This work reports a physical model that relates charge transport to the exciton formation rate in organic electroluminescent device. The model assumes ideal charge density profiles and the existence of an "optimal" charge separation for excitons to be formed. This allows the exciton formation rate to be dependent on the charge density profiles as well as parameters linked to the exciton formation process. Field dependent effect enters into this model through a "localization" parameter, which is related to the carrier mobility. When the model is applied to the ITO/CuPc/NPB/A1q3/Mg system, we have found it possible to address a number of observations not fully explained previously.
Current conduction in organic field-effect transistors (OFETs) has attracted much attention. The most
intriguing issue is that for most organic semiconductors only one carrier type is observed. To examine why
this is so, effort has been spent to study ambipolar conduction in OFETs under strong gate bias. For the
rubrene OFETs analyzed in this work, there is evidence that p-channel conduction, which occurs at low
positive gate bias, is closely associated with negative charge states present at the insulator-semiconductor
interface. Because of the somewhat small insulator capacitance only a fraction of the applied gate voltage
drops across the OFET channel making it very difficult to achieve n-channel conduction at low positive
gate voltage. Furthermore, electron conductivity is usually low due to the smaller density of states in the
LUMO. As computed, the negative charge states appear to have peak energy at 0.8 eV above the HOMO
and it is tempting to associate them with "polaron" states found in the insulator.
Charge relaxation in dispersive materials is often described in terms of the stretched
exponential function (Kohlrausch law). The process can be explained using a "hopping"
model which in principle, also applies to charge transport such as current conduction.
This work analyzed reported transient photoconductivity data on functionalized pentacene
single crystals using a geometric hopping model developed by B. Sturman et al and
extracted values (or range of values) on the materials parameters relevant to charge
relaxation as well as charge transport. Using the correlated disorder model (CDM), we
estimated values of the carrier mobility for the pentacene samples. From these results, we
observed the following: i) the transport site density appeared to be of the same order of
magnitude as the carrier density; ii) it was possible to extract lower bound values on the
materials parameters linked to the transport process; and iii) by matching the simulated
charge decay to the transient photoconductivity data, we were able to refine estimates on
the materials parameters. The data also allowed us to simulate the stretched exponential
decay. Our observations suggested that the stretching index and the carrier mobility were
related. Physically, such interdependence would allow one to demarcate between
localized molecular interactions and distant coulomb interactions.
High-brightness, inorganic light-emitting diodes (LEDs) have been successfully utilized for edge-lighting of large
displays for signage. Further interest in solid-state lighting technology has been fueled with the emergence of small
molecule and polymer-based organic light-emitting diodes (OLEDs). In this paper, edgelit inorganic LED-based displays
and state-of-the-art OLED-based displays are evaluated on the basis of electrical and photometric measurements. The
reference size for a signage system is assumed to be 600 mm x 600mm based on the industrial usage. With the
availability of high power light-emitting diodes, it is possible to develop edgelit signage systems of the standard size.
These displays possess an efficacy of 18 lm/W. Although, these displays are environmentally friendly and efficient, they
suffer from some inherent limitations. Homogeneity of displays, which is a prime requirement for illuminated signs, is
not accomplished. A standard deviation of 3.12 lux is observed between the illuminance values on the surface of the
display. In order to distribute light effectively, reflective gratings are employed. Reflective gratings aid in reducing the problem but fail to eliminate it. In addition, the overall cost of signage is increased by 50% with the use of these
This problem can be overcome by the use of a distributed source of light. Hence, the organic-LEDs are considered as a
possible contender. In this paper, we experimentally determine the feasibility of using OLEDs for signage applications
and compare their performance with inorganic LEDs. Passive matrix, small-molecule based, commercially available
OLEDs is used. Design techniques for implementation of displays using organic LEDs are also discussed. It is
determined that tiled displays based on organic LEDs possess better uniformity than the inorganic LED-based displays.
However, the currently available OLEDs have lower light-conversion efficiency and higher costs than the conventional,
inorganic LEDs. But, signage panels based on OLEDs can be made cheaper by avoiding the use of acrylic sheet and
reflective gratings. Moreover, the distributed light output and light weight of OLEDs and the potential to be built
inexpensively on flexible substrates can make OLEDs more beneficial for future signage applications than the inorganic
Organic light-emitting diodes (OLEDs) have been utilized successfully for various applications such as microdisplays in cell-phones and digital cameras. However, the application of OLEDs for large area signage displays has not yet been established. This paper presents novel design techniques for implementing OLEDs as light sources for signage application. The designs are examined on the basis of signage uniformity, cost and manufacturing complexity. Advantages and limitations of each design are described. It is determined that a trade-off is required to choose a design for implementation. After evaluation and comparison of the designs, the most optimal design is chosen and implemented. Measurement results with the optimal design are described.
Mobility in single-grain and polycrystalline organic field-effect transistors (OFETs) is of interest because it affects the performance of these devices. While reasonable values of the hole mobility has been measured in pentacene OFETs, relatively speaking, our understanding of the detailed transport mechanisms is somewhat weak and there is a lack of precise knowledge on the effects of the materials parameters such as the site spacing, the localization length, the rms width of the density of states (DOS), the escape frequency, etc. This work attempts to analyze the materials parameters of pentacene OFETs extracted from data reported in the literature. In this work, we developed a model for the mobility parameter from first principle and extracted the relevant materials parameters. According to our analyses, the transport mechanisms in the OFETs are fairly complex and the electrical properties are dominated by the properties of the trap states. As observed, the single-grain OFETs having smaller values of the rms widths of the DOS (in comparison with the polycrystalline OFETs) also had higher hole mobilities. Our results showed that increasing the gate bias could have a similar but smaller effect. Potentially, increasing the escape frequency is a more effective way to raise the hole mobility and this parameter appears to be affected by changes in the molecular structure and in the degree of "disorder".
Organic semiconductors have attracted significant interest because of their potential application in electronic devices. One of the most important parameters in such an application is the carrier mobility, which is low when compared with the inorganic semiconductors. In the past, significant effort has been spent to produce high mobility organic semiconductors, even though the best room temperature value reported is only a few cm<sup>2</sup>/V.s. In this work, we examined the field-effect carrier mobility in pentacene by correlating reported data on polycrystalline samples with simulations based on the correlated disorder model (CDM). Using the rms width of the density of states (DOS) as the variable, we were able to produce a good match. Our results suggested that the carrier mobility in polycrystalline pentacene might be primarily dependent on σ, the rms width of the DOS. Furthermore, a parameter extraction scheme was proposed and applied to pentacene data reported in the literature.
We simulated the negative capacitance-frequency curves for the organic semiconductor Alq3 using the Drude model equation to assess the applicability of the "universality of photocurrent transients". Using the data reported by Berleb et al, shape invariance was observed in these curves after normalization with respect to frequency and taking into account the bias dependence. Our results suggested that the reported frequency shifts in Berleb's measurements were primarily related to changes in the complex carrier mobility at the different biases. Normalization of the capacitance-frequency curves at different temperatures also showed shape invariance when normalized. The observed changes in the carrier mobility as a function of temperature were essentially the same as what one would expect from a disordered solid with Gaussian density of states. Our model therefore provided an alternative venue to determine the "universality of photocurrent transients" with focus on the complex carrier mobility.
Space charge is known to affect current transport in organic light emitting devices (OLEDs). This normally occurs at low bias when current density varies quadratically with the bias voltage. A transition exists when current flow changes from space-charge limited to “recombination” limited. In order to further investigate
this transition, we extended our model on exciton formation based on bimolecular recombination  to include the existence of space charge. In examining the model characteristics, we found that where the transition occurs depends on the density of the space charge in the active layer and the fraction of it associated with "recombination". This also relates to a parameter a<sub>0</sub> defining the optimal charge separation for "recombination" to take place. Our results indicated that the transition would vary for different organic semiconductors and would be an important factor affecting the device performance. In comparing our model with experimental data, we are of the opinion that space charge effect may be responsible for the occasional occurrence of an "anomalous" current reported in the J-V characteristics of OLEDs.
This work examined the relationship between the zero field carrier mobility and the change in potential distribution caused by different subgroups within a conjugated polymer such as OC1C10 - PPV. It was observed that the potential profiles with different values of energy minimum could have a strong bearing on the trapping of carriers and a shallow profile involving a lesser number deep (trap) states also gave rise to short retention time for the trapped carriers. This had the effect of enhancing the zero field mobility. For the structures examined, it appeared that shorter subgroups attached to the polymer backbone (Structure D) would result in a shallower energy trough and the same effect was observed for a geometrically balanced structure with two subgroups attached to the opposite ends in the polymer backbone (Structure C).
We have developed an extension of the Drude model for oscillators to determine capacitance in organic polymers. The extension covers the effect of time delay caused by dispersion in the carrier transport. This was done using a complex mobility parameter. Negative capacitance effect could be observed in simulation at low frequencies provided that model parameters are appropriately chosen. In matching the simulation results to a set of data reported, we have computed very reasonable values of the transport parameters in regimes where either positive capacitance or negative capacitance is dominant. Simplified equations based on the model were highlighted to accommodate for specific instances where the transport parameters could be directly extracted. We believe our model is a better approach to extract transport parameters from capacitance-frequency curves when detailed conduction processes could not be easily identified such as in an environment of disordered complex molecules.