Piergiulio Di Marco

Impact of high electric fields on the charge recombination process in organic light-emitting diodes

A study of electric field dependent spectra and the quantum electroluminescence yield of single-layer (SL) and double-layer (DL) light-emitting diodes (LEDs) based on a combination of an aromatic diamine (TPD) and an oxadiazole derivative (PBD) was carried out. It was shown that a high electric field weakens the recombination of Coulombically correlated electron-hole pairs in the bulk (SL LEDs) and enhances the process at the TPD-doped polymer/PBD interface (DL LEDs). The effects are discussed in terms of electric field mediated interplay between populations of localized (monomolecular) excitons, exciplexes and electroplexes in conjuction with their local environments.

Macrotrap model for charge-carrier transport in low-mobility solids

The neutral macrotrap model of charge carrier trapping has been developed to explain the electric field (F) and temperature (T) dependence of the effective charge carrier mobility (µeff) in low mobility solids. The potential barrier for the carriers localized in the neutral macrotraps (spatially extended trapping domains) can be effectively lowered by an external electric field accessible in the experiment, making the mobility and its thermal activation field dependent. The potential of a neutral macrotrap is approximated by Φ(r)=(3kT/eσ) ln (r0/r), where r is the distance from the center of the macrotrap, r0 stands for its radius, e is elementary charge and σ is a characteristic parameter of the exponential energy distribution of point traps composing the macrotrap. The lowering of the barrier is proportional to F at low fields and to ln F at high fields. The field behavior of the effective mobility is governed by this lowering of the barrier and by its field-dependent position which, therefore, can be reached by the thermally activated carriers with a field-dependent probability in the process of carrier diffusion. The latter introduces a factor exp (-3kT/σelDF) (lD-diffusion length of the carriers) responsible for the self-consistency of the field dependence of the mobility and the Arrhenius temperature term exp [-E(F, T)/kT]. This model provides a consistent description of representative experimental data reported for low-carrier mobility crystalline and disordered solids.

Absorption tail photoconductivity in solid films of C60

Abstract Steady-state photoconductivity measurements in solid C 60 films are reported. The observed action spectra are antibatic with the absorption spectrum and show well-resolved structure in the long-wavelength tail above 500 nm. They are analyzed in terms of the recombination limit model for the bulk-generated charge carriers. It is concluded that photocarrier production occurs by autoionization of localized intramolecular excited states.

Electroabsorption study of vacuum-deposited films of thionaphthenindole☆

Abstract Electric-field modulated absorption spectra in the range 270-380 nm have been studied in polycrystalline layers of thionaphthenindole (TNI) formed by vapour deposition onto room-temperature quartz substrates. The results allowed to refine several weak absorption transitions: (i) in addition to well-resolved vibronic satellites two Davydov components of the first 0-0 transition in the long-wavelength region were distinguished, (ii) the involvement of charge-transfer excitons resulted from electroabsorption behaviour in the short-wavelength region. The revealed nature of the excited states of crystalline TNI confirms previous suggestions that intrinsic photogeneration of charge carriers in the near-electrode layers within S 0 →S 1 transition proceeds via dissociation of Frenkel excitons on specific structural defects. The large values of the electroabsorption at the fundamental frequency of an applied field indicate the existence of a high degree of the orientational order in the layers.

Injection-controlled electroluminescence in organic light-emitting diodes based on molecularly-doped polymers : I. Single-layer devices

Organic single-layer light-emitting diodes (LEDs) based on molecularly-doped polymers (MDPs) have been characterized via their current-field characteristics and the dependence of light output, quantum efficiency, and spectrum of the electroluminescence (EL) on the applied electric field (F) and molecular composition of the organic layers. The results discussed within the framework of the thermionic carrier injection model prove the LEDs to operate in the injection-controlled EL mode. Analytic considerations are presented relating the light output and quantum EL efficiency to the charge recombination mechanisms. The results indicate that the quantum EL efficiency (EL) is determined by both the diffusion-controlled formation of correlated electron-hole (e-h) pairs and their fusion (ultimate recombination event) into an emitting molecular state. Thus, the increasing EL(F) at low fields is predicted to follow the Langevin-like recombination formalism, whereas the decreasing function EL(F) would be a consequence of the Thomson-like recombination prevailing at higher fields. These predictions are in good agreement with experiment. Information concerning the binding energy and charge separation in the correlated (e-h) pairs can be inferred from the high-field dependence of the EL(F).

Photoconductive Properties of Some Polyorganophosphazenes Doped With Trinitrofluorenone

Abstract Polyorganophosphazenes are electrical insulators and in general non photoconducting polymers. The doping with trinitrofluo-renone changes their optical and electrical characteristics: new optical absorption bands appear in the visible region and the doped polymers show photoconductive properties. The photoresponse seems to be related to the nature of the organic group attached to the polymer chain.

A new diamine as the hole‐transporting material for organic light‐emitting diodes

A new diamine, N,N′-diphenyl-N,N′-bis-biphenyl-(1,1′-biphenyl)-4,4′-diamine (PTPD), has been synthesised to be applied as a hole-transporting material for multilayer organic light-emitting diodes (LEDs). Electrical and optical characteristics of a series of double-layer (DL) LEDs using PTPD as hole-transporting layers (HTL) and Alq3 and Znq2 as electron-transporting layers have been measured and compared with those for DL LEDs based on other HTLs (TPD and NTPD). The differences are discussed in terms of charge injection at the electrodes and electron–hole recombination processes at the organic interfaces. The prerequisites based on the molecular structure of the compound, confirmed by preliminary experimental observations, suggest PTPD to be a stable hole-transporting material for high-durability organic LEDs. Copyright

Organic light-emitting device with a mixed ligand 8-quinolinolato aluminium chelate as emitting and electron transporting material

Abstract Double layer (DL) electroluminescent devices, made with TPD (75%) in polycarbonate (PC) and with a mixed ligand 8-quinolinolato aluminium chelate as electron transporting and emitting material, show higher electroluminescence (EL) quantum yield (2.2%) and an increased blue EL emission in comparison with Tris–(8-hydroxyquinolinate)Al(III) (Alq 3 ). The solid state photoluminescence (PL) quantum yield is as high as 60%, about five-fold the Alq 3 value. In spite of the high PL yield, EL efficiency is not as much high; this can be attributed to a strong electron mobility dependence on the electric field, which contributes to a slower increase with the field of the EL quantum yield.

Photoemission of electrons from metal electrodes into poly-N-vinylcarbazole

Abstract We have observed the photoemission of electrons from metallic electrodes (platinum, copper, gold and aluminium) into poly-N-vinylcarbazole (PVK). The experiments showed that the threshold values of internal photoemission change according to the work function of the metal and allowed us to determine the PVK band gap. We obtained a value of about 5.3 eV which is 0.7 eV greater than that obtained in previous calculations in which the polarization energy of PVK was probably overestimated.

Thomson-Like Electron-Hole Recombination in Organic Light-Emitting Diodes

The quantum electroluminescence (EL) yield (ϕEL) of a single-layer light-emitting diode (LED) based on a mixture of an oligothiophene (T5Ohex), an aromatic diamine (TPD) and bisphenol A polycarbonate (PC), and of a double-layer LED with a (TPD:PC) hole-transporting layer and an 8-hydroxyquinoline (Alq3) electron-transporting layer, was measured as a function of electric field (F). The devices exhibit maximum ϕEL(F) at Fmax around 106 V·cm-1. This behavior can be explained by the carrier motion-controlled Langevin recombination in the low-field region, and the Thomson-like recombination, controlled by the carrier capture, dominating the high-field regime.