by the formation of an electronic continuum band in the Me-BDT energy gap, which is caused by weak in-plane charge delocalization among the molecules forming the aggregates. To verify this model we also studied SAM molecular aggregate diodes using Al electrodes. The 1-eV difference in the electrode work function between Au and Al metals results in a pronounced E F shift with respect to the aggregate-related continuum band in the gap, and consequently, dramatically changes the device I-V characteristics.

Ab initioelectron propagators in molecules with strong electron-phonon interaction. I. Phonon averages

“…Strong temperature dependence of current-voltage characteristics [35][36][37][38][39][40] indicates that strong electron-phonon coupling is important in molecular transport junctions or correct calculation of ionization potential or affinities in molecules and ions. Such molecular systems contain many strong polar modes.…”

“…[35][36][37][38][39][40] The value of the current can be increased by several orders of magnitude from low to room temperatures. ⌫ L and ⌫ R are the matrices for the interaction between the molecule and the leads.…”

Molecular Photovoltaics in Nanoscale Dimension

“…The way to achieve epitaxial control in organic devices is not trivial too. The transport mechanism in low-dimensional organic structures is intimately related to the ordering and dimensionality of the underlying electronic system, which may transform with the molecular packing [ 24 ]. Therefore, a systematic research in organic semiconducting materials and structures with controlled order is a pre-requirement for efficient PVC fabrication.…”

Ab initioelectron propagators in molecules with strong electron-phonon interaction: II. Electron Green’s function

“…A broad variety of properties for small molecular systems depends on the electron-phonon interaction, such as current-voltage characteristics in transport molecular 1,2 and molecular aggregate [3][4][5][6] junctions, and tunneling spectroscopy. [7][8][9] One of the most spectacular examples of electron-phonon coupling in molecules is given by the behavior of electron transition in strong laser fields where coupling leads to acceleration or termination of the electron transfer by many orders of magnitude.…”

“…[45][46][47] In addition, the activation temperature dependence has been recently observed in molecular aggregate transport junctions. 3,6 However, we understand that electron correlation effects are also important to find true electronic and transport properties. Thus, in our Paper I ͑see Ref.…”