B. K. Crone

Morphology Effectively Controls Singlet-Triplet Exciton Relaxation and Charge Transport in Organic Semiconductors

We present a comparative study of ultrafast photoconversion dynamics in tetracene (Tc) and pentacene (Pc) single crystals and Pc films using optical pump-probe spectroscopy. Photoinduced absorption in Tc and Pc crystals is activated and temperature-independent, respectively, demonstrating dominant singlet-triplet exciton fission. In Pc films (as well as C60-doped films) this decay channel is suppressed by electron trapping. These results demonstrate the central role of crystallinity and purity in photogeneration processes and will constrain the design of future photovoltaic devices.

Device model investigation of bilayer organic light emitting diodes

Organic materials that have desirable luminescence properties, such as a favorable emission spectrum and high luminescence efficiency, are not necessarily suitable for single layer organic light-emitting diodes (LEDs) because the material may have unequal carrier mobilities or contact limited injection properties. As a result, single layer LEDs made from such organic materials are inefficient. In this article, we present device model calculations of single layer and bilayer organic LED characteristics that demonstrate the improvements in device performance that can occur in bilayer devices. We first consider an organic material where the mobilities of the electrons and holes are significantly different. The role of the bilayer structure in this case is to move the recombination away from the electrode that injects the low mobility carrier. We then consider an organic material with equal electron and hole mobilities but where it is not possible to make a good contact for one carrier type, say electrons. Th...

Device physics of single layer organic light-emitting diodes

We present experimental and device model results for electron only, hole only, and bipolar organic light-emitting diodes fabricated using a soluble poly (p-phenylene vinylene) based polymer. Current–voltage (I–V) characteristics were measured for a series of electron only devices in which the polymer thickness was varied. The I–V curves were described using a device model from which the electron mobility parameters were extracted. Similarly, the hole mobility parameters were extracted using a device model description of I–V characteristics for a series of hole only devices where the barrier to hole injection was varied by appropriate choices of hole injecting electrode. The electron and hole mobilities extracted from the single carrier devices are then used, without additional adjustable parameters, to describe the measured current–voltage characteristics of a series of bipolar devices where both the device thickness and contacts were varied. The model successfully describes the I–V characteristics of single carrier and bipolar devices as a function of polymer thickness and for structures that are contact limited, space charge limited, and for cases in between. We find qualitative agreement between the device model and measured external luminance for a thickness series of devices. We investigate the sensitivity of the device model calculations to the magnitude of the bimolecular recombination rate prefactor.

Device model investigation of single layer organic light emitting diodes

We present calculations of single layer organic light emitting diode (LED) characteristics using a device model which includes charge injection, transport, recombination, and space charge effects in the organic material. Contact limited and ohmic contacts, high and low carrier mobilities, and device thicknesses from 5 to 200 nm are considered. The scaling of device current with applied voltage bias and organic film thickness is described for contact limited and ohmic contacts. Calculated device current, light output, and quantum and power efficiency are presented for representative cases of material and device parameters. These results are interpreted using the calculated spatial variation of the electric field, charge density, and recombination rate density in the devices. We find that efficient single layer organic LEDs are possible for a wide range of organic material and contact parameters.

Charge injection and transport in single-layer organic light-emitting diodes

We present experimental and device model results for the current–voltage characteristics of a series of organic diodes. We consider three general types of structures: electron only, hole only, and bipolar devices. Electron and hole mobility parameters are extracted from the corresponding single carrier structures and then used to describe the bipolar devices. The device model successfully describes the experimental results for: electron only devices as thickness is varied, hole only devices as the contact metals are varied, and bipolar devices are both the thickness and the contact metals are varied.

Bulk photoconductive gain in poly(phenylene vinylene) based diodes

We observe large, bulk photoconductive gain (>100) in organic polymer diodes. Photoconductive gain was measured in diode structures employing the soluble polymer poly[2-methoxy,5-(2′-ethyl-hexyloxy)-1, 4-phenylene vinylene] (MEH-PPV) as the active layer. The MEH-PPV layer was either undoped or doped by incorporation of a soluble C60 derivative or PbSe quantum dots. The gain characteristics of the doped and undoped diodes are similar. We present the spectral response, transient response, and bias dependence of the gain. The photoconductive gain is due to the circulation of hole carriers through the diode in response to electrons trapped in the polymer layer. The bulk photoconductive gain reported here is distinct from the previous observations of gain in organic diodes that has been attributed to charge trapping near electrodes which increases the charge injection from that contact. The observed gain is consistent with estimates using previously established charge transport parameters of MEH-PPV.

A near infrared organic photodiode with gain at low bias voltage

We demonstrate an organic photodiode with near infrared optical response out to about 1100 nm with a gain of ∼10 at 1000 nm under 5 V reverse bias. The diodes employ a soluble naphthalocyanine with a peak absorption coefficient of ∼105 cm−1 at 1000 nm. In contrast to most organic photodiodes, no exciton dissociating material is used. At zero bias, the diodes are inefficient with an external quantum efficiency of ∼10−2. In reverse bias, large gain occurs and is linear with bias voltage above 4 V. The observed gain is consistent with a photoconductive gain mechanism.

Efficient plastic scintillators utilizing phosphorescent dopants

The authors demonstrate improved light yield from plastic scintillators utilizing a phosphorescent dopant to collect both singlet and triplet excitations created by ionizing radiation. They specifically considered poly(vinyltoluene) and poly(9-vinylcarbazole) doped with an Ir phosphor. They present the spectral, temporal, and integrated yield responses as a function of dopant concentration to pulses of 10keV electrons. Both doped plastics yield a maximum light output of ∼200% of anthracene with decay times <850ns. High light yield was obtained for Ir element fractions up to ∼10wt%, implying that these scintillators may be useful for gamma detection.

Efficient, visible organic light-emitting diodes utilizing a single polymer layer doped with quantum dots

We demonstrate organic light-emitting diodes (OLEDs) using a single active layer consisting of CdSe∕ZnS quantum dots (QDs) dispersed in poly (9,9-dioctylfluorene) (PFO). The diodes have an external quantum efficiency of ∼0.5% and reach 0.1A∕cm2 at 6.5V. These results are comparable to complex, multilayer QD OLEDs. Built-in potential measurements show that the QD valence levels are shifted to lower binding energy when compared to quantum confinement based estimates, and are close to PFO valence levels. Devices using red and green QDs emit predominantly from the QDs but the spectrum of blue QDs is perturbed by interactions with PFO.

Improving carrier injection in organic diodes by incorporating charge trapping molecules

We demonstrate improved charge injection in organic diodes by incorporating charge trapping molecules near the injecting electrode that dynamically alter the effective Schottky energy barrier to carrier injection between a metal electrode and the organic electronic material. Hole injection from Al and Cu anodes into the electroluminescent polymer poly[2-methoxy,5-(2’-ethyl-hexyloxy)-1,4-phenylene vinylene] was improved by incorporating C60 molecules into the polymer near the anode. In operation, electrons injected from the cathode are trapped by the C60 molecules, creating an induced dipole near the anode. We demonstrate these effects by measuring changes in diode current-voltage characteristics and built-in potentials.