Liam S C Pingree

Negative capacitance in organic light-emitting diodes

Negative capacitance has been characterized in organic light-emitting diode (OLED) heterostructures using impedance spectroscopy. Although similar inductive behavior has been previously reported for transient electroluminescence in OLEDs, definitive identification of negative capacitance in impedance spectroscopy data has been elusive due to the high concentration of distributed traps at the anode-organic interface. The addition of a layer of 4,4′-bis[(p-trichlorosilylpropylphenyl)phenylamino]-biphenyl at this interface minimizes these trapping sites, thus enabling the inductive nature of charge transport in OLEDs to be directly observable. By quantitatively correlating the resulting impedance spectroscopy data with equivalent circuit models, a detailed description of charge transport in OLEDs as a function of heterostructure composition is developed.

Bridge-enhanced nanoscale impedance microscopy

Nanoscale impedance microscopy and related methods, circuit and/or apparatus capable of quantitatively measuring magnitude and phase of alternating current flow.

Spatially resolved photocurrent mapping of operating organic photovoltaic devices using atomic force photovoltaic microscopy

A conductive atomic force microscopy (cAFM) technique, atomic force photovoltaic microscopy (AFPM), has been developed to characterize spatially localized inhomogeneities in organic photovoltaic (OPV) devices. In AFPM, a biased cAFM probe is raster scanned over an array of illuminated solar cells, simultaneously generating topographic and photocurrent maps. As proof of principle, AFPM is used to characterize 7.5×7.5μm2 poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester OPVs, revealing substantial device to device and temporal variations in the short-circuit current. The flexibility of AFPM suggests applicability to nanoscale characterization of a wide range of optoelectronically active materials and devices.

Spatially-resolved electroluminescence of operating organic light-emitting diodes using conductive atomic force microscopy

A conductive atomic force microscopy (cAFM) technique has been developed that concurrently monitors topography, charge transport, and electroluminescence. This cAFM approach is particularly well suited for probing the electroluminescent response characteristics of operating organic light-emitting diodes (OLEDs) over short length scales. In a typical experiment, charge is injected into individual OLED structures with the cAFM tip, and the resulting electroluminescence and current are measured with collecting optics and a variable gain photomultiplier tube. As a proof of principle, the real-time spatial and temporal current–voltage and electroluminescence–voltage properties of 8μm×8μm OLED pixels are simultaneously imaged.

Nanoscale impedance microscopy-a characterization tool for nanoelectronic devices and circuits

A recently developed conductive atomic force microscopy (cAFM) technique, nanoscale impedance microscopy (NIM), is presented as a characterization strategy for nanoelectronic devices and circuits. NIM concurrently monitors the amplitude and phase response of the current through a cAFM tip in response to a temporally periodic applied bias. By varying the frequency of the driving potential, the resistance and reactance of conductive pathways can be quantitatively determined. Proof-of-principle experiments show 10-nm spatial resolution and ideal frequency-dependent impedance spectroscopy behavior for test circuits connected to electron beam lithographically patterned electrode arrays. Possible applications of NIM include defect detection and failure analysis testing for nanoscale integrated circuits.

Field dependent negative capacitance in small-molecule organic light-emitting diodes

Frequency dependent charge transport in organic light-emitting diodes, including marked negative capacitance (NC), is reproduced through an equivalent circuit model. The robustness of the model is tested through impedance spectroscopy characterization as a function of bias changes and layer thickness modifications. Correlations with current-voltage measurements reveal that the NC occurs once trap assisted space charge limited transport is reached. Through variation of the organic layer thicknesses, the magnitude of the NC response can be precisely tuned. In particular, increasing the thickness of the electron transport layer increases the NC magnitude, whereas hole transport layer thickness modifications have little effect on the magnitude of NC. Subsequent modeling indicates that alterations in the distribution of the electric field across the individual organic layers account for the observed variations in NC. In addition, it is found that the time constants for the inductive elements of the model incre...