Z. V. Vardeny

Efficiency enhancement of an organic light-emitting diode with a cathode forming two-dimensional periodic hole array

We fabricated an organic light-emitting diode using a π-conjugated polymer emissive layer sandwiched between two semitransparent electrodes: an optically thin gold film anode, whereas the cathode was in the form of an optically thick aluminum (Al) film with patterned periodic subwavelength two-dimensional hole array that showed anomalous transmission in the spectral range of the polymer photoluminescence band. At similar current densities, we obtained a sevenfold electroluminescence efficiency enhancement with the patterned Al device compared with a control device based on unperforated Al electrode.

Evidence for excimer photoexcitations in an ordered π-conjugated polymer film

We report pressure-dependent transient picosecond and continuous-wave photomodulation studies of disordered and ordered films of 2-methoxy-5-(2-ethylhexyloxy) poly(para-phenylenevinylene). Photoinduced absorption (PA) bands in the disordered film exhibit very weak pressure dependence and are assigned to intrachain excitons and polarons. In contrast, the ordered film exhibits two additional transient PA bands in the midinfrared that blueshift dramatically with pressure. Based on high-order configuration interaction calculations we ascribe the PA bands in the ordered film to excimers. Our work brings insight to the exciton binding energy in ordered films versus disordered films and solutions. The reduced exciton binding energy in ordered films is due to new energy states appearing below the continuum band threshold of the single strand.

Unidirectional laser emission from π-conjugated polymer microcavities with broken symmetry

We report unidirectional laser emission from π-conjugated polymer microcavities with broken symmetry geometries such as spiral and microdisk containing a “line defect,” in comparison with plain microdisk cavity having isotropic emission. We found that the laser emission directionality contrast ratio is 8–10 and far field lateral divergence angle is 12°–15° for both broken symmetry geometries, with no significant increase in the laser threshold intensity. Fourier transform analysis of the laser emission spectra shows that unlike microdisks with line defect, the variation of light trajectories in the spiral microcavities leads to less defined laser modes.

Optical studies of metallodielectric photonic crystals: Bismuth and gallium infiltrated opals

We have fabricated three-dimensional metallodielectric photonic crystals that consist of bismuth and gallium metals infiltrated into porous synthetic opals that have 300 nm diam silica balls. The specular reflectivity was measured in a broad spectral range from 0.3 to 25 μm using a variety of light sources, detectors and spectrometers. In addition to broadening of the original Bragg stop bands in the visible spectral range that give rise to iridescence colors, we also found a dramatic decrease of reflectivity in the visible/near-infrared spectral range. For frequencies below a cut-off frequency in the mid-infrared range the reflectivity increases to its normal bulk value in the respective metal, as predicted by theory and fitted by a model calculation.

Transport studies of isolated molecular wires in self-assembled monolayer devices

We have fabricated a variety of isolated molecule diodes based on self-assembled monolayers (SAMs) of solid-state mixture (SSM) of molecular wires [1,4-methane benzene dithiol (Me-BDT)], and molecular insulator spacers [penthane 1-thiol (PT)] with different concentration ratios r of wires∕spacers, which were sandwiched between two gold (Au) electrodes. We introduce two specialized methods borrowed from surface science to (i) confirm the connectivity between the Me-BDT molecules with the upper Au electrode, and (ii) count the number of isolated molecular wires in the devices. The electrical transport properties of the SSM SAM diodes were studied at different temperatures via the conductance and differential conductance spectra. We found that a potential barrier caused by the spatial connectivity gap between the PT molecules and the upper Au electrode dominates the transport properties of the pure PT SAM diode (r=0). The transport properties of SSM diodes with r values in the range 10−8

Laser action and photoexcitations dynamics in PbI 2 films

We have grown high quality PbI2 films by thermal evaporation. We obtained in these films two amplified spontaneous emission (ASE) lines around ~2.23 eV at relatively low pump excitation threshold using 10 ns pulsed laser, along with a broad photoluminescence (PL) band that results from trapped electron and hole (e-h) pairs in shallow traps. We discuss the two ASE lines as due to the anisotropic PbI2 crystal structure. In addition, we also studied the ultrafast transient response of photoexcitations in the PbI2 films using a broadband pump-probe correlation spectroscopy in the broad spectral range of 0.3-2.4 eV with 150 fs time resolution. The transient photomodulation spectrum is dominated by two photoinduced absorption bands and a stimulated emission band that agrees with the obtained ASE band. We ascribed the low threshold laser action in PbI2 films as due to the e-h confinement in ‘quantum wells’ like structures formed between the main 2H-PbI2 crystalline and other polytypes with lower optical gap.

Enhanced emissive and lasing characteristics of nano-crystalline MAPbBr3 films grown via anti-solvent precipitation

Control of the nano-scale crystal size distribution in films of organic-inorganic lead-tri-bromide perovskites is achieved through a recently reported solution-based, anti-solvent treatment method [H. Cho et al., Science 350, 1222 (2015)]. The treated films are shown to be high quality, optically smooth with excellent emissive and optical gain properties including higher photoluminescence yield and reduced threshold for laser action. The improved lasing is shown to originate from a synergistic combination of a reduction in waveguide losses and a decrease in the non-radiative decay rate of the excited state population, compared to untreated films. The net gain is increased by a factor of two upon anti-solvent treatment and reaches a respectable value of ≈300u2009cm−1.